The 1978 Freeman Scholar Lecture: Buoyancy Induced Fluid Motions Characteristic of Applications in Technology

Abstract

Buoyancy induced flows abound in nature and in our enclosures and devices. They arise through the interaction of a body force, like gravity, with a density difference. The difference is often caused by the diffusion of thermal energy and/or chemical species. Here we are not principally concerned with environmental matters, but rather with the engineering of buoyancy-induced transport in technology, up to a size scale of a number of meters. Early pioneering insights concerning such transport began about a century ago. Increasingly intensive study in the last thirty years has further clarified our conceptual basis. A single generation of researchers has explained in detail many of the simplest flows and transport, those immersed in an extensive and quiescent ambient medium. Study is well along for transport within enclosed bodies of fluid, and, in addition, for the most subtle kinds of transport, those that combine the features of both the external and internal flows. This work considers only the first group, external flows. There is a large recent accumulation of information of great value to design, in a wide range of engineering application. However, the emphasis here is not on equations and correlations, but rather on concepts and on the basic nature of many of the processes which very commonly arise. This kind of information is the basis of understanding inter-relations, multiple flows and conceptual design. A discussion of the agency of buoyancy drive is followed by consideration of the mechanisms arising in different kinds of vertical and inclined laminar flows. Then the progression of laminar flows to turbulence is considered, with a quantitative parameter for transition. The effects of a temperature-caused density extremum on transport, as in cold water, is considered in terms of mechanisms and regimes. Commonly arising multi-buoyancy effect transport, as in the combined thermal transport and species diffusion which occurs around our bodies, is considered in a general way and also specifically for melting ice in saline water. A series of additional kinds of transport effects are next considered, first that of changing buoyancy forces due to ambient fluid density stratification. An example of the formation of one flow out of another is then given. A similar but different kind of multiple flow interaction is detailed in the mutual interaction mechanisms of multiple plumes and of plumes and surfaces. Then, the curious ways in which flows separate from the surfaces which generate them are clarified, along with the resulting transport behavior. The examples were chosen throughout to demonstrate most important aspects and mechanisms in common applications. Many of the basically different phenomena, among the well-understood ones, are included. Of course the choice of coverage is the writer’s and the following account does represent a single point of view.