What are Wind Tunnels?

The wind tunnel (wind tunnel) is a wind tunnel laboratory. It is an artificial way to generate and control airflow. It is used to simulate the flow of gas around an aircraft or an entity. It can measure the effect of airflow on an entity and observe physical phenomena. Pipe-like experimental equipment is one of the most commonly used and effective tools for conducting aerodynamic experiments.

The wind tunnel (wind tunnel) is a wind tunnel laboratory. It is an artificial way to generate and control airflow. It is used to simulate the flow of gas around an aircraft or an entity. It can measure the effect of airflow on an entity and observe physical phenomena. Pipe-like experimental equipment is one of the most commonly used and effective tools for conducting aerodynamic experiments.

Wind tunnel experiments are an indispensable part of aircraft development. It not only plays an important role in the research and development of aerospace and aerospace engineering. With the development of industrial aerodynamics, it is even more indispensable in transportation, housing construction, and wind energy utilization. With this experimental method, the flow conditions are easy to control. During the experiment, the model or object is often fixed in the wind tunnel and repeatedly blown, and the experimental data is obtained through measurement and control instruments and equipment.

In order to make the experimental results accurate, the flow during the experiment must be similar to the actual flow state, that is, the requirements of the similarity law must be met. However, due to the limitation of the size and dynamics of the wind tunnel, it is difficult to simulate all the similar parameters in a wind tunnel at the same time. Usually, according to the topic to be studied, some similar parameters with the greatest influence are selected for simulation.

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

Chinese name

Wind tunnel

Foreign name

wind tunnel

Full name

Wind tunnel laboratory

Make up

Cave, drive system and measurement control system

Wind tunnel history

The first wind tunnel recognized in the world was built by the Englishman E. Mariotte from 1869 to 1871, and measured the resistance to the relative movement of objects and air. It is a wooden box with two ends open, 45.7 cm x 45.7 cm in cross section, and 3.05 meters long. The brothers O. Wright and W. Wright of the United States built a wind tunnel in 1900 with a cross section of 40.6 cm x 40.6 cm, a length of 1.8 meters, and an air velocity of 40 to 56.3 thousand before they successfully carried out the world's first powered flight. Meters / hour. In 1901, the Wright brothers built a wind tunnel with a wind speed of 12 meters per second to conduct related experimental tests for their aircraft.

Wind tunnels appeared in the middle of the 20th century. So far, China already has low-speed, high-speed and ultra-high-speed wind tunnels such as shock waves and arcs.

Wind tunnels are the most widely used tools in aerodynamic research and testing. Its emergence and development are closely related to the development of aerospace science. Wind tunnels are widely used to study the basic laws of aerodynamics to verify and develop relevant theories, and directly serve the development of various aircraft. Wind tunnel experiments are used to determine the aerodynamic layout of aircraft and evaluate their aerodynamic performance. The design of modern aircraft relies heavily on wind tunnels. For example, the development of the US B-52 bomber in the 1950s carried out wind tunnel experiments of about 10,000 hours, while the development of the first space shuttle in the 1980s carried out about 100,000 hours of wind tunnel experiments.

Designing a new aircraft must go through wind tunnel experiments. The airflow in a wind tunnel needs to have different flow speeds, different densities, and even different temperatures in order to simulate the true flight status of various aircraft. The airflow velocity in a 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: wind tunnels with M <0.3 are called low-speed wind tunnels, and the air density in the airflow has almost no change at this time; wind tunnels in the range of 0.3 <M <0.8 are called subsonic wind tunnels. At this time, the density of the airflow has changed during the flow; wind tunnels in the range of 0.8 <M <1.2 are called transonic wind tunnels; wind tunnels in the range of 1.2 <M <5 are called supersonic wind tunnels; M5 The wind tunnel is called a hypersonic wind tunnel. Wind tunnels can also be classified by purpose, structure type, and experiment time.

