Divergence problem
"Already posted this yesterday but no response that's why" :(
Hello, I am getting this problem with the 3D VOF (air jet impinging over water). The simulation is good till 1.4e2s but after that it gives the following error. The time step size i am using is 1e6 with 40 time steps. The setting are as follows: 1. Double with SST 2. Surface tension 0.073 n/m 3. inlet velocity 96.672 m/s 4. PISO Scheme with VF(Mod HRIC) Momentum (2nd ord) 5. URF Pressure 0.3 Density 0.5 Body Forces 1 VF 0.5 In 2D for the same thing, i had mass loss problem but i was able to find the solution with the help of few people here. Would be thankful for your kind reply. Regards, Smaras ===>>>>turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 240257 cells # Divergence detected in AMG solver: xmomentum > Increasing relaxation sweeps! Divergence detected in AMG solver: xmomentum Divergence detected in AMG solver: ymomentum Divergence detected in AMG solver: zmomentum Divergence detected in AMG solver: vof1 Divergence detected in AMG solver: xmomentum Divergence detected in AMG solver: ymomentum Divergence detected in AMG solver: zmomentum Divergence detected in AMG solver: vof1 Divergence detected in AMG solver: xmomentum Divergence detected in AMG solver: ymomentum Divergence detected in AMG solver: zmomentum Divergence detected in AMG solver: vof1 Divergence detected in AMG solver: xmomentum Divergence detected in AMG solver: ymomentum Divergence detected in AMG solver: zmomentum Divergence detected in AMG solver: vof1 Divergence detected in AMG solver: xmomentum Divergence detected in AMG solver: ymomentum Divergence detected in AMG solver: zmomentum Divergence detected in AMG solver: vof1 Divergence detected in AMG solver: xmomentum Divergence detected in AMG solver: ymomentum Divergence detected in AMG solver: zmomentum Divergence detected in AMG solver: vof1 turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 251038 cells Primitive Error at Node 0: floating point exception Primitive Error at Node 1: floating point exception Primitive Error at Node 2: floating point exception Primitive Error at Node 3: floating point exception Primitive Error at Node 4: floating point exception Primitive Error at Node 5: floating point exception<<<<==== 
1) Does each time step converge until these errors appear? Can you show a residual plot?
2) Did you make a solution animation? Can you see something curious in the results until the errors? 
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1. Yes, from very start it was converging, but just before the diverging occurs three or four time steps has no convergence rather turbulent viscosity. 2. Yes, i did only did for the penetration depth. Please see the attachments. I thinks it's more clear in that. http://imageshack.us/a/img526/7353/velocity.th.jpg http://imageshack.us/a/img839/6499/residuals.th.jpg http://imageshack.us/a/img201/6622/p...ondepth.th.jpg http://imageshack.us/a/img827/964/fluent1410780.th.jpg Thanks once again and looking forward for your reply. Regards, Azam 
hi
Sometimes divergence problem is due to bad meshing that you have used .
Try to refine your mesh and run the simulation and let me know. try to decrease the under relaxation factors also tell what is the courunt number in your simulation. Regards shk 
I think shk is right here. It looks like the fluid firstly moves through a domain of "good mesh" where the solver is able to converge sufficiently, but then enters some bad cells, where the mess starts.
You should a) post a picture of your mesh b) show us the courant number (i think Fluent can show cell courant number) c) in the meantime let the simulation run with a smaller time step. Edit: before you do c) please try, if you can reduce the residuals right from the start each time step better than now. It looks like you get "continuity" to about 1.0e3. Can you get better convergence? How? By increasing the number of iterations per time step or only by reducing the time step? 
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This is my mesh a) http://imageshack.us/a/img833/6899/meshj.th.png Now just as shk as said i rechecked in fluent i am getting mesh http://imageshack.us/a/img685/8620/fluent.th.jpg http://imageshack.us/a/img708/7589/meshw.th.png http://imageshack.us/a/img855/1880/fluent1.th.jpg Rodriguez i have used smaller steps but this is stall point where it crashes. No matter the step size is small or large. I even tried number of iteration but no use. Now what should i do???? regards, Smaras 
1) Post a picture of the Courant number shortly before it crashes.
2) Why do you use a prism layer at the wall? I don't think you expect any flow along that wall, right? As I understand it, a prism layer isn't helpful in such a case. 3) Can you use SIMPLE algorithm for your model? In my experience this is the most robust one. 4) Do you have ICEM for meshing? 
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http://imageshack.us/a/img51/6087/courantno1.th.jpg 2,3) Ok i will try that without prism layers. (after your reply) 4) Yes i am using ICEM for meshing. 
If you have ICEM, you could make an excellent hexamesh. Your geometry is really simple and blocking would be straight forward. Don't use tet meshs unless you are not able to do the blocking. They are numerically lowgrade.

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I know Rodriguez but this isn't the real geometry i will be working on more complex geometry, and the requirement for my thesis is tetra meshing. This is just a mockup or example to make practice which are based on previous research papers. I am today remeshing it without the prism layer and plus more refined mesh. Further i have 2 questions: 1. I need to know one more thing i am using densities to get precise result for the jet flow and its impingement. as shown previously. Is it creating the problem???? 2. Further after opening opening the geometry in fluent there is seperation line within the air region. and when i saw the velocity profile @ 1.4e2sec the velocity is reaching the line. Is this meshing problem?????? Regards, Smaras 
1) I don't understand the question. Do you mean, if a variable density can cause problems? Then, the answer is yes.
2) What do you mean by "separation line"? 
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2.) http://imageshack.us/a/img28/6784/meshinfluent.th.jpg The line that starts from top till the water bed. 
But what kind of line is this? Did you paint it?
Also I am curious why the courant number at the top, next to the needle is so high. I guess the whole top area is a pressure outlet, right? Why would you have such a high courant number there? 
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2. The top area is outlet. And that i don't know. It's appeared just after divergence http://imageshack.us/a/img571/8189/courantno2.th.jpg this is the image before divergence 
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