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.

Due to the large parameter space that is involved in the design of synthetic-jet actuators used for flow separation control, it is essential to develop low-dimensional models for actuator-performance prediction and establish the conditions for design optimisation so that candidate actuators can be identified at the initial design stage. In Part 2 of this paper, three low-dimensional models that are capable of predicting actuator performance for synthetic jet actuators of different scales are described. Using these models the parametric relationships required for optimising the actuator performance are established. Finally, based on a better understanding of the fluid mechanics of synthetic jets (described in Part 1) and the improved modelling capacity, a methodology for designing synthetic-jet actuators used for flow separation control at full-scale flight conditions is developed. This methodology is illustrated in the design of actuators for delaying flow separation on the leading and trailing edge devices of a multi-element high-lift system of a typical commercialaircraft wing at take-off conditions.

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Dye visualisation of both inclined synthetic jets and skewed synthetic jets was undertaken in a cross-flow experiment and the results were compared with those of normal synthetic jets. The process of vortex roll-up near the orifice exit and how the structure develops and interacts with the cross-flow as it propagates downstream was investigated so as to obtain an understanding of the effect of orifice orientation on the behaviour of synthetic jets. The effects of varying Reynolds number, velocity ratio and Strouhal number due to changes in diaphragm displacements and freestream velocities on the characteristics of synthetic jets were also examined. It is observed that in comparison to the normal jets vortical structures produced by both inclined and skewed jets tend to stay closer to the near wall region where maximum flow control effect is required. In both cases, at a relatively low Reynolds number and velocity ratio the active structures produced by the synthetic jet appear to be hairpin vortices which turn into vortex rings that migrate away from the wall as the Reynolds number and velocity ratio increase. These hairpin vortices are persistent in the near wall region hence are believed to be desirable structures for delaying flow separation.

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