LES-AIDED SHAPE OPTIMISATION OF U-BEND CHANNEL

Abstract

Reynolds-Averaged Navier-Stokes (RANS) simulations are inaccurate in predicting complex flow features (ex: Separation regions), and therefore deriving an optimised shape using the RANS-adjoint framework does not yield a truly optimal geometry. With the purpose of obtaining accurate sensitivity to objective function of interest, we improve the RANS flowfield using the strategy of Singh et al. [1]. This involves multiplying a corrective factor β to the production term in the Spalart-Allmaras (SA) turbulence model equation and solving the inverse problem to determine the appropriate β field, which enables the RANS solution to match the high-fidelity data. The geometry of our interest is the U-Bend which is widely studied in literature in the context of gas turbine cooling, and which is known to be a challenging case for RANS simulations to reproduce. We use the mean flowfield from a large-eddy simulation of the U-Bend geometry as the high-fidelity data to which the RANS flowfield is fit using the β strategy outlined above. We observe a clear improvement in the RANS flowfield by optimising for the β field, the objective function to be minimized being L2-norm of the mean velocity difference between RANS and LES. We further show that adding an additional corrective factor (γ) to the destruction term in the SA turbulence equation and simultaneously optimising for the γ field alongside the β field results in a better match of the RANS flowfield with the corresponding LES flowfield. We also show that surface sensitivity map for the improved LES-aided flowfield varies significantly in comparison to the baseline SA-based flowfield for an objective function of interest, the total pressure loss in the U-Bend.