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Dye flow visualisation of circular synthetic jets was carried out in laminar boundary layers developing over a flat plate at a range of actuator operating conditions and freestream velocities of 0·05 and 0·1ms -1 . The purpose of this work was to study the interaction of synthetic jets with the boundary layer and the nature of vortical structures produced as a result of this interaction. The effects of Reynolds number (Re), velocity ratio (V R ) and Strouhal number (St) on the behaviour of synthetic jets were studied. At low Re and V R , the vortical structures produced by synthetic jets appear as highly stretched hairpin vortices attached to the wall. At intermediate Re and V R , these structures roll up into vortex rings which experience a considerable amount of tilting and stretching as they enter the boundary layer. These vortex rings will eventually propagate outside the boundary layer hence the influence of the synthetic jets on the near wall flow will be confined in the near field of the jet exit. At high Re and V R , the vortex rings appear to experience a certain amount of tilting but no obvious stretching. They penetrate the edge of the boundary layer quickly, producing very limited impact on the near wall flow. Hence it is believed that the hairpin vortices produced at low Re and V R are likely to be the desirable structures for effective flow separation control. In this paper, a vortex model was also described to explain the mechanism of vortex tilting.

Despite the proven capability of synthetic-jet actuators in delaying boundarylayer separation in laboratory experiments, a capability that allows the geometry and operating conditions of these devices to be designed and selected for maximum flowcontrol effectiveness in full-scale flight conditions has yet to be developed. In this twopart paper, the key results obtained during a 3-year research programme aiming at establishing such a capability based on a better understanding of the fluid mechanics of synthetic jets and an improved modelling capacity are reported. In Part 1 of this paper, the experimental studies of the behaviour of synthetic jets in both quiescent flow and a boundary layer are described. The work has led to an improved understanding of the dimensionless parameters that determine the formation and development of vortex rollup and how the strength of rollup can be enhanced by optimizing the geometry and operating condition. Based on the study of the nature of vortical structures produced as the result of the interaction with a boundary layer and their impact in the near-wall region where flow control is desired, the conditions for producing effective vortical structures for delaying flow separation were established. The finding from this work forms the basis of a number of criteria used for designing synthetic jet actuators for full-scale flight condition to be presented in Part 2.