CFD Online Discussion Forums - venturi tube help!

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venturi tube help!

Dear Foamers,

I am trying to simulate the 2-D flow of air through a venturi tube (the geometry shown in the image attached). The dimensions of the tube are small; to give you an rough estimate, the outline of the mesh measures 10 mm by 20 mm. The solver I am using is sonicFoam. The starting point of my simulation was 'fowardStep' tutorial case in the sonicFoam subdirectory in tutorials.

My boundary conditions are somewhat similar to the fowardStep case that is already setup, so I changed the mesh geometry from fowardStep to the one I have, and I immediately run into trouble with I run sonicFoam: the error states the max number of iterations exceeded. How do I fix this? I tried making the time step larger but that didn't work.

Now for the real boundary conditions, I want to setup a higher pressure at the inlet, which on the left side of the image, (it's value is 27 % higher than atmospheric pressure) and atmospheric pressure at the outlet; I want to do this so I can flow air at roughly mach 1 through the tube. Does this mean that I'll have to set up the inlet pressure boundary condition to (1.27)*(101325) ? When I setup pressure like this, I get same error. I have no clue how to fix this.

Please help me in any way you can. If you need a reference, my case is just like the foward step case in the sonicFoam directory but with modified geometry and some BC conditions changed. I've read the manual but I still can't get anywhere with this problem. I can provide any information that might be necessary for you to help me.

THANKYOU!

warm regards,
Ammar.

Ammar,
You should decrease the time step to fix your error. Did you check your modified mesh with checkMesh? You can try to specify your inlet and outlet pressures in 0/p as:

Code:

inletPatch

{

  type fixedValue;

  value uniform (128682.75 0 0); // 27% above atm

}



outletPatch

{

  type waveTransmissive;

  field p; // name of field

  phi phi; // volumetric flux field

  rho rho; // density field

  psi psi; // density field 

  gamma 1.4; // ratio of specific heats

  fieldInf 101325; // far-field value

  lInf 0.1; // distance to far-field

  value uniform 101325;

}

You might want to calculate the critical pressure for choked flows first. I guess your ratio of 1.27 is too small and the flow will not reach Mach 1.

Kind regards, Radko

Thank you Radko, it works now!

Regards,
Ammar.

Hi,

small question here, because i want to solve similar case.

What happens, if i increase the pressure further more?
According theory, the maximum velocity above criticial pressure ration is limited to sonic speed, hence, should be limited to mach 1, or?

KR