Because of its good controllability and high repeatability, wind tunnels are now widely used in automotive aerodynamics and wind engineering tests, such as wind loads and vibrations of structures, building ventilation, air pollution, wind power generation, and environmental wind fields. , Flow conditions in complex terrain, the effectiveness of wind protection facilities, etc. These problems can all use the principle of geometric similarity to place terrain and ground objects in a wind tunnel with a scale model, and then measure the wind or wind speed to which the model is subjected with an instrument. Some studies have also pointed out that the results of wind tunnel experiments are similar to the results of on-site wind field observations, so wind tunnel experiments are the most commonly used method for studying many wind engineering problems. Wind tunnel experimental data can also be used to verify the validity of the numerical model and find better model parameters.

The total number of wind tunnels in the world has reached more than a thousand. The largest low-speed wind tunnel is the National Full Size Equipment (NFSF) of NASA Ames Center. The experimental section size is 24.4 × 36.6 m 2, which is enough to test one. Complete real aircraft; the largest transonic wind tunnel with the highest Reynolds number is the National Transonic Equipment (NTF) at the Langley Center in the United States. It is a low-temperature wind tunnel with an experimental section size of 2.5 × 2.5 m 2 and uses injection fluid Nitrogen technology is used to reduce the temperature of the experimental gas, so that the Reynolds number of the wind tunnel experiment reaches or approaches the actual flight value of the aircraft. The largest gas piston wind tunnel with modern high Mach number and high Reynolds number is also equipped with advanced measurement display instruments and data acquisition and processing systems. The development trend of wind tunnels is to further increase the simulation capability of wind tunnels and improve the quality of flow fields, eliminate wall interference at transonic speeds, and develop self-correcting wind tunnels.

Wind tunnel ^[1]

Wind tunnel structure

A wind tunnel is mainly composed of a cave body, a driving system, and a measurement control system. The form of each part differs according to the type of the wind tunnel.

Wind tunnel

It has an experimental section that makes the necessary measurements and observations of the model. There is a stable section upstream of the experimental section to improve airflow uniformity and reduce turbulence, and a constriction section or nozzle to accelerate the airflow to the required flow rate. Downstream of the experimental section are an expansion section that reduces flow velocity and energy loss, and a discharge section that directs airflow outside the wind tunnel or a return section that leads back to the wind tunnel inlet. Sometimes in order to reduce the noise inside and outside the wind tunnel, a muffler is installed in the stable section and the exhaust port.

Wind tunnel drive system

There are two types of drive systems.

One type is composed of a controllable motor unit and a fan or an axial flow compressor driven by it. Fan rotation or compressor rotor rotation increases the air pressure to maintain a stable flow in the duct. Changing the fan speed or blade installation angle, or changing the damping of the airflow, can adjust the speed of the airflow. DC motors can be powered by AC-DC motors or thyristors. It has a long operating time and low operating cost, and is mostly used in low-speed wind tunnels. Wind tunnels using this type of drive system are called continuous wind tunnels, but the driving power required increases sharply as the air velocity increases. For example, the power required to generate transonic airflow per square meter of experimental area is about 4000 kilowatts, which generates supersonic speed. The air flow is about 16,000 to 40,000 kilowatts.

The other type is to use a low-power compressor to pre-pressurize and store the air in the air storage tank, or use a vacuum pump to evacuate the vacuum tank connected to the outlet pipe of the wind tunnel. During the experiment, quickly open the valve to allow high-pressure air to pass directly or through The ejector enters the cavity or the air is sucked into the cavity by a vacuum tank, so there are various forms of blowing, ejecting, inhaling, and their combination. A wind tunnel using this drive system is called a temporary wind tunnel. The impulse wind tunnel has a short construction period and low investment. Generally, the [[Reynolds number]] is high, and its working time can be from a few seconds to tens of seconds. It is mostly used for transonic, supersonic and hypersonic wind tunnels. For pulsed wind tunnels with an experimental time of less than 1 second, the temperature of the experimental gas can be increased by arc heaters or shock waves, so that the energy consumption is small and the simulation parameters are high.

Wind tunnel measurement control system

Its role is to control various valves, moving parts, model states, and instruments according to predetermined experimental procedures, and measure air flow parameters, model states, and related physical quantities through sensors such as balances, pressures, and temperatures. With the development of electronic technology and computers, wind tunnel measurement and control systems began in the late 1940s from the early use of crude instruments, through manual and manual recording, to the use of electronic hydraulic control systems, real-time acquisition and processing data systems.

