Experimental Analysis of Bio-Inspired Anisotropic Permeable Propulsive Substrates on Turbulent Separation Bubbles

The performance impact of bio-inspired anisotropic permeable propulsive substrates (APPS) attached to a flat plate model on adverse pressure gradient (APG) and suction-induced turbulent separation bubble (TSB) is investigated. The incoming turbulent flow has an approximate Reynolds number of Re_θ = 780 and a boundary layer thickness δ ~ 7 mm, with a mean suction velocity relative to the free stream u_s/u_∞ ~ 0.5 . Two APPS substrate designs are considered: a shark denticle substrate and a shark denticle-inspired substrate consisting of an array of flat plates supported by cylinders. The substrates are fabricated using Masked Stereolithography 3D printing, which provides sufficient resolution to produce small geometric features. Both denticle and cylindrical-flat plate structures have a height of δ_h = 0.14 δ = 1 mm (limited by the fabrication method) and are in a staggered layout. The substrate spans the width of the flat plate model and extends both upstream and downstream of the time-averaged separation bubble. An ATI Nano 25 load cell is attached to the bottom of the strut supporting the flat plate model to measure integral drag force, with the flat plate model isolated from the tunnel. Drag reduction up to 30% relative to the baseline is observed under TSB conditions with the substrates attached, while a negligible variation is observed under APG conditions, suggesting that the addition of the APPS substrate reduces the adverse effects of TSB while not having a negative impact under APG conditions. The effects of APPS substrates under increased incoming turbulent boundary layer thickness will be investigated in the future, such that the APPS feature height δ_h< 0.10 δ.