Abstract
In the present work, an investigation on the hydrodynamics of wave-current orthogonal combined flow has been carried out. The work focuses on the effects of the oscillatory flow superposed on the current steady boundary layer, and on how the oscillatory flow affects the current velocity distribution. A laboratory experimental campaign of wave-current orthogonal interaction has been carried out in a shallow water basin at DHI Water and Environment (Horsholm, Denmark), in order to investigate the orthogonal combined flow in the presence of different roughness beds. Mean flow has been investigated by computing time- and space-averaged velocity profiles. Friction velocity and equivalent roughness have been inferred from the velocity profiles by best fit technique, in order to quantify the shear stress experienced by the current mean flow. Tests in the presence of only current, only waves and combined flow have been performed. Instantaneous velocities have been Reynolds-averaged in order to obtain turbulent fluctuations time series and compute turbulence related quantities, such as Reynolds stresses. The mean current velocity profiles have been also compared with a selection of analytical models in order to assess their validity for the case of wave-current orthogonal flow for the considered wave and current condition ranges. The analysis of the mean flow revealed a complex interaction of the waves and currents combined flow. Depending on the relative strength of the current with respect to the waves, the superposition of the oscillatory flow may determine an increase or a decrease of the bottom friction experienced by the current. Such a behavior is also strictly related to the bed physical roughness. Analysis of the turbulence Reynolds stresses seems to confirm the results of the mean flow investigation.Recorded Presentation from the vICCE (YouTube Link): https://youtu.be/GbtOgeLlVTU
Highlights
Waves and currents are usually simultaneously present in coastal waters
The work has been focused on the effects of waves on the current boundary layer, and on how the oscillatory flow affects the current velocity distribution
Mean flow has been investigated by computing double-averaged velocity profiles, by means of timeand space-averaging of the velocity time series for the wings dataset, and time-averaged profiles for the acclive dataset
Summary
Waves and currents are usually simultaneously present in coastal waters. Their combined flow plays a fundamental role in several coastal processes such as sediment transport, mixing processes, diffusion etc. The two flows feature very different time and length scales, resulting in a thin wave boundary layer close to the bed, being embedded in a larger, steady current boundary layer. The presence of the wave boundary layer has been found to significantly affect the bottom flow, determining the current to experience an ’apparent’ roughness increase (Grant and Madsen, 1979), which affects its velocity vertical distribution. Wave-current hydrodynamics is furtherly complicated by the interaction with the sea bed, which can be fixed or movable and feature the presence of time-evolving bedforms (e.g. ripples). In the last decades several studies contributed to the current knowledge of the wave-current interaction hydrodynamics, which includes laboratory, field and numerical investigations
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