Turbulent duct flow with polymers

TL;DR

This study uses direct numerical simulation to analyze how polymer additives affect turbulence and secondary flow in a square duct, revealing modifications in flow structures and vorticity with implications for drag reduction.

Contribution

It provides new insights into the impact of polymers on turbulent duct flow, especially on secondary motion and vortex structures, using the FENE-P model at a fixed Reynolds number.

Findings

Polymer additives modify Prandtl's secondary flow and vortex circulation.

In-plane fluctuations decrease while streamwise fluctuations increase with polymers.

Flow elasticity, characterized by Weissenberg number, significantly alters vortex size and velocity fluctuations.

Abstract

We have performed direct numerical simulation of the turbulent flow of a polymer solution in a square duct, with the FENE-P model used to simulate the presence of polymers. First, a simulation at a fixed moderate Reynolds number is performed and its results compared with those of a Newtonian fluid to understand the mechanism of drag reduction and how the secondary motion, typical of the turbulent flow in non-axisymmetric ducts, is affected by polymer additives. Our study shows that the Prandtl's secondary flow is modified by the polymers: the circulation of the streamwise main vortices increases and the location of the maximum vorticity move towards the center of the duct. In-plane fluctuations are reduced while the streamwise ones are enhanced in the center of the duct and dumped in the corners due to a substantial modification of the quasi-streamwise vortices and the associated near-wall low- and high-speed streaks; these grow in size and depart from the walls, their streamwise coherency increasing. Finally, we investigated the effect of the parameters defining the viscoelastic behavior of the flow and found that the Weissenberg number strongly influences the flow, with the cross-stream vortical structures growing in size and the in-plane velocity fluctuations reducing for increasing flow elasticity.