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• Is it unsteady or transient? Does CFL matter in PISO & PIMPLE?

Posted by on September 9, 2023 at 7:43 am

Good day prof. Baris and collegues,

I have some inquiries related to openfoam on non-steady simulations…

In my knowledge (I could be wrong), ‘transient’ word is from moving from empty domain to an filled domain or from a flow with U=0 to motion, ‘unsteady’, for already domain filled in motion, but it some OpenFOAM documentation looks like for any case non-steady, uses the word transient, are my concepts wrong?

Thinking that OpenFOAM takes unsteady=transient, the only application solvers for ‘non-steady’ cases are PISO & PIMPLE, right?

As PISO and PIMPLE are described as ‘implicit’ in time (Crank-Nicholson / Euler…), do we have to take care of CFL <1 as we’ll do in an explicit scheme?

Can weÂ  in OpenFOAM run ‘explicit time’ schemes?

Thank you,

• BarÄ±ÅŸ Bicer

Moderator
September 11, 2023 at 8:58 pm

Hi Maria,

Unsteady or transient ( I didnt often heard non-steady) flows where you take into account time dependent change of any fields (as you can see in the unsteady term (rho dV/dt) of navier stoke eqn.)

Definitely, in OpenFOAM unsteady means transient simulation as you explained that used in PISO and PIMPLE which are implicit and of course at the beginning you should start CFL<1 and in some cases you can slightly increase CFL number in order to increase the time step in implicit solvers. that is one of the advantage of implicit solver compared to explicit one.

In OpenFOAM, I saw some explicit solvers where you have to use so small-time steps to catch e.g. shock formations for compressible high-speed flows. In these solvers, Shocks are explicitly considered when
solving equations. (e.g rhoCentralFoam)

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