1. a Department of Mechanical Engineering, Pohang University of Science and Technology, Pohang, Gyungbuk 790-784, South Korea
2. b Department of Information and Communication Engineering, Kookje College, Pyeongtaek, Gyeonggi 459-070, South Korea
3. c Department of Mechanical Engineering, Pohang University of Science and Technology, Pohang, Gyungbuk 790-784, South Korea;, Email: wkchung@postech.ac.kr