Wind tunnel classification

There are many types of wind tunnels and there are different classification methods. According to the magnitude of the air velocity in the experimental section , it can be divided into low speed, high speed and high supersonic wind tunnels.

Low speed wind tunnel

Many countries have successively built many large-scale low-speed wind tunnels. There are basically two forms, one is a DC wind tunnel designed by the French A.-G. Eiffel; the other is a reflow wind tunnel designed by the German L. Plante. Figure 1 shows these two types of wind tunnels. Schematic diagram of the hole structure. The largest low-speed wind tunnel in the world today is the full-scale low-speed wind tunnel at the National Aeronautics and Space Administration's (Ames) Ames Research Center at 12.2 meters by 24.4 meters. After the wind tunnel was completed, a new experimental section of 24.4 meters × 36.6 meters was added, and the fan motor power was also increased from the original 25 MW to 100 MW.

The low-speed wind tunnel experimental section has two forms: open and closed. The cross-sectional shape includes rectangular, circular, octagonal, and oval. The length depends on the type of wind tunnel and the experimental object. Since the 1960s, double-experimental wind tunnels and even third-experimental wind tunnels have been developed.

A wind tunnel is a duct used to generate artificial airflow (artificial wind). In this kind of pipeline, which can cause a section of uniform air flow, the automotive wind tunnel test is carried out in this section of the wind tunnel.

In low-speed wind tunnels, the commonly used energy ratio Er is a measure of the economic efficiency of wind tunnel operation. Where v0 and A0 are the airflow velocity and cross-sectional area of the experimental section; is the air density; and N are the drive system efficiency and the input power of the motor, respectively. For the closed experimental section, the wind tunnel Er is 3-6. Reynolds number Re is the main simulation parameter for low-speed wind tunnel experiments, but because of different experimental objects and projects, there are some other parameters that need to be simulated. In some cases where gravity works (such as tail spin, launch and dynamic model experiments, etc.), Froude number Fr needs to be simulated, and flight Mach number and rotor wing tip Mach number need to be simulated in helicopter experiments.

There are many types of low-speed wind tunnels. In addition to general wind tunnels, there are ice wind tunnels that specialize in researching aircraft anti-icing and de-icing, vertical wind tunnels that study the spiral formation and recovery methods of aircrafts, and research on the real aerodynamics of aircrafts under close flight conditions. Performance full-scale wind tunnels. V / STOL wind tunnels that study the aerodynamic characteristics of vertical short-range landing aircraft (V / STOL) and helicopters, as well as high-Reynolds-charging wind tunnels. In order to study the external noise of the engine, a dynamic model experiment was performed, and some wind tunnels were modified to meet the requirements of acoustic experiments and dynamic experiments. In order to carry out industrial aerodynamic research, in addition to retrofitting aviation wind tunnels and adding auxiliary equipment, countries have also built a number of dedicated wind tunnels, such as long experimental sections and velocity profiles that simulate atmospheric flow, turbulent structures, and temperature stratification. Atmospheric boundary layer wind tunnel with a minimum wind speed of about 0.2 m / s, research on full-scale vehicle performance, automotive wind tunnels that simulate climatic conditions, and sand wind tunnels affected by sand movement.

The low-speed wind tunnel in the DC closed experimental section is a typical low-speed wind tunnel. In this type of wind tunnel, the fan blasts to the right and causes air to enter the stable section of the wind tunnel from the outside at the left. The honeycomb and damping net in the stable section comb and smooth the airflow, and then the airflow is accelerated by the contraction section to form a stable airflow with uniform flow direction and uniform speed in the experimental section. In the experimental section, an air blowing experiment of an aircraft model can be performed to obtain aerodynamic experimental data acting on the model. The airflow speed of this wind tunnel is controlled by the speed of the fan. China Aerodynamics Research and Development Center has built a large-scale low-speed wind tunnel with an open-circuit closed-end tandem double test section. The size of the first experimental section is 12 × 16 × 25 m 3, the maximum wind speed is 25 m / s, and the size of the second experimental section It is 8 × 6 × 25 meters 3 and the maximum wind speed is 100 meters / second.

