Different routes to turbulence in high Prandtl number convection and in rectangular cells: Influence of the aspect ratio and of the structure

Abstract

The permanent control of the whole structure using optical methods combined with the study of the dynamics of the velocity has provided a considerable gain of knowledge of the mechanisms related to some routes to turbulence.

We can clearly separate two different kinds of turbulence. The first, the phase turbulence, is present in large boxes ; the presence of structure defects induces slow erratic motions of the rolls pattern. This kind of turbulence obtained directly from the steady convection is obviously related to that occurring in systems with a high number of degrees of freedom.

On the contrary in small boxes where this phase turbulence cannot take place, the routes to turbulence show a finite number of bifurcations from the steady to the chaotic regime. The observed behaviours are in agreement with the properties of strange attractors occurring in systems with small number of degrees of freedom. Furthermore, it is striking to note the analogy of results obtained here with that 2 corresponding to convection in low Prandtl number fluid^{2 3 5}.