hi,I am simulating in a buoyant jet.Can I use Boussinesq model if the buoyant diven-flow is not caused by temperature difference but by two materials difference.If yes,how can I set the operating density? thank you.

I don't think it's going to work. Boussinesq uses a coefficient of thermal expansion (Beta).

thank you. you are right.but if there also exit difference of temperture,how it can be?

Lets back up a step here. First, if you have two different fluids then I assume that you're using a version of FLUENT that can handle that. Second, I assume that you are performing a transient problem dealing with dispersion of one fluid into another (i.e., well injection into groundwater, etc.). Third, I assume that your two species are non-reacting.

Instead of using the approximation, why don't you just define densities (either constant or perhaps piecewise linear in temperature) for both species, apply the correct gravity vector and let the thing run. The density difference between species should give you a body force.

thank you very much.It is just the problem that I am solving.To calculate the concentration of burning production from the chimney into the air. thank you again, I will try it.