The backflow wind tunnel actually connects the DC wind tunnel end to end to form a closed loop. The airflow circulates back and forth in the wind tunnel, which saves energy and is not affected by external interference. Wind tunnels can also use other special gases or fluids instead of air. Variable-density wind tunnels use compressed air instead of normal-pressure air. Water instead of air is called a water tunnel (see sinks and water tunnels).

High-speed wind tunnel

A wind tunnel with an Mach number of 0.4 to 4.5 in the experimental section. According to the Mach number range, high-speed wind tunnels can be divided into subsonic wind tunnels, transonic wind tunnels, and supersonic wind tunnels.

Wind tunnel subsonic wind tunnel

The Mach number of the wind tunnel is 0.4 to 0.7. The structural form and working principle are similar to low-speed wind tunnels, except that the power required for operation is greater than that of low-speed wind tunnels.

Wind tunnel across sonic wind tunnel

The Mach number of the wind tunnel is 0.5 to 1.3. When the airflow in the wind tunnel reaches the speed of sound at the smallest cross section in the experimental section, even if the driving power or pressure is increased, the airflow speed in the experimental section does not increase any more. This phenomenon is called congestion. Therefore, in the early transonic experiments, the model could only be mounted on the convex surface of the aircraft wing or the bottom wall of the wind tunnel. Not only can the model not be too large, but the airflow is not uniform. Later research found that the experimental section used vented walls or ventilated walls slitted in the direction of the airflow to allow part of the airflow in the experimental section to flow out through the holes or slits, which could eliminate the congestion in the wind tunnel and generate low-sonic flow. This experimental section with vented walls can also reduce cavity wall interference and reduce or eliminate cavity wall reflection wave systems at low supersonic speeds. The shock wave generated by the model is reflected as a shock wave on the solid wall and as an expansion wave on the free boundary. If the ventilated wall has a suitable free boundary, it can greatly reduce or eliminate the reflected wave system of the cave wall.

In order to effectively reduce the reflected wave in various experimental situations, variable opening and closing ratios (the ratio of openings or slits to the wall area of the experimental section) and breathable walls that can change the opening and closing ratios along the airflow direction have been developed. The first transonic wind tunnel was built by the American Aviation Advisory Committee (NACA) in 1947. It is an open-wall wind tunnel with an opening-closing ratio of 12.5% and an experimental section diameter of 308.4 mm. Since then, transonic wind tunnels have developed rapidly. By the 1950s, a large number of model experimental wind tunnels with experimental calibers greater than 1 meter had been constructed.

Wind tunnel supersonic wind tunnel

A wind tunnel with an airflow Mach number of 1.5 to 4.5. Before entering the experimental section, the airflow in the wind tunnel passed a Laval tube and reached supersonic speed. As long as the pressure ratio between the front and rear of the nozzle is large enough, the velocity of the air flow in the experimental section depends only on the ratio of the cross-sectional area of the experimental section to the cross-sectional area of the nozzle throat. A two-dimensional nozzle consisting of two planar side walls and two profiles is usually used.

There are many types of nozzles, such as: fixed nozzles with two side walls and two profiles assembled into a rigid semi-permanent assembly and directly connected to the cavity; replaceable profile blocks and nozzle box sides The wall constitutes a nozzle, and the nozzle nozzle box is connected with the cavity to form a solid block nozzle; the flexible tube nozzle surface is composed of two flexible plates, and the flexible plate shape surface can be adjusted (Figure 3) ). The supersonic diffuser downstream of the experimental section is composed of a constricted section, a second throat, and a diffusion section (Figure 4). By changing the area of the throat, the supersonic flow passes through a weak shock wave system and becomes a subsonic flow to reduce the flow. Total pressure loss. The first supersonic wind tunnel was built in 1905 in Göttingen, Germany, and the experimental horse number can reach 1.5.

