An experimental study of synthetic jet actuators with multiple and rectangular orifices in quiescent conditions

The results of an investigation into the effect of both the number of orifices and the geometry of these orifices on the fluid structures exiting a synthetic jet actuator are presented. The experiments have used smoke and laser visualisation and single hotwire measurement techniques to establish the flow field exiting the various orifices and the fluid velocities in the exit plane of the orifice. The results have shown that the use of two small orifices can produce two coherent vortex rings with a total circulation greater than that found in the ring produced by a large, single orifice of equivalent area. However they have also shown that there are three different types of interaction between two adjacent orifices, related to the spacing between them and the level of excitation applied to the diaphragm, and that these interactions may affect the overall level of circulation in different ways. Finally the results have shown that a rectangular orifice of a given exit plane area will be more likely to produce a turbulent ring than the equivalent circular orifice due to a ‘smoothing’ process which endeavours to create an axisymmetric ring from a nonaxisymmetric slug of fluid. This work has shown that when using synthetic jet actuators in flow control applications, care should be taken in selecting the spacing and geometry of the orifices used, as certain setups may be more suited to certain applications than others.