Coherent vortical structures in large-eddy simulations of buoyant plumes in a crossflow

The structure and dynamics of buoyant plumes arising from isolated surface-based heat sources in the presence of a vertically sheared ambient atmospheric flow are examined using a large-eddy simulation (LES) model. Results from the LES model indicate that the interaction between the vorticity in the crossflow boundary layer and the buoyancy generated vorticity in the plume results in the generation of several types of coherent vortical structures that appear to dominate the structure and behavior of the plume.

Specifically, these simulations exhibit: (i) a counter-rotating vortex pair aligned with the plume trajectory that can lead to a downstream bifurcation of the plume; (ii) shear-layer (i.e., transverse) vortices on the upstream face of the plume; and (iii) vertically oriented wake vortices that form with alternating sign and regular period in the turbulent wake between the ground and the bent-over plume. In most cases, the simulated plumes do not appear to exhibit self-similar Gaussian structure, even in a time-averaged sense. The basic structure and behavior of the simulated plumes will be presented, along with the dependence of this structure and behavior on parameters such as the speed of the crossflow and the intensity of the heat source (i.e., buoyancy forcing).

Laboratory experiments of buoyant plumes in a crossflow using a recirculating water flume are also presently being planned, and it is hoped that preliminary results of these experiments will also be presented.