Multi-scale Simulation of a Very Large Fire Incident. Computation From the Combustion to the Atmospheric Meso-scale
Résumé
Valle male fire devastated more than 3000ha of Mediterranean maquis and pine forest in July 2009. Simulation of combustion processes as well as atmospherics dynamics represents a challenge for such scenarios because of the scale of the phenomenon. A coupled approach between Meso-NH (Non-Hydrostatic) LES (Large Eddy Simulation) meso/microscale scale atmospheric model and ForeFire area simulator is proposed for predicting fine-scale to large-scale phenomenon's involved in such wildfire, showing that using current supercomputers such simulation is possible in a reasonable time. To be representative of the phenomenon, typical resolution required for the simulation of a fire front combustion must be sub-meter (to represent an explicit flame thickness) while atmospheric simulation of a typical domain (several tens of square kilometres) may not be performed at a resolution of finer than 50 meters in a reasonable computational time. The two-way coupling in a Meso-NH/ForeFire simulation typically involve the surface wind to drive the fire, heat (combustion) and water vapour fluxes to be injected in the atmosphere at each atmospheric time step. The ForeFire code has been built so that several front velocity function could be easily defined and applied at different locations of the surface (e.g. a fire front velocity model could be different in forest with canopy than in grassland), likewise surface combustion models can be added and defined in the same way to force the atmospheric model. Meso-NH and ForeFire resolutions are independent and the computational time needed by the surface model is a typically a fraction of the atmospheric simulation. The most active part of the Valle male fire lasted 10 hours, while the computation of the 24 millions grid points took 9 hours on 900 computer cores.