The role of dynamic pressure in generating fire wind

Abstract

Earlier models of fire plumes based on simple entrainment laws and neglecting dynamic pressure have failed to produce the relatively shallow inflow over the fire perimeter known as fire wind. This inflow is of prime importance in fire modelling as it normally provides much of the air required for combustion; for this reason we have carried out a very simple numerical experiment on two-dimensional natural convection above a strip heat source with the intention of simulating those aspects of fire behaviour involved in the generation of fire wind without attempting the formidably difficult task of detailed fire modelling. Our results show clearly that fire wind is driven by the dynamic pressure field which is generated by and intimately related to the region of strong buoyant acceleration close above the ground boundary. Throughout our parametric range there is a concentrated region of large horizontal pressure gradient in a neighbourhood above the perimeter of the fire, and elsewhere the pressure gradients play a lesser role.

We have investigated also the dependence of our solution on the boundary conditions, particularly those at the lateral boundary, where we have imposed as little constraint as possible on flow into and out of the computational region. Considerable effects even of such weak side-boundary constraints persist throughout the solution region at moderate values of the pseudo-Rayleigh number (based on eddy diffusivities), but these can be limited by an appropriate choice of the thermal conditions and kept within acceptable bounds at large pseudo-Rayleigh numbers. Similar effects of boundary conditions are likely to appear in other mesoscale convectively driven atmospheric models, including sea breezes, katabatic winds and locally concentrated convective columns.