1. Professor, 1ga, Anam-dong, Seongbuk-gu, Korea Univ., Seoul 136-701, Republic of Korea.
2. Graduate Student, 1ga, Anam-dong, Seongbuk-gu, Korea Univ., Seoul 136-701, Republic of Korea.
3. Associate Professor, Mechanical and Aeronautical Engineering Dept., 8 Clarkson Ave., Clarkson Univ., Potsdam, NY 13699 (corresponding author).