There are three classifications of modern wind tunnels that you should consider in determining what type is best suitable for your experiment. Note that a single wind tunnel can have one or more classifications.
A classification of wind tunnels by which tunnels are classified by their speed in the test section relative to the speed of sound or their Mach numbers.
Subsonic Wind Tunnels
These are wind tunnels used to test objects at low Mach numbers or speeds slower than the speed of sound.
Example #01: TecQuipment Subsonic Wind Tunnel – AF1300
An open circuit suction subsonic wind tunnel with a working section of 300 mm by 300 mm and 600 mm long is used for studying aerodynamics. The wind tunnel saves time and money compared with full-scale wind tunnels or airborne laboratories, and it offers a wide variety of experiments. The wind tunnel gives accurate results and is suitable for undergraduate study and research projects. TecQuipment offers a comprehensive range of optional models and instrumentation, including a computer-based data acquisition system.
Example #02: NASA Ames 40 by 80 Subsonic Wind Tunnels
The National Full-Scale Aerodynamics Complex (NFAC) 40- by 80 Foot Wind Tunnels are managed and operated by the U.S. Air Force’s Arnold Engineering Development Center under a long-term lease agreement with NASA. The Air Force conducts NFAC test programs for the Department of Defense, NASA, other government agencies, and industry. The wind tunnels are primarily used for aerodynamic and acoustic tests of rotorcraft and fixed-wing, powered-lift V/STOL aircraft, and developing advanced technologies for these vehicles.
Advantage of subsonic wind tunnels
- It can be used for the research and development of objects that are not reaching speeds of up to the speed of sound as it will be inefficient to use other wind tunnels for testing below the speed of sound vehicles or subsonic.
Disadvantage of subsonic wind tunnels
- It can’t be used to test aircraft that has exceeded its limitation and only has a certain range and functionality.
Transonic Wind Tunnels
Transonic Wind Tunnels are wind tunnels used to study high-speed aerodynamic flows around aircraft and to investigate how these flows interact with the stores and other attachments carried and released from the wings and fuselage. The transonic has a speed that is about equal to the speed of sound (Mach 1 760 miles per hour at sea level).
Examples #01: Calspan Wind Tunnel
The only independently owned and operated wind tunnel in the United States. The Calspan Wind Tunnel is a continuous flow, variable density, closed-circuit facility with an 8-foot x 8-foot test section. Test conditions are set via independent control of Mach number and one other pressure-dependent variable for constant Reynolds number, static pressure, dynamic pressure, or total pressure operations. Other details about our test facilities include:
- Maximum Mach Number of 1.30
- Stagnation Pressures of 0.25 to 3.25 Atmospheres
- Reynolds Numbers up to 4.5 x 10^6 per foot (Conventional)
- Reynolds Numbers up to 10.5 x 10^6 per foot (Ejectors)
Example #02: European Transonic Wind Tunnel
The European transonic wind tunnel (ETW) is a high-Reynolds-number transonic wind tunnel using nitrogen as a test gas.
It is one of the world’s largest cryogenic wind tunnels. It is situated in Cologne, Germany. ETW was constructed and is operated by four European countries France, Germany, Great Britain, and The Netherlands. The ETW is in operation since 1994. ETW provides real-flight Reynolds numbers by both increased pressure and decreased temperature. Independent variation of Reynolds number and aeroelastic loading can be done there. They specialize in Flight Reynolds number testing for full-span and semi-span models at cruise conditions and extreme borders of the flight envelope.
Advantage of transonic wind tunnels
- It helps in developing and improving the design and testing of airliners, as the range of testing of transonic tunnels is on par with the speed of these aircraft thus is suitable for these kinds of aircraft.
Disadvantage of transonic wind tunnels
- The lack of versatility and use restricts what the transonic tunnel design is capable and the use of the tunnel will be very specific.
