Experimental evidence of a phase transition in a closed turbulent flow.

We experimentally study the susceptibility to symmetry breaking of a closed turbulent von Kármán swirling flow from Re=150 to Re≃10⁶. We report a divergence of this susceptibility at an intermediate Reynolds number Re=Re(χ)≃90,000 which gives experimental evidence that such a highly space and time fluctuating system can undergo a "phase transition." This transition is furthermore associated with a peak in the amplitude of fluctuations of the instantaneous flow symmetry corresponding to intermittencies between spontaneously symmetry breaking metastable states.