Experimental investigation of the effect of coherent secondary structures upon a tip vortex

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A vortex occurs when some of the fluid within a larger volume is caused to rotate, and is one of the most important phenomena in fluid mechanics. Vortices may range in size from microns (such as those formed around the beating wings of a mosquito) to hundreds of kilometers (such as hurricanes), and are of great importance in a large number of engineering applications. Vortex flows are of particular interest in the aircraft industry, since part of an aircraft's drag is the result of the formation of large vortices by the wing tips. As the engines propel the aircraft forward, part of the engine power is used to no other benefit than stirring the air behind the aircraft. If the wing tip vortex could be reduced in strength, the aircraft drag would decrease, reducing the amount of fuel burned. This would lead to both decreased carbon emissions and operating costs. The large vortices in the aircraft wake also pose a danger to other aircraft, so a minimum distance must be kept between them. Since the vortices are strongest at low speed such as during takeoff and landing, this wake hazard is what limits runway capacity. If the vortex could be destabilized and forced to break up and dissipate more quickly, airports would be able to accommodate a larger number of flights without needing additional runways. The strength and stability of a wing-tip vortex can be greatly reduced by introducing turbulence (or random disturbances) into the flow ahead of the wing. These disturbances interact with the vortex, transport energy away from it and reduce its strength. However, in wind tunnel tests, the disturbances are usually introduced by placing a series of heavy bars ahead of the wing; on a real aircraft, this would not be possible. Instead, in this study, small and carefully designed 'bumps' will be strategically placed on the surface of the wing in order to generate similar disturbances.Since there are infinitely many possible combinations of bump geometries and locations, it is first necessary to study the vortex and how it is affected by smaller disturbances. To begin with, despite the engineering importance of these flows, it still isn't clear whether or not there are naturally-occurring disturbances inside a vortex. Also, vortices are in many ways analogous to the flow over flat walls. Though similar disturbances occur naturally in wall flows and play a vital role in their development, very little attention has been given to the role played by the disturbances in vortex flows. If it can be shown that very small disturbances can have an effect on large vortices, this in itself would be an extremely important result: many flow simulation computer codes assume that the direct interaction between very small disturbances and very large vortices is impossible. Finally, all vortices- regardless of how they were generated, how strong they are or how 'disturbed' they may be- appear to evolve in exactly the same way. While this similarity has already been noticed, it is still not clear why it happens. Once the vortex and the way it responds to disturbances is better understood, this understanding will be used to intelligently develop a wing surface which can reduce aircraft drag (cutting down both cost and carbon emissions) and increase airport capacity.