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Title: An investigation of turbulent boundary layers with streamwise and spanwise pressure gradients
Author: Pearce, Nicholas F.
ISNI:       0000 0004 2749 3175
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2013
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An experimental comparison is made between two turbulent boundary layers produced in a low-speed water channel subjected to different pressure gradient distributions. Both flows involve identical favourable streamwise components, generated via a lateral contraction of the flow area; and in the second case, an additional spanwise gradient is imposed by curving the walls. The measurement system and methods are developed in full, with rigorous testing and validation allowing the uncertainty and accuracy of the results to be estimated. Hot-wire anemometry is employed to take measurements of velocity using miniature single-film probes. A Hydrogen bubble visualisation system enables an inspection of the coherent turbulent structures in the boundary layer near the wall. The mean-velocity measurements show a continued rise in the Reynolds number downstream accompanied by a fall in the coefficient of friction, in spite of a relatively high streamwise acceleration. This unorthodox behaviour was found to occur for both flows. In response to the acceleration, changes in the statistical moments of streamwise velocity show an increased dominance of high velocity fluctuations near the wall. This corresponds with the results of the structure visualisations which reveal a rise in the mean spanwise spacing of the low-speed fluid elements. The pressure gradients of the two cases are generated using a novel approach which aims to make the effects from each strain easier to evaluate. The additional spanwise component in the second case induced a cross ow in the boundary layer which reached 11% that of the local external velocity. Despite this, the measurements and low-speed streaks show the turbulence to be relatively insensitive to this level of three-dimensionality. A simple numerical method is presented to model the development of the low-speed fluid streaks in different mean-velocity distributions. Using this model, greater cross flow magnitudes in the boundary layer are tested for which a positive streak dependence is found.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: TA Engineering (General). Civil engineering (General)