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Andrew April 19, 2006 04:25

Opening
 
Hi cfx users,

I have a problem with a opnenig boundary condition. The only inlet/outlet of my domain is set as an opening condition. The flow is expected to be sucked in at some location of this boundary and flowing out at some others. I use the condition "Static Pres. (entrain) = 0" (I haven't notice any difference with the other conditions available). The flow is driven by some moving wall inside the domain.

So, logically, I expect to have massflow = 0 at this boundary. And it is not!! The average velocity coming out at the opening is about 2-3% the velocity of the moving wall! This is physically impossible! Where can be the problem?? Can it be a mesh problem? I add also that I reach 10^-5/10^-6 for the rms residuals, so I don't think it's a convergence problem...

Thanks for help,

Andrew

Robin April 19, 2006 13:37

Re: Opening
 
Measure the mass flow, not the velocity.

Andrew April 19, 2006 15:28

Re: Opening
 
Yes, thatīs what I did. I have just converted the massflow into velocity to give a way to compare the difference...

Regards,

Andrew

Glenn Horrocks April 19, 2006 18:23

Re: Opening
 
Hi,

Use the massflow function either as a monitor point during convergence or in CFX-Post to calculate massflow rate. Then it uses the exact values at the integration points and gives the true simulated massflow.

Glenn Horrocks

Robin April 20, 2006 14:00

Re: Opening
 
Post will also use the IP flows.

Andrew, what is the result of the mass flow calc? Make sure you use massFlowABS, otherwise all the flows will be positive and you'll get the total flow through the face, not the net flow.

How did you compute the velocity from mass flow (i.e. what expression or equation did you use, as the details may introduce errors)?


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