Supersonic Wind Tunnels
A supersonic wind tunnel is a wind tunnel that produces supersonic speeds (1.2<M<5) The Mach number and flow are determined by the nozzle geometry. The Reynolds number is varied by changing the density level (pressure in the settling chamber). Therefore, a high-pressure ratio is required (for a supersonic regime at M=4, this ratio is of the order of 10). A supersonic wind tunnel has a large power demand, so most are designed for intermittent instead of continuous operation.
Example #01: NASA Glenn Hypersonic Test Facility
Designed to conduct research, development, and acceptance testing of hypersonic air-breathing propulsion systems, the NASA Glenn Hypersonic Test Facility is fully self-contained. Its experimental infrastructure includes a shop area for the fabrication of materials for facility subsystems and assembly of customer hardware. Due to the high-energy nature of the facility, it is operated remotely from a control room approximately one-quarter mile from the actual facility. The HTF contains a large stand-alone experimental infrastructure that can be readily reconfigured to test a variety of ground test applications including high-energy, high-risk testing.
Example #02: PHEDRA High Enthalpy Low-Density Wind Tunnel
PHEDRA is a plasma ground test facility used to simulate low-pressure flight conditions in the upper layer of the planetary atmospheres. An arc-jet generator operates in a cylindrical chamber of 1;1 m in diameter and 4.3 m in length, pumped with 3 primary pumps and 3 Roots pumps, which capacity (27 000 m3/h) insures a residual pressure ranging between 1 and 100 Pa.
Advantage of supersonic wind tunnels
- It can be its contribution in the development in terms of designing and creating not only high-speed regions of flights for aircraft but also help in enhancing the development of rocket designs.
Disadvantage of supersonic wind tunnels
- The compressible flows experience mass flow choking.
Hypersonic Wind Tunnels
These types of wind tunnels are used to test flight characteristics in a hypersonic region of Mach number 5 or more. To develop satellite launch rockets, space shuttles, etc., it is essential to test in the hypersonic range of Mach number 5 or more. MHI Group manufactures wind tunnels achieving various Mach numbers, depending on the purpose of the intended tests.
Example #01: MARHy, Hypersonic Wind Tunnel Facility
The MARHy Hypersonic low-density Wind Tunnel, located at the ICARE. Laboratory in Orléans, France, is a research facility used extensively for fundamental and applied research of fluid dynamic phenomena in rarefied compressible flows. Its name is an acronym for Mach Adaptable Rarefied Hypersonic and the wind tunnel is recorded under this name under the European portal MERIL. The facility was completed in 1963 and is one of the three facilities belonging to the FAST platform (composed of two other wind tunnels) and used with the aim of supporting aeronautics and aerospace research.
Example #02: LENS-X Hypersonic Tunnel
LENS-X is an 8 foot diameter by 100 foot expansion tunnel with a top speed of Mach 30. The drive chamber, filled with helium or hydrogen gas, is compressed to 3,000 psi at 1000 degrees Fahrenheit; this breaks the first diaphragm, causing the driven chamber to experience an influx of hot gas, generating pressures over 20,000 psi before the second diaphragm is ruptured. The LENS-X can handle aircraft up to 30 feet long and generate speeds above 25,000 mph.
Advantage of hypersonic wind tunnels
- Supersonic tunnel design contributes to the development in terms of designing and creating not only high-speed regions of flights for aircraft but also helps in enhancing the development of rocket designs.
Disadvantage of hypersonic wind tunnels
- The pressure, temperature, and density across the shock increase explosively at a point in which the molecules of the air that surround the aircraft start to change by breaking apart.
This type of classification denotes the design of the wind tunnel regardless of the wind tunnel’s Mach number.
Open Return Wind Tunnel
This type of tunnel is also called an Eiffel tunnel, after the French engineer, or an NPL tunnel, after the National Physical Laboratory in England, where the tunnel was first used. The Eiffel tunnel has an open test section, while the NPL tunnel has a closed test section. The original Wright Brother’s wind tunnelwas an open return design. In the open return tunnel, the air that passes through the test section is gathered from the room in which the tunnel is located. Open return tunnel offers a superior design for propulsion and smoke visualization at Low construction cost.
