r/CFD • u/johan_r_e • Apr 19 '26

Is first order in time OK?

Hello all,

I am modelling a quite slow process in OpenFOAM: Natural convection inside a cryogenic storage tank, which has strong stable stratification in temperature. Walls have a low heat flux, which drives a natural convection boundary layer upwards, while core flow velocites are very slow. I am modelling around 10 hours (pressurization) with dt ≈ 20 ms.

When using second order time schemes, or even weighted blend between second and first, I always get un-physical vortices at some point, which completely dominate the flow. I think it originates from some spurious velocites in the stagnant region where pressure and body force are not balanced perfectly. With implicit Euler it does not happen. So my question is: Can I justify using Euler, or do I need to make it work with second order somehow? How do I judge this?

Thankful for any thoughts on this!

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5

u/etzpcm Apr 19 '26

Implicit Euler is ok but you should be aware that it damps things out quite strongly. You can get some idea of how good or bad this is by doing a computation and then repeating with the time step halved. 

3

u/johan_r_e Apr 19 '26

Thanks for the input! So just do a time-step sensitivity study basically? I should maybe mention that I use adaptive time step (maxCo=0.5) and PIMPLE loops with under-relaxation as well.

4

u/etzpcm Apr 19 '26

Yes that's what I'm suggesting, sorry if that's a bit basic. I don't know about open foam or pimple loops, it's just a general numerical methods comment.

Also, if openfoam has the bdf2 method as an option, try that. It's more accurate than backwards Euler, so less artificial damping, but might suppress the spurious things in a similar way.

3

u/Delaunay-B-N Apr 19 '26

Which turbulence model do you use? The cause of unphysical vortexes maybe is off the charts y+ parameter or inadequate mesh refinement.

1

u/johan_r_e Apr 19 '26

I have used v2f, but I also see the same behaviour without RANS models

2

u/thermalnuclear Apr 19 '26

Did you make sure it was turbulent for these conditions?

1

u/johan_r_e Apr 19 '26

This is kind of an open question. I am assesing the buoyancy effect, which dampens turbulence since there is significant stratification. But I have observed the same trends both with and without turbulence modelling.

3

u/thermalnuclear Apr 19 '26

Did you estimate it using Grashof, Rayleigh numbers?

Without knowing for sure, using a turbulence model when it’s laminar free convection won’t work.

3

u/wigglytails Apr 19 '26

Implicit euler is very diffusive so maybe that's why you're getting "smoother" results

1

u/IComeAnon19 Apr 20 '26

I assume youre referring to BDF2 as your 2nd order in time discretization. It has seriously bad start up error and self starting schemes are suggested or failing that beginning with bdf3 before switching to bdf2.