Pressure Shielding Mechanisms in Bio-Inspired Unidirectional Canopy Surface Treatments

Reduction of surface pressure fluctuations is desirable in various aerodynamic and hydrodynamic applications. Over the past few years, studies on canopy surface treatments have been conducted to investigate the fundamental mechanisms of surface pressure attenuation termed as pressure shielding. This work talks about the design, development and experimental testing of unidirectional canopy surface treatments which are evenly spaced arrays of streamwise rods placed parallel to the wall without an entrance condition. The canopy designs are based on surface treatments tested by Clark et al. (2014) inspired by the downy coating on owl wings. The main objective of the work is to establish fundamental physical and mathematical basis for treatments that shield aerodynamic surfaces from turbulent pressure fluctuations, while maintaining the wall-normal transport of momentum and low aerodynamic drag.

Experimental testing of these canopy treatments are performed in the Anechoic Wall-Jet facility at Virginia Tech. Different canopy configurations are designed to understand the effect of various geometric parameters on the surface pressure attenuation. The treatment is found to exhibit broadband reduction in the surface pressure spectrum. Attenuation develops in two frequency regions which scale differently depending on two different mechanisms. Canopies seems to reduce the large-scale turbulent fluctuations up to nearly twice the height. Semi-analytical model is developed to predict surface pressure spectra in a wall-jet and canopy flow. The rapid term model shows that the inflection in the streamwise mean velocity profile is the most dominant source of surface pressure fluctuations. Synchronized pressure and velocity measurements elucidate significant features of the sources that could be affecting surface pressure fluctuations. Overall, this study explores the qualitative and quantitative physics behind pressure shielding mechanism which find application particularly in trailing edge noise reduction.