THE FORMATION AND EVOLUTION OF SYNTHETIC JETS

A nominally plane turbulent jet is synthesized by the interactions of a train of counter-rotating vortex pairs that are formed at the edge o f an orifice by the time-periodic motion of a flexible diaphragm in a sealed cavity. Even though the jet is formed without net mass injectio n, the hydrodynamic impulse of the ejected fluid and thus the momentum of the ensuing jet are nonzero. Successive vortex pairs are not subje cted to pairing or other subharmonic interactions. Each vortex of the pair develops a spanwise instability and ultimately undergoes transiti on to turbulence, slows down, loses its coherence and becomes indistin guishable from the mean jet flow. The trajectories of vortex pairs at a given formation frequency scale with the length of the ejected fluid slug regardless of the magnitude of the formation impulse and, near t he jet exit plane, their celerity decreases monotonically with streamw ise distance while the local mean velocity of the ensuing jet increase s. In the far field, the synthetic jet is similar to conventional 2D j ets in that cross-stream distributions of the time-averaged velocity a nd the corresponding rms fluctuations appear to collapse when plotted in the usual similarity coordinates. However, compared to conventional 2D jets, the streamwise decrease of the mean centerline velocity of t he synthetic jet is somewhat higher (similar to x(-0.58)) and the stre amwise increase of its width and volume flow rate is lower (similar to x(0.88) and similar to x(0.33), respectively). This departure from co nventional self-similarity is consistent with the streamwise decrease in the jet's momentum flux as a result of an adverse streamwise pressu re gradient near its orifice. (C) 1998 American Institute of Physics.