A Laboratory Investigation of the Structure of Multiple Vortices

Abstract

The structure of the multiple vortices formed in small scale atmospheric vortices is studied in a laboratory experiment. A guide vane type simulator is shown to be capable of producing the multiple vortices for large swirl ratios. The number of subsidiary vortices increases with the swirl ratio. A condition with 3 subsidiary vortices is examined closely with an improved smoke-wire and a two component hot-wire anemometer. Core sizes, velocity distributions and other properties of the subsidiary vortices are determined by the measurement. A subsidiary vortex rotates around the center of the parent vortex keeping its identity. But the ratio of the translational velocity of the subsidiary vortex to mean tangential velocity is about 0.9 in the layers far from the ground, and about 0.7 near the ground because of the vertical shear in the mean field. The cospectra between the two horizontal velocity components indicate that in the upper layers the subsidiary vortex transports momentum from the maximum velocity region of the parent vortex toward the center. The transport process is more complex near the ground.