Effect of component waves on drag coefficient in laboratory experiments

Rikuya Maekawa, Kindai University, Graduate School of Sience and Engineering, Osaka, Japan, Naoya Suzuki, Kindai University, Department of Mechanical Engineering, Osaka, Japan and Naohisa Takagaki, University of Hyogo, Himeji 671-2280, Japan
Sea surface conditions include wind waves, but also component waves such as regular and irregular waves, which may be mixed. Although the long period effect of component waves such as swell has been the subject of substantial research, the effect of various component waves on the drag coefficient has not been investigated. In this study, as a first approach, we investigated the effect of component waves on the drag coefficient by reproducing the mixed condition of the wind waves and component waves (regular and irregular waves) in the wind wave tank. The imposed conditions included the wave frequencies of the 0.5, 1, and 2 Hz, wave amplitude (voltage) of 1 and 2 V and, wind speeds at 10 m height, U10 , of 6.6, 12.4 and 21.8 m/s. The variation of the drag coefficient in the regular waves was large at 6.6 and 12.4 m/s, but small at 21.8 m/s. In the case of the high frequency regular waves, there was a significant difference in the shape of the wind wave spectrum. It was shown that the regular component waves affects the wind wave. In the high wind speed, the difference in the shape of the wind wave spectrum was small. We also investigated the effect of the irregular component wave on the drag coefficient and the growth of the wind wave. Furthermore, the effect of the component waves on the drag coefficient also investigate in detail by measuring the Reynolds stress using PIV. (Particle Image Velocimetry).