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-   -   Problem with cfx Solver Results (http://www.cfd-online.com/Forums/cfx/26309-problem-cfx-solver-results.html)

Ordoumpozanis August 26, 2008 05:00

Problem with cfx Solver Results
 
Hi

I am running a case of double facade with buoyancy. I have adiabatic walls except one witch have a heat flux and a boundary of p=patm at inlet and m= const at outlet. I noticed that at outlet, the velocity that I get is different from the velocity witch I expect. I tried to run the same case with boundary condition of normal velocity = const at outlet and the mass flow that I get is different from the one I except. In the problem I use a K-e model with constant density.

Can anyone think a reason for this situation ???

Tnanks


CycLone August 26, 2008 16:25

Re: Problem with cfx Solver Results
 
What velocity did you expect?

brunoc August 26, 2008 18:15

Re: Problem with cfx Solver Results
 
So, correct massflow = wrong velocity correct velocity = wrong massflow

Is the density of your material correct? Can you assume it to be constant?

Ordoumpozanis August 27, 2008 04:07

Re: Problem with cfx Solver Results
 
Hi, thnk for the reply. I managed to get the right results by adding the normal velocity only to the calculations on the boundary, witch is logical. but if I check each cell on the boundary, the equation of Acell = mass flow (m3/s)/ velocity (m/s) is not equal to the area of the cell on the boundary. The values for the calculations are output from post process

thnks again

CycLone August 27, 2008 11:54

Re: Problem with cfx Solver Results
 
Hi Ordoumpozanis,

This is actually expected. The solver calculates mass flow rates at integration points, where the velocity is not equal to the nodal velocity. If you calculate the mass flow rate as rho*A*V, you'll actually get the wrong results. Post avoids this error by using the integration point mass flows, which are written to the results file from the solver.

The actual calculation of mass flow rate at the integration point is rather involved and must include the spatial variation of both velocity and pressure. You can review how this is discretized in the solver theory guide.

-CycLone


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