MUT/MU CLIPPED-FineTurbo
 

I am performing a turbomachinery steady simulation for transonic compressor stage using FineTurbo. The inputs are
Inlet : Total pressure extrpolated(The profile values have been defined except for constant turbulent viscosity 0.0001 m2/s)
Outet : Static pressure extrpolated
Numerical Model : CFL -3
Midway during the simulation I recieve a message "MUT/MU CLIPPED FOR X CELLS" and the the convergence curve goes wild.
I increased the MUCLIP value as suggested in the manual. But it shows the same message and behaviour as mentioned.
My concern is that the the constant turbulent viscosity may be the reason for this behaviour. I am not aware of a method to
calculate the turbulent viscosity. Any suggestions on the possible causes for this error message/ methods to calculate
turbulent viscosity are welcome. Thanks

Hi,
granted that I use NUMECA just from a couple of months, and I've found the same problem in some simulations, but the residul didn't have a wild behaviour.
Personally I think that the reason is due to the profiles that you use as boundary conditions; they could be too "hard" for your machine. In particular if you have a transonic field, it doesn't help the convergence.
Let me know.

Hi,

Please do check your mesh quality and make sure the qualities are following the criteria.

Good luck.

@ Alberto
The stage is transonic field. I tried using constant profile conditions but the result is the same.

@ Old shoes
My mesh conforms to the numeca recommended values wrt aspect ratio, orthogonality etc. The current mesh size is about 700000 for the stage. i will refine the mesh and try checking the results.

Thanks
Vikram

Hi,

Is there any improve computed results after your refinement? What is the Reynolds Number of your case?

Help need to simulate flow around Contra rotating Fan
 

I am trying to simulate flow around Contra rotating Fan using Full Non Matching
Mixing Plane. I meshed my geometry in Autogrid5 and I am trying to simulate in FineTurbo. The flow is steady and incompressible with a blade speed of 1029 rpm. I use Spalart–Allmaras turbulence model with velocity inlet (68 m/s), pressure outlet (Radial equilibrium) as the boundary condition and I imposed Farfield velocity as 68 m/s. The Periodic angle for the front rotor is 32.7272 degree and 40 degree. There are nearly 6.5 million cells.

In the initialization of simulation, I gave 98500 pascal as inlet pressure and 90000 pascal as pressure in mixing plane. I ran simulation, after two iteration it say solution blow out, Process killed, Crashed. I tried with different boundary condition but nothing helped me. I am not able to figure out the problem.

Please Can anyone give me some suggestions to run my simulation.

Hello,
I think it is a problem with your boundary conditions. Which value of CFL are you using? try to reduce it :)
Sorry but i am not an expert in that problems...

Hi daxterss,
Thanks for your reply. The value of CFL that I use is 3. I tried to reduce it to 1 but again the calculation blows up after two iteration. Currently I am using velocity inlet and pressure outlet. Could you please tell me what mistake am I doing in boundary condition.

Sorry but I am not an expert in this topic.
Are you running your simulation in the finest grid? or a coarse level? Try to get run in the coarsest level. And when you get the convergence, move to the next level using as inital solution, the solution you got before.

Again thanks for your quick reply. I have started my calculation at coarsest grid level. I did as you said but still I have the blow up problem. I don't know what to do.

Ok, if you try to converge in the coarsest grid and you can't (the coarsest is the grid, the easiest is to converge) I am pretty sure that the problem is with your b.c, but I can't help with this... Sorry
Hope you can solve your problem

Thanks for your suggestion. one quick question. can i try with pressure inlet instead of velocity inlet. The problem is I don't know to set up pressure inlet. At inlet it show me only parameters associated to velocity.

Sorry but I don't know which b.c are the best for your problem

Thanks for your suggestion. one quick question. can i try with pressure inlet instead of velocity inlet. The problem is I don't know to set up pressure inlet. At inlet it show me only parameters associated to velocity.

ok I will work on it and post my issues with my simulation.

Hi,

What is the number of mesh you have within your fine grid and what is the number of mesh you have within your coarse grid?

