wind tunnel, apparatus for studying the interaction between a solid body and an airstream. A wind tunnel simulates the conditions of an aircraft in flight by causing a high-speed stream of air to flow past a model of the aircraft (or part of an aircraft) being tested. The model is mounted on wires so that lift and drag forces on it can be measured by measuring the tensions in the wire. The paths of the airstream around the model can also be studied by attaching tufts of wool (which align themselves with the wind direction) to various parts of the model, by injecting thin streams of smoke into the tunnel to render the airflow visible, or by using certain optical devices. Pressures on the model surface are measured through small flush openings in its surface. Forces exerted on the model may be determined from measurement of the airflow upstream and downstream of the model. In wind tunnels operating well below the speed of sound, the airstream is created by large motor-driven vanes. At velocities near or above the speed of sound, the airstream is created either by releasing highly compressed air from a tank at the upwind end of the tunnel or by allowing air to rush through the tunnel into a previously evacuated vacuum tank at its downwind end. Sometimes these methods are combined, especially for the production of hypersonic velocities, i.e., velocities at least five times as great as the speed of sound. The effect of wind on other vehicles, e.g., automobiles, and on stationary objects such as buildings and bridges may also be studied in wind tunnels. In many instances, wind tunnels have been rendered obsolete by computer modeling.

tunnel diode: see diode.

tunnel, underground passage usually made without removing the overlying rock or soil. Although tunnels are approximately horizontal, they must be built with sufficient gradient for proper drainage. Tunnels may be ventilated by shafts leading to the surface or by exhaust fans at the ends.

Design and Construction Techniques

Methods of tunneling vary with the nature of the material to be cut through. When soft earth is encountered, the excavation is timbered for support as the work advances; the timbers are sometimes left as a permanent lining for the tunnel. Another method is to cut two parallel excavations in which the side walls are constructed first. Arches connecting them are then built as the material between them is extracted. Portions of the unexcavated center, left temporarily for support, may be removed later. A tunnel cut through rock frequently requires no lining. Hard rock is removed by blasting.

In constructing tunnels under rivers, the ordinary methods can be used as long as a stratum of impermeable material lies between the tunnel and the riverbed. In all cases, however, pumping equipment must be installed. Where mud, quicksand, or permeable earth is present in underwater tunneling, it becomes necessary to provide some means of holding back the water while the enclosing sections of the tunnel are placed in position. For this purpose the shield was devised and first used in 1825 by the French-born engineer Sir Marc I. Brunel when boring between Wapping and Rotherhithe, in England. Considered unsuccessful, the device was not employed again until 1869, when the British engineer James H. Greathead and the American inventor Alfred E. Beach developed improvements at about the same time. Their shields were metal cylinders fitting around the outside of the tunnel, the forward end closed by a diaphragm plate. As the rock or earth was cut away, the shield was shoved forward into the earth by hydraulic rams, compressed air being used to keep seepage to a minimum. The use of the pneumatic shield is now universal in tunneling under rivers. The actual cutting is performed by huge rotating cutter heads, each with up to fifty separate cutters, capable of penetrating 10 mm (1/2 in.) per revolution.

River-crossing tunnels are also constructed by dredging a trench in the riverbed and then lowering prefabricated tunnel sections through the water into the trench, where they are connected to each other. The trench and tunnel are then covered over. In 1969, a tunnel was constructed across the Schelde River in Belgium, using sections 330 ft (100 m) long. Often, to speed construction, work is started at both ends. This poses no problem with the cut-and-cover method, but when the tunnel is bored from within, it must be assured that the tubes will actually meet in the center. Modern methods accomplish this with high precision.

Significant Historic and Modern Tunnels

The origin of tunnel building is disputed. The Egyptians built tunnels as entrances to tombs. The Babylonians built (c.2180 B.C.) a tunnel under the Euphrates using what is now called the "cut-and-cover" method; the river was diverted, a wide trench was dug across its bed, and a brick tube was constructed in it and covered up. The ancient Greeks and Romans built tunnels for carrying water and for mining purposes; some of the Roman tunnels are still in use. One of the first notable tunnels in Great Britain was part of the Grand Trunk Canal. It was nearly 2 mi (3.2 km) long and was completed in 1777. The Mont Cénis Tunnel, a railroad tunnel in the French Alps that opened in 1871 and is now 8.5 mi (13.7 km) long, was probably the first tunnel built using compressed-air drills.

The first tunnel of importance in the United States was the tunnel through the Hoosac Range in Massachusetts. There are hundreds of miles of tunnels in New York City and its vicinity, e.g., for subways, roads, water systems, and railroads. The Delaware Aqueduct, which provides part of New York City's water supply, is at 105 mi (168 km) the longest continuous tunnel in the world. Road tunnels include the Holland Tunnel and the Lincoln Tunnel, which connect New York City's Manhattan Island with New Jersey, and the Brooklyn-Battery Tunnel, which connects Manhattan Island with Brooklyn and is longest vehicular tunnel (1.7 mi/2.7 km) in the United States. The Chesapeake Bay Bridge-Tunnel in Virginia, opened in 1964, has a length of 17.6 mi (28.2 km) and includes two tunnel segments over a mile long.

The Simplon Tunnel (see under Simplon) through the Alps, for many years the longest railway tunnel in the world, consists of two parallel single-line tunnels (both 12.3 mi/19.8 km) with connecting tunnels at short intervals. The Channel Tunnel (Chunnel) under the English Channel is much longer, at 31 mi (50 km), and the Seikan Tunnel in Japan, the world's deepest underwater tunnel, is also the longest railroad tunnel at 33.5 mi (53.6 km). The world's longest vehicular tunnel, the Lærdal Tunnel (15.2 mi/24.5 km long), connects Lærdal and Aurland, Norway, and is an important overland link between Oslo and Bergen. The St. Gotthard Tunnel (10.2 mi/16.4 km long), in the Swiss Alps, was formerly the longest vehicular tunnel.

carpal tunnel syndrome: see repetitive stress injury.

