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Supersonic free jets from CD nozzle

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Old   April 22, 2013, 11:12
Default Supersonic free jets from CD nozzle
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Naveen S Das
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Hi Friends !!

I am new to Fluent.. I am trying to simulate a 2D supersonic free jet from a Convergent Divergent nozzle.This is how i set up my problem

I am using 2d axisymmetric, steady implicit formulation. Operating pressure=98950Pa. I am expecting oblique shocks at the exit. I went for a Density based solver and realizable K-e Model for turbulence.
Nozzle Inlet pressure=465250Pa (Pressure Inlet B.C). The upper part of the nozzle, I went for wall b.c and the right end of the domain is pressure outlet and the bottom part is the axis.
My domain is 22D in axial direction and 5D in the radial direction
For Left and the upper part of my domain, I had initially gone for pressure inlet B.C with gage pressure=0. But I started to get warnings “temperature limited to 5000K”. I found out that the sharp edge was giving the problem. I refined the mesh close to the sharp edge. But still divergence occurred. Then I got a suggestion from a friend to change the left and upper domain b.c to pressure outlet. With that, I was not having that much a problem with sharp edge. But still my residuals are diverging after initially showing a downward trend.

I even started with a courant number of 0.05 and reduced the URFs. I used first order difference schemes. But I cannot get a converged solution..
Is there any problem with the B.C’s or turbulence model? Please help me..
Thanks in advance !!
Naveen

Last edited by naveensdas; April 22, 2013 at 13:01.
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Old   April 22, 2013, 13:26
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Ghazlani M. Ali
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can we take a look at your mesh first ?
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New to ICEM CFD, try this document --> http://goo.gl/G2gkE
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Old   April 22, 2013, 22:42
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Hii Ali

Thanks for replying.. !! i am posting the screen shot of my mesh. will send you the dbs file by mail if you want
Attached Images
File Type: jpg CD nozzle.JPG (52.3 KB, 32 views)

Last edited by naveensdas; April 23, 2013 at 04:10.
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Old   April 23, 2013, 00:22
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did you set the initial pressure as "0"?

or outlet pressure as "0"??

because, what I know is that you can't get exact solution of rarefied gas dynamics with fluent

sorry about my weird English, I'm not good at this language :z



or you can set the nozzle-inlet condition gradually
(set 1 atm for the condition and iterate, set 1.2 atm when the changes of residuals seeme stable, .... set real condition)
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Old   April 23, 2013, 00:27
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Quote:
Originally Posted by cartman View Post
did you set the initial pressure as "0"?

or outlet pressure as "0"??

because, what I know is that you can't get exact solution of rarefied gas dynamics with fluent

sorry about my weird English, I'm not good at this language :z



or you can set the nozzle-inlet condition gradually
(set 1 atm for the condition and iterate, set 1.2 atm when the changes of residuals seeme stable, .... set real condition)
Hii Chris,

I have given operating pressure as 98950 Pa (which is the ambient pressure) .. so if i give the gage pressure as 0, then is it a problem?
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Old   April 23, 2013, 00:39
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I'm not sure, but.. I heard that 0 pa for operating pressure is good for compressible flow
(I always set like that)
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Old   April 23, 2013, 02:04
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Quote:
Originally Posted by cartman View Post
I'm not sure, but.. I heard that 0 pa for operating pressure is good for compressible flow
(I always set like that)
Thanks Chris ..!!

I will try with operating pressure=0.. so i guess i have to keep my initial guess pressure at 98950 Pa.. am i right?
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Old   April 23, 2013, 02:41
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keep the initial condition as ambient will be nice
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Old   April 23, 2013, 03:05
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Quote:
Originally Posted by cartman View Post
keep the initial condition as ambient will be nice
Hii Chris !!

I have tried to run the simulation with operating pressure=0 and setting initial guess pressure as ambient.. i also went for first order discretization schemes to begin with and a courant number of 0.5.. residuals were decreasing to 5*10^-2.. but then i get the warning "time step reduced in cells" and the number of such cells goes on increasing and with that residuals too started increasing...

Last edited by naveensdas; April 23, 2013 at 03:53.
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Old   April 24, 2013, 12:49
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I would also like to know if my B.Cs are correct..?
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Old   April 25, 2013, 01:15
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I think so,

If you don't mind, could you pass your case file?
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Old   April 25, 2013, 02:38
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Quote:
Originally Posted by cartman View Post
I think so,

If you don't mind, could you pass your case file?
Thanks Chris !! shall i mail it to you?
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Old   April 25, 2013, 04:49
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sure,

hdkam@pknu.ac.kr
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Old   April 25, 2013, 06:00
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Hii Chris

i have mailed you the case file
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Old   May 2, 2013, 12:07
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Hii Chris,

Could you identify any mistakes?
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