Unsteady Flow Simulations in Compressors

The aim of this study is to propose a computing method to obtain a detailed simulation of the unsteady flow that develops in multistage turbomachines. The three-dimensional unsteady Reynolds-Averaged Navier-Stokes equations are solved using a structured multiblock decomposition method. Although this kind of flow solver is very popular in the turbomachine community nowadays, the complex block connectivities used in meshes of industrial configurations can be penalizing for parallel computing. The computing strategy implemented in a modern flow solver is investigated in this paper, with a particular interest for mesh partitioning, communications and load balancing. Advantages and drawbacks of different computing platforms are then discussed, ranging from vector supercomputers to massively scalar platforms. Comparisons are performed regarding criteria such as the elapsed time and the electric power consumption. The results show that the use of a large number of computing cores (>128) is largely penalized by communications and load balancing errors, whereas computing performance with a moderate number of computing cores (<128) is mainly driven by the peak power of the architecture. To help users estimate a priori the parallel performance of a task, a tool based on an extension of the Amdahl's law is proposed, showing satisfying results when compared with observations. Finally, an unsteady flow simulation is performed in a whole three-stage compressor at the design operating point. While still far beyond industrial resources, this numerical flow simulation shows that potential breakthroughs in the design of compression systems can be expected.