Synthetic Jets in Cross-flow. Part 1; Round Jet

Results of an experimental investigation on synthetic jets fro m round orifices with and without cross -flow are presented. Jet Reynolds number up to 46 ,000 with a fully turbulent approach boundary layer, and Stokes number up to 400, are covered. The threshold of ‘stroke length’ for synthetic jet formation, in the abse nce of the cross -flow, is found to be L0 /D � 0.5. Above L0 /D � 10, the profiles of normalized centerline mean velocity appear to become invariant. It is reasoned that the latter threshold may be related to the phenomenon of ‘saturation’ of impulsively ge nerated vortices. In the presence of the cross -flow, the penetration height of a synthetic jet is found to depend on the ‘momentum -flux ratio’. When this ratio is defined in terms of the maximum jet velocity and the cross -flow velocity, not only all data c ollapse but also the jet trajectory is predicted well by correlation equation available for steady jets -in -cross -flow. Distributions of mean velocity, streamwise vorticity as well as turbulence intensity for a synthetic jet in cross -flow are found to be si milar to those of a steady jet -in -cross -flow. A pair of counter -rotating streamwise vortices, corresponding to the ‘bound vortex pair’ of the steady case, is clearly observed. Mean velocity distribution exhibits a ‘dome’ of low momentum fluid pulled up fro m the boundary layer, and the entire domain is characterized by high turbulence.