Wind tunnel

Wind tunnels are machines in which objects are held stationary inside a tube, and air is blown around it to study the interaction between the object and the moving air. They are used to test the aerodynamic effects of aircraft, rockets, cars, and buildings. Different wind tunnels range in size from less than a foot across, to over 100 feet (30 m), and can have air that moves at speeds from a light breeze to hypersonic velocities.

Usually, large fans move air through the wind tunnel, while the object being tested is held stationary. The object can be an aerodynamic test object such as a cylinder or an airfoil, an individual component of an aircraft, a small model of the vehicle, or, in the largest tunnels, even a full-sized vehicle. Different measurements can be taken from these tests. The aerodynamic forces on the entire object can be measured, or on individual components of it. The air pressure at different points can be measured with sensors. Smoke can be introduced into the airstream to show the path that air takes around the object. Or, small threads can be attached to specific parts to show the airflow at those points.

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. The wind tunnel reversed the usual situation. Instead of the air standing still and an aircraft moving, an object would be held still and the air moved around it. In this way, a stationary observer could study the flying object in action, and could measure the aerodynamic forces acting on it.

The development of wind tunnels accompanied the development of the airplane. Large wind tunnels were built during World War II, and as supersonic aircraft were developed, supersonic wind tunnels were constructed to test them. Wind tunnel testing was considered of strategic importance during the Cold War for development of aircraft and missiles.

Other problems are also studied with wind tunnels. The effects of wind on man-made structures need to be studied when buildings became tall enough to be significantly affected by the wind. Very tall buildings present large surfaces to the wind, and the resulting forces have to be resisted by the building's internal structure or else the building will collapse. Determining such forces was required before building codes could specify the required strength of such buildings and these tests continue to be used for large or unusual buildings.

Wind tunnel testing was first applied to automobiles as early as the 1920s, on cars such as the Rumpler Tropfenwagen, and later the Chrysler Airflow. Initially, automakers would test out scale models of their cars, but later, full scale automotive wind tunnels were built. Starting in the 1960s, wind tunnel testing began to receive widespread adoption for automobiles,^{[additional citation(s) needed]} not so much to determine aerodynamic forces in the same way as an airplane, but to increase the fuel efficiency of vehicles by reducing the aerodynamic drag. In these studies, the interaction between the road and the vehicle plays a significant role, and this interaction must be taken into consideration when interpreting the test results. In the real world, the vehicle is moving while the road and air are stationary. In a wind tunnel test, the road must also be moved past a vehicle along with air being blown around it. This has been accomplished with moving belts under the test vehicle to simulate the moving road, and very similar devices are used in wind tunnel testing of aircraft take-off and landing configurations.

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Sporting equipment has also studied in wind tunnels, including golf clubs, golf balls, bobsleds, cyclists, and race car helmets. Helmet aerodynamics is particularly important in open cockpit race cars such as Indycar and Formula One. Excessive lift forces on the helmet can cause considerable neck strain on the driver, and flow separation on the back side of the helmet can cause turbulent buffeting and thus blurred vision for the driver at high speeds.

The advances in computational fluid dynamics (CFD) modelling on high-speed digital computers has reduced the demand for wind tunnel testing, but has not completely eliminated it. Many real-world problems can still not be modeled accurately enough by CFD to eliminate the need for physical tests in wind tunnels.