wind tunnel

wind tunnel

This entry is compiled and applied by "popular science China" Science Encyclopedia

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Wind tunnel is a kind of pipe like experimental equipment, which is used to generate and control air flow artificially, simulate the flow of air around aircraft or entity, measure the effect of air flow on entity and observe physical phenomena. It is one of the most commonly used and effective tools for aerodynamic experiments.

Wind tunnel test is an indispensable part of aircraft development. It not only plays an important role in the research and development of aviation and aerospace engineering, but also plays an indispensable role in transportation, housing construction, wind energy utilization and other fields with the development of industrial aerodynamics. In this way, the flow condition is easy to control. During the experiment, the model or object is often fixed in the wind tunnel for repeated blowing, and the experimental data is obtained by measuring and controlling instruments and equipment.

In order to make the experimental results accurate, the experimental flow must be similar to the actual flow state, that is, it must meet the requirements of the similarity law. However, due to the limitation of wind tunnel size and power, it is very difficult to simulate all the similar parameters in one wind tunnel at the same time. Usually, the most influential similar parameters are selected according to the subject to be studied.

In addition, the flow field quality of the experimental section of the wind tunnel, such as the uniformity of the velocity distribution, the deviation of the average flow direction from the axis of the wind tunnel, the pressure gradient along the axis of the wind tunnel, the uniformity of the cross-section temperature distribution, the turbulence degree and the noise level of the air flow, must meet certain standards, and be checked and measured regularly.

history

The first wind tunnel recognized in the world was built by E. Mariotte in 1869-1871, and the resistance of objects moving relative to air was measured. It is a wooden box with openings at both ends, with a section of 45.7 cm × 45.7 cm and a length of 3.05 M. O. Wright and W. Wright brothers of the United States built a wind tunnel in 1900, with a section of 40.6 cm × 40.6 cm, a length of 1.8 m, and an air velocity of 40-56.3 km / h, before they successfully carried out the world's first powered flight. In 1901, the Wright brothers built a wind tunnel with a wind speed of 12 m / s to carry out relevant experimental tests for their aircraft.

Wind tunnels appeared in large numbers in the middle of the 20th century. So far, China has owned low-speed, high-speed, ultra-high speed, shock wave, arc and other wind tunnels.

Wind tunnel is the most widely used tool in Aerodynamics Research and test. Its emergence and development are closely related to the development of Aerospace Science. Wind tunnel is widely used to study the basic laws of aerodynamics, to verify and develop relevant theories, and directly serve the development of various aircraft. Through wind tunnel experiments, the aerodynamic layout and aerodynamic performance of aircraft can be determined. The design of modern aircraft depends heavily on wind tunnel. For example, in the 1950s, about 10000 hours of wind tunnel tests were carried out in the development of the U.S. B-52 bomber, while about 100000 hours of wind tunnel tests were carried out in the development of the first space shuttle in the 1980s.

Wind tunnel tests are necessary to design new aircraft. The airflow in wind tunnel needs different velocity, density and temperature to simulate the real flight state of various aircraft. The velocity of airflow in wind tunnel is generally measured by the Mach number (M number) of the experimental airflow. Wind tunnels are generally classified according to the range of flow velocity: the wind tunnel with m < 0.3 is called low speed wind tunnel, when the air density in the air flow has almost no change; the wind tunnel with 0.3 < m < 0.8 is called subsonic wind tunnel, when the air density in the air flow has changed; 0.8 < m < 1.2 The wind tunnel in the range of 1.2 < m < 5 is called transonic wind tunnel; the wind tunnel in the range of 1.2 < m < 5 is called supersonic wind tunnel; the wind tunnel with m ≥ 5 is called hypersonic wind tunnel. Wind tunnels can also be classified according to their uses, structural types and test time.

Because of its good controllability and high repeatability, wind tunnels are widely used in automotive aerodynamics and wind engineering tests, such as wind load and vibration of structures, building ventilation, air pollution, wind power generation, environmental wind field, flow conditions in complex terrain, and the effectiveness of wind proof facilities. All these problems can be solved by using the principle of geometric similarity to place the terrain and objects in the wind tunnel as scale models, and then measure the wind or wind speed of the model with instruments. Some studies also point out that the results of wind tunnel test are similar to the results of local wind field observation, so wind tunnel test is the most commonly used method to study many wind engineering problems. The wind tunnel test data can also be used to verify the validity of the numerical model and find the better model parameters.

