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Re: Underestimation of velocity, compressible model

Posted: Thu Nov 18, 2021 12:56 pm
by Christopher
Hi Yvan,

anything new from this side?

Kind regards,
Christopher

Re: Underestimation of velocity, compressible model

Posted: Fri Nov 19, 2021 12:45 am
by Yvan Fournier
Hello,

Sorry, I had forgotten. Thanks for reminding me.

The attached user parameters sets up cv0 where needed, though I used the same value for CV that you set for Cp (just for checking). With that value, I have a negative pressure after 2 iterations, so I'll let you check and set the correct value.

If things still fail, I'll need to check with colleagues who know the compressible module better...

Best regards,

Yvan

Re: Underestimation of velocity, compressible model

Posted: Mon Nov 22, 2021 9:32 am
by Christopher
Hi Yvan,

thank you. Right now a calculation is running. After that I will give it a try.

By the way, is there a limit to the calculation steps in CS7.1? I am running a compressible hot air flow simulation on a locally very refined mesh (smallest mesh size = 0,1 mm). With a max CFL <= 0,4, the achievable time step is quite small. So it goes up to more than one million time steps to be calculated. However, the solver hangs up after about 1.500.000 steps and stops responding. The desired physical time is 0,15 s, but it reaches only 0,8 s before it hangs up.

Kind regrads,
Christopher

Re: Underestimation of velocity, compressible model

Posted: Tue Nov 23, 2021 6:01 pm
by Yvan Fournier
Hello,

No, there is no theoretical time step limit.
Or at least, the limit would be ar 2^32, so a little over one billion.

Did you try using a computation restart ? What happens if you increase the reference time step a bit. You might not lose stability immediately (though you can't use a too high CFL, you can test a bit higher).

Best regards,

Yvan

Re: Underestimation of velocity, compressible model

Posted: Wed Nov 24, 2021 9:33 am
by Christopher
Hi Yvan,

I will try that as well and inform you about the results.

So far, I haven't tried a computation restart. Might be a good idea to calculate to 1.000.000 steps and then restart from there to continue. Unfortunatly, I can't increase the reference time step because of negativ energy encountered in a few cells. The max. t <= 1e-07.

However, the hang up could be caused by Ubuntu and not CS.

Kind ragards,
Christopher

Re: Underestimation of velocity, compressible model

Posted: Tue Nov 30, 2021 9:11 am
by Christopher
Hello Yvan,

so far, I tried the computation restart. It works. I can go above 1.500.000 steps by splitting the caluculation in parts with checkpoints at each ends.

However, one more question.

I wanted to start the calculation with a coarse mesh for 600,000 iterations and a larger time step and then restart with a fine mesh for the rest using the "Different mesh" option in CS. However, the restart aborts with an error message. It says that the calculation cannot start with different cell counts.

Is there an assignment (mapping) option to transfer the results of the first calculation with the coarse mesh to the second with the finer mesh with more cells? Or what is the option for different meshes for?

Kind regards,
Christopher

Re: Underestimation of velocity, compressible model

Posted: Tue Nov 30, 2021 10:48 am
by Yvan Fournier
Hello,

Yes, in v7.0, there is an option to restart on a different mesh. You have a box to check for this in the restart definition section in the GUI, and must provide a link to the original mesh there (many users get confused and invert the initial and new mesh, so if things do not work when checking the box, try inverting the 2).

Best regards,

Yvan

Re: Underestimation of velocity, compressible model

Posted: Tue Nov 30, 2021 12:42 pm
by Christopher
Hi Yvan,

alright. So to do it right, I have to replace the old (original) mesh with the new one under the mesh menu and then place a link to the old mesh under restart => different mesh, right?

Kind regards,
Christopher

Re: Underestimation of velocity, compressible model

Posted: Tue Nov 30, 2021 6:21 pm
by Yvan Fournier
Hello,

Yes, if I remember correctly, that is how it works. In any case, there are not so many possible combinations, and only one of them can work :-).

Best regards,

Yvan

Re: Underestimation of velocity, compressible model

Posted: Wed Dec 22, 2021 12:14 pm
by Christopher
Hi Yvan,

so far I can say that your cs_user_params works. I can't see any negative pressure. The difference between cp and cv is just the isentropic exponent k.

Next I will compare the calculation results with our experimental results. I will make two runs. One with the stiffend gas model and another with a strong mesh refinement at the nozzle outlet. It could be that a millimeter mesh size and a bit below is still too coarse to correctly capture all these physical properties. Therefore, this could be another reason for the underestimation. But for now it is hard to say.

Once the calculations are complete, I will post the result here and let you and other participants know.

I wish everyone a Merry Christmas and a Happy (and healthy) New Year.

Yours sincerely,
Christopher