Laboratory Simulation of Downbursts

As a part of the laboratory experiment on a microburst, the circumferential instability and the associated instantaneous velocity field of a vortex ring impinging onto a normal wall have been investigated. Vortex rings have been generated at the circular orifice by the impulsive motion of the speaker corn and have been visualized by filling ethylene glycol smoke. The instantaneous velocity fields induced by a vortex ring were measured by employing the PIV in which the cross correlation method was used. The wave number of the circumferential instability increases with an increase of the initial Reynolds number. The wave number at the same initial Reynolds number decreases as the distance from the orifice increases. It was shown that the development of induced secondary vortex ring influences the change of velocity vector fields.