Large eddy (LES) and Reynolds-Averaged Navier-Stokes (RANS) simulations were performed to investigate modeling weaknesses in two popular RANS models, realizable k-ε and shear-stress transport (SST), in predicting film cooling of a flat plate. The cooling jets issue from a plenum through one row of circular holes of diameter D and length 4.7D that are inclined at 35° relative to the plate. The following parameters were studied: blowing ratio (BR =0.5 and 1.0) and density ratio (DR = 1.1 and 1.6). The LES and RANS solutions generated were validated by comparing with data from PIV and thermal measurements. The LES results agreed well, while RANS results showed large errors. Results obtained show the turbulent and thermal structure of the jets predited by the two RANS models to differ cconsiderably. However, both models are consistent in under predicting the spread of the film-cooling jet. The counter-rotating vortex pair (CRVP) dominates the interaction of the jet and crossflow in the near-wall region, and neither RANS model could predict the strength and structure of the interactions. The gradient-diffusion and Boussinesq hypotheses in the two RANS models were evaluated by using the LES data; regions of counter-gradient diffusion and stress-strain misalignment were identified. Comparing LES and RANS results show regions near the film-cooling hole where the realizable k-ε and SST models predict eddy-viscosities that lead to incorrect Reynold's stresses.
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