5.3. How-To
This "how-to" section focuses on how the material subsystem works, how the user can control what materials are applied to what objects, how and where materials are defined, and how to tell DIRSIG which materials to use.
5.3.1. Assigning materials to scene elements
The assignment of materials to elements in the scene is part of the scene construction process. The basics of this process is discussed in the chapter entitled "Basic Scene Construction". To summarize, the object geometry files that the DIRSIG model loads (see the section describing Geometric Database Files) contain geometry elements (usually facets) that were each assigned a material ID number. This material ID number is a reference to a material description in the material database file described in the next section.
The material ID number assigned to a specific geometry element is usually chosen from an existing material database file. At this time, there is not a standard taxonomy used to define the material ID system. Therefore, a user "A" can create a material database where material ID #1 is a variety of concrete, and user "B" can create another database where material ID #1 is a variety of grass. Therefore, it is very important for the user to insure that the logical connection between the material database and a given object is not broken. With the lack of a user community-wide material ID system, it is strongly suggested that companies and organizations with multiple users establish their own internal "master material database" file that everyone uses.
5.3.2. The materials database file
The materials that are used in a specific DIRSIG simulation are pulled from a user-supplied "material database". This database takes the form of another structured file that can be created and modified by the user. The format of this file is described in the "Support Application Manuals" part of this manual in the Reference section entitled "Material Database File".
Table 5-1. Basic Material Properties
| Variable Name | Usage | Units | Description | |
| MATERIAL_ID | Required | Material ID number | ||
| MATERIAL_NAME | Required | Name of material | ||
| SPECIFIC_HEAT | Required | [L/C] | Specific heat | |
| THERMAL_CONDUCTIVITY | Required | [W/m*K] | The thermal conductivity | |
| MASS_DENSITY | Required | [gm/cm3] | Mass density | |
| THICKNESS | Optional | [cm] | Slab thickness | |
| SOLAR_ABSORPTION | Optional | (fraction) | Mean, broad-band solar absorptivity | |
| THERMAL_EMISSIVITY | Optional | (fraction) | Mean, broad-band thermal emissivity | |
| EXPOSED_AREA | Required | (fraction) | Surface exposed area term | |
| SELF_GENERTAED_POWER | Optional | [L/hr] | Internal (self-generated) power source term | |
| OPTICAL_DESCRIPTION | Optional | (see below) | Controls surface opacity | |
| EMISSIVITY_FILE | Required | Name of spectral emissivity file | ||
| SPECULARITY | Required | (fraction) | Surface specularity | |
| EXTINCTION_FILE | Required * | Name of spectral extinction file | ||
| DOUBLE_SIDED | Optional | Boolean | Control double-sided attribute | |
The units for the specific heat are [L/C] where [L] is a Langley unit and [C] is degrees Celsius. A Langely is equal to 1 calorie/cm^2 or 1 watt/m^2 is equal to 0.086 L/hour. Since several of the material properties are unique to the model, the more uncommon variables are discussed in the following sections.
5.3.2.1. Exposed Area
The exposed area term is used to communicate the amount of exposure the facet has to the ambient air. For example, a facet may be a slab of parking lot asphalt that is exposed to the air only on the top side. In contrast, a facet may be a vertical panel or fence that is exposed to the ambient air on both the front and back sides. The differences between these two cases is very important since the convective load on each surface differs drastically. The sign of this exposure term is used to indicate one of these two possibilities. In addition to controlling the convective processes, this term plays a roll in the radiational loading of a surface (for example, describing how much of a surface is exposed to the cold sky). Using the original documentation from the DCS Corporation, we provide the Table Table 5-2 as a guideline:
5.3.2.2. Self-Generated Power
The thermal model allows the user to specify an internal or self-generated power term that can be used to model a simple internal heat source within the surface. Setting the optional SELF_GENERATED_POWER term to a non-zero value generally biases the temperature solution in the same signed direction. This can be a useful tool for modeling internal heat sources in buildings or even the biological activity in living vegetation.
5.3.2.3. Broad-Band Absorption/Emissivity Values
The thermal model utilizes solar absorption and thermal emissivity coefficients in the radiational loading computations. These two coefficients are derived from the spectral emissivity curves assigned to each material. If the emissivity file assigned to a given material contains multiple curves, then each coefficients will be computed for each curve and used in conjunction with the texture mapping routines.
The SOLAR_ABSORPTION and THERMAL_EMISSIVITY variables allow the user to shift the mean value of these respective coefficients. The most common reason values are set is to account for biased values that are derived from incomplete spectral emissivity curves. For example, in cases where the user has only thermal spectral emissivity values, the SOLAR_ABSORPTION variable will have to be set so that the thermal model knows how much solar insolation is absorbed by the surface.
5.3.2.4. Emissivity File
The primary optical property for a material is the "emissivity" of the surface. The spectral emissivity defines efficiency of the surface to radiate thermal emitted photons with respect to a blackbody radiator. The reason that emissivity is the primary optical property and not reflectance dates back to the origins of the DIRSIG model, which was originally written to perform strictly in the thermal infrared region of the spectrum. When the model was extended to the reflective regions of the EO spectrum, the concept of using emissivity as the primary optical property remained. The model assumes that surfaces are at thermodynamic equilibrium. Therefore, Kirchoff's law holds and the reflectance can be computed as 1 - emissivity. There is a tutorial at the end of this chapter that provides an example on how to incorporate a reflectance curve into the model.
The emissivity for the surface is supplied by a DIRSIG specific spectral emissivity file. The format of this file is described in the "Spectral Emissivity File" section in the "Support Application Manuals" part of the manual.
5.3.2.5. Extinction File
Spectral extinction applies to the two transmissive material types that were discussed at the start of this chapter. The spectral extinction coefficients allow the transmission for an arbitrary path (of length "d") to be computed as:
Regardless if you are modeling a plate-style transmission surface or a volume-style transmission surface, you need to supply a spectral extinction file. If the surface of interest can be described by a transmission curve that you have in hand, you will need to convert it to a set of extinction coefficients. See the one of the tutorials in this chapter for an example.
The extinction coefficients for the surface is supplied by a DIRSIG specific spectral extinction coefficient file. The format of this file is described in the "Spectral Extinction File" section in the "Support Application Manuals" part of the manual.
5.3.2.6. Specularity
The SPECULARITY variable describes the ratio of the reflected radiance from the specular direction over the total reflected radiance. For example, a material that is extremely diffuse will have a SPECULARITY value of 0.0 and a very specular material will have a value of 1.0. In future releases, this term will be replaced by the true bi-directional reflectance distribution function for the material.
5.3.3. Run-time material assignment
The material database is supplied to the simulation via the MATERIAL_FILENAME variable in the SCENE section of the Input Configuration File. The file is searched for using the MATERIAL_PATH in the PATHS section of the Input Configuration File.
When the model initializes at the start of program execution, it reads all of the supplied Geometric Database Files into memory. As each file is loaded, the required material ID for each geometric element (facet) is recorded in "material shopping list". When all of the geometry is loaded, the specified material database file will be opened and the material IDs in the "shopping list" are searched for. If one of the requested material IDs is not found in the file, an error is issued.