Formation of fire whirls: Experimental verification that a counter-rotating vortex pair is a possible origin of fire whirls

One of the formation mechanisms of fire whirls, which often occur in a cross flow downwind of a fire, was studied experimentally in a wind tunnel using a flow visualisation technique. We observed that vortices forming along and just downwind of a flame were occasionally shed downwind. Such vortices differed markedly from wake vortices, which occurred downwind of the flame and exhibited periodic shedding behaviour in the downwind direction. We demonstrated that these vortices were a counter-rotating vortex pair (CVP) of the plume of the flame. This result suggests that the CVP of a large fire is a possible origin of the fire whirls that occur downwind of a fire area and are shed downwind.