1. Associate Professor, College of Aeronautical Engineering, Civil Aviation Univ. of China, Tianjin 300300, China.
2. Preliminary Year Tutor in Mathematics, Centre for English Language Education, Univ. of Nottingham Ningbo China, Zhejiang 315100, China (corresponding author).
3. Professor, Dept. of Mechanical Science and Engineering, Univ. of Illinois at Urbana-Champaign, Urbana, IL 61801.