Boundary layer transition near actuated deformable membranes (Conference Presentation)

Abstract

Marine animals are known to have developed adaptations to minimize drag and energy expenditure. Among these are passive material properties, such as the streamwise-aligned riblets found on shark skin, as well as the active modulation of the viscoelastic skin layer, which is thought to give dolphins their hydrodynamic edge. These adaptations serve to delay the transition from laminar to turbulent flow in the boundary layer around the body, minimize boundary layer turbulence, and reduce frictional drag. Transition to turbulence in the boundary layer happens via the development of two-dimensional instabilities, so-called Tollmien-Schlichting (TS) waves, which break down into fully developed turbulence. One mechanism to delay transition, is to counteract TS waves, reduce their amplitude, and delay their breakdown. This can be achieved by actively modulating a deformable membrane as part of the boundary near which the instabilities develop. We investigate boundary layer flow and transition to turbulence, and the effect of actuated boundaries, in a laminar to turbulent flow tank. To test the impact of a deformable boundary on the flow, a hydrofoil is outfitted with fluid chambers overlaid by an optical quality PDMS membrane, which can be actuated in response to the flow. Flow over the hydrofoil is visualized with dye experiments and quantified with Particle Image Velocimetry. The impact of boundary actuation, as well as different boundary materials, on the flow is characterized. Achieving a reduction of boundary layer turbulence on operational scales would have profound implications for platform energy efficiency, as well as signature and acoustic noise reduction.