Sub-grid-scale parameters in computational fluid dynamics modelling of compartment fires

The increasing use of computational fluid dynamics-based computer programs in the fire engineering industry requires a corresponding increase in validation work on these models. The Natural Fire Safety Concept compartment fire tests carried out at Cardington offered large-scale fire test data which could be utilised in this sense. The tests were modelled with the Fire Dynamics Simulator (FDS V4.0) using the mass loss data as the basis for the rate of heat release input, and comparing the predicted compartment gas temperatures across the domain against the measured temperature profiles from the thermocouple data. The sub-grid-scale (SGS) Smagorinsky model constant, Cs, was varied in order to firstly investigate the effect of such a variation on the output temperature predictions, and secondly to suggest an optimum value for this constant when modelling large-scale compartment fires. The results suggest a stronger dependence on Cs for coarser grids, but for finer grids the output temperatures show less sensitivity to the varying value of Cs. Two further SGS constants, the Prandtl number, Pr, and the Schmidt number, Sc, were varied together with Cs. A matrix of 200 simulations was carried out to investigate the combination factor of these three SGS parameters. The results suggest that for any given scenario different combinations of these factors can result in markedly different output predictions. © 2005 Civil-Comp Press.