Wind tunnel

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NASA wind tunnel with the scale model of an airplane Windkanal.jpg
NASA wind tunnel with the scale model of an airplane
A model Cessna with helium-filled bubbles showing pathlines of the wingtip vortices Cessna 182 model-wingtip-vortex.jpg
A model Cessna with helium-filled bubbles showing pathlines of the wingtip vortices

Wind tunnels are large tubes with air moving inside. The tunnels are used to copy the actions of an object in flight. Researchers use wind tunnels to learn more about how an aircraft will fly. NASA uses wind tunnels to test scale models of aircraft and spacecraft. Some wind tunnels are big enough to hold full-size versions of vehicles. The wind tunnel moves air around an object, making it seem like the object is really flying.

Contents

Most of the time, powerful fans move air through the tube. The object to be tested is fastened in the tunnel so that it will not move. The object can be a small model of a vehicle. It can be just a piece of a vehicle. It can be a full-size aircraft or spacecraft. It can even be a common object like a tennis ball. The air moving around the still object shows what would happen if the object were moving through the air. How the air moves can be studied in different ways. Smoke or dye can be placed in the air and can be seen as it moves. Threads can be attached to the object to show how the air is moving. Special instruments are often used to measure the force of the air on the object.

The earliest wind tunnels were invented towards the end of the 19th century, in the early days of aeronautic research, when many attempted to develop successful heavier-than-air flying machines. The wind tunnel was envisioned as a means of reversing the usual paradigm: instead of the air standing still and an object moving at speed through it, the same effect would be obtained if the object stood still and the air moved at speed past it. In that way a stationary observer could study the flying object in action, and could measure the aerodynamic forces being imposed on it.

The development of wind tunnels accompanied the development of the airplane. Large wind tunnels were built during World War II. Wind tunnel testing was considered of strategic importance during the Cold War development of supersonic aircraft and missiles.

Later, wind tunnel study came into its own: the effects of wind on man-made structures or objects needed to be studied when buildings became tall enough to present large surfaces to the wind, and the resulting forces had to be resisted by the building's internal structure. Determining such forces was required before building codes could specify the required strength of such buildings and such tests continue to be used for large or unusual buildings.

Building code set of rules that specify the standards for constructed objects such as buildings and nonbuilding structures

A building code is a set of rules that specify the standards for constructed objects such as buildings and nonbuilding structures. Buildings must conform to the code to obtain planning permission, usually from a local council. The main purpose of building codes is to protect public health, safety and general welfare as they relate to the construction and occupancy of buildings and structures. The building code becomes law of a particular jurisdiction when formally enacted by the appropriate governmental or private authority.

Wind engineering is a subsets of mechanical engineering, structural engineering, meteorology, and applied physics to analyze the effects of wind in the natural and the built environment and studies the possible damage, inconvenience or benefits which may result from wind. In the field of engineering it includes strong winds, which may cause discomfort, as well as extreme winds, such as in a tornado, hurricane or heavy storm, which may cause widespread destruction. In the fields of wind energy and air pollution it also includes low and moderate winds as these are relevant to electricity production resp. dispersion of contaminants.

Still later, wind tunnel testing was applied to automobiles, not so much to determine aerodynamic forces per se but more to determine ways to reduce the power required to move the vehicle on roadways at a given speed. 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 an actual situation the roadway is moving relative to the vehicle but the air is stationary relative to the roadway, but in the wind tunnel the air is moving relative to the roadway, while the roadway is stationary relative to the test vehicle. Some automotive-test wind tunnels have incorporated moving belts under the test vehicle in an effort to approximate the actual condition, and very similar devices are used in wind tunnel testing of aircraft take-off and landing configurations.

