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44 ventilation effects. The upstream condition appears to be the critical value. Consequently, 2D simulations require spe- quite limited, because it forces the flow in one direction, cific precautions and can be used in specific situations only. especially if the velocity's components are imposed. For modeling, it may be more reasonable to specify differ- ent pressure boundary conditions at the portals, such as FINDINGS ON NUMERICAL MODELING BASED ON LITERATURE REVIEW wind effects. The main advantage of the CFD models is to allow the study Simplifying the model can also have an effect on the results. of cases for which no experimental data are available. After a preliminary validation has been made from full-scale tests, Another challenge is in setting the time step for transient simulations are used to study many other situations. This tech- modeling. Sensitivity analyses are suggested to select the nique provides a general description of the various phenomena. appropriate time step. The use of simplified models may be This is the only method that offers such possibilities, even if applicable for long tunnels for which the boundary conditions the results must be considered as orders of magnitude. are difficult to describe. A simplified model, such as a one- dimensional (1D) model, can be used to estimate globally the The major restriction is the time needed for the calculation flow in the complete tunnel and derive the boundary conditions and the complexity of the model. Lengthy preliminary vali- to impose at both ends of the mesh. Simplified 1D models dations and skillful users are necessary; otherwise the obtained can evaluate the critical fire locations, depending on the results may be misleading. tunnel geometry and ventilation scheme. The CFD models are therefore adapted principally to certain As a first approach a simplified model, such as a 1D model, specific uses: is used to estimate globally the flow in the complete tunnel and derive the boundary conditions to impose at both ends of To set up general design rules by simulating typical the mesh for 2D and 3D models. Simplified 1D models are cases. used to evaluate the critical fire locations depending on the To investigate new or especially complex situations. tunnel geometry and ventilation scheme, evaluate ventilation To obtain a thorough understanding of actual fires and requirements, and provide ample sensitivity information. to analyze them. Although 3D simulations require long calculation times, Moreover, there are many advantages that can be drawn 2D simulations may appear as attractive alternatives. This from the use of CFD models in conjunction with other study simplification of the problem by utilizing a flow between two methods. For instance, full-size testing will benefit from some planes requires some precautions, such as: preliminary computational simulations (if necessary, very approximate) to assess the expected phenomena and their orders of magnitude. Also, a test program on scale models To take into account the reduction of the friction forces, could advantageously be prepared by calculations aimed at and evaluating the quality of similarities and orders of magnitude. To perform modeling using similarities based on the The reduced-scale model will then allow for studies by varying Reynolds and Froude numbers and energy dimensionless the useful parameters. Last, new calculations can be made to parameters. calibrate the computational code in a first step, and in a second step to understand, even extrapolate the model measurements. The energy dimensionless parameters represent the energy released by the fire or the quantity of fuel injected per square To simulate fires in long tunnels or complex underground meter (square foot) of tunnel cross-sectional area. Therefore, networks it may be useful to couple a 3D simulation with simplified ones, such as 2D or 1D models, to determine Qfire Stunnel is the name in 2D or 3D (21), boundary conditions. This technique appears as a potential development of the numerical simulation. where: The CFD models include several physical models that have Qfire is the heat release rate, and been validated against fundamental experiences, but where Stunnel is the tunnel cross-sectional area. few global validations have been made on full-scale fire tests. Therefore, the databases drawn from the EUREKA 499 and These constraints influence the geometrical and gravita- the Memorial Tunnel experiments are very useful for validat- tional terms. A validation assessment has been performed on ing CFD models (35). the basis of Ofenegg Tunnel experiments. A plan to set up a CFD model that has been calibrated and Some important limitations must be mentioned. For exam- fitted with the numerous parameters that can easily be used by ple, this technique does not correctly describe the stratification an unskilled user is probably not realistic. A preliminary val- in the instance when the longitudinal velocity is lower than idation work must compare qualitatively and quantitatively