Keywords: thermal, dynamic

Summary

This demo shows how the built-in THERM thermal model can model dynamic heating and cooling conditions. In this scenario, a pair of aircraft are repositioned. The first plane has been parked all day and then pulls forward to reveal thermal shadow "scar" on the concrete. A second, identical plane has been inside the hangar all day and then pulls forward into the sunlight to reveal its cooler surface temperatures. After the two planes quickly reposition, the simulation captures a series of frames over the next 100 minutes. During that hour the shadow left by the first plane slowly fades, the second plane heats up and new shadows develop beneath both planes.

video

Related Materials

The following demos, manuals and tutorials can provide additional information about the topics at the focus of this demo:

Details

This scenario is focused on the temporal nature of the thermal heating and cooling in the scene. The first plane blocked the sun for most of the day, during which the background under the plane stayed cooler due to the decrease in radiational loading. This results in a "thermal shadow". The thick, high heat capacity (high "thermal inertia") material used for the background will react slowly to changes in the insolation (like the plane leaving) because it needs to absorb a lot of energy in order to change temperature. Once the first plane moves, it takes a while for that shadow to fade. The second plane is cooler because it was shadowed by the hangar. Once the plane is moved into the open, the plane starts to heat up.

Important Files

The key to this simulation is letting the THERM thermal model do it’s job and compute the solar insolation histories to get the temperature phenomenology. This scene is a stripped down version of the Foxbat scene. The moving geometry is driven by the Generic Motion model being associated with the two instances of the aircraft.gdb base geometry in the geometry/lists/planes.glist file:

<geometrylist>
  <object>
    <basegeometry>
      <gdb><filename>objects/aircraft.gdb</filename></gdb>
    </basegeometry>
    <dynamicinstance>
      <motion type="generic">
        <filename>$SCENE_DIR/geometry/lists/plane1.ppd</filename>
      </motion>
    </dynamicinstance>
    <dynamicinstance>
      <motion type="generic">
        <filename>$SCENE_DIR/geometry/lists/plane2.ppd</filename>
      </motion>
    </dynamicinstance>
  </object>
</geometrylist>

The two Generic motion model PPD files each define the motion as a pair of location/orientation entries — one for the starting location and one for the ending location. This motion happens over the course of 1 second (unrealistic, but a realistic scenario would work the fine).

As far as the tasking,

  • The single-frame simulation (see demo.sim) collects that frame after both planes have repositioned.

  • The multi-frame simulation (see video.sim) collects a series of 10 frames during the 1 second it takes the planes to reposition (10 Hz clock), and then a series of frames every 5 minutes after that for 100 minutes.

Setup

Single-Frame Simulation

To run the single-frame simulation, perform the following steps:

  1. Run the DIRSIG demo.sim file

  2. Load the resulting demo-t0000-c0000.img file in the image viewer.

Multi-Frame (Video) Simulation

To run the multi-frame (video) simulation, perform the following steps:

  1. Run the DIRSIG video.sim file

  2. Load the resulting demo-t0000-c0000.img, …​, demo-t0000-c0009.img, demo-t0000-c0009.img, demo-t0001-c0000.img, demo-t0002-c0000.img, …​ demo-t0030-c0000.img files in the image viewer.

The make_mp4.sh script can be used on the UNIX and Mac platforms to make an MPEG-4 file using FFmpeg.

Results

Single-Frame Simulation

The single-frame simulation generates a single output image. The simulation captures the scene shortly after the two planes have moved from their original positions. The "outside" plane is warm (from being in the sun) and has left behind a shadow on the ground (from being shaded). The "inside" plane is cool (from being shaded). Neither plane has a shadow under them because they just arrived at these locations and the ground is still warm from being in the sun. The temperature truth for this single image is shown below:

demo
Figure 1. Output of the single-frame (static) simulation.

Multi-Frame (Video) Simulation

The multi-frame simulation will generate a series of output images. This simulation spans a period of time including the relocation of the two planes and a fair bit of time after. During the time sequence you can see the shadow left by the "outside" plane fade as the sun warms the ground, the "inside" plane warms up and the shadows under the relocated planes develop as they shade the ground. Toward the end, the overall scene starts to cool as the sun gets lower in the sky. The temperature truth for this sequence of images is shown below:

video
Figure 2. Animation of the planes repositioning and the 100 mins that follow.