Specification of Dynamic Pressure at the Opening Boundary

[swiss_zhang@hotmail.com]

Hello Everyone:

I got a problem about Specification of Dynamic Pressure at the Opening Boundary.

The problem is following:

When i define a Open Vent at the boundary of a domain, the Dynamic Pressure should be specificated. But the dynamic pressure depends on the local velocity of the flow at the boundary and i can''t give a concrete value of dynamic pressure, which should be a function of the local velocity.

But, when i follow the PyroSim Example Guide Chapter of Burner Fire, i found that the dynamic pressure at the open vent is specified as zero, which equivalents the flow velocity zero. The consequence is the soot will be circulated in the flow domain and will not flow out. But this is not the realistic case.

Does anyone knows how to solve this problem? Can i specify the dynamic pressure at the open vent as a function a local velocity and give it as text input in FDS?

Thanks very much

Yang

[Charlie Thornton]

When i define a Open Vent at the boundary of a domain, the Dynamic Pressure should be specificated.

Dynamic pressure can be specified, if you''d like to use dynamic pressure instead of the default OPEN behavior.  If you don''t turn on dynamic pressure (by setting a non-zero value), the OPEN vent should behave as a normal opening to ambient conditions.

PyroSim only writes out the DYNAMIC_PRESSURE value if it has been changed from the default (0.0).  This is consistent with many of FDS''s feature that only "turn on" when you specify namelist values.  For more on open vents and dynamic pressure - See: section 7.4.2 "Special Vents", section 9.3 "Special Topic: Pressure Boundary Conditions", and table 15.27 "VENT (Vent Parameters)"

Of course, this is all based on my reading of the FDS manual.  I''ve been wrong before.  The part about PyroSim and not writing the default value is definitely accurate though.

[swiss_zhang@hotmail.com]

Actually i didn''t input any value for the dynamic pressure. It is default as zero. But when i run the simulation of burner fire in the tutorials, i can''t understand the flow behaviours at the open vent.

It seems the flow or soot is circulated, somehow they are reflected by the open vent.

[Charlie Thornton]

I''ve attached a couple pictures of a burner run with velocity vectors showing.  Are you feeling like the simulation is giving the wrong answer in this case (assuming a passive, open vent)?

[swiss_zhang@hotmail.com]

i just can''t inmage the flow behaviour at the open vent. You can see a strong flow circulation in the attachment, which the soot mass fraction at the open vent was measured by using Gas Phase Devices. And such a strong flow ciruclation can only occur when there is a forece or pressure applied at the oppisite direction.

Can you explain the phenomen?

Thanks

Yang

[Charlie Thornton]

I''m more of a programmer, so I''m not the guy to explain phenomena.  You might ask over at the FDS forum: http://groups.google.com/group/fds-smv/topics?pli=1

You could do a quick check by replacing the open vent with an actual opening into a large volume.  Then you can compare the behavior of an OPEN vent to an opening directly.