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Author Topic: Difference beetween using vent or multiple surfaces  (Read 2658 times)

Feyd

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Difference beetween using vent or multiple surfaces
« on: July 23, 2014, 01:32:36 am »

hello everyone,

From my personnal experience, there is two possibilities for creating a specific surface in the model:
- You can put a vent on an obstacle
- or you can use the multiple surface option in the obstacle properties.

Is there a difference between this 2 solutions?

Thanks in advance.
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Bryan Klein

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Re: Difference beetween using vent or multiple surfaces
« Reply #1 on: July 23, 2014, 09:47:26 am »

The primary difference can come into play when VENTS don't align well to the obstructions.  Sometimes you will get an error about the VENT not on a Solid Surface, which usually can be resolved by moving the VENT closer to the cell edge that the OBST will occupy.  Assigning the SURF to the OBST properties will never result in this kind of error.

Behind the scenes, when you use a VENT, FDS is replacing the OBST SURF properties with whatever properties are assigned to the VENT in each cell that the VENT covers.  When you assign the SURF to the OBST, no such substitution occurs.

Also, it is good to remember that if you have more than one VENT overlapping the same OBST cells, the first VENT listed in the FDS input file (the VENT higher up in the PyroSim Object Tree in the left panel), will be assigned to the cells.  All subsequent VENTS and SURFS are rejected.
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michael

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Re: Difference beetween using vent or multiple surfaces
« Reply #2 on: August 08, 2014, 07:53:47 am »

The primary difference can come into play when VENTS don't align well to the obstructions.  Sometimes you will get an error about the VENT not on a Solid Surface, which usually can be resolved by moving the VENT closer to the cell edge that the OBST will occupy.  Assigning the SURF to the OBST properties will never result in this kind of error.

Behind the scenes, when you use a VENT, FDS is replacing the OBST SURF properties with whatever properties are assigned to the VENT in each cell that the VENT covers.  When you assign the SURF to the OBST, no such substitution occurs.

Also, it is good to remember that if you have more than one VENT overlapping the same OBST cells, the first VENT listed in the FDS input file (the VENT higher up in the PyroSim Object Tree in the left panel), will be assigned to the cells.  All subsequent VENTS and SURFS are rejected.

Will that speed up the process of simulation if you use SURF rather than a VENT?
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