Wind tunnel

From Free net encyclopedia

A wind tunnel is a research tool developed to assist with studying the effects of air moving over or around solid objects.

Air is blown or sucked through a duct equipped with a viewing port and instrumentation where models or geometrical shapes are mounted for study. Various techniques are then used to study the actual airflow around the geometry and compare it with theoretical results, which must also take into account the Reynolds number and Mach number for the regime of operation.

For example:

Threads can be attached to the surface of study objects to detect flow direction and relative speed of air flow.
Dye or smoke can be injected upstream into the airstream and the streamlines that dye particles follow photographed as the experiment proceeds.
Probes consisting of a Pitot tube can be inserted at specific points in the air flow to measure static and dynamic air pressure.

A vertical wind tunnel is a recreational facility for indoor skydiving.

1 History
2 Windtunnel classification
3 Well known wind tunnels around the world
4 External links

[edit]

History

English military engineer and mathematician Benjamin Robins (1707–1751) invented a whirling arm apparatus to determine drag and did some of the first experiments in aviation theory.

Sir George Cayley (1773-1857), the 'father of aerodynamics', also used a whirling arm to measure the drag and lift of various airfoils. His whirling arm was 5 feet long and attained tip speeds between 10 and 20 feet per second. Armed with test data from the arm, Cayley built a small glider that is believed to have been the first successful heavier-than-air vehicle to carry a man in history. Cayley went on to lay all the design foundations for the later development of aircraft, and would almost certainly have created a powered one if engine technology had been sufficiently advanced at the time.

However, the whirling arm does not produce a reliable flow of air impacting the test shape at a normal incidence. Centrifugal forces and the fact that the object is moving in its own wake mean that detailed examination of the airflow is difficult. Francis Herbert Wenham (1824-1908), a Council Member of the Aeronautical Society of Great Britain, addressed these issues by inventing, designing and operating the first enclosed wind tunnel in 1871.

Once this breakthrough had been achieved, detailed technical data was rapidly extracted by the use of this tool. Wenham and his colleague Browning are credited with many fundamental discoveries, including the measurement of l/d ratios, and the revelation of the beneficial effects of a high aspect ratio.

Carl Rickard Nyberg used a wind tunnel when designing his Flugan from 1897 and onwards.

In a classic set of experiments, the Englishman Osborne Reynolds (1842-1912) of the University of Manchester demonstrated that the airflow pattern over a scale model would be the same for the full-scale vehicle if a certain flow parameter were the same in both cases. This factor, now known as the Reynolds Number, is a basic parameter in the description of all fluid-flow situations, including the shapes of flow patterns, the ease of heat transfer, and the onset of turbulence. This comprises the central scientific justification for the use of models in wind-tunnels to simulate real-life phenomena.

It is often claimed that the Wright brothers use of a simple wind tunnel in 1901 to study the effects of airflow over various shapes while developing their Wright Flyer was in some way revolutionary, it can be seen from the above that they were simply using the accepted technology of the day, though this was not yet a common technology in America.

Subsequent use of wind tunnels proliferated as the science of aerodynamics and discipline of aeronautical engineering were established and air travel and power were developed.

Wind tunnels were often limited in the volume and speed of airflow which could be delivered.

The wind tunnel used by German scientists at Peenemünde prior and during WWII is an interesting example of the difficulties associated with extending the useful range of large wind tunnels. It used some large natural caves which were increased in size by excavation and then sealed to store large volumes of air which could then be routed through the wind tunnels. This innovative approach allowed lab research in high speed regimes and greatly accelerated the rate of advance of Germany's aeronautical engineering efforts.

Later research into airflows near or above the speed of sound used a related approach. Metal pressure chambers were used to store high pressure air which was then accelerated through a nozzle designed to provide supersonic flow. The observation or instrumentation chamber was then placed at the proper location in the throat or nozzle for the desired airspeed.

Computational fluid dynamics has augmented, and is starting to replace, the use of wind tunnels. For example, the experimental rocket plane SpaceShipOne was designed without any use of wind tunnels. (However, on one test flight threads were attached to the surface of the wings, performing a wind tunnel type of test during an actual flight in order to refine the computational model.)

An area that is still much too complex for the use of Computational fluid dynamics is determining the effects of flow around buildings and bridges. Boundary layer wind tunnels are the state of the art method to test such structures. These wind tunnels are also used to simulate and measure wind characteristics at the pedestrian level and exhaust gas dispersion patterns for laboratories and other emitting sources. Wind tunnel tests in a boundary layer wind tunnel allow for the natural drag of the earth's surface to be simulated. For accuracy, it is important to simulate the mean wind speed profile and turbulence effects within the atmospheric boundary layer. Most codes and standards recognize that wind tunnel testing can produce reliable information for designers, especially when their projects are in complex terrain or on exposed sites.

[edit]