Wind tunnel testing of sporting equipment has also been prevalent over the years, including golf clubs, golf balls, Olympic bobsleds, Olympic cyclists, and race car helmets. Helmet aerodynamics is particularly important in open cockpit race cars (Indycar, 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. [1]

The advances in computational fluid dynamics (CFD) modelling on high-speed digital computers has reduced the demand for wind tunnel testing. However, CFD results are still not completely reliable and wind tunnels are used to verify CFD predictions.[ citation needed ]

Computational fluid dynamics branch of fluid mechanics that uses numerical analysis and data structures to solve and analyze problems that involve fluid flows

Computational fluid dynamics (CFD) is a branch of fluid mechanics that uses numerical analysis and data structures to analyze and solve problems that involve fluid flows. Computers are used to perform the calculations required to simulate the free-stream flow of the fluid, and the interaction of the fluid with surfaces defined by boundary conditions. With high-speed supercomputers, better solutions can be achieved, and are often required to solve the largest and most complex problems. Ongoing research yields software that improves the accuracy and speed of complex simulation scenarios such as transonic or turbulent flows. Initial validation of such software is typically performed using experimental apparatus such as wind tunnels. In addition, previously performed analytical or empirical analysis of a particular problem can be used for comparison. A final validation is often performed using full-scale testing, such as flight tests.

Measurement of aerodynamic forces

Air velocity and pressures are measured in several ways in wind tunnels.

Air velocity through the test section is determined by Bernoulli's principle. Measurement of the dynamic pressure, the static pressure, and (for compressible flow only) the temperature rise in the airflow. The direction of airflow around a model can be determined by tufts of yarn attached to the aerodynamic surfaces. The direction of airflow approaching a surface can be visualized by mounting threads in the airflow ahead of and aft of the test model. Smoke or bubbles of liquid can be introduced into the airflow upstream of the test model, and their path around the model can be photographed (see particle image velocimetry).

Bernoullis principle Relates pressure and flow velocity in fluid dynamics

In fluid dynamics, Bernoulli's principle states that an increase in the speed of a fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid's potential energy. The principle is named after Daniel Bernoulli who published it in his book Hydrodynamica in 1738. Although Bernoulli deduced that pressure decreases when the flow speed increases, it was Leonhard Euler who derived Bernoulli's equation in its usual form in 1752. The principle is only applicable for isentropic flows: when the effects of irreversible processes and non-adiabatic processes are small and can be neglected.

Dynamic pressure is the increase in a moving fluid's pressure over its static value due to motion. In incompressible fluid dynamics, it is indicated as , or Q, defined by:

In fluid mechanics the term static pressure has several uses:

Aerodynamic forces on the test model are usually measured with beam balances, connected to the test model with beams, strings, or cables.

The pressure distributions across the test model have historically been measured by drilling many small holes along the airflow path, and using multi-tube manometers to measure the pressure at each hole. Pressure distributions can more conveniently be measured by the use of pressure-sensitive paint, in which higher local pressure is indicated by lowered fluorescence of the paint at that point. Pressure distributions can also be conveniently measured by the use of pressure-sensitive pressure belts, a recent development in which multiple ultra-miniaturized pressure sensor modules are integrated into a flexible strip. The strip is attached to the aerodynamic surface with tape, and it sends signals depicting the pressure distribution along its surface. [2]

Pressure distributions on a test model can also be determined by performing a wake survey, in which either a single pitot tube is used to obtain multiple readings downstream of the test model, or a multiple-tube manometer is mounted downstream and all its readings are taken.

The aerodynamic properties of an object can not all remain the same for a scaled model. [3] However, by observing certain similarity rules, a very satisfactory correspondence between the aerodynamic properties of a scaled model and a full-size object can be achieved. The choice of similarity parameters depends on the purpose of the test, but the most important conditions to satisfy are usually:

In certain particular test cases, other similarity parameters must be satisfied, such as e.g. Froude number.

History

Origins

English military engineer and mathematician Benjamin Robins (17071751) invented a whirling arm apparatus to determine drag [4] and did some of the first experiments in aviation theory.

Sir George Cayley (1773–1857) also used a whirling arm to measure the drag and lift of various airfoils. [5] His whirling arm was 5 feet (1.5 m) long and attained top speeds between 10 and 20 feet per second (3 to 6 m/s).

Otto Lilienthal used a rotating arm to accurately measure 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. [6]

Replica of the Wright brothers' wind tunnel WB Wind Tunnel.jpg
Replica of the Wright brothers' wind tunnel
Eiffel's wind tunnels in the Auteuil laboratory Windtunnel2.JPG
Eiffel's wind tunnels in the Auteuil laboratory

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. [7] Once this breakthrough had been achieved, detailed technical data was rapidly extracted by the use of this tool. Wenham and his colleague John 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.

