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Old   April 29, 2003, 03:14
Default FREE BOUNDARY
  #1
John
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Hi:

I want to ask a question about free boundary condition. The question is in a face there may be inflow and outflow. For example, the opening of fire compartment. In the upper part, the air could flow out while in the lower paer the air enters the room. How can we define the opening?

Thanks
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Old   April 29, 2003, 09:29
Default Re: FREE BOUNDARY
  #2
John Ludwig, CHAM
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This should be defined as a fixed pressure boundary - in VR terms as an OUTLET object.

In those cells where the internal pressure is greater than the set external pressure, the flow will be outward, and it will take with it the in-cell values of temperature, velocity and all other solved variables.

In those cells where the internal pressure is below the set external pressure, the flow will come in. In these cells, the values convected in will be those set as the external values in the outlet attributes dialog.

The external values can be a user-set constant, or 'in-cell'. This means use the current value in the cell as the incoming value; it approximates to a zero gradient condition. In most cases the external values shouldeb set to the known ambient or free-stream values.

The source reported in the RESULT file will be the net source for the object. Positive R1 is a net inflow in kg/s, negative R1 is a net outflow.
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Old   April 30, 2003, 04:04
Default Re: FREE BOUNDARY
  #3
John
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Dear John Ludwig:

Can you tell me the meaning of the 'cofficient' in the VR outlet? How can we determine its real value? Thanks

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Old   April 30, 2003, 05:02
Default Re: FREE BOUNDARY
  #4
John Ludwig, CHAM
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On all the object dialog boxes, there is a ? in the top-right corner. If you click on the ? and then click on a button or data-entry box, you will get a help message explaining that item. The help entry for the coefficient on the outlet attributes dialog says:

"Pressure Coefficient

The mass flow through the opening is:

M" = C * (Pext - Pint)

Large values of C hold the pressure very close to the external. Small values allow more variation.

A value of 1000 usually gives pressures close enough to the set value without rounding-errors."

When you create an OUTLET object in the VR Editor, it generates a PATCH and COVAL to tell the Earth Solver what to do. These can be seen in the Group 13 echo in the RESULT file. The coefficient set in the outlet attributes dialog is the Coefficient in the COVAL for P1 - COVAL(patch_name,P1,C,Pext). There is an Encyclopaedia entry on COVAL which explains more. The link is http://www.cham.co.uk/phoenics/d_pol...nc/coval.htm#e.

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