Zhang, Xuan; (2018) Structure of turbulence in combined wave-current boundary layers. Doctoral thesis (Ph.D), UCL (University College London).

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Text PhD Thesis Xuan Zhang Final Version 2018.09.24.pdf - Accepted Version Download (30MB) | Preview

Abstract

This thesis is based on an experimental programme conducted in two laboratory flumes with different scales, to examine the near bed boundary layer under combined surface waves and a turbulent current. Flow conditions have covered a range of wave frequencies and amplitudes, and current conditions. The results give an enhanced understanding of turbulence structures close to the bed in a combined wave-current flow, in terms of coherent vortices and eddy viscosities. A two-dimensional Particle Image Velocimetry system was used to measure the velocity field. Average properties of turbulence characteristics were determined. Hydrogen bubble techniques were employed to provide flow visualisation of the near-wall flow patterns. Results for the unidirectional current are consistent with many previous studies on the existence of coherent vortices. In particular, the measured flow velocity vectors and contours of magnitude demonstrate the presence of low-speed and high-speed streaks in the wall region and bursts in the outer flow. Further quantitative analysis also shows that the characteristic length scales agree well with previous research. For tests with combined wave-current flow, results demonstrate that streamwise vortices are also present. However, the spacing between adjacent low-speed streaks changes periodically within one wave cycle. In addition, measurements also reveal that the maximum Reynolds shear stress induced by ejections is increased when waves are superimposed on the turbulent current. The increase is observed to be more significant for waves with higher frequencies and amplitudes. In order to relate turbulent behaviour with time-averaged velocities in a combined wave-current boundary layer, an eddy viscosity distribution was computed based on experimental results from the flumes and a large oscillating water tunnel from previous tests. These results show a different distribution from those in existing models of wave-current interaction. These results are expected to improve the accuracy on prediction of the hydrodynamics and sediment transport in combined wave-current flows.

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