  Dilute polymer solutions are known to exhibit purely elastic instabilities
even when the fluid inertia is negligible. Here we report the quantitative
evidence of two consecutive oscillatory elastic instabilities in an elongation
flow of a dilute polymer solution as realized in a T-junction geometry with a
long recirculating cavity. The main result reported here is the observation and
characterization of the first transition as a forward Hopf bifurcation resulted
in a uniformly oscillating state due to breaking of time translational
invariance. This unexpected finding is in contrast with previous experiments
and numerical simulations performed in similar ranges of the $Wi$ and $Re$
numbers, where the forward fork-bifurcation into a steady asymmetric flow due
to the broken spatial inversion symmetry was reported. We discuss the plausible
discrepancy between our findings and previous studies that could be attributed
to the long recirculating cavity, where the length of the recirculating cavity
plays a crucial role in the breaking of time translational invariance instead
of the spatial inversion. The second transition is manifested via time
aperiodic transverse fluctuations of the interface between the dyed and undyed
fluid streams at the channel junction and advected downstream by the mean flow.
Both instabilities are characterized by fluid discharge-rate and simultaneous
imaging of the interface between the dyed and undyed fluid streams in the
outflow channel.
