1. Tohoku University, 15-4 Kitatakamori, Izumi-ku, Sendai 981-3202, Japan
2. Institute of Fluid Science, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
3. Tohoku Electric Power Co. Inc., 7-1 Honcho, 1-chome, Aoba-ku, Sendai 980-8550, Japan