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 Ahmet May 6, 2007 04:30

Density based compressible flow solution

Hi all,

I made a converging diverging nozzle solution with density based solution. But I can not get the correct results. I have a big differance between mass flow inlet and mass flow outlet. I tried lots of thing but I could'nt the problem.

Thanks a lot for helps.

 George May 6, 2007 11:22

Re: Density based compressible flow solution

Because the flow is compressible, this difference is a reasonable result.

 Dominic May 6, 2007 12:36

Re: Density based compressible flow solution

Probably you can try refining the mesh and see whether this condition persists. You should also try the flow inside a rectangular channel (parallel walls) to see if mass_in=mass_out.

 ll May 6, 2007 12:44

Re: Density based compressible flow solution

You mean that, because of compressibility, there is no more mass conservation ?

I don't think it is a reasonable result.

 Djeghri Noureddine May 6, 2007 19:19

Re: Density based compressible flow solution

Hi Ahmet

it is your code or you use a commercial CFD software ?

 ag May 6, 2007 20:28

Re: Density based compressible flow solution

I would look at my boundary conditions, and also check to see if total enthalpy is being conserved. Also, try running without boundary conditions to see if your solver can maintain a uniform freestream in the absence of boundary conditions.

 Ahmet May 7, 2007 01:21

Re: Density based compressible flow solution

I use Fluent. I made better mesh but I got the same problem. I think there is mass conservation also in compressible flow.

 Dominic May 7, 2007 03:26

Re: Density based compressible flow solution

Plot (mass_out - mass_in) as a function of time and see whether this difference is still dropping when you had taken the reading.

 andy May 7, 2007 03:56

Re: Density based compressible flow solution

What you are calling "mass" and what your commercial code is conserving via the mass conservation equation may not be the same thing. Look in the manual and find the form of the continuity equation at the end of the solution procedure:

* what is numerically conserved?

* where is it numerically conserved (e.g. face or centre).

* does it include pressure smoothing terms?

* are there splitting errors at the end of an iteration?

When you check your boundary conditions the same questions apply.

 George May 7, 2007 05:15

Re: Density based compressible flow solution

In transient calculations Mass_in will not be equal to Mass_out. For steady-state calculations the inlet and outlet mass will be equal. Is your code for transient calculations?

 Mani May 7, 2007 08:44

Re: Density based compressible flow solution

No mention of unsteady application; assume classical steady-state Laval nozzle: Of course, the mass flow has to be conserved as Ahmet expects. No question.

Ahmet, how do you evaluate the mass flow? Could there be an error in your evaluation? Could you examine the cross-sectional mass flow at any axial location, and if yes, does the mass flow vary continuously inside the nozzle or is there a jump in mass flow right at the boundary? How well is it converged?

How big is "big", i.e. what is the relative difference between the inlet and outlet mass flows?

If this were true, it would be a HUGE embarassment for the Fluent guys, but because this is such a standard test case and surely has been evaluated many times I am guessing that there is something wrong in your setup and/or post-processing.

 reza May 22, 2007 03:48

Re: Density based compressible flow solution

hi i need the information about compressible flow solution by ansys cfx.

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