Studies of the Interaction Between Drag-Reducing Polymers and a Turbulent Flow Field Using Pulsed Image Velocimetry and FENE Bead-Spring Model
Massah Bavani, Heshmatollah
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https://hdl.handle.net/2142/72208
Description
Title
Studies of the Interaction Between Drag-Reducing Polymers and a Turbulent Flow Field Using Pulsed Image Velocimetry and FENE Bead-Spring Model
Author(s)
Massah Bavani, Heshmatollah
Issue Date
1993
Doctoral Committee Chair(s)
Hanratty, Thomas J.
Department of Study
Civil Engineering
Discipline
Civil Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Applied Mechanics
Engineering, Chemical
Engineering, Mechanical
Abstract
An understanding of polymer drag-reduction, discovered by Toms (1948), has remained a challenge to researchers in turbulence and polymer fluid mechanics. The intriguing aspect of this phenomenon is that the addition of high molecular weight, long chain polymer molecules in amounts as small as 1 ppm can reduce the drag of a turbulent flow on a wall by as much as 25%. A popular argument is that drag-reduction is associated with the unraveling of polymer molecules by the turbulent velocity fluctuations causing an increase in the viscosity in the buffer zone but not in the viscous sublayer. This increase in the viscosity results in damping of small eddies and an increase in the thickness of the buffer layer and reducing the drag.
This thesis addresses two basic questions underlying this phenomenon. What is the effect of turbulence on polymers? What is the effect of polymers on turbulence? A 3-D direct numerical simulation of turbulent channel flow and a particle tracking code were utilized to follow the paths of fluid particles and to record the changes of flow field. A FENE bead-spring model was used to calculate the changes in the average configuration of polymer molecules affined to the fluid particles whose path were being followed. Stresses created by polymers were calculated. For future work, it is suggested that these stresses be added to the Navier-Stokes equations in order to simulate turbulent flow of a dilute solution of polymers. Pulsed image velocimetry (PIV) measurements were carried out to compare the flow fields between water and a polymer solution of 3ppm concentration, in planes parallel and perpendicular to the top wall of a 2" x 24" channel.
The computer experiments show that polymer molecules unravel the most in the viscous sublayer and orient in the direction of flow. They add, mainly, elongational stress to the flow in this region. In the buffer zone, polymers unravel and orient in different directions and create both elongational and shear stresses. Therefore, the flow becomes more dissipative and eddies increase in size. This results in a thickening of the viscous wall region and a reduction of drag. PIV measurements show that the similarities between flow fields with and without polymers are more than the differences; both flows are highly turbulent. However, in the polymeric regime, spanwise fluctuations are significantly dampened and the streaky structures appear farther away from the wall. This, also, indicates that the viscous wall region has been thickened as a result of polymer injection.
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