EXPERIMENTAL-EVIDENCE OF THE RAPID DISTORTION OF TURBULENCE IN THE AIR-FLOW OVER WATER-WAVES

Detailed observations of the air flow velocity, pressure and Reynolds stresses above water waves in a wave flume are presented. The static p ressure fluctuations induced by the waves are observed following a new procedure that eliminates acoustical contamination by the wave maker. The measurements are analysed by comparing them with numerical simula tions of the air flow over waves. In these numerical simulations the s ensitivity to the choice of turbulence closure is studied. We consider ed both first-order turbulence closure schemes based on the eddy visco sity concept, and a second-order Reynolds stress model. The comparison shows that turbulence closure schemes based on the eddy viscosity con cept overestimate the modulation of the Reynolds stress in a significa nt part of the vertical domain. When an eddy viscosity closure is used , the overestimated modulation of the Reynolds stress gives a signific ant contribution to the wave growth rate. Our results confirm the conc lusions Belcher & Hunt reached on the basis of the rapid distortion th eory. The ratio of the wind speed to the phase speed of the paddle wav e in the experiment varies between 3 and 6. The observed amplitudes of the velocity and pressure perturbation are in excellent agreement wit h the simulations. Comparison of the observed phases of the pressure a nd velocity perturbations shows that the numerical model underpredicts the downwind phase shift of the undulating flow. The sheltering coeff icients for the flow over hills and the growth rates of waves that are slow compared to the wind calculated with the Reynolds stress model a re in excellent agreement with the analytical model of Belcher & Hunt. Extending the calculations to fast waves, we find that the energy flu x to waves travelling almost as fast as the wind is increased on going from the mixing length turbulence closure to the Reynolds stress mode l.