PhD Thesis - staffweb - University of Greenwich

APPENDIX 3 : Conference Paper "The Development and Application of Group Solversin the SMARTFIRE Fire Field Model", Ewer J., Galea E., Patel M. and Knight B.,Proceedings of Interflam '99, Edinburgh, UK, 1999, Vol. 2, pp 939-950.The CFD engine of SMARTFIRE, called CWNN++, is written in C++ and has beendeveloped in-house from an existing FORTRAN code [6]. CWNN++ uses validatednumerical methods from the legacy FORTRAN code enhanced by object orienteddevelopments in C++ and additional physics features that are required for fire field modelling[11]. CWNN++ uses three-dimensional unstructured meshes, enabling complex irregulargeometries to be meshed without the need for body fitted co-ordinate grids. Unlikeconventional CFD technology such as PHOENICS [7], FLOW3D [8], JASMINE [9] andSOFHIE [10], this allows extremely complex geometries to be meshed efficiently.The CFD engine is under continual development and evaluation. As part of the testingprocedure, model predictions are compared with other commercial CFD codes and againstdata generated through physical experimentation. Test cases [11] include standard CFDbenchmark cases such as the backward facing step and moving lid problems and the Stecklerroom fire experiments [12]. Results produced by SMARTFIRE for the Steckler experiments[5, 11] compare favourably with those produced using commercial CFD codes such asCFDS-FLOW3D and PHOENICS [13].The current release of the software is V2.01 [11]. In this version, the software representsfires as a transient volumetric heat and mass source. Standard models for gaseouscombustion (i.e. diffusion and eddy-dissipation models), while not enabled in the generalrelease software, are implemented and undergoing testing. The code uses the SIMPLE [14]algorithm and can solve turbulent (two equation K-Epsilon closure with buoyancymodification) or laminar flow problems under transient or steady state conditions. Radiationis represented through the use of an enhanced six-flux radiation model [11]. Furtherinformation concerning SMARTFIRE including a demonstration may be found on the WorldWide Web [15].BACKGROUND TO GROUP SOLVERSSince SMARTFIRE is a truly unstructured mesh CFD code there are a limitednumber of reliable and general purpose numerical techniques available to solve the systemsof algebraic equations for each of the primary field variables. Structured mesh CFD codescan exploit the structured nature of the data (e.g. using lines or planes) in various solvers togive more efficient solution than for the point-by-point iterative solvers commonly used inunstructured codes. One of the goals of this work has been to investigate and, if possible,exploit reliable techniques that prove to be of benefit to fire modelling within unstructuredmesh CFD codes. One such technique, developed by the authors, is the group solvers.In previous published research [2], it was demonstrated that a multiple region twodimensionalfire scenario could use static group solver techniques to save 31% of theprocessing time. The demonstration consisted of two rooms, one containing a fire , the other(separated by a wall) was initially free of fire or its influence. Under a given set ofconditions, the partition was removed allowing the fire to spread into both compartments.The saving in computational effort was achieved by effectively removing the need to performthe computations for half of the solution domain during part of the simulation, prior to thepartition removal. In this way approximately half of the solution domain was fully decoupledfrom the simulation for part of the simulation. This work used the concept of astatic “geometric” group to dictate the solution strategy in each region. Here weAppendix 11.3 Page 143-3 3