Variation of the Drag Coefficient Investigated Using Tower-Based Long Period Measurements: Condition with Following and Cross Swell

摘要

The drag coefficient C_D is generally expressed as functions only ofwind speed U_(10). However, there exists considerable disagreementamong the observed values of C_D. In this study, we investigate thevariation of C_D, by use of data sets including directional wave spectraldata in long period measurements. Wind and wave data observed at acoastal tower operated by Shirahama Oceanographic Observatory ofDisaster Prevention Research Institute, Kyoto University, Japan,together with data sets observed by Suzuki et al. (2002) are used. Thedirectional wave spectra were separated into four different groups: thefollowing and cross swell cases, mixed swell case and pure wind-wavecase. The data set was also shown as variation in wind speed regionshigher than 20 m/s on the C_D-U_(10) diagram. It is shown that the variationhas effects of natural fluctuation of wind. We have also analyzedobserved C_D values using the windsea Reynolds number RB proposedby Toba et al. (2006). In the following swell cases, C_D values areslightly smaller. In the cross swell cases, when difference of directionbetween the wind and waves is larger than 70 degree, C_D has largervalues. However C_D has similar values with the following swell cases,when smaller than 70 degree.