Inertio-elastic instabilities in channel flow*

The addition of small amounts of long-chain polymers to a Newtonian solvent can lead to frictional drag reduction in inertial shear flows. The interplay of viscoelasticity and inertia in a dilute polymer solution results in the emergence of inertio-elastic instabilities. The nonlinear evolution of these instabilities engenders a state of turbulence with significantly different spatiotemporal features compared to Newtonian counterpart, termed elasto-inertial turbulence (EIT). Recent numerical simulations suggest the emergence of two distinct instabilities as pathways to EIT. The first route is an inertio-elastic wall-mode instability that appears as amplified Tollmien-Schlichting waves, while the second route is an elastic center-mode instability that continues to exist at infinitesimally small levels of inertia. Despite these advancements, there is a lack of experimental work on viscoelastic channel flows. We explore the emergence and evolution of these elastic and inertio-elastic instabilities in a channel flow using schlieren imaging. The spatio-temporal structures of a viscoelastic channel flow are visualized at the centerline and in proximity to the wall in a Lagrangian manner and are compared to the turbulent structures present in the corresponding Newtonian channel flow.