Understanding Pressure Shielding by Canopies

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.