We would like to thank the Technip company for financial sup-port, especially J. Denegre, P. Espinasse and T. Parenteau. F.R.
thanks Colette Gauthier for fruitful discussion. We are particularly indebted to M. Joulin from the DynFluid laboratory of Arts et Me-tiers ParisTech, as well as to the students J.-R. De Céa and C. Valet for their assistance in the design and development of the experi-ment. The main part of the experiments have been carried out with the help of A. Lemaire.