Butterfly - Wind pressure coeffients (Cp's values) for AFN models

Hello, Butterfly/OpenFoam experts,

I am running a natural ventilation study with EnergyPlus using the AirflowNetwork (AFN) Model.
To get more accurate wind pressure coefficients (Cp) I modelled the case with Butterfly as well.

I have followed the workflow suggested in this post to get the desired results (Cp’s): Error with wind simulation when adding awning window panes

However, I am quite lost regarding the coefficient values that need to be informed at the following fields: pRef; pInf; UInf; rhoInf, … , for instance.

I found some guidance here, but no reference values: https://www.openfoam.com/documentation/guides/latest/api/classFoam_1_1functionObjects_1_1pressure.html

I appreciate any help and hints on the matter.
Thank you all very much.


Hi Nayara. I know it has been a long time since you asked this question but I’m also trying to do the same thing with an AFN model. Did you manage to get the Cp values at the end through BF?

Hello, @Shadab_mdr

We’ve had a few updates in this area since then.
Yes, I was able to calculate the Cp values with BF. I even cover a bit of that subject in this article here: https://www.researchgate.net/publication/370325934_CFD-based_wind_pressure_coefficients_on_building_facades_with_ButterflyOpenFOAM_for_airflow_network_models#fullTextFileContent

Also, a grasshopper plugin came out that does this as well: Eddy 3D

More details here: Eddy3D - Pressure on building facades on Vimeo

Finally, if you work with Energy+ models, I recommend this site, which calculates the pressure coefficients for the shared .idf file.


Thank you Nayara for your help! I’ve found your introduced methods incredibly useful. However, I’m a bit confused about something (either in Eddy3D or Butterfly). I’m not sure what the wind velocity should be for all wind directions while calculating Cp (for an annual analysis of natural ventilation). Is this reference velocity a fixed constant, such as the 10m/s I came across in some resources? Or does it vary depending on the weather file? or could it be linked to the height of the opening?
Since the goal of using a coupled CFD-EnergyPlus analysis is to get precise results for a specific building, I’d like to make sure I use the right input.