The behavior of viscoelastic flow behind a two-dimensional slits was examined using direct numerical simulations (DNS). We performed DNS at five different conditions with changing the Reynolds number and the Weissenberg number, to investigate the parametric dependence of several characters of the viscoelastic flows (e.g., Toms effect and Barus effect) accompanied by the separation and reattachment. In the present conditions, the drag reduction rate was achieved from 15.1% to 19.7%. It was found that the wall-normal viscoelastic stress mainly enhanced the Barus effect in the present geometry and the streamwise viscoelastic force caused an increase of the drag. We found that, at a Weissenberg number higher than a certain level, the drag reduction rate should be decreased despite the reduced turbulent frictional drag. Moreover, we observed that, in the Newtonian flow, the spanwise vortices were dominant in a downstream region of the slits, while the streamwise vortices were dominant there in the case of the viscoelastic flow.
Drag enhancement and drag reduction in viscoelastic flow
