Abstract The advent of Unmanned Underwater Vehicles (UUVs) has enabled scientific research at the depths of the oceans. At such depths ( 100 m), with freestream turbulence intensity Tu 1.5 %, the predominant drag on a stream-lined UUV is the skin-friction drag. Inspired by dolphin skin, we propose a two-layer compliant coating comprising a thick blubber undergoing wall-normal compression, and a thin dermis undergoing bending. For the proposed coating , the energy exchange at the wall due to compliant surface deformation attenuates the growth of Tollmien-Schlichting (TS) waves. This stabilizing effect of the coating 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. Numerical simulations of the flow around this UUV have been performed at cruise Reynolds number ReL = 1.185 × 107, using pisoFoam to characterize the boundary-layer. The Falkner-Skan solution (βw = 24.8°) 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.
Das et al. (Sat,) studied this question.