contd...

Figure 4.54: Isotherms on horizontal planes of the fluid layer, Ra=1.39x10⁴.

The local Nusselt number along the direction of the projection angle shows definite variations with distance. This suggests three-dimensionality in the longitudinal rolls which are no longer two-dimensional at the Rayleigh number studied. The loss of two dimensional structure in the rolls can also be confirmed from other projections (Figure 4.55 (c) - 4.55(f)). Since the rolls identified in the projection data are parallel to the axis, projections symmetric with respect to the axis will be identical at the limit of strict two-dimensionality. A comparison of projections obtained from summetrically placed angles, namely (30, 150) and (60, 120) shows qualitative similarity. This strengthens the suggestion of rolls, but also highlights their unequal sizes and their three-dimensional nature along the roll axis.

Figure 4.55: Experimentally obtained line-integrals of Nusselt numbers for both the walls,

The Nusselt surface obtained from the reconstructed temperature field is shown for each of the hot and cold surfaces in Figure 4.56. Along the rolls axis, the Nusselt number surfaces of the top and bottom plates are oppositely oriented. Heat transfer form the lower to the top plate by a buoyancy-driven roll is associated with the peaks and valleys of the Nusselt number surface.