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Title: Deliverable D2.5-2 The effect of three-dimensionality on the building response to a forest fire
Subject: [FIRE PARADOX Deliverables, Fire Paradox Modules, Module 2 - Physical mechanisms, WP2.5 - Fire effects on buildings and people]
Description: In the practical applications of computational fluid dynamics in the prediction of the effects of forest fire on structures and people, the computational cost associated with the three dimensional simulations may sometimes restrain the usability of the simulations. Making the simulation two-dimensional reduces the computational cost by one or two orders of magnitude but may introduce additional errors to the results. In this work, the effects of the 2D assumption are studied by performing a series of simulations in both 3D and 2D, monitoring both convective and radioactive heat fluxes on the surfaces of the building surface, and reporting the difference between the 2D and 3D predictions of these heat fluxes. When doing the comparison between three- and two-dimensional simulations, special care must be taken to ensure that the two simulations actually represent the same fire scenario, which is the infinitely wide fire front. The performance of three-dimensional model at different boundary conditions and numerical parameters was first studied. The mirror boundaries on the sides of the fire front were found practical and valid for the reduction of computational cost. The grid sensitivity study performed in 2D showed that the independence of the grid resolution can be reached at 0.25 m. Due to the limited computational resources, only part of the final simulations were actually performed at this grid resolution. For the simulations of the crown fires, a coarser grid had to be used. From the viewpoint of radiation predictions, the 2D simulations were found to be better suited for the purpose than 3D simulations because in 2D, the smoothness of the radiation fields is much easier to achieve. At least 1000 angular directions were found necessary in 3D simulations. The validity of 2D simulations was studied by running a series of simulations in both 2D and 3D at different boundary conditions. The differences between the 2D and 3D simulations were studied by computing differences in convective and radioactive heat fluxes and by averaging them over the heat release rate ranges covered in the simulations. The conclusion was drawn based the cases where significant heat fluxes were found. The conclusion is that 2D simulations can be used for order-of-magnitude type of analysis, for which purpose they are well suited due to the small computing times. However, the differences seem to be too large for accurate predictions of the building and human response. Therefore, the critical simulations of the future analysis should be made in three dimensions.
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Language: English
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Creator: VTT
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Rights restrictions: restricted
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Audience: public
Download: Deliverable D2.5-2 The effect of three-dimensionality on the building response to a forest fire