Rove Tunnel, southern section of the Marseilles-Rhône Canal, 4.5 mi (7.2 km) long and 72 ft (22 m) wide, Bouches-du-Rhône dept., SE France; opened 1927. Starting near the village of Le Rove, it cuts through the Chaǐne de l'Estaque at sea level. It is considered one of the greatest pieces of engineering since the Panama Canal.

Moffat Tunnel, railroad tube, 24 ft (7.3 m) high, 18 ft (5.5 m) wide, and 6.4 mi (10.3 km) long, N central Colo., in the Continental Divide, NW of Denver. One of the country's longest railroad tunnels, it was built between 1922 and 1928. At an elevation of 9,094 ft (2,772 m), it pierces James Peak. An adjacent bore carries water to Denver.

Chesapeake Bay Bridge-Tunnel, officially the Lucius J. Kellam, Jr. Bridge-Tunnel, 17.6 mi (28.2 km) long, across the mouth of Chesapeake Bay, E Va., connecting Cape Charles with Norfolk, Va. Opened in 1964, the original roadway consisted of a chain of low trestle bridges, two high bridges, and two tunnels (each 1 mi/1.6 km long) under the shipping channels. The tunnels are anchored on four artificial islands. A set of parallel bridges, but not tunnels, opened in 1999.

Channel Tunnel, popularly called the "Chunnel," a three-tunnel railroad connection running under the English Channel, connecting Folkestone, England, and Calais, France. The tunnels are 31 mi (50 km) long. There are two rail tunnels, each 25 ft (7.6 m) in diameter, and a central tunnel, 16 ft (4.8 m) in diameter, that is used for maintenance and ventilation. The depth of the tunnels below the seabed averages about 150 ft (45 m). The project is a joint English and French venture, with a 55-year concession granted to Eurotunnel, a private company, and is the centerpiece of a high-speed rail link between London and Paris. The project began with the signing of the Channel Tunnel Treaty between France and Britain in 1986; passenger service began in 1994. A higher than expected cost of digging the tunnels and lower than predicted tunnel traffic levels left Eurotunnel with huge debts, the repayment of which strained the finances of the company, threatened the company with insolvency, and led a French court to grant (2006) Eurotunnel creditor protection while the company reorganized (as Groupe Eurotunnel in 2007) and restructured its debt.

Device for producing a controlled stream of air to study the effects on objects such as aircraft moving through air or the effects of moving air on models of stationary objects such as buildings. Applications of wind-tunnel research range from testing of airframes (the structures of aircraft and spacecraft) to research on the boundary layer, turbulence, drag, and lift. Measurements of air pressure and other characteristics at many points on the model yield information about how the total wind load is distributed. In addition to testing the effects of wind on aircraft and spacecraft, studies in wind tunnels have been used to solve design problems in automobiles, boats, trains, bridges, and buildings. Seealso aerodynamics.

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or barrier penetration

In physics, the passage of a particle through a seemingly impassable energy barrier. Though a particle's energy may be too low to surmount a barrier in classical physics, the particle may still cross the barrier as a consequence of its quantum-mechanical wave properties. An important application of this phenomenon is in the operation of the scanning tunneling microscope.

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Horizontal or nearly horizontal underground or underwater passageway. Tunnels are used for mining, as passageways for trains and motor vehicles, for diverting rivers around damsites, for housing underground installations such as power plants, and for conducting water. Ancient civilizations used tunnels to carry water for irrigation and drinking, and in the 22nd century BC the Babylonians built a tunnel for pedestrian traffic under the Euphrates River. The Romans built aqueduct tunnels through mountains by heating the rock face with fire and rapidly cooling it with water, causing the rock to crack. The introduction of gunpowder blasting in the 17th century marked a great advance in solid-rock excavation. For softer soils, excavation is accomplished using devices such as the tunneling mole, with its rotating wheel that continuously excavates material and loads it onto a conveyor belt. Railroad transportation in the 19th–20th century led to a tremendous expansion in the number and length of tunnels. Brick and stone were used for support in early tunnels, but in modern tunneling steel is generally used until a concrete lining can be installed. A common method of lining involves spraying shotcrete onto the tunnel crown immediately after excavation.

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Painful condition caused by repetitive stress to the wrist over time. The median nerve and the tendons that bend the fingers pass through the carpal tunnel on the inner side of the wrist, between the wrist (carpal) bones on three sides and a ligament on the fourth. Repetitive finger and wrist movements rub the tendons against the walls of the carpal tunnel and may make the tendons swell, squeezing the nerve. Numbness, tingling, and pain in the wrist and hand may progress to loss of muscle control. CTS is most common in assembly-line workers and computer keyboard users. Treatment may include avoidance of the causative activity, ergonomic workplace design, anti-inflammatory drugs, brace or splint use, and surgery.

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or Eurotunnel

Rail tunnel that runs beneath the English Channel between Folkestone, England, and Sangatte (near Calais), France. A rail tunnel was chosen over proposals for a very long suspension bridge, a bridge-and-tunnel link, and a combined rail-and-road link. The 31-mi (50-km) tunnel, which opened in 1994, consists of three separate tunnels, two for rail traffic and a central tunnel for services and security. Trains, which carry motor vehicles as well as passengers, can travel through the tunnel at speeds as high as 100 mph (160 kph).

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