Experimental investigation on the structure of turbulence in the bottom wave-current boundary layers

This paper presents insights into the structure of turbulence in combined wave-current boundary layers, based on experiments performed in flumes of different scale using Particle Image Velocimetry and hydrogen bubble visualisation. Flow conditions covered a range of wave frequencies, wave amplitudes and mean flow conditions. Results show that the spacing between turbulent streaks varies periodically with the passage of each wave, increasing when the flow accelerates and decreasing when the flow decelerates. A new formula has been put forward, relating the streak spacing variation and the wave-induced orbital displacements. The near-wall flow structure suggests a rhythmic pattern in terms of the velocity gradients across the flume. Waves with higher frequencies and larger amplitudes lead to a greater reduction of mean streak spacing, together with a greater increase of the maximum Reynolds shear stress induced by ejections. These results can be useful for better predictions of the hydrodynamics and sediment transport in combined wave-current flows. •Low-speed streaks observed in unidirectional current boundary layers are also found in combined wave-current boundary layers.•The spacing between turbulent streaks varies with the passing of wave crest and trough.•Velocity gradients across the flume change periodically within one wave cycle.