1. Department of Engineering Mechanics, Tsinghua University, Beijing 100084, P.R.C.
2. School of Aerospace Engineering and Applied Mechanics, Tongji University, Shanghai 200092, P.R.C.; Department of Engineering Mechanics, Tsinghua University, Beijing 100084, P.R.C.
3. Department of Mechanical Engineering, University of Hong Kong, Hong Kong, P.R.C.