Characteristics Of Streaky Thermal Footprints On Wind Waves

Coherent vortices in the aqueous surface layer beneath wind waves manifest themselves by inducing elongated high-speed streaks on the interface. Analyses of thermal images taken in the wind-wave flume reveal that the mean streak spacing scaled by the viscous length, lambda+ over bar , depends strongly on the wind-wave condition. This contradicts the wall-bounded turbulent flow in which the scaled mean streak spacing approaches a canonical value, lambda+ over bar approximate to 100. Comparative numerical simulations of shear flow bounded by flat and wavy surfaces are conducted to explain the variation. In the low-wind range with insignificant surface waves, lambda+ over bar <100; the reduction of lambda+ over bar is attributed to the insufficient shear rate to form the elongated streaks. For the moderate-wind range in which surface waves become pronounced but remain to be unbroken, lambda+ over bar is still less than 100. Analysis of the vorticity transport in the simulated wavy flows reveals that the presence of non-breaking waves enhances the formation of quasi-streamwise vortices and the surface streaks. For the high-wind range, surface waves break and the breakers wipe out the streaks. The streaks quickly reestablish in the wake of the breaker with mean spacing lambda+ over bar approximate to 100 but are destructed again by the following breaker.