Turbulence structures observed during experimental fires in forest and grassland environments

Abstract

Fire-atmosphere interactions can occur at spatial scales on the order of tens of meters at the fire front and to kilometers in plumes during large fires. However, few studies have focused on the observed turbulence structure in the immediate environment of propagating fires to further understand the relative importance of ambient and fire-induced turbulence on fire front propagation. In this study, turbulence structures during the passage of fire fronts were investigated using the data obtained from two field experiments. One fire was conducted over grass fuel under strong ambient mean winds and the other conducted in a forest sub-canopy environment under light ambient winds. The main objective of this research is to investigate scale-averaged variances over different fuels before, during, and after the fire front passage in relation to different fire intensities and atmospheric conditions. Time series data from in-situ sonic anemometer arrays are analyzed using wavelet signal processing to treat non-stationary process of the fire front passage. Only a small increase in u variance over the 2-4 s band was found during the fire front passage in the grass fire environment under the strong mean winds. The variance associated with low intensity sub-canopy fire was found to increase over 2-4 s and 8-16 s bands, due to intermittent convective smoke plume pulses of the low intensity sub-canopy fire.