Pattern selection in Benard convection experimental results in a annulus.

Benard convection is experimentally studied in an annular cell with free-free boundary conditions: a silicon oil floating on mercury and in contact with air. Surface tension forces are comparable to buoyancy forces in driving the convection. The transition to a cellular pattern of rolls is via a forward bifurcation. Above the onset there is a broad basin of stable roll patterns differing in their wavelength. Inserting a radial wall in the cell reduces considerably the basin of stable patterns and modulates their wavelength.