@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{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{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{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}, journal={NUCLEAR SCIENCE AND ENGINEERING}, 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{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{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{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{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{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={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{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={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 char...}, 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={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 o...}, 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} }
 @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={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 in...}, 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={Abstract 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={Abstract 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{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={Abstract 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} }
 @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{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={Abstract 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{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={Abstract 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{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={Abstract 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 R e b = 35 . The coupled phenomenon of bubble deformation and wall effect is investigated as well. The net lift sign change is observed at E o = 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{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={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 ...}, 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{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={Abstract 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{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_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={Abstract 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{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{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={Abstract 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={Abstract 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{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{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={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 record...}, 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 flux. This separate effect study provides new guidelines on how the heat flux in subcooled boiling regime affects the turbulence behaviour near the wall and guides the computational fluid dynamics model development for boiling two-phase flows.}, 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={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. Curren...}, 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={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={Abstract 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 Re τ = 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={Abstract 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} }