Laboratory experiment on the impact of wind on water turbulence in shallow areas

Abstract

A series of laboratory experiments focused on the wind impact on the vertical turbulence structure in shallow water has been carried out. The turbulence characteristics in the mid-lower water column under relatively strong wave conditions are investigated. For different experimental conditions (i.e., waves only, wind only, combinations of wind and waves) in a wind-wave flume, the effects of wind and waves were investigated in detail by decomposing the total energy into different terms (i.e., wind-driven currents, wind waves, wind-induced turbulence). The results show that shallow water waves play a major role in transferring energy by non-zero wave-induced Reynolds stress ($\overline{\tilde{u}\tilde{w}}$), turbulent diffusion ($\overline{u^{\prime }{w}^{\prime }}$). The superimposed wind can further modify the energy transference due to its impact on wave asymmetry and skewness as well as through homogenizing the time-average velocity profiles. Subsequently, the impact of wind on turbulence structure was explored in detail. The most important finding is that the wind can directly influence water turbulent diffusion ($\overline{u^{\prime }{w}^{\prime }}$) along with wave-induced turbulence. The vertical turbulence intensity (σ_w) is more sensitive to wind than the horizontal turbulence intensity (σ_u). Furthermore, the major way that wind affects water turbulence is by introducing nonlinear wind and wave interactions, which exhibit a maximum effect (∼60 % compared to the respective effect of wind and wave) at the edge of the bottom boundary layer. This study demonstrates that the wind can transfer momentum downward to mid-lower water columns even under strong waves in shallow waters, which differs from that in deep water systems.