We would like to thank the Technip company for financial support, 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 Metiers ParisTech, as well as to the students J.-R. De Céa and C. Valet for their assistance in the design and development of the experiment. The main part of the experiments have been carried out with the help of A. Lemaire.