PLOT OF THE MONTH
Visualizing Chemical Dispersion in Populated Regions
Los Alamos, NV
Contributed by:
David S. DeCroix
Decision Applications Division
Nuclear Design and Risk Analysis
Los Alamos National Laboratory
Using Tecplot, David visualized a large eddy dispersion simulation in an urban area. Visualizing 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.
- Application code development
- Facility safety analysis
- Human system optimization
- 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 characterizing 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 initialization 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 analyze and visualize 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 Analyzer 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 Analyzer. 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 zeros and ones - zero 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, Z, 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.