# always disconvergent

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 April 23, 2012, 22:28 always disconvergent #1 New Member   Wang Join Date: Dec 2010 Posts: 8 Rep Power: 7 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.

 April 24, 2012, 02:07 #2 Senior Member   Join Date: May 2011 Location: Germany Posts: 130 Rep Power: 7 Have you tried to lower the under-relaxation factors?

 April 24, 2012, 20:54 #3 New Member   Wang Join Date: Dec 2010 Posts: 8 Rep Power: 7 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.

 April 25, 2012, 02:01 #4 Senior Member   Join Date: May 2011 Location: Germany Posts: 130 Rep Power: 7 Just decrease the under-relaxation factors more ..... I had problems with under-relaxation of 1/3 or less of default values

 April 25, 2012, 03:09 #5 New Member   Wang Join Date: Dec 2010 Posts: 8 Rep Power: 7 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

 April 25, 2012, 03:14 #6 Senior Member   Join Date: May 2011 Location: Germany Posts: 130 Rep Power: 7 okay .... have you checked the mesh in this region?

 April 25, 2012, 08:43 #7 New Member   Wang Join Date: Dec 2010 Posts: 8 Rep Power: 7 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.

 April 29, 2012, 21:53 #8 Member   Krishna Join Date: Oct 2009 Posts: 34 Rep Power: 8 You should use density based solver for compressible flows. Have you tried decreasing the courants number?

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