Experimental investigation of time-invariant eddy viscosity in wave-current interaction

An experimental study of the turbulent boundary layer, where waves propagate with a current, is presented in this paper. A wide range of test conditions have been covered, namely, flows over a rough bed and a smooth bed, combined flows in a large-scale oscillatory water tunnel and combined waves and currents in two flumes with different scales. Particle Image Velocimetry and Laser Doppler Velocimetry were employed to obtain the velocity field. Detailed analysis of eddy viscosity profiles calculated from the experiments leads to the conclusion that previous assumed profiles do not always accurately describe eddy viscosity distributions in a combined wave current flow. The distributions of eddy viscosity are categorised into two types (two-layer or three-layer), based on the influence of wave motions superimposed. For those cases in the current-dominated regime, eddy viscosity profiles are similar to unidirectional turbulent currents. When combined flows are in the wave dominated regime, three-layer eddy viscosity distributions are observed. For both types, a linear eddy viscosity profile is found to be present in the bottom 10 per cent of the turbulent boundary layer. Above this, the classic parabolic profile is observed, over the whole turbulent boundary layer for the first type and over 40 per cent of the turbulent boundary layer thickness for the second type. An empirical eddy viscosity distribution in the outer region is proposed for the second type. This newly developed eddy viscosity distribution provides guidance for numerical modellers in the field of wave-current interaction and for coastal engineers wishing to predict sediment transport.