Mechanism of tornado funnel formation

Abstract

The mechanism of formation of a tornado funnel was observed in a specially designed facility for tornado simulation. In this facility, a tornado cyclone is first simulated at the exit of a vortex generator. Interaction of this vortex with a ground plane is found to be responsible for the funnel formation. Time-averaged velocity measurements indicate that a smaller, but highly concentrated vortex core is developed upward from the ground plane due to the horizontal convergence motion induced by the above interaction. The flow visualization technique has revealed that the funnel cloud is the downward extension of its parent cloud inside this invisible concentrated vortex core. This is possibly the fluid mechanical phenomenon of axial flow reversal usually occurring in a converging or diverging swirling flow.