[naveensdas]

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

[diamondx]

can we take a look at your mesh first ?

[naveensdas]

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

[cartman]

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)

[naveensdas]

Originally Posted by cartman

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?

[cartman]

I'm not sure, but.. I heard that 0 pa for operating pressure is good for compressible flow
(I always set like that)

[naveensdas]

Originally Posted by cartman

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?

[cartman]

keep the initial condition as ambient will be nice

[naveensdas]

Originally Posted by cartman

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...

[naveensdas]

I would also like to know if my B.Cs are correct..?

[cartman]

I think so,

If you don't mind, could you pass your case file?

[naveensdas]

Originally Posted by cartman

I think so,

If you don't mind, could you pass your case file?

Thanks Chris !! shall i mail it to you?

[cartman]

sure,

hdkam@pknu.ac.kr

[naveensdas]

Hii Chris

i have mailed you the case file

[naveensdas]

Hii Chris,

Could you identify any mistakes?