@article{tai_yuan_merzari_bolotnov_2024, title={High-Fidelity Simulation of the Light-to-Dense Stratification Transient in the HiRJET Facility}, volume={8}, ISSN={["1943-7471"]}, DOI={10.1080/00295450.2024.2382621}, journal={NUCLEAR TECHNOLOGY}, author={Tai, Cheng-Kai and Yuan, Haomin and Merzari, Elia and Bolotnov, Igor A.}, year={2024}, month={Aug} }
 @article{bolotnov_iskhakov_nguyen_tai_wiser_baglietto_dinh_shaver_merzari_2024, title={NEAMS IRP challenge problem 1: Flexible modeling for heat transfer for low-to-high Prandtl number fluids for applications in advanced reactors}, volume={428}, ISSN={["1872-759X"]}, DOI={10.1016/j.nucengdes.2024.113467}, journal={NUCLEAR ENGINEERING AND DESIGN}, author={Bolotnov, Igor A. and Iskhakov, Arsen S. and Nguyen, Tri and Tai, Cheng-Kai and Wiser, Ralph and Baglietto, Emilio and Dinh, Nam and Shaver, Dillon and Merzari, Elia}, year={2024}, month={Nov} }
 @article{zhang_rafique_ding_bolotnov_hampel_2023, title={A direct numerical simulation study to elucidate the enhancement of heat transfer for nucleate boiling on surfaces with micro-pillars}, volume={147}, ISSN={["1879-0178"]}, DOI={10.1016/j.icheatmasstransfer.2023.106943}, abstractNote={Recent experimental studies have demonstrated great potential of surface engineering in enhancing nucleate boiling heat transfer performance. However, the underlying mechanism remains unclear, especially the role of microlayer evaporation underneath bubbles. In this work, we investigate the heat transfer from microlayer evaporation underneath a growing bubble on micro-pillar arrayed surfaces using Direct Numerical Simulations (DNS). The evolution of the microlayer is reproduced in the DNS by considering a bubble growth driven by the local temperature gradient. The effects of micro-pillar structures on the microlayer profile and the heat transfer performance are systematically studied and analyzed. Our simulation results reveal three distinct microlayer morphologies related to micro-pillar structures: the undisturbed microlayer, the disturbed microlayer, and the disrupted microlayer. It can be further generalized as the greater the spacing and height of the micro-pillars, the more disrupted the microlayer becomes. Due to the reduction of microlayer thickness, more disruption means higher microlayer heat transfer coefficient. However, this accelerates microlayer depletion and thus reduces the overall heat transfer potential from microlayer evaporation during its life cycle in nucleate boiling. Based on these findings, a strategy is revealed for the design of micro-pillar arrayed surfaces to achieve optimal heat transfer performance of the microlayer.}, journal={INTERNATIONAL COMMUNICATIONS IN HEAT AND MASS TRANSFER}, author={Zhang, Jinming and Rafique, Maimuna and Ding, Wei and Bolotnov, Igor A. and Hampel, Uwe}, year={2023}, month={Oct} }
 @article{iskhakov_tai_bolotnov_nguyen_merzari_shaver_dinh_2023, title={Data-Driven RANS Turbulence Closures for Forced Convection Flow in Reactor Downcomer Geometry}, volume={3}, ISSN={["1943-7471"]}, url={https://doi.org/10.1080/00295450.2023.2185056}, DOI={10.1080/00295450.2023.2185056}, abstractNote={Recent progress in data-driven turbulence modeling has shown its potential to enhance or replace traditional equation-based Reynolds-averaged Navier-Stokes (RANS) turbulence models. This work utilizes invariant neural network (NN) architectures to model Reynolds stresses and turbulent heat fluxes in forced convection flows (when the models can be decoupled). As the considered flow is statistically one dimensional, the invariant NN architecture for the Reynolds stress model reduces to the linear eddy viscosity model. To develop the data-driven models, direct numerical and RANS simulations in vertical planar channel geometry mimicking a part of the reactor downcomer are performed. Different conditions and fluids relevant to advanced reactors (sodium, lead, unitary-Prandtl number fluid, and molten salt) constitute the training database. The models enabled accurate predictions of velocity and temperature, and compared to the baseline k−τ turbulence model with the simple gradient diffusion hypothesis, do not require tuning of the turbulent Prandtl number. The data-driven framework is implemented in the open-source graphics processing unit–accelerated spectral element solver nekRS and has shown the potential for future developments and consideration of more complex mixed convection flows.}, journal={NUCLEAR TECHNOLOGY}, author={Iskhakov, Arsen S. and Tai, Cheng-Kai and Bolotnov, Igor A. and Nguyen, Tri and Merzari, Elia and Shaver, Dillon R. and Dinh, Nam T.}, year={2023}, month={Mar} }
 @article{tai_nguyen_iskhakov_merzari_dinh_bolotnov_2023, title={Direct Numerical Simulation of Low and Unitary Prandtl Number Fluids in Reactor Downcomer Geometry}, volume={6}, ISSN={["1943-7471"]}, DOI={10.1080/00295450.2023.2213286}, abstractNote={Mixed convection of low and unitary Prandtl fluids in a vertical passage is fundamental to passive heat removal in liquid metal and gas-cooled advanced reactor designs. Capturing the influence of buoyancy in flow and heat transfer in engineering analysis is hence a cornerstone to the safety of the next-generation reactor. However, accurate prediction of the mixed convection phenomenon has eluded current turbulence and heat transfer modeling approaches, yet further development and validation of modeling methods is limited by a scarcity of high-fidelity data pertaining to reactor heat transfer. In this work, a series of direct numerical simulations was conducted to investigate the influence of buoyancy on descending flow of liquid sodium, lead, and unitary Prandtl fluid in a differentially heated channel that represents the reactor downcomer region. From time-averaged statistics, flow-opposing/aiding buoyant plumes near the heated/cooled wall distort the mean velocity distribution, which gives rise to promotion/suppression of turbulence intensity and modification of turbulent shear stress and heat flux distribution. Frequency analysis of time series also suggests the existence of large-scale convective and thermal structures rising from the heated wall. As a general trend, fluids of lower Prandtl number were found to be more susceptible to the buoyancy effect due to stronger differential buoyancy across the channel. On the other hand, the effectiveness of convective heat transfer of the three studied fluids showed a distinct trend against the influence of buoyancy. Physical reasoning on observation of the Nusselt number trend is also discussed.}, journal={NUCLEAR TECHNOLOGY}, author={Tai, Cheng-Kai and Nguyen, Tri and Iskhakov, Arsen S. and Merzari, Elia and Dinh, Nam T. and Bolotnov, Igor A.}, year={2023}, month={Jun} }
 @article{bolotnov_2023, title={Direct Numerical Simulation of Single- and Two-Phase Flows for Nuclear Engineering Geometries}, volume={7}, ISSN={["1943-7471"]}, DOI={10.1080/00295450.2023.2232222}, abstractNote={Abstract The significant progress in the last decade of high-resolution single- and two-phase flow simulations of reactor-relevant flows is summarized in this review paper. The rapid development of high-performance computing capabilities creates exciting opportunities to study complex reactor thermal-hydraulic phenomena. Today’s advances in thermal-hydraulic analysis, interface capturing simulations, and advanced data processing and analysis approaches will help pave the way to the next level of understanding of two-phase flow behavior in nuclear reactors. This paper discusses two major topics: (1) a brief review of interface-capturing simulations in recent years and (2) several opportunities to advance these numerical research tools in the future. The first part discusses typical computational methods used for these simulations and provides some examples of past work, as well as computational cost estimates and affordability of such simulations for research and industrial applications. In the second part, some specific examples are discussed that could be analyzed using exascale supercomputers being designed and projected to be online in the next several years. New-generation methodologies are required to take full advantage of these capabilities to greatly enhance the scientific understanding of complex two-phase flow phenomena in various conditions relevant to industrial applications.}, journal={NUCLEAR TECHNOLOGY}, author={Bolotnov, Igor A.}, year={2023}, month={Jul} }
 @article{li_liao_bolotnov_zhou_lucas_li_gong_2023, title={Direct numerical simulation of heat transfer on a deformable vapor bubble rising in superheated liquid}, volume={35}, ISSN={["1089-7666"]}, DOI={10.1063/5.0137675}, abstractNote={Heat transfer on a vapor bubble rising in superheated liquid is investigated by direct numerical simulation. The vapor–liquid system is described by the one-fluid formulation with the level set method capturing the interface. The proportional-integral-derivative controller is employed to keep the bubble's location fixed and evaluate interfacial forces. The heat transfer performance featured by the Nusselt number is evaluated based on the energy balance. Simulations are carried out for the bubble Reynolds number ranging from 20 to 500 and Morton number from 1.10 × 10−10 to 3.80 × 10−4. The aim of this paper is to shed some light on the effect of bubble deformation and oscillation on interfacial heat transfer. The results show that the front part of the bubble contributes to the majority of the interfacial heat transfer, while the rear part mainly affects the oscillation amplitude of the total heat transfer. The interface stretch during bubble oscillation is considered as a key mechanism in enhancing the instantaneous Nusselt number. The potential flow solution of the averaged Nusselt number is corrected by considering the influence of the aspect ratio. This research provides additional insights into the mechanism of interfacial heat transfer, and the results apply equally to interfacial mass transfer.}, number={2}, journal={PHYSICS OF FLUIDS}, author={Li, Jiadong and Liao, Yixiang and Bolotnov, Igor A. and Zhou, Ping and Lucas, Dirk and Li, Qing and Gong, Liang}, year={2023}, month={Feb} }
 @article{zhang_rafique_ding_bolotnov_hampel_2023, title={Direct numerical simulation of microlayer formation and evaporation underneath a growing bubble driven by the local temperature gradient in nucleate boiling}, volume={193}, ISSN={["1778-4166"]}, DOI={10.1016/j.ijthermalsci.2023.108551}, abstractNote={Recently, experiments carried out with high-resolution measurement techniques showed a thin liquid microlayer (∼μm) formation underneath a growing bubble in nucleate boiling. However, a deep understanding of the heat transfer enhancement induced by this microlayer is still lacking. In the present work, we investigate the microlayer dynamics and its effect on microlayer evaporation during the microlayer formation stage. Using direct numerical simulations with the PHASTA solver, a fully resolved microlayer underneath a growing bubble driven by the local temperature gradient in nucleate boiling is reproduced. The simulation results are compared and largely validated against recent experimental observations and Mikic's model. The detailed microlayer dynamics indicates that the microlayer formation can be considered a quasi-steady process. In addition, the hydrodynamic effect shows limited influences on the microlayer thickness, thus suggesting a rather constant amount of microlayer evaporation during the entire microlayer life cycle under different hydrodynamic conditions. Here, the maximum evaporative heat flux of the microlayer occurs near the contact line and exceeds 1 MW/m2. Evaporation of the microlayer can account for ∼50% bubble volume growth after the onset of nucleate boiling. This value emphasizes the significance of microlayer evaporation in nucleate boiling modeling.}, journal={INTERNATIONAL JOURNAL OF THERMAL SCIENCES}, author={Zhang, Jinming and Rafique, Maimuna and Ding, Wei and Bolotnov, Igor A. and Hampel, Uwe}, year={2023}, month={Nov} }
 @article{iskhakova_kondo_tanimoto_dinh_bolotnov_2023, title={Interface Capturing Flow Boiling Simulations in a Compact Heat Exchanger}, volume={145}, ISSN={["2832-8469"]}, DOI={10.1115/1.4056688}, abstractNote={Abstract
