Turbulent Drag Reduction Using Dynamic Free-Slip Surfaces
The phenomenon of wall-bounded turbulent flow critically affects the performance of many engineering transportation systems. For example, the turbulent drag force acting on the ocean liners accounts for more than 80% of the total energy expenditure. As such, understanding the fundamental wall-bounded turbulent flow and achieving turbulent drag reduction have been a long-time pursuit for both scientists and engineers. Over the past several decades, although many promising drag reduction techniques have been developed (e.g., by employing a passive super-hydrophobic surface), sustainable drag reduction remains a fundamental challenge. In this talk, I will present a novel dynamic free-slip surface method that can reduce the turbulent drag force by more than 40%. This method employs an array of air-water interfaces that are attached to the wall boundary, and dynamically oscillates the free-slip interfaces at the desired frequency and amplitude. The dynamic free-slip surfaces substantially modify the momentum and energy transfer processes within the turbulent flow. The turbulent vorticity and shear motions were effectively lifted away from the wall boundary and a counterintuitive local propulsion force (rather than drag force) was generated. In addition, as shown by an analysis of the energy exchange process, the turbulent flow in the regions close to the wall boundary partially re-laminarized through an inversed energy cascade process. These physical observations all support the existence of a strong drag reduction effect.