Characteristics of Streaky Thermal Footprints on Wind Waves
Journal
Journal of Geophysical Research: Oceans
Journal Volume
126
Journal Issue
7
Date Issued
2021
Author(s)
Abstract
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, (Formula presented.), 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, (Formula presented.). 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, (Formula presented.); the reduction of (Formula presented.) 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, (Formula presented.) 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 (Formula presented.) but are destructed again by the following breaker. ? 2021. American Geophysical Union. All Rights Reserved.
Subjects
image processing
infrared image
numerical simulation
turbulence
wind waves
flume experiment
numerical method
shear flow
simulation
surface layer
turbulent flow
vortex
wind wave
Type
journal article