Hi all,
I’m modelling a large naturally ventilated space and have openings on either end of the space. I’m not using the AFN (for now).
I’m just trying to double check my results by doing some hand calcs. My understanding is that E+ can output a single mass (or volume) flow rate per zone, rather than per opening, and that this total zone flow rate is calculated from the equations shown in the notes of HB Window Opening.
I just wondered how the wind coefficient is determined (maybe related to the terrain input), and whether the wind speed is taken directly from the epw? Is the wind coefficient not different for different elevations of the building? Also, for the opening area, is this the total, or per opening?
If someone could clarify that would be greatly appreciated!!
Thanks
Joe
Hey @jroberts,
For simplified ventilation without the AFN, it’s not possible to output the flow rate per window, only per zone, as you said.
The wind coefficient (or opening effectiveness) only depends on the wind’s angle of incidence (wind direction) and the window’s orientation, not on wind speed or height. More info here.
What does vary with height is the wind speed, which gets corrected from the EPW value (assumed at 10m) for each surface, based on its height and the terrain type. More info here.
If I get your question right, you’re asking whether E+ considers all apertures in a zone as a single object (with a total area) or keeps them separated. You can see in the IDF that all apertures are separate and each window has its own area and effective angle. HB only lets you set controls, discharge coefficient and operable fraction at room level, which get then applied to every window in that zone. If you need to control each window separately, you can do that by editing the generated IDF manually.
Hope this helps!
Matteo
Hi @MatteoMerli
Many thanks for your detailed answer, you’ve definitely put me on the right track and taught me a few new things!
Nonetheless I still can’t manage to replicate the ventilation rates calculated by E+ in my hand calc. I’ve posted separately about this on Unmet Hours, if anyone fancies a challenge! Trying to understand Ventilation by Wind and Stack with Open Area - Unmet Hours
Thanks,
Joe