Example #01: 1901 Wright Brothers Wind Tunnel
After building and testing a small wind tunnel, the Wright brothers completed a larger, more sophisticated one in October 1901. They used it extensively to carry out aerodynamic research that proved essential in designing their 1903 airplane.
The wind tunnel consisted of a simple wooden box with a square glass window on top for viewing the interior during testing. A fan belted to a one-horsepower engine, which ran the machinery in their bicycle shop, provided an airflow of about 30 miles per hour.
Example #02: Gustave Eiffel’s Wind Tunnel
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-meter test section is still operational today. 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.
Advantages of open return wind tunnel
- Low construction cost
- Superior design for propulsion and smoke visualization. There is no accumulation of exhaust products in an open tunnel
Disadvantages of open return wind tunnel
- Poor flow quality is possible in the test section. Flow turning the corner into the bellmouth may require extensive screens or flow straighteners.
- High operating costs. The fan must continually accelerate the flow through the tunnel.
- Noisy operation. Loud noise from the fan may limit the times of operation.
Closed Return Wind Tunnel
A type of wind tunnel geometry where contrary to open circuits, the air circulates continuously within the wind tunnel. In the closed return tunnel, the air is conducted from the exit of the test section back to the fan by a series of turning vanes. Exiting the fan, the air is returned to the contraction section and back through the test section. Air is continuously circulated through the ductwork of the closed return tunnel.
Example #01: FMA-Pc16 Closed-Circuit Wind Tunnel
OMEGA’s state-of-the-art wind tunnel is designed to give a highly uniform flow rate over a 152 mm (6″) test section. A powerful 12-amp motor with variable speed from 0 to 10,000 RPM is adjustable to give a particular flowrate by a precise motor control unit. Each wind tunnel is supplied with a NIST traceable certificate. The uniform flow rate is determined by monitoring a highly repeatable differential pressure sensor that has been calibrated to each wind tunnel as a system. Each wind tunnel is supplied with two restrictive plates for achieving optimum low flow rates. The established differential pressure measurements versus flow rates are listed from 25 to 9000 FPM. The temperature ranges from 5⁰C to 45⁰C. Calibration sheets are included, which makes calibrating different flow sensors simple.
Example #02: ARA Transonic Closed-Circuit Wind Tunnel
Since 1956 ARA has provided wind tunnel data of the highest quality by developing and utilizing highly efficient, world-class techniques for our global civil and military aerospace customer base. The ARA Transonic Wind Tunnel (TWT) is a closed circuit, continuous flow tunnel with the following operational features; Test section: 2.74m wide x 2.44m high (9ft x 8ft). Four porous walls 22% maximum porosity. Mach number: 0 to 1.4Stagnation pressure: 0.8 to 1.2 atmospheres (standard operation is 1 atmosphere) Reynolds number: 3.5 to 16.7 million/m (dependent upon Mach number).
Advantages of closed return wind tunnel
- Superior flow quality in the test section. Flow turning vanes in the corner and flow straighteners near the test section ensure relatively uniform flow in the test section.
- Low operating costs. Once the air is circulating in the tunnel, the fan and motor only need to overcome losses along the wall and through the turning vanes. The fan does not have to constantly accelerate the air.
- Quiet operation relative to an open return tunnel.
Disadvantages of closed return wind tunnel
- Higher construction cost because of the added vanes and ducting.
- Inferior design for propulsion and smoke visualization. The tunnel must be designed to purge exhaust products that accumulate in the tunnel.
- Hotter running conditions than an open return tunnel. The tunnel may have to employ heat exchangers or active cooling.
A type of wind tunnel by which the wind tunnels are classified by their length of operation.
