Abstract
Viscoelastic turbulent flows of particle suspensions are studied using direct numerical simulations. In the presence of a moderate concentration of particles, the beneficial effect of polymers in terms of drag reduction are lost, and an increase in drag is observed for sufficiently dense suspensions.
Highlights
We study the effect of spherical particles on the turbulent flow of a viscoelastic fluid and find that the drag reducing effect of polymer additives is completely lost for semidense suspensions, with the drag increasing more than for suspensions in Newtonian fluids
Drag increase is provided by the growth of the particle and polymeric shear stresses with the particles, due to larger shear rates in the vicinity of the particle surface
Polymers dampen near-wall vortices but enhance the streamwise kinetic energy of the near-wall streaks, with the net balance of these two opposite actions leading to a self-sustained drag-reduced turbulent flow: The polymers reduce turbulence by opposing the downwash and upwash flows generated by near-wall vortices, while they enhance streamwise velocity fluctuations in the very near-wall regions
Summary
We study the effect of spherical particles on the turbulent flow of a viscoelastic fluid and find that the drag reducing effect of polymer additives is completely lost for semidense suspensions, with the drag increasing more than for suspensions in Newtonian fluids. Polymers dampen near-wall vortices but enhance the streamwise kinetic energy of the near-wall streaks, with the net balance of these two opposite actions leading to a self-sustained drag-reduced turbulent flow: The polymers reduce turbulence by opposing the downwash and upwash flows generated by near-wall vortices, while they enhance streamwise velocity fluctuations in the very near-wall regions Recent studies on this mechanism relate the stretching of the polymers to dampening of active and enhancement of hibernating turbulent phases [11,12].
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