Aerodynamics

The word aerodynamics combines 'aero' meaning air, atmosphere or gases, from the Greek aero or lower atmosphere and 'dynamic', relating to mechanical forces not in equilibrium from the Greek dynamikos meaning powerful.

1 Brief history of aerodynamics in theory
2 Brief history of aerodynamics in flight
3 Brief history of aerodynamics in structures
4 Brief history of aerodynamics in buildings
5 Aerodynamics in buildings today
6 Related articles on Designing Buildings

[edit] Brief history of aerodynamics in theory

Aerodynamics have been a consideration in the construction of sailing boats and windmills for many years, whilst aerodynamic concepts date back to Aristotle and Archimedes in the 2nd and 3rd centuries BC. Sir Isaac Newton was the first to develop a theory of air resistance in the early 1700's, expanded some years later in Hydrodynamica which describes the relationships between pressure, density, and flow velocity, known as Bernoulli's principle after its author, it also provides a method to calculate aerodynamic lift.

[edit] Brief history of aerodynamics in flight

It was at the turn of the next century that Sir George Cayley identified the inter-related aerodynamic forces of flight as being, weight, lift, drag, and thrust, whilst towards the end of the century Francis Herbert Wenham constructed the first wind tunnel, allowing precise measurements of aerodynamic forces. Aerodynamic principles were then first put into practice in 1871 by the French inventor, Alphonse Pénaud with his model aeroplane called the Planophore, the first aerodynamically stable flying mode with a wind up rear propeller.

In the years prior to their historic flight of 1903 the Wright brothers themselves built a small, 6-foot-long wind tunnel to test scale models of wing sections before embarking on their first sustained flight by a manned heavier-than-air powered and controlled aircraft in the Wright Flyer (or Kitty Hawk).

Gustave Eiffel built the first open-return wind tunnel in 1909, at Champs-de-Mars, near the foot of the tower that bears his name, it ran about 4,000 tests up to 1912, setting new standards for aeronautical research. In 1907 the relationship between fluid dynamics and aerodynamics became clearer with the foundation of an experimental facility by Ludwig Prandtl, who was considered the founder of fluid dynamics. He built the first Göttinger type of wind tunnel, which unlike Eiffel's wind tunnel featured a closed cycle for liquids or gases to flow in and was the starting point of the later Aerodynamical Experimental Station founded in Göttinger in 1915.

As the early beginnings of the aviation industry had commenced, some 15 years later the first full-scale wind tunnel building was constructed and able to test the aerodynamics of flight on the ground. At Langley Field near Hampton, Virginia a 9x18 metre aerodynamic test centre was built in the 1930's, testing World War II fighters and space capsules to submarines and jets.

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 later the aerodynamics of buildings.

[edit] Brief history of aerodynamics in structures

Aerodynamics only really started to be considered for fixed objects following the construction and subsequent failure of a number of large structures. The first, most significant and most dramatic failure was that of the Tacoma Narrows Bridge built in the USA in 1940, which had a span of over 1,645 metres and shortly after opening became known as Galloping Gertie due to the movement of its deck. About 5 months after completion, as wind speeds in the area picked up to around 40 miles an hour, the wave movements of the deck increased and as the public watched from the shore the bridge eventually collapsed. As it took such a long time and withheld such movements the extraordinary event was actually captured on film and can be watched here as a silent fim recording.

Similar issues were found with the The Bronx–Whitestone suspension Bridge (or Whitestone Bridge) in New York City, opened in 1939. As a result. extra stiffening trusses were added around the same time as the Tacoma Narrows Bridge collapse and it was widened to six lanes. Likewise the The Deer Isle suspension bridge in the state of Maine, also completed in 1939 encountered wind stability problems, and was modified with cable stays connecting cables to the tower and tower to the deck as a result of what happened in Tacoma.

Sometime later in 1965 at Ferrybridge in England cooling towers collapsed because of what was later described as a group effect, in that building structures in a group under wind loads behaved diametrically opposite to how they might have behaved individually. This was an example of the aerodynamic group effect of surroundings, very much like the localised built environment surrounding buildings.

The Millennium Bridge in London, opened in 2000, had similar problems and was nicknames the Wobbly Bridge. Its oscilation was corrected by the addition of mass dampers.

Finally in 2009 a seven km long section of the bridge over the river Volga in Volgograd, Russia was put into operation, it was a section just under 30 km of a continuous beam composite steel-concrete bridge. One section of the bridge around 1 km long started oscillating with amplitudes of up to 1 m, the cause again was wind, but in this kind of concrete hybrid bridge ( as opposed to a suspension bridge) this kind of movement had never been seen before. Video footage of the moving bridge can be found here.

[edit] Brief history of aerodynamics in buildings

In the 1930's, working in Denmark, Irminger and Nokkentved, investigated the nature of air movement over buildings, in particular the accuracy of wind-tunnel methods for reproducing the correct air-flow separation and reattachment positions of the vortices formed by winds passing over buildings.

Ludwig Prandtl, the founder of fluid dynamics and the boundary layer theory, with the Aerodynamical Experimental Station in Göttinger and the Kaiser Wilhelm Institute for Fluid Dynamics developed the fundamentals of current techniques of aerodynamics. Experiments at this time, were performed in flows with low turbulence, and without boundary layers, and the need to model the atmospheric turbulence properly lead to the design of so called Atmospheric Boundary Layer Wind Tunnels. Recognised in the 1950s, this led to longer working sections of wind tunnels in which experiments with the wind loads on buildings were performed.

The impact of the Ferrybridge cooling towers collapse in 1965 and what was accepted as the aerodynamic group effect, lead to further studies of aerodynamic effects on buildings, especially as heights increased and loads changed. However the phenomenon had been studied some in the early 1900s by the Hungarian physicist Theodore von Kármán, who was investigating why bridge supports and buildings vibrate in high winds. He found that air flowing around an obstacle broke away into distinct whirlwinds, at a certain wind speeds, the forces involved produce resonance, causing wires to sing in the wind, or towers to wobble and in the case of Ferry bridge to collapse..

The impacts of the aeroelastic effect which had impacted the various suspension bridges that had been built, gradually highlighted the importance aerodynamic study being carried on a number of major projects.

In physics and engineering, both aerodynamics and fluid dynamics as well as hydrodynamics are considered subdisciplines of fluid mechanics that describes the flow of fluids—liquids and gases. The first scientific paper that used a 3D model in fluid flow problems was published by John Hess and A.M.O. Smith in 1967 and marked the birth of what is known as Computational Fluid Dynamics (CFD) which is the digital form of aerodynamic analysis. It allowed aerodynamicists to gain a more detailed understandings of flow fields around or through objects and in particular helped in the modelling of larger objects that could not be modelled directly iin wind tunnels, such as buildings.

For example here is a CFD analysis of a Kármán vortex, which was theorised in the 1900's, impacted the towers at Ferrybridge in 1965 and modelled in the twentieth century by Cyclone Fluid Dynamics.

[edit] Aerodynamics in buildings today

Today fluid mechanics, aerodynamics, fluid dynamics as well as on occasions hydrodynamics and in particular benefitting from CFD analysis techniques has a significant role to play in the design of buildings and is used in a number of specific areas: