Elia Merzari

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.

Numerical Simulation and Proper Orthogonal Decomposition of the Flow in a Counter-Flow T-Junction

Large eddy simulations (LES) of the turbulent mixing in a T-junction have been carried out with the spectral element code Nek5000 at two inlet velocity ratios. Numerical results have been compared with an available experiment. Proper orthogonal decomposition (POD) has then been used to identify the most energetic modes of turbulence for both the velocity and temperature fields. Since POD was also performed on the experiment particle image velocimetry (PIV) data, a further means of verification and validation was available. The structure of the numerical POD modes and the time histories of the projection of each mode on the velocity field offer additional insight into the physics of turbulence in T-junctions. In particular, in the case of identical inlet velocities (T-junction velocity ratio equal to 1.0) the dynamics appears to be richer than might be expected and additional diagonal modes are present.

On the Strong Scaling of the Spectral Element Solver Nek5000 on Petascale Systems

The present work is targeted at performing a strong scaling study of the high-order spectral element fluid dynamics solver Nek5000. Prior studies such as [5] indicated a recommendable metric for strong scalability from a theoretical viewpoint, which we test here extensively on three parallel machines with different performance characteristics and interconnect networks, namely Mira (IBM Blue Gene/Q), Beskow (Cray XC40) and Titan (Cray XK7). The test cases considered for the simulations correspond to a turbulent flow in a straight pipe at four different friction Reynolds numbers Reτ = 180, 360, 550 and 1000. Considering the linear model for parallel communication we quantify the machine characteristics in order to better assess the scaling behaviors of the code. Subsequently sampling and profiling tools are used to measure the computation and communication times over a large range of compute cores. We also study the effect of the two coarse grid solvers XXT and AMG on the computational time. Super-linear scaling due to a reduction in cache misses is observed on each computer. The strong scaling limit is attained for roughly 5000-10,000 degrees of freedom per core on Mira, 30,000 - 50,0000 on Beskow, with only a small impact of the problem size for both machines, and ranges between 10,000 and 220,000 depending on the problem size on Titan. This work aims at being a reference for Nek5000 users and also serves as a basis for potential issues to address as the community heads towards exascale supercomputers.

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.

Algebraic Turbulent Heat Flux Model for Prediction of Thermal Stratification in Piping Systems

Abstract The effect of stratification on the flow in bounded geometries is studied through computational fluid dynamics and two different modelings of the turbulent heat flux: constant turbulent Prandtl number and Algebraic Heat Flux Model (AHFM). The main feature of the work is evaluation of the effect of buoyancy on the thermal quantities, velocity field, and related pressure drop. For evaluation of the turbulent heat flux and temperature field, AHFM has been demonstrated to be superior to the simple eddy diffusivity approach. However, serious concerns remain for the prediction of the velocity field in both isothermal and nonisothermal conditions, since greater uncertainties for the obtained pressure drop and related Fanning friction factor can be introduced. Incremental pressure drop is also investigated in conditions deviating from fully developed flows, in order to study stratification effects qualitatively using an engineering method.

Numerical Simulation of a Completely Passive Spent Fuel Pool: Lessons Learned

As part of the design and safety analyses of the KIPT accelerator driven subcritical assembly system of Ukraine, a passive cooled spent fuel pool has been conceived, designed and analyzed numerically. The total decay power of the pool is low and The maximum heat load is 0.5 kW. Air cooling of the spent fuel pool tank through a natural convection thermo-siphon is deemed sufficient to provide sufficiently low temperatures. Natural convection of the water within the tank removes the decay heat from the fuel elements to the tank surface. The present work discusses the numerical simulations of such facility by the means of CFD. While the system has low power and it is relatively simple, it poses significant challenges for the CFD simulations. In fact the presence of two natural convection patterns is a source of numerical instability at such low power. These issues and the obtained solutions are discussed in this paper. Since the problem (the simulation of two coupled natural convection systems) is general and likely to be of significant relevance to the design of future power plants, this paper is targeted to a broader audience. Rather than the specific design the focus will be on the theoretical and the practical problems involved with this kind of simulations. The problem is analyzed theoretically and numerically. For CFD simulations, the range of meshes used ranges from 1 million points to 40 million points. Several turbulence models and wall modeling approaches have been tried and tested. Several set of simulations have been performed: sets of simplified simulations considering only the external air thermo-siphon assuming a constant heat flux at the tank wall as well as a set of simulations of the coupled system using a porous medium approach in the fuel tank. All simulations provided consistent predictions and helped confirm that the temperature within the pool is below boiling point.Copyright

