The response of waves to an opposing wind

Abstract

The results of a laboratory study of the response of waves to an opposing wind are presented. The study was conducted in a special purpose wind-wave flume which allowed mechanically generated waves to propagate in opposition to wind. The mechanically generated waves were created by a wedge wave maker for which an extensive set of measurements were performed to determine its transfer function. A wave follower which could hold pressure and velocity probes a fixed distance above the moving water surface was designed and constructed. Extensive evaluation experiments indicated that the wave follower tracked the water surface with little error in the frequency range of interest. The transfer functions of all intermediate devices in the pressure and velocity recording systems were determined and subsequently used to correct all pressure and velocity records. Measurements of the wave induced pressure at various heights above the waves-indicated that the pressure and water surface were 180º out of phase, consistent with the predictions of potential flow theory. Such a result indicates that there is no air-water energy flux due to normal stresses. Measurements of the velocity field above the waves indicated that the Reynolds stress term -p ǖǖ can cause waves to decay in an opposing wind. The magnitude of the decay is proportional to the wave slope squared and the term (1 - U ∞/C) squared. Hence, high frequency waves will decay far more rapidly than low frequency waves. Typical results indicate that low frequency ocean swell is almost uneffected by opposing winds whereas high frequency wind waves will be attenuated very rapidly.