A coupling approach between resolved and coarse-grid sub-channel CFD

As a follow-up development of the Computational Fluid Dynamics (CFD)-based sub-channel analysis tool, i.e. coarse-grid Sub-Channel CFD (SubChCFD), this paper aims at developing a coupling between SubChCFD and resolved CFD, thereby enhancing the performance and application flexibility of the coarse-grid model. A time-explicit domain-overlapping method is used to achieve the coupling, which ensures good flexibility and reasonable numerical stability. In such a coupling framework, embedded resolved sub-models are to be placed arbitrarily into a SubChCFD baseline model in regions selected for refinement. Two coupling modes are available: the one-way coupling mode, where the SubChCFD model provides the boundary conditions for the resolved sub-models, but no feedback from the resolved sub-model to the SubChCFD model is carried out; the two-way coupling mode, where feedback is enabled from the resolved sub-model back to the SubChCFD model to improve the solution of the latter. The coupling methodology has been first tested using 2-D flow cases, including an internal flow at a T-junction and an external flow passing a square cylinder. It has then been applied to 3-D cases of nuclear rod bundles with complex conditions. One is a 7 × 7 rod bundle with locally ‘ballooned’ fuel rods where complex flow phenomena occur due to the blockage effect caused by area reduction in flow passages. The other is a 5 × 5 rod bundle with inward jet flow at one corner of the housing walls resulting in a strong cross flow. In all of the test cases, the results of the coarse-grid SubChCFD model with the two-way coupling approach are consistently improved compared with those of the uncoupled SubChCFD simulations

A multiscale model of a rod bundle using subchannel CFD

This paper deals with a coarse-mesh Computational Fluid Dynamics (CFD) approach, referred to as Subchannel CFD (SubChCFD), which combines features of the traditional 1-D subchannel analysis tools used in nuclear thermal hydraulic analyses and modern CFD. The SubChCFD, which has been previously developed for flow simulations, has been extended to perform thermal simulations and is then applied to a complex nuclear fuel bundle to explore and demonstrate the capacity of this new tool. SubChCFD allows resolved CFD sub-models to be nested into the regions that are of interest to locally improve the prediction, and can be coupled with the porous media approach to deal with any sub-scale fine structures that are difficult to be handled using a coarse mesh. In the present case studied, a SubChCFD baseline model is created, covering the entire heated length of the test geometry, to capture the axial developments of the flow and heat transfer in the rod bundle. Spacer grids that are used to keep the rods in place are modelled as embedded porous media to account for the associated blockage effect and pressure losses. In addition, a resolved CFD sub-model is created and coupled with the baseline model to improve the prediction for the region where high fidelity experimental measurements were performed. Through the present test, SubChCFD shows good predictability, flexibility and scalability in modelling large nuclear reactor components with complex internal structures. With the advanced coupling functionality, it is able to produce comparable simulation results to that of conventional CFD methods for regions of interest, with greatly reduced computational cost

Effect of a nonuniform distribution of voids on the plastic response of voided materials: a computational and statistical analysis

This study investigates the overall and local response of porous media composed of a perfectly plastic matrix weakened by stress-free voids. Attention is focused on the specific role played by porosity fluctuations inside a representative volume element. To this end, numerical simulations using the Fast Fourier Transform (FFT) are performed on different classes of microstructure corresponding to different spatial distributions of voids. Three types of microstructures are investigated: random microstructures with no void clustering, microstructures with a connected cluster of voids and microstructures with disconnected void clusters. These numerical simulations show that the porosity fluctuations can have a strong effect on the overall yield surface of porous materials. Random microstructures without clusters and microstructures with a connected cluster are the hardest and the softest configurations, respectively, whereas microstructures with disconnected clusters lead to intermediate responses. At a more local scale, the salient feature of the fields is the tendency for the strain fields to concentrate in specific bands. Finally, an image analysis tool is proposed for the statistical characterization of the porosity distribution. It relies on the distribution of the ‘distance function’, the width of which increases when clusters are present. An additional connectedness analysis allows us to discriminate between clustered microstructures

