Only limited information is currently available on the evolution of waves generated by wind that varies in time, and in particular on the initial stages of wind-wave growth from rest under a suddenly applied wind forcing. The emerging wind-wave field varies in time as well as in space. Detailed knowledge of wave parameter distributions under those conditions contributes to a better understanding of the mechanisms of wind wave generation. In the present study, the instantaneous surface elevation and two components of the instantaneous surface slope were recorded at various fetches in a small-scale experimental facility under nearly impulsive wind forcing. Numerous independent realizations have been recorded for each selection of operational conditions. Sufficient data at a number of fetches were accumulated to calculate reliable ensemble-averaged statistical parameters of the evolving random wind-wave field as a function of the time elapsed from activation of wind forcing. Distinct stages in the wave evolution process from appearance of initial ripples to emergence of a quasi-steady wind-wave field were identified. The experimental results during each stage of evolution were analysed in view of the viscous instability theory by Kawai (J.

[1] Results of an experimental investigation of the turbulent boundary layer in airflow over evolving young wind-waves are presented. The experiments were conducted in a laboratory wind-wave flume consisting of a closed-loop wind tunnel capable of generating wind speed that may exceed 15 m/s, atop of a 5 m long wave tank. Simultaneous measurements of mean wind velocity and of instantaneous fluctuations of the horizontal and vertical air velocity components were carried out along the test section at different airflow rates and at numerous heights above the highest wave. Instantaneous surface elevation at the air sensors' location was simultaneously recorded. The friction velocities at all locations and for all airflow rates were determined by two independent methods: by fitting the logarithmic velocity profiles and by extrapolating the measured Reynolds shear stresses to mean water surface level. The variation with height and along the test section of the fluctuations of two velocity components, in the mean flow and in the vertical directions, was also studied and the results compared with flow behavior over rough and smooth plates. Wave-induced airflow parameters were then investigated by application of cross-spectral analysis. Results on the vertical extent of wave-induced boundary layer, on the phase relation between the wave-induced velocity fluctuations and the surface elevation, as well as on the wave-induced Reynolds shear stress are reported.Citation: Zavadsky, A., and L. Shemer (2012), Characterization of turbulent airflow over evolving water-waves in a wind-wave tank,

Experiments on wind-waves in a laboratory tank are carried out for a variety of operational conditions. In addition to using a conventional wave gauge to determine the temporal variation of the surface elevation at the sensor location, independent measuring techniques such as 3D reconstruction of stereo video images and 2D laser slope gauge are applied in order to gain information on the threedimensional structure of the wind-wave field. The consistency of results obtained by different methods under identical forcing conditions is examined to establish the limits of applicability and accuracy of each method. The accumulated results on the spatial and temporal coherence of the surface elevation variation, on the directional wave spectra, and on the probability distribution of the instantaneous surface slope directions demonstrate that the wind-wave field is essentially three-dimensional and short-crested.

Detailed investigation of wind-generated water waves in a 5-m-long wind wave flume facility is reported. Careful measurements were carried out at a large number of locations along the test section and at numerous airflow rates. The evolution of the wind wave field was investigated using appropriate dimensionless parameters. When possible, quantitative comparison with the results accumulated in field measurements and in larger laboratory facilities was performed. Particular attention was given to the evolution of wave frequency spectra along the tank, distinguishing between the frequency domain around the spectral peak and the high-frequency tail of the spectrum. Notable similarity between the parameters of the evolving wind wave field in the present facility and in field measurements was observed.

Statistical parameters of water waves generated by wind in a small scale facility are studied using extensively a Laser Slope Gauge (LSG), in addition to conventional measuring instruments such as a wave gauge and Pitot tube. The LSG enables direct measurements of two components of the instantaneous surface slope. Long sampling duration in a relatively small experimental facility allowed accumulating records of the measured parameters containing a large number of waves. Data were accumulated for a range of wind velocities at multiple fetches. Frequency spectra of the surface elevation and of the instantaneous local slope variation measured under identical conditions are compared. Higher moments of the surface slope are presented. Information on the waves' asymmetry is retrieved from the computed skewness of the surface slope components.