Input of Cw in the "Set EP Air Flow"


I’m trying to use the “Set EP Air Flow” component to do the simulation under natural ventilation. As it uses the fomula below and I set the natural ventilation type to 1, does anyone know where I can input the Cw in this component? Or should I set the natural ventilation type to 2 as I see the input of the wind coefficient?


@HongningLan ,

When the natural ventilation type is set to 1 for “window-based natural ventilation”, the input for Cw defaults to “Autocalculate.” This means that E+ will look at the angle between the window normal vector and the direction of the wind at each hour of the simulation hour to dynamically determine what the Cw should be at every hour. The Honeybee components have another level of intelligence built into them that checks if a zone possesses windows that have normal vectors that are > 90 degrees different from one another, thereby enabling wind driven cross ventilation. If there are no windows on your zone that have > 90 degree difference in normal vector, the Cw will default to 0 for no wind-driven cross ventilation (thus only accounting for bouyancy-driven flow). If you know that a zone will have cross ventilation but it does not have windows at > 90 degree difference from one another, you can always force the components to use the “Autocalculate” method by setting windDrivenCrossVent_ to True:

Another thing to note is that, if you set a number for Cw in E+, it will have this number for all hours of the simulation regardless of the direction that the wind is blowing in the epw file. So this is not a good way to model window-based natural ventilation but it might be a good way to model something like a cowl, which promotes ventilation using the wind when it blows from any direction. If you are actually trying to model a cowl, I suggest that you use option 2 on the “Set EP Airflow” component, which is for “Custom Stack / Wind Ventilation” and allows you to set a number for Cw.



Thanks for your detailed explanation. I think window-based natural ventilation is more suitable for my simulation and would it possible to send me the link of the formula about the autocalculate Cw in Type 1?
Since the Airflow Network Group is more complicated and accurate for the indoor natural ventilation, is there any further progress about this?


@HongningLan ,

The reference for the Autocalculate feature is the same as where the image that you posted came from:

I’ll also add that Airflow Networks are only more accurate if they are set up correctly and well-understood by the modeler. The fact that I see many people still getting the simple natural ventilation objects under their belt tells me that the pool of people who can make correct use of AFN simulations is still pretty small. This said, the pool is clearly growing and the fact that OpenStudio team has added the AFN into the latest versions of OpenStudio SDK means that we will definitely make it a part of HB[+] when we roll out energy simulation features within a year or so.