Wind-generated water waves in a wind tunnel: Free surface statistics, wind friction and mean air flow properties

Abstract

This paper describes the systematic measurements of wind and water waves in a wind tunnel with a water tank inside. The velocity fields are measured using a 2-D Laser Doppler velocimetry (LDV) in air and the instantaneous water levels are measured using resistive twin-wire probes. The paper gives details on the free surface statistics and the mean airflow structures, including the statistics of the wind-generated water waves, phase velocity and group velocity of the waves, waves grouping, and interface friction. Comparisons are made between these results with those for wind flows over a fixed solid surface. The wind-generated waves show the typical growing trend with fetch and wind speed and are also asymmetric, with crests more pronounced than troughs. A model is developed to account for the relative variation of phase velocity and group velocity, which includes a dependence of the drift velocity from the wave steepness. The statistics of the wave groups suggest that a separate treatment of the envelopes of the crests and of the troughs should be necessary. The air flow boundary layer over the water waves shows a logarithmic profile with a wake near the free stream and the apparent roughness is related to the wave amplitude. The transition to turbulence occurs at an earlier stage with respect to the transition of a boundary layer over a smooth, flat rigid wall. The thickness of the boundary layer over the water waves grows much faster than for the boundary layer over a plain, smooth solid wall.