Abstract
A high-performance gas kinetic solver using multi-level parallelization is developed to enable pore-scale simulations of rarefied flows in porous media. The Bhatnagar–Gross–Krook model equation is solved by the discrete velocity method with an iterative scheme. The multi-level MPI/OpenMP parallelization is implemented with the aim to efficiently utilize the computational resources to allow direct simulation of rarefied gas flows in porous media based on digital rock images for the first time. The multi-level parallel approach is analyzed in detail confirming its better performance than the commonly-used MPI processing alone for an iterative scheme. With high communication efficiency and appropriate load balancing among CPU processes, parallel efficiency of 94% is achieved for 1536 cores in the 2D simulations, and 81% for 12288 cores in the 3D simulations. While decomposition in the spatial space does not affect the simulation results, one additional benefit of this approach is that the number of subdomains can be kept minimal to avoid deterioration of the convergence rate of the iteration process. This multi-level parallel approach can be readily extended to solve other Boltzmann model equations.
Highlights
Gas transport in ultra-tight porous media has recently received extensive attention due to its important role in extracting unconventional gas resources and developing new technologies such as fuel cells, filters and catalytic converters [1,2]
MPI/Open Multi-Processing (OpenMP) approach with excellent scalability is required for the kinetic solver to be able to directly simulate gas flows in the porous media with flow passages provided by high-resolution images
Convergence properties of the kinetic solvers. Both the 2D and 3D MPI/OpenMP parallel implementations of the current algorithm are compared against the OpenMP parallelization in terms of the convergence rate and converged permeability
Summary
Gas transport in ultra-tight porous media has recently received extensive attention due to its important role in extracting unconventional gas resources and developing new technologies such as fuel cells, filters and catalytic converters [1,2]. The accuracy of conventional LBM for porous media flows in the transition and free-molecular regimes is still questionable and other gas kinetic methods are required to simulate rarefied gas flows in these regimes. Rapid advances in massively parallel computing technology have paved the way for gas kinetic solvers for direct simulations of flows in porous media [25,26,27,28,29,30,31,32,33]. MPI/OpenMP approach with excellent scalability is required for the kinetic solver to be able to directly simulate gas flows in the porous media with flow passages provided by high-resolution images. Parallel scalability of two-level MPI/OpenMP approach is analyzed and compared with that of the pure MPI approach
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