Turbulence Measurements and Coherent Structures in a PIV Experiment on Grassland Fires

High frequency (30 Hz) two-dimensional particle image velocimetry (PIV) data, exploring fire spread in ignited hand-spread pine needles under calm ambient wind conditions, are analyzed in this study. As the flame spreads away from the ignition point in the absence of ambient wind forcing, it entrains cooler ambient air into the warmer fire core and experiences the dynamic pressure offered by the entrained air. Coherent structures describe the initial shape of the fire-front and its response to local wind shifts while also revealing possible fire-spread mechanisms. Vortex tubes originating outside the fire spiral inward and get stretched thinner at the fire-front leading to higher vorticity there. These tubes comprise circulation structures that induce a radially outward velocity close to the fuel bed, which pushes hot gases outward, thereby causing the fire to spread. Such structures also confirm the presence of counter-rotating vortex pairs, known to be a key fire-spread mechanism. Precessing of the axis of the vortex tubes causes them to be kinked. The strong updraft observed at the fire-front could potentially advect and tilt the kinked vortex tubes vertically upward leading to fire-whirl formation. As the fire evolves, its perimeter disintegrates in response to flow instabilities to form smaller fire "pockets". These pockets are confined to certain points in the flow field that remain relatively fixed for a while and resemble the behavior of a chaotic system in the vicinity of an attractor. Increased magnitudes of the turbulent fluxes of horizontal momentum, computed at certain such fixed points along the fire-front, are symptomatic of irregular fire bursts and help contextualize the fire spread. Most importantly, the time-varying transport terms of the turbulent kinetic energy (TKE) budget equation computed at adjacent fixed points imply that horizontal turbulent transport is the primary mechanism for interaction between local fires along the fire-front.