THERMALLY INDUCED INSTABILITIES OF TWO-LAYER FLUID SYSTEMS

Thermally induced instabilities of two-layer fluid systems have been studied by the use of both a linear and non-linear analysis. Two essentially different destabilizing mechanisms have been identified, namely buoyancy and surface tension. A general procedure has been developed to determine the relative importance of each mechanism.; There are many problems, formerly analyzed as single layers which truly are problems of two-layer systems. For example, the original Benard experiment was a two-layer system. Previous investigators had to impose stress and heat transfer conditions at the interface which could not be known a priori. This study attempts to elevate these problems from the qualitative to the quantitative realm by accurately formulating the interfacial boundary conditions. Comparisons with experiment were performed to attain a level of confidence.; The investigation was motivated from the study of the geophysical problem of a body of water cooled by the atmosphere. Two distinct modes of the instability were identified. In one mode, the air is in convection by itself while the water is essentially stagnant. In the other mode, both the water and air are in convective motion. The stability curves, associated with each mode, actually intersect each other. This is a new phenomenon which does not occur in the analysis of single layers.; The effects of surface tension and surface curvature were explored in depth. The reinforcing nature of the destabilizing mechanisms, surface tension and buoyancy, was demonstrated. The relationship between the mean surface tension and surface curvature was also investigated.