how can we calculate time step for buoyancy induced (created because of species concentration) flows?

Not sure what the standard method for doing this is, but I reckon a quick calculation could be: Force=(delta_rho*g)/cell_volume=rho*flux_area*u**2 and then use u to calculate your cell residence time. You don't indicate whether you are just doing a quick hand calc for an initial guess at a set time-step, or whether you're trying to do something clever in code. Like I say, I just made this up, so if anyone has a better idea feel free. This assumes that the buoyant momentum transfer is the most significant factor, but it might to turbulence, in which case your Strouhal number might be important. Also, I haven't include anything for the volume fraction of your species concentration, which should go somewhere near the delta_rho*g bit.

hi there, refer to computational heat transfer by Jaularia, Torrance, c 1986, page 219 if u are using fd method, but otherwise you should use your time scale or CFL criterion, but scale is not meant to equal so time step <= there is a certain constant * time scale, in that case you have to find a paper which do sclaings and found what the right constant is. The other is guess any time step and work throgh it till you get grid independence result, hope I have been of help bye taw

Thank you, Its FVM code, so as you r telling I calculated time step using CFL criterion, since convection is domiant in my case, but later i would like to simulate combustion along with flow. So, does this holds for the later case also or is there any differece.