Development and decay of streamwise vorticity in the laminar boundary layer due to a step input by an array of plasma actuators*

The temporal development of the streamwise vorticity and resulting velocity streaks generated by a spanwise array of plasma actuators that are operated in a burst mode within a Blasius boundary layer is studied. The effect of the actuators is modeled by a momentum source using a commercial CFD code. The source is calibrated with an experimental dataset for steady actuation. Using a burst mode, the Blasius boundary layer is given a step input to the body force for 100 boundary layer turnover and then permitted to relax to the baseline flow. Near the wall an inverse response in the streamwise velocity is detected. The streamwise vorticity data are used to explain the corresponding inverse shear stress observed during the simulated activation and deactivation of the plasma actuators. It is shown that secondary streamwise vortices occurring below the primary titled flow structures lead to the inverse flow response during unsteady forcing.