Aleksandr Obabko

Characterization of the secondary flow in hexagonal ducts

In this work we report the results of DNSs and LESs of the turbulent flow through hexagonal ducts at friction Reynolds numbers based on centerplane wall shear and duct half-height Reτ,c ≃ 180, 360, and 550. The evolution of the Fanning friction factor f with Re is in very good agreement with experimental measurements. A significant disagreement between the DNS and previous RANS simulations was found in the prediction of the in-plane velocity, and is explained through the inability of the RANS model to properly reproduce the secondary flow present in the hexagon. The kinetic energy of the secondary flow integrated over the cross-sectional area 〈K〉yz decreases with Re in the hexagon, whereas it remains constant with Re in square ducts at comparable Reynolds numbers. Close connection between the values of Reynolds stress uw¯ on the horizontal wall close to the corner and the interaction of bursting events between the horizontal and inclined walls is found. This interaction leads to the formation of the secon...

High-resolution coupled physics solvers for analysing fine-scale nuclear reactor design problems

An integrated multi-physics simulation capability for the design and analysis of current and future nuclear reactor models is being investigated, to tightly couple neutron transport and thermal-hydraulics physics under the SHARP framework. Over several years, high-fidelity, validated mono-physics solvers with proven scalability on petascale architectures have been developed independently. Based on a unified component-based architecture, these existing codes can be coupled with a mesh-data backplane and a flexible coupling-strategy-based driver suite to produce a viable tool for analysts. The goal of the SHARP framework is to perform fully resolved coupled physics analysis of a reactor on heterogeneous geometry, in order to reduce the overall numerical uncertainty while leveraging available computational resources. The coupling methodology and software interfaces of the framework are presented, along with verification studies on two representative fast sodium-cooled reactor demonstration problems to prove the usability of the SHARP framework.

A performance analysis of ensemble averaging for high fidelity turbulence simulations at the strong scaling limit

Abstract We analyze the potential performance benefits of estimating expected quantities in large eddy simulations of turbulent flows using true ensembles rather than ergodic time averaging. Multiple realizations of the same flow are simulated in parallel, using slightly perturbed initial conditions to create unique instantaneous evolutions of the flow field. Each realization is then used to calculate statistical quantities. Provided each instance is sufficiently de-correlated, this approach potentially allows considerable reduction in the time to solution beyond the strong scaling limit for a given accuracy. This paper focuses on the theory and implementation of the methodology in Nek5000, a massively parallel open-source spectral element code.

Large Eddy Simulation of Thermo-Hydraulic Mixing in a T-Junction

Unsteady heat transfer problems that are associated with non-isothermal flow mixing in pipe flows have long been the topic of concern in the nuclear engineering community because of the relation to thermal fatigue of nuclear power plant pipe systems. When turbulent flow streams of different velocity and density rapidly mix at the right angle, a contact interface between the two mixing streams oscillates and breaks down because of hydrodynamic instabilities, and large-scale unsteady flow structures emerge (Figure 1). These structures lead to low-frequency oscillations at the scale of the pipe diameter, D, with a period scaling as O(D/U), where U is the characteristic flow velocity. If the mixing flow streams are of different temperatures, the hydrodynamic oscillations are accompanied by thermal fluctuations (thermal striping) on the pipe wall. The latter accelerate thermal-mechanical fatigue, damage the pipe structure and, ultimately, cause its failure.

Proposed experiment for validation of CFD methods for advanced SFR design: Upper plenum thermal striping and stratification

In response to the goals outlined by the U.S. Department of Energy’s Advanced Fuel Cycle Initiative, an effort is underway to develop an integrated multi-physics, multi-resolution thermal-hydraulic simulation tool package for the evaluation of nuclear power plant design and safety. As part of this effort, initial guidance has been proposed for the development of experiments to supply validation data sets for the CFD-based thermo-fluid simulation capability. To demonstrate that the proposed data requirements can be achieved using current generation measurement methods and to refine correlation and data comparison methods suitable for very large data sets, an initial experiment focused on turbulent mixing in the upper plenum of an advanced sodium fast reactor has been proposed. Prior validation efforts to support the use of one-dimensional lumped parameter models in the analysis of reactor safety performance relied primarily on data from carefully scaled integral system experiments to validate and tune correlations used to represent the physics associated with a particular transient in a particular reactor design. Unlike the correlation-based lumped parameter codes, computational fluid dynamics simulations reduce the reliance on experimentally derived correlations to the prediction of local turbulence effects rather the prediction of integral quantities like pressure drop and heat transfer coefficients. As a consequence, simpler separate effects experiments, which capture the turbulence effects but not necessarily the integral effects within a specific component of a system, can be utilized as the primary validation basis for the CFD codes. However, while the need for large carefully scaled integral experiments is reduced, the high spatial and temporal resolution of these codes requires that experimental data be collected at fine spatial and temporal resolutions. An initial series of simulations has been completed to support the development of the proposed experimental facility using air as a surrogate for the sodium coolant. Design options considered in RANS simulations using the commercial CFD code Star-CCM+ include mixing facility dimensions, the number of inlet jets to be included and outlet position. The use of RANS simulations is supported by an initial benchmarking comparison with predictions from the spectral element large eddy simulation code Nek5000 for the nominal experimental geometry.© 2009 ASME

