SCALING OF TURBULENT BOUNDARY LAYER PROFILES IN AIRFLOW OVER YOUNG WIND-WAVES

Understanding the air velocity profile over wind-waves is crucial for evaluation of momentum and energy transfer between air and water. It is well accepted that the airflow above water waves has logarithmic profile that is usually expressed in terms of an effective roughness parameter. This parameter however cannot be evaluated directly from the wave measurements at the water surface. Determination of accurate air velocity profile in field conditions is nearly impossible; the existing estimates are based on rough data. We suggest an alternative approach based on the extensive experimental data accumulated in our wind-wave facility. Combined measurements of the finely resolved mean air velocity profile above water surface and of the characteristics of the wind-wave field were performed at 6 wind forcing conditions and at 7 fetches. We demonstrate that the spatially developing boundary layer over the complicated three-dimensional, random and moving wind-wave surface is characterized by wall similarity observed in turbulent flow above rough solid surfaces. The results enable more accurate estimations of the actual drag coefficient.