Hi! All.

I am not so sure on how to decidee the time step size in Finite Volume Method for 3 D natural convection (laminar flow) case. Could you please give me some information?

Thanks.

It's about the time for flow passing from inlet to outlet.

I THINK YOU NEED TO TRY FROM THE SMALL TIME STEP TO LARGE TIME STEP UNTIL IT CAN NOT CONVERGE

I am assuming your question refers to solving the unsteady equations with a real "time step" and not the steady equations with "iteration steps".

In general there are two reasons why the time step is important: numerical stability and accuracy. The first one is an issue only with explicit time marching, and the answer to your question will depend on the scheme you are using. The second one (accuracy) is actually more difficult to address. You'll need to know a little bit about the flow before you can decide what "small" and "large" even means, regarding the time step. For starters, it would be good to know some fundamental length and velocity in your problem, which could help you define a fundamental time scale. I suppose a proper velocity might not be easy to define in the case of natural convection, but you might start with a steady-state solution.

Then, when you are going to resolve some unsteady phenomenon (vortex shedding for example), you'll need to roughly know what (range of) frequencies to expect. Of course, you don't need to know this in exact terms, but estimations are needed to get you started. Experimental data on similar problems might help you out, here. Once you have a reasonable (but inaccurate) unsteady solution, you can refine your time step to increase accuracy.

you use a implicit method or explicit method .