The increase in fuel price and the enforcement of more restrictive regulations have presented significant challenges to the aircraft industry and forced the development and implementation of revolutionary technologies so as to reduce operational costs and environmental impact of aircraft transportation. The development of active flow control is believed to be one of the key measures to meet these challenges.Synthetic jet actuators, as a promising method for flow separation control, are capable of injecting momentum to the external flow with zero-net-mass-flux, which negate the need for air supplies and complex piping systems. So far, the capability of synthetic jets in controlling flow separation has been demonstrated in many laboratory-based investigations. However, our understanding of the fluid mechanics of synthetic jets is still limited. An improved understanding is essential to the optimisation of the synthetic jet actuator operating conditions for a given flow so as to maximise their flow control effectiveness for a practical setting. The present research is aimed at achieving this goal through a series of carefully designed experiments.Firstly, PIV measurements and smoke-wire flow visualisation of circular synthetic jets in quiescent air are undertaken to study the effect of dimensionless parameters on the formation and the extent of vortex roll-up. The results show the dimensionless stroke length (L) affects the onset of suction cycle on the vortex ring and the Stokes number (S) determines the strength of vortex roll-up. A criterion for vortex roll-up is also established. Based on PIV (Particle Image Velocimetry) measurements of both micro-scale and macro-scale synthetic jets, it is confirmed that the characteristic of synthetic jets of different scales will be identical when the L, S and ReL are the same.Dye visualisation of round synthetic jets issued into a zero-pressure gradient laminar boundary layer is carried out in a water flume. Three different types of vortical structures are found to be produced as a result of the interaction of a synthetic jet with a laminar boundary layer as the operating condition of synthetic jets changes, i.e., hairpin vortices, stretched vortex rings and tilted vortex rings. Hairpin vortices and stretched vortex rings are formed as the result of the upstream branches of the vortex rings being weakened by the resident vorticity in the boundary layer. In the case of tilted vortex rings, the vortex rings remain intact as they emerge from the boundary layer due to an increased jet strength.Finally, laser induced fluorescent flow visualisation and PIV measurement are undertaken to study the flow separation control effectiveness of a synthetic jet array issued upstream of a separated laminar flow over a flap in water. Based on the results from the experiment, parameter maps showing the effectiveness of separation control at different actuator operating conditions are produced. It is found that a good control effect is associated with the presence of hairpin type of structures and a velocity ratio (VR) in the range of 0.3 and 0.7. For the experimental conditions tested in this thesis, operating the synthetic jets at a dimensionless stroke length (L) around 2 and velocity ratio (VR) around 0.6 would deliver the best flow control effect with the least energy consumption.
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