We use large-eddy simulation (LES) to perform a mechanistic study on the momentum transfer between turbulent flows and a traveling wave boundary. Several previous field observations showed that this momentum flux varies with the different wave boundary conditions related to the shoaling effect. In this study, we conduct wall-resolved LES to investigate the canonical problem of turbulence over streamwise traveling wave surfaces with the shoaling effect incorporated. Our result shows that shoaling induces a noticeable difference in the mean profile and the velocity fluctuations of the turbulence field. An analysis on the momentum budget shows that, in the wave boundary layer, the wave-induced momentum flux terms become much higher in the turbulence field as shoaling occurs. By comparison with a third computational case which is used as a control case, we find that the shoaling-induced wave age reduction contributes the most to the enhanced form drag, through the changes in the wave-coherent surface pressure asymmetry and amplitude, while the increased wave steepness also leads to the form drag increase but plays a secondary role. Our result identifies the roles of the wave age and steepness in the shoaling-induced momentum flux increase, which is valuable for the future development of parametrizations. We also observe quasiperiodic large-scale motions, which indicate the correlation between the friction generation and the turbulence self-sustaining process.
Bibliographical noteFunding Information:
This work is supported by the Office of Naval Research through Award No. N00014-20-1-2103. The authors gratefully acknowledge the anonymous referees for their valuable comments.
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