1. Doctor, Institute of Mechanics, Chinese Academy of Sciences, No. 15 Beisihuanxi Rd., Beijing, China 100190.
2. Professor, Institute of Mechanics, Chinese Academy of Sciences, No. 15 Beisihuanxi Rd., Beijing, China 100190.
3. Professor and Caroline & William N. Lehrer Distinguished Chair in Water Engineering, Dept. of Biological and Agricultural Engineering, Texas A & M Univ., College Station, TX 77843-2117 (corresponding author).