Oops, forgot the link in my original post.. you can get them at..
http://www.scienceenthusiast.com/product/tornado-demonstration-model.html
I was going to build one, but a TV Met gave me the link where to buy that saying to just buy that, that I would save time, money, and a lot of frustration.
It has 2 fans in the top, one acts as the updraft pulling the air up and then there is a fan acting as the downdraft. Then each of the poles have holes that helps create the circulation.. it's very well designed and works great. My nieces came to my apartment and saw it and they loved it, the oldest one is already wanting to take it to show and tale. lol.