In 1920, A. Buzeman improved the nozzle design and obtained a uniform supersonic flow field. In 1945, Germany already had a supersonic wind tunnel with a diameter of about 1 meter in the experimental section. In the 1950s, a number of supersonic wind tunnels for aircraft model experiments appeared in the world, the largest of which was a supersonic wind tunnel of 4.88 meters by 4.88 meters in the United States.

Many wind tunnels built often break through the above-mentioned ranges of subsonic, transonic, and supersonic velocities, and can perform subsonic, transonic, and supersonic experiments in one wind tunnel. This type of wind tunnel is called a three-sonic wind tunnel. The 1.2m × 1.2m transonic, supersonic wind tunnel at the China Aerodynamics Research and Development Center (Figure 5) is a three-sonic wind tunnel.

Since the 1960s, raising the Reynolds number of wind tunnels has received widespread attention. Transonic sonic wind tunnel model experiments usually have a Reynolds number less than 1 × 109. Large-scale aircraft development requires the construction of a transonic wind tunnel with a higher Reynolds number (for example, greater than 4 × 109). Wind tunnels, such as low-temperature wind tunnels that lower the temperature of experimental gases and increase the Reynolds number by injecting liquid nitrogen. Low temperature wind tunnels have the ability to independently change Mach number, Reynolds number, and dynamic pressure, so they have developed rapidly.

Wind tunnel hypersonic wind tunnel

Supersonic wind tunnel with Mach number greater than 5. It is mainly used for model experiments of missiles, artificial satellites and space shuttles. Experimental items usually include aerodynamic, pressure, heat transfer measurement and flow field display, as well as dynamic stability, low melting model ablation, mass ejection, and particle erosion measurement. Hypersonic wind tunnels mainly include conventional hypersonic wind tunnels, low-density wind tunnels, shock wind tunnels, and thermal shock wind tunnels.

If a hypersonic wind tunnel is to obtain a higher M-number airflow in the wind tunnel (for example, M5), generally speaking, the sufficient pressure difference cannot be generated by the blowing effect of the high-pressure air upstream. A large-volume vacuum container is connected to the outlet, and a large pressure difference can be formed by punching up and down, thereby generating a hypersonic airflow with M5. However, when the airflow is accelerated through the nozzle to a hypersonic speed, the temperature will rapidly decrease, causing the gas to liquefy. In order to avoid liquefaction or simulate the required temperature, a heating device must be installed at the equivalent section of the hypersonic wind tunnel. There are many types of hypersonic wind tunnels depending on the heating principle and application. Spontaneous conventional hypersonic wind tunnels are more typical, and they are very similar to conventional supersonic wind tunnels. Other types of wind tunnels include shock wind tunnels, artillery wind tunnels, thermal tunnel wind tunnels, long tunnel wind tunnels, gas piston wind tunnels, and arc wind tunnels (see ultra-high-speed experimental equipment). China Aerodynamics Research and Development Center's high-pressure, ejection-driven, temporary, hypersonic wind tunnel experimental section has a diameter of 0.5 meters. The center has also built a shock tunnel with a diameter of 2 meters in the experimental section.

Wind tunnel conventional hypersonic wind tunnel

It was developed on the basis of a supersonic wind tunnel. Figure 6 is a schematic diagram of a hypersonic wind tunnel. Figure 7 is a photograph of a temporary hypersonic wind tunnel with a diameter of 0.5 meters in an experimental section.

The operating principle of a conventional hypersonic wind tunnel is similar to that of a supersonic wind tunnel. The main difference is that the former must heat the gas. Because at a given steady-state temperature, the static temperature of the airflow in the experimental section decreases with the increase of the Mach number, so that the airflow in the experimental section will liquefy. In fact, due to the rapid expansion of the airflow, there are varying degrees of supersaturation under certain experimental conditions.

Therefore, the temperature of the stable section actually used may be lower than the temperature obtained from the air saturation curve. Different heating methods are used for the experimental gas according to different stable section temperatures. Under normal circumstances, the heating temperature of gas combustion heaters can reach 750 K; Ni-Cr resistance heaters can reach 1000 K; iron-Cr-Al resistance heaters can reach 1450 K; alumina pebble bed heaters can reach 1670 K; Pebble bed heaters can reach 2500 Kelvin; tungsten resistance heaters with high purity nitrogen as the experimental gas can reach 2200 Kelvin; graphite resistance heaters can reach 2800 Kelvin.

