SimScale CAE Forum

Error in simulation of compressible flow that was performing well before

I am lecturer at the UPC (Spain) and I am using Simscale since several years ago for introducing my students to CFD. One of the tutorials I use is the simulation of a shock wave in a De Laval nozzle. Until some moths ago, I was able to simulation a Mach 5 shock wave, but then the density-based solver disappeared from Simscale, and only the pressure-based solver was available. So, I adapted the tutorial to capture a 1.15 Mach number shock wave. No problem.
But now, last week indeed, something more changed. My students reported that simulations that where performing well are giving run errors.
The error is:

Pressure field started diverging. Please check the mesh quality near the reported location and try refining the mesh. If the problem occurred near a boundary, please check the boundary conditions. In case of doubt, please ask for assistance via our support chat.

Please check the project here:
https://www.simscale.com/projects/castilla/de_laval_nozzle_2/
The simulation is “Copy of Compressible” (the first one was with the strong shock wave)
The Run13 is identical to Run11(second) with two weeks of difference. The Run11 performed without problem, the first crashed.
I am now trying with a refined mesh, but I (and my students) would like to know why this simulation is now failing.
With best regards
Robert

Hi Robert!

I am a bit busy at the moment but will have a look at it this evening/over the weekend. For your interest, check out this doc entry on the laval nozzle: https://www.simscale.com/docs/validation-cases/compressible-flow-de-laval-nozzle/ - hope that helps!

Jousef

Hey Robert,

I copied your project and started a run with custom inlet & custom outlet BCs, based on your run 11. So far it’s looking quite similar to your results, if you want to have a look: https://www.simscale.com/workbench/?pid=3753262523291023370&mi=run%3A32%2Csimulation%3A30&mt=SIMULATION_RUN

Let me know your thoughts!

Hi Robert!

I am working with the same case and I had the same problem. To solve it I have followed the same procedure as this tutorial:

It seem that the problem was with the boundary conditions, the solver does not like a high increment from the initial conditions and the boundary conditions. So I have ramped the pressure.

Take a look at Run 4 and Run 5, the ones with a pressure table (different Pinlet). The results are not perfect but at least they are closer than before.

https://www.simscale.com/projects/Sguares/nozzle_compressible_flow/

Thanks,

Santi

Hi, Santi!

Thak you so much for your response. Yes, you are right. I was performing this pressure ramp in the pressure in the previous simulations, and it was working fine with density-based solver. But it started to fail, , even with the pressure inlet ramp, and I hade to switch to pressure-based solver.
I have been looking at your project, and it seems to work smoothly. Nevertheless, now the course with my student is over, and I have modified the simulation with a much lower inlet pressure. Also, I am quite busy with exams. I save it to study it later.
Again, thanks.
Regards
Robert