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ENVIRONMENT |
han a constant-velocity wind
the measurementstations, and dot
beginning of spill 1. The crosses in-
The ATMAScode models atmospheric transport by solving the
advection-diffusion equation,
which simply states that the con-
tions. Figure 2 shows a contour plot
of the concentration isopleths 1 m
above the ground 60s after the
this experiment, the wind carried
model.
plots of marker-particle distribu-
centration of pollutants at a given
400
point is determined by two
I
dicate the measurementstations.In
muchof the plume to theleft of the
detector array.
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|
processes: diffusion (the simple
spreading that takes place without
wind and the effects of gusts and
eddies) and advection (blowing
away in the average wind). ATMAS
uses the particle-in-cell method,°
which represents the mass of the
emitted pollutant by marker particles whose trajectories through
space are calculated according to
_an equation of motion derived
from the advection-diffusion equation. The array of marker particles
gives us a graphic threedimensional representation of the
vapor plume.
To use the ATMAScode, we
must specify boundary conditions,
the characteristics of the LNG
vapor source, and measuredvalues
of wind speed and direction as a
function of time. Forthe first three
experiments we assumed a constant emission rate from a round
LNG pool of constant size for a
fixed period of time. In the fourth
0.585 g/m®
300 + |
~
E
i
8 200 -
—
a
2
—
100 }-
29.2 g/m?
test, because of significant changes
87.7 g/m"
in wind velocity, we found it
necessary to simulate the way in
which the LNG pool spreads over
175 g/m?
0
the water surface. We did this by
turning on a series of vapor sources
with successively larger areas from
time to time during the calculation.
The various 5-m* tests are
described in Table 1.
The ATMAScode displays the
x
0
i
.
50
D
|
100
Distance — m
|
150
;
200
Concentration isopleths calculated by the
Oi,
ATMAScode for the vapor plume 60 s
after the start of thefirst 5-m’ spill experiment. The lower
flammability limit (33 g/m? at 300 K) eventually extended to about 380 m.
results of its calculations in contour
plots of gas concentration, graphs
of concentration vs time for each of
31