Large-Eddy Simulation Investigation of Impact of Roughness on Flow in a Low-Pressure Turbine

Surface roughness can make boundary-layers separate in diffusing flow. Most roughness Reynolds Averaged Navier Stokes (RANS) models which change the boundary conditions to model the roughness effects cannot predict this phenomenon. In this study, Large-Eddy Simulation (LES) is performed to predict the roughness induced separation and investigate the flow physics to improve our understanding of the underlying phenomena. Flow over a roughened low-pressure turbine airfoil was simulated by LES with WALE subgrid-scale model [15]. The roughness is modeled as regularly placed roughness elements with a similar equivalent roughness height following Schlichting [1]. The roughness elements are gridded in a body-fitted and multi-block structured way. Over a range of Reynolds number, the LES correctly predicted the behavior — with the flow separation occurring only at the high Reynolds number. Analysis revealed that surface drag and boundary layer thickness increase as Reynolds number increases, which is opposite to the conventional smooth wall boundary-layer behavior. In the end, the thickened boundary layer undergoes separation in the diffusing section. RANS simulations are also conducted with a roughness model — over a smooth airfoil grid — and without a roughness model — by using the same rough airfoil grid. In all cases, no separation was observed. The boundary layer thickness predicted with RANS is thinner than those of LES, suggesting that models that only modify surface stress at the boundary do not properly capture the flow physics over the rough surface of an airfoil.