Computational Model for the Rise and Dispersion of Wind-Blown, Buoyancy-Driven Plumes. Part 1. Neutrally Stratified Atmosphere.
Computational Model for the Rise and Dispersion of
Wind-Blown, Buoyancy-Driven Plumes. Part 1. Neutrally
Stratified Atmosphere.
(1609 K)
Zhang, X.; Ghoniem, A. F.
Atmospheric Environment, Vol. 27A, No. 15, 2295-2311,
1993.
Sponsor:
National Institute of Standards and Technology,
Gaithersburg, MD
Minerals Management Service, Herndon, VA
Keywords:
buoyant flows; computation; entrainment; fire phases;
large fires; simulation; urban fires; wildland fires;
wind effects
Abstract:
A multi-dimensional computational model for the rise and
dispersion of a wind-blown, buoyancy-driven plume in a
calm, neutrally stratified atmosphere is presented.
Lagrangian numerical techniques, based on the extension
of the vortex method to variable density flows, are used
to solve the governing equations. The plume rise
trajectory and the dispersion of its material in the
crosswind plane are predicted. It is found that the
computed trajectory agrees well with the two-thirds
power law of a buoyancy-dominated plume, modified to
include the effect of the initial plume size. The
effect of small-scale atmospheric turbulence, modeled in
terms of eddy viscosity, on the plume trajectory is
found to be negligible. For all values of buoyancy
Reynolds number, the plume cross-section exhibits a
kidney-shaped patern, as observed in laboratory and
field experiments. This pattern is due to the fomation
of two counter-rotating vortices which develop as
baroclinically generated vorticity rolls up on both
sides of the plume cross-section. Results show that the
plume rise can be described in terms of three distinct
stages: a short acceleration stage, a long
double-vortex stage, and breakup stage. The induced
velocity field and engulfment are dominated by the two
large vortices. The effect of a flat terrainon the
plume trajectory and dispersion is found to be very
small. The equivalent radii of plumes with different
initial cross-sectional aspect ratios increase at almost
the same rate. A large aspect-ratio plume rises slower
initially and then catches up with smaller aspect-ratio
plumes in the breakup stage. The Boussinesq
approximation is found to be valid if the ratio of the
density perturbation to the reference density is less
than 0.1.
