DEVELOPMENT OF A NEW RANS-BASED SURFACE ROUGHNESS MODEL FOR MULTI-FIDELITY 3D CFD

Abstract

This paper deals with the CFD-based modelling of surface roughness, a challenging topic which is increasingly gaining popularity in turbomachinery applications. One approach to model the impact of roughness on the flow over a surface is to derive a roughness wall-function responsible for shifting the boundary layer log-law downwards, which is indeed a result of the increased wall shear stress caused by roughness. Another approach is to modify the turbulence model boundary conditions at the wall, so that the produced turbulent viscosity mimics the roughness-generated turbulence. In this work, a new roughness model belonging to the latter category has been implemented in an in-house RANS solver, based on the k-ω SST turbulence model. Using the equivalent sand-grain roughness parameter as input, the model modifies accordingly the specific dissipation rate (ω) at the wall, which ultimately leads to an increased turbulent viscosity. The model has been trained to match pressure losses generated in roughness-resolved channel flow simulations, the roughness of which is representative of deposition-generated roughness and was obtained from previous investigations on in-service high-pressure compressor blades. Verification has been performed on various zero and non-zero pressure gradient channel flow configurations, and experimental validation on a high-pressure turbine cascade. Finally, as a simple modification to the commonly employed k-ω SST turbulence model, this roughness modelling capability can also be extended to high-fidelity scale-resolving simulations, as demonstrated here via coupling with a hybrid URANS-LES methodology.