1. Institute of Fluid Mechanics and Heat Transfer, TU Wien, Vienna 1060, Austria; Complex Fluids and Flows Unit, Okinawa Institute of Science and Technology, Graduate University, Okinawa 904-0495, Japan
2. Institute of Fluid Mechanics and Heat Transfer, TU Wien, Vienna 1060, Austria; D.P.I.A., University of Udine, Udine 33100, Italy