               High-fidelity flow boiling simulations are conducted in a vertical mini channel with offset strip fins (OSF) using R113 as a working fluid. Finite-element code PHASTA coupled with level set method for interface capturing is employed to model multiple sequential bubble nucleation using transient three-dimensional approach. The code performance is validated against experiments for a single nucleation site in a vertical rectangular channel. To assess code performance, a study on the bubble departure from the wall in a mini channel with OSF is carried out first. Contributions from the microlayer are not considered due to low heat flux values applied to the channel (1 kW/m2). The influence of surface characteristics, such as contact angle and liquid superheat on bubble dynamics, is also analyzed as well as the local two-phase heat transfer coefficient. For higher void fractions, two conical nucleation cavities are introduced in the same channel with OSF. Observed bubble characteristics (departure diameter, bubble departure frequency) are evaluated and bubble trajectories are presented and analyzed. The local heat transfer coefficient is then evaluated for each simulation. The results show approximately a 2.5 time increase in the local heat transfer coefficient when the individual bubbles approach the wall. With smaller bubble nucleation diameters, the heat transfer coefficient can increase by up to a factor of two. Thus, the current work demonstrates the flow modeling capability of the boiling phenomena in complex geometry with OSF.}, number={4}, journal={ASME JOURNAL OF HEAT AND MASS TRANSFER}, author={Iskhakova, Anna and Kondo, Yoshiyuki and Tanimoto, Koichi and Dinh, Nam T. and Bolotnov, Igor A.}, year={2023}, month={Apr} }
 @article{tai_mao_petrov_manera_bolotnov_2023, title={Study of Stable Stratification in HiRJET Facility With Direct Numerical Simulation}, volume={6}, ISSN={["1943-748X"]}, DOI={10.1080/00295639.2023.2197656}, abstractNote={Stable density stratification in a large enclosure could significantly hamper the effectiveness of natural convection cooling in pool-type liquid metal or gas-cooled advanced reactors. In addition, accurate prediction of stratified front behavior remains to be a challenging task for turbulence modeling. With the rapid growth of high-performance-computing capabilities in recent years, conducting high-fidelity simulations for a large-timescale transient has become more affordable and hence a valuable data source to support turbulence modeling as well as to gain further physical insights. In this work, direct numerical simulation is performed at experiment-consistent conditions to simulate the density stratification transient High-Resolution Jet (HiRJET) facility. Specifically, we focus on the case where an injected aqueous sugar solution has 1.5% density higher than that in the enclosure. In the early stage of the transient, the impingement of the denser jet to the bottom surface of the enclosure promoted turbulent mixing locally. This rendered the establishment of the mixture layer, formation and swift upward propagation of the stratified front, and elevation with (locally) the highest vertical concentration gradient. As the front rose, the diminishing turbulent mass flux slowed down the propagation, and a larger vertical concentration gradient was established. In this stage, both the velocity and the concentration scalar showed large-timescale fluctuation behavior around the stratified front. For the concentration time signal, the characteristic frequency in the power spectral density was found to agree well with the Brunt-Väisällä frequency. The preliminary validation endeavor showed that the stratified front location and the corresponding concentration gradient magnitude in the simulation agreed well with the experiment data. Further validation will mainly revolve around benchmarking against high-resolution density measurement and high-order flow statistics.}, journal={NUCLEAR SCIENCE AND ENGINEERING}, author={Tai, Cheng-Kai and Mao, Jiaxin and Petrov, Victor and Manera, Annalisa and Bolotnov, Igor A.}, year={2023}, month={Jun} }
 @article{nguyen_merzari_tai_bolotnov_jackson_2023, title={Toward Improved Correlations for Mixed Convection in the Downcomer of Molten Salt Reactors}, volume={7}, ISSN={["1943-7471"]}, DOI={10.1080/00295450.2023.2223036}, abstractNote={Developing heat transfer correlations for buoyancy-driven flows and mixed convection is challenging, especially if the fluid's Prandtl (Pr) number is not close to 1. For advanced nuclear reactor (Generation IV) designs, the downcomer plays a crucial role in normal operation and loss-of-power scenarios. The fluid-flow behavior in the downcomer can involve forced, mixed, or natural convection. Characterizing the heat transfer for these changing regimes is a serious challenge, especially in the heat transfer deterioration region. In this paper, the downcomer is simplified to heated parallel plates. The high–Pr number fluid FLiBe (a mixture of lithium fluoride and beryllium fluoride) is considered for all simulations. Direct numerical simulations using the graphics processing unit–based spectral element code NekRS are performed for a wide range of the Richardson number, from 0 to 400, at two different FLiBe Pr numbers (12 and 24). This results in an unprecedented 74 cases in total. Each case's Nusselt number is calculated to evaluate existing heat transfer correlations.Moreover, we propose several new modifications for cases without satisfactory choice. As a result, several novel mixed-convection heat transfer correlations have been built for high–Pr number fluids. The correlations are expressed as a function of the buoyancy number, covering several mixed-convection regimes. The Pr number effect on the Nusselt number behavior is also analyzed in detail. We also propose a novel method to evaluate the heat transfer deterioration region. Modified Reynolds-Gnielinski forced-convection correlations are defined for the laminarization region, and a free-convection correlation is used for the natural-convection-dominated region. These correlations can describe well the trend in the heat transfer–deficient region.}, journal={NUCLEAR TECHNOLOGY}, author={Nguyen, Tri and Merzari, Elia and Tai, Cheng-Kai and Bolotnov, Igor A. and Jackson, Brian}, year={2023}, month={Jul} }
 @article{mao_vishwakarma_welker_tai_bolotnov_petrov_manera_2023, title={Validation of RANS-Based Turbulence Models Against High-Resolution Experiments and DNS for Buoyancy-Driven Flow with Stratified Fronts}, volume={8}, ISSN={["1943-748X"]}, DOI={10.1080/00295639.2023.2241800}, abstractNote={AbstractTo provide computational fluid dynamics (CFD)–grade experimental data for studying stratification, measurements on the High-Resolution Jet (HiRJet) facility at the University of Michigan have been conducted with density differences of 1.5% and −1.5%, respectively. Fluid with a density different from the fluid initially present in the HiRJet tank was injected, and the propagation of the time-dependent density stratification was captured on a two-dimensional plane with the aid of the wire-mesh sensor technique for Reynolds numbers near 5000 and Richardson numbers near 0.29. Direct numerical simulations (DNSs) of the two cases have also been conducted to expand the multifidelity database. The novel experimental and DNS data were then used to assess the predictive capabilities of the Standard k−ε (SKE) model and the Reynolds Stress Transport (RST) model. In particular, the propagation speed and thickness of the stratification fronts were assessed by comparing the CFD results against the experimental and DNS data. It was found that the general trends of the stratified density fronts were well predicted by the CFD simulations; however, slight overprediction of the thickness of the stratification layer was found with the SKE model while the RST model gave a larger overprediction of the mixing. Examination of the turbulent statistics showed that the turbulent viscosity was largely overpredicted by the RST model compared to the SKE model.Keywords: StratificationexperimentsDNSCFD RANS, HiRJet Disclosure StatementNo potential conflict of interest was reported by the author(s).Additional informationFundingThis work was cofunded by the NEUP and the IRP entitled "Center of Excellence for Thermal-Fluids Applications in Nuclear Energy: Establishing the Knowledgebase for Thermal-Hydraulic Multiscale Simulation to Accelerate the Deployment of Advanced Reactors," IRP-NEAMS-1.1, "Thermal-Fluids Applications in Nuclear Energy," from DOE. The authors are thankful for the financial support from the DOE office and appreciate the insightful suggestions from all colleagues under these two projects.}, journal={NUCLEAR SCIENCE AND ENGINEERING}, author={Mao, J. and Vishwakarma, V. and Welker, Z. and Tai, C. K. and Bolotnov, I. A. and Petrov, V. and Manera, A.}, year={2023}, month={Aug} }
 @article{iskhakov_tai_bolotnov_dinh_2022, title={A Perspective on Data-Driven Coarse Grid Modeling for System Level Thermal Hydraulics}, volume={9}, ISSN={["1943-748X"]}, url={https://doi.org/10.1080/00295639.2022.2107864}, DOI={10.1080/00295639.2022.2107864}, abstractNote={Abstract In the future, advanced reactors are expected to play an important role in nuclear power. However, their development and deployment are hindered by the absence of reliable and efficient models for analysis of system thermal hydraulics (TH). For instance, mixing and thermal stratification in reactor enclosures cannot be captured by traditional one-dimensional system codes, yet usage of high-resolution solvers is computationally expensive. Recent developments of coarse grid (CG) and system codes with three-dimensional capabilities have shown that they are promising tools for system-level analysis. However, these codes feature large turbulence model form and discretization errors and require further improvements to capture turbulent effects during complex transients. Improvements can be achieved by using data-driven (DD) approaches. This paper provides an overview of recent applications of DD methods in the areas of fluid dynamics and TH. It is demonstrated that they are being widely applied for engineering-scale analysis (e.g., closures for large eddy simulations/Reynolds-averaged Navier-Stokes using direct numerical simulation data). However, they cannot be directly employed for the system scale because of some features of the latter: usage of CG, transient nature of the considered phenomena, nonlinear interaction of multiple closures, etc. At the same time, accumulated experience allows outlining of potential frameworks for further developments in DD CG modeling of system-level TH.}, journal={NUCLEAR SCIENCE AND ENGINEERING}, publisher={Informa UK Limited}, author={Iskhakov, Arsen S. and Tai, Cheng-Kai and Bolotnov, Igor A. and Dinh, Nam T.}, year={2022}, month={Sep} }
 @article{zhu_dinh_saini_bolotnov_2022, title={An adaptive knowledge-based data-driven approach for turbulence modeling using ensemble learning technique under complex flow configuration: 3D PWR sub-channel with DNS data}, volume={393}, ISSN={["1872-759X"]}, DOI={10.1016/j.nucengdes.2022.111814}, abstractNote={This work describes a new approach to increase the accuracy of Reynolds-averaged Navier–Stokes (RANS) in modeling turbulence flow leveraging the machine learning technique. Traditionally, different turbulence models for Reynolds stress are developed for different flow patterns based on human knowledge. Each turbulence model has a certain application domain and prediction uncertainty. In recent years, with the rapid improvements of machine learning techniques, researchers start to develop an approach to compensate for the prediction discrepancy of traditional turbulence models with statistical models and data. However, the approach has deficiencies in several aspects. For example, the amount of human knowledge introduced to the statistical model couldn't be controlled, which makes the statistical model learn from a very naïve stage and limits its application. In this work, a new approach is developed to address those deficiencies. The new approach uses the "ensemble learning" technique to control the amount of human knowledge introduced into the statistical model. Therefore, the new approach could be adaptive to the multiple application domains. According to the results of case study, the new approach shows higher accuracy than both traditional turbulence models and the previous machine learning approach.}, journal={NUCLEAR ENGINEERING AND DESIGN}, author={Zhu, Yangmo and Dinh, Nam T. and Saini, Nadish and Bolotnov, Igor A.}, year={2022}, month={Jul} }
