Validation of Large-Eddy Simulation for the Prediction of Compressible Flow in an Axial Compressor Stage

A better understanding of unsteady flows (including turbulence) is a necessary step towards a breakthrough in the design of modern gas turbine components. LES emerges as a very promising method to improve both knowledge of complex physics and reliability of flow solver predictions. However, there is still a lack of evidences in the literature that LES is applicable for rotor/stator configurations at industrial Reynolds numbers. In that context, the objective of the present work is to investigate the capability of LES to predict the turbulent flow in a stage of an axial compressor. The studied configuration operates at conditions relevant to industrial applications (M = 0.5, Re = 5 × 10⁵). The whole 3D flow is considered, including the rotor tip clearance, as well as the natural periodicity of the compressor (i.e. the exact ratio between the number of rotor blades and stator vanes). First a validation of LES is obtained by comparing results on different mesh densities, in order to evaluate the grid requirements necessary to perform a wall-resolved LES (the finest grid is made of o(10⁹) points to represent 3 rotor and 4 stator passages). Typical mesh scales are compared to the Kolmogorov length scale to estimate the regions where the subgrid scale model acts. Results obtained with both URANS and LES are then compared at nominal operating conditions. The analysis focuses on two dataset: time-averaged quantities (boundary layer profiles, turbulent kinetic energy, etc). and unsteady flow data. An evaluation of the flow physics shows that URANS predicts much more losses than LES in this configuration (discrepancy on efficiency is about 6%). The analysis indicates that LES predicts a laminar to turbulent transition at 50% of the rotor chord, leading to thinner boundary layers in both rotor and stator than with URANS. Actually LES also predicts the development of high-energy frequencies in the tip region that are uncorrelated to the blade passing frequency, related to the pulsation of the tip leakage flow.