Wind tunnel

NASA transonic wind tunnel

[edit] Introduction

A wind tunnel is a device which produces a strong, controlled stream of air to enable the study of the effects of winds and air flow on objects such as cars, aircraft and buildings. It was invented in 1912 by Gustav Eiffel, the designer and engineer of the tower in Paris which bears his name. Although not the world’s first such device, the ‘aerodynamic dynamic blast engine’ was the first to have a diffuser and a collector, thereby setting the mould for modern versions of the machine.

The wind tunnel played a major role during the Second World War and saw further development for the testing of supersonic aircraft during the Cold War. It has developed largely in parallel with the development of aircraft generally and the increasing height of buildings. Wind tunnels can show the optimal design for aerodynamics and wind loading on the ever-increasing surface areas of tall buildings.

The basic structure of a wind tunnel comprises a tube of circular section with a large fan at one end. Circular section, smooth-surfaced tunnels have become the norm as being square produces turbulence due to the corners. Later variants involve air being sucked out at great force to achieve the same effect and may have a series of fans depending on the windspeed required

[edit] Observing air flows

Once a stream of air is pushed through the tunnel, a means is required to observe the air flows around the placed objects. This is usually achieved by various means such as:

• Adding smoke or dye to the air.
• Attaching threads or tufts of yarn to the object.
• Attaching gauges to measure the air pressure at various points.
• Using helium-filled bubbles to show vortices paths on wing tips.

. Typically, wind tunnels may be able to replicate a variety of windspeeds:

• Subsonic (Mach* 0.8).
• Transonic (Mach 1).
• Supersonic (approximately Mach 6).
• Hypersonic (Mach 6-12)
• Hypervelocity (Mach 12+).

(Mach is a term used to describe the ratio of an object's speed to the speed of sound. So, Mach 1 is equal to the speed of sound, Mach 2 is twice the speed of sound, Mach 3 three times….and so on.)

High-speed travel usually generates high temperatures on wing tips and other areas. Such scenarios are replicated by heating the air passing through the tunnel to an appropriate temperature, often higher than the melting point of the material being tested.

In the case of buildings and aircraft, scale models are positioned inside the tube and secured tightly in place to determine the effects of applied wind or movement through the air, however some wind tunnels are large enough to accommodate full-size cars, car body parts and even full-size aircraft.

[edit] Buildings

Wind tunnel testing of buildings can improve occupant comfort and safety, save on construction costs and help achieve highly individual architectures that might otherwise not have been possible. It is also increasingly used to improve the pedestrian experience, to avoid being blown off course by strong winds which can occur at ground level due to tall-building aerodynamics.

Wind tunnel tests may be conducted on:

• Tall buildings and skyscrapers.
• Other tall structures.
• Clusters of buildings to determine aerodynamic interference.
• Cladding assemblies.
• Long-span or unusual-shaped roofs.
• Long-span bridges.

Typically, wind tunnel testing is usually undertaken during the early planning stages and where the building:

• Is over 22-storeys in height (or 10 storeys if in a hurricane zone).
• Has an unusual geometry or construction.
• May be adversely affected by surrounding structures.
• Is located amid unusual terrain.

Wind tunnel testing can show how wind flows up, down and around a building, and the sorts of loadings that will have to be borne by the structural frame, whether pushing or suction forces, particularly important for the highest reaches.

Building codes have responded in kind – once wind loads acting on a building are established, codes can be drawn up to ensure structures are robust enough to withstand them. Wind-tunnel testing is particularly useful where a building has an irregular shape.

NB BREEAM UK New Construction, Non-domestic Buildings (United Kingdom), Technical Manual, SD5078: BREEAM UK New Construction 2018 3.0, published by BRE Global Limited defines wind tunnel modelling as: ‘…a versatile physical technique which allows a large number of variables (for example building design, intake and exhaust positions, local pollutant sources, wind speed and direction), to be investigated for complex urban areas. In particular, wind tunnel modelling provides reliable and detailed data, both visual and quantitative, on outdoor pollution distribution. This enables effective siting of intakes and exhausts for both mechanically and naturally ventilated buildings.’

[edit] Related articles on Designing Buildings Wiki

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• Burj al Arab, Dubai.
• CFD.
• Computational fluid dynamics in building design: An introduction FB 69.
• CN Tower.
• Empire State Building.
• Integrating CFD into the design process.
• Lifts for buildings.
• Lotte World Tower.
• Petronas Twin Towers.
• Skyscraper.
• Structural principles.
• Taipei 101.
• Tallest buildings in the world.
• The Shard.
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