In October (2012) I started a workshop at CGSociety to learn the basics of Houdini.
In the fifth week we looked deeper into particle manipulation to create some cool simple effects. As you see, one of those effects is a vortex (aka tornado) effect. The setup starts with a tube to create the shape from which a volume is created. In that volume, points get scattered. With a VOP SOP, the normal information of those points is created and manipulated to get the downward spiraling direction. That normal information is put into a volume to reference from in the PopNet and steer the particles. Those particles also get some noise in a Force POP and get killed once they hit the ground.
The cool thing with setups like this, is that you can put in a different source, leave everything else and you get a whole new variation on your effect. I show this by instead of using the scattered points, I plug in a circle from which the points of the corners are used as the source. (I use cache nodes to quickly switch between the variations, instead of having to sim them during recording).
The boxes that get copied onto the particles receive the normal information for their orientation and get scaled by the length of the particle velocity.
And I suppose those other 2 variation setups kind of speak for themselves :)

This was one of two effects we did that week. You can view the other effect here:

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