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-   -   ODD: no UDS flux across interior boundary (https://www.cfd-online.com/Forums/fluent/29402-odd-no-uds-flux-across-interior-boundary.html)

 Bob February 25, 2002 09:15

ODD: no UDS flux across interior boundary

Dear,

I have the following problem when implementing a UDS (Fluent 5.5) for the settling velocity for solids in a clarifier.

The clarifier has an "interior" boundary. First, I solved (steady-state) the flow field without consideration of the UDS. Afterwards, I solved (unsteady) the UDS transport equation with the obtained flow field. The scalar was indeed transported as it should be, but it seems that the "interior" thread was acting as a wall for the UDS with zero concentration; the UDS simply did not cross the thread! If I look at the mass flow rate across this interior thread, I get a result but the net mass flow is zero (report -> fluxes), which is of course unrealistic...

Does someone have a wild idea what the cause might be? Is this an error inherent to Fluent?

Thanks a lot in advance! Cheers, Bob

 Greg Perkins February 25, 2002 20:15

Re: ODD: no UDS flux across interior boundary

Hi Bob,

Not sure exactly what's happening, but here's some thoughts:-

Fluent uses control volumes, so it doesn't actually store the face values - and doesn't always store face fluxes. Thus, if you decouple the calculations, it may be that Fluent is no longer storing the face flux of the gas phase mass flow.

You can test this, by printing out the face fluxes returned by fluent and seeing if they are zero - or do something equivalent.

If this is the case, you may need to either: (1) re-couple the calculations or (2) re-calculate the face flux by interpolating the cell values to the faces. This is not that hard, but its a bit of a pain to do. (If you look in the Fluent 6 udf manual, they have some details on the gradient operators you can use - in Fluent 5 manuals here's no advanced documentation).

Alternatively, there might be a way to store the Fluent values somewhere during the first phase of calculation - but this will require a bit of thinking on how to do since there are multiple face fluxes per cell.

Well to be honest I'm not sure if this will help but it might give you some ideas.

Regards

Greg

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