Investigation of the effect of wave age on the wind turbulence over breaking waves

Wind turbulence over breaking waves is investigated using direct numerical simulation (DNS) of two-fluids flows.  We focus on analyzing the effect of wave age on the statistics of wind turbulence over breaking waves.  It is found that before wave breaking, the vertical gradient of mean streamwise velocity is positive at small and intermediate wave ages, but it becomes negative near the wave surface at large wave ages. The turbulence kinetic energy (TKE) is enhanced during wave plunging at small and large wave ages, but at intermediate wave ages, the transient enhancement of TKE is absent.  The analyses of TKE production shows that at small wave ages, the TKE enhancement is caused by the magnitude increase of turbulence momentum flux. At large wave ages, the TKE enhancement is correlated to the sign change in the wave-coherent momentum flux. At intermediate wave ages, both of these two processes are absent, such that the TKE enhancement does not occur.