Identifying Weaknesses in Eddy-Viscosity Models for Predicting Film Cooling via Large-Eddy Simulations

Abstract

Large-eddy simulations (LES) and Reynolds-averaged Navier–Stokes (RANS) simulations were performed to investigate modeling weaknesses in two popular RANS models, realizable and shear-stress transport (SST), in predicting film cooling of a flat plate. The film-jets issue from a plenum through one row of circular holes of diameter and length that are inclined at 35° relative to the plate. The LES and RANS solutions generated were validated by comparing with data from PIV and thermal measurements. The LES results agree well, whereas RANS results show large errors. Results obtained show the turbulent and thermal structure of the jets predicted by the two RANS models to differ considerably. However, both models are consistent in underpredicting the spread of the film-cooling jet. The counter-rotating vortex pair dominates the interaction of the jet and crossflow in the near-wall region, and neither RANS model could predict the strength and structure of that interaction. The gradient-diffusion and Boussinesq hypotheses were evaluated by using the LES data. Comparing LES and RANS results shows that tends to overpredict eddy viscosity, whereas SST tends to underpredict the eddy viscosity. Additionally, both models predict very low values of eddy viscosity near the wall, which leads to incorrect Reynolds stresses. Although regions of counter-gradient diffusion and stress-strain misalignment were found in the near-wall region, further above the wall, the jet behaved according to the hypotheses.