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