  We report experimental results on elastic instability in a viscoelastic
channel shear flow due to only a natural non-smoothed inlet and small holes
along the channel for pressure measurements. We show that non-normal mode
instability results in elastic waves and chaotic flow self-organized into
periodically cycled stream-wise streaks synchronized by elastic wave frequency.
The chaotic flow persists above the transition with increasing $Wi$ further
into elastic turbulence and drag reduction regimes. Thus, we resolve the recent
puzzle whether strong prearranged perturbations are necessary to get an elastic
instability in parallel shear viscoelastic flow. Moreover, flow resistance,
velocity spectra decay, and elastic wave speed reveal the same scaling with Wi
as obtained in the case of strong disturbances. The remarkable result is that
all scaling behavior and streaks are found in the entire channel with small
attenuation, in sharp contrast to the flow with strong prearranged
perturbations.
