Polymer flexibility and turbulent drag reduction

Gillissen, J.J.J.

Polymer flexibility and turbulent drag reduction

Title

Polymer flexibility and turbulent drag reduction

Author

Gillissen, J.J.J.

Faculty

Applied Sciences

Department

Multi-Scale Physics

Date

2008-10-24

Abstract

Polymer-induced drag reduction is the phenomenon by which the friction factor of a turbulent flow is reduced by the addition of small amounts of high-molecular-weight linear polymers, which conformation in solution at rest can vary between randomly coiled and rodlike. It is well known that drag reduction is positively correlated to viscous stresses, which are generated by extended polymers. Rodlike polymers always assume this favorable conformation, while randomly coiling chains need to be unraveled by fluid strain rate in order to become effective. The coiling and stretching of flexible polymers in turbulent flow produce an additional elastic component in the polymer stress. The effect of the elastic stresses on drag reduction is unclear. To study this issue, we compare direct numerical simulations of turbulent drag reduction in channel flow using constitutive equations describing solutions of rigid and flexible polymers. When compared at constant or2, both simulations predict the same amount of drag reduction. Here o is the polymer volume fraction and r is the polymer aspect ratio, which for flexible polymers is based on average polymer extension at the channel wall. This demonstrates that polymer elasticity plays a marginal role in the mechanism for drag reduction.

Subject

channel flow
drag reduction
elasticity
friction
molecular configurations
polymer solutions
turbulence

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Publisher

American Physical Society

ISSN

1539-3755

Source

Physical Review E, 78 (4 pt 2), 2008

Part of collection

Institutional Repository