12.3. How-To

12.3.1. Required data

The gas absorption spectra to be used in the simulation are located in a directory that is identified to DIRSIG through the ABSORPTION_PATH variable in the PATHS section of the configuration file. This can be entered in the PATHS section of the cfg_edit command or entered into the configuration file by hand (using the appropriate path for your installation) as:

PATHS {
  .......
  ABSORPTION_PATH = /dirs/common/dirsig/data/absorption
  .......
}
    

The absorption spectra files follow a naming convention requiring the file to have the suffix .gas. The prefix in the name of the file is the variable name identifying the gas species in the release file (shown below). An example name for a gas absorption file is NH3_50T.gas. As with all DIRSIG data files, the gas absorption spectra are in ASCII format. The gas data are in two columns, the first column is the wavenumber [cm-1] and the second is the absorption [1/ppm-m] (this file is described in more detail in the "Gas Absorption File" section in the "Support Application Manuals" part of the manual).

In addition to the gas absorption spectrum, the gas release parameters are required to describe the concentration, temperature, and dispersion of the plume (see the "Plume Release File" section in the "Support Application Manuals" part of the manual). The required parameters are entered into the plume release file (by hand) and are described (with appropriate units) below:

The "atmospheric stability" is the Pasquill-Gifford stability number and is related to the standard deviation in the wind direction. It can take on values of [10, 20, 30, 40, 50, 60], with 10 equal to an unstable atmosphere (high wind direction variability) and 60 representing a highly stable atmosphere (low wind direction variability). The parameters, along with an example plume release file, are described below.

12.3.2. Adding a stack to the scene

The source of the plume, typically a stack, is a physical object within the scene just like all other objects. It requires a three-dimensional, facetized representation with material properties assigned to it in the form of a Geometric Database File. The stack model must also be included in the overall scene description which is described to the model via the Object Database File. Remember to attribute the inside of the stack if the experimental conditions indicate the the interior of the stack will be viewed through the plume.

12.3.3. Setting up the configuration file

To enable plume modeling within DIRSIG, the configuration file must be set up accordingly. If you are using cfg_edit, under the Options tab, Model complexity options, press the button to Enable Plume Models. If you wish to output the truth maps associated with the plume model, make sure to press Generate truth images under the Output options section of this window as well. The Options window of cfg_edit is shown below.

If you are editing the configuration file by hand, it will look like this:

OPTIONS {
    .......
    ENABLE_PLUME =               TRUE
    ENABLE_TRUTH_IMAGES =        TRUE
    .......
}
    

To run the plume simulation, you must provide a separate release file. Once the Enable Plume Model is selected in the Options window, the "Plume" window is accessible from cfg_edit. This will open the following window where the name identifying the particular release is entered as well as the type of plume (only the Classic EPA/JPL model is currently supported) and the name of the release file describing the plume are entered.

If editing the configuration file by hand, include the following lines to point DIRSIG to the appropriate file:

PLUME {
    GPI_INSTANCE{
	MODEL_FILENAME = jpl_gpi
	RELEASE_FILENAME = plume.rel
    }
}
    

This will tell DIRSIG to run the JPL/EPA plume model using the release parameters contained in the file plume.rel. The name of this file is arbitrary but it must reside in the same directory as the DIRSIG configuration file unless an explicit path is specified in the RELEASE_FILENAME variable.

12.3.4. Release File Description

The release file describing the plume is an ASCII file typically residing in the same directory as the configuration file. It contains source information for the plume such as location in the scene, gas name, and release parameters (see the "Plume Release File" section in the "Support Application Manuals" part of the manual). An example release file is shown below:

GPI_RELEASE

#
# Information about the stack:
#     LOCATION - 3D location of stack base
#     HEIGHT - the height of the stack [m]
#     DIAMETER - the diameter of the stack [m]
STACK {
  LOCATION = 582.0, 741.0, 118.754
  HEIGHT   = 20.25
  DIAMETER = .8128
}

#
# Information about the effluent compounds released by the stack.  Each
# COMPOUND section includes the name of the gas, its molecular weight [g/mol]
# and the name of the spectral absorption coefficient file.
#
EFLUENTS {
  COMPOUND {
      NAME = sf6_25t
      WEIGHT = 146.0
  }
}
#
# Temporal information about the stack release.
# [time] [exit vel] [exit temp] [air temp] [atm stab] [rate1]
#
# TIME - ISO8601 date and time
# EXIT VELOCITY = Flow velocity at stack [m/s]
# EXIT TEMPERATURE - Effluent temperature at the stack [K]
# AIR TEMPERATURE - Ambient air temperature [K]
# WIND VELOCITY - Wind velocity [m/s]
# WIND DIRECTION - Wind direction [degrees]
# ATM STABILITY - Pasquill-Gifford stability number (10, 20, 30, 40, 50, 60)
# RATES - List of release rates for each gas [g/s]
#
# [time] [exit vel] [exit temp] [air temp] [wind vel] [wind dir] [atm stab] [rate1]
RELEASE {
0.0 5.0 350. 301.15 3.6 0 40 4.0
}
    

The format of the file was originally set up to allow for multiple time instances for the wind variables, but is not used in that way now. Also, if the "AIR TEMPERATURE" is not the same as that computed by MODTRAN, the air temperature computed by MODTRAN will be used and a message to that effect will be written to standard output by DIRSIG. Note that the "NAME" of the gas is also the name of the gas absorption file name, minus the .gas suffix. In this case, DIRSIG will look a file named sf6_25t.gas in the directory defined by the ABSORPTION_PATH variable.

12.3.5. Available truth outputs

Similar to the other aspects of the DIRSIG simulation, the truth used to create the simulated plume image can be written to a truth image. For the plume, the values that are written are the band integrated "Plume Radiance", the band averaged "Plume Transmission", the average "Plume Temperature", the "Plume Dilution Factor", and the "Plume Column Density" for each species in the release. Because these are written as an image, the spatial description of the plume is fully described in the plume truth outputs.

To enable the plume truth outputs through the cfg_edit GUI, once Enable Plume Model and Generate Truth Outputs have been selected (see above), in the Truth Images section of the main menu, Plume Maps can be selected to output the plume truth images described above.

If editing the configuration file by hand, the PLUME_MAPS variable must be set to true under the TRUTH_IMAGES section:

TRUTH_IMAGES {
  .......
  PLUME_MAPS  =               TRUE
  .......
}
    

With all of the above options set, run DIRSIG as normal (i.e. dirsig file.cfg >& log.out &) to create a synthetic image including the effects of a gaseous plume.