Convection at Ra=4.02 x 10⁴

Figure 4.56: Nusselt number surfaces for top and bottom walls, Ra=1.39x10⁴

Analysis of results from the experiments at a Rayleigh number of 4.02 x 10⁴ are presented next. In the higher Rayleigh number experiment, formation of a hot buoyant plume arising from the bottom plate and advancing towards the cold top plate has been observed. Since the discussion is based on two views, structures referred to below are to be interpreted are representative of the flow field. The temperature surfaces, namely the temperature variation over horizontal planes, are shown in Figure 4.57. Three horizontal planes, namely , have been considered. A clearer picture emerges when isotherms at the corresponding planes over an assembly of four adjacent cells are examined. This is shown in Figure 4.58. Here, each cell corresponds to the portion of the cavity reconstructed from the interferograms. On assembly, the collection of temperature surfaces over the three planes clearly shows the structure of a rising plume. The repeating roll-like structure seen the from the both the view angles suggests that a cubic cell exists inside the cavity. Mukutmoni and Yang [102] have shown the formation of an oscillatory polygonal planform structure for intermediate aspect ration boxes for a fluid of Prandtl number equal to 3.5 at a Rayleigh number of 2400. The present observation that flow organizes in the form of cubic cells in the cavity is similar to their conclusion. It is also in agreement with the authors’ previous study based on partial projection data with two view angles [79].

Figure 4.57: Temperature surface in the cavity at three horizontal planes, Ra=4.02x10⁴

Figure 4.58 shows that the size of the heated region increases as one moves towards the heated lower wall. The buoyant plume rises from the center of the cluster of four adjacent repeated cubic cells. Each cubic cell can be visualized as being divided along its diagonal plane with high- and low-bulk-fluid temperatures on each side. Thus, the fluid rising along the center descents uniformly around the plume in the four quadrants. The assembly of four cells encloses a set of four rolls, all of which raise hot fluid jointly along the central vertical axis, which after being cooled, descents all around towards the lower surface. While it is possible to identify a cubic cell that is isolated from a thermal viewpoint (for example from the interferograms ), it should be noted that the associated velocity field need not be isolated. In fact, the velocity field in individual cubic cells will interact and an orderly pattern for flow can be discerned only over a collection of cells.