A mechanism for turbulent drag reduction by polymers
Seminar Room 1, Newton Institute
Minute quantities of long chained polymers, of order 10 ppm, added to liquids like water or oil are capable of cutting the turbulent wall friction by half. This startling effect- the "Toms phenomenon" -has been known for more than 60 years, but a detailed explanation of how such small amounts of polymer alter the structure of turbulence so dramatically has been lacking. To explore this question, direct numerical simulations have been performed based on a visco-elastic model of the fluid that uses a finite extensible non-linear elastic-Peterlin (FENE-P) constituitive equation. It is found that the stresses due to the polymers circulating around turbulent vortices produce counter-torques that inherently oppose the rotation. Vortices creating the turbulent transport responsible for drag are weakened and the creation of new vortices is inhibited. Thus, both coherent and incoherent turbulent Reynolds stresses are reduced. Interesting, the viscoelastic stresses of the FENE-P model rely upon the vortices being asymmetric and such deviations from axisymmetry occur where the vortices are strained by themselves or by adjacent vortices.
Kim, K, Li, C.-F., Sureshkumar, R., Balachandar, S. and Adrian, R. J., “Effects of polymer stresses on eddy structures in drag-reduced turbulent channel flow,” J. Fluid Mech. 584, 281 (2007).
Kim, K,, Adrian, R. J., Balachandar, S. and Sureshkumar, R., “Dynamics of hairpin vortices and polymer-induced turbulent drag reduction,” Phys Rev. Lett. 100 (2008). LJ11563
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