Wind in a Natural and Artificial Wildland Fire Fuel Bed

Fuel beds represent the layer of fuel that typically supports continuous combustion and wildland fire spread. We examine how wind propagates through and above loose and packed pine needle beds and artificial 3D-printed fuel beds in a wind tunnel. Vertical profiles of horizontal velocities are measured for three artificial fuel beds with prescribed porosities and two types of fuel beds made with long-leaf pine needles. The dependence of the mean velocity within the fuel bed with respect to the ambient velocity is linked to the porosity. Experimental results show significant structure to the vertical profile of mean flow within the bed, and suggest that small-scale sweeps and ejections play a role in this system redistributing momentum similar to larger-scale canopy flows.

Suitability of fibre-optic distributed temperature sensing for revealing mixing processes and higher-order moments at the forest–air interface

The suitability of a fibre-optic distributed temperature sensing (DTS) technique for observing atmospheric mixing profiles within and above a forest was quantified, and these profiles were analysed. The spatially continuous observations were made at a 125 m tall mast in a boreal pine forest. Airflows near forest canopies diverge from typical boundary layer flows due to the influence of roughness elements (i.e. trees) on the flow. Ideally, these complex flows should be studied with spatially continuous measurements, yet such measurements are not feasible with conventional micrometeorological measurements with, for example, sonic anemometers. Hence, the suitability of DTS measurements for studying canopy flows was assessed. The DTS measurements were able to discern continuous profiles of turbulent fluctuations and mean values of air temperature along the mast, providing information about mixing processes (e.g. canopy eddies and evolution of inversion layers at night) and up to third-order turbulence statistics across the forest–atmosphere interface. Turbulence measurements with 3D sonic anemometers and Doppler lidar at the site were also utilised in this analysis. The continuous profiles for turbulence statistics were in line with prior studies made at wind tunnels and large eddy simulations for canopy flows. The DTS measurements contained a significant noise component which was, however, quantified, and its effect on turbulence statistics was accounted for. Underestimation of air temperature fluctuations at high frequencies caused 20 %–30 % underestimation of temperature variance at typical flow conditions. Despite these limitations, the DTS measurements should prove useful also in other studies concentrating on flows near roughness elements and/or non-stationary periods, since the measurements revealed spatio-temporal patterns of the flow which were not possible to be discerned from single point measurements fixed in space.

Suitability of fiber-optic distributed temperature sensing to reveal mixing processes and higher-order moments at the forest-air interface

Suitability of fibre-optic distributed temperature sensing (DTS) technique to observe atmospheric mixing profiles within and above forest was quantified and these profiles were analysed. The spatially continuous observations were made at a 125 m tall mast in a boreal pine forest. Air flows near forest canopies diverge from typical boundary layer flows due to the influence of roughness elements (i.e. trees) on the flow. Ideally these complex flows should be studied with spatially continuous measurements, yet such measurements are not feasible with conventional micrometeorological measurements with e.g. sonic anemometers. Hence the suitability of DTS measurements for studying canopy flows was quantified. The DTS measurements were able to discern continuous profiles of turbulent fluctuations and mean values of air temperature along the mast providing information about mixing processes (e.g. canopy eddies, evolution of inversion layers at night) and up to third order turbulence statistics across the forest-atmosphere interface. Turbulence measurements with 3D sonic anemometers and Doppler lidar at the site were also utilised in this analysis. The continuous profiles for turbulence statistics were in line with prior studies made at wind tunnels and large eddy simulations for canopy flows. The DTS measurements contained a significant noise component which was however quantified and its effect on turbulence statistics was accounted for. Underestimation of air temperature fluctuations at high frequencies caused 20...30 % underestimation of temperature variance at typical flow conditions. Despite these limitations, the DTS measurements should prove useful also in other studies concentrating on flows near roughness elements and/or non-stationary periods, since the measurements revealed spatio-temporal patterns of the flow which were not possible to discern from single point measurements fixed in space.

Large Eddy Simulation of Wind Flow over A Realistic Urban Area

A high-resolution large eddy simulation (LES) of wind flow over the Oklahoma City downtown area was performed to explain the effect of the building height on wind flow over the city. Wind flow over cities is vital for pedestrian and traffic comfort as well as urban heat effects. The average southerly wind speed of eight meters per second was used in the inflow section. It was found that heights and distribution of the buildings have the greatest impact on the wind flow patterns. The complexity of the flow field mainly depended on the location of buildings relative to each other and their heights. A strong up and downflows in the wake of tall buildings as well as large-scale coherent eddies between the low-rise buildings were observed. It was found out that high-rise buildings had the highest impact on the urban wind patterns. Other characteristics of urban canopy flows, such as wind shadows and channeling effects, are also successfully captured by the LES. The LES solver was shown to be a powerful tool for understanding urban canopy flows; therefore, it can be used in similar studies (e.g., other cities, dispersion studies, etc.) in the future.