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.

Danish inventor Poul la Cour applied wind tunnels in his process of developing and refining the technology of wind turbines in the early 1890s. 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. However, there are limitations on conditions in which dynamic similarity is based upon the Reynolds number alone.

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 ways revolutionary. [8] It can be seen from the above, however, that they were simply using the accepted technology of the day, though this was not yet a common technology in America.

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

Between 1909 and 1912 Eiffel ran about 4,000 tests in his wind tunnel, and his systematic experimentation set new standards for aeronautical research. In 1912 Eiffel's laboratory was moved to Auteuil, a suburb of Paris, where his wind tunnel with a two-metre test section is still operational today. [9] Eiffel significantly improved the efficiency of the open-return wind tunnel by enclosing the test section in a chamber, designing a flared inlet with a honeycomb flow straightener and adding a diffuser between the test section and the fan located at the downstream end of the diffuser; this was an arrangement followed by a number of wind tunnels later built; in fact the open-return low-speed wind tunnel is often called the Eiffel-type wind tunnel.

Widespread usage

German aviation laboratory, 1935 Bundesarchiv Bild 102-17158, Deutsche Versuchsanstalt fur Luftfahrt.jpg
German aviation laboratory, 1935

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.

The US Navy in 1916 built one of the largest wind tunnels in the world at that time at the Washington Navy Yard. The inlet was almost 11 feet (3.4 m) in diameter and the discharge part was 7 feet (2.1 m) in diameter. A 500 hp electric motor drove the paddle type fan blades. [10]

In 1931 the NACA built a 30-foot by 60-foot full-scale wind tunnel at Langley Research Center in Langley, Virginia. The tunnel was powered by a pair of fans driven by 4,000 hp electric motors. The layout was a double-return, closed-loop format and could accommodate many full-size real aircraft as well as scale models. The tunnel was eventually closed and, even though it was declared a National Historic Landmark in 1995, demolition began in 2010.

Until World War II, the world's largest wind tunnel, built in 1932-1934, was located in a suburb of Paris, Chalais-Meudon, France. It was designed to test full-size aircraft and had six large fans driven by high powered electric motors. [11] The Chalais Meudon wind tunnel was used by ONERA under the name S1Ch until 1976 in the development of, e.g., the Caravelle and Concorde airplanes. Today, this wind tunnel is preserved as a national monument.

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. [12] :44 The vortex street of turbulence downstream of a cylinder was tested in the tunnel. [12] :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. Progress at Aachen, I felt, would be virtually impossible without a good wind tunnel. [12] :76

When von Kármán began to consult with Caltech he worked with Clark Millikan and Arthur L. Klein. [12] :124 He objected to their design and insisted on a return flow making the device "independent of the fluctuations of the outside atmosphere". It was completed in 1930 and used for Northrop Alpha testing. [12] :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." [12] :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." [12] :302,3

World War II

In 1941 the US constructed one of the largest wind tunnels at that time at Wright Field in Dayton, Ohio. This wind tunnel starts at 45 feet (14 m) and narrows to 20 feet (6.1 m) in diameter. Two 40-foot (12 m) fans were driven by a 40,000 hp electric motor. Large scale aircraft models could be tested at air speeds of 400 mph (640 km/h). [13]

The wind tunnel used by German scientists at Peenemünde prior to 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. By the end of the war, Germany had at least three different supersonic wind tunnels, with one capable of Mach 4.4 (heated) airflows. [14]

A large wind tunnel under construction near Oetztal, Austria would have had two fans directly driven by two 50,000 horsepower 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. With its 8m test section and airspeed up to Mach 1 it is the largest transonic wind tunnel facility in the world. [15]

On 22 June 1942 Curtiss-Wright financed construction of one of the nation's largest subsonic wind tunnels in Buffalo, N.Y. The first concrete for building was poured on 22 June 1942 on a site that eventually would become Calspan, where the largest independently-owned wind tunnel in the United States still operates. [16]

By the end of World War II, the US had built eight new wind tunnels, including the largest one in the world at Moffett Field near Sunnyvale, California, which was designed to test full size aircraft at speeds of less than 250 mph [17] and a vertical wind tunnel at Wright Field, Ohio, where the wind stream is upwards for the testing of models in spin situations and the concepts and engineering designs for the first primitive helicopters flown in the US. [18]

After World War II

NACA wind tunnel test on a human subject, showing the effects of high wind speeds on the human face

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 ("test section") was then placed at the proper location in the throat or nozzle for the desired airspeed.

