The interaction between synthetic jet and adverse pressure gradient boundary layer flows

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This research project aims to investigate the interaction between the synthetic jets and the separating flow in a boundary layer under adverse pressure gradient conditions. The thesis provides an understanding of how the jets work to modify the boundary layer flow structure and information for designing synthetic jet actuators. To develop an understanding of the actuator operation, three actuators were first tested with variable frequencies and voltages under conditions of no cross-flow. At a fixed y/d0 location, the relationships between the jet velocity and the forcing frequency and between the jet velocity and the forcing voltage were examined. It was found that the jet velocity increased with the increase of forcing voltage, whilst several peaks were observed in the jet velocity distributions with frequency. Furthermore, it was demonstrated that the peak velocity could occur at different locations relative to the actuator orifice, depending on the forcing frequency. Experimental investigation to the effectiveness of the synthetic jet actuators were performed in a wind tunnel and measurements were taken in a boundary layer under adverse pressure. Experimental results demonstrated that the boundary layer flow separation was effectively resisted by the synthetic jet actuation. The results also showed that the control effectiveness of synthetic jets might be more strongly affected by the forcing frequency than the forcing voltage, depending on the particular actuator characteristics. The results also indicated that synthetic jets might play dual roles; in resisting separation by increasing momentum near the wall and amplifying the natural frequencies in the flow. The effectiveness of multiple jets in boundary layer separation control was investigated experimentally. When the actuators were operated individually, it was found that the performance of the actuator in the absence of cross-flow played a major role in determining the control effectiveness. For the same forcing frequency and forcing voltage, the effect of flow control was determined by locations of actuator. For multiple actuators, the control effect was increased relative to the interaction of individually operated actuator. Actuators located at different positions had their different 'most effective' forcing frequency. The results seem to indicate that the expected effect of flow control can be accomplished by varying forcing frequencies and voltages, but it may also be achieved by using different combined actuators. Generally, in combined actuators cases, the separation was controlled, with more effectiveness than individually operated actuators. In all cases, the boundary layer seems to approach the same downstream state, which indicates that both single and multiple actuators may have limited control effectiveness under the experimental conditions for present study for the cases considered.
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