Journal of Fluid Mechanics

Papers

Effects of polymer stresses on eddy structures in drag-reduced turbulent channel flow

KYOUNGYOUN KIMa1, CHANG-F. LIa2 p1, R. SURESHKUMARa2, S. BALACHANDARa3 and RONALD J. ADRIANa1

a1 Department of Mechanical and Aerospace Engineering, Arizona State University, Tempe, AZ 85287, USA

a2 Department of Energy, Environmental and Chemical Engineering, Washington University, Saint Louis, MO 63130, USA

a3 Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, FL 32611, USA

Abstract

The effects of polymer stresses on near-wall turbulent structures are examined by using direct numerical simulation of fully developed turbulent channel flows with and without polymer stress. The Reynolds number based on friction velocity and half-channel height is 395, and the stresses created by adding polymer are modelled by a finite extensible nonlinear elastic, dumbbell model. Both low- (18%) and high-drag reduction (61%) cases are investigated. Linear stochastic estimation is employed to compute the conditional averages of the near-wall eddies. The conditionally averaged flow fields for Reynolds-stress-maximizing Q2 events show that the near-wall vortical structures are weakened and elongated in the streamwise direction by polymer stresses in a manner similar to that found by Stone et al. (2004) for low-Reynolds-number quasi-streamwise vortices (‘exact coherent states: ECS’). The conditionally averaged fields for the events with large contribution to the polymer work are also examined. The vortical structures in drag-reduced turbulence are very similar to those for the Q2 events, i.e. counter-rotating streamwise vortices near the wall and hairpin vortices above the buffer layer. The three-dimensional distributions of conditionally averaged polymer force around these vortical structures show that the polymer force components oppose the vortical motion. More fundamentally, the torques due to polymer stress are shown to oppose the rotation of the vortices, thereby accounting for their weakening. The observations also extend concepts of the vortex retardation by viscoelastic counter-torques to the heads of hairpins above the buffer layer, and offer an explanation of the mechanism of drag reduction in the outer region of wall turbulence, as well as in the buffer layer.

(Received May 30 2006)

(Revised March 11 2007)

Correspondence:

p1 Present address: School of Energy and Power Engineering, Jiangsu University, Zhenjiang, Jiangsu 212013, P. R. China

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