Imitating dolphins: Nature-inspired boundary modulation to reduce frictional drag

In recent years, there has been a renewed interest in mechanisms of flow and turbulence control inspired by aquatic animals. This includes passive mechanisms such as the boundary roughness changes found on shark skin, as well as active boundary modulation, inspired by the viscoelastic properties of dolphin skin. In this work, we present a novel device to control boundary morphology and impact near-boundary flow. We developed this device as a 5-by-5 actuator array, using voice coil motors, and implemented it in the wall of a laminar-to-turbulent flow channel. The actuators are overlaid by a flexible membrane and embedded in the wall of the channel. Each actuator can be controlled individually, and the system can generate wave patterns on the boundary along or across the flow direction. The boundary deformation patterns were quantified using a computer vision approach. The system was tested to assess its impact on near-boundary velocity and turbulence. A DPIV (Digital Particle Image Velocimetry) system was used to quantify velocity fields. Velocity decomposition showed that boundary actuation impacts the mean boundary layer velocity profile. The impact on near-boundary velocity was found to be consistent with a reduction in frictional drag. A numerical tank was set up using Computational Fluid Dynamics, and the results from the model support the laboratory findings. To our knowledge, this is the first such device allowing boundary modulation using individual actuator control that has been implemented and tested in direct contact with water with the impact on flow assessed. A successful implementation of such a device in a natural underwater environment could lead to wide-ranging benefits including increased platform energy-efficiency and reduction of flow-induced noise.