Entrainment and stirring in viscous plumes

Abstract

Buoyant plumes in very viscous fluids, although laminar, entrain their surroundings as a result of conduction of heat. Once heated, the fluid takes part in the plume motion and is stirred within the plume by internal circulation. Entrainment modifies the behavior of plumes by enlarging and cooling them. Similarity solutions for several cases, including isolated ‘‘thermals’’ having fixed buoyancy, thermals having a constant internal heat generation, the initial stages of a plume formed by a continuous volume flux (a starting plume), and steady plume conduits fed by a constant source flux are reviewed. Both laboratory experiments and computations of particle paths show that stirring in thermals and starting plumes also leads to well-defined shapes for passive tracers, shapes that depend upon the magnitude of an appropriate Rayleigh number. The computations of particle paths are extended to the case of a steady plume conduit ascending through a horizontal shear flow in order to show the internal structure of the plume. Implications of these models for mantle plumes and volcanic hotspots are discussed briefly. It is possible that the structured chemical heterogeneities produced by stirring within plumes may contribute to observed chemical variability of lavas generated by plume activity.