Help with Dust Wave

Hey! I’m using the smoke simulator to do a dust wave/shockwave type thing. I pretty much want most of the smoke to expand sideways, but I came up with this instead: http://www.youtube.com/watch?v=OEdqrtmn3z8

as you can see, it does start out right, but then that big mushroom cloud looking thing comes out of nowhere. I try different settings but can’t get a shockwave, the smoke always rises up no matter what I set the gravity too.

Can someone kinda give me some tips on how I can fix it? Thanks

I don’t recall the setting. But you can assign the smoke a value that will make it heavier. I don’t think you need to touch gravity as such. There are few really great tutorials on smoke that explain particles that behave like dust wave so google, google.