this should improve convergence due to tighter coupling however you tripple your ram requirements

Are you still talking about openfoam?

https://openfoam.org/release/2-2-0/matrix-solvers/ Probably this.

Ahhh okay. This is only through the Laplacian operator, though. Quite an overhead for very little benefit

Is it correct that $p_1 + \gamma_W h = p_2 +\gamma_w (2\tan 30) + \gamma_m h$? https://cdn.discordapp.com/attachments/726377664213942323/1368901998010826752/Screenshot_2025-04-24_at_14.49.33.png?ex=6819e8d5&is=68189755&hm=e675b3a56e69650cef504a4abbf32ff266452ac29927e1fc8e37d8deee8f7e15&

mathmo learning intro fluid mech https://cdn.discordapp.com/attachments/726377664213942323/1368902008047800330/711934347934040125.png?ex=6819e8d7&is=68189757&hm=f34e5fd9ad650333071fa36bbec45592e69100ecc46ab69c7f4006a3b16a76e1&

I.e. the pressure at the 3 and 4 on my diagram is the same

yes, the pressure at 3 and 4 on your diagram is the same

between 1 and 3 is a difference according to the density of water, between 4 and 5 is a difference according to the density of mercury, and between 2 and 5 is a difference according to the density of water

yes, this equation appears to be correct

Hey friends, just seeking some advice on pimple.I have an interFoam case, so multiphase volume of fluid. I have almost exclusively run PISO based sims in the past but in this particular case, I'd like to do bigger delta T and skip intial transients quickly. Ive seen that we can do high Co with pimple and I'm getting quick convergence at Co=0.8 right now with 2-3 outerCorrs needed per timestep. I wonder how much can/should I safely stretch the Co limit? Any advice?

I have feeling the multiphase discretization scheme may have some limits (from vague recall of moukalled's chapter on TVDNVD schemes)

Are you using turbulence scheme ?

hey btw

If I recal for LES I think I saw somewhere in tutorials from Jozsef Nagy (or someone else) that usually you don't want to have Co > 0.7

Either way for turbulence I think it was around 1 and 2 max for some simulation. I am outta work now but I can have a look at it tomorrow morning if you are interested

Its RANS, k epsilon standard...

Yeah okay. I'll stay conservative at 0.8 for now, coz this is a lonnng sim and the only priority above speed is that the solution shouldn't turn out totally crap.

So after asking my seniors, ideal is 0.5 for RANS and never above 1

(my group works in thermodynamics applied in water boiling)

I'm now at Co=0.3 😬 although a completely different case

So just my 50cts that I would not trust at all. Basically each time we solve an equation in OpenFOAM we implicitly solve a linearised version of that equation. This means that to deal with the nonlinearity of the equation we must repeat this process multiply times, and with each repitition the temporal "solution" behaviour of our solver gets closer to a "truly non linear euler implicit" solver. Which is unconditionaly stable.

Now stable is very far away from accurate. Euler implicit is only accurate for processes that are "slow" with regards to its timestep. Thats why to get accurate results you must have a low Courant Number, whereas when just stability is a concern you can crank up the corrector steps, and take advantage of the high stability.

Since you mentioned being afraid of creating an unphysical solution by running with a high co number at first, from which you cant recover afterwards even if you lower the co number I would argue, that this risk allways exists. Since your initial guess is most likley a unphysical one. We most allways assume that the solver will converge to a physical solution even when starting with a unphysical one therefore Id say starting with a large timestep, even though that will lead to fast processes being way to strongly dampened is not a problem.

Yeah I just need to get past the initial transient phase even it is not very physical in that period. But I'm not sure if I should do a Co>1 in multiphase as the interface may begin to fly off into oblivion.

I fear that is case dependent but of cause with keeping it low your on the safe side

not a multiphase expert, but could it help to use local time stepping?

No unfortunately I don't think this aligns with my use case.

First simulate with a coarse mesh, and then map the solution to fine mesh and continue

I really appreciate the suggestion 🙂 I wish I could do that - its an annular pipe with core of one phase and annulus of another, so I cannot do without a certain resolution - I do have super-coarse mesh everywhere that I could afford (everywhere away from the region of interest - after having convinced myself that solution remains largely unaffected by super coarse away mesh) I did think of breaking it down into AMI type two domains, generating one part the problematic annulus by itself to steadiness and then using its surface as an interface with downstream part to develop it separately. But unfortunately I don't have so much time to experiment with these tools at the moment so I'm just brute forcing through the whole domain. Unfortunately it comes with the cost of time, as may be expected.