 @article{tai_evdokimov_schlegel_bolotnov_lucas_2022, title={Development of machine learning framework for interface force closures based on bubble tracking data}, volume={399}, ISSN={["1872-759X"]}, DOI={10.1016/j.nucengdes.2022.112032}, abstractNote={Interfacial force closures in the two-fluid model play a critical role for the predictive capabilities of void fraction distribution. However, the practices of interfacial force modeling have long been challenged by the inherent physical complexity of the two-phase flows. The rapidly expanding computational capabilities in the recent years have made high-fidelity data from the interface-captured direct numerical simulation become more available, and hence potential for data-driven interfacial force modeling has prevailed. In this work, we established a data-driven modeling framework integrated to the HZDR multiphase Eulerian-Eulerian framework for computational fluid dynamics simulations. The data-driven framework is verified in a benchmark problem, where a feedforward neural network managed to capture the non-linear mapping between bubble Reynolds number and drag coefficient and reproduce the void distribution resulting from the baseline model in the test case. The second focus is on utilizing the bubble tracking data set to form a closure for the bubble drag in the turbulent bubbly flow, in which the drag coefficient is set to be correlated with the bubble Reynolds number and the Eötvös number. Pseudo-steady state filtering in the Frenet Frame was carried out to obtain the drag coefficient from the turbulent bubbly flow data. The performance of the data-driven drag model is also examined through a case study, where improvement of model's prediction near-wall is regarded necessary. Discussion and further plans of investigation are provided.}, journal={NUCLEAR ENGINEERING AND DESIGN}, author={Tai, Cheng-Kai and Evdokimov, Ilya and Schlegel, Fabian and Bolotnov, Igor A. and Lucas, Dirk}, year={2022}, month={Dec} }
 @article{fan_li_pointer_bolotnov_2022, title={High-fidelity pool boiling simulations on multiple nucleation sites using interface capturing method}, volume={399}, ISSN={["1872-759X"]}, DOI={10.1016/j.nucengdes.2022.112004}, abstractNote={Boiling has proved to be one of the most efficient means for heat transfer and is a very important phenomenon during severe accident scenarios in light water reactors. High-fidelity pool boiling simulations can provide a numerical database for improving mechanistic boiling models by allowing for specific evaluation of interactions among bubbles. Previously published pool boiling simulations investigated two nucleation sites in which bubble growth at one site suppressed nucleation at the other site. Based on previous study results, more complicated interface-capturing simulations on pool boiling were conducted using PHASTA code with locally refined unstructured mesh. First, different boundary conditions (BCs) were assessed to support robustness and reproducibility of the boiling model. Then, a scale study was conducted at a larger domain with nine nucleation sites where either nine or four nucleation sites are activated. Involving more nucleation sites increased the complexity of bubble interactions from surrounding sites. Finally, bubble departure behavior influenced by wall heat flux was investigated. When heat flux was increased, the order of bubble departure changed, but diagonal bubbles always departed after one another. The departure time interval between the first and second bubble reduced as heat flux increased. The corresponding frequency was almost linearly proportional to the heat flux. In addition, bubble departure behavior was found to be greatly influenced by the nucleation site pattern. Multiple nucleation sites resulted in superimposed inhibitive effects from surrounding sites to each bubble, which extensively delayed the departure. This new observation was not discussed in previously published works. The work presented here provides new insight on the fundamental understanding of boiling phenomena, contributes to the development of a 3D multiphase computational fluid dynamics (M-CFD) model, and provides a more comprehensive database for data-driven pool boiling studies.}, journal={NUCLEAR ENGINEERING AND DESIGN}, author={Fan, Yuqiao and Li, Mengnan and Pointer, William D. and Bolotnov, Igor A.}, year={2022}, month={Dec} }
 @article{pillai_sponsel_mast_kushner_bolotnov_stapelmann_2022, title={Plasma breakdown in bubbles passing between two pin electrodes}, volume={55}, ISSN={["1361-6463"]}, url={https://doi.org/10.1088/1361-6463/ac9538}, DOI={10.1088/1361-6463/ac9538}, abstractNote={Abstract
               The ignition of plasmas in liquids has applications from medical instrumentation to manipulation of liquid chemistry. Formation of plasmas directly in a liquid often requires prohibitively large voltages to initiate breakdown. Producing plasma streamers in bubbles submerged in a liquid with higher permittivity can significantly lower the voltage needed to initiate a discharge by reducing the electric field required to produce breakdown. The proximity of the bubble to the electrodes and the shape of the bubbles play critical roles in the manner in which the plasma is produced in, and propagates through, the bubble. In this paper, we discuss results from a three-dimensional direct numerical simulation (DNS) used to investigate the shapes of bubbles formed by injection of air into water. Comparisons are made to results from a companion experiment. A two-dimensional plasma hydrodynamics model was then used to capture the plasma streamer propagation in the bubble using a static bubble geometry generated by the DNS The simulations showed two different modes for streamer formation depending on the bubble shape. In an elliptical bubble, a short electron avalanche triggered a surface ionization wave (SIWs) resulting in plasma propagating along the surface of the bubble. In a circular bubble, an electron avalanche first traveled through the middle of the bubble before two SIWs began to propagate from the point closest to the grounded electrode where a volumetric streamer intersected the surface. In an elliptical bubble approaching a powered electrode in a pin-to-pin configuration, we experimentally observed streamer behavior that qualitatively corresponds with computational results. Optical emission captured over the lifetime of the streamer curve along the path of deformed bubbles, suggesting propagation of the streamer along the liquid/gas boundary interface. Plasma generation supported by the local field enhancement of the deformed bubble surface boundaries is a mechanism that is likely responsible for initiating streamer formation.}, number={47}, journal={JOURNAL OF PHYSICS D-APPLIED PHYSICS}, author={Pillai, Naveen and Sponsel, Nicholas L. and Mast, J. T. and Kushner, Mark J. and Bolotnov, Igor A. and Stapelmann, Katharina}, year={2022}, month={Nov} }
 @article{bolotnov_benhamadouche_2022, title={Selected papers from the 2020 International Topical Meeting on Advances in Thermal Hydraulics (ATH'20) Foreword}, volume={208}, ISSN={["1943-7471"]}, DOI={10.1080/00295450.2022.2086385}, number={8}, journal={NUCLEAR TECHNOLOGY}, author={Bolotnov, Igor and Benhamadouche, Sofiane}, year={2022}, month={Aug}, pages={III-III} }
 @article{fan_fang_bolotnov_2022, title={Complex bubble deformation and break-up dynamics studies using interface capturing approach (vol 3, pg 139, 2021)}, volume={4}, ISSN={["2661-8877"]}, DOI={10.1007/s42757-021-0127-1}, abstractNote={The article “Complex bubble deformation and break-up dynamics studies using interface capturing approach” written by Yuqiao Fan, Jun Fang, and Igor Bolotnov, was originally published electronically on the publisher’s internet portal (currently SpringerLink) on 18 July 2020 without open access. After publication in Volume 3, Issue 3, page 139–151, the author(s) decided to opt for Open Choice and to make the article an open access publication. Therefore, the copyright of the article has been changed to © The Author(s) 2021 and the article is forthwith distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits use, duplication, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.}, number={2}, journal={EXPERIMENTAL AND COMPUTATIONAL MULTIPHASE FLOW}, author={Fan, Yuqiao and Fang, Jun and Bolotnov, Igor}, year={2022}, month={Jun}, pages={191–191} }
 @article{saini_bolotnov_2021, title={Detailed Analysis of the Effects of Spacer Grid and Mixing Vanes on Turbulence in a PWR Subchannel Under DFFB Conditions Based on DNS Data}, volume={12}, ISSN={["1943-7471"]}, DOI={10.1080/00295450.2021.1974279}, abstractNote={Abstract Spacer grids and mixing vanes exhibit a significant role in the thermal hydraulics of pressurized water reactors (PWRs), especially in the post loss-of-coolant accident regimes. A detailed analysis of the contrasting upstream and downstream turbulent flow features is of great importance to both system codes and computational fluid dynamics (CFD)–Reynolds-averaged Navier–Stokes (RANS) modeling. Further, with the advent of supercomputing resources and machine learning research, a data-driven approach to turbulence modeling is gaining popularity. However, owing to the complexities associated with large-scale, high-fidelity data collection capabilities, the application of machine learning–based turbulence models has been limited to simple geometries. In this work, using a highly scalable CFD code PHASTA, we have collected data from direct numerical simulations of a PWR subchannel with high spatial and temporal resolution. From the collected data we extract key turbulent flow features, including mean velocities and Reynolds stresses that highlight the effects of spacer grids and mixing vanes on downstream turbulence in a typical PWR subchannel. An invariant analysis of the anisotropic stress tensor is also presented, which further elucidates their effect on the nature of turbulence in the immediate upstream and downstream vicinity. The high-resolution data from the simulations are archived and intended for the development of data-driven RANS closure models that are capable of capturing the evolution of anisotropy in PWR subchannels.}, journal={NUCLEAR TECHNOLOGY}, author={Saini, Nadish and Bolotnov, Igor A.}, year={2021}, month={Dec} }
 @article{pillai_sponsel_stapelmann_bolotnov_2022, title={Direct Numerical Simulation of Bubble Formation Through a Submerged "Flute" With Experimental Validation}, volume={144}, ISSN={["1528-901X"]}, DOI={10.1115/1.4052051}, abstractNote={Abstract
               Direct numerical simulation (DNS) is often used to uncover and highlight physical phenomena that are not properly resolved using other computational fluid dynamics methods due to shortcuts taken in the latter to cheapen computational cost. In this work, we use DNS along with interface tracking to take an in-depth look at bubble formation, departure, and ascent through water. To form the bubbles, air is injected through a novel orifice geometry not unlike that of a flute submerged underwater, which introduces phenomena that are not typically brought to light in conventional orifice studies. For example, our single-phase simulations show a significant leaning effect, wherein pressure accumulating at the trailing nozzle edges leads to asymmetric discharge through the nozzle hole and an upward bias in the flow in the rest of the pipe. In our two-phase simulations, this effect is masked by the surface tension of the bubble sitting on the nozzle, but it can still be seen following departure events. After bubble departure, we observe the bubbles converge toward an ellipsoidal shape, which has been validated by experiments. As the bubbles rise, we note that local variations in the vertical velocity cause the bubble edges to flap slightly, oscillating between relatively low and high velocities at the edges.}, number={2}, journal={JOURNAL OF FLUIDS ENGINEERING-TRANSACTIONS OF THE ASME}, author={Pillai, Naveen and Sponsel, Nicholas L. and Stapelmann, Katharina and Bolotnov, Igor A.}, year={2022}, month={Feb} }
 @article{zimmer_bolotnov_2021, title={Evaluation of Length Scales and Meshing Requirements for Resolving Two-Phase Flow Regime Transitions Using the Level Set Method}, volume={143}, ISSN={["1528-901X"]}, DOI={10.1115/1.4049934}, abstractNote={Abstract