There are more than 1000 wind tunnels in the world. The largest low-speed wind tunnel is the national full scale facility (NFSF) of NASA Ames center. The size of the experimental section is 24.4 × 36.6 m2, The largest transonic wind tunnel with the highest Reynolds number is the national transonic facility (NTF) of Langley center in the United States. It is a low temperature wind tunnel with a test section size of 2.5 × 2.5 m2, which uses liquid nitrogen injection technology to reduce the temperature of the test gas, so that the Reynolds number of the wind tunnel test can reach or approach the actual flight value of the aircraft. The largest modern high Mach number and high Reynolds number gas piston wind tunnel is also equipped with advanced measurement and display instruments and data acquisition and processing system. The development trend of wind tunnel is to further increase the simulation ability of wind tunnel, improve the flow field quality, eliminate the wall interference under transonic speed, and develop self correcting wind tunnel.

structure

The wind tunnel is mainly composed of tunnel body, driving system and measurement control system. The form of each part varies with the type of wind tunnel.

Cave body

It has an experimental section which can measure and observe the model. In the upstream of the experimental section, there is a stable section to improve the flow uniformity and reduce the turbulence, and a contraction section or nozzle to accelerate the flow to the required velocity. In the downstream of the test section, there is a diffuser to reduce the flow velocity and energy loss, and a discharge section to direct the airflow out of the tunnel or a return section to the entrance of the wind tunnel. Sometimes in order to reduce the noise inside and outside the wind tunnel, a muffler is installed at the stable section and the exhaust port.

driving system

There are two types of drive systems.

One is composed of controllable electric unit and fan or axial compressor driven by it. Fan rotation or compressor rotor rotation makes the air pressure increase to maintain stable flow in the pipeline. The speed of air flow can be adjusted by changing the fan speed or blade installation angle, or changing the damping of air flow. DC motor can be supplied by AC / DC motor group or silicon controlled rectifier. Because of its long running time and low cost, it is often used in low speed wind tunnels. The wind tunnel using this kind of driving system is called continuous wind tunnel, but with the increase of air velocity, the driving power required increases sharply. For example, the power required to produce transonic air flow per square meter of experimental section area is about 4000 kW, and the power required to produce supersonic air flow is about 16000-40000 kW.

The other is to use a small power compressor to pressurize and store the air in the air tank in advance, or use a vacuum pump to vacuumize the vacuum tank connected with the outlet pipe of the wind tunnel, and quickly open the valve during the experiment, so that the high-pressure air can enter the cavity directly or through the ejector, or the air can be sucked into the cavity by the vacuum tank, so there are various forms of blowing, injection, suction and their combination. The wind tunnel using this driving system is called a temporary impulse wind tunnel. The construction period of the temporary impulse wind tunnel is short, the investment is small, and it is generally high. Its working time can be from a few seconds to dozens of seconds. It is mostly used in transonic, supersonic and hypersonic wind tunnels. For the pulse wind tunnel whose experimental time is less than 1 second, the temperature of the experimental gas can be increased by arc heater or shock wave, so that the energy consumption is less and the simulation parameters are higher.

Measurement control system

Its function is to control all kinds of valves, moving parts, model states and instruments according to the predetermined experimental procedures, and measure the air flow parameters, model states and related physical quantities through the balance, pressure and temperature sensors. With the development of electronic technology and computer, since the late 1940s, wind tunnel measurement and control system has developed from the early use of simple instruments, through manual and manual recording, to the use of electronic hydraulic control system, real-time data acquisition and processing system.

classification

There are many kinds of wind tunnels with different classification methods. It can be divided into low speed wind tunnel, high speed wind tunnel and hypersonic wind tunnel.

Low speed wind tunnel

Many large-scale low-speed wind tunnels have been built in many countries. There are basically two types of wind tunnels, one is the direct current wind tunnel designed by A. - G. Eiffel of France, and the other is the reflux wind tunnel designed by L. Prandtl of Germany. Fig. 1 is the structural diagram of the two wind tunnels. Now the largest low-speed wind tunnel in the world is the 12.2m × 24.4m full-scale low-speed wind tunnel of Ames Research Center of NASA. After the completion of the wind tunnel, a new experimental section of 24.4m × 36.6m was added, and the power of the fan motor was increased from 25MW to 100MW.

The experimental section of low speed wind tunnel has two shapes: open and closed

Chinese PinYin : Feng Dong