The strucuture of a multiple-vortex type tornado realized in a supercell simulator

A multiple-vortex type tornado is thought to cause severe disaster. Church et al. (1977) experimentally demonstrated such tornado-like vortices in their tornado simulator. However, the velocity field of the multiple-vortex type tornado has not yet been measured in detail. We can also realize a multiple-vortex type tornado in our originally designed supercell simulator which simulates a rotating updraft of mesocyclone and a gust front by rear flank downdraft (Sassa et al. 2009). Our experimental study aims to clarify the detailed structure of the multiple-vortex type tornado by measuring its horizontal velocity fields. Our supercell simulator is composed of a mesocyclone simulator and a gust generator. The mesocyclone simulator of D=900 mm in diameter has four fans and 48 guide vanes to generate a rotating updraft. It is similar to the ordinal tornado simulator but it has free space between its bottom and the floor. The gust generator spew out dense mist made from dry ice. The mist flows into the mesocyclone simulator like as an outflow accompanied by rear flank downdraft. We can observe the multiple-vortex type tornado when we set that the attack angle of guide vanes and the height of the mesocyclone simulator are 60 deg. and 20 mm, respectively. We filmed the outline and the several horizontal planes of the vortex by a hi-speed camera. The horizontal velocity fields were measured by using a PIV system. The suction vortices were clearly observed whereas the main vortex was not visualized as shown in Fig.1. These suction vortices incline outward in upper layer and bend toward opposite to rotating direction. Such shape is quite similar to that of actual multiple-vortex type tornado. We observed that 2, 3, and 4 suction vortices existed simultaneously as shown in Fig. 2. Its number depends on the velocity of the gust not on the attack angle of guide vanes. The characteristics of the suction vortex were evaluated from 30 suction vortices as shown in Fig. 3. The radius of the suction vortex became larger with height but its relative size to that of the main vortex was 0.2 independent of the height. The maximum tangential velocity of the suction vortex was 0.6 times of that of the main vortex. It means that the maximum tangential velocity is accelerated up to 1.6 times of mean value when the suction vortex paths through. The suction vortices located at 0.7 times of the core radius from the center axis of the main vortex. Conclusively, we can illustrate the schematic view of the multiple-vortex type tornado as shown in Fig. 4.