Forewords
The main research themes I have worked on are the topics considered in the PhD I have achieved in the School of Civil Engineering and the Environment in the Hydraulics Research Group, at the University of Southampton. This project has been conducted under the supervision of professor John R. Chaplin.
PhD Subject
Oscillatory wave-induced boundary layer flow over a rippled bed.
Abstract
The aim of this project is to study the wave-induced flow above a rippled bed. Seabed ripples do not have a major impact on the main flow pattern, but they strongly influence the structure of the boundary layer. Therefore they are of interest in coastal engineering as they are part of the sediment transport process and have influence on surface wave dissipation and flow friction in coastal areas. By both numerical and experimental means, the flow characteristics were studied. A two-dimensional computer model solving this particular flow was developed and compared to other numerical and experimental data and the available theory. Experiments were conducted in a wave tank fitted with a rigid rippled bed, and flow visualizations were done using fluorescent dye filmed by a digital video camera. Different flow regimes were identified and were classified in terms of relevant parameters such as the ripple slope, the wave orbital amplitude to ripple wavelength ratio and the Taylor number. For a weak flow oscillation to ripple wavelength ratio, two-dimensional structures tend to develop in the form of recirculation cells. For a stronger flow oscillation and a medium to steep ripple slope, the flow separates giving rise to vortices ejected from the ripple crests every half-wave period. Three-dimensional instabilities also appear in two different forms. The most common form observed is a structure of "rings". The other form called brick-pattern is suspected to involve centrifugal instabilities and play a major role in the building of three-dimensional ripple shapes. A final summary of the flow characteristics was done, using the numerical and experimental results.
Keywords
Rippled bed, oscillatory flow, three-dimensional instabilities, vortex, flow visualizations.
Publications
(2005) Yann
Ourmières and D. Mouazé,
Wave-induced boundary layer flows
over a flat and rippled bed.
Journal of Hydraulics Research (submitted).
(2004) Yann Ourmières and John
R. Chaplin,
Visualizations of the disturbed laminar wave-induced flow
above a rippled bed.
Experiments in Fluids, Vol 36, p 908-918.
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(2003) Yann Ourmières and John R. Chaplin, Visualizations of wave-induced flow over ripples, Euromech colloquium n°451 Sea Wave Bottom boundary layer, Taormina, Italy.
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(2002) Dominique Mouazé, Yann Ourmières and John R. Chaplin, Steady and unsteady flow in wave induced boundary layers, Proc. 17th Int. Workshop on Water Waves and Floating Bodies, Cambridge, UK, p139-142.
Summary:This paper describes measurements of velocities inside the seabed boundary layer in conditions where the boundary is slightly rough. Agreement with linear and non-linear theory is found to deteriorate as the higher harmonic components of the waves become more important. In all cases the steady streaming at the edge of the boundary layer is considerably weaker than predicted.