               New criteria for fully resolving two-phase flow regime transitions using direct numerical simulation with the level set method for interface capturing are proposed. A series of flows chosen to capture small scale interface phenomena are simulated at different grid refinements. These cases include droplet deformation and breakup in a simple shear field, the thin film around a Taylor bubble, and the rise of a bubble toward a free surface. These cases cover the major small scale phenomena observed in two-phase flows: internal recirculation, interface curvature, interface snapping, flow of liquid in thin films, and drainage/snapping of thin films. The results from these simulations and their associated grid studies were used to develop new meshing requirements for simulation of two-phase flow using interface capturing methods, in particular the level set method. When applicable, the code used in this work, PHASTA, was compared to experiments in order to contribute to the ongoing validation process of the code. Results show that when the solver meets these criteria, with the exception of resolving the nanometer scale liquid film between coalescing bubbles, the code is capable of accurately simulating interface topology changes.}, number={6}, journal={JOURNAL OF FLUIDS ENGINEERING-TRANSACTIONS OF THE ASME}, author={Zimmer, Matthew D. and Bolotnov, Igor A.}, year={2021}, month={Jun} }
 @article{lahey_baglietto_bolotnov_2021, title={Progress in multiphase computational fluid dynamics}, volume={374}, ISSN={["1872-759X"]}, DOI={10.1016/j.nucengdes.2020.111018}, abstractNote={This paper is primarily concerned with the development of three dimensional (3-D) multiphase computational fluid dynamics models for use in Pressurized Water Nuclear Reactor (PWR) design and analysis. These models were developed during the last 40 years, that is, during the time since the first NURETH meeting in 1980. The major topics in this paper include: the development of a 3-D two-fluid model for the MCFD prediction of phase distribution in turbulent adiabatic and diabatic bubbly flows. The mechanistic prediction of departure from nucleate boiling (DNB), and the direct numerical simulation (DNS) of bubbly flows. While significant progress has been made in the modeling and prediction of bubbly flows, specific recommendations are made in this paper for further improvements and for the extension of this type of MCFD model to other flow regimes.}, journal={NUCLEAR ENGINEERING AND DESIGN}, author={Lahey, Richard T., Jr. and Baglietto, Emilio and Bolotnov, Igor A.}, year={2021}, month={Apr} }
 @article{saini_bolotnov_2021, title={Two-Phase Turbulence Statistics from High Fidelity Dispersed Droplet Flow Simulations in a Pressurized Water Reactor (PWR) Sub-Channel with Mixing Vanes}, volume={6}, ISSN={["2311-5521"]}, url={https://www.mdpi.com/2311-5521/6/2/72}, DOI={10.3390/fluids6020072}, abstractNote={In the dispersed flow film boiling regime (DFFB), which exists under post-LOCA (loss-of-coolant accident) conditions in pressurized water reactors (PWRs), there is a complex interplay between droplet dynamics and turbulence in the surrounding steam. Experiments have accredited particular significance to droplet collision with the spacer-grids and mixing vane structures and their consequent positive feedback to the heat transfer recorded in the immediate downstream vicinity. Enabled by high-performance computing (HPC) systems and a massively parallel finite element-based flow solver—PHASTA (Parallel Hierarchic Adaptive Stabilized Transient Analysis)—this work presents high fidelity interface capturing, two-phase, adiabatic simulations in a PWR sub-channel with spacer grids and mixing vanes. Selected flow conditions for the simulations are informed by the experimental data found in the literature, including the steam Reynolds number and collision Weber number (Wec={40,80}), and are characteristic of the DFFB regime. Data were collected from the simulations at an unprecedented resolution, which provides detailed insights into the continuous phase turbulence statistics, highlighting the effects of the presence of droplets and the comparative effect of different Weber numbers on turbulence in the surrounding steam. Further, axial evolution of droplet dynamics was analyzed through cross-sectionally averaged quantities, including droplet volume, surface area and Sauter mean diameter (SMD). The downstream SMD values agree well with the existing empirical correlations for the selected range of Wec. The high-resolution data repository from the simulations herein is expected to be of significance to guide model development for system-level thermal hydraulic codes.}, number={2}, journal={FLUIDS}, author={Saini, Nadish and Bolotnov, Igor A.}, year={2021}, month={Feb} }
 @article{fang_purser_smith_balakrishnan_bolotnov_jansen_2020, title={Annular Flow Simulation Supported by Iterative In-Memory Mesh Adaptation}, volume={194}, ISSN={["1943-748X"]}, DOI={10.1080/00295639.2020.1743577}, abstractNote={Abstract Various flow regimes exist in a boiling water reactor (BWR) as the steam quality increases in the uprising coolant flow, from bubbly flow, slug/churn flow, to annular flow. The annular flow is characterized by the presence of a fast-moving gas core and the surrounding liquid film flowing on the conduit wall. In addition, entrained droplets can be observed in the gas core with ingested bubbles in the liquid film. The dynamics occurring on the wavy interface between the liquid film and gas core plays a crucial role in affecting the heat transfer rate and pressure drop within the BWR core. However, a fundamental understanding of annular flow is still lacking, partly due to the difficulty in obtaining detailed local data in annular flow experiments. In the current study, a novel simulation framework is developed for the annular flow by coupling a computational fluid dynamics flow solver with state-of-the-art meshing software. The gas-liquid interface is tracked with the level set method. Based on the computed flow solutions, the computational mesh is dynamically adapted in memory to meet the local mesh resolution requirement. This iterative simulation-adaptation framework can ensure the fine mesh resolution across the interface, which not only helps mitigate the mass conservation degradation known to level set methods but also improves the representation of dramatic interface topological changes such as wave breaking and droplet entrainment. The present investigation will shed light onto the complex interfacial processes involved in annular flow and generate much needed simulation data for annular flow modeling.}, number={8-9}, journal={NUCLEAR SCIENCE AND ENGINEERING}, author={Fang, Jun and Purser, Meredith K. and Smith, Cameron and Balakrishnan, Ramesh and Bolotnov, Igor A. and Jansen, Kenneth E.}, year={2020}, month={Sep}, pages={676–689} }
 @article{zimmer_bolotnov_2020, title={Exploring Two-Phase Flow Regime Transition Mechanisms Using High-Resolution Virtual Experiments}, volume={194}, ISSN={["1943-748X"]}, DOI={10.1080/00295639.2020.1722543}, abstractNote={Abstract Recent advancements in computing power allow utilization of state-of-the-art direct numerical simulations (DNSs), coupled with interface tracking techniques, to perform fully resolved simulations of complex two-phase flows, such as flow regime transitions. Studying the highly resolved temporal and spatial information produced from these virtual experiments can advance our understanding of the phenomenon and inform coarser models. With these improved models, better predictions of flow regime behavior and location in boiling water reactors can be made. The presented research uses the PHASTA code, which employs the level set method for interface tracking, to examine the mechanisms of flow regime transition, specifically the slug-to-bubbly and slug-to–churn-turbulent regime transitions. The DNS was validated using theoretical and experimental work found in open literature. Different geometries, including pipes and minichannels, were explored in order to improve the fundamental understanding of the complex flow phenomenon. Using advanced analysis techniques, the transient flow properties were analyzed at resolutions not available to other methods. The numerical data analysis allows for calculation of both time and spatially averaged properties as well as local instantaneous properties. Possible mechanisms for the transition are discussed. Examples include liquid kinetic energy/surface tension energy balance and interfacial shear forces in the liquid film. It is also noted that the transition out of slug flow can take at least two pathways: interfacial wave-induced instability development in the Taylor bubble, leading to its disintegration, or strong bubble shearing at the tail of the bubble.}, number={8-9}, journal={NUCLEAR SCIENCE AND ENGINEERING}, author={Zimmer, Matthew D. and Bolotnov, Igor A.}, year={2020}, month={Sep}, pages={708–720} }
 @article{cambareri_fang_bolotnov_2020, title={Interface capturing simulations of bubble population effects in PWR subchannels}, volume={365}, ISSN={["1872-759X"]}, DOI={10.1016/j.nucengdes.2020.110709}, abstractNote={As the computational power of high-performance computing (HPC) facilities grows, so too does the feasibility of using first principle based simulation to study turbulent two-phase flows within complex pressurized water reactor (PWR) geometries. Direct numerical simulation (DNS), integrated with an interface capturing method, allows for the collection of high-fidelity numerical data using advanced analysis techniques. The presented research employs the massively parallel, finite-element based, unstructured mesh code, PHASTA, to simulate a set of two-phase bubbly flows through PWR subchannel geometries including auxiliary structures (spacer grids and mixing vanes). The main objective of the presented work is to analyze bubble dynamics and turbulence interactions at varying bubble concentrations to support the development of advanced two-phase flow closure models. Turbulent two-phase flows in PWR subchannels were simulated at hydraulic Reynolds numbers of 81,000 with bubble concentrations of 3%–15% by gas volume fraction (768–3928 resolved bubbles, respectively) and compared against a 1% void fraction case (262 bubbles) that had been previously simulated. The finite element mesh utilized for the study at higher bubble concentrations was composed of 1.55 billion elements, compared to the previous study which employed 1.11 billion elements, ensuring all turbulence scales and individual bubbles within the flow are fully resolved. For each case, the resolved initial bubble size was 0.65 mm in diameter (resolved with 25 grid points across the diameter). The simulations were analyzed to find flow features such as the mean velocity profile, bubble relative velocity and the effect of the bubbles on the turbulent conditions.}, journal={NUCLEAR ENGINEERING AND DESIGN}, author={Cambareri, Joseph J. and Fang, Jun and Bolotnov, Igor A.}, year={2020}, month={Aug} }
 @article{li_bolotnov_2020, title={The evaporation and condensation model with interface tracking}, volume={150}, ISSN={["1879-2189"]}, DOI={10.1016/j.ijheatmasstransfer.2019.119256}, abstractNote={Interface tracking simulation (ITS) is one of the promising approaches to describe heat transfer of boiling phenomena and their underlying mechanisms. Better understanding and modeling of this process will benefit various engineering systems relying on two-phase heat transfer. The presented research implements, verifies and validates the modeling capability of the evaporation process using a massively parallel unstructured grid flow solver, PHASTA. The verification of the evaporation and condensation model has been performed by comparing the bubble growth rate with analytical solutions. Both pool boiling and flow boiling simulations are performed using this ITS evaporation and condensation model. The bubble nucleation frequency in pool boiling simulation is validated against experimentally-based correlations. The bubble evolution and growth rate are compared with experimental data to validate the model performance under flow boiling condition. The authors propose an innovative ITS based-boiling model which can conduct boiling simulation with 3D unstructured computational mesh. This capability will serve as one of the most important building blocks for high resolution boiling simulation in realistic engineering geometries being developed under the PHASTA simulation framework. Compared to the structured-grid-based solvers, which are challenging to apply to complex engineering geometries, this boiling model implementation is capable of conducting high-resolution boiling simulations in engineering geometries and resolving the detailed hydrodynamics and thermal information for quantities of interest at/around the interface. This approach may help fulfill the numerical data gap between the local physical phenomena and the engineering scale ITS applications in the future.}, journal={INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER}, author={Li, Mengnan and Bolotnov, Igor A.}, year={2020}, month={Apr} }
