On Boundary-Layer Stabilization by Bio-Inspired Compliant Coatings for Underwater Drones

Abstract

Abstract. The advent of unmanned underwater vehicles (UUVs) has enabled scientific research, resource exploration, and infrastructure development at the depths of the oceans. At such depths (>100 m), with freestream turbulence intensity Tu<1.5 %, predominant drag component on a streamlined UUV is the skin friction drag. Inspired by dolphin skin, we propose a two-layer silicone-based compliant coating comprising a thick blubber that undergoes wall-normal compression, and a thin dermis that undergoes bending deformation. For the proposed choice of material and coating parameters, the energy exchange at the wall, due to deformation of the compliant surface, serves to attenuate the growth of Tollmien–Schlichting (TS) waves. This stabilizing effect of the compliant surface allows the vehicle to maintain low-drag laminar boundary-layer flow over a greater portion of the wetted surface. The efficacy of the proposed coating has been studied on the X-35 UUV in this work. Numerical simulations of the flow around this UUV have been performed at a cruise Reynolds number ReL=1.185×107, using pisofoam to characterize the boundary layer. The Falkner–Skan solution with wedge angle βw=24.8deg was found to be a close approximation of the fore-body laminar boundary layer. Using this self-similar baseflow, the Orr–Sommerfeld equation was solved with compliant wall boundary conditions to compute the amplification of TS waves. For the best-performing coating, with blubber thickness hB=7 mm, the transition point shifts from 18% to 47% of the UUV length, without triggering any fluid-induced surface instabilities (FISI). This delay in laminar-to-turbulent transition allows for potentially 31% reduction in skin-friction drag on the UUV.