Nonmodal elastic instability and elastic waves in weakly perturbed channel flow

Adding tiny amounts of long polymer chains to a fluid dramatically alters its behavior. In this work, we show that channel flows such viscoelastic fluids at vanishingly small Reynolds numbers are highly sensitive weak disturbances, even though they linearly stable. We thus experimentally disturbance elicits strong fluctuations with continuous spectrum in three different regimes and elastic waves far from the excitation location through non-modal bifurcation.