Pedestrian Comfort (AIJ Niigata)

Pedestrian wind comfort studies evaluate whether wind speeds at ground level - in plazas, arcades, and passages between buildings - are acceptable for the intended activities: walking, standing, sitting outdoors. The key metric is the velocity ratio \(U/U_\text{ref}\), the ratio of the local mean wind speed at pedestrian height (typically 1.5–2 m above ground) to a reference wind speed at a standard height (usually building height or 10 m).

AeroSim delivers velocity ratio maps and probe-level time-averaged velocities across an urban domain in a single simulation run. This guided case uses the AIJ Niigata benchmark as the reference: a well-documented urban block with on-site measurements and a published experimental dataset, making it ideal for validating the workflow before applying it to a real project.

Case Description

The AIJ Niigata case is an urban cluster centred on a building of reference height \(H\) = 15.9 m, set within a rotative table of diameter 420 m. The benchmark includes four cardinal wind directions - 0° (East), 90° (North), 180° (West), 270° (South) - with pedestrian probe measurements at ground level throughout the urban area.

This guided case covers a single wind direction: 0° (East), which is the standard starting direction for the AIJ benchmark. Once the workflow is validated for one direction, additional directions are run with identical setup by rotating the geometry or the wind direction input.

Parameter

Value

Reference building height

\(H\) = 15.9 m

Rotative table diameter

420 m

Wind direction (this guide)

0° (East)

Pedestrian probe height

1.5 m above ground

Terrain category

II (open country, \(z_0\) = 0.05 m)

The primary output is the mean velocity ratio \(U/U_\text{ref}\) at each pedestrian probe position, compared against the AIJ experimental measurements.

Prerequisites

Before setting up this case you must have a completed and validated ABL case for Terrain Category II. Refer to ABL (Atmospheric Boundary Layer) and confirm:

  1. The mean velocity profile and turbulence intensity match the target Category II profiles.

  2. You have retained the inlet profile CSV and the roughness fin configuration.

Empty-domain ABL validation at the site. Before introducing the urban geometry, run an empty-domain ABL simulation with the full domain length and confirm that the inlet profile is preserved at the location where the urban cluster will be placed (approximately at domain centre). This step validates the inflow at the site and is required before the results can be compared to experimental data.

Setting Up the Case in AeroSim

Import the Geometry

Import the AIJ Niigata urban geometry (or your urban block model) as a closed, watertight surface mesh. Position the cluster so that the centre of the rotative table aligns with the intended domain centre. For 0° (East) wind, the inlet face is the west boundary of the domain.

Todo

Add UI steps for importing the urban geometry, orienting it for the 0° wind direction, and confirming placement within the domain. Include screenshots.

Boundary Conditions

The boundary conditions follow the same pattern as the validated ABL case:

  • Inlet: Synthetic Eddy Method (SEM) using the Category II profile CSV (columns: z, Ux, Rxx, Ryy, Rzz, Rxz, Rxy, Ryz).

  • Ground: IBM EqLog wall model with \(z_0\) = 0.05 m (Category II) and roughness fins (fin height 2 m, width 6 m, spacing 16 × 32 m).

  • Building and urban surfaces: IBM with smooth-wall condition.

  • Top and lateral faces: Free-slip Neumann condition (RegularizedNeumannSlip).

  • Outlet: Zero-gradient Neumann condition (RegularizedNeumannOutlet).

Todo

Add UI steps for assigning boundary conditions. Include screenshots of the BC panel.

Refinement Levels

The Niigata case uses five refinement levels (lvl 0–4) with a 1:2 ratio between successive levels. The finest level (lvl 4) targets the urban geometry and pedestrian zone with \(\Delta x\) = 0.75 m. Coarser levels cover the outer domain and ABL region.

Todo

Add UI steps for configuring the five refinement levels and the refinement regions in AeroSim. Include screenshots of the mesh refinement panel.

Domain Guidelines

Todo

Write the internal guidelines for domain sizing for the pedestrian comfort case: upstream fetch (multiple of H), downstream length, domain height, lateral width, blockage ratio.

Refinement Guidelines

Todo

Write the internal guidelines for mesh refinement: cell sizes per level, coverage of the urban cluster, pedestrian zone cell size, total node count, and expected GPU memory budget.

Exports

Todo

Describe what to export from the pedestrian comfort case: velocity fields at pedestrian height, probe time series, convergence history. Mention any AeroSim export presets for pedestrian comfort cases.

Post-Processing and Visualizing Results

Velocity Ratio at Probe Points

Extract the time-averaged streamwise and total velocity magnitude at each pedestrian probe location. Compute the velocity ratio:

\[ \frac{U}{U_\text{ref}} = \frac{\overline{|\mathbf{u}|}_\text{probe}}{U_\text{ref}(z_\text{ref})} \]

where \(U_\text{ref}\) is the mean wind speed at the reference height \(z_\text{ref}\) (building height \(H\) or 10 m depending on the applicable standard). Plot the simulated ratios against the AIJ experimental values. Good agreement is typically within ±15% at most probe locations.

Velocity Colormap at Pedestrian Height

Generate a horizontal slice of mean velocity magnitude at 1.5 m above ground. This colormap immediately identifies sheltered zones (low \(U/U_\text{ref}\)) and acceleration corridors (high \(U/U_\text{ref}\)) across the urban area, and is the primary deliverable for wind comfort assessments.

The AeroSim validation results for the AIJ Niigata benchmark, including probe comparisons for all four wind directions, are available at docs.aerosim.io/validation.

Todo

Add AeroSim-specific instructions for extracting probe velocities and generating pedestrian-height colormaps in the post-processing interface. Include screenshots of the result panel.

Advanced analyses - including turbulence intensity maps, gust velocity spectra, and directional frequency weighting for comfort criteria (Lawson, Davenport, NEN 8100) - are covered in a separate guide.

Next Steps