In the United States, concern over the lagging of American research facilities compared to those built by the Germans led to the Unitary Wind Tunnel Plan Act of 1949, which authorized expenditure to construct new wind tunnels at universities and at military sites. Some German war-time wind tunnels were dismantled for shipment to the United States as part of the plan to exploit German technology developments. [19]

For limited applications, Computational fluid dynamics (CFD) can supplement or possibly 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. Where external turbulent flow is present, CFD is not practical due to limitations in present-day computing resources. For example, an area that is still much too complex for the use of CFD is determining the effects of flow on and around structures, bridges, terrain, etc.

Preparing a model in the Kirsten Wind Tunnel, a subsonic wind tunnel at the University of Washington Kirsten wind tunnel 05.jpg
Preparing a model in the Kirsten Wind Tunnel, a subsonic wind tunnel at the University of Washington

The most effective way to simulative external turbulent flow is through the use of a boundary layer wind tunnel.

There are many applications for boundary layer wind tunnel modeling. For example, understanding the impact of wind on high-rise buildings, factories, bridges, etc. can help building designers construct a structure that stands up to wind effects in the most efficient manner possible. Another significant application for boundary layer wind tunnel modeling is for understanding exhaust gas dispersion patterns for hospitals, laboratories, and other emitting sources. Other examples of boundary layer wind tunnel applications are assessments of pedestrian comfort and snow drifting. Wind tunnel modeling is accepted as a method for aiding in Green building design. 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 U.S. Green Building Council.

Fan blades of Langley Research Center's 16 foot transonic wind tunnel in 1990, before it was retired in 2004 16 Foot Transonic Tunnel Rehabilitation - GPN-2000-001300.jpg
Fan blades of Langley Research Center's 16 foot transonic wind tunnel in 1990, before it was retired in 2004

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.

In the United States, many wind tunnels have been decommissioned in the last 20 years, including some historic facilities. Pressure is brought to bear on remaining wind tunnels due to declining or erratic usage, high electricity costs, and in some cases the high value of the real estate upon which the facility sits. On the other hand, CFD validation still requires wind-tunnel data, and this is likely to be the case for the foreseeable future. Studies have been done and others are underway to assess future military and commercial wind tunnel needs, but the outcome remains uncertain. [20] More recently an increasing use of jet-powered, instrumented unmanned vehicles ["research drones"] have replaced some of the traditional uses of wind tunnels. [21]

How it works

Six-element external balance below the Kirsten Wind Tunnel Kirsten wind tunnel 08A.jpg
Six-element external balance below the Kirsten Wind Tunnel

Air is blown or sucked through a duct equipped with a viewing port and instrumentation where models or geometrical shapes are mounted for study. Typically the air is moved through the tunnel using a series of fans. For very large wind tunnels several meters in diameter, a single large fan is not practical, and so instead an array of multiple fans are used in parallel to provide sufficient airflow. Due to the sheer volume and speed of air movement required, the fans may be powered by stationary turbofan engines rather than electric motors.

The airflow created by the fans that is entering the tunnel is itself highly turbulent due to the fan blade motion (when the fan is blowing air into the test section – when it is sucking air out of the test section downstream, the fan-blade turbulence is not a factor), and so is not directly useful for accurate measurements. The air moving through the tunnel needs to be relatively turbulence-free and laminar. To correct this problem, closely spaced vertical and horizontal air vanes are used to smooth out the turbulent airflow before reaching the subject of the testing.

Due to the effects of viscosity, the cross-section of a wind tunnel is typically circular rather than square, because there will be greater flow constriction in the corners of a square tunnel that can make the flow turbulent. A circular tunnel provides a smoother flow.

The inside facing of the tunnel is typically as smooth as possible, to reduce surface drag and turbulence that could impact the accuracy of the testing. Even smooth walls induce some drag into the airflow, and so the object being tested is usually kept near the center of the tunnel, with an empty buffer zone between the object and the tunnel walls. There are correction factors to relate wind tunnel test results to open-air results.

