Onset and universality of turbulent drag reduction in von Karman swirling flow
Department of Physics of Complex Systems, Weizmann Institute of Science - Rehovot 76100, Israel
Received: 6 September 2012
Accepted: 21 September 2012
We report the results of experiments on turbulent drag reduction (TDR) in swirling flow of water and water-sucrose polymer solutions, where Re and Wi as well as polymer concentration ϕ are varied. The friction coefficients Cf and Cp defined through average torque and rms of pressure fluctuations prms for different elasticity El = Wi/Re and ϕ vs. Re/Rec collapse onto universal curves in accord with theory, where Rec is Re at TDR onset. The transition lines to the TDR state, Rec − El and Rec − ϕ, are measured and relevant physics is discussed. Power spectra for Γ and p at Re/Rec > 1 show a drastic reduction of low-frequency noise and the emergence of a peak corresponding to the main vortex frequency in accord with TDR.
PACS: 47.50.-d – Non-Newtonian fluid flows / 47.27.Jv – High-Reynolds-number turbulence / 47.57.Ng – Polymers and polymer solutions
© EPLA, 2012