  Directed percolation (DP), a universality class of continuous phase
transitions, has recently been established as a possible route to turbulence in
subcritical wall-bounded flows. In canonical straight pipe or planar flows, the
transition occurs via discrete large-scale turbulent structures, known as puffs
in pipe flow or bands in planar flows, which either self-replicate or
laminarize. However, these processes might not be universal to all subcritical
shear flows. Here, we design a numerical experiment that eliminates discrete
structures in plane Couette flow and show that it follows a different, simpler
transition scenario: turbulence proliferates via expanding fronts and decays
via spontaneous creation of laminar zones. We map this phase transition onto a
stochastic one-variable system. The level of turbulent fluctuations dictates
whether moving-front transition is discontinuous, or continuous and within the
DP universality class, with profound implications for other hydrodynamic
systems.