The lighting is usually embedded into the circular walls of the tunnel and shines in through windows. If the light were mounted on the inside surface of the tunnel in a conventional manner, the light bulb would generate turbulence as the air blows around it. Similarly, observation is usually done through transparent portholes into the tunnel. Rather than simply being flat discs, these lighting and observation windows may be curved to match the cross-section of the tunnel and further reduce turbulence around the window.

Various techniques are 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.

Pressure measurements

Pressure across the surfaces of the model can be measured if the model includes pressure taps. This can be useful for pressure-dominated phenomena, but this only accounts for normal forces on the body.

Force and moment measurements

A typical lift coefficient versus angle of attack curve Lift curve.svg
A typical lift coefficient versus angle of attack curve

With the model mounted on a force balance, one can measure lift, drag, lateral forces, yaw, roll, and pitching moments over a range of angle of attack. This allows one to produce common curves such as lift coefficient versus angle of attack (shown).

Note that the force balance itself creates drag and potential turbulence that will affect the model and introduce errors into the measurements. The supporting structures are therefore typically smoothly shaped to minimize turbulence.

Flow visualization

Because air is transparent it is difficult to directly observe the air movement itself. Instead, multiple methods of both quantitative and qualitative flow visualization methods have been developed for testing in a wind tunnel.

Qualitative methods

Compilation of images taken during an alpha run starting at 0 degrees alpha ranging to 26 degrees alpha. Images taken at the Kirsten Wind Tunnel using fluorescent mini-tufts. Notice how separation starts at the outboard wing and progresses inward. Notice also how there is delayed separation aft of the nacelle. GIF Flow visualization.gif
Compilation of images taken during an alpha run starting at 0 degrees alpha ranging to 26 degrees alpha. Images taken at the Kirsten Wind Tunnel using fluorescent mini-tufts. Notice how separation starts at the outboard wing and progresses inward. Notice also how there is delayed separation aft of the nacelle.
Fluorescent mini-tufts attached to a wing in the Kirsten Wind Tunnel showing air flow direction and separation. Angle of attack ~ 12 degrees, speed ~120 Mph. Wing with minitufts.jpg
Fluorescent mini-tufts attached to a wing in the Kirsten Wind Tunnel showing air flow direction and separation. Angle of attack ~ 12 degrees, speed ~120 Mph.
China clay on a wing in the Kirsten Wind Tunnel showing reverse and span-wise flow Wing air flow pattern.jpg
China clay on a wing in the Kirsten Wind Tunnel showing reverse and span-wise flow
Oil flow visible on a straight wing in the Kirsten Wind Tunnel. Trip dots can be seen near the leading edge. Oil flow vis on straight wing.jpg
Oil flow visible on a straight wing in the Kirsten Wind Tunnel. Trip dots can be seen near the leading edge.
Fog (water particle) wind tunnel visualization of a NACA 4412 airfoil at a low-speed flow (Re=20.000) Fog visualization.jpg
Fog (water particle) wind tunnel visualization of a NACA 4412 airfoil at a low-speed flow (Re=20.000)

High-speed turbulence and vortices can be difficult to see directly, but strobe lights and film cameras or high-speed digital cameras can help to capture events that are a blur to the naked eye.

High-speed cameras are also required when the subject of the test is itself moving at high speed, such as an airplane propeller. The camera can capture stop-motion images of how the blade cuts through the particulate streams and how vortices are generated along the trailing edges of the moving blade.

Quantitative methods

Classification

There are many different kinds of wind tunnels. They are typically classified by the range of speeds that are achieved in the test section, as follows:

Wind tunnels are also classified by the orientation of air flow in the test section with respect to gravity. Typically they are oriented horizontally, as happens during level flight. A different class of wind tunnels are oriented vertically so that gravity can be balanced by drag instead of lift, and these have become a popular form of recreation for simulating sky-diving:

Wind tunnels are also classified based on their main use. For those used with land vehicles such as cars and trucks the type of floor aerodynamics is also important. These vary from stationary floors through to full moving floors, with smaller moving floors and some attempt at boundary level control also being important.

