Onset of Periodic Convection in a Vertical Slot.

New observations of the convective states in a vertical slot in the neighborhood of the transition to multi-cellular motion are reported. Experiments in large Prandtl number fluids reveal that the departure from unicellular flow develops gradually in the central region of the slot, with an initial closure of the distorted primary flow. Subsequently, as the Rayleigh number increases, a multiplicity of transverse stationary eddies occur in the same vicinity within the primary flow. This motion is a precursor to the familiar vertically stacked transverse rolls originally found by Elder (1965). Similar observations in an air-filled slot display a very different response. In this case the initial perturbation of the base flow, resulting in closure of the unicellular motion, takes place near the end walls.