Open Access
Effects of polymer stresses on eddy structures in drag-reduced turbulent channel flow
Kyoungyoun Kim,Chang F. Li,Chang F. Li,Radhakrishna Sureshkumar,S. Balachandar,Ronald J. Adrian +5 more
- 01 Nov 2006
Vol. 59
115
TL;DR: In this paper, 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, and conditionally averaged flow fields for Reynolds-stress-maximizing Q2 events are shown that the nearwall vortical structures are weakened and elongated in the streamwise direction by polymer stresses in a manner similar to that found by Stone et al.
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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.
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Citations
Drag reduction and the dynamics of turbulence in simple and complex fluidsa)
TL;DR: In this paper, the authors describe simulations of turbulent minimal channel flow of Newtonian fluids and viscoelastic polymer solutions and show that there are intervals of hibernating turbulence that display very low drag as well as many other features of the maximum drag reduction observed in polymer solutions.
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On the mechanism of elasto-inertial turbulence
TL;DR: Elasto-inertial turbulence provides new insights on the nature of the asymptotic state of polymer drag reduction (maximum drag reduction), and explains the phenomenon of early turbulence, or onset of turbulence at lower Reynolds numbers than for Newtonian flows observed in some polymeric flows.
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Turbulent drag reduction by polymer additives: Fundamentals and recent advances
TL;DR: The problem of polymer-induced drag reduction has attracted continuous attention over the seven decades since its discovery as discussed by the authors, however, changes in research paradigm and perspectives have triggered a wave of new advancements in the past decade.
Simulations of three-dimensional viscoelastic flows past a circular cylinder at moderate Reynolds numbers
TL;DR: In this article, the effects of polymer extensibility and viscoelasticity during 3D cylinder wake transition were investigated. But the results from a numerical investigation of inertial visco-elastic flow past a circular cylinder are presented which illustrate the significant effect that dilute concentrations of polymer additives have on complex flows.
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Strong polymer-turbulence interactions in viscoelastic turbulent channel flow
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