[roberto]

Hello

I'm just to trying a verification case, I tried to study a venturi injector for water. This is an existing part, I know the main feature from testing ( inlet pressure, outlet pressure, flow, suction flow).
I generated the mesh with gambit.
I used the turbFoam application finding reasonable velocity fields, but very low pressure fields, (1000 times or more smaller than expected) I
Running the same simulation with Fluent, with the same mesh, boundary condition and viscosity, I found very similar velocity fields but the pressure fields were much more high, matching test values.
some more information:
For openfoam setup I started from turbFoam/cavity case
I never input water density in openfoam (have to be done? how?)
I used gambitToFoam to convert the mesh

anyone have some idea?

Thanks
Roberto

[gschaider]

I know, that was not what you're asking: if you're generating your mesh with gambit it's better to use fluentMeshToFoam to convert the mesh.

The other thing: turbFoam is an incompressible solver. In OF they don't specify a density for those, it's all done with the viscosity (dynamic vs kinematic)

[hjasak]

Hi Bernhard,

Any particular reason for preferring the Fluent converter - if there are bugs I should know about, please let me know and I'll have a look. Both converters should be doing their job equally well.

Hrv

[mattijs]

Hi roberto,

check turbFoam for whether it solves for p or p/rho (since incompressible). The source code is in $FOAM_SOLVERS/incompressible/turbFoam. Or check the dimensions on 0/p.

[gschaider]

Of course your right (no bugs). My reasons for recommending the fluent-converter were:

- The fluent-converter preserves boundary-information (wall, symmetry etc) where possible

- Don't know what the gambit-converter does with "internal"-boundaries (the fluent-Converter at least writes faceSets/cellSets) and one can work with that (for instance http://openfoamwiki.net/index.php/Howto_importing _fluent_mesh_with_internal_walls)

- if he's comparing with Fluent he can use the same .msh-file he propably imported into Fluent (I think the gambit-converter reads the Neutral-Files). I would prefer that because there are less oportunities to mess things up with just one "version" of the grid lying around (but I tend to mistrust myself -for -good reasons- so this might not apply to others)

[roberto]

Hello and thanks to everybody!

Mattijs,
I checked p dimension, it's [ 0 2 -2 0 0 0 0]
Now I understand the problem, it isn't a pressure!
How can I obtain the pressure?
thanks again

[gschaider]

Roberto: If you multiply it with a density it becomes a pressure

[roberto]

Hi Bernhard

Great!
Now the pressure results have lot of sense!
(and you can understand my emarrassment for not finding density input...)
Is there a way to convert p field in a pressure field in order to have parafoam rappresentations with pressure?

[gschaider]

@embarassment: there's no need. At least you were looking at the numbers (not just the colors).

For the pressure there are two ways:

- the easy way: in paraFoam use the calculator filter to multiply the p-field with a constant (the density would be the best choice ;-) ) and vizualize that

- rewrite the solver to write out an additional field pReal (or similar) that is p*density

[roberto]

Thank you very much for everithing!
For almost all commercial programs this usefull and quick answers are a dream.

Embarassment was generated by the consciousness that density enter in problem equations, even with uncompressable fluid, so I didn't found phisical sense to results.
Now I know what p means for openfoam and all is very clear and full of sense.
I'm used to solve my mechanical problems by pen and paper, so I don't care too much about colors, anyway results presentation is alwais important.

If someone is interested, after simulation,sampling and testing I will share the results for validation purposes.

[gschaider]

Roberto: one nice place to put your validation case would be:

http://openfoamwiki.net/index.php/Main_ContribExa mples

[ravirajsharma]

A Venturi Injector works on the principle of differential pressure. Water enters the venturi at a higher pressure than it exits. The difference in entry and exit pressures creates a vacuum, and the higher the difference, the greater the vacuum. It increases the efficiency of the mixing.
Automat provides this product with maximum injector efficiency, suction capacity, and fertilizer mixing capabilities. They also have an in-built check valve for protecting the injected liquid against backflow.