SOPHIA: Development of Lagrangian-based CFD code for nuclear thermal-hydraulics and safety applications

Lagrangian-based meshless CFD methods have recently been developed and are being gradually applied to various research areas. In nuclear engineering, the meshless methods are gaining attention in modeling natural disasters and severe accident phenomena, such as tsunami, molten corium behaviors, etc., because of its flexible computational domain and effective treatment of non-linear deformations. This paper introduces recent progress and on-going activities in Lagrangian-based CFD code development at Seoul National University (SNU). The code, named SOPHIA, is based on the smoothed particle hydrodynamics (SPH) method, one of the best-known meshless numerical methods, based on a Lagrangian framework that can easily handle various types of physics because of its simplicity in expressing and solving mathematical equations. The SOPHIA code currently incorporates basic conservation equations and various physical models, including fluid flow, heat transfer, turbulence, multi-phase, phase change, elastic solid, diffusion and so on. To handle multi-phase, multi-component, and multi-resolution flows simultaneously, the SOPHIA code formulates density and continuity equations based on the normalized-density form, which has been recently developed and is proposed in the current study. The SOPHIA code also adapts CUDA GPU architectures for parallelization and performance improvement. Based on the benchmark calculations, the parallelized GPU code shows much higher computational speed by two orders of magnitude compared to the single CPU code for 1.0 million particles. This paper summarizes the overall features of the SOPHIA code, including governing equations, algorithms, and parallelization methods, along with some benchmark simulations.