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Make sure you are using the current release of FDS. The lower the free area, the more the drag and the higher the likelihood that a particle based CFL will start to limit the timestep. As stated in the guide, the screen drag routine is based on data for window screens with 30 to 60 % porosity. If you are dropping below 30 % free area or going above 60 %, you are extrapolating the correlation beyond the range of data used to develop it. You may not be getting reasonable drag beyond the limits of the correlation. Window screens are not perforated ceiling panels. I would recommend you test your inputs in case similar to the screen drag verification cases to verify that you are getting an appropriate pressure drop for the various free area fractions for your ceiling panels. The thickness of your ceiling panel is likely much larger than the typical wire thickness for a window screen which could also be a factor. You may need to develop custom drag parameters to reflect the manufacturer reported pressure drop through the ceiling. |
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Here's tip -- put a |
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Can you also try running the case with an input of PARTICLE_CFL_MAX = 0.9 in the &MISC line? |
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I am using porosity screen function, as a way to mimic a false ceiling with various percentage free area. However when i adopt a low % free area, my wall time increased dramatically whenever i go below 30% free area.
Is this a limitation of the modelling equations? or there is a better way to do what i am doing ?
I want to simulate a porous screen between two compartments for a range of % free area, 10-100%
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