Ali, Reza (2003) Receptivity & transition in boundary-layers over rigid & compliant surfaces. PhD thesis, University of Warwick.

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Abstract

The generation of freestream turbulence by minute particulates and surface roughness over compliant walls in flat-plate, laminar and turbulent boundary layers is numerically investigated. The findings fill the gap in knowledge concerning the effect of such disturbances and are important for the development of compliant walls for marine applications.

The three-dimensional, boundary-layer disturbances, are modelled using a mixed spectral/ finite-difference, velocity-vorticity formulation of the Navier-Stokes equations. The compliant wall is modelled using a plate-spring model, where the displacement of the surface is restricted to the vertical direction. The fluid and wall codes are interactively coupled and numerical stability is achieved by combining the inertia terms together. The integrity of the code for rigid and compliant walls was demonstrated through the generation of Tollmien-Schlichting (T/S) waves. The particles were modelled as both stationary and moving body force(s), and roughness modelled spectrally as a wavy surface. The findings highlight favourable effects of wall-compliance. Both compliant walls, stretching across the entire domain, and a finite compliant panel embedded in a rigid wall were investigated. With regards to the latter, it was observed that, in general, the joins did not exert any additional adverse effects on the particle- and roughness-induced phenomena, and those that were induced could be easily controlled.

Klebanoff modes, which manifest themselves as streaky structures, were simulated in a laminar boundary layer, generated by a single stationary body force. The model was extended to show the development of similar near-wall structures in a turbulent boundary layer, providing for the first time quantitative agreement with experimental studies. The findings suggest compliant walls are less susceptible to bypass transition. The simulations also provide a possible explanation for observation of skin-friction drag reductions. A simple model describing the behaviour of moving particles, demonstrated local suppression of induced perturbations at the wall and revealed the faster-growing disturbance travelling with the particle.

Over rough surfaces, an oscillating body force was used as a T/S driver and the stability of the subsequent wave explored. In general, compliance has a stabilising effect, which is reduced as amplitude of the waviness is increased. The model was extended to ascertain the hydrodynamic effects of the minute cutaneous ridges observed over dolphin skin.

Item Type:	Thesis (PhD)
Subjects:	Q Science > QA Mathematics T Technology > TA Engineering (General). Civil engineering (General)
Library of Congress Subject Headings (LCSH):	Fluid mechanics, Turbulence, Boundary layer, Boundary layer control, Surfaces
Official Date:	October 2003
Dates:	Date Event October 2003 Submitted
Institution:	University of Warwick
Theses Department:	Department of Engineering
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Carpenter, P. W. (Peter William)
Extent:	xiv, 224 leaves : illustrations, charts
Language:	eng

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