Viscoelastic instability of Poiseuille flow

• Applicant: Professor Dr. Christian Wagner
• Subject Area: Statistical Physics, Nonlinear Dynamics, Complex Systems, Soft and Fluid Matter, Biological Physics ; Experimental Condensed Matter Physics
• Term: from 2010 to 2017
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 157269621

The flow of simple liquids in channels is known to undergo an instability transition from laminar to turbulent flow if inertial effects overcome viscous effects. The ratio of these two quantities is estimated by the so-called Reynolds number. For the case of elastic liquids (e.g. polymer solutions) it has been found recently that flows with curved streamlines become linearly unstable when the Weissenberg number - that measures the effect of elasticity - exceeds a certain critical value. This happens even at negligible Reynolds numbers, i.e. without any effect of inertia. Still, the flow pattern shows characteristics like a continuous spectrum of wave modes and an energy cascade, similar to the one that is found in inertial turbulence. For the case of straight streamlines the situation was less clear until recently. Theoretical considerations proposed the existence of a weakly nonlinear instability scenario where finite amplitude distortions should destabilize the flow. An unambiguous experimental verification of such a weakly nonlinear instability in polymer solutions has only been obtained in the first funding period, but a quantitative characterization of the bifurcation scenario and of the type and amplitude of the finite distortion that is necessary to trigger the instability is still missing. Furthermore, there exist no data on coherent flow structures that are theoretically expected close to the transition point. This experimental project will help to increase the understanding of turbulence and the flow dynamics of complex liquids in general. In addition, we hope to learn more about technical problems in industrial processes like e.g. the effect of melt fracture when a polymer melt becomes unstable at very low flow rates and the sample cannot be processed further.

• DFG Programme: Research Grants