Experimental investigation of turbulent wave boundary layers under irregular coastal waves

In this study full-scale experiments of wave boundary layers under irregular coastal waves are conducted using an oscillatory water tunnel. The flow conditions cover two rough bottoms, three types of wave shapes, i.e. sinusoidal, skewed and asymmetric waves, and two types of irregular-wave sequences. The instantaneous turbulent velocity fields are measured with a 2-dimensional Particle Image Velocimetry system. The measured turbulence statistical values show that the residual turbulence at the end of wave cycle can persist into the next wave cycle, until the next cycle's self-produced turbulence becomes sufficiently strong. Consequently, the Reynolds-averaged flow at the beginning of a wave cycle can behave as if the flow "memorizes" the previous wave cycle. However, this memory effect quickly vanishes, and therefore does not have a significant influence on some key boundary layer characteristics, e.g. bottom shear stress. For irregular wave boundary layers with skewed and asymmetric free stream velocities, the measured mean current and the associated mean bottom shear stress confirm the existence of a well-known boundary layer streaming due to the imbalance of turbulence between the two halves of a wave cycle, and the measurements of bottom shear stress of individual waves closely resemble those for periodic wave conditions. These experimental results suggest that modeling irregular wave boundary layers in a wave-by wave manner is plausible.