Effect of the fluctuation of the wind speed and direction on the drag coefficient investigated using tower-based measurements

Naoya Suzuki, Kindai University, Department of Mechanical Engineering, Osaka, Japan and Takuji Waseda, Univ Tokyo, Frontier Sciences, Chiba, Japan
Drag coefficient CD is generally expressed as a function of only wind speed U10. However, for a single U10 value there exists a considerable range of observed CD values. In this study, we observed the wind stress at a coastal tower: the Hiratsuka Offshore Experimental Tower of the University of Tokyo, Japan. Three-axis sonic anemometers were installed atop the tower, which was 20 m above mean sea level. The observation period was from September 15, 2015 to December 31, 2018. The friction velocity was obtained every 10 min. using the eddy correlation method. The pooled data from the observation period revealed a large variation in CD with respect to U10. The variation of CD was reduced by excluding data points from periods of large fluctuation of wind speed and/or direction in time series within one hour. Furthermore, the wind speed fluctuation in time series was found to affect the CD variation in the region of high wind speed, while the wind direction fluctuation influenced the CD variation in the region of low wind speed. These results show that the CD variability factors include not only the effects of the wind wave growth, swell, wave breaking, etc., but also those of fluctuation in wind speed and direction. We also investigated the influence of wave effects on the CD.