An experimental investigation of the effect of owl-inspired velvety coating on trailing edge noise

Many owl species have three adaptations on their wings that are responsible for their quiet flight: the comb-like serrations at the leading edge, the soft fringes at the trailing edge, as well as the velvety structure on the upper wing surface. This study was inspired by the third feature. An experimental investigation was conducted at the HKUST low-speed wind tunnel, UNITED, to study the effect of artificial velvety structure on the boundary layer flow, as well as the trailing edge noise of a flat plate model. The chord-based Reynolds number ranged between 160,000 to 300,000. Turbulent strips were used to suppress instability noise. Four different velvety coatings with different hair dimensions were tested. The induced noise source distribution and the noise level were acquired by a 56-channel microphone array. Acoustic measurements show that all the tested velvety coatings can reduce the trailing edge noise above a critical chord-based Strouhal number, which depends on both the hair length and the location of the coating. However, the velvety coatings also increases the induced noise below that Strouhal number. The coating thickens the boundary layer, elevates the overall turbulent spectrum in the boundary layer at all frequencies, but reduces the velocity fluctuation in the vicinity of the trailing edge. If the acoustic measurement results were extended to the case of owls’ flight (below 10 m/s), then the velvety coating can reduce noise above a critical frequency that is well below 2 kHz, which is the lower hearing threshold of owls’ prey. © 2019, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.