1. Faculty of Engineering, University of the Ryukyus, 1, Senbaru, Nishihara-cho ,Okinawa 903-0213, Japan
2. Department of Mechanical Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka, Fukuoka 819-0395, Japan
3. Institute of Fluid Science, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, Miyagi 980-8577, Japan