Question-and-Answer Resource for the Building Energy Modeling Community
 | 1 | initial version |
To your questions:
1) You can read up more on the Irradiance Cache (which is stored in the ambient file) in the following sources:
2) Do you still use an irradiance cache (i.e., a non-zero ambient accuracy (-aa) value) when you don't use ambient files? In my experience, you will get higher illuminance values when you use irradiance caching as opposed to turning it off, and the higher values are more correct. That's because using an irradiance cache effectively increases the number of bounces in ray paths beyond the constraints you set with -ab, -lr, and -lw for those values cached late in the process, because they can sample the values that were cached earlier. This effective increase is a good thing, because real physics does not place a limit on ray bounces.
SImilarly, when you use a previously created ambient file, any additional ambient values that are created early in the run also get the benefit of sampling previously calculated ambient values that were saved in the file.
In your case, your zone 2 simulation is reusing the ambient values calculated by your zone 1 simulation that were saved into the ambient file. You could consider using an overture pass: running rtrace twice for each zone, so that the first time you create the ambient file, and the second time you read the ambient file and use it to calculate that values you will use.
Note that setting the number of ambient bounces to 50 requires that you also set your ray relays (-lr) to 50 or higher and set your minimum ray weight (-lw) extremely low. Otherwise, I doubt very much that your large ab simulation is very different from your low ab simulation.