Turbulence statistics in a spectral element code: a toolbox for High-Fidelity Simulations

In the present document we describe a toolbox for the spectral-element code Nek5000 Fischer et al. (2008), aimed at computing turbulence statistics. The toolbox is presented for a small test case, namely a periodic hill with Lx = 9h, Ly = 3.035h and Lz = 4.5h, where x, y and z are the horizontal, vertical and spanwise directions, respectively, and h is the hill height. The number of elements in the xy−plane is 442, and the number of elements in z is 19, leading to a total of 8,398 spectral elements. A polynomial order of N = 5 is chosen, and the mesh is generated using ICEM-CFD. In this case, transition is triggered by tripping the boundary layers on both surfaces by means of a random-volume force implemented in the subroutine userf in per hill.usr. The toolbox presented here allows to compute mean-velocity components, the Reynoldsstress tensor as well as turbulent kinetic energy (TKE) and Reynolds-stress budgets. Note that the present toolbox allows to compute turbulence statistics in turbulent flows with one homogeneous direction (where the statistics are based on time-averaging and averaging in the homogeneous direction), and also in fully three-dimensional flows (with no periodic directions, where only time-averaging is considered). The structure of the StatsToolbox folder is as follows:

Turbulent Flow-Field Comparisons of RANS and LES for a Twisted Pin Lattice Geometry at Low Reynolds Number

The accurate evaluation of fuel and cladding peak temperatures is of prime importance for nuclear reactor design and safety. The Global Threat Reduction Initiative reactor conversion program often encounters exotic flow geometries in its mission to aid in converting reactors from high-enriched to low-enriched fuel. These geometries can pose modeling challenges. Analysis presented here concerns a reactor with twisted fuel pins that are in direct contact with each other in a large, hexagonal-pitch lattice. The Reynolds number for a unit cell is only 7500. Such flow conditions can present difficulties for standard approaches based on Reynolds-Averaged Navier-Stokes (RANS). Moreover there are no available experimental data and a small expected margin to the limiting cladding surface temperature. Given some of the geometric uncertainties, reducing the turbulence model uncertainty is thus important for meaningful calculations. A computational fluid dynamics model of a full-length unit cell was built using the commercial code STAR-CCM+. Multiple RANS models were employed, which gave disparate results. To provide higher-fidelity data for comparison, given the lack of experimental data, a periodic single-helical-pitch simulation with a Large Eddy Simulation (LES) approach was performed using Nek5000, a massively-parallel spectral-element code. This was compared with single-pitch RANS simulations from STAR-CCM+. Stream-wise velocity profile shape was generally well-represented by RANS. Cross-velocities and peak turbulent kinetic energy (TKE) were underestimated for most of the turbulence models with respect to LES, while mean flow TKE was universally underestimated. The overall results suggest that the Realizable k-e Two-Layer model, which was the best at reproducing the LES TKE distribution, would likely be the most appropriate turbulence model choice for this flow. Future work includes full conjugate heat transfer simulations of 1/6 sectors of fuel assemblies featuring this type of pin lattice.Copyright

A POD-based solver for the advection-diffusion equation

We present a methodology based on proper orthogonal decomposition (POD). We have implemented the POD-based solver in the large eddy simulation code Nek5000 and used it to solve the advection-diffusion equation for temperature in cases where buoyancy is not present. POD allows for the identification of the most energetic modes of turbulence when applied to a sufficient set of snapshots generated through Nek5000. The Navier-Stokes equations are then reduced to a set of ordinary differential equations by Galerkin projection. The flow field is reconstructed and used to advect the temperature on longer time scales and potentially coarser grids. The methodology is validated and tested on two problems: two-dimensional flow past a cylinder and three-dimensional flow in T-junctions. For the latter case, the benchmark chosen corresponds to the experiments of Hirota et al., who performed particle image velocimetry on the flow in a counterflow T-junction. In both test problems the dynamics of the reduced-order model reproduce well the history of the projected modes when a sufficient number of equations are considered. The dynamics of flow evolution and the interaction of different modes are also studied in detail for the T-junction.© 2011 ASME

The three-dimensional structure of swirl-switching in bent pipe flow

Swirl-switching is a low-frequency oscillatory phenomenon which affects the Dean vortices in bent pipes and may cause fatigue in piping systems. Despite thirty years worth of research, the mechanis ...