Effect of Fluid Viscoelasticity on Turbulence and Large-Scale Vortices behind Wall-Mounted Plates

Direct numerical simulations of turbulent viscoelastic fluid flows in a channel with wall-mounted plates were performed to investigate the influence of viscoelasticity on turbulent structures and the mean flow around the plate. The constitutive equation follows the Giesekus model, valid for polymer or surfactant solutions, which are generally capable of reducing the turbulent frictional drag in a smooth channel. We found that turbulent eddies just behind the plates in viscoelastic fluid decreased in number and in magnitude, but their size increased. Three pairs of organized longitudinal vortices were observed downstream of the plates in both Newtonian and viscoelastic fluids: two vortex pairs were behind the plates and the other one with the longest length was in a plate-free area. In the viscoelastic fluid, the latter vortex pair in the plate-free area was maintained and reached the downstream rib, but its swirling strength was weakened and the local skin-friction drag near the vortex was much weaker than those in the Newtonian flow. The mean flow and small spanwise eddies were influenced by the additional fluid force due to the viscoelasticity and, moreover, the spanwise component of the fluid elastic force may also play a role in the suppression of fluid vortical motions behind the plates.