New research finds new turbulence

In the paper published this week in PNAS, IST Austria professor Björn Hof and his collaborators at the Universities of Saarbrücken, Vermont, Edinburgh and the Max Planck Institute for Dynamics and Self Organization unravel the mystery surrounding drag reduction through polymer addition, showing that an instability is responsible for this effect. (DOI: 10.1073/pnas.1219666110). In turbulent liquids, a reduction of drag is most efficiently achieved by adding small amounts of polymers to the liquid. This trick is used in oil pipelines, sewage, heating and irrigation to achieve a smooth flow of liquid. Empirical data has shown there to be a limit to the possible reduction of drag, the so-called MDR asymptote. However, how the addition of polymers reduces drag and where its limit comes from is not understood yet. Employing both experimental and numerical approaches, Björn Hof and his team showed that a new type of turbulence is involved.

Reynolds numbers measure how turbulent a fluid is, while the Weissenberg number measures the elastic stresses in a fluid, which can be caused by added polymers. Recently, it has been demonstrated that a new type of disordered motion arises at high Weissenberg numbers, so-called “elastic turbulence”. Björn Hof and his team studied the interaction between Newtonian and elastic turbulence during polymer addition. Their experiments show that a small amount of added polymers causes a delay in the transition from smooth, laminar flow to Newtonian turbulence. At higher shear rates, a new state occurs, dubbed elasto-inertial turbulence (EIT). This EIT is related to elastic turbulence, but takes into account inertia of the fluid. When a larger amount of polymers is added to the liquid, EIT sets in at lower Reynolds numbers than the transition to Newtonian turbulence, and this EIT suppresses Newtonian turbulence. Contrary to previous assumptions, the researchers show that polymers postpone or suppress the transition from smooth flow to Newtonian turbulence through EIT. Björn Hof and his team thus show experimentally and numerically (simulations by Prof. Yves Dubief, Vermont University) that a new type of turbulence, elasto-inertial turbulence, is caused by polymer addition.