Pedestrian wind comfort: making the city liveable.

Wind, the blind spot of urban projects

A project can be perfectly designed on paper yet deliver an unusable forecourt: wind is the one outdoor-comfort parameter that appears on no site plan.

Every building reshapes the airflow far beyond its own plot. A tower captures fast high-altitude winds and sends them back to the ground; two closely spaced buildings compress the flow and speed it up; an open square becomes a corridor. At pedestrian scale, these phenomena translate very concretely: a hard-to-open entrance door, a deserted terrace, an unbalanced pushchair, a forecourt crossed at a run. It is this invisible path of the air that CFD simulation makes visible, before works begin.

The stake is also economic. A ground-floor retail unit with a windy forecourt lets less well; an unusable terrace is a programmed area that produces nothing; a deserted public space is a lost development investment. The wind-comfort study secures the usability value of these outdoor surfaces, just as a structural study secures the building.

What the wind does to a city

Urban wind is not a uniform flow slowed by buildings: it is a system of accelerations and recirculations governed by geometry. When it meets a façade, the air splits: part rounds the corners and accelerates, part plunges along the façade down to the ground, part rushes into the transverse streets. Every urban configuration produces its own aeraulic signature.

Simplified diagram of the effects of wind in the city

These mechanisms combine: a tower's downwash feeds a Venturi effect, which opens onto a square in a turbulent wake. The result: public squares turned into windy corridors, building entrances subject to sudden gusts, unusable terraces, and sometimes dangerous loads on glazed surfaces or lightweight structures.

How is wind comfort judged?

Wind comfort is not decreed, it is measured against standardised references. The most widely used is the Lawson criterion, complemented by the Dutch standard NEN 8100: both associate a wind speed and an admissible frequency of occurrence with each use. The more static the use, the stricter the requirement: more wind is tolerated on a transit pavement than on a café terrace.

The final map crosses these thresholds with the local wind statistics: each point of the site receives the use class it can genuinely host. It is a programming tool as much as a verification tool: it says where to place the terrace, the play area or the entrance, and where protection will be needed.

The choice of reference is made at kick-off, with the client and the inspection office where relevant: some tenders require Lawson, others NEN 8100, others a city criterion. Our maps are produced in the reference required by your file, and can be issued in several references for the same site.

The digital wind tunnel: what the model reconstructs

Our digital wind tunnel reproduces the site as it will be: the project geometry and that of its urban surroundings over several hundred metres, because it is the neighbouring buildings that condition the incident wind. The model is then exposed to the site's statistical winds, direction by direction, and returns the speed field at every point at pedestrian height.

On the most sensitive sites, we bring in LES (Large Eddy Simulation): instead of averaging the turbulence, the computation resolves the gusts themselves. The wind is then seen living in the city, puff by puff, and instantaneous peaks are captured that an averaged approach underestimates. It is today the closest thing to a physical wind tunnel, with information at every point of the site.

“At a tower corner, the wind speed can approach twice the incident wind: that is exactly where the entrances are.”

The output is not limited to a mean map: we produce maps by wind direction (to understand which sector creates each discomfort zone), seasonal maps (a summer terrace is not judged on January storms) and vertical sections at the singular points, from the ground to the top of the towers. It is this granularity that then allows targeted protections to be designed rather than generic remedies.

The full approach follows a constant sequence: collection of climate data, construction and computation of the model, analysis of the speed fields, then translation into development recommendations, re-simulated until validation.

From diagnosis to corrected project

A comfort map only has value if it changes the project. Each hard spot identified (corner overspeed, Venturi, exposed terrace) is reworked with the design team and translated into testable solutions: windbreak screens and fences, canopies and overhangs, façade setbacks, vegetation sized for its porosity, or simply the relocation of sensitive uses. Each variant is re-simulated: we recommend only what works in the computation.

Windbreak fence — comfort of a public square

A well-sized windbreak fence channels the flow and creates a sheltered zone at use height, without generating troublesome wake turbulence downstream. The sizing (height, porosity, placement) is settled in the simulation, not by guesswork.

The same approach applies outside the dense city. In rural or peri-urban settings, the air circulates more freely: a summer breeze cools, but strong winds accentuate wind chill and erosion. The strategic placement of hedges, trees and natural barriers reduces the impact of gusts without compromising natural ventilation, taking account of seasonal variations and the terrain topography.

Sizing activities according to wind zones

The comfort map is not only a verification tool: it is a plan for programming uses. Each zone of the site receives a use class (prolonged sitting, standing, walking), and this class says very concretely what the space can sell: a restaurant terrace is only profitable on a "prolonged sitting" zone, a market or kiosks require a "standing" zone, a simple pedestrian link is content with a "walking" zone. Rather than placing activities on the site plan and then noting the problems, the programming is made to coincide with the real climate of the site.

This is a direct stake for property value: at equal area, a usable balcony, a terrace operated year-round or a welcoming building base let and sell better. On high-rises in particular, where exposure grows with height, the study determines floor by floor the type of outdoor space it is honest to promise.

More broadly, this reading opens onto an urbanism of wind: designing the city as a climatic environment whose uses are organised. Pedestrian routes take the calm zones, static activities concentrate in the sheltered pockets, ventilated corridors are preserved to cool and cleanse rather than be built over. Wind ceases to be a constraint endured and becomes a programming input, on a par with sunlight or noise.

Wind safety and regulatory files

Beyond comfort, the criteria set a distinct safety threshold: above about fifteen metres per second, a gust can unbalance a pedestrian, all the more so an elderly person, a child or a cyclist. The study identifies the zones where this threshold risks being crossed, particularly near high-rises, car-park exits and cycle continuities, and sizes the protections needed.

This analysis also has a file value. More and more local authorities expect a wind-comfort study at the permit stage for high-rise projects or large developments. Our maps, backed by the standardised references, fit into planning files and specifications; they extend naturally towards the calculation of wind pressures on façades per Eurocode 1 when the structure is at stake.

Beyond wind: outdoor thermal comfort

Wind is only one of the components of comfort felt outdoors. The same simulation integrates, when the project requires it, sunlight and cast shadows, the radiant temperature of mineral surfaces and the humidity, to assess an overall outdoor thermal comfort: a forecourt sheltered from the wind but crushed by the sun stays uncomfortable in summer, and conversely a well-oriented breeze cools a mineral square.

This cross reading guides fine design trade-offs: orienting the resting spaces to catch the summer breeze and shelter from cold winter winds, positioning vegetation where it both cools and shelters, and checking that the dispersion of pollutants nearby (roads, ventilation outlets, loading docks) does not degrade the seating zones. It also prepares projects for future summers, where the passive cooling of public spaces becomes a design criterion in its own right.

The EOLIOS method: engineers who also do urbanism

For us, CFD is not a visualisation exercise: it is an urban design tool, placed in the hands of engineers who work daily with architects, landscapers and developers.

Rather than handing over a report, we optimise the project with the design teams. Each hard spot is reworked in workshops, translated into costed recommendations, re-simulated until a solution integrated into the architecture is reached. We intervene from sketch to permit, on business-district towers, schools, rooftops, sports facilities and public spaces, in France and internationally. The results take the form of images and 3D animations understandable by all the project stakeholders, usable in a design meeting as much as in a regulatory file.

This expertise does not work alone: it articulates with our other Air & Wind skills. The same district model feeds the calculation of wind pressures on façades for the structure and the glazing, the study of the dispersion of pollutants and odours nearby, and the analysis of heat islands. For a client, it is one model, several answers, and one point of contact.