Large eddy simulation of unstably stratified boundary layer flow for understanding the structure of wildland fire flames

摘要

We study unstably stratified boundary layer flows produced by establishing an external crossflow over a horizontal heated plate using large eddy simulations (LES) with the objective of gaining insight into the structure and dynamics of wildland fire flames, to evaluate the relative weight of external/wind-driven versus internal/buoyancy-driven motions and to identify the characteristic scales. Numerical simulations in the present study are conducted with an advanced LES solver called FireFOAM. The simulations are wall-resolved and therefore provide both a detailed description of the flow and temperature fields and a direct evaluation of the wall convective heat flux. The flow is initially laminar and due to buoyancy, transitions to an unsteady laminar flow featuring longitudinal vortices which subsequently transitions to turbulent flow with intermittent occurrence of reversed flow close to the wall. The spanwise length scale of the flow structures is seen to continuously increase in the streamwise direction. Furthermore, regions in the boundary layer containing buoyancy-or shear-driven turbulence are identified using the Monin-Obukhov length scale. Finally, the results suggest that the convective velocity scale is an important scaling parameter.