Exact coherent state in purely elastic pressure-driven channel flow

Dilute polymer solutions do not flow like Newtonian fluids: Their flows exhibit instabilities at very low Reynolds numbers that are driven not by inertia, but rather by anisotropic elastic stresses. Further increase of the flow rate results in a chaotic flow, often referred to as purely elastic turbulence. The mechanism of this new type of chaotic motion is poorly understood.

In this talk we present the first coherent state in purely elastic parallel shear flows. We perform direct numerical simulations of a model shear-thinning viscoelastic fluid driven by an applied pressure gradient through a two-dimensional channel. By starting from a linearly unstable mode recently discovered by Khalid et al. (arXiv:2103.06794) at very large flow rates and very low polymer dilution, we demonstrate that this instability sub-critically connects to significantly higher values of polymer concentration and lower flow rates, rendering these structures experimentally relevant. We explain the physical mechanism of their stabilisation and discuss their relevance to purely elastic turbulence.