 @misc{fang_cambareri_li_saini_bolotnov_2020, title={Interface-Resolved Simulations of Reactor Flows}, volume={206}, ISSN={["1943-7471"]}, DOI={10.1080/00295450.2019.1620056}, abstractNote={Abstract This critical review paper outlines the recent progress in high-resolution numerical simulations of two-phase coolant flow in light water reactor–relevant geometries by resolving the water-vapor interface. Rapid development of capabilities in high-performance computing is creating exciting opportunities to study complex reactor thermal-hydraulic phenomena. Today’s advances in thermal-hydraulic analysis and interface-resolved simulations will help pave the way to the next level of understanding of two-phase flow behavior in complex geometries. This paper consists of two major parts: (1) a brief review of direct numerical simulation and interface tracking simulation and (2) several opportunities in the near future to apply cutting-edge simulation and analysis capabilities to address the nuclear-related multiphase flow challenges. The first part will discuss typical computational methods used for the simulations and provide some examples of the past work as well as computational cost estimates and affordability of such simulations for research and industrial applications. In the second part specific application examples are discussed, from adiabatic bubbly flow simulations in pressurized water reactor subchannel geometry to the modeling of nucleate boiling. The uniqueness of this study lies in the specific focus on applications with nuclear engineering interest as well as new generation modeling and analysis methodologies. Together with the ever-growing computing power, the related large-scale two-phase flow simulations will become indispensable for the improved scientific understanding of complex two-phase flow phenomena in nuclear reactors under normal operation and postulated accident conditions.}, number={2}, journal={NUCLEAR TECHNOLOGY}, author={Fang, Jun and Cambareri, Joseph J. and Li, Mengnan and Saini, Nadish and Bolotnov, Igor A.}, year={2020}, month={Feb}, pages={133–149} }
 @article{hanna_dinh_youngblood_bolotnov_2020, title={Machine-learning based error prediction approach for coarse-grid Computational Fluid Dynamics (CG-CFD)}, volume={118}, ISSN={["1878-4224"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85072240790&partnerID=MN8TOARS}, DOI={10.1016/j.pnucene.2019.103140}, abstractNote={Computational Fluid Dynamics (CFD) is one of the modeling approaches essential to identifying the parameters that affect Containment Thermal Hydraulics (CTH) phenomena. While the CFD approach can capture the multidimensional behavior of CTH phenomena, its computational cost is high when modeling complex accident scenarios. To mitigate this expense, we propose reliance on coarse-grid CFD (CG-CFD). Coarsening the computational grid increases the grid-induced error thus requiring a novel approach that will produce a surrogate model predicting the distribution of the CG-CFD local error and correcting the fluid-flow variables. Given sufficiently fine-mesh simulations, a surrogate model can be trained to predict the CG-CFD local errors as a function of the coarse-grid local flow features. The surrogate model is constructed using Machine Learning (ML) regression algorithms. Two of the widely used ML regression algorithms were tested: Artificial Neural Network (ANN) and Random Forest (RF). The proposed CG-CFD method is illustrated with a three-dimensional turbulent flow inside a lid-driven cavity. We studied a set of scenarios to investigate the capability of the surrogate model to interpolate and extrapolate outside the training data range. The proposed method has proven capable of correcting the coarse-grid results and obtaining reasonable predictions for new cases (of different Reynolds number, different grid sizes, or larger geometries). Based on the investigated cases, we found this novel method maximizes the benefit of the available data and shows potential for a good predictive capability.}, journal={PROGRESS IN NUCLEAR ENERGY}, author={Hanna, Botros N. and Dinh, Nam T. and Youngblood, Robert W. and Bolotnov, Igor A.}, year={2020}, month={Jan} }
 @article{cambareri_fang_bolotnov_2020, title={Simulation scaling studies of reactor core two-phase flow using direct numerical simulation}, volume={358}, ISSN={["1872-759X"]}, DOI={10.1016/j.nucengdes.2019.110435}, abstractNote={Tremendous growth in supercomputing power in recent years has resulted in the emergence of high-resolution flow analysis methods as an advanced research tool to evaluate single and two-phase flow behavior. In particular, unstructured mesh-based methods have been applied to analyze flows in complex reactor core geometries, including those of light water reactors (LWR). The finite-element based code, PHASTA, is utilized to perform large-scale simulations of two-phase bubbly flows in LWR geometries. Given the large computational cost of direct numerical simulation (DNS) coupled with interface tracking methods (ITM), typical domains encompass a portion of a single subchannel. In the presented research, the state-of-the-art analysis of turbulent two-phase flows in complex LWR subchannel geometries are demonstrated at both prototypical reactor parameters as well as scaled low pressure conditions. Three different cases are studied, a high-pressure simulation in prototypical reactor subchannel geometry, a low-pressure case in prototypical geometry and a final low-pressure case in a geometry scaled up to conserve the ratio between the bubble size and the domain pitch. Utilizing advanced statistical processing tools, these simulation conditions are compared to shed light on the relevancy of two-phase flow characteristics given the significant differences between LWR and low-pressure conditions. These findings can lead to the generation of useful guiding principles when researchers need to scale the two-phase flow behavior captured at low pressure and temperature conditions to those at reactor operating conditions.}, journal={NUCLEAR ENGINEERING AND DESIGN}, author={Cambareri, Joseph J. and Fang, Jun and Bolotnov, Igor A.}, year={2020}, month={Mar} }
 @article{zimmer_bolotnov_2019, title={Slug-to-churn vertical two-phase flow regime transition study using an interface tracking approach}, volume={115}, ISSN={["1879-3533"]}, DOI={10.1016/j.ijmultiphaseflow.2019.04.003}, abstractNote={Direct numerical simulation (DNS) coupled with an interface tracking method (ITM) is used to demonstrate the applicability of DNS to the study of vertical two-phase flow regime transition. The study focuses on the slug flow to churn-turbulent regime transition. PHASTA, a finite element based flow solver which utilizes the level set method for interface tracking, has been used to perform the presented simulations. A domain size study has been conducted in order to find pipe dimensions that match the natural wavelength of the periodic slug flow. A mesh resolution study has been completed. The results show that using DNS to simulate the slug-to-churn transition is within the capabilities of the code as the simulations agree well with experimental data and empirical knowledge. The simulations were analyzed to find flow features such as the velocity profile in the wake of a Taylor bubble and the bubble interface shape evolution during breakdown. An understanding of such features could help identify the driving physics behind the transition phenomenon.}, journal={INTERNATIONAL JOURNAL OF MULTIPHASE FLOW}, author={Zimmer, Matthew D. and Bolotnov, Igor A.}, year={2019}, month={Jun}, pages={196–206} }
 @article{li_zeng_wonnell_bolotnov_2019, title={Development of a New Contact Angle Control Algorithm for Level-Set Method}, volume={141}, ISSN={["1528-901X"]}, DOI={10.1115/1.4041987}, abstractNote={A contact angle control algorithm is developed and implemented in the multiphase interface tracking flow solver—phasta. The subgrid force model is introduced to control the evolving contact angle. The contact angle force is applied when the current contact angle deviates from the desired value (or range of values) and decreases to zero when it reaches the desired value. The single bubble departure simulation and the capillary flat plates simulation are performed for verification purpose. The numerical results are compared with the analytical solution with good agreement. The mesh resolution sensitivity analysis and parametric study are conducted for both simulations. Coupled with the other existing capabilities in phasta like evaporation and condensation algorithm, the contact angle control algorithm will allow us to investigate the boiling phenomenon in various conditions with lower cost (by utilizing localized mesh refinement for bubble growth region) compared to uniformly refined structured meshes and in engineering geometries.}, number={6}, journal={JOURNAL OF FLUIDS ENGINEERING-TRANSACTIONS OF THE ASME}, author={Li, Mengnan and Zeng, Kaiyue and Wonnell, Louis and Bolotnov, Igor A.}, year={2019}, month={Jun} }
 @article{fang_cambareri_brown_feng_gouws_li_bolotnov_2018, title={Direct numerical simulation of reactor two-phase flows enabled by high-performance computing}, volume={330}, ISSN={["1872-759X"]}, DOI={10.1016/j.nucengdes.2018.02.024}, abstractNote={Nuclear reactor two-phase flows remain a great engineering challenge, where the high-resolution two-phase flow database which can inform practical model development is still sparse due to the extreme reactor operation conditions and measurement difficulties. Owing to the rapid growth of computing power, the direct numerical simulation (DNS) is enjoying a renewed interest in investigating the related flow problems. A combination between DNS and an interface tracking method can provide a unique opportunity to study two-phase flows based on first principles calculations. More importantly, state-of-the-art high-performance computing (HPC) facilities are helping unlock this great potential. This paper reviews the recent research progress of two-phase flow DNS related to reactor applications. The progress in large-scale bubbly flow DNS has been focused not only on the sheer size of those simulations in terms of resolved Reynolds number, but also on the associated advanced modeling and analysis techniques. Specifically, the current areas of active research include modeling of subcooled boiling, bubble coalescence, as well as the advanced post-processing toolkit for bubbly flow simulations in reactor geometries. A novel bubble tracking method has been developed to track the evolution of bubbles in two-phase bubbly flow. Also, spectral analysis of DNS database in different geometries has been performed to investigate the modulation of the energy spectrum slope due to bubble-induced turbulence. In addition, the single- and two-phase analysis results are presented for turbulent flows within the pressurized water reactor (PWR) core geometries. The related simulations are possible to carry out only with the world leading HPC platforms. These simulations are allowing more complex turbulence model development and validation for use in 3D multiphase computational fluid dynamics (M-CFD) codes.}, journal={NUCLEAR ENGINEERING AND DESIGN}, author={Fang, Jun and Cambareri, Joseph J. and Brown, Cameron S. and Feng, Jinyong and Gouws, Andre and Li, Mengnan and Bolotnov, Igor A.}, year={2018}, month={Apr}, pages={409–419} }
