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-   -   supersonic with ccm? (http://www.cfd-online.com/Forums/star-ccm/64238-supersonic-ccm.html)

askzlynn May 4, 2009 13:49

supersonic with ccm?
 
hye,could you help me with ccm? i am trying to predict thrust produced by CD nozzle with this software. my residual plot is tend to increase and has no sign to decrease. plese..help me..anybody?thanx

vishyaroon May 4, 2009 16:41

Which of your residuals starts diverging first?

Have you checked the quality of your mesh and removed the invalid cells?

Also if you have problems with the convergence of the turbulence equations try activating 'Cell Quality Remediation' in your physics continua and see if this makes a difference.

askzlynn May 4, 2009 22:37

the X momentum and continuity...mm,can you tell me what sa_nut is for?
then,sometimes my simulation having error:floating poit exception(divided by zero)..
i'll try at the moment with my meshing and others..
thank you

askzlynn May 5, 2009 02:00

once again the simulation terminated with floating error..
this time it was overflow

one more thing,how can i removed the invalid cells?

thanx in advance

vishyaroon May 5, 2009 08:54

1. sa_nut is the turbulence viscosity. You seem to be using the S-A RANS model, which gives the turbulent viscosity.

2. To remove the invalid cells, right click on the region in the left hand side menu. The 8th option is Remove Invalid cells. You can set the minimum face validity between 0.90 and 0.95, min. cell quality to be around 1e-6 to 1e-9 and min. volume change between 1e-3 and 1e-6. Preview the number of cells that will be removed to make sure that it is not a significant number before you hit 'Ok' to remove these cells.

3. Cell quality remediation is under physcis continua you have defined. If you right click on the physics model you are using, then you can choose Cell Quality Remediation.

Hope this helps

askzlynn May 5, 2009 14:21

owh...ok..ic..erm..thank you..
you are helping me a lot..by replying me..
i am really appreciate that..
thanx once again.. =)

but, if there is no cell to be delete..then?

vishyaroon May 5, 2009 17:19

Hello Adzlin,

A couple of things you may want to check:

1. In general check the solution set-up to see if everything is ok.

2. When the solution starts blowing up, stop the solution and look for the regions where some flow parameter like pressure is the abnormal. This might give you an idea. I guess you are working with external flows. Can you check if your meshing domain is continuous, i.e, make sure that there are no regions that have been meshed and not linked to the main flow region. This generally results in the continuity to blow up easily.

askzlynn May 5, 2009 22:29

hi Mr. Aroon,
my project is about simulation on heated compressed air passing through convergent divergent nozzle.
then, my nozzle should involve internal flow, right?
the CD nozzle is 3D, and my friend says it may involved with sa turbulence model..
I just cannot predict the thrust produced.

several modification had been done
1. Courant number = 0.5- 5.0
2. the plane section, [-1, 0, 0]
3. the velocity and mass flowrate for xy report [-1,0,0] in position.

did you have any ideas for other modification that need to consider?

azt May 6, 2009 12:29

Hi

Set your inlet boundary condition to a mass flow inlet, not velocity, and set your reference pressure to the outlet pressure. Then set your outlet pressure on the boundary to zero.

try that

allan

askzlynn May 6, 2009 12:49

oo..
thank you mr.allan
will try that..
=)


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