Separation Delay in Turbulent Boundary Layers via Model Predictive Control of Large-Scale Motions*

Turbulent boundary layers are dominated by large-scale motions (LSMs) that contain a significant fraction of the turbulent kinetic energy and Reynolds shear stresses. These LSMs, mostly residing in the outer region of the boundary layer, have the potential to increase near-wall mixing and reenergize the boundary layer, if brought closer to the wall. Following previous and concurrent efforts on steering synthetic LSMs in a laminar boundary layer, we explore the effect of manipulating LSMs in a moderate Reynolds number turbulent boundary layer for separation delay. We employ direct numerical simulations of a turbulent boundary layer in an adverse pressure gradient at a Reynolds number sufficiently high for LSMs to naturally occur. The volume these LSMs occupy is identified from the fluctuations in the 3D velocity field and a downwash-inducing force field – which mimics a plasma actuator and is controlled via a downwash-maximizing model predictive control scheme – is used to push these LSMs closer to the wall. The effect of targeting LSMs with the induced downwash versus a naïve periodic actuation scheme that does not account for the presence of LSMs is studied.