 @article{fang_cambareri_rasquin_gouws_balakrishnan_jansen_bolotnov_2018, title={Interface Tracking Investigation of Geometric Effects on the Bubbly Flow in PWR Subchannels}, volume={193}, ISSN={0029-5639 1943-748X}, url={http://dx.doi.org/10.1080/00295639.2018.1499280}, DOI={10.1080/00295639.2018.1499280}, abstractNote={Abstract Absorbing heat from the fuel rod surface, water as coolant can undergo subcooled boiling within a pressurized water reactor (PWR) fuel rod bundle. Because of the buoyancy effect, the vapor bubbles generated will then rise along and interact with the subchannel geometries. Reliable prediction of bubble behavior is of immense importance to ensure safe and stable reactor operation. However, given a complex engineering system like a nuclear reactor, it is very challenging (if not impossible) to conduct high-resolution measurements to study bubbly flows under reactor operation conditions. The lack of a fundamental two-phase-flow database is hindering the development of accurate two-phase-flow models required in more advanced reactor designs. In response to this challenge, first-principles–based numerical simulations are emerging as an attractive alternative to produce a complementary data source along with experiments. Leveraged by the unprecedented computing power offered by state-of-the-art supercomputers, direct numerical simulation (DNS), coupled with interface tracking methods, is becoming a practical tool to investigate some of the most challenging engineering flow problems. In the presented research, turbulent bubbly flow is simulated via DNS in single PWR subchannel geometries with auxiliary structures (e.g., supporting spacer grid and mixing vanes). The geometric effects these structures exert on the bubbly flow are studied with both a conventional time-averaging approach and a novel dynamic bubble tracking method. The new insights obtained will help inform better two-phase models that can contribute to safer and more efficient nuclear reactor systems.}, number={1-2}, journal={Nuclear Science and Engineering}, publisher={Informa UK Limited}, author={Fang, Jun and Cambareri, Joseph J. and Rasquin, Michel and Gouws, Andre and Balakrishnan, Ramesh and Jansen, Kenneth E. and Bolotnov, Igor A.}, year={2018}, month={Aug}, pages={46–62} }
 @article{fang_bolotnov_2017, title={Bubble tracking analysis of PWR two-phase flow simulations based on the level set method}, volume={323}, ISSN={["1872-759X"]}, DOI={10.1016/j.nucengdes.2017.07.034}, abstractNote={Bubbly flow is a common natural phenomenon and a challenging engineering problem yet to be fully understood. More insights from either experiments or numerical simulations are desired to better model and predict the bubbly flow behavior. Direct numerical simulation (DNS) has been gaining renewed interests as an attractive approach towards the accurate modeling of two-phase turbulent flows. Though DNS is computationally expensive, it can provide highly reliable data for model development along with experiments. The ever-growing computing power is also allowing us to study flows of increasingly high Reynolds numbers. However, the conventional simulation and analysis methods are becoming inadequate when dealing with such 'big data' generated from large-scale DNS. This paper presents our recent effort in developing the advanced analysis framework for two-phase bubbly flow DNS. It will show how one can take advantage of the 'big data' and translate it into in-depth insights. Specifically, a novel bubble tracking method has been developed, which can collect detailed two-phase flow information at the individual bubble level. Due to the importance of subcooled boiling phenomenon in pressurized water reactors (PWR), the bubbly flow is simulated within a PWR subchannel geometry with the bubble tracking capability. It has been demonstrated that bubble tracking method significantly improves the data extraction efficiency for level-set based interface tracking simulations. Statistical analysis was introduced to post-process the recorded data to study the dependencies of bubble behavior with local flow dynamics.}, journal={NUCLEAR ENGINEERING AND DESIGN}, author={Fang, Jun and Bolotnov, Igor A.}, year={2017}, month={Nov}, pages={68–77} }
 @article{talley_zimmer_bolotnov_2017, title={Coalescence Prevention Algorithm for Level Set Method}, volume={139}, ISSN={["1528-901X"]}, DOI={10.1115/1.4036246}, abstractNote={An algorithm to prevent or delay bubble coalescence for the level set (LS) method is presented. This novel algorithm uses the LS method field to detect when bubbles are in close proximity, indicating a potential coalescence event, and applies a repellent force to simulate the unresolved liquid drainage force. The model is introduced by locally modifying the surface tension force near the liquid film drainage area. The algorithm can also simulate the liquid drainage time of the thin film by controlling the length of time the increased surface tension has been applied. Thus, a new method of modeling bubble coalescence has been developed. Several test cases were designed to demonstrate the capabilities of the algorithm. The simulations, including a mesh study, confirmed the abilities to identify and prevent coalescence as well as implement the time tracking portion, with an additional 10–25% computational cost. Ongoing tests aim to verify the algorithm's functionality for simulations with different flow conditions, a ranging number of bubbles, and both structured and unstructured computational mesh types. Specifically, a bubble rising toward a free surface provides a test of performance and demonstrates the ability to consistently prevent coalescence. In addition, a two bubble case and a seven bubble case provide a more complex demonstration of how the algorithm performs for larger simulations. These cases are compared to much more expensive simulations capable of resolving the liquid film drainage (through very high local mesh resolution) to investigate how the algorithm replicates the liquid film drainage process.}, number={8}, journal={JOURNAL OF FLUIDS ENGINEERING-TRANSACTIONS OF THE ASME}, author={Talley, Matthew L. and Zimmer, Matthew D. and Bolotnov, Igor A.}, year={2017}, month={Aug} }
 @article{feng_bolotnov_2018, title={Effect of the wall presence on the bubble interfacial forces in a shear flow field}, volume={99}, ISSN={["1879-3533"]}, DOI={10.1016/j.ijmultiphaseflow.2017.10.004}, abstractNote={Understanding the interactions between the bubbles and solid walls is important for predicting how the bubbly flow characteristics, such as void fraction and statistical distribution of bubble sizes, are affected by the presence of solid boundaries. In the present research, the numerical simulations with interface tracking method are performed to evaluate the interfacial forces’ closures in shear flow field under well-controlled conditions. The proportional-integral-derivative bubble controller is adopted to keep a single bubble at fixed position near the wall by balancing the forces in all three directions. This allows the extraction of the interfacial forces and the calculation of the corresponding coefficients. Validation tests have previously been conducted by the authors to compare the interfacial closures’ results against the experimental data. In this work, the influence of wall distance, relative velocity and bubble deformation on the interfacial forces are investigated. The results show that as the bubble approaches the wall, it experiences a transition from attractive force to repulsive force normal to the wall. The bubble Reynolds number also has significant impact on the interfacial forces: the net lateral force changes direction at Reb=35. The coupled phenomenon of bubble deformation and wall effect is investigated as well. The net lift sign change is observed at Eo=2.3 when the dimensionless wall distance (the ratio between the distance from the bubble center to the wall and the bubble diameter) is equal to 1. The presented studies complement the state-of-the-art knowledge and those are aimed to help improve the closure laws for interfacial forces and contribute to the multiphase computational fluid dynamics (M-CFD) closure model development.}, journal={INTERNATIONAL JOURNAL OF MULTIPHASE FLOW}, author={Feng, Jinyong and Bolotnov, Igor A.}, year={2018}, month={Feb}, pages={73–85} }
 @article{feng_bolotnov_2017, title={Evaluation of bubble-induced turbulence using direct numerical simulation}, volume={93}, ISSN={["1879-3533"]}, DOI={10.1016/j.ijmultiphaseflow.2017.04.003}, abstractNote={The presented research evaluates the interaction between a single bubble and homogeneous turbulent flow using direct numerical simulation (DNS) approach. The homogeneous single-phase turbulence is numerically generated by passing a uniform flow through grid planes. The turbulence decay rate is compared with experiment-based correlation. The single phase turbulence is then used as an inflow boundary condition for a set of single bubble studies. By estimating the turbulent field around the fully resolved bubble, the effects of bubble deformability, turbulent intensity and relative velocity on the bubble-induced turbulence are investigated. The existence of bubble creates new vortices in the wake region and the enhancement of turbulence is observed in the region behind the bubble. The results show that the magnitude of the turbulence enhancement would increase as the bubble encounters larger liquid turbulent intensity or higher relative velocity. Set of bubble Weber numbers from 0.34 to 3.39 are used to investigate the effect of bubble deformability. The more deformable bubble is the higher the increase in the magnitude of the turbulence enhancement behind the bubble. This research provides systematic insight on the bubble-induced turbulence (BIT) mechanism and is important for multiphase computational fluid dynamics (M-CFD) closure model development.}, journal={INTERNATIONAL JOURNAL OF MULTIPHASE FLOW}, author={Feng, Jinyong and Bolotnov, Igor A.}, year={2017}, month={Jul}, pages={92–107} }
 @article{peeples_magerl_o'brien_doster_bolotnov_wieland_stokely_2017, title={High Current C-11 Gas Target Design and Optimization Using Multi-Physics Coupling}, volume={1845}, ISSN={["0094-243X"]}, DOI={10.1063/1.4983547}, abstractNote={A high current conical C-11 gas target with a well characterized production yield was designed and optimized using multi-physics coupling simulations. Two target prototypes were deployed on an IBA 18/9 cyclotron, and the experimental results were used to benchmark the predictive simulations.}, journal={WTTC16: PROCEEDINGS OF THE 16TH INTERNATIONAL WORKSHOP ON TARGETRY AND TARGET CHEMISTRY}, author={Peeples, J. L. and Magerl, M. and O'Brien, E. M. and Doster, J. M. and Bolotnov, I. A. and Wieland, B. W. and Stokely, M. H.}, year={2017} }
 @article{feng_bolotnov_2017, title={Interfacial force study on a single bubble in laminar and turbulent flows}, volume={313}, ISSN={["1872-759X"]}, DOI={10.1016/j.nucengdes.2016.12.034}, abstractNote={Two-phase flows are present in various industrial processes in engineering fields ranging from light water reactor engineering to petrochemical engineering. In this paper, we conduct the interfacial force study on a single bubble under both laminar and turbulent flow scenarios. Advanced finite-element based flow solver (PHASTA) with level-set interface tracking method is used to perform the studies. The interface tracking approach is verified and validated by analyzing the interfacial forces, i.e., drag and lift forces, and comparing the results with the experiment-based data and correlations. The sign change of transverse migration direction is observed at Eo=3.4 which is close to the experimental observations. A set of parametric studies, including relative velocity, bubble deformability and turbulent intensity, are performed to analyze the impact of homogeneous turbulent flow on the drag force. A new drag coefficient closure model is proposed which agrees well with the DNS data considering both laminar and turbulent flow. Those studies can complement the experimental database to obtain improved closure laws for interfacial forces and are important contribution to the multiphase computational fluid dynamics (M-CFD) closure model development.}, journal={NUCLEAR ENGINEERING AND DESIGN}, author={Feng, Jinyong and Bolotnov, Igor A.}, year={2017}, month={Mar}, pages={345–360} }
