SimScale CAE Forum

Simulation only runs for one time step

Hello! New user here,
I am trying to model converging-diverging rocket nozzles, but no matter what I’ve done the simulation only runs for one time step and gives the error “Maximum number of iterations exceeded when calculating temperature from a thermodynamic potential.” So far I’ve tried:
-smoothing out corners in the geometry (I’m not modeling a specific nozzle, just a test piece)
-modeling half the nozzle with symmetry
-decreasing delta t to .01 sec and .001 sec (still running for 1000 steps though)
-increasing maximum run time
-inflating the boundary layer
-refining the symmetry surface (it had some meshing artifacts)
-refining the throat region
-increasing the pressure/velocity absolute tolerances

A pair of expert eyes on this would be a great help.
Project link:
https://www.simscale.com/projects/nbabusis/smooth_bell_nozzle_test/

Hi @nbabusis, will have a look at your project later on.

In the meantime, can you have a look at this setup here: https://www.simscale.com/docs/validation-cases/compressible-flow-de-laval-nozzle/ - maybe it gives you an idea how to set up a compressible fluid flow analysis. If there are still problems, the @CFD-SQUAD and me would be happy to jump back in.

Best,

Jousef

Thank you for the quick advice!
That tutorial was some help. I am trying a new version of the simulation here as a wedge (although I don’t know how to mesh the wedge in 2D, so I am using symmetry for now):
https://www.simscale.com/projects/nbabusis/bell_nozzle_wedge_-test-/

I managed to get the non-wedge simulation to work by using a velocity inlet at very low velocity, but whenever I set the velocity to its real value or ramp it up, the quantities diverge, particularly the temperature, which goes to <0 K. I’m not sure why this is happening, since I am using a fine mesh with no apparent defects.
Any advice?

@CFD-SQUAD
Still having problems with the simulation. I am now working with the “real nozzle” geometry but the problems seem to be the same for both geometries. I can get the simulation to run without errors, but only for subsonic inlet velocities and absolute U and P tolerances 100x higher than the default. The convergence plots for these simulations also seem pretty rough and all over the place, so that may be a symptom of the problem. Whenever I ramp the velocity up to 800 m/s (~Mach 1), I get the error that the temperature is some huge negative number at the outlet or wall.
Any idea what’s going wrong?

Hello nbabusis,

Looking at your simulation it appears you are using a very large nozzle, 30 meters in length. One thing I may suggest to do is to add length to the model. So what you could do here is extend the outlet, for some further length, for your model I would look at extending to maybe 120 meters, maybe even further if this doesn’t produce adequate results. The extension can have the same cross section area and shape as your outlet currently, it just needs more tube length per say.

This will allow the residuals more time to converge and also allow the solver more time to come to equilibrium. When you do this you wouldn’t have to have the same mesh fineness for your throat region as you would this outlet region some so mesh refinements may be of use with this approach.

Edit: You should remove the boundary condition for the exhaust on face 522. You just want the end of your tube to be the exhaust not a side wall. Also I would remove the Wall boundary condition you are using. This does not seem to make sense in the realm of what you are simulating, but if it is necessary for your analysis keep it.

Let me know what you think.

Best,
Jake