  We experimentally investigate the flow of a viscoelastic fluid in a parallel
shear geometry at low Reynolds number. As the flow becomes unstable via a
nonlinear subcritical instability, velocimetry measurements show non-periodic
fluctuations over a broad range of frequencies and wavelengths, consistent with
the main features of elastic turbulence. Using the same experimental setup, we
compare these features to those in the flow around cylinders, which is upstream
to the parallel shear region; we find significant differences in power spectra
scaling, intermittency statistics, and flow structures. We propose a simple
mechanism to explain the growth of velocity fluctuations in parallel shear
flows based on polymer stretching due to fluctuations in streamwise velocity
gradients.
