pisoFoam basics...
 

Hello Foamers,

How do we know what time step or mesh density to use with pisoFoam? I know this should be introductory CFD basics, but it escapes me.

The pisoFoam tutorial, for instance... it is currently set to deltaT = 0.005. If I re-run the simulation with the time step 1/10 the size (deltaT = 0.0005), the results are different. The pressure field differs by 15% and phi differs by 8%.

Any insights would be much appreciated.

-Dave

I don't think there's a simple answer to that. There are a priori error estimates (e.g. http://www.ams.org/journals/mcom/199...18-96-00701-6/), but their use is limited. In most cases, people have a posteriori error estimates, such as rerunning with a smaller timestep or higher mesh resolution, and then estimating the error e.g. with GCI or Richardson extrapolation. Jasak's thesis also might be a good read.

Thanks. So it is just plain old trial and error, then. Man, CFD is littered with it.

Hii,

Do you have any idea about what the Mean/Max Courant Number (hence specifying the time step accordingly) should be for getting meaningful results with pisoFoam?

If this is regarding the problem I was facing, the max was very small... around 10e-6. If this is a general question, then no, that's partly what I wanted to know.

-Dave

That was just a general question I was curious about.