Delaying Disorder in Flow via Ordered Phononic Subsurfaces

Turbulence in flow is distinguished by chaotic changes in pressure and flow velocity, and this behavior leads to turbulent drag on air vehicles, producing high fuel costs. Historically, there are passive flow control measures where static materials are added to the fluid-structure interface to manipulate the flow characteristics in the boundary layer to decrease drag. There have also been effective active control methodologies where energy is added into the flow to manipulate it. The goal of this work is to develop a novel passive flow control method by engineering the structural dynamics of a material interfacing with a flow, so that the material autonomously reacts in a beneficial manner to delay turbulence. In this effort, a phononic crystal (PnC), also called a phononic subsurface (PSub), was embedded within a plate in a subsonic flow with a single interaction surface where energy can be exchanged between the PSub interaction surface and the flow. The formation of an instability in the flow causes a forcing on the interaction surface of the PSub, and leads to a displacement of the interaction surface back into the flow. The importance of the phase between the oscillatory forcing by the flow instability, and the response of the PSub interface will be discussed for transition delay. Iterations on PSub design will be discussed based on predictions from fully-coupled high-fidelity CFD simulations. Wind tunnel experiments of a variety of phononic subsurfaces in subsonic flow will be presented.