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Mechanism of polymer drag reduction using a low-dimensional model
Anshuman Roy1, Alexander Morozov, Wim van Saarloos
1Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109, USA.
Physical Review Letters
|February 7, 2007
Summary
Polymer elasticity initially enhances turbulent coherent structures but suppresses them at higher Weissenberg numbers (Wi). This drag reduction effect is linked to biaxial extensional viscosity, crucial for understanding wall-bounded turbulence.
Area of Science:
- Fluid Dynamics
- Polymer Physics
- Turbulence Research
Background:
- Wall-bounded turbulence relies on coherent vortical structures for energy transfer.
- Polymer additives are known to reduce drag, but the underlying mechanisms are complex.
- Understanding polymer elasticity's role in turbulence is key to optimizing drag reduction.
Purpose of the Study:
- To develop a low-dimensional model for polymer stress in wall-bounded turbulence.
- To investigate how polymer elasticity affects coherent streamwise vortical structures.
- To identify the key rheological properties governing drag reduction.
Main Methods:
- Utilized a retarded-motion expansion to model polymer stress.
- Derived a low-dimensional model for polymer elasticity effects.
- Analyzed the impact of Weissenberg number (Wi) on turbulence dynamics.
Main Results:
- At low Weissenberg numbers (Wi), polymer elasticity enhances coherent structures.
- At high Wi, polymer stresses suppress streamwise vortices by stabilizing streak instabilities.
- Nonmonotonic dependence of biaxial extensional viscosity on Wi was observed.
Conclusions:
- Polymer elasticity has a dual effect on coherent structures in wall-bounded turbulence.
- Biaxial extensional viscosity is identified as the critical rheological property for drag reduction.
- The study provides insights into polymer-induced drag reduction mechanisms.
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