[afnene]

Please I am simulating transient natural convection using fluent, and I would really appreciate your help.
is it better to use Piso or Simple in pressure-velocity coupling?
What is the meaning of skeweness correction and neighbor coupling in piso scheme?

I put the default relation factor to the half default values....is it the best choice?

In solution methods, there is an option where the relaxation factor is set to 0.75 by default, do I need to change it?

thank you for your help in advance.

[LuckyTran]

PISO is generally preferred for transient simulations because you can use higher urf's (faster convergence) and bigger time-steps (fewer time steps need to be computed) compared to SIMPLE. But SIMPLEC is also faster than SIMPLE and might be worth considering. If you want small time-steps anyway, then the advantages of PISO are lost, because it is more expensive than SIMPLE.

It's best to keep the default urf's unless you are having problems with convergence. Reduce them only if you run into trouble. You might even want to increase the urf's to get faster convergence.

The additional neighbor correction step is what makes PISO different than SIMPLE (or call it a momentum correction). Skewness correction refers to updating the fluxes for an additional pressure correction. Skewness correction can be done on SIMPLE also, but the neighbor correction is a little unique to PISO.

All these corrections are a result of using predictor-corrector approach used in the segregated pressure based solver. If you used a coupled solver, you wouldn't need these corrections.