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Title: An experimental study of the development and growth rate of short-wave instabilities on a vortex dipole
Author: Nicholas, Patrick Taylor
Awarding Body: Imperial College London (University of London)
Current Institution: Imperial College London
Date of Award: 2007
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Abstract:
Aircraft wakes ar~ dominated by a pair of strong counter-rotating vortices that give rise to the well known 'wake hazard' wherein an aircraft encountering the wake may be subjected to severe forces and moments. Eventually, naturally occurring instabilities grow and cause the breakdown of the wake vortices. The purpose of this current study is to gain a further understanding of instabilities which are supported by and develop in counter-rotating vortex pairs (vortex dipoles). Particularly, it is of importance to try to ascertain under which conditions such instabilities develop and grow and whether it is possible to promote their occurrence by means that could bOe practically implemented on an aircraft. Also ofinterest is whether there are conditions under which such instabilities may not occur. This research is mainly of an experimental nature in which the studies are conducted using water as a medium. A rig has been designed and built to generate a pair of counter rotating vortices in a water tank. The vortices are generated by rotating two flat plates submerged in the water. As the plates rotate, the fluid boundary layer rolls off of the plate edges and thus generates a vortex. The plates are controlled via a stepper motor and code written in LabView. Vortex strength, r, core radius, a, and separation distance between the vortex pair, h, can be controlled by rotating the plates at various angular velocities and though various angular displacements. The rig described above generates vortices with no axial velocity, besides those which occur due to end effects. In order to compare the behaviour of vortices with axial velocity to those without, experiments have also been conducted on a delta wing which was towed through a large (8m long) water tank. The vortices have been characterised by computing a velocity field from experimental data derived using the PlY technique. Subsequent to this, the vortex characterisation parameters, circulation, r, core radius, a, and separation distance, b, were extracted by means of a least squares fit to a Lamb-Oseen vortex profile. In order to observe instabilities which may occur, the colunmar vortex centre line was mapped by means of flow visualisation using hydrogen bubbles as markers. A new data processing algorithm has been developed to extract frequencies and amplitudes of the 'wave' like vortex centre line disturbance from the flow visualisation and has provided a new insight into this phenomenon. A full discussion of the accuracy of the techniques used is given in the thesis. The results presented are for vortices with Rer (where Rer = r/v) ranging between 2000 and 10000, core radius, a, ranging between 5mm and 15mm and the ratio of core radius to separation distance, alb, ranging between 0.15 and 0.3.
Supervisor: Not available Sponsor: Not available
Qualification Name: Imperial College London, 2007 Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.486613  DOI: Not available
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