  The volatile transition from quiescent laminar to strongly fluctuating
turbulent dynamics in shear flows remains only poorly understood despite its
practical importance and more than a century of intense research. The
theoretical understanding of the transition process has been complicated by the
lack of a linear instability mechanism and additionally by a catastrophic
collapse of turbulence which can occur after extremely long lifetimes.
Turbulence close to onset is investigated experimentally in three different
geometries: pipe, duct and channel flow. The reverse transition from turbulent
to laminar flow is observed to be a general feature of shear flows. A critical
point is uncovered at slightly higher flow rates, where the nature of the flow
changes abruptly to the generic case of expanding turbulence. The critical
exponent associated with this phase transition is found to be universal. This
confirms a conjecture made over 20 years ago based on an analogy between fluid
turbulence and discrete models in statistical physics.
