Numerical investigation of noise reduction mechanisms in a bio-inspired airfoil

Abstract

This paper presents a numerical analysis of an airfoil geometry inspired by the down coat of the night owl. The objective is to understand the mechanisms of airfoil trailing edge noise reduction that has been observed with such designs in previous experiments. The bioinspired geometry consists of an array of finlet “fences” that are applied near the trailing edge of the baseline (NACA 0012) airfoil. Wall-resolved large eddy simulations are performed over the baseline and the bioinspired airfoil geometries and the aeroacoustic performance of the two geometries are contrasted. Both models are simulated at chord-based Reynolds number Rec = 5 × 105, flow Mach number, M∞ = 0.2, and angle of attack, α = 0°. Unsteady surface pressure spectra near the airfoil trailing edge show large reductions at high frequencies but an increase in low frequencies with the bioinspired airfoil, consistent with previous measurements. Farfield noise spectra comparisons between the baseline and the bioinspired airfoil show reductions of up to 10 dB with the fences. The simulations reveal that the fences lift the turbulence eddies away from the airfoil trailing (scattering) edge hence reducing the scattering efficiency. These findings suggest that one of the mechanisms of noise reduction is the increased source-scattering edge separation distance. Two-point correlations show that the fences reduce the spanwise coherence at low frequencies for separation distances greater than a fence pitch. Reduction in spanwise coherence is another potential mechanism of farfield noise reduction at low frequencies.