Vegetation in a wildfire simulation

[Leobardo Valera]

Hello everyone,

I am trying to simulate a wildfire in a mountain. The vegetation I am using is a compound of grass, I realized that the fire extinguishes too fast when the fire reaches a ridge. Any suggestion of what kind of vegetation should I use the make the fire last longer. 

Thanks

Leobardo Valera

[Ruddy]

Simulating a fire spreading directly over a ridge (spread direction is perpendicular to the ridge line) is going to require a finer computational mesh than a fire spreading up slope. Have you adjust the grid size? Changing the vegetation to make the simulated fire behavior be what you want can lead to unrepresentative results. You may want to consider using the level set model, bearing in mind its limitations.

[Leobardo Valera]

Ruddy: Do you mean in the region close to de the ridge: I should work with a finer mesh? I am not familiar with this: "you may want to consider using the level set model, bearing in mind its limitations"

Thanks

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[Anthony Marcozzi]

Hi Leobara,

Page 243 of the new FDS User Guide (https://pages.nist.gov/fds-smv/manuals.html) will have more information on the level set model that Ruddy mentioned. Our group has had mixed success applying the full-physics model to simulations with terrain features. The level set method is a good alternative, given, as Ruddy also mentioned, its limitations.

[Ruddy]

Yes, a finer mesh would be needed along and below the ridge. Unless the ambient wind is strong enough, the fire will be spreading into the wind as it passes over the ridge. This results in a smaller fire. The level set model is described in the FDS user guide. It is a simplified approach for fire in surface vegetation. The simplifications allow simulations on much coarser grids than would be possible for the full physics model. In addition, you also need to have confidence that the wind is properly modeled, at least from a grid dependence point of view. The would require grid resolution tests of wind-only simulations. There has not been a lot of work on this topic, especially at the scales relevant to fire/wind interaction. Also, as you are probably aware, an important mechanism for a fire to spread over a ridge is spot fires from the deposition of firebrands on the downwind side of the ridge. This is also a modeling topic that has received very little attention.

[Jimmy Landmesser, Jr.]

Good morning Mr. Marcozzi,

I am working with Leobardo on potential impacts of soil moisture on wildland fire spread.  I noticed some of the comments that you and Mr. Ruddy have recommended he explore on his simulation.  I am supporting this project from a fire protection standpoint, but not proficient with FDS.  I was wondering if you and/or Mr. Ruddy may have some time to discuss some opportunities for improvements via the Level Set Model (and understand its limitations), modeling of the wind and exploring the use of firebrands for WUI spread.  I want to make sure we don't "build-design" our simulation, basically making the output fit our need at the cost of accurate modeling and replicability.

I do not want to be presumptive, but would you be willing to have a conversation or two offline?  

Feel free to call me or send me an email (both listed below).

Thank you and have a nice day,

Jimmy Landmesser, Jr.

865-607-5331
jimmyla...@gmail.com