 @article{magolan_baglietto_brown_bolotnov_tryggvason_lu_2017, title={Multiphase turbulence mechanisms identification from consistent analysis of direct numerical simulation data}, volume={49}, ISSN={1738-5733}, url={http://dx.doi.org/10.1016/J.NET.2017.08.001}, DOI={10.1016/J.NET.2017.08.001}, abstractNote={Direct Numerical Simulation (DNS) serves as an irreplaceable tool to probe the complexities of multiphase flow and identify turbulent mechanisms that elude conventional experimental measurement techniques. The insights unlocked via its careful analysis can be used to guide the formulation and development of turbulence models used in multiphase computational fluid dynamics simulations of nuclear reactor applications. Here, we perform statistical analyses of DNS bubbly flow data generated by Bolotnov (Reτ = 400) and Lu–Tryggvason (Reτ = 150), examining single-point statistics of mean and turbulent liquid properties, turbulent kinetic energy budgets, and two-point correlations in space and time. Deformability of the bubble interface is shown to have a dramatic impact on the liquid turbulent stresses and energy budgets. A reduction in temporal and spatial correlations for the streamwise turbulent stress (uu) is also observed at wall-normal distances of y+ = 15, y/δ = 0.5, and y/δ = 1.0. These observations motivate the need for adaptation of length and time scales for bubble-induced turbulence models and serve as guidelines for future analyses of DNS bubbly flow data.}, number={6}, journal={Nuclear Engineering and Technology}, publisher={Elsevier BV}, author={Magolan, Ben and Baglietto, Emilio and Brown, Cameron and Bolotnov, Igor A. and Tryggvason, Gretar and Lu, Jiacai}, year={2017}, month={Sep}, pages={1318–1325} }
 @article{guillen_cambareri_abboud_bolotnov_2018, title={Numerical comparison of bubbling in a waste glass melter}, volume={113}, ISSN={["0306-4549"]}, DOI={10.1016/j.anucene.2017.11.044}, abstractNote={Radioactive tank waste is scheduled for vitrification at the Waste Treatment and Immobilization Plant (WTP) being constructed at the Hanford Site. Testing of the pilot-scale DuraMelter 1200 at the Vitreous State Laboratory at the Catholic University of America has demonstrated that bubbling increases the melt rate of the batch material, and as a result, melter throughput. Computational fluid dynamics (CFD) models of this pilot-scale waste glass melter are being developed to improve our understanding of the processes that occur within the melter to aid in process optimization and troubleshooting of the WTP melters. Unfortunately, model validation is complicated by the difficulty of obtaining suitable experimental data for operational melters due to the inaccessibility for direct observation and measurements of the high-temperature, opaque fluid through the water-jacketed, refractory-lined steel vessel. This study focuses on assessing the fidelity of the CFD models to accurately predict the bubbling behavior. Because of the paucity of experimental data at the resolution required for CFD validation, a code comparison was used to evaluate two common approaches for simulating flows of two immiscible Newtonian fluids on numerical grids and resolving multiphase interfaces. Here, the volume of fluid and level set methods are used to resolve the dynamically evolving interfaces between the molten glass and the air bubbles. To aid in the validation of the results of these codes, a comparison of the bubble behavior, growth, and frequency of bubble generation are presented and a grid convergence study is performed for the two approaches. The predictions from the two codes are within 6% for the average bubble radius of curvature within the bubble channels, within 2% for average terminal rise velocity of the bubbles, and within 4% for the area mean of the local maxima at the free surface. These parameters are of interest since they affect the convection within the melter and at the interface between the glass and batch layer. Ultimately, the results of this work can assist in confirming the predictive ability of waste glass melter models and provide a better understanding of the flow patterns within the WTP melters.}, journal={ANNALS OF NUCLEAR ENERGY}, author={Guillen, Donna Post and Cambareri, Joseph and Abboud, Alexander W. and Bolotnov, Igor A.}, year={2018}, month={Mar}, pages={380–392} }
 @article{brown_shaver_lahey_bolotnov_2017, title={Wall-resolved spectral cascade-transport turbulence model}, volume={320}, ISSN={["1872-759X"]}, DOI={10.1016/j.nucengdes.2017.06.001}, abstractNote={A spectral cascade-transport model has been developed and applied to turbulent channel flows (Reτ = 550, 950, and 2000 based on friction velocity, uτ; or Reδ = 8500; 14,800 and 31,000, based on the mean velocity and channel half-width). This model is an extension of a spectral model previously developed for homogeneous single and two-phase decay of isotropic turbulence and uniform shear flows; and a spectral turbulence model for wall-bounded flows without resolving the boundary layer. Data from direct numerical simulation (DNS) of turbulent channel flow was used to help develop this model and to assess its performance in the 1D direction across the channel width. The resultant spectral model is capable of predicting the mean velocity, turbulent kinetic energy and energy spectrum distributions for single-phase wall-bounded flows all the way to the wall, where the model source terms have been developed to account for the wall influence. The model has been implemented into the 3D multiphase CFD code NPHASE-CMFD and the latest results are within reasonable error of the 1D predictions.}, journal={NUCLEAR ENGINEERING AND DESIGN}, author={Brown, C. S. and Shaver, D. R. and Lahey, R. T., Jr. and Bolotnov, I. A.}, year={2017}, month={Aug}, pages={309–324} }
 @inproceedings{li_bolotnov_2016, title={Interface tracking simulation of phase-change phenomena: boiling and condensation verification}, DOI={10.1115/fedsm2016-7701}, abstractNote={Interface tracking simulation (ITS) is one of the promising approaches to describe heat transfer of boiling phenomena and their underlying mechanisms. Better understanding and modeling of this process will benefit various engineering systems. In modern nuclear reactors, study on nucleate boiling phenomena is very important for the prediction of the Critical Heat Flux (CHF) phenomena.
               The presented research will implement and verify the capability of evaporation process modeling by the massively parallel research code, PHASTA. The comparison of the numerical results and the analytical results demonstrates that the overall behavior of the simulation compares well with the analytical solution. A second simulation of the single bubble growth with non-uniform temperature distribution demonstrates both condensation and evaporation modeling. In the third simulation flow boiling capabilities were preliminary tested with laminar flow demonstration case. These results will be applied to a larger scale, multi-bubble simulations and help modeling of nucleate boiling phenomena.}, booktitle={Proceedings of the asme fluids engineering division summer meeting, 2016, vol 1a}, author={Li, M. N. and Bolotnov, Igor}, year={2016} }
 @article{fang_rasquin_bolotnov_2017, title={Interface tracking simulations of bubbly flows in PWR relevant geometries}, volume={312}, ISSN={["1872-759X"]}, DOI={10.1016/j.nucengdes.2016.07.002}, abstractNote={The advances in high performance computing (HPC) have allowed direct numerical simulation (DNS) approach coupled with interface tracking methods (ITM) to perform high fidelity simulations of turbulent bubbly flows in various complex geometries. In this work, we have chosen the geometry of the pressurized water reactor (PWR) core subchannel to perform a set of interface tracking simulations (ITS) with fully resolved liquid turbulence. The presented research utilizes a massively parallel finite-element based code, PHASTA, for the subchannel geometry simulations of bubbly flow turbulence. The main objective for this research is to demonstrate the ITS capabilities in gaining new insight into bubble/turbulence interactions and assisting the development of improved closure laws for multiphase computational fluid dynamics (M-CFD). Both single- and two-phase turbulent flows were studied within a single PWR subchannel. The analysis of numerical results includes the mean gas and liquid velocity profiles, void fraction distribution and turbulent kinetic energy profiles. Two sets of flow rates and bubble sizes were used in the simulations. The chosen flow rates corresponded to the Reynolds numbers of 29,079 and 80,775 based on channel hydraulic diameter (Dh) and mean velocity. The finite element unstructured grids utilized for these simulations include 53.8 million and 1.11 billion elements, respectively. This has allowed to fully resolve all the turbulence scales and the deformable interfaces of individual bubbles. For the two-phase flow simulations, a 1% bubble volume fraction was used which resulted in 17 bubbles in the smaller case and 262 bubbles in the larger case. In the larger simulation case the size of the resolved bubbles is 0.65 mm in diameter, and the bulk mesh cell size is about 30 microns. Those large-scale simulations provide new level of details previously unavailable and were enabled by the excellent scaling performance of our two-phase flow solver and access to the state-of-the-art supercomputing resources. The presented simulations used up to 256 thousand processing threads on the IBM BG/Q supercomputer "Mira" (Argonne National Laboratory).}, journal={NUCLEAR ENGINEERING AND DESIGN}, author={Fang, Jun and Rasquin, Michel and Bolotnov, Igor A.}, year={2017}, month={Feb}, pages={205–213} }
 @article{brown_bolotnov_2016, title={Spectral Analysis of Single- and Two-Phase Bubbly DNS in Different Geometries}, volume={184}, ISSN={["1943-748X"]}, DOI={10.13182/nse15-126}, abstractNote={Abstract The spectral analysis of turbulent single- and two-phase direct numerical simulation (DNS) data in flat plane channel, circular pipe, and reactor subchannel geometries is performed using the recorded DNS velocity fluctuations as a function of time and applying the fast Fourier transform. This results in an energy spectrum of the liquid turbulence in a frequency domain. The complexity of multiphase flow results in a mixed velocity time history coming from either the liquid or the gas phase. A modified single-phase signal that mimics the presence of bubbles (“pseudo-void”) is developed to quantify the effect of the liquid signal intermittency as the bubble passes through a virtual probe. Comparisons of single-phase, pseudo-void, and two-phase results quantify the changes to the expected #x2013;5/3 slope of the energy spectrum for single-phase flows due to turbulent interactions caused by the wakes behind a bubble. The two-phase energy spectra show a slope close to #x2013;3 and similar shape in the different geometries while single-phase energy spectra exhibit the expected #x2013;5/3 slope. Pseudo-void results indicate that the change to the energy spectrum in bubbly two-phase flows is due entirely from liquid turbulence interactions with the bubble wakes. A comprehensive spectral analysis for different geometries and different Reynolds number flows at varying distances from the wall is an essential step in developing physically sound closure models for bubble-liquid interactions. The comparison between different geometries demonstrates the direct applicability of various models to reactor-relevant geometries.}, number={3}, journal={NUCLEAR SCIENCE AND ENGINEERING}, author={Brown, C. S. and Bolotnov, I. A.}, year={2016}, month={Nov}, pages={363–376} }
 @article{mishra_bolotnov_2015, title={DNS of turbulent flow with hemispherical wall roughness}, volume={16}, ISSN={["1468-5248"]}, DOI={10.1080/14685248.2014.989231}, abstractNote={The present study of the effect of roughness density on the mean flow turbulence parameters is motivated by the need for new generation of boundary conditions for multiphase computational multiphase fluid dynamics (CMFD) models applied to boiling flows. Effect of roughness element density on the turbulent flow in a channel is quantified through direct numerical simulations (DNSs). The Navier--Stokes equations are solved using finite element method and bubbles are approximated as rigid near-hemispherical obstacles at the wall. Six different cases were analysed including channel flow with smooth wall and channel flow with rough wall for five different bubble nucleation site densities. Friction factor and the law of the wall was calculated and compared with the previously published results. Existing correlations for nucleating bubble site density dependency on a wall heat flux were used to obtain a relation between the heat flux and the friction factor, leading to the law of the wall dependency on the heat f...}, number={3}, journal={JOURNAL OF TURBULENCE}, author={Mishra, Anand V. and Bolotnov, Igor A.}, year={2015}, pages={225–249} }
