At , the interferogram shows nearly straight dense fringes parallel to the horizontal walls. These are indicative of near-parallel flow near the hot and the cold walls.  The flow turns at the corners, leading further to fringe curvature and separation. The flow field is symmetric about the centerline of the cavity.  These trends are well-reproduced by schlieren and shadowgraph images.

The movement of the fluid medium in the cavity cross-section has the shape of a roll.  The roll size can be determined directly from the optical images since the fringes as well as the intensity fields should turn along at corners with the local velocity vector. The roll sizes of Figure 5.22 have been compared in Table 3, and are found to be quite close. The three experimental techniques reveal a reduction in the roll size with increasing Rayleigh number.

Table 3: A comparision of roll sizes of convection seen relative to the cavity height in the three optical techniques; effect of increasing Rayleigh Number.

Rayleigh Number	Interferometry	Schlieren	Shadowgraph
	0.270	0.278	0.310
	0.258	0.252	0.252
	0.210	0.242	0.220