Circulation enhancement of synthetic jet actuators via multiple orifices

Abstract

Synthetic jet has shown potential benefits in laboratories as a flow control technique which is an important issue in the performance of aircraft and many fluid mechanical devices. It has attracted a lot of interest in recent years especially because of their zero net mass flux characteristic. However, higher momentum delivering capability is essential for synthetic jet to become a practical flow control technique. From this consideration, several investigations on various aspects of synthetic jet actuators have been conducted in the past. Some qualitative studies using flow visualisation have shown that when fluid passes through two smaller orifices, the resulting total circulation of the fluid appears to be greater than it is when the same fluid passes through a single but larger orifice. In this thesis, numerical investigations using computational fluid dynamics were performed in order to explore the benefits of multiple circular orifice synthetic jets over a single one for higher circulation production.

The spacing between the orifices which is an important factor in multiple-orifice actuators was investigated. In order to prevent the interaction between the vortex rings and avoid possible circulation cancelation, Lambda-2 criterion was used in this study to capture the three dimensional vortex core regions from multiple-orifice actuators. It was found that the generation of distinct and non-interacting vortex rings have direct dependency on the spacing between the orifices. As a result, a simple relationship for the minimum spacing which is based on the diameter of the orifice and the dimensionless stroke length was obtained. The effect of multiple-orifices on the total circulation produced was also investigated. It was found that for a given actuator with a specific input energy, it is possible to increase substantially the efficiency of the actuator, in terms of circulation produced, simply by increasing the number of orifices. From the results of the present study, decreasing the orifice diameter but increasing the number of orifices simultaneously from one orifice to five orifices with specified diameters, the efficiency increases by more than 300%. This is an important finding for synthetic jet applications and the results of the present study suggests that optimal and efficient synthetic jets can be generated to achieve higher circulation and momentum with lower input energy than has, hitherto, been possible.