1. Department of Mechanical Engineering, The Univ. of Sheffield, Sheffield, S1 3JD, GB.
2. Health & Safety Laboratory, Harpur Hill, Buxton, Derbyshire, SK17 9JN, GB.
3. Department of Mechanical Engineering, The Univ. of Sheffield, Sheffield, S1 3JD, GB (Corresponding author), e-mail: m.j.carre@shef.ac.uk