Wind tunnel

Wind tunnel test sections range in size from less than a foot across, to over 100 feet (30 m), and with air speeds from a light breeze to hypersonic.

The earliest wind tunnels were invented towards the end of the 19th century, in the early days of aeronautical research, as part of the effort to develop heavier-than-air flying machines.

Advances in computational fluid dynamics (CFD) have reduced the demand for wind tunnel testing, but have not completely eliminated it.

Moreover, confidence in a numerical simulation tool depends on comparing its results with experimental data, and these can be obtained, for example, from wind tunnel tests.

Research in wind tunnels produces accurate results and is done rapidly and economically compared to flight testing of full-scale aircraft.

Car fuel consumption is of secondary importance to drivers when starting and driving in extreme cold and wind-driven snow.

English military engineer and mathematician Benjamin Robins (1707–1751) invented a whirling arm apparatus to determine drag[3] and did some of the first experiments in aerodynamics.

Otto Lilienthal used a rotating arm to make measurements on wing airfoils with varying angles of attack, establishing their lift-to-drag ratio polar diagrams, but was lacking the notions of induced drag and Reynolds numbers.

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.

Konstantin Tsiolkovsky built an open-section wind tunnel with a centrifugal blower in 1897, and determined the drag coefficients of flat plates, cylinders, and spheres.

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 parameter, now known as the Reynolds number, is used in the description of all fluid-flow situations, including the shape of flow patterns, the effectiveness of heat transfers, and the onset of turbulence.

In France, Gustave Eiffel (1832–1923) built his first open-return wind tunnel in 1909, powered by a 67 hp (50 kW) electric motor, at Champs-de-Mars, near the foot of the tower that bears his name.

In 1912 Eiffel's laboratory was moved to Auteuil, a suburb of Paris, where his wind tunnel with a 7-foot (2 m) test section is still operational today.

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.

[citation needed] It was designed to test full-size aircraft and had six large fans driven by high powered electric motors.

Ludwig Prandtl was Theodore von Kármán's teacher at Göttingen University and suggested the construction of a wind tunnel for tests of airships they were designing.

[13]: 63  When he later moved to Aachen University he recalled use of this facility: I remembered the wind tunnel in Göttingen was started as a tool for studies of Zeppelin behavior, but that it had proven to be valuable for everything else from determining the direction of smoke from a ship's stack, to whether a given airplane would fly.

[13]: 169 In 1939 General Arnold asked what was required to advance the USAF, and von Kármán answered, "The first step is to build the right wind tunnel.

"[13]: 226  On the other hand, after the successes of the Bell X-2 and prospect of more advanced research, he wrote, "I was in favor of constructing such a plane because I have never believed that you can get all the answers out of a wind tunnel.

A large wind tunnel under construction near Oetztal, Austria would have had two fans directly driven by two 50,000 hp (37,000 kW) hydraulic turbines.

The installation was not completed by the end of the war and the dismantled equipment was shipped to Modane, France in 1946 where it was re-erected and is still operated there by the ONERA.

Metal pressure chambers were used to store high-pressure air which was then accelerated through a nozzle designed to provide supersonic flow.

Studies have been done and others are underway to assess future military and commercial wind tunnel needs, but the outcome remains uncertain.

[21] More recently an increasing use of jet-powered, instrumented unmanned vehicles, or research drones, have replaced some of the traditional uses of wind tunnels.

Lift, drag, and lateral forces, as well as yaw, roll, and pitching moments are measured over a range of angle of attack.

Nets are installed above and below the test section to prevent the model from moving too high and to catch it when the air stops flowing.

[35] The aerodynamic principles of the wind tunnel work equally on watercraft, except the water is more viscous and so sets greater forces on the object being tested.

Another significant application for boundary layer wind tunnel modeling is for understanding exhaust gas dispersion patterns for hospitals, laboratories, and other emitting sources.

For instance, the use of boundary layer wind tunnel modeling can be used as a credit for Leadership in Energy and Environmental Design (LEED) certification through the US Green Building Council.