  We report the presence of traveling Elastic Waves in experiments featuring a
shear flow in the very low Reynolds number regime (Re < 0.1) with no external
curvatures or internal perturbations in the channel design. The classic
Kelvin-Helmholtz type shear flow with an inflection point in its velocity
profile is generated by co-moving dilute polymeric solutions moving at
different speeds within a Hele-Shaw cell. Notably, this is the first
observation of Elastic waves in straight channels, with no internal or external
wall curvatures or physical perturbation sources present in the flow or on the
channel walls, making the differential velocity of the parallel streams the
only possible source of generation of Elastic Waves. The interface between the
co-moving streams displays a wavy and unsteady nature for all studied cases.
Furthermore, the frequency and wave speed of the Elastic Waves are found to
scale with the Weissenberg number, which is based on the velocity difference
between the streams. This finding contributes to the emerging body of evidence
regarding the presence of Elastic Waves in straight channels with minimal
perturbation sources (Steinberg 2022).
