# Problem calculating pressure coefficient on facade

Hi all,

I am trying to calculate the wind pressure coefficient, Cp, at specific points on the facade of a test building. This is to validate my setup compared to existing research so I can continue to use it further in my thesis. I made a simple python script to calculate Cp according to the equation Cp = (P - Pref) / (0.5 * rho * Uref^2) however, the results don’t make sense (Cp > 1). I am sampling Pref and Uref at a point 1.5H away from the top of the building. The setup I am emulating specifies U = 12.7m/s at building height (182.88m) which I set in the Create Wind Tunnel Component, however, it seems it still makes 12.7m/s at 10m when I run the analysis.

I tried to follow the advice for getting Cp here: Error with wind simulation when adding awning window panes but BF does not run when I input the function object. I attached my GH file please any help would be appreciated.

PressureCoeffTests.gh (438.5 KB)

1 Like

Z0 in wind tunnel is currently always set to 0. You can manually edit that value inside the folder. I believe there is an open issue on this topic on GitHub.

@errone92

When you input a velocity value to the wind tunnel component it always corresponds to the 10m wind velocity, thus your findings.

What you can do is use a logarithmic calculator (LB has one or one from the net) and find your 10m wind velocity, for a 12.7m/s velocity at 182.88m. Then plug in that number in BF and all should work okay, assuming your ground starts at 0 (if not then you also have the issue mostapha noted).

Kind regards,
Theodore.

Hi Theodor. Does this mean that the wind tunnel inherently considers the power law? so there is no need to define the wind speed variation with height manually when using the wind speed from a weather data file?