Transient two dimensional natural convective flows around a pair of heated vertical parallel plates in an isothermal enclosure are simulated using the CSIRO finite element package Fastflo for a wide range of Ra.
The air is initially at a uniform temperature T0 when the heating and cooling commence. The evolution of the resulting transient flows are simulated. At low and moderate Rayleigh numbers (Ra < 105) the flows are asymptotically steady. At higher Ra, the plume becomes unstable and commences a wavering motion. The amplitude of the motion increasing rapidly with Ra. The fully developed high Ra unsteady flows involve a highly active plume that alternatingly generates small intense vortices that convect around the main recirculation zones.
Nu are calculated and compared to those of earlier steady state simulations and with experimental measurements. At intermediate Ra there is close agreement. This deteriorates somewhat at higher Ra. We interpret this difference as arising from the growth of three dimensional structures in the flow. Surprisingly the steady and unsteady flows have identical Nu for the same Ra. This indicates that the plume motion and the resulting unsteadiness does not affect the heat transfer rates for this particular application.
The stability of the plume, the onset of plume motion and the stabilisation of the plume by net heating of the air are discussed. The structure of the asymptotic unsteady flows are also examined and the plume activity and the small scale flow structure are related to a turbulence intensity.