Case Studies
Los Alamos, NV - June 2003
Using Tecplot, David visualised a large eddy dispersion simulation
in an urban area. Visualising dispersion may help protect people against
chemical accidents or biological weapons attacks.
The Engineer
David S. DeCroix is a technical staff member within the Decision Applications
Division, Nuclear Design and Risk Analysis Group at Los Alamos National
Laboratory (LANL). David has worked at LANL for five years. His group
consists of 65 technical staff members and 35 contractors.
The Nuclear Design and Risk Analysis Group's mission is to provide state-of-the-art
risk assessment and risk management solutions within LANL,
as well as to the U.S. Department of Energy, the Department of Defense,
and the Nuclear Regulatory Commission.
Their solutions promote safe, reliable, and cost-effective construction
and operation of complex components, systems, and facilities.
Their core competencies include:
- Application code development
- Facility safety analysis
- Human system optimisation
- Logic-evolved decision models
- Nuclear reactor safety and reliability analysis
- Nuclear weapon system studies
- Project and program risk management
- Probabilistic system and vulnerability modeling
URBAN2000 Field Program and Simulation
The main image is a large-eddy
simulation of dispersion in an urban area. The simulation was performed
to support the URBAN2000 field program sponsored by the U.S. Department
of Energy's Chemical and Biological National Security Program (CBNP). The
field program's goals were to:
- Take turbulence and dispersion measurements to provide insight into
dispersion mechanisms.
- Provide a data set for computer model simulation comparisons and validation.
The field program measured mean temperatures, winds, turbulence within the
city street canyons as well as winds above downtown. An inert tracer gas,
sulfur hexaflouride (SF6), was then released and measured at night.
SF6 is a passive tracer not typically found in the atmosphere. Measuring
SF6 concentrations versus something like CO2, which could potentially be
ambient car exhaust, assures them that their measurements are not contaminated.
SF6 could represent any type of passive gas (not chemically reactive in
air or water vapor). This could include something like natural gas from
a ruptured pipeline. The resulting dispersion from the rupture would behave
similarly to SF6.
They were also interested in characterising night time dispersion. Night
time is one of the most difficult flow regimes to model because the atmosphere
is stably stratified — this stability tends to suppress turbulence. However,
due to the presence of buildings, the flow tends to trip, causing turbulence
and mixing. Turbulence and mixing in the stable planetary boundary layer
is not well understood and is a subject of much research in the atmospheric
science and meteorological communities.
Using the field measurements for initialisation and boundary conditions,
David modeled specific releases during the field program. This allowed his
group to better understand the dispersion mechanisms and compare field measurements
with simulation results.1
The large-eddy simulation (LES) code used to develop the model is called
HIGRAD. The advantage of LES and HIGRAD are the accurate representations
of velocity and concentration fluctuations.
Tecplot
David has used Tecplot since 1984 to analyse and visualise his complex flow
simulations. Typically he uses a series of 2-D plots, that compare the simulated
and measured winds and concentrations, and 3-D animations to view his solutions.
He also uses CFD
Analyser to calculate variables such as vorticity, helicity, as well
as to perform on-the-fly integrations.
David says, "I have been using Tecplot for many years and have yet to think
of something I want to plot that it cannot do. Tecplot allows me to quickly
assess the state of the simulation, diagnose what's occurring and make changes
as needed. It also produces report quality plots that are very easy to incorporate
in LaTeX and Microsoft Word."
According to David, Tecplot's three greatest strengths as it relates to
his work are:
- The layout files and macros. These tools enable him to plot, save
the layout file, and regenerate the exact same plot with updated simulation
data.
- CFD Analyser. This tool saves him a lot of time and effort.
- Multi-platform support. David uses Tecplot on both Linux and Windows
— the layout and macro files work on both systems without a problem.
The Plot
The main plot consists of three frames. The first frame contains building
information — essentially a 3-D matrix of seros and ones — sero if there
is a building and one if there is no building.
The shading effect on the buildings is produced with value-banking. David
enables primary corner blanking on his data (X, Y, s, building) where
building is greater than or equal to constant 0.5. He then turns off shading
and the buildings are drawn.
The second overlapping frame is a 2-D slice within the domain displaying
surface wind vectors and instantaneous ground level tracer concentrations.
The third frame uses the complete 3-D domain to display concentration
iso-surfaces with different translucency. David extracted the iso-surfaces
within the frame for each iso-surface and assigned varying values of opacity.
The buildings are shaded completely opaque.
 Notes:
- Allwine, K.J. et. al., "Overview of URBAN2000: A multiscale field
study of dispersion through an urban environment," Bulletin of the American
Meteorological Society, April 2002, Vol. 83, No. 4, pp. 521-536.
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