How to model window/opening for natural ventilation

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There are a few lists out there, including in old ASHRAE handbooks, but those can be hard to find. The best resource that I know of is for NIST's CONTAM software here. Some caveats, though: most of that data is going to be for closed doors and windows and it's in the form needed for use with CONTAM. Some work will be needed to use the data in EnergyPlus. There are references to source data at the NIST link, so you can dig in deeper and get more information.

Validation is pretty difficult. You can use CFD or do measurements, but neither of those is necessarily very easy to do. Studies have been done, but the problem is likely to be that unless you're lucky enough to find work that uses exactly your window type(s) it won't be directly applicable. Searching via Google for "natural ventilation rate window" turned up a large number of results, I'd expect similar results in any of the literature search engines. Even if you can't find exactly what you want, you should be able to find similar enough windows that it'll give you an idea of what reasonable values are.