| This Model of the Month highlights the modeling of natural environments and canopy clutter with our software. Modeling the forest canopy can be important for generating accurate temperatures or IR signatures of objects in such environments. For example, some of our users study the use of tree canopies to create passive "cool zones" in urban park development. Through simulation, the ideal size and location of trees can be planned to provide improved thermal comfort at specific times of the day.
IR Signature researchers utilize clutter to predict its effects on signature in a particular IR wave band. For example, searchers may use IR cameras to locate lost hikers that have holed up in a tent on a mountainside. The model shown here could be used to determine the best time of day (or night) to search for the tent and from what distance it would be visible through the clutter of the tree canopy.
Tents with tree canopy.
We began our workflow by creating our mesh geometry in Alias|Wavefront's Maya 4.5 software, which allows direct creation of mesh geometry without surfaces. Deciduous and evergreen trees were created, then populated and scaled into a small forest. The mesh was exported to our software using DXF file format.
Three tents were created separately in Rhino3D software, and imported, and placed onto the terrain. The large wall tent included a wood stove with vent through the roof. Though most tent chimney systems use a neoprene interface between the stove pipe and tent roof, this component was left out, and the chimney surface was placed adjacent to the canvas. The chimney pipe was modeled as a double wall pipe, an adaptation often used to improve the safety of the heating system.
Tents set up on Terrain.
5000 BTU/hr (1400 watt) of imposed energy on a simple box geometry and chimney inside the tent. The external chimney was heated by a 0.1 m/s flow of 500C air. The chimney vent plume was not modeled. The terrain was defined as fresh snow cover, to simulate a mid-winter situation.
Wood stove and chimney modeled inside large 5mX5m (hidden) wall tent. Tent is hidden in this view.
Heat sources in the small tents were modeled based on human metabolic activity. In this case, there was assumed to be 1 person in the smallest tent (100 Watts imposed on the air inside the tent), 3 persons in the medium-sized dome tent with rain fly (300W), and 5 persons inside the largest tent (500W).
RadTherm Terrain Editor
Terrain definition in MuSES/RadTherm. For areas without a frost layer in the ground, deep snow is often melting at the interface; hence the core temp is set to 0 degrees Centigrade.
The surface conditions were defined using our paint code system through MuSES. This method assigns a single value of the surface emissivity for thermal solutions but includes specular dependent emissivity for IR signature processing. See an example of a paint definition below.
Specular Paint Definition and Assignment to the Medium-Sized Tent
MuSES Screen shot with paint code assignment for the medium-sized tent surface and the paint specular curve in the floating window. Click on the image above for a high-resolution version.
The weather file for this analysis was developed from the TMY2 data for Houghton, MI, for February 9th to11th. Plots of the weather data are below. Temperature and wind speed are two main drivers in this case. Air Temperature is in Centigrade and Wind Speed is Meters per Second. Cloud cover range is 0 (clear) to 10 (overcast).
Thermal Results
Thermal sample results for each of the tents are plotted below.
IR Signature Results
The IR signature results below are shown for the 8-12 micron wavelength over the intensity scale shown at the bottom of the image. The clearest signature item is the hot chimney on the large wall tent. Although partially occluded by the forest, it shows the greatest contrast against the background snow terrain, which is often reflecting the cold sky above.
Transient IR Signature Results
Click on the images below to view animated GIFs of the scene from two different views (Northwest and Southwest, respectively). NOTE: The animations begin with a still image of the mesh for a few seconds, so that you can visualize the scene. The diurnal animation follows.
This model is available to qualified research teams. To request a sample on CD-ROM, please contact us.
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