Rayleigh-Bénard convection for viscoplastic fluids

The influence of rheological and interfacial properties of yield stress fluids is investigated on the onset of the Rayleigh-Benard convection. Different Carbopol registered (B.F. Goodrich) gels are used in a circular cell for Rayleigh-Benard experimental setup. The influence of the boundary conditions is also investigated by controlling either slip or no-slip conditions. The onset of thermoconvection is shown by measuring temperature differences and also by using shadowgraph flow visualization. Experimental results show that convection occurs in the range of our experiments. Considering Carbopol gels as elasto-plastic materials with a yield stress tau sub(y), a generalized Rayleigh number is obtained: Ra sub(g) = Y super(-1), with Y the yield number, which represents the balance between the yield stress of the gel and the buoyancy effects. The results show that the Rayleigh number is proportional to d, the height of the setup, and that the control parameter is the yield number at the onset of convection. Critical values of Y super(-1) have been determined for slip conditions 1/Y sub(c) super(S) approximately 40 as well as for no-slip conditions 1/Y sub(c) super(NS) approximately 80. It highlights that the change in surface conditions affect significantly the critical conditions.