Three Types of Horizontal Vortices Observed in Wildland Mass and Crown Fires

Abstract

Observation shows that three types of horizontal vortices may form during intense wildland fires. Two of these vortices are longitudinal relative to the ambient wind and the third is transverse. One of the longitudinal types, a vortex pair, occurs with extreme heat and low to moderate wind speeds. It may be a somewhat common structure on the flanks of intense crown firm when burning is concentrated along the fire's perimeter. The second longitudinal type, a single vortex, occurs with high winds and can dominate the entire fire. The third type, the transverse vortex, occurs on the upstream side of the convection column during intense burning and relatively low winds. These vortices are important because they contribute to fire fighter and are a threat to fire fighter safety. This paper documents field observations of the vortices and supplies supportive meteorological and fuel data. The discussion includes applicable laboratory and conceptual studies in fluid flow and heat transfer that may apply to vortex formation.