Search for: All records

Previous studies have demonstrated that structures such as a canopy or finlets placed within a boundary layer over an aerodynamic surface can attenuate pressure fluctuations on the surface without compromising aerodynamic performance. This paper describes research into the fundamental mechanisms of this pressure shielding. Experiments and analysis are performed on elemental canopy configurations which are arrays of streamwise rods placed parallel to the wall in order to eliminate the confounding effects of a leading edge support structure. Experiments show that such a canopy produces attenuation in three distinct frequency ranges. At low frequencies, where convective scales are much greater than the canopy height, attenuation spectra scale on the canopy height Strouhal number, but at high frequencies, a dissipation type frequency scaling appears more appropriate. There is mid-freqeuncy region which shows reduction in attenuation and is observed for all canopy structures tested. Attenuation in this region appears to scale with Strouhal number based on canopy spacing. 

Shear sheltering is defined as the effect of the mean flow velocity profile in a boundary layer on the turbulence caused by an imposed gust. It has been studied extensively in applications involving boundary layer transition, where the primary concern is flow instabilities that are enhanced by turbulence in the flow outside the boundary layer. In aeroacoustic applications turbulent boundary layers interacting with blade trailing edges or roughness elements are an important source of sound, and the effect of shear sheltering on these noise sources has not been studied in detail. Since the surface pressure spectrum below the boundary layer is the primary driver of trailing edge and roughness noise, we will consider the effect that shear sheltering has on the surface pressure spectrum below a boundary layer. We will model the incoming turbulence as vortex sheets at specified heights above the surface and show, using classical boundary layer profiles and approximations to numerical results, how the mean flow velocity can be manipulated to alter the surface pressure spectrum and hence the radiated trailing edge noise.