Early conventional hypersonic wind tunnels often used two-dimensional nozzles. Under high Mach number conditions, the size of the throat is small, the thermal deformation caused by high heat flow on the surface makes the size of the throat unstable, and the boundary layer distribution is very uneven, which will affect the uniformity of the airflow. Therefore, most late-stage hypersonic wind tunnels were fitted with conical or profile axisymmetric nozzles. Conical nozzles are easy to process, but produce a cone-shaped flow field, so they are gradually replaced by profile nozzles. When the Mach number is greater than 7, the nozzle throats that work under high temperature and pressure are generally cooled with water.

The typical aerodynamic performance of conventional hypersonic wind tunnels is characterized by experimental Mach numbers and unit Reynolds numbers. The experimental Mach number of a typical wind tunnel using air as the experimental gas is 5-14, and the order of Reynolds number per meter is 3 × 10 ⁶ . In order to further increase the experimental Mach number and Reynolds number, helium with a very low condensation temperature (4 Kelvin) is used as the experimental gas. The Mach number can reach 25 at room temperature and the Mach number can reach 42 when heated to 1000 Kelvin.

The world's first conventional hypersonic wind tunnel was built by Germany during World War II. This is a temporary wind tunnel. The upper limit of Mach number is 10, and the size of the experimental section is 1m × 1m. Germany was defeated and the wind tunnel was not fully completed. After the war, the United States built a number of conventional hypersonic wind tunnels with a size of more than 0.45 meters. A few were continuous and most were temporary.

Low-density wind tunnel

A hypersonic wind tunnel with thin (low density) gas flow is formed. It provides the aerodynamic environment for the development of spacecraft at high altitudes, and is also an experimental tool for studying the dynamics of thin gases. Low-density wind tunnels are mainly used for experiments in slip flow regimes and transitional flow regimes, and mainly simulate the ratio of Knudsen number, Mach number, average surface temperature and stagnation temperature (temperature when the gas velocity becomes zero) It is 0.06 1), and the real gas effect under high temperature and low pressure. The principle and structure of a low-density wind tunnel are similar to conventional hypersonic wind tunnels. Compared with the conventional hypersonic wind tunnel, it has the following characteristics: the pressure in the stable section and the size of the experimental model are reduced by orders of magnitude compared with the conventional hypersonic wind tunnel; it has a huge vacuum pumping system and excellent wind tunnel sealing performance; The cryogenic Laval tube or small hole free jet experimental technology is commonly used to solve the problem of nozzle boundary layer thickening caused by low Reynolds number and high Mach number, so that it can be obtained for experiments under a larger Knudsen number A thin airflow area of sufficient size is used; at the same Mach number, the heating requirement for preventing the liquefaction of the working gas is lower than that of a general hypersonic wind tunnel. However, in the low-density wind tunnel experiment, due to the small airflow density and the small size of the experimental model, the aerodynamic force, heat, and pressure of the model are very weak, and the measurement technology is difficult. Non-contact measurement technologies such as electromagnetic suspension balances and electron beam devices have been used for related measurements. Figure 8 is a schematic diagram of a low-density wind tunnel.

Wind tunnel

A shock wave is used to compress the experimental gas, and then a steady expansion method is used to generate a wind tunnel with a hypersonic experimental airflow. It consists of a shock tube and a main part of the wind tunnel such as a nozzle connected behind it. A diaphragm (second diaphragm) is used to separate the shock tube and the nozzle, and the back of the nozzle is evacuated. FIG. 9 is a schematic diagram of a reflective shock wave tunnel. The working process of a shock wave tunnel is as follows: when the wind tunnel starts, the main diaphragm is broken first, causing the expansion of the driving gas, generating an expansion wave propagating upstream, and generating a shock wave in the experimental gas. When this shock wave moves downstream to the nozzle inlet, the second diaphragm is flushed away, so the experimental gas that reaches the high temperature and high pressure after the shock wave compression enters the nozzle to expand and accelerate, and flows into the experimental section for experiment use. The experiment ends when the experimental conditions are destroyed due to wave reflections or the end of the test gas flow.

