Direct Numerical Simulation of Turbulence Structure at a Wavy-Sheared Air-water Interface

Abstract

Turbulence structure near a wavy-sheared air-water interface was numerically investigated using a direct numerical simulation (DNS). The unsteady three-dimensional Navier-Stokes equations were numerically solved on both air and water sides for a wind-driven turbulent flow with a Reynolds number Re=6700, based on the maximum wave height and uniform velocity on the air side. Two boundary conditions [1] were used to determine the instantaneous shape of air-water interface. One was the dynamical boundary condition determined from the balance of the normal and tangential stresses acting on the wavy air-water interface, and the other was the kinematic boundary condition that describes the Lagrangian behavior of the air-water interface. The results show that the organized motion intermittently appears on the front side of the wavy interface in the air flow. The organized motion in the air flow gives high shear and pressure on the interface. The high shear and pressure acting on the wavy air-water interface induce the organized surface-renewal motion in the water flow beneath the air-water interface, which controls heat and mass transfer across the interface. The predictions well explain the previous measurements in the wind wave tank [2].