@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{saini_bolotnov_2021, title={Detailed Analysis of the Effects of Spacer Grid and Mixing Vanes on Turbulence in a PWR Subchannel Under DFFB Conditions Based on DNS Data}, volume={12}, ISSN={["1943-7471"]}, DOI={10.1080/00295450.2021.1974279}, abstractNote={Abstract Spacer grids and mixing vanes exhibit a significant role in the thermal hydraulics of pressurized water reactors (PWRs), especially in the post loss-of-coolant accident regimes. A detailed analysis of the contrasting upstream and downstream turbulent flow features is of great importance to both system codes and computational fluid dynamics (CFD)–Reynolds-averaged Navier–Stokes (RANS) modeling. Further, with the advent of supercomputing resources and machine learning research, a data-driven approach to turbulence modeling is gaining popularity. However, owing to the complexities associated with large-scale, high-fidelity data collection capabilities, the application of machine learning–based turbulence models has been limited to simple geometries. In this work, using a highly scalable CFD code PHASTA, we have collected data from direct numerical simulations of a PWR subchannel with high spatial and temporal resolution. From the collected data we extract key turbulent flow features, including mean velocities and Reynolds stresses that highlight the effects of spacer grids and mixing vanes on downstream turbulence in a typical PWR subchannel. An invariant analysis of the anisotropic stress tensor is also presented, which further elucidates their effect on the nature of turbulence in the immediate upstream and downstream vicinity. The high-resolution data from the simulations are archived and intended for the development of data-driven RANS closure models that are capable of capturing the evolution of anisotropy in PWR subchannels.}, journal={NUCLEAR TECHNOLOGY}, author={Saini, Nadish and Bolotnov, Igor A.}, year={2021}, month={Dec} }
 @article{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} }
 @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{vaish_kleinstreuer_kolanjiyil_saini_pillalamarri_2016, title={Laminar/turbulent airflow and microsphere deposition in a patient-specific airway geometry using an open-source solver}, volume={22}, ISSN={["1752-6426"]}, DOI={10.1504/ijbet.2016.079145}, abstractNote={Using the open-source software OpenFOAM as the solver, airflow and microsphere transport have been simulated in a patient-specific lung-airway model. A suitable transitional turbulence model was validated and implemented to accurately simulate airflow fields, as the laryngeal jet occurring in the throat region may induce turbulence immediately downstream. Furthermore, a modified eddy interaction model with a generalised near-wall correction factor is presented that more accurately simulates the particle trajectories and subsequent deposition phenomena which are especially affected by near-wall velocity fluctuations. Particle depositions in the realistic lung-airway configuration are compared with those in an idealised upper airway model. The results indicate that for microsphere deposition in turbulent airflow regions, selection of an appropriate near-wall correction factor can reduce the problem of subject variability for different lung-airway configurations. Open-source solvers for lung-aerosol dynamics simulations, such as OpenFOAM, are predictive tools which are basically cost-free, flexible, largely user-friendly, and portable.}, number={2}, journal={INTERNATIONAL JOURNAL OF BIOMEDICAL ENGINEERING AND TECHNOLOGY}, author={Vaish, Mayank and Kleinstreuer, Clement and Kolanjiyil, Arun V. and Saini, Nadish and Pillalamarri, Narasimha R.}, year={2016}, pages={145–161} }