Variation of the Drag Coefficient Investigated by Using Tower-based Long Period Measurements - Condition of High Wind Speed with Following and Cross Swell

The drag coefficient C_D is generally expressed as a function only of the wind speed U₁₀. However, there exists considerable disagreement among the observed values of C_D. In this study, we investigate the variation of C_D by using data sets, including directional wave spectral data in long period measurements. Wind and wave data obtained at a coastal tower operated by the Shirahama Oceanographic Observatory of the Disaster Prevention Research Institute (DPRI) at Kyoto University in Japan, together with data sets obtained by Suzuki et al. (2002), were used. The directional wave spectra were separated into four different groups: the following and cross swell cases, mixed swell case, and pure wind-wave case. The data sets also showed variation in wind speed regions higher than 20 m/s in the C_D - U₁₀ diagram. It was shown that this variation is the effect of the natural fluctuation of wind. We also observed C_D values by using the windsea Reynolds number R_B proposed by Toba et al. (2006). In the following swell cases, C_D values are slightly smaller. In the cross swell cases, when the difference in direction between the wind and waves is greater than 70 degrees, C_D has larger values. However, C_D has similar values in the following swell cases when the difference in direction between the wind and waves is less than 70 degrees.