The role of wave-induced pressure fluctuations in the transfer processes across an air–water interface

Abstract

The structure of the pressure and velocity fields in the air above mechanically generated water waves was investigated in order to evaluate their contribution to the transfer of momentum and energy from wind to water waves. The measurements were taken in a transformed Eulerian wave-following frame of reference, in a wind-wave research facility at Stanford University.The organized component of the fluctuating static pressure at the channel roof was found to contain contributions from both the sound field and the reflected water wave. The acoustic contributions were accounted for by appropriately correcting the pressure amplitude and phase (relative to the wave) and its contribution to the momentum and energy exchange. The wave-induced pressure coefficient at the fundamental mode shows in general an exponential decay behaviour with height, but the rate of decay is different from that predicted by potential-flow theory. The wave-induced pressure phase relative to the wave remains fairly constant throughout the boundary layer, except when the ratio of the wave speed to the freestream velocity, c/Uδ0 = 0.78 and 0.68. This phase difference was found to be about 130° during active wave generation, with the pressure lagging the wave. The momentum and energy transfer rates supported by the waves were found to be dominated by the wave-induced pressure, but the transfer of the corresponding total quantities to both waves and currents may or may not be so dominated, depending on the ratio c/Uδ0. The direct contribution of the wave-induced Reynolds stresses to the transfer processes is negligible.

Experimental study of the initial stages of wind waves' spatial evolution

“…Presenting the non-dimensional temporal growth rates of wave energy as a function of the reversed wave age u * /c set the style for data comparison that was widely used in later works. Hsu & Hsu (1983) and Papadimitrakis, Hsu & Street (1986) collected experimental data on the structure of the wave-induced velocity and pressure fields in the airflow over mechanically generated waves. Investigation of the wave-induced vertical profile of the air velocity and pressure was performed by Hare et al (1997).…”

“…Among the most prominent is the possibility to create stable and steady airflow at a wide range of air velocities. Positioning the sensors at accurate vertical locations is possible both in respect to the mean water surface and to the instantaneous surface elevation using wave followers (Shemdin & Hsu 1967;Papadimitrakis et al 1986;Mastenbroek et al 1996;Donelan et al 1999Donelan et al , 2005Donelan et al , 2006. To attain reasonably high airflow velocities matching the open sea values, relatively large blowers may be needed.…”

Airside velocity measurements over the wind-sheared water surface using particle image velocimetry

“…Kawamura and Toba (1988) reported that the wave follower could not perfectly follow waves especially at the forward face of the wave crest. Papadimitrakis et al (1986) observed that the wave follower generates acoustic waves that travel in both upstream and downstream directions, which contaminate the measured data. They concluded that neglecting the acoustic contamination leads to errors in the momentum and energy exchange between 18% and 32%.…”

Near-surface flow structure over wind-generated water waves, part I: wave-induced flow characteristics

“…They argued that in the near-surface region, the wave-induced Reynolds stress is produced from the stretching and vertical oscillation of the turbulent velocity with the waves. Kawamura and Toba (1988) and Papadimitrakis et al (1986) are the other examples of the airside velocity measurements. They performed velocity measurements using hot-wire anemometer, mounted over the wavefollower system in the laboratory.…”

“…Kawamura and Toba (1988) reported that the wave follower could not perfectly follow waves especially at the forward face of the wave crest. Papadimitrakis et al (1986) observed that the wave follower generates acoustic waves that travel in both upstream and downstream directions, which contaminate the measured data. They concluded that neglecting the acoustic contamination could cause 18-32% errors in the momentum and energy exchange.…”