Active control of a free jet using a synthetic jet

The control of an axisymmetric free jet (ReUe=6600) using a single synthetic jet was investigated experimentally. The interaction was examined for a range of momentum coefficients, Strouhal numbers, and synthetic jet orientations (with respect to the main jet). To better explore the complex flow field resulting from the interaction, a rendering technique was used where three-dimensional flow fields were calculated from multiple two-dimensional measurement planes. The synthetic jet deflects the majority of the main jet flow away from it, while drawing some of the flow back toward it. Also, the synthetic jet is shown to appreciably raise the main jet's turbulent quantities, suggesting that mixing has been enhanced. Using triple decomposition, it was shown that the random and coherent motions have similar contributions to the turbulent stresses near the interaction region; whereas the coherent motions prevail farther downstream (and along the shear layers). Measurements of the streamwise vorticity showed that the interaction results in the formation of counter-rotating streamwise vortices, similar to the effect of passive tabs. The size and strength of these structures can be controlled by changing the synthetic jet's momentum coefficient, actuation frequency, or orientation. At low momentum coefficients, the largest effect is obtained for a Strouhal number of 0.32; while at higher momentum coefficients saturation is obtained due to the high excitation level. A steady control jet, which only utilizes the direct impact mechanism, results in vectoring and a deep penetration into the main jet. However, it yields decreased spreading compared to a synthetic jet with the same momentum coefficient.