1. Technical Univ. of Denmark, Dept. of Mechanical Engineering, Produktionstorvet b.425, DK 2800 Kgs. Lyngby, DK
2. Technical Univ. of Denmark, Dept. of Mechanical Engineering, Produktionstorvet b.425, DK 2800 Kgs. Lyngby, DK, (Corresponding author) e-mail: tch@mek.dtu.dk