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-   -   Continuity (http://www.cfd-online.com/Forums/fluent/31473-continuity.html)

DAE May 14, 2003 11:35

Continuity
 
Hi

I am modelling the flow of an incompressible fluid through a structure.

Due to the complexity of this structure, the use of hexahedral elements is not practical, thus tetrahedral are being used.

My question to anyone that can help is, do you know why I am getting a net gain of mass of 1.6kg/s when the mass-flowrate is 6kg/s.

The solver is steady state.

The inlet is a velocity inlet, and the outlets are velocity inlets with negative velocities. I have tried pressure outlets, but for my problem, setting the correct pressure on the boundary is not easy to get continuity right, thus the use of pressure outlets is not practical either.

Each velocity inlet has a minimum of 6 cells across the diameter, thus providing one either side of the inlet for a crude boundary layer, and 4 for the bulk of the flow.

Thanks

ap May 14, 2003 15:23

Re: Continuity
 
Why don't you try using Outflow as boundary conditions for your outlets?

Outflow BC imposes a zero diffusion flux of all variables and applies a mass balance correction. All others variables are calculated supposing a fully developed flow.

This boundary can't be used if flow density changes, if you're using Pressure Inlets, or if you're modeling compressible flows (not your case).

Hi

ap

Shiping Liu May 14, 2003 16:33

Re: Continuity
 
I think the solution is not convergent.

Alex Munoz May 15, 2003 00:17

Re: Continuity
 
Hi

The solver is given to you a solution for the condition that you impose. In other word, you must to

1) increase the number of elements 2) impose a velocity profile at the inlet 3) inpose a turbulent kinetic energy at the inlet 4) impose a dissitation rate of the TKE at the inlet. 5) Use second order upwind 6) Use Presure outlet 7) Use pressure velocity SIMPLEC

Finally, Tetrahedral mesh are characterizad for numerical diffusion. Therefore, You must to increase the number of elements per volume. Keep in mind that not too many works has been than with it, and also the ones reported are 2-D. It is my guess that yo have a large domain in 3-D.

By the way, Mr Liu coment about lack of convergence take it as a lack of convergence to the true solution not to a mathematical solution.

Regards

Alex Munoz

John May 15, 2003 06:41

Re: Continuity
 
I don't think your comments are right at all: the mass is always conserved in finite-volume based method regardless the mesh density, mesh topology, turbulent or laminar, SIMPLE or SIMPLEC, upwind or HOD.

Anton May 15, 2003 17:13

Re: Continuity
 
If you are specifying velocity at the both the inlet and outlets of your model, then you are fixing the mass coming in and leaving, assuming constant density. Go back and double check that your inlet velocity multiplied by your inlet area is IDENTICALLY equal to your outlet velocity multiplied by your outlet area.

Ashish May 20, 2003 12:33

Re: Continuity
 
Hi DAE,

Are you getting reverse flow at the outlet ?

As you don't know the exact pressure outlet bc, you try outflow bc. Run for about 100 to 200 iterations. take the total pressure value at the outlet and put it as pressure outlet bc. (i have tried this for my problem this works very well)

bye, Ashish

satyendra October 26, 2011 01:14

hi ashish,

i am trying to solve an incompressible ideal gas problem having two mass flow inlet conditions and two pressure outlet conditions ( i cannot use outflow BC since according to chapter 7, fluent user guide this is not allowed ). hence I have set target mass flow at outlet. But the continuity is not converging. any idea where i am going wrong.

rana

amir.mofakham October 26, 2011 08:57

I think you cannot define the velocity outlet and inlet.
For example, if the velocity inlet is specified the outlet velocity is less than inlet velocity because of the drop pressure (viscosity).
So one of them can be defined and the other one will be calculated.


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