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Computational Model for the Rise and Dispersion of Wind-Blown, Buoyancy-Driven Plumes. Part 2. Linearly Stratified Atmosphere.


pdf icon Computational Model for the Rise and Dispersion of Wind-Blown, Buoyancy-Driven Plumes. Part 2. Linearly Stratified Atmosphere. (2341 K)
Zhang, X.; Ghoniem, A. F.

NIST GCR 93-637; 47 p. December 1993.

Sponsor:

National Institute of Standards and Technology, Gaithersburg, MD

Available from:

National Technical Information Service
Order number: PB94-143427

Keywords:

buoyant flows; computation; entrainment; fire phases; predictive models; large fires; simulation; urban fires; wildland fires; wind effects

Abstract:

A multi-dimensional computational model of wind-blown, buoyancy-driven flows is applied to study the effect of atmospheric stratification on the rise and dispersion of plumes. The model utilizes Lagrangian tranpsort elements, distributed in the plane of the plume cross section normal to the wind direction, to capture the evolution of the vorticity and density field, and another set of elements to model the dynamics in the atmosphere surrounding the plume. Solutions are obtained for a case in which atmospheric density changes linearly with height. Computational results show that, similar to the case of a neutrally stratified atmosphere, the plume acquires a kidney-shaped cross section which persists for a long distance downstream the source and may bifurcate into separate and distinct lumps. Baroclinic voricity generated both along the plume boundary and in the surroundings are used to explain the origin of the distortion experienced by the plume and inhibiting effect of a stratified atmosphere, respectively. The vorticity within the plume cross section forms two large-scale coherent eddies which are responsible for the plume motion and the entrainment. Prior to reaching the equilibrium height, the computed plume trajectory is found to follow the two-thirds law, when extended to include the initial plume size, reasonably well. The entrainment and the added mass coefficients, 0.49 and 0.7, respectively, are obtained from the numerical results over a wide range of the buoyancy ratio, defined as the ration between the plume buoyancy and the degree of background stratification. In the case of strong stratification, the plume trajectory shows weak, fast decaying oscillations around the equilibrium height. The origin and decay of these oscillations are explained using a simple analytical model.