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

Abstract

Despite a significant progress and numerous publications over the last few decades a comprehensive understanding of the process of waves' excitation by wind still has not been achieved. The main goal of the present work was to provide as comprehensive as possible set of experimental data that can be quantitatively compared with theoretical models. Measurements at various air flow rates and at numerous fetches were carried out in a small scale, closed-loop, 5 m long wind wave flume. Mean airflow velocity and fluctuations of the static pressure were measured at 38 vertical locations above the mean water surface simultaneously with determination of instantaneous water surface elevations by wave gauges. Instantaneous fluctuations of two velocity components were recorded for all vertical locations at a single fetch. The water surface drift velocity was determined by the particle tracking velocimetry (PTV) method. Evaluation of spatial growth rates of waves at various frequencies was performed using wave gauge records at various fetches. Phase relations between various signals were established by cross-spectral analysis. Waves' celerities and pressure fluctuation phase lags relative to the surface elevation were determined. Pressure values at the water surface were determined by extrapolating the measured vertical profile of pressure fluctuations to the mean water level and used to calculate the form drag and consequently the energy transfer rates from wind to waves. Directly obtained spatial growth rates were compared with those obtained from energy transfer calculations, as well as with previously available data.

Characterization of turbulent airflow over evolving water‐waves in a wind‐wave tank

“…The computer‐controlled blower enables maximum wind speed in the test section that may exceed 15 m/s. The mapping of the mean airflow through the cross section in this facility carried out by Liberzon [2010] demonstrated that the boundary layer thickness at the sidewalls of the tank did not exceed about 70 mm, with an essentially flat mean velocity spanwise distribution in the central part of the cross‐section. To make possible velocity measurements in turbulent airflow using thermo‐anemometry, it is essential to maintain constant temperature in the test section.…”

“…In the second series of experiments the hot‐film sensor was removed, thus allowing mean airflow velocity measurements by Pitot tube closer to air water interface. In both experimental series, prior to initiation of the data accumulation process at each fetch and blower setting, the maximum possible crest height was first determined using the maximum wave height sensor and an iterative computer‐controlled procedure described in Liberzon and Shemer [2011]. Detailed vertical profiles of the measured parameters were obtained at numerous fetches along the test section; in the 1st experimental series measurements of the mean air velocity and of the turbulent fluctuations were performed at elevations that ranged from 5 mm to 130 mm relative to the highest wave crest.…”

“…The values of the derived from those logarithmic fit curves friction velocities u * and effective roughness values z 0 , as well as the dimensionless surface roughness , are presented in Table 2. The results of Table 2 are in agreement with Nordeng [1991] and with Liberzon and Shemer [2011].…”

Separation of Upslope Flow over a Plateau

“…Afterwards, the velocity components were calculated using the track information. Additional PTV algorithm details can be found in Hocut et al [7] and Liberzon and Shemer [24]. Prior to each experiment, the water tank was allowed to settle for two hours in order to allow the residual motion to decay, after which a prescribed current was applied to the two heating units.…”

“…Afterwards, the velocity components were calculated using the track information. Additional PTV algorithm details can be found in Hocut et al [7] and Liberzon and Shemer [24].…”