Wind-induced Stresses on Water Surfaces: A Wind-tunnel Study

Abstract

SummaryA laboratory wind tunnel has been used to study the effect of wind on a water surface. The surface shearing stress 7'0 and the slope of the surface induced by wind have been measured. Values of the surface stress, in good agreement with each other, have been obtained from: (a) the velocity profile of the wind above the water surface, (b) the measured values of surface slope or set-up, and (c) the spreading characteristics of surface films. The drag coefficient, C h = TO/ pU~, was found to be constant for wind speeds up to about 412 cm/s and then to rise gradually for higher wind speeds.When the surface waves are damped out by the addition of small amounts of surface-active material to the water, the shearing effect of the wind on the surface is somewhat modified. If detergent solution is used for this purpose, there is a critical concentration at which conditions in the boundary layer near the water surface appear to undergo a marked change in character.

The suppression of wind-generated waves by a surface film

“…Our results may be compared to some extent with the experiments of Fitzgerald (1963). He measured the surface stress 70 for flow of air over water in a rectangular channel, with and without surface films.…”

“…The friction velocity is given by U i = U;Y, = rO/pa, where T,, is the steady stress on the surface. A conservative value of the velocity at the edge of the laminar sublayer (y = yl) is U, = 5U, and it is convenient to note that if U, is the wind velocity far from the interface, U, = UJc) so that with a typical value of 0.0025 for cf (Francis 1954;Fitzgerald 1963), U, + 20U,.…”

“…Wind tunnel experiments have been carried out by Keulegan (1951) and Fitzgerald (1963). Both of these authors used surface active agents for suppressing wave formation, to enable them to measure velocity profiles over smooth water surfaces.…”

Wave damping by a thin layer of viscous fluid

“…23 For applications such as distinguishing oil slicks from natural films by their radar backscatter characteristics, [17][18][19][20][21][22] it will also be necessary to consider in detail the atmospheric boundary layer processes leading to wave generation by wind forcing, and the nonlinear interaction processes between the different Fourier components of the surface-wave field. 7,8,[34][35][36][37] FIG. 1.…”