Soft Matter and Complex Systems Seminar

Elastic turbulence, a chaotic flow state occurring in viscoelastic fluids at negligible Reynolds numbers, enhances fluid mixing on small scales, making it valuable for applications like lab-on-a-chip devices. Here, we show the influence of active and passive control on elastic turbulence using numerical simulations of the Oldroyd-B model in OpenFOAM®. In a two-dimensional Taylor-Couette geometry, the onset of elastic turbulence is characterised by a critical Weissenberg number, marking a transition from laminar to turbulent flow. Flow resistance and secondary-flow strength, serving as order parameters, increase with turbulence, correlating strongly with enhanced fluid mixing. Power-law scaling of velocity fluctuations aligns with experimental results, confirming the turbulent nature of the flow. Active control, implemented through imposed shear-rate modulations, shows promise for managing turbulence. Slow modulations induce complex behaviours, while fast modulations suppress turbulence. The state diagram of Weissenberg and Deborah numbers highlights the transition to turbulence and suggests an effective critical Weissenberg number to approximate the transition line. In three-dimensional von Kármán flow, elastic turbulence exhibits a subcritical transition, marked by bistable behaviour and hysteresis. Flow resistance and order parameters increase significantly, confirming turbulence. Active control applied here reduces turbulence and relaminarises the flow, mirroring results from the Taylor-Couette system. These findings underscore the potential of controlling elastic turbulence to optimise fluid mixing and stability in viscoelastic systems.