COMBINED THERMOCAPILLARY-BUOYANCY CONVECTION IN A CAVITY .2. AN EXPERIMENTAL-STUDY

The problem of buoyant-thermocapillary convection in cavities is gover ned by a relatively large number of nondimensional parameters, and the re is consequently a large number of different types of flow that can be found in this system. Previous results give disjoint glimpses of a wide variety of qualitatively and quantitatively different results in widely different parts of parameter space. In this study, we report ex periments on the primary and secondary instabilities for acetone as th e working fluid with a Prandtl number of 4.44, and in a geometry with equal aspect ratios in the range from 1 to 8 in both the direction alo ng and perpendicular to the applied temperature gradient. We thus comp lement previous work that mostly involved either fluid layers of large extent in both directions, or consisted of investigations of strictly two-dimensional disturbances. We investigate the qualitative and quan titative features of the fluid velocity field by flow visualization an d particle tracking techniques. We observe the primary transition from an essentially two-dimensional flow to steady three-dimensional longi tudinal rolls. The critical Marangoni number for this first transition is found to depend on the aspect ratios of the system, and varies fro m 4.6x10(5) at aspect ratio 2.0 to 5.5x10(4) at aspect ratio 3.5. The structure of the steady three-dimensional how far above the transition is found to involve a nonintuitive reverse flow against the temperatu re driving due to the strongly nonlinear three-dimensional flow associ ated with the longitudinal rolls. Further, we have investigated the st ability of this three-dimensional flow at larger Marangoni numbers, an d find a novel oscillatory flow at critical Marangoni numbers of the o rder of 6x10(5). We suggest possible mechanisms that give rise to the oscillations. (C) 1997 American Institute of Physics.