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-   -   "Volume Flow Rate" Boundary Condition (http://www.cfd-online.com/Forums/flow-3d/123585-volume-flow-rate-boundary-condition.html)

Marcus.filho September 16, 2013 15:48

"Volume Flow Rate" Boundary Condition
 
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Dear all,

Im trying simulate the intake of a powerhouse of a Hydroeletric Power Dam. My simulation consists of having a part of the resevoir upstream and the intake itself. See attachment for a picture.

Im trying to use, as upstream BCs, the option Volume Flow Rate. I would like to understand better how does it work. Ive done a simulation and the velocity entering the domain at the BC varies at each element. So my question is how does FLOW3D understands the value of discharge Ive described? My first thought was that he would calculate the open area and put a constant velocity into the domain but it doesnt seem that way.

I would like to take the opportunity and also ask if anyone think of a better BC for this kind of problem.

Thank you in advance.
Marcus

JBurnham September 26, 2013 15:10

Marcus - you are correct that FLOW-3D calculates the open area and finds velocity from the flow rate. The velocity will be the same in all cells, except where the specified fluid elevation is lower than the actual fluid elevation: in this case fluid can leave the simulation. Also, in cells that contain solid boundary (like river bed), the velocity will be modified by the near-wall effect. If you are seeing different velocities in every cell, it means that the flow is interfering with the boundary condition, and the boundary condition should be moved farther away from the region of interest.

JBurnham September 26, 2013 15:23

Marcus - one other note: based on the image you sent, it appears you have two volumetric-flow-rate boundaries that are adjacent and 90 degrees from each other. They will interfere with the flow rate near the corner, unless you specify 45-degree flow rate vectors on the boundary condition dialog. See the User Manual for instructions on how FLOW-3D interprets the flow rate vectors. Also make sure that the free surface elevation (fluid "height") on the boundary conditions is the same for both flow rate boundaries. Ideally, you would like to have only one inflow boundary, but you can work with two. - Jeff

Marcus.filho October 10, 2013 15:30

Dear Jeff,

Thank ou for your remarks. Ive been able to sucessfully run the simulation now. Ive change my BC`s to stagnation pressure at the reservoir and VFR at the outlet. Its working perfectly.

Thank you for your help.

Marcus

JBurnham October 17, 2013 10:55

Awesome! Thanks for asking your question so clearly. - Jeff

Marcus.filho October 22, 2013 08:01

Thank you for the prompt reply!

Marcus


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