Generation of boundary-layer disturbances by freestream forcing

We present an investigation based on simplified DNS of the generation of boundary layer disturbances by free-stream vorticity in Falkner-Skan boundary layers. The free-stream vorticity is generated by a source of the form $\delta (x)\delta (z-z_{f})$exp(\textit{i$\beta $y}) derived from a body force, where ($x$,$y$,$z)$ are the streamwise, spanwise and wall-normal co-ordinates, $z_{f }$is located above the boundary layer and near its edge, and \textit{$\beta $} is the spanwise wave-number. Both streamwise and spanwise vorticity and stationary and oscillating sources are used. A steady streamwise vorticity source creates a diffused and corrugated sheet of streamwise vorticity that drives the boundary layer with a wall-normal velocity, thereby generating streak-like structures within the boundary layer. Oscillating the source acts to generates two different structures; one similar to a Klebanoff mode and the other, depending on the frequency of oscillation, either grows exponentially or decays and cannot be seen.