Hi,
I dont know how to check number of mesh in fine grid and coarse grid. My Mesh has around 5.8 million points. The Number of grid level in quality report shows that 4 3 2. I stated the calculation with coars grid initialization with a CFL number 3. I dont know much about preconditioning parmater so I kept the default value. I will post some images so that you can check my boundary conditions and figure out my mistake. The flow is incompressible and Steady.

In Preconditoning parameter I also tried Merkel method but it does not work,solution blow up after two iteration.

Hi,
My Calculation is running now. I reduced the CFL number to 0.0000001. In the user manual it was suggested to have a minimum CFL number between 0.5 to 1.
Problem is that MAX_RES (Maximum residual) increases with iteration. MIN_RES (Minimum Residual) decreases with iteration which is a normal thing. There is almost no variation in the Force quantities with increase in iteration.

Dont know whether the calculation is running in right path. How can we check whether the calculation is running in right path.

I ran the calculation with CFL number = 0.000001. I started the calculation in coarse grid for 200 iteration. the residuals were decreasing slowly. After 200 iteration, I started to run in fine grid but the solution blew up after 213 iteration.
Can you tell me the reason why the solution blew up in finest grid. thanks in advance for suggestions

Dear venkat;
When the CFL falls below 0.1 while the convergence problems still exists, the source of problem is definitely somewhere else. So, reducing the CFL to 0.0000001 does not help at all!
Following suggestions come to my mind regarding your case:

- Try total pressure at inlet boundary instead of velocity. it is a more stable condition at inlet.
- Check if your boundary conditions are consistent and physically meaningful. moving walls, direction of rotation, amount of pressure, etc.
- Change the output pressure condition from radial equilibrium to averaged one. radial equilibrium condition sometimes induces instabilities within the flow field.
- Make sure that you have set a proper initial velocity field in the axial direction so that the fluid can find an initial stream from inlet to outlet.
- Check the mass flow and make sure that it is physical.
- Check that if your have properly defined rotor/stator interface.
- Check that if you have correctly defined moving and stationary parts.

In general, early divergence in CFD simulations is caused by a mistake in setting boundary conditions, initial conditions or wrong geometry arrangements.
I hope it helps.

Regrads

A bit late on the reply here, but my experience with the viscosity clipping is that it tends to be an issue with your initial solution. Here's why I think that:

The documentation states that the viscosity clipping occurs to prevent the solution from blowing up when mu and mu_turb are very different. This happens most frequently when there is a velocity spike in the flow field. That being said, it isnt a bad thing if the solver prints this message out, so long as it stops printing before you use the solution (it is artificially changing the solution when this message is printing). If you can initialize the flow field to something closer to what you expect the final answer to look like you can sometimes avoid this issue altogether.

Have you tried running an initialization case with easier BCs? With the rotor at 0 rpm?

If you ever did solve the issue, let us know what you did.

Hi, I have similar problem with clipping Mu/Mt and convergency. My case is high pressure centrifugal compressor with IGV (row 1), impeller (row 2) diffuser (row 3) and straighter (row 4) vanes.
Could somebody tell me, may such mesh quality be the reason of numerical unstability?

http://img.leprosorium.com/1628117

So it's very strange that i have good convergence at low pressure at outlet, pretty convergence at high pressure at outlet, but between them calculation is blew up. there is no mass flow or pressure ratio pikes during calculation, it's just message

! DENSITY NEGATIVE IN DOMAIN 10
! PRESSURE NEGATIVE IN DOMAIN 10 1 1 1
! DENSITY NEGATIVE IN DOMAIN 22
! PRESSURE NEGATIVE IN DOMAIN 22 1 1 1

and so on at some iterration, then solution crushed.

maybe some changes in expert parameters needed?
I change just
MVRELF 0.9-> 0.45 (rel/f for meshes with high Aspect ratios/Skew angles)
RQSTDY 1->0 rel/f for mixing plane int/
MUCLIP 5000->50000 (as recommended in manual)


BC's are total pressure/temp at inlet, static pressure at outlet