Airfoil Tonal Noise Reduction by Roughness Elements. Part II -- Direct Simulations

TL;DR

This study uses direct numerical simulations to analyze how cylindrical roughness elements on airfoils can reduce tonal noise by disrupting flow structures and stabilizing shear layer instabilities, aligning with experimental findings.

Contribution

It provides detailed numerical insights into the noise reduction mechanisms of roughness elements on airfoils, complementing experimental results and identifying flow-structure interactions.

Findings

Roughness elements attenuate and suppress tonal noise at higher Reynolds numbers.

Spectral POD reveals dominant flow structures affected by roughness.

Stability analysis shows roughness stabilizes shear layer instabilities.

Abstract

In a combined experimental and numerical effort we investigate airfoil tonal noise generation and reduction. The means of noise control are streak generators in form of cylindrical roughness elements. These elements are placed periodically along the span of airfoil at the mid chord streamwise position. Experiments are performed for a wide range of Reynolds number and angle of attack in a companion work (Alva et al. 2024). In the present work we concentrate on numerical investigations for a further investigation of selected cases. We have performed wall-resolved large-eddy simulations for a NACA 0012 airfoil at zero angle of attack and Mach 0.3. Two Reynolds numbers (80,000 and 100,000) have been investigated, showing acoustic results consistent with experiments at the same Reynolds but lower Mach numbers. Roughness elements attenuate tones in the acoustic field and, for the higher Reynolds number, suppress them. Through Fourier decomposition and spectral POD analysis of streamwise velocity data, dominating structures have been identified. Further, the coupling between structures generated by surface roughness and instability modes (Kelvin-Helmholtz) of shear layer has been identified through stability analysis, suggesting stabilisation mechanisms by which the sound generation by the airfoil is reduced by the roughness elements.