Journal ArticleDOI
Added stresses because of the presence of FENE-P bead-spring chains in a random velocity field
TLDR
In this article, the average behavior at different times of an ensemble of bead-spring chains, contained in a fluid particle that is moving around in a random velocity field obtained from direct numerical simulation of turbulent flow of a Newtonian fluid in a channel was studied.Abstract:
FENE-P bead–spring chains unravel in the presence of large enough velocity gradients. In a turbulent flow, this can result in intermittent added stresses and exchanges of energy between the chains and the fluid, whose magnitudes depend on the degree of unravelling and on the orientations of the bead–spring chains. These effects are studied by calculating the average behaviour at different times of an ensemble of chains, contained in a fluid particle that is moving around in a random velocity field obtained from direct numerical simulation of turbulent flow of a Newtonian fluid in a channel. The results are used to evaluate theoretical explanations of drag reduction observed in very dilute solutions of polymers.In regions of the flow in which the energy exchange with the fluid is positive, the possibility arises that turbulence can be produced by mechanisms other than the interaction of Reynolds stresses and the mean velocity gradient field. Of particular interest, from the viewpoint of understanding polymer drag reduction, is the finding that the exchange is negative in velocity fields representative of the wall vortices that are large producers of turbulence. One can, therefore, postulate that polymers cause drag reduction by selectively changing the structures of eddies that produce Reynolds stresses. The intermittent appearance of large added shear stresses is consistent with the experimental finding of a stress deficit, whereby the total local shear stress is greater than the sum of the Reynolds stress and the time-averaged shear stress calculated from the time-averaged velocity gradient and the viscosity of the solvent.read more
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Journal ArticleDOI
Influence of drag-reducing polymers on turbulence: effects of Reynolds number, concentration and mixing
TL;DR: In this paper, the authors argue that the interaction of turbulence with the polymers introduces mean and fluctuating polymer stresses which can create turbulence, and that the effect of turbulence modification depends on the manner by which polymers are introduced into the flow.
Journal ArticleDOI
Turbulent channel flow near maximum drag reduction: simulations, experiments and mechanisms
PK Ptasinski,Bendiks Jan Boersma,Ftm Nieuwstadt,MA Martien Hulsen,van den Bhaa Ben Brule,Jcr Hunt +5 more
TL;DR: In this article, a simulation of a turbulent channel flow is presented, where the polymers are modelled as elastic dumbbells using the FENE-P model and the simulation results show that at approximately maximum drag reduction the slope of the mean velocity profile is increased compared to the standard logarithmic profile in turbulent wall flows.
Journal ArticleDOI
Drag reduction by polymer additives in a turbulent channel flow
TL;DR: In this paper, the authors investigated the effect of polymer additives on turbulent drag reduction in a channel using direct numerical simulation, where the dilute polymer solution is expressed with an Oldroyd-B model that shows a linear elastic behaviour.
Journal ArticleDOI
New Answers on the Interaction Between Polymers and Vortices in Turbulent Flows
Yves Dubief,Yves Dubief,Vincent Terrapon,Christopher White,Eric S. G. Shaqfeh,Parviz Moin,Sanjiva K. Lele +6 more
TL;DR: In this paper, the authors interpreted the data of polymer drag reduced flows in terms of modification of near-wall coherent structures, and showed that polymers are shown to reduce drag by damping nearwall vortices and sustain turbulence by injecting energy onto the streamwise velocity component in the very nearwall region.
Journal ArticleDOI
Roles of drag reducing polymers in single- and multi-phase flows
TL;DR: In this article, a review of the use of polymers as drag reducing agents in turbulent single and multiphase flows is presented. But, despite the voluminous works on drag reducing polymers, there are no universally accepted models and hence the mechanisms of drag reduction by polymers.