Influence of Surface Roughness Geometry on Trailing Edge Wall Pressure Fluctuations and Noise

Abstract

Surface roughness elements are commonly used in wind tunnel testing to hasten the laminar-turbulent transition of the boundary layer in model tests to mimic the aerodynamic effects present in the full-scale application. These devices can alter the characteristics of the turbulent boundary layer, such as the spanwise correlation length, the boundary layer thickness, etc. This not only affects the aerodynamic performance but also the aeroacoustic characteristics of the tested model. Few studies have investigated the effects of surface roughness elements on the trailing edge near- and far-field noise. So far, the influence of roughness on the wall pressure fluctuations and spanwise coherence at the trailing edge has been left unexplored. Thus, this research addresses the effects of surface roughness geometries of different heights on the trailing edge wall pressure fluctuations, the spanwise coherence, and the far-field noise. Wind tunnel experiments were performed adopting zigzag strips and novel sharkskin-like surface roughness installed in a NACA 0012 at zero angle of attack. The tested surface roughness heights ranged from 29% to 233% of the undisturbed boundary layer thickness. The wall pressure fluctuations and the far-field noise were measured for Reynolds numbers from 1.3x10^5 to 3.3x10^5. It was observed that the surface roughness affects the low- and high-frequency range of the wall pressure spectrum, with trip heights in the range from 50% to 110% of the undisturbed boundary layer thickness having a slight level increase for low frequencies and no difference for high frequencies. The far-field noise increased for low and high frequencies as the trip height increased. The low-frequency increase is a consequence of the trip effects on the trailing edge wall pressure fluctuations, whereas for high frequencies the increase is due to the noise generated by the trip itself.