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Title: Transition to turbulence in cylinder wakes
Author: Williams, Patrick Richard Giles
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2007
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Abstract:
The laminar to turbulent transition of a circular cylinder wake was investigated experimentally, examining a range of features leading to an aperiodic three-dimensional structure and a possible mechanism causing the route to turbulence was postulated. Following the analysis of the Stuart-Landau equations defining a Hopf bifurcation at the primary instability, a new relationship between the Strouhal number (St) and the Reynolds number (Re) was formed, providing agreement with a range of equations defined experimentally. The St-Re relationship was examined for a range of mechanisms modifying the cylinder's end termination and a variation in the free-stream turbulence level. It was ascertained that the end termination had a direct influence on the wake dynamics, producing a change in the wake structure leading to the possibility of parallel vortex shedding and also a modification of the critical Reynolds number. Following the introduction of specific boundary conditions, it was ascertained that the critical Reynolds number could be delayed to a value of 192. Although an increased level of free-stream turbulence demonstrated a reduction in the critical Reynolds number for all end conditions imposed. Within a small range of Reynolds numbers prior to transition, random wake irregularities \vere identified within the laminar velocity signal. These were ascertained to be generated from finite three-dimensional perturbations within the wake, introduced by the method of end termination and the increased level of free-stream turbulence. Following the analysis of these intermittent aperiodic bursts within the transition regime, they were demonstrated to relate to the occurrence of turbulent flow prior to transition. This produced an alternate switching between the two states relating to the hysteretic bifurcation to the mode A wake structure. A possible transition mechanism causing a route to turbulence was considered, relating to the statistics of these intermittent bursts. Leading to a proposal that the cylinder wake transition undergoes a noise induced dynamic forcing, which is consistent with on-off intermittency.
Supervisor: Not available Sponsor: Not available
Qualification Name: University of Warwick, 2007 Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.487916  DOI: Not available
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