Aeronautical wind tunnels

The main subcategories in the aeronautical wind tunnels are:

High Reynolds number tunnels

Reynolds number is one of the governing similarity parameters for the simulation of flow in a wind tunnel. For mach number less than 0.3, it is the primary parameter that governs the flow characteristics. There are three main ways to simulate high Reynolds number, since it is not practical to obtain full scale Reynolds number by use of a full scale vehicle.

  • Pressurised tunnels: Here test gases are pressurised to increase the Reynolds number.
  • Heavy gas tunnels: Heavier gases like freon and R-134a are used as test gases. The transonic dynamics tunnel at NASA Langley is an example of such a tunnel.
  • Cryogenic tunnels: Here test gas is cooled down to increase the Reynolds number. The European transonic wind tunnel uses this technique.
  • High-altitude tunnels: These are designed to test the effects of shock waves against various aircraft shapes in near vacuum. In 1952 the University of California constructed the first two high-altitude wind tunnels: one for testing objects at 50 to 70 miles above the earth and the second for tests at 80 to 200 miles above the earth. [22]

V/STOL tunnels

V/STOL tunnels require large cross section area, but only small velocities. Since power varies with the cube of velocity, the power required for the operation is also less. An example of a V/STOL tunnel is the NASA Langley 14' x 22' tunnel. [23]

Spin tunnels

Aircraft have a tendency to go to spin when they stall. These tunnels are used to study that phenomenon.

Automotive tunnels

Automotive wind tunnels fall into two categories:

Wunibald Kamm built the first full-scale wind tunnel for motor vehicles. [24]

For external flow tunnels various systems are used to compensate for the effect of the boundary layer on the road surface, including systems of moving belts under each wheel and the body of the car (5 or 7 belt systems) or one large belt under the entire car, or other methods of boundary layer control such as scoops or perforations to suck it away. [25]

Aeroacoustic tunnels

These tunnels are used in the studies of noise generated by flow and its suppression.

Vertical wind tunnel T-105 at Central Aerohydrodynamic Institute, Moscow, built in 1941 for aircraft testing Vertical wind tunnel at TsAGI.jpg
Vertical wind tunnel T-105 at Central Aerohydrodynamic Institute, Moscow, built in 1941 for aircraft testing

High enthalpy

A high enthalpy wind tunnel is intended to study flow of air around objects moving at speeds much faster than the local speed of sound (hypersonic speeds). "Enthalpy" is the total energy of a gas stream, composed of internal energy due to temperature, the product of pressure and volume, and the velocity of flow. Duplication of the conditions of hypersonic flight requires large volumes of high-pressure, heated air; large pressurized hot reservoirs, and electric arcs, are two techniques used. [26]

Aquadynamic flume

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. A looping flume is typically used for underwater aquadynamic testing. The interaction between two different types of fluids means that pure wind tunnel testing is only partly relevant. However, a similar sort of research is done in a towing tank.

Low-speed oversize liquid testing

Air is not always the best test medium for studying small-scale aerodynamic principles, due to the speed of the air flow and airfoil movement. A study of fruit fly wings designed to understand how the wings produce lift was performed using a large tank of mineral oil and wings 100 times larger than actual size, in order to slow down the wing beats and make the vortices generated by the insect wings easier to see and understand. [27]

Fan testing

Wind tunnel tests are also performed to precisely measure the air movement of fans at a specific pressure. By determining the environmental circumstances during measurement, and by revising the air-tightness afterwards, the standardization of the data is ensured.

There are two possible ways of measurement: a complete fan, or an impeller on a hydraulic installation. Two measuring tubes enable measurements of lower air currents (< 30.000 m³/h) as well as higher air currents (< 60.000 m³/h). The determination of the Q/h curve of the fan is one of the main objectives. To determine this curve (and to define other parameters) air technical, mechanical as well as electro technical data are measured:

Air technical:

Electro technical:

The measurement can take place on the fan or in the application in which the fan is used.

