Wind-induced growth of water waves

Spatial growth of mechanically generated water waves under the action of wind has been measured in a laboratory wind-wave flume both for pure water and for water containing a surfactant (sodium lauryl sulphate, concentration 2.6 × 10 −2 %). I n the latter case, no wind waves develop on the surface of the mechanically generated waves as well as on the still water surface for wind speeds up to U 10 ≈ 15 m/s, where U 10 is the wind velocity at the height Z = 10 m. Therefore we can study the wind-induced growth of monochromatic waves without the effects of co-existing short wind waves. The mechanically generated waves grew exponentially under the action of the wind, with fetch in both cases. The measured growth rate β for the pure water can be fitted by β/ f = 0.34( U * /C) 2 0.1 [lsime ] U * /C [lsime ] 1.0, where f is the frequency of the waves, C is the corresponding phase velocity, and U , is the friction velocity obtained from vertical wind profiles. The effect of the wave steepness H / L on the dimensionless growth rate β/ f is not clear, but seems to be small. For water containing the surfactant, the measured growth rate is smaller than that for pure water, but the friction velocity of the wind is also small, and the above relation between β/ f and U * /C holds approximately if the measured friction velocity U * is used for the relation.