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Reza September 5, 2002 19:36

Compressible flow
 
Ok, I have asked this before but no one answered. I am solving a compressible flow (Gas) in a pipeline. I am providing the inlet velocity and outlet pressure. The problem is I get lots of ossilations in pressure and outlet flow rate. when I ignore the term d rho/dt in the continuity equation the ossilation disappears but the obviously the mass balance is not conserved. I use SIMPLE method to solve the pressure and velocity field. Anyone knows what should I do?

sri September 6, 2002 14:40

Re: Compressible flow
 
Hi,

Do make sure that the flow is laminar and if it's not I wonder you have to solve for turbulence.

sri

Reza September 6, 2002 22:30

Re: Compressible flow
 
Thanks,

The flow is laminar, it seems the problem comes from d(rho)/dt. There are some papers on SIMPLE compressible flow with they account the correction equation for density but even with applying them I have the problem.

Abhijit September 9, 2002 02:56

Re: Compressible flow
 
Hi,

What is your exit condition ? do u know it apriori? If it is subsonic your exit pressure is fixed..it cannot change with iteration,unless u have a transient flow. (This comes from charateristics theory.) If your exit is supersonic you need to extrapolate all variables from interior, you cannot fix the exit pressure. First make sure u apply correct exit B.C. Hope this helps.

Regards,

Abhijit.

Reza September 9, 2002 20:11

Re: Compressible flow
 
Thanks,

the oulet B.C. is fixed pressure and it is subsonic. I am mainly interested in the transient part. I have plotted the outlet flow and it exceeds the inlet flow and the ossilates and then it becomes equal to the inlet flow rate. seems that at some iteration times the pressure is over estimated.

gita September 10, 2002 10:29

Re: Compressible flow
 
Hi,

There is something called "artificial compressiblity factor". Since you're using SIMPLE algorithm first you need to take care of the term d(rho)/dt. There are papers related to that. Check out,if not I will let you know the author's name(which I don't have it with me now).

gita


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