Turbulent Thermal Image Velocimetry at the Immediate Fire and
We present novel in-field vegetation fire observations, and the analyses
used to process the data, using brightness temperatures recorded by
longwave infrared camera and thermal image velocimetry. The brightness
temperatures from a wind-driven stubble wheat fire were obtained in
video format with a 60 frames per second (fps) acquisition rate.
Multi-level sonic anemometers mounted on a 10m in-fire tower were used
for in-situ measurements of turbulent velocity and air temperatures,
while fuel level air and flame temperatures were collected by an array
of thermocouples. The camera’s image pixel resolution was adequate to
resolve dynamics and in accordance with the in-fire thermocouple spacing
distances. The in-situ and remotely measured flaming zone dynamics were
derived using two different methodologies, Thermal Image Velocimetry
(TIV) and Image Segmentation (IS). The results highlight spatial and
spectral information of coherent turbulent and mean velocity structures.
The power spectra decomposition of the thermal image velocimetry showed
similar spectral characteristics to the sonic velocity measurements
during the fire passage under the tower with a similar inertial subrange
slope. This result reveals plausible evidence of interaction between the
flaming zone and wind turbulence for a prescribed rapidly moving stubble
wheat fire. This research presents a new field measurement methodology
for understanding fire-atmospheric interactions between the flaming zone
and the immediate overlying atmospheric turbulent boundary layer.