Heights: Meteorological, Probe, and Pedestrian
Overview
Three distinct heights govern the ArchiWind workflow. They serve different roles and must not be confused with one another:
| Symbol | Name | Role |
|---|---|---|
| $z_{\text{met}}$ | Meteorological reference height | Height at which the wind data source’s statistics were recorded |
| $h_{\text{probe}}$ | Upstream CFD probe height | Height of the upstream reference probe placed inside the CFD domain |
| $z_{\text{proj}}$ | Pedestrian projection height | Height at which comfort and safety results are evaluated |
This page defines each height, how it is determined, and how they combine to produce the per-direction velocity scaling factor used across ArchiWind’s climate-weighted outputs. For the underlying log-law wind profile and per-direction roughness determination, see Inlet Boundary Conditions and Reference Point Handling.
1. Meteorological Reference Height ($z_{\text{met}}$)
$z_{\text{met}}$ is the height AGL at which the wind statistics were recorded at the meteorological station. It is a property of the wind data source, not of the CFD model.
| Wind data source | How $z_{\text{met}}$ is determined |
|---|---|
| GovMap CSV | User-specified at project setup (default 150 m). The file contains parameter blocks at multiple heights; the chosen value selects the correct block. |
.tim and .tab files | Must be provided explicitly as a separate simulation parameter, since neither format carries height metadata. |
2. Upstream CFD Probe Height ($h_{\text{probe}}$)
$h_{\text{probe}}$ is the height AGL at which an upstream reference probe is placed inside the CFD domain. It is a property of the simulation geometry and is entirely independent of $z_{\text{met}}$.
Its purpose is to anchor the post-processing velocity scaling: by sampling the CFD-computed speed at a point that lies in undisturbed upstream flow, the workflow establishes a per-direction ratio between the expected real-world wind speed and the normalised CFD speed.
Placement
One probe is generated per wind direction. Its height above local terrain is determined as follows:
- Automated (default): $h_{\text{probe}} = 1.5 \times H_{\text{tallest}}$, where $H_{\text{tallest}}$ is the height of the bounding box of the built models (building or surroundings). The result is clamped to $[10\text{ m}, 150\text{ m}]$ AGL.
- User override: the operator may supply an explicit value, bypassing the $1.5\times$ rule, for example when a more precise height is known. The same $[10\text{ m}, 150\text{ m}]$ clamp still applies.
The probe’s absolute elevation also varies per wind direction, placed at $z_{\text{terrain}}(x, y) + h_{\text{probe}}$, the local terrain elevation at the upstream location for that direction. This keeps the probe at a consistent height above ground regardless of topographic variation around the site.
Per-direction velocity scaling factor
For each direction $d$, the velocity scaling factor is computed by comparing the theoretical ABL log-law speed at $h_{\text{probe}}$, referenced to $z_{\text{met}}$, against the CFD-simulated speed at the same point:
$$ SF[d] = \frac{u_{\text{ABL}}(h_{\text{probe}}, z_{0,d})}{U_{\text{CFD}}(\text{probe}, d)} $$
where
$$ u_{\text{ABL}}(h, z_0) = u_{\text{ref}} \cdot \frac{\ln\left(h_{\text{probe}} / z_0\right)}{\ln\left(z_{\text{met}} / z_0\right)} $$
| Symbol | Description |
|---|---|
| $U_{\text{CFD}}(\text{probe}, d)$ | CFD-simulated speed at the probe for direction $d$ |
| $z_{0,d}$ | Upstream roughness length for direction $d$ (from land cover) |
| $z_{\text{met}}$ | Meteorological reference height (wind data source) |
$SF[d]$ is applied uniformly to all surface cell velocities in direction $d$ during the exceedance calculation.
The two heights appear together in the formula: $z_{\text{met}}$ sets the normalisation of the ABL profile, while $h_{\text{probe}}$ determines the specific elevation at which that profile is evaluated and compared to the CFD output.
3. Pedestrian Projection Height ($z_{\text{proj}}$)
$z_{\text{proj}} = 1.5$ m AGL is the height at which comfort and safety results are evaluated on all exposed surfaces, in accordance with IS 5281. This is where CFD velocity and turbulence fields are sampled to produce the exceedance maps. See Pedestrian Wind Comfort and Safety for how these fields are classified.