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kuhbuh April 6, 2010 10:52

wall condition for extended outlet
Hi everyone!

In my calculation, I extended the outlet to prevent inflow as recommend in the CFX documentation. I just want to ask if it's appropriate to assign the wall of the additional region as free slip (no slip in the main domain) so as not to further retard the flow. Is this correct? thanks a lot guys!


dvolkind April 6, 2010 12:55

Depends on what You are modelling. Why don't You want to retard the flow?

kuhbuh April 6, 2010 13:19

Hi! Thank you for your reply. I'm simulating 2 phase flow and there is some recirculation near the outlet. My outlet condition is based on experimental data and I'm afraid that further extending it would have some inaccuracy.

dvolkind April 6, 2010 13:35

What are Your BCs?

kuhbuh April 6, 2010 13:37

My BCs are velocity in and pressure out. Tnx!

dvolkind April 6, 2010 13:58

Then moving outlet further away most likely won't affect accuracy. With inlet speed specified You will get a higher inlet pressure then You would expect without moving the boundary. Thus, absolute pressure values might slightly change, but basically pressure differences are important. So, if the absolute pressure level does not have much effect for Your problem and extending the domain won't introduce new geometry features then I think it's OK to move the boundary.

By the way, wall shear stress at non-sleep wall is not the only cause of pressure loss, so there's no guarantee that You won't get any pressure drop in free-slip region.

kuhbuh April 6, 2010 14:02

Thank you very much for the input! I'll run few more simulations and check if there's effect. Thanks again!

ghorrocks April 6, 2010 18:41

The best way to look at it is you want to model what the experimentalists measured. If they had a long runner downstream of the test section, then model that. If they dumped it into a big tank then model that.

kuhbuh April 8, 2010 10:30

Thanks again Sir Glenn! I'll take note of that.

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