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Title: Vortex formation in a two-dimensional periodic wake
Author: Wiese Nielsen, K. Th.
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 1970
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Flow visualization techniques have been used for quantitative determination of the distribution of vorticity in a two-dimensional incompressible periodic wake formed behind a bluff body at a Reynolds number of 1.5 x 104. Following the suggestion of Gerrard (1966) the vorticity distribution in the wake was considered as an ordered subdivision of the shed circulation into no more than three identifiable and measurable parts. This permitted a single algebraic summation of the circulation quantities involved. To trace the history of the transport of circulation in the formation region of the wake, a technique was developed which used small countable particles ejected from the point of separation into one of the shear layers. The number rate of ejection of particles was known and controlled accurately so that the particle density could be related to the vorticity in the shear layer. Having established that the particles remain implicitly with the rotational fluid, it is shown that the quantity of circulation transferred to any location is pro- portional to the number of particles carried into the same region. By application of flow visualization techniques developed for this specific purpose, the number of particles carried to any region of interest in the wake could be determined. By means of these measurements, all the terms in the proposed circulation distribution equation was evaluated independently and the equation shown to be satisfied. Not only do these results verify the Gerrard flow model; but also they provide the first known quantitative information concerning the actual distribution and history of the circulation within the formation region. In the case of the final vortex strength comparison is possible with the work of other investigators and here good agreement was found. As the particles are a vital tool in the investigation considerable efforts were devoted towards establishing the validity of their use as a flow marker in accelerating and shearing flows. This phase included also the solution of the mechanical problems of handling and measurement associated with the controlled ejection of particles into the shear layer. This work is described separately in Part II of the thesis. The third part of the thesis is devoted to a general wake inquiry in which the particle technique is employed to explore certain subsidiary aspects of the wake flow not covered in Part I. These topics included the determination of the size of the dead fluid region, measurements of the injection mode of circulation into the dead fluid region and also effects of the ambient turbulence upon the formation of region and residual vortex strength in the fully formed vortices.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available