Transitional pipe flow of shear-thinning fluids*

Shear-thinning fluid properties can substantially delay the onset of turbulence in pipe flow. In Newtonian pipe flow, transition to turbulence is subcritical, resulting from finite amplitude disturbances. While experiments find a symmetry breaking instability of the laminar base flow prior to the onset of turbulence. Simulations using idealized shear-thinning fluids, such as the Carreau-Yasuda model applied in the present computations, do not encounter such instability. This suggests more complex fluid properties in the polymer solutions used in experiments.

The focus of the present study is the effect of the idealized shear-thinning case on the transition to turbulence. We focus on a recent observation that body forces, which generate sequivalent profile shapes as the shear-thinning case, in general suppress spatio-temporal intermittency and that eventually the transition to turbulence encountered does not involve puffs or slugs that dominate the transition regime in ordinary pipe flow. Instead a sharp, discontinuous transition has been suggested for pipe flows subject to body forces. We address the question if despite the different underlying physical mechanism puffs and slugs may also be suppressed for the case of sufficiently strong shear-thinning and if the transition may eventually switch from continuous to discontinuous.