1. Faculty of Science and Technology and J M Burgers Centre for Fluid Dynamics, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands
2. Faculty of Science and Technology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands
3. Institute de Mécanique des Fluides de Toulouse (IMFT), 1 Allée du Professeur Camille Soula, 31400 Toulouse, France
4. Faculty of Science and Technology and J M Burgers Centre for Fluid Dynamics, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands and Department of Mechanical Engineering, Johns Hopkins University, Baltimore, Maryland 21218