Fire behavior in multiple burning shrubs separated horizontally and vertically

Fire interactions between multiple 1 m tall, 0.7 m diameter chamise shrubs was studied utilizing the WildlandUrban Interface Fire Dynamics Simulator (WFDS, Mell et al., 2009). Two shrub arrangements were investigated. First, nine shrubs were placed in a 3 x 3 horizontal region. The shrub separation distance and wind speed were varied. Two competing interaction mechanisms were identified: heat feedback enhancement, primarily due to thermal radiative heating, and air entrainment restriction. Shrub burning characteristics were examined and a global average burning rate was analyzed. For the no wind condition, the peak mass loss rate of the center shrub is 23% higher than the others, indicating that heat feedback enhancement is dominant. Air entrainment causes the surrounding shrubs to burn less intensely. At an imposed wind speed, air entrainment effects are dominant. Shrubs that are best shielded from the wind burn most intensely. Second, the vertical separation between two shrubs was varied under different wind conditions. With no ambient wind, nearly no interaction between the two shrubs was observed. At a wind speed of 1 m/s, significant interaction between shrubs occurred due to flame tilting. The downwind shrubs burned the most vigorously for vertical separation distances between 0.2 and 0.8 m.