Fire Progression - Level Set

[George Balkman]

Hi, 

I have a couple of questions relating to the level set method that is implemented within FDS. I'd be delighted if you could help as I don't believe this info is available in the User Guide. 

1) When using the Rothermel model with atmospheric coupling how is the mid flame height velocity defined?  If my understanding is correct, in the original derivation of Rothermel this was not a local velocity but a wind speed that is not directly influenced by the fire i.e specified velocity in a wind tunnel or info from a weather tower. Is there a particular distance behind the fire front(when coupled) where the velocity is calculated and inputted into the ROS equation? If so, does this automatically update depending on the residence time (fire zone depth) and can the area/method of interpolation be changed? This question also relates to question 3. 

2)It is outlined in the user guide that custom ROS equations can be developed - is this still based upon the derivation of Rothermel, that is fuel properties give a no wind/slope ROS value which is the modified by the wind/slope vector?

3) Can other ROS spread models be implemented into FDS? Such as CSIRO grass model, California chapparal or an alternative user defined ROS function dependent on the wind?

4) Can the heat of combustion be altered? Currently I believe it is fixed at 18.6MW/kg. It might not have a large impact for the ROS for grass fire spread but could be interesting to test.

Thanks alot for your help. 

[William Mell]

'The implementation of the level set in FDS is at its first stages. Regarding your questions:

1) The wind speed in the direction of the local fire spread is input into the Rothermel equation. This wind speed is taken from the first grid cell above ground level. This means there's no attempt to find the mid-flame wind speed and the input wind speed will depend on the grid resolution. You are correct that the Rothermel model was developed based on a wind speed unaffected by the fire. For this reason, any implementation of the Rothermel model that uses a wind speed that's influenced by the fire is an inconsistent use of the Rothermel model. See answer to your question 3.

2) Yes, you are correct.

3) Other models can, and have, been implemented. See Appendix 1 of this report (reply to me if you cannot download the report) for an example. However, this currently requires modification of the source code. 

4) Yes, you can modify the heat of combustion in the &REAC line.

Ruddy

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William (Ruddy) Mell
U.S. Forest Service
Pacific Wildland Fire Sciences Lab
400 N. 34^th St., Suite 201
Seattle, WA 98103
willia...@usda.gov
rudd...@gmail.com
206-430-2072

[Kevin]

The level set methodology currently in FDS is the same as that in Farsight. The appendix of this paper 

A.S. Bova, W.E. Mell, and C.M. Hoffman. A comparison of level set and marker methods for the
simulation of wildland fire front propagation. International Journal of Wildland Fire, 25(2):229–241,
2015. http://dx.doi.org/10.1071/WF13178.

provides the equations and assumptions. It is coded up in vege.f90 of the FDS source code.