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-   -   always disconvergent (http://www.cfd-online.com/Forums/fluent/100314-always-disconvergent.html)

tmac1kobe8 April 23, 2012 22:28

always disconvergent
 
hello,everyone. I was simulating the flowfield of supersonic crossflow and normal sonic jet. The configuration was a 2d plate,
By using 1st scheme of flow in FLUENT, I can get the convergent solution. however, when the 2nd SOU scheme was applied, the calculation always unstable.
Please give some advise, thank you very much.

Zigainer April 24, 2012 02:07

Have you tried to lower the under-relaxation factors?

tmac1kobe8 April 24, 2012 20:54

yes, I have lowered under-relaxation factors from the default value(0.8) to 0.5, it failed still.
the boundary condition:
crossflow: pressure inlet(supersonic) p0=2.4e5 Pa, T0=309K, M=3.5
jet: pressure inlet(sonic): p0=3.8e5 Pa, T0=293K,
upper boundary: farfield pressure: M=3.5, T=86.5K
out: pressure outlet: p=3145 Pa.

Zigainer April 25, 2012 02:01

Just decrease the under-relaxation factors more ..... I had problems with under-relaxation of 1/3 or less of default values

tmac1kobe8 April 25, 2012 03:09

I tried the value of under-relaxatioin factors of 0.1, it didn't work. I observed the field, when I turned 1st scheme to 2nd scheme, after several iteration, there was high temperatrue region right behind the mach disc of jet plume.
I think it was the source of disconvergence.
But I don't konw how to deal with this problem

Zigainer April 25, 2012 03:14

okay .... have you checked the mesh in this region?

tmac1kobe8 April 25, 2012 08:43

yes, mesh is very simple, it is H-type, I think the problem should not lie in mesh.
Now I am trying calculating with pressure-based solver instead of density-based solver.

t.krishnamohan April 29, 2012 21:53

You should use density based solver for compressible flows. Have you tried decreasing the courants number?


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