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Closest BC similar to 'Opening' BC in CFX

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Old   July 13, 2017, 10:15
Default Closest BC similar to 'Opening' BC in CFX
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Praveen Gambhir
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Hey All!
I am trying to replicate a run in CFX, which had Entrainment openings in CFX. It has been a while since I've come back to use Fluent, and wanted to refresh my memory on the boundary condition which closely resembles an Entrainment Opening in Fluent.

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Praveen
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Old   July 15, 2017, 10:02
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Justin
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Praveen,

I have never used CFX but I looked in the ANSYS Documentation to see exactly what the 'opening' bc is. In Fluent, it's not available but I wish it was. You could use either 'pressure_inlet' or 'pressure_outlet' depending on your project but the latter is probably the best. I'll use my current project as an example.

I have a 3D domain with a converging-diverging nozzle. The overall surrounding domain is a rectangular block. I assigned the following bc's:

- One face of the outer domain as symmetry (my model is symmetric about one plane)

- The faces above, to the side, below, and at the end of the jet were all assigned as 'pressure_outlet's. Be careful on how you assign 'backflow direction' because it can lead to error. Since I was not sure of the backflow direction, I selected 'from neighboring cell' which allows Fluent to determine the backflow direction.

- The remaining face of the outer fluid domain nearest the nozzle inlet was assigned as a 'pressure-inlet'. I wasn't sure of backflow direction and for pressure inlets the 'from neighboring cell' option is not available so I left it as 'normal to boundary'.

Beware that if reversed flow occurs at either of these boundaries, it will be reported in the Fluent console convergence history as 'reversed flow in X face on pressure inlet 9' as it has in my project. The reason this is occurring in my project is because the jet from the nozzle is supersonic and pressure waves emanate from the interactions at the jet shear layer (where high speed fluid meets with low speed fluid) and propagate upstream eventually reaching the pressure inlet of the outer domain. Whether or not this leads to solution inaccuracy, I have no idea. Hope this helps!

Thanks,
Justin

Last edited by DA6righthand; July 15, 2017 at 10:53. Reason: added more detail
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boundary conditions, multiple reference frames, opening bc-entrainment

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