Boundary condition effects on oscillatory momentum generators

The effects of boundary conditions on the performance of compact oscillatory momentum generators were studied experimentally. The boundary conditions included: unrestricted and restricted entrainment, relative directions between the excitation and the surface, and the effect of an incoming laminar boundary layer. The excitation signals included a pure sine wave or an amplitude modulated sine wave, in order to generate low frequency excitation through non-linearity. When the actuator operates in still air, a quasi-2D vortex pair is generated due to the extreme shear at the edges of the ejected flow during the blowing stage of the cycle. A threshold slot exit velocity was identified, under which the vortices are either sucked back into the actuator's cavity or canceled due to the opposite shear generated during the suction portion of the cycle. The vorticity flux exiting the slot determines the resulting vortex circulation, while the vortex convection speed approximately scales with the peak velocity at the slot exit. Vortex pairing was identified for amplitude modulated signals, when faster, larger circulation vortices accelerate and catch-up with slower, weaker ones and the far-field only senses the low frequency modulation. When even a very short extension is attached to one "lip" of the actuator exit, operating in still air, the jet is deflected in the direction opposite the extended lip, due to the restriction on the entrainment process. When a long extension is attached, the coherence of the vortices increases, their phase speed and magnitude decrease. The effects of high-frequency excitation, ejected perpendicular to the wall into a laminar boundary layer for active tripping, were investigated. It was found that shallow angle downstream excitation is more effective for transition promotion, as well as for increasing the skin friction, than wall-normal excitation. Detailed PIV measurements of excitation-laminar boundary layer (LBL) interaction reveal that unsteady vortices are being shed and convected downstream, as long as the actuation magnitude is supercritical. The "mean bubble" identified using simple averaging does not represent the flow at any given time. A "bubble" of separated flow exists for only a fraction of the cycle and is convected downstream while the boundary layer is completely "washed away" at other times. The mechanism by which amplitude modulated signals generate low frequency at a distance of only several slot widths or LBL thicknesses was demonstrated and explained.