 @article{thomas_fang_feng_bolotnov_2015, title={ESTIMATION OF SHEAR-INDUCED LIFT FORCE IN LAMINAR AND TURBULENT FLOWS}, volume={190}, ISSN={["1943-7471"]}, DOI={10.13182/nt14-72}, abstractNote={Abstract The goal of the present study is to demonstrate that direct numerical simulations (DNS) coupled with interface tracking methods can be used to estimate interfacial forces in two-phase flows. Current computational multiphase fluid dynamics codes model interfacial forces utilizing closure laws that are heavily dependent on limited experimental data and simplified analytical approximations. In the present work, a method for improving the current interfacial force database has been developed by using DNS to quantify the lift and drag forces on a single bubble in laminar and turbulent shear flows. A proportional-integral-derivative–based controller was implemented into the finite element–based, multiphase flow solver [PHASTA (Parallel, Hierarchic, higher-order accurate, Adaptive, Stabilized, finite element method Transient Analysis)] to control the bubble position. This capability allowed for utilization of a steady-state force balance on the bubble to determine lift and drag coefficients in various shear flows. Specifically, for low shear flows (2.0 s−1), the effect of the wall presence is analyzed, and for high shear flows, the effect of turbulence is studied. A number of uniform shear (10.0 to 470.0 s−1) laminar flows were simulated to assess lift and drag force behavior as the kinetic energy of the flow increased. Two high shear (236.0 and 470.0 s−1) turbulent flows were simulated to understand bubble-turbulence interaction influence on the drag and lift phenomena. Two uniform shear rates (20.0 and 100 s−1) were simulated utilizing pressurized water reactor fluid properties. The lift and drag coefficients estimated in this work are in agreement with models developed for low shear laminar flows, whereas for high shear laminar and turbulent flows, bubble-turbulence interaction became a dominating influence in the lift and drag coefficient estimation. The novel results and method presented in this paper offer a path to simulating full-fledged reactor coolant environments where the lift and drag forces on a single bubble can be studied.}, number={3}, journal={NUCLEAR TECHNOLOGY}, author={Thomas, Aaron M. and Fang, Jun and Feng, Jinyong and Bolotnov, Igor A.}, year={2015}, month={Jun}, pages={274–291} }
 @article{behafarid_shaver_bolotnov_antal_jansen_podowski_2013, title={COUPLED DNS/RANS SIMULATION OF FISSION GAS DISCHARGE DURING LOSS-OF-FLOW ACCIDENT IN GENERATION IV SODIUM FAST REACTOR}, volume={181}, ISSN={["1943-7471"]}, DOI={10.13182/nt13-a15755}, abstractNote={Abstract The objective of this paper is to give an overview of a multiscale modeling approach to three-dimensional (3-D) two-phase transient computer simulations of the injection of a jet of gaseous fission products into a partially blocked sodium fast reactor (SFR) coolant channel following localized cladding overheat and breach. The phenomena governing accident progression have been resolved at two different spatial and temporal scales by the intercommunicating computational multiphase fluid dynamics codes PHASTA (at direct numerical simulation level) and NPHASE-CMFD (at Reynolds-averaged Navier-Stokes level). The issues discussed in the paper include an overview of the proposed 3-D two-phase-flow models of the interrelated phenomena that occur as a result of cladding failure and the subsequent injection of a jet of gaseous fission products into partially blocked SFR coolant channels and gas-molten-sodium transport along the channels. An analysis is presented on the consistency and accuracy of the models used in the simulations, and the results are shown of the predictions of gas discharge and gas-liquid-metal two-phase flow in a multichannel fuel assembly. Also, a discussion is given of the major novel aspects of the overall work.}, number={1}, journal={NUCLEAR TECHNOLOGY}, author={Behafarid, F. and Shaver, D. and Bolotnov, I. A. and Antal, S. P. and Jansen, K. E. and Podowski, M. Z.}, year={2013}, month={Jan}, pages={44–55} }
 @article{bolotnov_2013, title={Influence of Bubbles on the Turbulence Anisotropy}, volume={135}, ISSN={["1528-901X"]}, DOI={10.1115/1.4023651}, abstractNote={Direct numerical simulation (DNS) with interface tracking of turbulent bubbly flows is becoming a major tool in advancing our knowledge in the area of multiphase modeling research. A comprehensive analysis of the turbulent flow structure allows us to evaluate the state-of-the-art computational multiphase fluid dynamics (CMFD) models and to propose new closure laws. The presented research will demonstrate how the multiphase DNS data can inform the development of computational fluid dynamics (CFD) models. In particular, the Reynolds stress distribution will be evaluated for single- and two-phase bubbly flows and the level of turbulence anisotropy will be measured in several scenarios. The results will help determine if the isotropic turbulent models are suitable for bubbly flow applications or if there is a strong need to apply and develop Reynolds-stress turbulent models for two-phase flow CFD modeling.}, number={5}, journal={JOURNAL OF FLUIDS ENGINEERING-TRANSACTIONS OF THE ASME}, author={Bolotnov, Igor A.}, year={2013}, month={May} }
 @inproceedings{bolotnov_2012, title={Influence of bubbles on the turbulence anisotropy}, DOI={10.1115/fedsm2012-72412}, abstractNote={Direct numerical simulations (DNS) with interface tracking of turbulent bubbly flows are becoming a major tool in advancing our knowledge in the multiphase modeling research area. Comprehensive analysis of turbulent flow structure allows to evaluate the state-of-the-art computational multiphase fluid dynamics (CMFD) models and to propose new closure laws.
               The presented research will demonstrate how the multiphase DNS data can inform the development of computational fluid dynamics (CFD) models. In particular, Reynolds stress distribution will be evaluated for single and two-phase bubbly flows and the level of turbulence anisotropy will be measured in several scenarios. The results will help determine if the isotropic turbulent models are suitable for bubbly flow applications or there is a strong need to apply and develop Reynolds-stress turbulent models for two-phase flow CFD modeling.}, booktitle={Proceedings of the asme fluids engineering division summer meeting, 2012, vol 1, pts a and b, symposia}, author={Bolotnov, Igor}, year={2012}, pages={695–702} }
 @article{bolotnov_antal_jansen_podowski_2012, title={Multidimensional analysis of fission gas transport following fuel element failure in sodium fast reactor}, volume={247}, ISSN={["1872-759X"]}, DOI={10.1016/j.nucengdes.2012.02.004}, abstractNote={Significant progress in several areas will have to be made to achieve the required technological and safety standards for future Gen. IV reactors, including both novel experimental methods (starting with separate-effect, then followed by integral experiments) and high performance computational models characterized by the necessary level of modeling detail and high accuracy of predictions. Furthermore, it is important that the experimental and theoretical/computational research complement each other, so that the results of measurements could be directly used for model validation purposes, whereas the results of simulations should provide input to identify modeling uncertainties and provide guidelines for prioritizing future experiments. The purpose of this paper is to present the modeling concept for mechanistic computer simulations of the injection of a jet of gaseous fission products into a partially blocked SFR coolant channel following localized cladding overheat and breach. A three-dimensional model of gas/liquid-sodium interaction has been developed based on a multifield modeling framework implemented in the NPHASE-CMFD code. The boundary conditions used as input to NPHASE-CMFD have been obtained by averaging the results of direct numerical simulations (DNS) performed using the PHASTA code. The novel aspects of the results discussed in the paper include the demonstration of advantages of using a multiscale approach to model local phenomena governing gas/liquid-sodium two-phase flow inside reactor coolant channels following cladding breach, as well as the observations about areas where future experiments are needed to improve the predictive capabilities of two-phase flow models.}, journal={NUCLEAR ENGINEERING AND DESIGN}, author={Bolotnov, Igor A. and Antal, Steven P. and Jansen, Kenneth E. and Podowski, Michael Z.}, year={2012}, month={Jun}, pages={136–146} }
 @article{bolotnov_jansen_drew_oberai_lahey_podowski_2011, title={Detached direct numerical simulations of turbulent two-phase bubbly channel flow}, volume={37}, ISSN={["1879-3533"]}, DOI={10.1016/j.ijmultiphaseflow.2011.03.002}, abstractNote={DNS simulations of two-phase turbulent bubbly channel flow at Reτ = 180 (Reynolds number based on friction velocity and channel half-width) were performed using a stabilized finite element method (FEM) and a level set approach to track the air/water interfaces. Fully developed turbulent single-phase solutions obtained previously using the same stabilized FEM code were used as the initial flow field, and an appropriate level-set distance field was introduced to represent the air bubbles. Surface tension and gravity forces were used in the simulations to physically represent the behavior of a bubbly air/water two-phase flow having a liquid/gas density ratio of 858.3. The simulation results were averaged to obtain the liquid and gas mean velocity distributions, the local void fractions as well as the local turbulent kinetic energy and dissipation rate of the liquid phase. The liquid phase parameters were compared with the corresponding single-phase turbulent channel flow to determine the bubbles’ influence on the turbulence field.}, number={6}, journal={INTERNATIONAL JOURNAL OF MULTIPHASE FLOW}, author={Bolotnov, Igor A. and Jansen, Kenneth E. and Drew, Donald A. and Oberai, Assad A. and Lahey, Richard T., Jr. and Podowski, Michael Z.}, year={2011}, month={Jul}, pages={647–659} }
 @article{bolotnov_lahey_drew_jansen_oberai_2010, title={Spectral analysis of turbulence based on the DNS of a channel flow}, volume={39}, ISSN={0045-7930}, url={http://dx.doi.org/10.1016/j.compfluid.2009.11.001}, DOI={10.1016/j.compfluid.2009.11.001}, abstractNote={The development and assessment of spectral turbulence models requires knowledge of the spectral turbulent kinetic energy distribution as well as an understanding of the terms which determine the energy distribution in physical and wave number space. Direct numerical simulation (DNS) of turbulent channel flow yields numerical "data" that can be, and was, analyzed using a spatial Fast Fourier Transform (FFT) to obtain the various spectral turbulent kinetic energy balance terms, including the production, dissipation, diffusion, and the non-linear convective transfer terms.}, number={4}, journal={Computers & Fluids}, publisher={Elsevier BV}, author={Bolotnov, Igor A. and Lahey, Richard T., Jr. and Drew, Donald A. and Jansen, Kenneth E. and Oberai, Assad A.}, year={2010}, month={Apr}, pages={640–655} }
 @article{bolotnov_lahey_drew_jansen_oberai_2008, title={A spectral turbulent cascade model for single- and two-phase uniform shear flows}, volume={9}, ISSN={1468-5248}, url={http://dx.doi.org/10.1080/14685240802261102}, DOI={10.1080/14685240802261102}, abstractNote={A spectral turbulent cascade-transport model was developed and applied to single-and two-phase turbulent uniform shear flows. This model tracks the development of the turbulent kinetic energy spectrum by splitting the turbulent kinetic energy into wave number bins. A separate transport equation accounts for the spectral production, dissipation and transport terms and is solved for each wave number bin. The predicted evolution of the turbulence level, turbulent production and turbulent dissipation is shown to be consistent with experimental data for turbulent uniform shear flows. The shear rate in the various experiments ranges from 12.9 to 46.8 s− 1 for air flows, 0.96 to 1.23 s−1 for water flows and was 2.9 s− 1 for bubbly two-phase air/water flow.}, journal={Journal of Turbulence}, publisher={Informa UK Limited}, author={Bolotnov, Igor A. and Lahey, Richard T., Jr. and Drew, Donald A. and Jansen, Kenneth E. and Oberai, Assad A.}, year={2008}, month={Jan}, pages={N26} }