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.
Intense Atmospheric Vortices Associated with a 1000 MW Fire
Abstract Observations of vortices of various types produced in a large thermal plume are described. The apparatus used to generate the plume is the Meteotron, an array of 105 fuel oil burners with a total heat output of approximately 1000 MW. Three types of vortices have been observed: 1) large counter-rotating rolls in the downstream plume, 2) intense small-scale vortices resembling very strong dust devils seen at the surface on the downwind side of the plume, and 3) very large columnar vortices produced when the lower portion of the plume goes into rotation as a whole. Three mechanisms leading to the concentration of vorticity necessary to produce these vortex types are discussed. These include tilting and stretching of horizontal vorticity present in the environmental wind field, generation of vorticity within the plume by the action of buoyancy and drag forces, and convergence of preexisting background vorticity from the environment. It is concluded, based on these observations and physical considerat...
AN EXPERIMENTAL TEST OF MASS FIRE SCALING PRINCIPLES
Abstract : The large fires which follow nuclear detonations over a city present an important civil defense problem. The high temperature, high winds, high thermal radiation flux, and high noxious gas concentrations at street level present an adverse environment for the escape of people, for fighting fire, and for the operation of shelters. It is essential that civil defense planning be based on as reliable a knowledge of this environment as possible. Since the local conditions will be influenced by the entire mass fire, it is necessary to determine the characteristics of mass fires. The purpose of this research was to find out if the air velocities and temperatures that exist in the gross flow field above a mass fire could be determined from measurements made on a small scale laboratory-sized model of the mass fire, using electrical heating elements to provide the heat input. (Author)