Smoke Rising in Still Air: Physics Explanation

[Chris R]

Hi all,
I have what I believe is a simple physics question. I was burning a sprig of incense in my apartment, and noticed that the smoke was rising from the burning section perfectly straight up into the air for a good meter, and then started to dissipate into the air. I know that a gas will disperse into the air, but I was slightly puzzled because the smoke is a solid. My question is if there was a room with perfectly still air, would the smoke rise straight up all the way to the ceiling, or would it behave more like a gas and dissipate into the air regardless? Also, what properties would explain this?
Thanks!
CR

 

[russ_watters]

Welcome to PF!

This is actually an interesting demonstration of laminar vs turbulent flow. Most fluid flow streams (the smoke is a fluid -- it's air and exhaust gases that you can see because of suspended ash particles in it) start of laminar (linear/coherent) and after a while transition to turbulent. The distance until transition is actually highly predictable and depends on velocity, size/length of the flow stream, and the properties of the fluid. It is also an important part of our understanding of how air behaves as it flows over a wing.

Here's more on the phenomena:
http://profs.sci.univr.it/~zuccher/research/blstability/

Here's the number/equation that characterizes the behavior of a flow stream:
http://en.wikipedia.org/wiki/Reynolds_number

 

[Chris R]

Very interesting, thank you very much!