Mechanism of polymer drag reduction using a low-dimensional model |
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Authors: | Roy Anshuman Morozov Alexander van Saarloos Wim Larson Ronald G |
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Institution: | Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109, USA. |
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Abstract: | Using a retarded-motion expansion to describe the polymer stress, we derive a low-dimensional model to understand the effects of polymer elasticity on the self-sustaining process that maintains the coherent wavy streamwise vortical structures underlying wall-bounded turbulence. Our analysis shows that at small Weissenberg numbers, Wi, elasticity enhances the coherent structures. At higher Wi, however, polymer stresses suppress the streamwise vortices (rolls) by calming down the instability of the streaks that regenerates the rolls. We show that this behavior can be attributed to the nonmonotonic dependence of the biaxial extensional viscosity on Wi, and identify it as the key rheological property controlling drag reduction. |
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