Non-Newtonian turbulent jets at low-Reynolds number

TL;DR

This study uses direct numerical simulations to explore how non-Newtonian fluids with shear-thinning and viscoelastic properties affect the transition from laminar to turbulent jets at low Reynolds numbers, revealing complex flow regimes.

Contribution

It provides a comprehensive analysis of flow regimes in non-Newtonian jets, highlighting the effects of shear-thinning and viscoelasticity on turbulence transition at low Reynolds numbers.

Findings

Viscoelasticity and shear-thinning can induce turbulence transition.

Oldroyd-B fluid becomes disordered at lower Weissenberg numbers.

Bulk flow statistics follow Newtonian jet scaling laws.

Abstract

We perform direct numerical simulations of planar jets of non-Newtonian fluids at low Reynolds number, in typical laminar conditions for a Newtonian fluid. We select three different non-Newtonian fluid models characterized by shear-thinning (Carreau), viscoelasticity (Oldroyd-B) and shear-thinning and viscoelasticity together (Giesekus), and perform a thorough analysis of the resulting flow statistics. We observe that, as the Weissenberg number is increased, the jet transitions from a laminar flow at low Weissenberg number, to a turbulent flow at high Weissenberg number. We show that the different non-Newtonian features and their combination give rise to rather different flowing regimes, originating from the competition of viscous, elastic and inertial effects. We observe that both viscoelasticity and shear-thinning can develop the instability and the consequent transition to a turbulent flowing regime; however, the purely viscoelastic Oldroyd-B fluid exhibits the onset of disordered fluid motions at a lower Weissenberg number than what observed for the purely shear-thinning Carreau fluid. When the two effects are both present, an intermediate condition is found, suggesting that, in this case, the shear-thinning feature is acting against the fluid elasticity. Despite the qualitative differences observed in the flowing regime, the bulk statistics, namely the centerline velocity and jet thickness, follow almost the same power-law scalings obtained for laminar and turbulent Newtonian planar jets.