Dispersion and deposition of firebrands in a turbulent boundary layer

Abstract

The dispersion and deposition of cylindrical shaped firebrands of constant mass in a turbulent boundary layer was investigated by large eddy simulation of air flow, with firebrands individually tracked in a Lagrangian framework. The carried firebrands experienced both translation and rotation as gravity, drag and lift forces acted on them. Simulations were carried out in a turbulent boundary layer with a free stream wind velocity of 18.8 m/s, an initial boundary layer height of 25 m, with firebrand densities of 70, 230, and 570 kg/m3 and release heights of 20 m and 40 m. Identical firebrands were consecutively released under identical initial conditions after the turbulent flow reached a statistically stationary state. The motion and deposition locations of firebrands were characterized by tracking tens of thousands of firebrands in each simulation and computing statistical quantities such as dispersion and diffusion of flying firebrands, and the joint probability density function of the coordinates of the deposited firebrands. For all simulated cases, the distribution of deposited firebrands exhibited symmetry in the spanwise direction. The normalized mean position of the deposited firebrands in the streamwise direction increased approximately by a factor of two when the density of firebrands decreased from 570 kg/m3 to 70 kg/m3. Also, the ground distribution of firebrands that were released from a higher elevation was found more leptokurtic with a multivariate kurtosis deviating more than 5% of that of a normal distribution.