Wind engineering testing

In wind engineering, wind tunnel tests are used to measure the velocity around, and forces or pressures upon structures. [28] Very tall buildings, buildings with unusual or complicated shapes (such as a tall building with a parabolic or a hyperbolic shape), cable suspension bridges or cable stayed bridges are analyzed in specialized atmospheric boundary layer wind tunnels. These feature a long upwind section to accurately represent the wind speed and turbulence profile acting on the structure. Wind tunnel tests provide the necessary design pressure measurements in use of the dynamic analysis and control of tall buildings. [29] [30]

See also

Related Research Articles

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Wing Surface used for flight, for example by insects, birds, bats and airplanes

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Parasitic drag is drag that acts on an object when the object is moving through a fluid. In the case of aerodynamic drag, the fluid is the atmosphere. Parasitic drag is a combination of form drag and skin friction drag. Parasitic drag does not result from the generation of lift on the object, and hence it is considered parasitic.

Ducted fan

A ducted fan is an air moving arrangement whereby a mechanical fan, which is a type of propeller, is mounted within a cylindrical shroud or duct. The duct reduces losses in thrust from the tips of the propeller blades, and varying the cross-section of the duct allows the designer to advantageously affect the velocity and pressure of the airflow according to Bernoulli's principle. Ducted fan propulsion is used in aircraft, airships, airboats, hovercraft and fan packs.

In fluid dynamics, drag is a force acting opposite to the relative motion of any object moving with respect to a surrounding fluid. This can exist between two fluid layers or a fluid and a solid surface. Unlike other resistive forces, such as dry friction, which are nearly independent of velocity, drag forces depend on velocity. Drag force is proportional to the velocity for a laminar flow and the squared velocity for a turbulent flow. Even though the ultimate cause of a drag is viscous friction, the turbulent drag is independent of viscosity.

Turbulence modeling is the construction and use of a mathematical model to predict the effects of turbulence. Turbulent flows are commonplace in most real life scenarios, including the flow of blood through the cardiovascular system, the airflow over an aircraft wing, the re-entry of space vehicles, besides others. In spite of decades of research, there is no analytical theory to predict the evolution of these turbulent flows. The equations governing turbulent flows can only be solved directly for simple cases of flow. For most real life turbulent flows, CFD simulations use turbulent models to predict the evolution of turbulence. These turbulence models are simplified constitutive equations that predict the statistical evolution of turbulent flows.

Ira Abbott American aerospace engineer

Ira H. Abbott was an American aerospace engineer. A graduate of MIT, Abbott worked for Langley Aeronautical Laboratory in 1929. He was a Director of Aeronautical and Space Research at NASA during the middle of the twentieth century and before that was employed by the National Advisory Committee for Aeronautics (NACA). Abbott supervised the X-15, supersonic transport, nuclear rocket and advanced reentry programs. He retired in 1962. As Assistant Director of NACA, Abbott was decisive in keeping Ames Research Center focused on research instead of moving into operations during the development of the proposed Orbiting Astronomical Observatory in 1960. In recognition for his "outstanding contributions" to airfoil research and his leadership, he was inducted into the first round of the NACA/NASA Hall of Fame on August 13, 2015.

The Air Movement and Control Association International, Inc. (AMCA) is a long-established American trade body that sets standards for Heating, Ventilation and Air Conditioning (HVAC) equipment. It is best known for its ratings in fan balance and vibration, aerodynamic performance, air density, speed and efficiency.

Sting (fixture)

In experimental fluid mechanics, a sting is a test fixture on which models are mounted for testing, e.g. in a wind tunnel. A sting is usually a long shaft attaching to the downstream end of the model so that it does not much disturb the flow over the model. The rear end of a sting usually has a conical fairing blending into the model support structure.

Variable Density Tunnel United States historic place

The Variable Density Tunnel (VDT) was the second wind tunnel at the National Advisory Committee for Aeronautics (NACA) Langley Research Center. Proposed by German aerospace engineer, Max Munk in May, 1921, it was the world's first variable density wind tunnel and allowed for more accurate testing of small-scale models than could be obtained with atmospheric wind tunnels. It was actively used as a wind tunnel from 1923 until its retirement in the 1940s. Langley Research Center historian, James R. Hansen, wrote that the VDT provided results superior to the atmospheric wind tunnels used at the time and was responsible for making NACA, the precursor to NASA, "a world leader in aerodynamic research". It is now on display on the Langley grounds, near the old Reid Conference Center and is a National Historic Landmark.

