Most of us prefer to stay as dry as we can in the rain. Some urban streets use overhangs to provide pedestrians with protection from this as well as the other elements, and of course the greater the area of the overhangs, the greater the coverage. While intuitively we expect lower overhangs to provide more protection, what would be the quantitative relationship between the height of the overhang and the degree of protection from the rain under realistic physical and environmental conditions?
To examine this relation, scSTREAM, computational fluid dynamics (CFD) software by Cradle, was used to simulate rainfall on overhangs at different heights while monitoring rain accumulation below.
First, a model of an urban setting with a mixture of medium to high-rising buildings was created. An overpass was placed at the center of the model which served as a street crossing for pedestrians, and on top of the overpass, an overhang roof was attached to provide rain cover. The height of the overhang was set to change between 3.5m, 4.0m, and 4.5m from the floor where the amount of raindrops beneath the overhang was then monitored with respect to each height to indicate the coverage effectiveness. In addition, the rain was driven by a cross wind, so the first simulation was to establish the steady-state flow field caused by the wind.
After the steady-state wind flow was established, rainfall was initiated. Raindrop diameters typically range between 0.5mm – 6mm, and depending on the diameter, the velocity and drag coefficient of the raindrops will change, affecting the amount of raindrops as a percentage of the total. Based on these studies, the raindrops were categorized into three different size bins designated as small (1.0mm), medium (2.4mm), and large (4.0mm). With these considerations, this should provide adequate representation of typical rainy weather.
The three different sized raindrops were represented as particles with mass using the particle tracking methodology in scSTREAM. The particles were given appropriate material properties and entered the computational domain above the buildings across the sky. At the start of the simulation, they entered this domain with an initial velocity equal to their terminal velocity. Immediately after entry, the particles were carried by the wind and rained down onto the building model. Every particle reaching the monitoring region below the overhang was recorded. The simulation was repeated for each overhang height.
The graphs below show the accumulated number of raindrops measured under the overhang. As expected, the lower the overhang, the more effectively it protects the area below. The 3.5m overhang improves rain coverage by nearly 50% compared to the 4.5m overhang.
If the only purpose of the overhang was to protect pedestrians from the rain, a low overhang height would be desirable. However, other factors must also be considered, including cost, aesthetics, practicality, and expected pedestrian density. Realistically, one of the best protections from the rain is still a good umbrella.
 Gunn R, Kinzer GD. The terminal velocity of fall for water droplets in stagnant air. J Meteorol 1949;6:243-8