Three-dimensional streaming in sea bed boundary layer
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- Institutt for marin teknikk 
The objective of the work carried out in the present thesis is to obtain new insights into wave- current seabed boundary layers by studying the wave-current interaction for the general case of a non-zero angle between the waves and the current using numerical modelling. The first part of this work focuses on the hydrodynamics, while the second part deals with the resulting sea bed sediment transport. The effect of three-dimensional wave-induced streaming on seabed boundary layers is investigated for following and opposing waves and current where the wave propagation forms a non-zero angle with the current. It is shown that the sea bed boundary layer flow results from an interaction between the classical wave-current interaction (reducing the mean velocity relative to current alone), Longuet-Higgins streaming (forcing the flow in the wave propagation direction) and streaming caused by turbulence asymmetry in successive wave half-cycles (forcing the flow against the wave propagation direction). For waves and current which are not colinear, the mean velocity profile exhibits a veering behaviour which is strongly affected by streaming, particularly for the most wave-dominated situations. The effect of streaming on the boundary layer flow has been investigated for different wave-current conditions and bottom roughnesses. Visualisations are given by mean Eulerian and Lagrangian velocity profiles, as well as three-dimensional seabed boundary layer particle trajectories. The effect of streaming decreases as the flow becomes more current-dominated. The mean velocity in the current direction decreases as the roughness increases. However, the mean velocity orthogonal to the current direction increases as the roughness increases due to the lack of wave-current interaction in this direction. An excellent agreement between the predicted and recently measured velocity profiles (Yuan and Madsen, 2015) beneath horizontally uniform asymmetric forcing is obtained. The effect of three-dimensional wave-induced streaming on the resulting seabed boundary layer sediment transport (i.e. bedload and suspended sediment transport) has been investigated for following and opposing waves and current where the wave propagation forms a nonzero angle with the current. The mean sediment transport results from an interaction between Longuet-Higgins streaming, asymmetric streaming and the current. It appears that the mean sediment transport decreases as the angle between the waves and the current increases and for a given angle, the sediment transport is largest for second order Stokes waves, followed by linear propagating waves, horizontally uniform Stokes forcing and horizontally uniform linear forcing. The mean sediment transport direction is rotated from the wave propagation direction towards the current and this rotation is largest for horizontally uniform linear forcing, followed by horizontally uniform asymmetric forcing, linear propagating waves and second order Stokes waves. The effect of wave asymmetry on the mean sediment transport has been investigated showing that the mean sediment transport increases as the wave asymmetry increases. An excellent agreement between predictions and existing measurements of the mean bedload transport beneath second order Stokes waves is obtained, while the mean suspended sediment concentration is well predicted near the bed and poorer predicted farther up in the water column. To authors knowledge, the numerical model developed in the current thesis is the first one- dimensional vertical (1DV) Reynolds-averaged Navier-Stokes (RANS) solver that accounts for wave-current interaction at an arbitrary angle between the waves and the current.