When Wind Carries the Rain 🌧️💨

We often picture rain as something that falls vertically. Yet, as soon as the wind picks up, raindrops are carried horizontally and can hit building façades. This is the principle behind wind-driven rain.

The video below illustrates this phenomenon in a deliberately simplified scene: two buildings, a turbulent wind field, and raindrops tracked one by one using a Lagrangian approach.

Simulation setup

Although this simulation is intended for educational purposes, it was produced using advanced tools:

• OpenFOAM simulation using LES ,
• Lagrangian coupling with DPMFoam ,
• Mesh size of about 5 cm, for a domain approximately 120 m long and 60 m high,
• The tallest building is 12 m high.

A deliberately simple scene

The scene shown here is intentionally minimal: two buildings, a 2D domain, a turbulent inlet, and raindrops injected into the flow.

The raindrops are tracked using a Lagrangian approach: their trajectories are computed individually, as particles transported by the wind field. The fluid itself is represented on a computational grid: this is the Eulerian part of the problem.

This separation is useful for understanding wind/rain coupling:

• the wind imposes a local velocity on the raindrops,
• gravity continues to make them fall,
• the buildings disturb the flow,
• wake and recirculation regions locally modify the raindrop trajectories.

The role of the buildings

The two buildings are not just geometric obstacles. They completely transform the local flow.

Upstream, the wind is disturbed by the presence of the first building. At the edges, the flow separates. Downstream, wake and recirculation regions appear. These structures change the way raindrops are transported.

In the video, some raindrops are strongly deflected, while others pass through calmer or partially sheltered areas. Rain therefore becomes a very local phenomenon: two nearby points in space can receive very different amounts of water depending on their position relative to the wind and the obstacles.

This is one of the key messages of this visualization: exposure to wind-driven rain does not depend only on the weather. It also depends on the shape of the built environment, the orientation of the façades, and the organization of the flow around the volumes.

This animation mainly helps visualize three mechanisms:

1. Raindrop deflection by the wind
   • As soon as the wind increases, rain acquires a horizontal component. The trajectories become oblique.
2. The shielding effect of buildings
   • Some areas are less exposed because they are located in the wake or behind an obstacle.
3. Local redistribution of rain
   • Recirculation zones and accelerations around buildings can concentrate or divert raindrops.

These mechanisms are central to wind-driven rain studies in urban environments. They explain why a purely meteorological approach, based only on wind speed and rainfall intensity, may be insufficient to understand the actual exposure of a façade.

To go further

We describe the physical background and calculation methods in more detail on the dedicated thematic page: Wind-driven rain .