School of Mechanical Engineering Seminar
Wednesday, May 3, 2017 at 14:00
Wolfson Building of Mechanical Engineering, Room 206
Experimental Study of Water Waves Generated by Wind
Andrey Zavadsky
PhD. Student of Prof. Lev Shemer
School of Mechanical Engineering, Tel Aviv University, 69978 Tel Aviv, Israel
Only limited information is currently available on evolution of waves generated by wind that varies in time, and in particular on the initial stages of wind-waves growth from rest under suddenly applied wind forcing. The emerging wind-wave field varies in time as well as in space. Detailed knowledge of wave parameters distribution under those conditions contributes to better understanding of mechanisms of wind wave generation. Present research aims to advance our knowledge of the details of this fundamental phenomenon by performing a series of experiments in a small-scale wind-wave facility.
In order to accumulate reference data for the unsteady wave field, extensive experiments on waves under steady wind forcing were performed. Characteristic parameters of turbulent airflow as well as of the water surface were carefully studied. Prolonged measurements of wind-generated water waves using standard wave-gauge 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. 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 (LSG) were applied in order to gain information on the three dimensional structure of the wind-wave field.
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.
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.
The 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 obtained by processing of the reconstructed 3D surfaces demonstrate that the wind-wave
field, though generated in relatively narrow test section, is essentially three-dimensional and short-crested. The LSG-derived results corroborate these conclusions.
Temporal variation of instantaneous main wave parameters in small scale facility under impulsive wind forcing were experimentally studied. Temporal variation of the wave field from the appearance of initial ripples, to emergence of quasi-steady wind-wave field was obtained. Distinct stages in the temporal and spatial wave evolution process were identified for waves excited by impulsive application of wind forcing. The experimental results during each stage of evolution were analyzed in view of the viscous instability theory by Kawai (1979) and the resonance model by Phillips (1957).
