Experimental Investigation Of Turbulent Oscillatory Boundary Layers

Author : Jing Yuan (Ph. D.)
Publisher :
ISBN 13 :
Total Pages : 244 pages
Book Rating : 4.:/5 (872 download)

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Download or read book Experimental and Theoretical Study of Turbulent Oscillatory Boundary Layers written by Jing Yuan (Ph. D.) and published by . This book was released on 2013 with total page 244 pages. Available in PDF, EPUB and Kindle. Book excerpt: Sediment transport is of crucial importance to engineering projects in coastal regions, so it is of primary interest in coastal engineering. The driving forces for sediment transport are mostly determined by the hydrodynamics of oscillatory turbulent bottom boundary layers, which is still not well understood. Therefore, the goal of this thesis is to improve the present experimental and theoretical understandings on this subject. A high-quality experimental study including a large number of tests which correspond to full-scale coastal boundary layer flows is performed using a state-of-the-art oscillating water tunnel (OWT) for flow generations and a Particle Image Velocimetry system for velocity measurements. The experimental results suggest that the logarithmic profile can accurately represent the boundary layer flows in the very near-bottom region, so the log-profile fitting analysis can give highly accurate determinations of the hydrodynamic roughness, the theoretical bottom location and the bottom shear stress. The current velocity profiles in the presence of sinusoidal waves indicate a two-log-profile structure suggested by the widely-used Grant-Madsen model. However, for weak currents in the presence of nonlinear waves, the two-log-profile structure is contaminated or even totally obliterated by the boundary layer streaming which is related with the temporal variation of the turbulent eddy viscosity. This, together with some other experimental evidence, motivates the development of a new theoretical model which adopts a rigorous way to account for a time-varying turbulent eddy viscosity. The model accurately predicts the mean and leading Fourier components of the velocity and the bottom shear stress for various flow conditions. Most importantly, the boundary layer streaming related to the time-varying turbulent eddy viscosity is reasonably predicted, which leads to successful predictions of the mean velocity embedded in nonlinear-wave tests and the current velocity profiles in the presence of either sinusoidal or nonlinear waves. The predictions reveal significant differences between boundary layer flows in OWTs and in the coastal environment, which must be considered when interpreting OWT results for sediment transport.