LES of the trailing-edge flow and noise of a NACA0012 airfoil near stall

Abstract

Reynolds-averagedNavier-Stokes (RANS) simulations and large-eddy simulations (LES) of flow over a low-speed airfoil are performed at very high angle of attack in near-stall or stall conditions. In the parallel experimental effort at Ecole Centrale de Lyon (ECL), such flow conditions were obtained at 16◦ and 18◦ of angle of attack, respectively, on the selected NACA0012 airfoil. In the ECL small anechoic wind tunnel, the wall pressure noise sources have been measured simultaneously with the far field acoustic pressure. The stall condition is found to have an extraneous sound source at low frequencies on top of the trailing-edge noise. It is characterized by two specific tones at Strouhal number of 0.31 and 0.56. Both noise sources are found to be of dipolar nature. In the RANS k − ω SST simulations, the airfoil goes into stall at a much higher angle of attack of 25◦. Yet, during convergence at 18◦, the flow solution temporarily goes through a fully separated flow on the suction side, hinting at the observed experimental stall hysteresis. Even when this initial condition is selected, the corresponding LES returns to attached flow, yielding unrealistic unsteady lift on the airfoil. For the stall noise acoustic radiation, a simple model for a compact dipole is proposed based on the experimental evidences. From the experimental data collected on the airfoil, an analytical model is given for the spanwise coherence length.

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