@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{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{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={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{chang_fang_dinh_2020, title={Reynolds-Averaged Turbulence Modeling Using Deep Learning with Local Flow Features: An Empirical Approach}, volume={194}, ISSN={["1943-748X"]}, url={https://doi.org/10.1080/00295639.2020.1712928}, DOI={10.1080/00295639.2020.1712928}, abstractNote={Abstract Reynolds-Averaged Navier-Stoke (RANS) models offer an alternative avenue in predicting flow characteristics when the corresponding experiments are difficult to achieve due to geometry complexity, limited budget, or knowledge. RANS models require the knowledge of subgrid scale physics to solve conservation equations for mass, energy, and momentum. Mechanistic turbulence models, such as k-ε, are generally evaluated and calibrated for specific flow conditions with various degrees of uncertainty. These models have limited capability to assimilate a substantial amount of data due to model form constraints. Meanwhile, deep learning (DL) has been proven to be universal approximators with the potential to assimilate available, relevant, and adequately evaluated data. Moreover, deep neural networks (DNNs) can create surrogate models without knowing function forms. Such a data-driven approach can be used in updating fluid models based on observations as opposed to hard-wiring models with precalibrated correlations. The paper presents progress in applying DNNs to model Reynolds stress using two machine learning (ML) frameworks. A novel flow feature coverage mapping is proposed to quantify the physics coverage of DL-based closures. It can be used to examine the sufficiency of training data and input flow features for data-driven turbulence models. The case of a backward-facing step is formulated to demonstrate that not only can DNNs discover underlying correlation behind fluid data but also they can be implemented in RANS to predict flow characteristics without numerical stability issues. The presented research is a crucial stepping-stone toward the data-driven turbulence modeling, which potentially benefits the design of data-driven experiments that can be used to validate fluid models with ML-based fluid closures.}, number={8-9}, journal={NUCLEAR SCIENCE AND ENGINEERING}, publisher={Informa UK Limited}, author={Chang, Chih-Wei and Fang, Jun and Dinh, Nam T.}, year={2020}, month={Sep}, pages={650–664} }
 @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{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{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} }
 @inproceedings{isukapati_list_2016, title={Synthesizing route travel time distributions considering spatial dependencies}, booktitle={2016 IEEE 19th International Conference on Intelligent Transportation Systems (ITSC)}, author={Isukapati, I. K. and List, G. F.}, year={2016}, pages={2143–2149} }
 @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} }