Experimental Investigations of the Turbulent Boundary Layer for Biomimetic Surface with Spine-Covered Protrusion Inspired by Pufferfish Skin

Abstract

Pufferfish skin is known for its spine-covered surface, which differs significantly from that of common fish species. Recent research has shown that such rough surfaces may have potential applicability in saving energy. In order to verify the drag reduction effects of pufferfish skin, a flat sample and six samples featuring biomimetic spine-covered protrusions (BSCPs) with combinations of three different protrusion heights (0.2, 0.4, and 0.8 mm) and two array patterns (average and staggered) were manufactured. Force measurements and particle image velocimetry (PIV) were introduced with a free-stream velocity of 0.65 ms−1. Drag results suggested that the staggered surface with the shortest BSCPs achieved the best performance, exhibiting the maximum drag reduction of 5.9% compared to the flat sample. Lower Reynolds shear stress and turbulence intensity were achieved over the staggered array according to the PIV results. Less retrograde vortex structures existed inside a viscous buffer sublayer over the BSCPs sample, about 20 ~ 30% in quantity lower than flat one; these prominently influenced the drag reduction effect, as determined by a combination of the $$ \upomega $$ -criterion and Q-criterion. Furthermore, the coherent structure appeared orderly over the staggered array in the streamwise direction. As a result, the BSCPs height and array pattern are considered influential factors that remain to be optimized in the future for drag reduction.