Expansion and shock tunnels are aerodynamic testing facilities with a specific interest in high speeds and high temperature testing. Shock tunnels use steady flow nozzle expansion whereas expansion tunnels use unsteady expansion with higher enthalpy, or thermal energy. In both cases the gases are compressed and heated until the gases are released, expanding rapidly down the expansion chamber. The tunnels reach speeds from Mach 3 to Mach 30 to create testing conditions that simulate hypersonic to re-entry flight. These tunnels are used by military and government agencies to test hypersonic vehicles that undergo a variety of natural phenomenon that occur during hypersonic flight.

Wind-turbine aerodynamics

The primary application of wind turbines is to generate energy using the wind. Hence, the aerodynamics is a very important aspect of wind turbines. Like most machines, there are many different types of wind turbines, all of them based on different energy extraction concepts.

Pressure-sensitive paint (PSP) is a method for measuring air pressure or local oxygen concentration, usually in aerodynamic settings. PSP is paint-like coating which fluoresces under a specific illumination wavelength in differing intensities depending on the external air pressure being applied locally to its surface.

Reynolds number Dimensionless quantity that is used to help predict fluid flow patterns

The Reynolds number is an important dimensionless quantity in fluid mechanics used to help predict flow patterns in different fluid flow situations. At low Reynolds numbers, flows tend to be dominated by laminar (sheet-like) flow, while at high Reynolds numbers turbulence results from differences in the fluid's speed and direction, which may sometimes intersect or even move counter to the overall direction of the flow. These eddy currents begin to churn the flow, using up energy in the process, which for liquids increases the chances of cavitation. The Reynolds number has wide applications, ranging from liquid flow in a pipe to the passage of air over an aircraft wing. It is used to predict the transition from laminar to turbulent flow, and is used in the scaling of similar but different-sized flow situations, such as between an aircraft model in a wind tunnel and the full size version. The predictions of the onset of turbulence and the ability to calculate scaling effects can be used to help predict fluid behaviour on a larger scale, such as in local or global air or water movement and thereby the associated meteorological and climatological effects.

Airflow, or air flow is the movement of air from one area to another. The primary cause of airflow is the existence of pressure gradients. Air behaves in a fluid manner, meaning particles naturally flow from areas of higher pressure to those where the pressure is lower. Atmospheric air pressure is directly related to altitude, temperature, and composition.

NACA Report No. 964 - The effects of variations in Reynolds number between 3.0 x 106 and 25.0 x 106 upon the aerodynamic characteristics of a number of NACA 6-series airfoil sections was published by the United States National Advisory Committee for Aeronautics in 1950. It contained a series of graphs showing the resulting lift and drag of several NACA 6-series airfoil sections from tests performed in a variable-density wind tunnel, in which the Reynolds number (RN) was set at three different values.

Aerodynamics is a branch of dynamics concerned with the study of the motion of air. It is a sub-field of fluid and gas dynamics, and the term "aerodynamics" is often used when referring to gas dynamics.

A flow straightener, sometimes called a honeycomb, is a device used to straighten the air flow in a wind tunnel. It is a passage of ducts, laid along the axis of main air stream to minimize the lateral velocity components caused by swirling motion in the air flow during entry. The cross-section shapes of these "honeycombs" may be of square, circular and regular hexagonal cells.

References

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    • That Wenham and Browning were attempting to build a wind tunnel is briefly mentioned in: Sixth Annual Report of the Aeronautical Society of Great Britain for the Year 1871, p. 6. From p. 6: "For this purpose [viz, accumulating experimental knowledge about the effects of wind pressure], the Society itself, through Mr. Wenham, had directed a machine to be constructed by Mr. Browning, who, he was sure, would take great interest in the work, and would give to it all the time and attention required."
    • In 1872, the wind tunnel was demonstrated to the Aeronautical Society. See: Seventh Annual Report of the Aeronautical Society of Great Britain for the Year 1872, pp. 6-12.
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  20. Goldstein, E., "Wind Tunnels, Don't Count Them Out," Aerospace America, Vol. 48 #4, April 2010, pp. 38-43
  21. Benjamin Gal-Or, Vectored Propulsion, Supermaneuverability & Robot Aircraft, Springer Verlag, 1990, ISBN   0-387-97161-0, ISBN   3-540-97161-0
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Further reading

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