Intermittent Wind Tunnel
It is a type of wind tunnel in which energy is stored in the form of pressure and allowed to drive the tunnel only a few seconds out of each pumping hour. The intermittent wind tunnels can yield higher compression ratios. So starting the intermittent tunnel is not a problem. Intermittent blowdown and in draft tunnels are normally used for Mach numbers from 0.5 to 5.0.and the intermittent pressure -vacuum tunnels are normally used for higher Mach numbers.
Example #01: Af300-Experiment Supersonic Wind Tunnel (Intermittent)
An intermittent supersonic (up to Mach 1.8) wind tunnel for investigations into subsonic and supersonic airflow around two-dimensional models produced by TECQUIPMENT. A compressed air supply (AF300b, available separately) induces a flow in the working section of the wind tunnel. This gives a less turbulent and more stable flow for accurate results and comparison with theory. The optionally compressed air supply includes filters and air dryers to give a dust-free and dry air source needed for good results. The wind tunnel includes two analog pressure gauges. One measures the compressed air pressure available from the supply (for reference); the other measures the pressure delivered to the wind tunnel and includes an electronic transducer that connects to TecQuipment’s optional Versatile Data Acquisition System (VDAS®) to record the pressure. This allows accurate real-time data capture, monitoring, display, calculation, and charting of all the important readings on a suitable computer.
Example #02: Mhi 60 Trisonic Wind Tunnel
The MHI 60 Trisonic Wind Tunnel is an intermittent blowdown trisonic wind tunnel. The tunnel is capable of conducting tests in the subsonic, transonic, and supersonic speed ranges. The test gas is exhausted from the atmosphere. The tunnel is used to conduct six-component force tests; pressure distribution tests; half-model tests; flutter tests (half-wing and empennage); static and aeroelastic tests; air intake tests, including unsteady pressure measurements; power effect tests; and flow visualization tests. The angle of attack range is from -15 to 30 degrees when the sting support system is used for complete aircraft model tests, and a sidewall five-component balance is used for half-model tests. The air storage sphere is 8m in diameter with air pressure up to 15 kg/cm2. Tunnel upgrades in 1982 improved the flow uniformity and turbulence level as well as precise control, data acquisition, and processing systems with MX computers.
Continuous Wind Tunnel
Continuous wind tunnels are capable of operating for longer periods. The testing conditions can be held constant over a long time. However, it takes a considerable amount of time to pressurize the tunnel reservoir to the required value resulting in long starting times. In addition, the size of the air supply system required for its operation is much higher when compared to the other types.
Example #01: Af302 – Experiment Supersonic Wind Tunnel (Continuous)
A suction-type continuous-operation supersonic wind tunnel for investigations into subsonic and supersonic airflow was developed by TECQUIPMENT. It also allows students to study airflow in two dimensions around aerodynamic models. The working section of the wind tunnel is a convergent-divergent nozzle with a removable top part (‘liner’). The shape of the liner controls the maximum air velocity at the divergent part of the working section. Included are three different liners. An instrument frame (supplied) holds a remote control unit that controls a high-capacity vacuum pump (supplied). The pump creates a low pressure downstream of the working section to draw air into the wind tunnel. A bypass duct with a hand-operated valve allows the operator to reduce the airflow through the Working Section without disturbing the quality of the main airflow. This is useful for startup and shutdown and subsonic tests.
Example #02: German-Dutch Wind Tunnel
The cryogenic wind tunnel in köln is a cryogenic facility for research, development, and data set measurements.
- Type of wind tunnel: Closed-circuit, continuous, low-speed wind tunnel with a closed wall test section. Operation either at ambient temperature or cooled down by injection of liquid nitrogen.
- Integrated optical traversing system
- Typical tests
- 2D airfoil tests with high-lift and flow control devices (flaps, slats, vortex generators, trailing edge devices)
- Wind rotor blades
- Half-model tests of transport aircraft in high-lift configurations
- Surface vehicles (trains, trucks)
- Probe calibration, function, and reliability tests