Transitional region of a round synthetic jet

Spreading and decay rates of several synthetic jets are experimentally measured up to 70–100 diameters (d) away from the orifice. It is found that, consistent with previous studies, after 10d–15d from the nozzle a synthetic jet displays a self-similar velocity profile together with enhanced spreading and decay behaviors than a continuous jet. However, this enhancement does not persist throughout the downstream region, as the spreading and decay rates around 30d–50d away from the orifice decrease to asymptotic values that are comparable to continuous jets. This intermediate region with enhanced spreading and decay rates is dubbed the transitional region, while the region beyond 30d–50d , where the spreading and decay rates approach the continuous jet values, is referred to as the actual far field of a synthetic jet. Physically, the enhanced spreading and decay rates of the transitional region are caused by an enhanced mode of mixing related to the forced instability of the pulsed large vortices, which eventually break down into smaller eddies in the fully turbulent far field.