Igor A. Bolotnov

Influence of Bubbles on the Turbulence Anisotropy

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.

Estimation of Shear-Induced Lift Force in Laminar and Turbulent Flows

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.

A spectral turbulent cascade model for single- and two-phase uniform shear flows

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.

Spectral Analysis of Single- and Two-Phase Bubbly DNS in Different Geometries

Abstract The spectral analysis of turbulent single- and two-phase direct numerical simulation (DNS) data in flat plane channel, circular pipe, and reactor subchannel geometries is performed using the recorded DNS velocity fluctuations as a function of time and applying the fast Fourier transform. This results in an energy spectrum of the liquid turbulence in a frequency domain. The complexity of multiphase flow results in a mixed velocity time history coming from either the liquid or the gas phase. A modified single-phase signal that mimics the presence of bubbles (“pseudo-void”) is developed to quantify the effect of the liquid signal intermittency as the bubble passes through a virtual probe. Comparisons of single-phase, pseudo-void, and two-phase results quantify the changes to the expected #x2013;5/3 slope of the energy spectrum for single-phase flows due to turbulent interactions caused by the wakes behind a bubble. The two-phase energy spectra show a slope close to #x2013;3 and similar shape in the different geometries while single-phase energy spectra exhibit the expected #x2013;5/3 slope. Pseudo-void results indicate that the change to the energy spectrum in bubbly two-phase flows is due entirely from liquid turbulence interactions with the bubble wakes. A comprehensive spectral analysis for different geometries and different Reynolds number flows at varying distances from the wall is an essential step in developing physically sound closure models for bubble-liquid interactions. The comparison between different geometries demonstrates the direct applicability of various models to reactor-relevant geometries.

Coupled DNS/RANS Simulation of Fission Gas Discharge During Loss-of-Flow Accident in Generation IV Sodium Fast Reactor

Abstract The objective of this paper is to give an overview of a multiscale modeling approach to three-dimensional (3-D) two-phase transient computer simulations of the injection of a jet of gaseous fission products into a partially blocked sodium fast reactor (SFR) coolant channel following localized cladding overheat and breach. The phenomena governing accident progression have been resolved at two different spatial and temporal scales by the intercommunicating computational multiphase fluid dynamics codes PHASTA (at direct numerical simulation level) and NPHASE-CMFD (at Reynolds-averaged Navier-Stokes level). The issues discussed in the paper include an overview of the proposed 3-D two-phase-flow models of the interrelated phenomena that occur as a result of cladding failure and the subsequent injection of a jet of gaseous fission products into partially blocked SFR coolant channels and gas-molten-sodium transport along the channels. An analysis is presented on the consistency and accuracy of the models used in the simulations, and the results are shown of the predictions of gas discharge and gas-liquid-metal two-phase flow in a multichannel fuel assembly. Also, a discussion is given of the major novel aspects of the overall work.

DNS of turbulent flow with hemispherical wall roughness

The present study of the effect of roughness density on the mean flow turbulence parameters is motivated by the need for new generation of boundary conditions for multiphase computational multiphase fluid dynamics (CMFD) models applied to boiling flows. Effect of roughness element density on the turbulent flow in a channel is quantified through direct numerical simulations (DNSs). The Navier--Stokes equations are solved using finite element method and bubbles are approximated as rigid near-hemispherical obstacles at the wall. Six different cases were analysed including channel flow with smooth wall and channel flow with rough wall for five different bubble nucleation site densities. Friction factor and the law of the wall was calculated and compared with the previously published results. Existing correlations for nucleating bubble site density dependency on a wall heat flux were used to obtain a relation between the heat flux and the friction factor, leading to the law of the wall dependency on the heat f...

In-situ visualization and computational steering for large-scale simulation of turbulent flows in complex geometries

Large-scale simulations conducted on supercomputers such as leadership-class computing facilities allow researchers to simulate and study complex problems with high fidelity, and thus have become indispensable in diverse areas of science and engineering. These high-fidelity simulations generate vast amount of data which is becoming more and more difficult to transform into knowledge using traditional visual analysis approaches. For instance, there are tremendous challenges in analyzing big data produced by high-fidelity simulations in order to gain meaningful insight into complex phenomena such as turbulent two-phase flows. The traditional workflow, which consists in conducting simulations on supercomputers and recording enormous raw simulation data to disk for further post-processing and visualization, is no longer a viable approach due to prohibitive cost of disk access and considerable amount of time spent on data transfer. Visual Analytics approaches for big data have to be researched and employed to address the problem of knowledge discovery from such large-scale simulations. One approach to tackle this issue is to couple a numerical simulation with in-situ visualization so that the post-processing and visualization occurs while the simulation is running. This in-situ approach minimizes data storage by extracting and visualizing important features of the data directly within the simulation without saving the raw data to disk. In addition, in-situ visualization allows users to steer the simulation by adjusting input parameters while the simulation is ongoing. In this paper, we present our approach for in-situ visualization of simulation data generated by massively parallel finite-element computational fluid dynamics solver (PHASTA) instrumented and linked with ParaView Catalyst. We demonstrate our in-situ visualization and simulation steering capability with a fully resolved turbulent flow through 2×2 reactor subchannel complex geometry. In addition, we present results from our in-situ visualization for turbulent flow simulations conducted on the supercomputers Cray XK7 “Titan” at Oak Ridge National Laboratory and IBM BlueGene/Q “Mira” at Argonne National Laboratory up to 32,768 cores and examine the overhead of in-situ visualization and its effect on code performance.

Interface Tracking Investigation of Geometric Effects on the Bubbly Flow in PWR Subchannels

Abstract Absorbing heat from the fuel rod surface, water as coolant can undergo subcooled boiling within a pressurized water reactor (PWR) fuel rod bundle. Because of the buoyancy effect, the vapor bubbles generated will then rise along and interact with the subchannel geometries. Reliable prediction of bubble behavior is of immense importance to ensure safe and stable reactor operation. However, given a complex engineering system like a nuclear reactor, it is very challenging (if not impossible) to conduct high-resolution measurements to study bubbly flows under reactor operation conditions. The lack of a fundamental two-phase-flow database is hindering the development of accurate two-phase-flow models required in more advanced reactor designs. In response to this challenge, first-principles–based numerical simulations are emerging as an attractive alternative to produce a complementary data source along with experiments. Leveraged by the unprecedented computing power offered by state-of-the-art supercomputers, direct numerical simulation (DNS), coupled with interface tracking methods, is becoming a practical tool to investigate some of the most challenging engineering flow problems. In the presented research, turbulent bubbly flow is simulated via DNS in single PWR subchannel geometries with auxiliary structures (e.g., supporting spacer grid and mixing vanes). The geometric effects these structures exert on the bubbly flow are studied with both a conventional time-averaging approach and a novel dynamic bubble tracking method. The new insights obtained will help inform better two-phase models that can contribute to safer and more efficient nuclear reactor systems.

High current C-11 gas target design and optimization using multi-physics coupling

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.