Evidence and Mechanism of Turbulence Production by Non-Breaking Surface Waves

Abstract:

Numerical simulation of monochromatic, finite-amplitude surface waves propagating over an initial quiescent flow field is conducted to reveal the potential impact of non-breaking waves on turbulence production. The numerical model solves the primitive equations subject to the fully nonlinear boundary conditions on the exact water surface. The simulation result predicts growth rates of turbulent kinetic energy consistent with previous measurements and modeling. It also validates the observed horizontal anisotropy of the near-surface turbulence that the spanwise turbulent intensity exceeds the streamwise component. Such a flow structure is found to be attributed to the formation of an array of streamwise vortices with alternating orientations near the water surface, which also induce elongated surface streaks. These characteristic flow structures suggest mechanism akin to that induces Langmuir turbulence. The proposition is confirmed by stability analysis employing Craik-Leibovich phase-averaged equation: The spanwise wavelength of the most unstable disturbance is found to be close to the average spacing between adjacent counter-rotating vortex pairs.