Scaling of spacing between surface streaming on non-breaking and breaking wind waves

The high-speed, wind-aligned streaks on the wind waves are geometrically similar to the low-speed streaks observed in the turbulent wall layer. It is generally accepted that the spanwise spacing between the low-speed streaks in wall-bounded turbulent flow, when scaled by the viscous length, exhibit probability distribution conforming to lognormal behavior with a universal mean value of 100 independent on the wall friction velocity. Analyses of thermal images from wind-wave flume experiments, however, reveal that the scaling between the mean streak spacing and the surface friction velocity is different from that of wall-bounded flow. For non-breaking waves, the scaled mean streak spacing becomes notably narrower than that between low-speed streaks next to the solid wall. Comparative numerical simulations reveal that the presence of surface waves intensifies the generation of quasi-streamwise vortices that form the elongated streaks, and reduces the streak spacings. For breaking wind waves, analyses of the consecutive image sequences reveal that the breakers wipe out the existing surface streaks. After the passage of the breakers, the wind-aligned streaks reform immediately, which are then destructed again by the next breaking waves. In contrast to the streaks on the non-breaking waves, the scaled mean streak spacing in the wake of breakers is close to the canonical value of 100, which approximately follows the wall-flow scaling.