| Summary: | Active control of flow separation over multi-element high-lift systems of aircraft wings could result in the decrease of drag force, the increase of lift, and the reduction of system complexity. This, in return, lowers fuel consumption and increases profitability, making the goals of both the Kyoto Protocol and the European Vision for 2020 more achievable. To maximise the potential of this technology, forms of actuation with high efficiency, low power consumption, fast response, good reliability, and low cost, are required. The synthetic jet is a kind of jet that on one hand produces zero net mass flux across the orifice over an oscillation cycle, whereas on the other is capable of transferring momentum and vorticity to the external fluid. It is a promising form of actuation for flow separation control with demonstrated success in laboratory experiments. The aims of the present research are to achieve an improved understanding of the fluid mechanics of synthetic jet actuators and to obtain the desirable modelling capability, such that a methodology of designing synthetic jet actuators for flow separation control at full-scale flight conditions can be developed. The research is focused on synthetic jets issuing from normal circular orifices. 2D axisymmetric numerical simulations were conducted using a commercial CFD code, FLUENT. The computational results for laminar synthetic jets agreed well with the experimental data, and the RNG 1(-cand Standard 1(-OJ models produced the best match between the computational and experimental results for turbulent synthetic jets. The CFD study confirmed the capability of FLUENT in simulating the key features of synthetic jets and established confidence in using the simulation results from FLUENT to validate low-dimensional models when experimental data are insufficient. Three low-dimensional prediction models have been developed, which are capable of predicting the space-averaged jet velocity from which the other actuator performance parameters can be estimated. The Dynamic Incompressible (DI) model provides analytical expressions for calculating the performance parameters for a given actuator geometry and operating condition. The Static Compressible (SC)model yields important relations among the actuator geometry and operating frequency, which allows the peak jet velocity to be maximised for a given diaphragm displacement, in two different flow regimes, i.e. the Helmholtz resonance regime and the viscous flow regime. The Lumped Element (LE) model is the most accurate model among the three. It is able to predict the jet velocity, which is in good agreement with CFD simulations, for both micro-scale and macro-scale actuators. An improved understanding in the effects of the dimensionless actuator operating parameters on the vortex rollup of synthetic jets has been achieved. The Stokes number S, the dimensionless stroke length L, and the Reynolds number based on the stroke length ReL, are important parameters affecting the vortex rollup. A criterion for the occurrence of vortex rollup in terms of the Stokes number has been developed. Finally, on the basis of the enhanced modelling capability and the improved understanding of the fluid mechanics of synthetic jet actuators, the first methodology of designing synthetic jet actuators for flow control at full-scale flight conditions has been developed. This methodology has been illustrated in the design of actuators on the leading and trailing edge devices of a multi-element high-lift system of a typical commercial aircraft wing at take-off conditions.
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