Experimental investigation of turbulent boundary layer beneath a wind-driven water surface

Quantitative properties of turbulent boundary layer beneath a wind-driven water surface are investigated by means of laboratory experiments, which are carried out using a wind-wave tank, 17m long, 0.6m wide and 0.8m deep. Instantaneous streamwise, transverse and vertical velocities in the water are measured at a fetch of 10.75m by using a three-component acoustic Doppler velocimeter, and they are obtained at reference wind speeds of 5.0, 7.5 and 10m/s. Wind speeds and surface displacements are also measured using a Pitot-static tube and capacitance-type wave gauges, respectively. Power spectra of the velocity fluctuations show that a spectral region in proportion to – 5/3 power of the frequency f forms over a frequency range lower than the peak frequency due to dominant waves fp. Also, the turbulence in ranges f < 0.1Hz and f> fp is found to be anisotropic. It is seen from cospectra of the streamwise and vertical velocity fluctuations that in the region of low wind speeds, most of the Reynolds stress is contained in a lower frequency range than fp, and that the Reynolds stress around fp becomes large with increasing the wind speed and is decreased exponentially with increasing the water depth. Turbulent velocity fluctuations at lower frequencies than fp are separated from wave