A numerical study of turbulent upward flow of super critical water in a 2×2 rod bundle with non-uniform heating

This work is part of a benchmarking excise organized by IAEA CRP in SCWR thermal-hydraulics aimed at improving the understanding and prediction accuracy of the thermal-hydraulic phenomena relevant to SCWRs. An experiment carried out using a 2x2 SCWR bundle at University of Wisconsin-Madison was modeled using an open-source Computational Fluid Dynamics (CFD) code - Code_Saturne. The k-w SST model was employed to account for the buoyancy-aided turbulent flow in the fuel channel. Significant heat transfer deterioration (HTD) was observed in the boundary layer, which is commonly expected to occur in buoyancy-aided flows. For comparison, simulations were also conducted using ANSYS FLUENT with similar model setups

Coupled porous media approaches in sub-channel CFD

In this paper, the porous media method is introduced into a recently developed Computational Fluid Dynamics (CFD)-based sub-channel analysis tool, Sub-Channel CFD (SubChCFD) through direct coupling to improve its capability in modelling complex structures, such as spacers or distorted fuel rod bundles. Two coupling methods have been developed, namely, embedding and interfacing coupling. The former is suitable for rod bundles with attached fine structures which do not change the basic geometry of the sub-channels, so a SubChCFD baseline model can be created based on the bare bundle configuration. Porous media sub-models can then be embedded in this model at locations where fine structures are installed to account for their influence on the flow and pressure drop. The interfacing method is more general and flexible, and used in a wider range of applications. In this method, separate meshes are used for the sub-domains where the porous media method is applied. These sub-domains are then interfaced with the domain covered by SubChCFD using a meshing joining technique and they are simulated together using a single set of governing equations. The methods developed are tested and validated with simulations of two rod bundle configurations, a 7 × 7 rod bundle with locally ballooned fuel rods and a 5 × 5 rod bundle equipped with simple support grids. It has been shown that a suitably selected method of the coupling can greatly simplify the modelling of complex structures using SubChCFD, thus providing additional flexibility and functionality to this newly developed CFD-based sub-channel framework

Microstructural enrichment functions based on stochastic Wang tilings

This paper presents an approach to constructing microstructural enrichment functions to local fields in non-periodic heterogeneous materials with applications in Partition of Unity and Hybrid Finite Element schemes. It is based on a concept of aperiodic tilings by the Wang tiles, designed to produce microstructures morphologically similar to original media and enrichment functions that satisfy the underlying governing equations. An appealing feature of this approach is that the enrichment functions are defined only on a small set of square tiles and extended to larger domains by an inexpensive stochastic tiling algorithm in a non-periodic manner. Feasibility of the proposed methodology is demonstrated on constructions of stress enrichment functions for two-dimensional mono-disperse particulate media.Comment: 27 pages, 12 figures; v2: completely re-written after the first revie

HPC for sensitivity studies: simulations with TOMAWAC and TELEMAC-3D

High Performance Computing (HPC) is useful in a range of scientific applications, from running computationallyintensive high-resolution models to running a large number of smaller simulations either to calibrate the model (for example, using genetic algorithms) or to assess a range of design options or model scenarios. One particularly promising area for HPC is when carrying out sensitivity studies. In this case, a large number of simulations are erformed using a systematic variation in the values of certain input parameters in order to determine the effects of each of the parameters on the model results. This type of application is non-intrusive in the sense that no modifications are required to the model source code. Instead, the only changes that need to be made are to usergenerated subroutines and to the files that control the values of the input parameters. In this paper, we describe a script which has been developed for use on an iDataPlex cluster (STFC aresbury Laboratory) and a Cray XE6 supercomputer (UK National Supercomputing Service). The script enables concurrent simulations to be submitted to the system via the queuing facility available on parallel clusters. This is not only relevant for serial instances of the code but is also valid for parallel simulations. Using the script for parallel instances allows the elapsed time for sensitivity studies to be reduced by a factor of approximately NCSIMS´ ICORESOPT, where NCSIMS is the number of concurrent instances and ICORESOPT is the optimum number of cores used for running each parallel instance