Transitions to turbulence in helium gas

Experimental study in gaseous helium at low temperature (4 K) of thermal convection up to a Rayleigh number $R={10}^{11}$. Three regimes are observed, a chaotic state up to $R=2.5\mathrm{}\times {10}^5$, a soft-turbulence state up to $R=4\mathrm{}\times {10}^7$, and then a hard-turbulence state. We associate those three regimes to the boundary-layer formation and dynamics.