On the role of air flow separation for momentum flux transfer at water surface

Occurrence frequency of air flow separation over wind waves was measured to investigate relationship between the air flow separation and a drag coefficient. This experiment was performed by using a laboratory wind wave tank (8.0m long, 0.2m width, 0.45m depth, 0.25m height of the air flow section). The measurement points were placed at 2.30, 3.15, 4.30, 5.15m from the wind inlet, respectively. The maximum wind speed close to the wind inlet are about 4-11m/s. PIV measurement technique was used to visualize air flow distribution above wind waves. The air flow separations from wave crests in the PIV vector distributions were counted by visual observation. The occurrence frequencies of the air flow separation converged to a value of 10% with windsea Reynolds number, RB, ranging between 200 and 1000. This range (200 < RB < 1000) is called “transitional” by Toba et'al(2006). In that condition, the drag coefficients also have a constant value. Under moderate wind speed and short fetch condition, the air flow controls momentum flux transfer from air side to water side due to intermittent separation from a wave crest. As a result, despite growth of wind speed, wave breaking probability and wave steepness are still kept constant. This process is considered as a self-regulation mechanism between air and water without wave breaking.