Experimental Analysis of Pressure Shielding Mechanisms in Bioinspired Unidirectional Canopies

Previous studies have demonstrated that treatments such as a canopy or finlets placed within a boundary layer can shield surfaces from unsteady pressure fluctuations without substantially compromising the aerodynamic per-formance. This paper describes research into fundamental mechanisms of this phenomenon known as pressure shielding. Unidirectional canopy is an idealized surface treatment which consists of a streamwise array of rods cantilevered at the downstream end, inspired from the downy coating on owls’ wings. Experiments show that such a canopy attenuates the surface pressure in two distinct frequency ranges. At low frequencies associated with convective scales much greater than the canopy height, the attenuation spectra show scaling on the Strouhal number based on canopy height. At high frequencies, associated with convective scales of the order of the canopy height or lower, a dissipation-type frequency scaling appears more appropriate. The ratio of streamwise distance over the height is an important parameter at the low-frequency regions of attenuation, while the open-area ratio controls the broadband magnitude of attenuation spectra. Spatial and temporal correlations further shed light on the effects of the canopy in reducing the larger, energetic turbulent structures associated with the wall jet unsteady surface pressure fluctuations.