Shock wind tunnel experiments are short, usually in milliseconds. The name of the shock wave tunnel was proposed by Herzberger in 1951. Its development is closely related to the development of medium and long-range missiles and spacecraft. From the early 1950s to the mid 1960s, due to the urgent need to study the high temperature real gas effect in hypersonic flight, shock wave tunnels were mainly used to simulate high temperature conditions. After the mid-1960s, due to the need for strategic warheads to maneuver at low altitudes, it turned to simulated high Reynolds numbers, and in 1971, it first realized such simulated operations. Early shock wave tunnels used a through type (incident shock waves did not reflect at the nozzle entrance and passed directly through the nozzle), so the experimental time was very short (even shorter than 1 millisecond) and it was difficult to apply, so a reflective type was developed. Shock wind tunnel. This kind of wind tunnel has different operating methods. If the operating conditions are properly selected, usually 5-25 milliseconds of experimental time can be obtained. Shock wind tunnel experiments have been established as a standard hypersonic experimental technique and have become the main source of hypersonic aerodynamic data.

The experimental items are usually heat transfer, pressure, aerodynamic measurement, and flow field display. In addition, there are special items such as electronic density measurement. The highest parameters of the existing shock wind tunnel operation are: the driving pressure is about 3400 atmospheres (1 atmosphere equals 101325 Pa); it can simulate a flight speed of 6.7 km / s; the airflow Mach number reaches 24; the Reynolds number reaches 108 (when Mach When the number is 8).

Wind tunnel

The arc gas discharge is used to heat and compress the experimental gas in a constant volume to generate a wind tunnel with a high supersonic flow. The basic structure is shown in Figure 10. Before operation, the energy storage device stores electric energy, the arc chamber is filled with a certain pressure of gas, and various parts downstream of the diaphragm are pumped to a vacuum state (generally not lower than 105 Pa). During operation, the stored electrical energy is released in the arc chamber by an arc discharge in the time of one thousandth to tens of milliseconds to heat and compress the gas; when the pressure in the arc chamber rises to a predetermined value, the diaphragm is broken The gas expands and accelerates through the nozzle to form a hypersonic gas flow in the experimental section; then it is discharged into the vacuum box through the diffuser.

Unlike conventional hypersonic wind tunnels and shock wind tunnels, the experimental airflow of a hot impulse wind tunnel is a quasi-steady flow (see unsteady flow), and the experiment time is about 20 to 200 milliseconds; the arc chamber gas pressure and temperature during the experiment depend on The experimental conditions and time are about 10-50% lower than those of the hypersonic wind tunnel and the shock wind tunnel. Therefore, the air flow parameters and the aerodynamic characteristics on the model of the experimental section should be measured instantaneously and synchronously, and a set of special data processing techniques should be used. The development of the thermal shock wind tunnel started in the early 1950s, and slightly after the shock wind tunnel. The original was to use spark discharge to obtain a high-performance shock tube drive section, which later evolved into a thermal shock tunnel. The term "hot punch" was coined by RW Perry in 1958.

One of the key technical aspects of a thermal shock tunnel is to reduce material burnout and gas contamination to an acceptable level. The measures adopted are: replacing nitrogen as the experimental gas with nitrogen; reducing the arc chamber insulation area exposed to hot gas; rationally designing the electrode and throat baffle structure that burns out the precipitated material to generate particles; appropriately select the fuse for the arc initiation Wire; restrict the wind tunnel to operate under the temperature of the arc chamber gas below 4000. There are two types of energy storage devices for thermal shock tunnels: capacitance and inductance. The former is often used to store energy below 10 MJ, while the latter is mostly used to store energy between 5 and 100 MJ.

