1. Department of Mechanical Engineering, Imperial College London, Exhibition Road, South Kensington Campus, London SW7 2AZ, UK e-mail:
2. Dyson School of Design Engineering, Imperial College London, Exhibition Road, South Kensington Campus, London SW7 2AZ, UK e-mail:
3. Rolls-Royce plc., 7 Thomas Court, Derby DE1 1AJ, UK e-mail: