Intense Atmospheric Vortices Associated with a 1000 MW Fire

Abstract

Observations of vortices of various types produced in a large thermal plume are described. The apparatus used to generate the plume is the Météotron, an array of 105 fuel oil burners with a total heat output of approximately 1000 MW. Three types of vortices have been observed: 1) large counter-rotating rolls in the downstream plume, 2) intense small-scale vortices resembling very strong dust devils seen at the surface on the downwind side of the plume, and 3) very large columnar vortices produced when the lower portion of the plume goes into rotation as a whole. Three mechanisms leading to the concentration of vorticity necessary to produce these vortex types are discussed. These include tilting and stretching of horizontal vorticity present in the environmental wind field, generation of vorticity within the plume by the action of buoyancy and drag forces, and convergence of preexisting background vorticity from the environment. It is concluded, based on these observations and physical considerations, that the generation of vortices of moderate intensity is to be expected in large plumes, be their source a forest fire or an industrial operation.