How to keep spectators dry in open-air venues

Robin Stanfield, head of client management at BMT Fluid Mechanics, an international wind engineering consultancy, discusses how to overcome the technical challenges presented by open roof stadia.

FM pic 1Partial roofs at outdoor venues are primarily intended to keep spectators dry, without impeding anyone’s view or spoiling the open-air atmosphere. But, most are designed with only vertical rainfall in mind, leaving many fans at the mercy of the weather when the wind gets up and blows rain onto the stands.

A skim through recent articles I’ve read on the latest stadia designs reminds me that the reality can be quite different and clearly, the combination of wind and rain is not high up the priority list – as shown by a roof oculus where the diameter looks to be four times the roof span; large gaps between the upper tier and the outer roof edge; a horseshoe shaped roof with one stand uncovered; and finally, four independent stands with clear paths for wind, and thus rain, to blow directly into the stand and onto spectators.

In the same articles, there is a lot of discussion around climatic factors that need to be considered within the design phase including earthquakes, hurricanes and typhoons, snow and ice drift and build-up, and even sunlight availability for grass growth. However, nothing about rain. I should perhaps find that surprising, but the opposite is true. Wind-driven rain and its potential impact on spectators should be an important consideration within the design phase of any stadia project where rainfall is common.

FM pic 3Harnessing the combined power of wind tunnel testing and computational fluid dynamics (CFD) to understand how wind behaves inside open-air stadia informs designs that protect more spectators from driving rain.

Often, computational methods alone are used to simulate wind-driven rain, meaning that while informative, these predictions may only partially resolve highly complex and unsteady, turbulent flow patterns. To close the gap, combining CFD with established wind tunnel tests in tandem can offer rich design insights for improving spectator comfort, whatever the challenges of the local climate.

Together with the stadium’s designers, we proved the value of using both methods for a deeper understanding of how rain is driven around an arena, by carrying out tandem tests for the 55,000-seater San Mamés football stadium in Bilbao, Spain.

Based on the dual results for the football club, the design and engineering team recommended an extension to the stadium’s roof that has improved spectator comfort by 70%.

Individually, both testing techniques have their limitations as tools for understanding the behavior of wind-driven rain but studying CFD and wind tunnel visualizations side by side is uniquely informative.

This integrated approach, which has potential for further development, provides smarter, more robust solutions for keeping customers dry and happy.

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