On the evolution of amplitude modulated excitation in still air

The effects of boundary conditions on the performance of compact, zero-mass-flux, oscillatory momentum and vorticity generators (OMVG's) were studied experimentally. The slot exit boundary conditions allowed unrestricted and restricted entrainment. The excitation signals included a pure sine wave (as a reference) and amplitude modulated sine wave, in order to generate low frequency excitation through non-linearity. Detailed PIV and hot wire measurement are presented and discussed. When the actuator operates in still air, a train of quasi-2D vortex pairs 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 stage 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. Two main mechanisms were identified as responsible for the low frequency generation from amplitude modulated (AM) signals. The first is due to Sub-critical vortices, generated as part of an AM vortex train, that do not affect the far-field. The second is attributed to vortex pairing in amplitude modulated signals, when faster, larger circulation vortices accelerate and catch-up with slower, weaker vortices to form one vortex pair that affects the far field. As a result of both mechanisms, the far-field only senses the low frequency modulation.