[NingIt]

I analyzed transient solution of multicopter using PISO first, but the residual and coefficients diverged. However using SIMPLE calculated the solution stably.
Initial condition was from the steady state solution using MRF. That was calculated with SIMPLE because coupled method had too long time for convergence.
I've been aware of the PISO was intended for transient solution but why does this happened? Is it OK to use SIMPLE for transient solution?
I used sliding mesh method, turbulence model was SST k-omega and time step was 8.33e-0.5

[LuckyTran]

You can use SIMPLE or PISO for transient calculations, but the strategy is different. For practical engineering problems, the vast majority of transient calculations are done with SIMPLE, If I had to make an estimate, I'd say 90% of people use SIMPLE and only very few use PISO. With PISO you must take very small time-steps such that the courant number is around 1 or it will easily diverge. The finer the mesh the smaller the time-step you need to maintain PISO's accuracy and the computational cost skyrockets quickly. SIMPLE tolerates larger time-steps but...


SIMPLE needs multiple sweeps to make make the velocity correction ad pressure correction consistent with one another and for the non-orthogonal coupling to take place. PISO does not.

PISO is well suited for transient problems where you take small time-steps and perform 1, at most 2 iterations per time-step. However, the small time-step size needed can be prohibitive. When you must take large time-steps, or when you plan to do multiple iterations per time-step anyway, SIMPLE is generally preferred. If you are doing PISO and taking many iterations per time-step then you are doing CFD the wrong way.