Wind tunnel generation of horizontal roll vortices over a differentially heated surface
Wildland crown fires represent an extreme type of forest fire that advances from top to top of trees or shrubs. Some such fires generate energy comparable to several nuclear explosions. They occur most frequently in coniferous forests found in the United States and Canada, and in broad-leafed eucalyptus forests of Australia. Here we report visualization of wind tunnel experiments in which airflow along the perimeter of a simulated forest fire produced horizontal vortex pairs with axes in the downstream direction. These vortex pairs formed above a differentially heated surface simulating the thermal energy flux from a crown fire. The thermal energy was produced by an electrically heated surface ribbon at a temperature of ∼250°C and also, in a separate experiment, by an electrically heated embedded wire coil at a temperature of ∼825°C. Wind velocities ranged from 0.2 to 3.6 m s−1. Similar vortex action may cause the unburned tree-crown strips that are observed when crown fires, driven by a significant wind, burn over flat terrane.
Understanding flame structure in wildfires using Large Eddy Simulations
Title of thesis: UNDERSTANDING FLAME STRUCTURE IN WILDFIRES USING LARGE EDDY SIMULATIONS Salman Verma, Master of Science, 2014 Thesis directed by: Professor Arnaud Trouvé Department of Fire Protection Engineering The structure of wildfire flames in the presence of crossflow was analyzed by utilizing suitable non-reacting numerical experiments with low speed flow over a hot isothermal horizontal semi-infinite surface. FireFOAM, a Large Eddy Simulation (LES) based solver developed by FM Global for fire protection engineering applications, was employed for all the calculations. Early-time dynamics of Rayleigh-Taylor Instability (RTI) was first simulated using Direct Numerical Simulations (DNS) so that the solver could be verified against Linear Stability Theory (LST). Then attention was given to late-time dynamics in order understand the different stages (e.g., appearance of secondary instability, generation of larger scales due to interaction between structures) involved in the development of the instability. The onset of thermal vortex instability, in a configuration with low speed flow over a hot isothermal semi-infinite horizontal plate, predicted using DNS was compared with the literature. Spatial evolution of various terms in the streamwise vorticity equation was used to identify the dominant mechanisms responsible for the generation/evolution of vorticity. Streamwise evolution of the instabilities was studied and the effects of the changes in temperature and orientation of the plate on the thermal instabilities were also investigated. Finally, a configuration with low speed flow over a hot isothermal semi-infinite horizontal strip was used to understand the effects of upstream Boundary Layer (BL) height and the length of the strip on both the thin horizontal and larger structures (analogous to Flame Towers (FT) observed in real wildfires and laboratory experiments). UNDERSTANDING FLAME STRUCTURE IN WILDFIRES USING LARGE EDDY SIMULATIONS
Thermal stability of reconstituted collagen fibrils. Shrinkage characteristics upon in vitro maturation
Thermal stability measured as area shrinkage without tension during heating was determined for membranes of collagen fibrils reconstituted from solutions of highly purified rat skin collagens. Shrinkage in per cent of area at 25 degrees C and shrinkage temperature were quantitated in a standardized way and determined as a function of in vitro maturation time for 11 to 104 days after aggregation for the collagen membranes. Similar to reports on intact rat skin, shrinkage temperature remained constant and shrinkage per cent declined with a rate decreasing with time during maturation. Solubility in water at 80 degrees C for 2 hours was 95-96% and remained unchanged for the maturation time (about 2 months) studied. The decreased shrinkage reflecting a lower degree of collapse is ascribed to an increasing thermal stability of the membranes during maturation. Development of heat-stable bonds in the reconstituted collagen fibrils is taken up to be amenable to this increased stability. Similarity in changes of shrinkage characteristics during in vivo and in vitro maturation indicates that maturation changes in reconstituted collagen fibrils reflect those occurring in intact collagen during in vivo aging.