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October 20, 2015, 04:30 
Solution diverges

#1 
New Member
gio
Join Date: Oct 2015
Posts: 4
Rep Power: 4 
Hello everyone
I am new to Prostar. I am trying to do a stationary mass flow simulation using these settings: Stagnation Inlet: Pstag = 0.000E+00 for inlet Pressure boundary Constant static pressure: P =6.300E+03 for outlet. SOLUTION PROCEDURE SIMPLE RESIDUAL TOLERANCE 1.00E03 FLUID FLOW TURBULENT INCOMPRESSIBLE TURBULENCE MODEL HIGH RE KEPS MODEL REFERENCE PRESSURE PREF = 1.000E+05 Pa The problem is simple because I know only pressure for inlet and outlet. The solution diverges. I also tried PISO solution algorithm but the solution diverges. I did the same simulation using CCM and the solution does not diverges(the result is ok!) I tried to change in prostar the inlet boundary using pressure boundary and it works but the results is not what I expect. Using stagnation inlet in prostar there are master processor reported warnings in file star.info Warning 020: The center of cell adjacent to boundary is outside the cell, 220768 or cell is concave Can you help me? Thanks in advance 

October 30, 2015, 07:31 

#2 
Senior Member
Blanco
Join Date: Mar 2009
Location: Torino, Italy
Posts: 159
Rep Power: 10 
Are you using the same mesh between CCM+ and StarCd?
The warning you get is not so serious, anyway I would check mesh quality on the inlet boundary (you have some concave cells there). Try to modify underelaxation factors for pressure and velocity, then note that a pressurepressure boundary is not the best setup from a numerical perspective, if I remember well this is written also in the userguide. Double check your simulation setup. If your setup is really correct, then at last you can try to start with a small pressure difference between inlet and outlet, solve for it, then use the result to start a new simulation changing only the pressure difference (going to your desired value). This could help but I'm pretty sure this would work by changing only the underelax factors. Regards, Andrea 

November 3, 2015, 12:31 

#3 
New Member
gio
Join Date: Oct 2015
Posts: 4
Rep Power: 4 
Hi Andrea,
Thank you for your answer, Yes, I am using the same mesh between CCM+ and StarCd. I tried to change underelax factor using PISO solution algorithm but there is this error: Error076 negative densities found at more than 100 cells. I used underelax factor=0.5, 0.7, 1. I did the same simulation(stagnation inlet/pressure outlet) using CCM and the result is ok. Can it be a mesh problem? Regards, Gio 

November 4, 2015, 05:27 

#4 
Senior Member
Blanco
Join Date: Mar 2009
Location: Torino, Italy
Posts: 159
Rep Power: 10 
Ok, the negative densities error means that something is wrong somewhere in the setup. I know that 2 pressure boundaries destabilize the sim from the numerical point of view, however if CCM+ can manage to give you a solution, also Star should be able to do it. This even if the mesh is not so good (in any case I would double check the mesh. Remember that you can see "where" you get negative densities...)
I would put down underelax factor for pressure to 0,1, 0,5 for velocity, to stabilize as much as possible the numerics. Then I would check boundary conditions and initial conditions: are boundary conditions very different compared to initial conditions? If yes, this could cause instability in the runup of the sim (and hence negative densities). You could use a better initialization or a varying boundary conditions to start the sim. The other option is to proceed with the other method I suggested (do a first run with reduced pressure difference between inlet and outlet, modify boundary cond., restart). Good luck! Andrea 

November 5, 2015, 13:04 

#5 
New Member
gio
Join Date: Oct 2015
Posts: 4
Rep Power: 4 
I partially solved the problem:
If I use SIMPLE solution algorithm and compressible flow changing also fluid initialization, it works. If I try to run solver using the same setup but incompressible flow, it not works(Error012). PISO solution gives Error076. I'll try your suggestions and tell you, thank you very much! 

Tags 
diverges, mass flow, prostar 
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