Mechanism of vorticity amplification by elastic waves in a viscoelastic channel flow

Abstract Inertia-less viscoelastic pipe and channel flows exhibit a non-normal mode elastic instability independent of perturbation strength and despite their linear stability. The non-modal instability is identified mostly by a direct transition from laminar to chaotic flows. At higher velocities, transitions to elastic turbulence and further drag reduction occurs accompanied by elastic waves in three regimes. Here we demonstrate experimentally that the elastic waves play a key role in amplifying wall-normal vorticity fluctuations by resonant pumping of energy, withdrawn from the mean flow, into wall-normal fluctuating vortices. Indeed, the flow resistance and rotational part of the wall-normal vorticity fluctuations depend linearly on the intensity of elastic waves in three regimes. The higher (lower) the elastic wave intensity, the larger (smaller) the flow resistance and rotational vorticity fluctuations. The suggested physical mechanism of the resonant vortex amplification by the elastic waves above the instability onset in three flow regimes recalls the Landau damping in magnetized relativistic plasma. It is universal not only for various viscoelastic parallel shear flows but also generally for flows with both transverse waves and wall-normal vortices, such as the Alfven waves interacting with vortices in turbulent magnetized plasma.