Identification and modelling of coherent structures in trailing vortex break-up

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Trailing vortices are an inherent feature of aircraft flight and are primarily responsible for the swirling air motions observed in an aircraft wake. They present an in-flight hazard to aircraft and therefore place constraints on flight paths and the timing of take-off and landings at airports. Trailing vortices also have a strong influence on the dispersion of jet engine exhaust pollutants and pesticides in aerial spray applications. There is a substantial ongoing research effort to predict and, if possible, control the development of trailing vortices. Generally the objective is to limit vortex longevity by exciting the inherent modes of instability that exist within them. There has been extensive theoretical study of these instabilities but relatively few attempts to directly test for their presence in the laboratory. In spite of this fact many devices have been proposed to limit vortex longevity and some have been moderately successful. One device that has demonstrated the ability to reduce the rate of swirl velocity and increase the rate of vortex diffusion is spanwise directed air jets mounted on the wing tips. The present research proposal is to conduct laboratory testing on a trailing vortex formed with tip jets and identify the modes of vortex instability excited and the subsequent coherent structures of vortex break-up that they produce. A comparison will then be made between the findings of this study and the existing theories of vortex instability with the expectation that the phenomenon can be better understood. In particular it may be possible to reduce the energy required to achieve vortex break-up if the disturbance is more precisely designed. The experimental methods used will include flow imaging and advanced pattern recognition techniques. Once identified attempts will also be made to model the effect of these "coherent structures" on vortex development. Ultimately an improved understanding of vortex dynamics will lead to more effective vortex control.