1. Graduate Student, School of Mechanical Engineering, Kyungpook National Univ., 1370 Sankyuk-Dong, Buk-Gu, Daegu 702-701, South Korea.
2. Professor, School of Mechanical Engineering, Kyungpook National Univ., 1370 Sankyuk-Dong, Buk-Gu, Daegu 702-701, South Korea (corresponding author).