Cp on Facades
The pressure coefficient $C_p$ is a dimensionless quantity that describes the local wind pressure on a surface relative to the free-stream dynamic pressure:
$$ C_p = \frac{p}{\frac{1}{2} \rho U_\text{ref}^2} $$
ArchiWind computes $C_p$ on each face of the building envelope from the CFD pressure field and displays it as a spatial map across the facade.
This feature requires a building geometry to be uploaded as part of the simulation input, and is available in detailed quality simulations.
Interpreting Cp Values
| Value | Meaning |
|---|---|
| $C_p = 1$ | Stagnation point: the flow comes to a complete stop and all kinetic energy is converted into a static pressure rise equal to the free-stream dynamic pressure. This is the theoretical maximum and occurs at the centre of windward faces. |
| $C_p > 0$ | The flow is decelerating and pressure builds up on the surface. Typical of windward faces directly exposed to the incoming flow. |
| $C_p = 0$ | The local static pressure equals the free-stream static pressure and the local velocity equals $U_\text{ref}$. |
| $C_p < 0$ | The flow accelerates around the building, creating suction. Typical on leeward faces, sides, and roof edges. |
More negative values correspond to stronger suction. At sharp corners and edges, the flow cannot follow the geometry and separates, producing the most intensely negative $C_p$ regions. These areas are often critical for facade and cladding design, as suction loads can govern the structural requirements of the building envelope.
Reference Velocity
$U_\text{ref}$ is the free-stream wind speed used to normalise the pressure field. It is selected by the user directly in the 3D viewer after the simulation has completed, and changing it updates the $C_p$ map in real time.