Influence of Viscoelasticity on Turbulent Flow Over a Backward-Facing Step

We studied viscoelastic turbulent flow over a backward-facing step of the expansion ratio ER = 1.5 using DNS (direct numerical simulation) at a friction Reynolds number Re_τ0 of 100. We chose the Giesekus model as a viscoelastic constitutive equation, and the Weissenberg number is Wi_τ0 = 10 and 20. Visualized instantaneous vortices revealing that a few vortices occur only above the recirculation regions in the viscoelastic fluid flow compared to those in the Newtonian flow. This phenomenon might be caused by the fluid viscoelasticity that would suppress the Kelvin-Helmholz vortex emanating from the step edge. The reattachment length from the step is 6.80h for the Newtonian fluid, 7.82h for Wi_τ0 = 10, and 8.82h for Wi_τ0 = 20, where h is the step height. In the mean velocity distributions normalized by maximum inlet velocity, we have observed no significant difference among the three fluids, except for region near the upper or bottom wall, i.e., the recirculation and recovery regions at the front and behind the reattachment point. The streamwise turbulent intensity u’_rms is weaken in the recirculation region of the viscoelastic flows. In terms of v’_rms, its magnitude in the recirculation region becomes largest in the case of Wi_τ0 = 10, not for the Newtonian fluid flow or more viscoelastic case of Wi_τ0 = 20.