Accelerating the High-Fidelity Simulation of Turbulence: Ensemble Averaging

Computational fluid dynamics (CFD) is increasingly used to simulate complex industrial systems. Most CFD analysis relies on the Reynolds-averaged Navier-Stokes (RANS) approach and traditional two-equation turbulence models. Higher-fidelity approaches to the simulation of turbulence such as wallresolved large eddy simulation (LES) and direct numerical simulation (DNS) remain limited to smaller applications or to large supercomputing platforms. Nonetheless, continued advances in supercomputing are enabling the simulation of physical systems of increasing size and complexity. These simulations can be used to gain unprecedented insight into the physics of turbulence in complex flows and will become more widespread as petascale architectures become more accessible. As the scale and size of LES and DNS simulations increase, however, the limitations of current algorithms become apparent. For larger systems, more temporal and spatial scale must be resolved, thus increasing the time-scale separation. While the smaller time scales dictate the size and the computational cost associated with each time step, the larger time scales dictate the length of the transient. An increased time-scale separation leads to smaller time steps and longer transients, eventually leading to simulations that are impractical or infeasible. In practice the presence of multiple and strongly separated time scales limits the effectiveness of CFD algorithms for LES and DNS applied to large industrial systems. Moreover, the situation is likely to become worse on future computer architectures, as even larger systems will be simulated, thus increasing the size and length of transients. At the same time transients currently simulated on petascale architectures are unlikely to become any faster on exascale architectures. In this paper we consider a technique to accelerate current transient simulations aimed at collect averaged turbulent statistics. The focus is on ergodic flows and simulations. This technique is ensemble averaging, commonplace in machine learning and artificial neural networks. Ensemble averaging is the process of creating multiple models and combining them to produce a desired output. It is also at the basis of RANS/URANS turbulence modeling. In the proposed approach, multiple instances of the same ergodic flows are simulated in parallel for a short timeframe and summed to create an ensemble. Provided each instance is sufficiently statistically decorrelated, this allows considerable reduction in the time to solution. This paper focuses on the theory and implementation of the methodology in Nek5000, a massively parallel open-source spectral element code. Moreover, we present the application of the method to the DNS and LES simulation of channel flow and pipe flow. INTRODUCTION Computational fluid dynamics (CFD) is increasingly used to simulate turbulent flows. Most CFD analysis, especially in industry, relies on the Reynolds-averaged Navier-Stokes (RANS) approach and traditional two-equation turbulence models. Higher-fidelity approaches to the simulation of turbulence, such as wall-resolved large eddy simulation (LES) and direct numerical simulation (DNS), remain limited to smaller applications or to large supercomputing platforms. In fact, since the Reynolds number dictates the local resolution, large machines are currently necessary to simulate engineering systems with turbulence-resolving techniques. Nonetheless, continued advances in supercomputing are enabling the simulation of physical systems of increasing size and complexity. Current supercomputers can accommodate grids that reach tens to hundreds of billions of points, enabling the simulation of entire rod bundles with wall-resolved LES using CFD algorithms with good scalability properties. As supercomputer become more powerful and larger, LES and DNS simulations become possible. With traditional algorithms, however, little can be done to make the solution of traditional problems run faster once the strong-scaling limit is reached. Therefore, the time to solution of traditional algorithms is

On minimum aspect ratio for experimental duct flow facilities

To the surprise of some of our colleagues, we recently recommended aspect ratios of at least 24 (instead of accepted values over last few decades ranging from 5 to 12) to minimize effects of sidewa ...

Multi-Physics Demonstration Problem with the SHARP Reactor Simulation Toolkit

This report describes to employ SHARP to perform a first-of-a-kind analysis of the core radial expansion phenomenon in an SFR. This effort required significant advances in the framework Multi-Physics Demonstration Problem with the SHARP Reactor Simulation Toolkit used to drive the coupled simulations, manipulate the mesh in response to the deformation of the geometry, and generate the necessary modified mesh files. Furthermore, the model geometry is fairly complex, and consistent mesh generation for the three physics modules required significant effort. Fully-integrated simulations of a 7-assembly mini-core test problem have been performed, and the results are presented here. Physics models of a full-core model of the Advanced Burner Test Reactor have also been developed for each of the three physics modules. Standalone results of each of the three physics modules for the ABTR are presented here, which provides a demonstration of the feasibility of the fully-integrated simulation.

Full Core Multiphysics Simulation with Offline Mesh Deformation

In this report, building on previous reports issued in FY13 we describe our continued efforts to integrate thermal/hydraulics, neutronics, and structural mechanics modeling codes to perform coupled analysis of a representative fast sodium-cooled reactor core. The focus of the present report is a full core simulation with off-line mesh deformation.

A Novel Variant of the K-ω URANS Model for Spectral Element Methods: Implementation, Verification, and Validation in Nek5000

Unsteady Reynolds-averaged Navier-Stokes (uRANS) models can provide good engineering estimates of wall shear and heat flux at a significantly lower computational cost compared with LES simulations. In this paper, we discuss the implementation of two novel variants of the k-ω turbulence model, the regularized k-ω standard and the regularized k-ω SST model, in a spectral element code, Nek5000. We present formulation for the specific dissipation rate (ω) in the standard k-ω model, which would obviate the need for ad hoc boundary conditions of ω on the wall. The regularized approach is designed to lead to grid-independent solutions as resolution is increased. We present a detailed comparison of these novel methods for various standard problems including the T-junction benchmark problem. The two approaches presented in this work compare very well with the standard k-ω model and experimental data for all the cases studied.Copyright