Ansys Employee

OK, I'd go with steady state then. However, depending on the plume behaviour you could find convergence isn't ideal as the flow may have transient features as Raef mentions. Note, I'm using the software to get a "good enough" answer: you need to decide to what level you need to defend your work, and (possibly more importantly) how long you've got.  A velocity of zero probably isn't easily found, plot the axial velocity along the plume centre and see if literature has any half-life type correlations. 

You are going to need to be very careful with the mesh and choice of turbulence model, so there's quite a lot of the manual etc to read up on.