Energy amplification and coherent structure evolution due localized forcing in flat plate boundary layer flow

Input-output analysis of the linearized Navier Stokes equations is used to evaluate the effect of localized forcing for laminar and turbulent flat plate boundary layer base flows. First, a stochastic forcing is applied at different heights above the plate to determine the wall-normal location that leads to the largest steady-state variance (output energy). Next, the time evolution of streamwise coherent structures due to impulse forcing at two wall-normal locations is studied. Forcing at the location associated with maximum energy amplification is compared with forcing at the wall, where inputs are typically applied in experiments. The inclination angles of the coherent structures as well as the wall-normal positions associated with maximum energy are compared for the laminar and turbulent base flows. The results show that the wall-normal location of the forcing matters more for turbulent base flows than for laminar ones. This study may provide insights for determining actuator placement in flow control applications.