SimScale CAE Forum

Supersonic flow on a rocket


#1

Hello, everyone!

I am trying to run a simulation of a rocket flying at Mach 2 with approximately 8000 Pa of atmospheric pressure por a school project.
I keep getting problems when I start the simulation run, as you can see in the solver log down below. I searched the forum for similar cases and all I could find was that the mesh was having too many illegal faces. I don´t think that is my case here, but I could try to refine it just to be sure.

The project link is https://www.simscale.com/workbench/?pid=4242617502015661596&mi=spec%3A26b3d619-61a4-49a7-bf2f-6f65ab375a71%2Cservice%3AMESHING%2Cstrategy%3A3&sh=1

Thanks a lot in advance!!
Pablo.


#2

Hi Pablo - nice project you have there!

Would be nice if we can make this work, have the Falcon Heavy in my Workspace which also needs to be finished at some point :smiley: I saw that you have used a density based formulation which sometimes struggles for strong shocks. I remember that my colleagues @Get_Barried & @1318980 worked on compressible simulations in the past who might add their two cents here. Setup wise the simulation looks quite good but I would consider optimizing the mesh (not only have a bigger distance in front of the rocket from the inlet but only work with less refinements which sometimes helps) - however you can see that the layers are not uniformly distributed along the rocket…

All the best!

Jousef


#3

Hi jousefm!

Thanks a lot for the answer. I will take into consideration the suggestions you made, but do you really think it is necessary to have a bigger distance in front of the rocket from the inlet? I made this small distance on purpose because the air in front of the rocket does not “know” that the rocket is coming because it is going supersonic.

Cheers!
Pablo


#4

Hey Pablo!

Indeed you can try it with the small distance and see if that works - I usually give it a bit more space. Let us know how your progress goes and if you need any assistance.

Best,

Jousef


#5

Hi @puboldi,

We’ve had some difficulty getting supersonic flow to behave properly. I suggest trying out a run and we’ll work out solutions from there.

And yes that is mostly true that the inlet distance can be small provided you are using a laminar model.

Cheers.

Regards,
Barry


#6

Hi, guys!

I re-meshed the model giving it a little bit more space in front of the rocket and making it a little simpler, then I made another run (run 8).

The problem now was that the instance storage run out of space. The suggestion says that I reduce the number of timesteps that are stored by increasing the write interval in the simulation control settings. This is very strange, because I specifically set the write interval big enough so that it writes only 7 times.

I made this calculation:
start time: 0 s
end time: 0.35 s
delta t: 0.005 s

So, there should be 0.35/0.005 = 70 steps. I set the write interval to 10 timesteps, so it should only write 7 times, but the simulation returned the error anyway.

I will make another run setting the “Write control” parameter to “Adjustable runtime” and setting the “Write interval” value to “0.175 s” so that it only writes the result twice.

I hope I’m doing this right.

Cheers!
Pablo


#7

Hi @puboldi,

I suggest deleting all of your runs that you do not require and other projects/geometries that you also may not require to free up the storage space allocated to you.


#8

Oh! I didn’t know I had limited storage space. Thanks for the tip, I deleted what I didn’t need.

I think I know what was wrong with my setup.
Reading the solver log and investigating here https://www.simscale.com/blog/2017/08/cfl-condition/ I could figure out that my simulation crashes because it reaches a Courant number higher than 1 and I think this is a combination of a thin mesh, a fast speed flow and a big “delta t”. I will have to find a balance between this 3 variables in order to find a good result because a thinner mesh will necesarily require a smaller “delta t” and that will result in a longer simulation run.

Whatever progress I make, I will update it here in case someone has the same problems I had.

Thanks a lot!

Cheers!
Pablo