Ansys Employee

As ANSYS staff I'm limited in scope when it comes to offering advice. 

In your case you need to determine what goal you wish to achieve: do you want to know how far the plume goes or how long it takes to develop? The former is steady state, the latter is transient. Now consider that a steady solution takes a few hundred to a few thousand iterations to converge and has one answer: a transient solution takes thousands of timesteps each of 15-20 iterations so will take significantly longer to run. 

The successful application of simulation is as much down to planning the work as it is to driving the software. Once you know the project goals, you can plan the modelling strategy and then do it.