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Title: Instability and transition of boundary layer flows disturbed by steps and bumps
Author: Wang, Yaxing.
Awarding Body: Queen Mary, University of London
Current Institution: Queen Mary, University of London
Date of Award: 2004
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An investigation of the influence of various wall perturbations on the development of instability waves has been carried out through a series of experiments to study different aspects of the stability/transition problem. The first experiment was devoted to an investigation of the effects of an isolated step on the transition of the boundary layer over a flat plate. Steps of various heights, both positive and negative, produced an upstream movement of the transition point, which was interpreted as arising from an increase in the amplification of the instability waves. The transition Reynolds number and the resulting change in the so-called 'N-factor', used for transition prediction, has been correlated with the step height to provide a useful empirical engineering design tool. The second experiment concerned the effect of a two-dimensional (2-D) sharpedged short bump on instability waves (Tollmien-Schlichting (TS) waves). The experiment was designed to study the mechanisms by which a sharp-edged surface roughness element, in the form of a 2-D bump, modified the evolution of TS wave and hence affected the process of transition. This investigation demonstrated that the interaction of a 2-D TS wave with a 2-D bump was a many-sided complex problem. The bump played multiple roles as a disturbance generator, an amplifier and a promoter of three-dimensionality. The crucial parameter was the height. Nonlinearity was the main factor that caused the enhancement of boundary layer transition and manifested itself in the appearance of higher harmonics in the spectra. In the last experiment the bump was three-dimensional (3-D) and in the form of a cylinder. The experimental results revealed that the effect of a 3-D bump was confined to a wedge-shaped region downstream of the bump. The mean velocity distortion had a distinct spanwise distribution from which a pair of streamwise vortex structures could be inferred. The interaction of a 2-D TS wave with a 3-D bump generated oblique waves with a dominant band of spanwise wavenumbers as predicted by theoretical analysis. Three thresholds were found that defined bump height ranges where there was no-effect, a linear behaviour and a nonlinear one.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available