Control of large-scale motions in boundary layers*

Turbulent flows are dominated by large-scale motions with temporal and spatial coherence. In particular, LSMs in boundary layers, which can span several times the boundary layer thickness in the streamwise direction, contain a large fraction of the turbulent kinetic energy of the streamwise velocity component, contribute significantly to the average Reynolds shear stresses, and transport momentum within the turbulent boundary layer. Given their importance, we pose the question of whether we can manipulate the movement of these LSMs and bring them closer to the wall in order to energize a boundary layer. To answer that question, we introduce a set of synthetic LSMs in a direct numerical simulation of a laminar boundary layer by generating a series of aligned hairpin vortices via force fields. We then target these structures via an actuator modeled as a body force field with a pitch angle towards the wall that generates a region of downwash that traps and pushes the synthetic LSMs toward the wall. The body force required to move the oncoming LSMs is computed via a model predictive control framework (Tsolovikos et al., AIAA J. 2021). The effect of moving these coherent structures toward the wall on the vorticity and momentum transport in the boundary layer is studied.