The intrinsic symmetry of the cavity and associated boundary conditions affords the formation of many cells whose axes are parallel to the cavity length. A single cell is preferred over other configurations owing to several factors that include mild imperfections in the experimental apparatus. A qualitative explanation can be given in terms of the energy requirement to sustain multicellular flow. In a square cross-section and at modaerate Rayleigh numbers, one can expect unicellular flow to consume the least energy and hence be most likely to appear in the cavity. The present set of experiments, however, provide evidence of bicellular flow for small-time and is discussed in the following sections

Figure 4.44: Fringes near the cold wall during transient convection. (a) Original, (b) Fourier-filtered and (c) Fringe-extracted Image,