Non Boussinesq convective structures in water near 4 °C

The convective properties of a water layer near 4 °C have been examined by means of local velocity measurements. The non linear density profile of such a layer produces qualitatively different behaviour from that observed in « Boussinesq » fluids. In particular a hexagonal dissipative structure is observed corresponding to an abnormal bifurcation near the threshold ; at higher Rayleigh numbers, these hexagons transform into asymmetric two-dimensional rolls. The measured velocity field is in agreement with that expected from the breakdown in the symmetry properties of the water layer.