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.
Horizontal Roll Vortices and Crown Fires
Abstract Observational evidence from nine crown fires suggests that horizontal roll vortices are a major mechanism in crown-fire spread. Post-burn aerial photography indicates that unburned tree-crown streets are common with crown fire. Investigation of the understory of these crown streets after two fires showed uncharred tree trunks along a center line. This evidence supports a hypothesis of vortex action causing strong downward motion of air along the streets. Additionally, photographs of two ongoing crown fires show apparent horizontal roll vortices. Discussion also includes laboratory and numerical studies in fluid dynamics that may apply to crown fires.
Stability of buoyancy-driven convection in a tilted slot
Abstract The stability of buoyancy-driven convection in a slot, slightly tilted with respect to the horizontal, is investigated analytically on the basis of linear theory. For mathematical simplicity, the boundaries are assumed free and isothermal. It is shown that the Rayleigh number and the wave number at the critical point have the same values as for an exactly horizontal slot, however, the predicted motion, rather than being indeterminate, is one of longitudinal rolls with their axes aligned in the direction of the mean flow. This is in contrast to the analogous problem of convection in a vertical slot in which the secondary flow pattern is known to consist of transverse rolls, i.e. rolls with their axes normal to the mean motion.