Another method is direct power supply from the power grid, whose energy is generally on the order of 10 megajoules. Different electric energy utilization methods require corresponding charging and discharging systems. The simulation range of the thermal shock tunnel can generally reach: Mach number 8-22, Reynolds number per meter 1 × 10 ⁵ 2 × 10 ⁸ . The experiment time of hundreds of milliseconds not only enables it to complete a static wind tunnel experiment with all angles of attack of the model in one run, but also can perform dynamic experiments of the wind tunnel, measure dynamic stability, and use air as the experimental gas (temperature is generally Below 3000 Kelvin) for hypersonic stamping engine experiments.

In addition to the above-mentioned wind tunnels, there are also a hypersonic wind tunnel, a nitrogen wind tunnel, a helium wind tunnel, a gun wind tunnel (light piston wind tunnel), a long punch wind tunnel (heavy piston wind tunnel), a gas piston wind tunnel, and an expansion wind tunnel. And hypersonic Ludwig duct wind tunnel.

Wind tunnel dedicated

In order to meet the needs of various special experiments, various special wind tunnels can also be used. Ice wind tunnels are used to study the phenomenon of local icing on the aircraft surface when the aircraft flies through the clouds. The tail cyclone is used to study the flight characteristics of aircraft tail spin. The experimental section of this kind of wind tunnel is placed vertically, and the air blows up in a dish-shaped speed distribution, and the wind speed can be changed quickly, which can support the tail spin model so that it will not fall.

Wind tunnel

Natural wind tunnel (5 photos)

A natural wind tunnel refers to a natural cave formed by nature. The wind is blown out from the mouth of the cave. The specific location is the wind tunnel on the old house at the source of Xintang Village, Youjia Town, Xinhua County, Hunan Province. Stop the wind. Only the summer will be windy. The wind temperature is very low, only a few degrees. The grass at the entrance of the cave will not grow. People cannot stay at the entrance of the cave for a long time. Otherwise, the whole body will be cold. At night, a humming wind will be heard. Strong, the older generations blocked the entrance of the cave in the 1950s and 1960s, but the wind still opened a hole, but the wind speed was significantly reduced, but the temperature of the wind was not changed. Inside the cave is a cat-like animal with a body-like pattern resembling a zebra. No one has solved the mystery about the formation of the wind tunnel, and it has become a shadow locally with signs of ominousness.

Wind tunnel vertical wind tunnel

In March of Yangchun, the reporter walked into the largest vertical wind tunnel in Asia independently designed and built by China, and experienced the unique scenery in the wind tunnel.

Exposure to artificial "sky"

At the top of Qinling Mountain, there is still a little bit of snow, and there are peach blossoms at the foot of the mountain. As the car drove across a winding mountain road, the vision seemed suddenly bright: among the green mountain forests, a 5-storey building stood up.

"We are here. This is the largest vertical wind tunnel in Asia." The reporter was disappointed to hear the accompanying staff, because the scene in front of him was completely different from what he had imagined. The newly-built vertical wind tunnel is not tall, nor does it look very mighty, even worse than the skyscrapers commonly found in cities.

From the exterior, the only difference from ordinary houses is that the building "carries" a thick iron pipe. A technician explained to the reporter: "You can't look down on this iron guy, it is the main channel for generating airflow."

In fact, the ordinary appearance of the wind tunnel has a magical "heart". Stepping into it, the reporter found that this artificial "sky" was completely stacked with high-tech achievements.

Wind tunnel construction is a multi-disciplinary and inter-professional system integration project, covering more than 20 professional fields including aerodynamics, materials science, and acoustics. The entire vertical wind tunnel took only two and a half years from ground breaking to the first ventilation test, creating a miracle in the history of wind tunnel construction in China.

In the hall, the spiral staircase is surrounded by two huge pipes, so spectacular! It is not so much experimental equipment as avant-garde architectural art.

Along the way, the reporter found that the wind tunnel has a lot of "bright spots": it realized the simultaneous acquisition of test images by two cameras, and the computer automatically interpreted and processed it; it was the first to use the world's most advanced medium-voltage variable frequency speed regulation technology for the control of the wind tunnel main drive system. Speed accuracy increased by 50% ...

The person in charge said that the vertical wind tunnel is the most striking new star in China's huge wind tunnel family, and only a few developed countries have such a wind tunnel.

Feel the charm of "wind"

The wind, no shadow, no trace, extreme freedom. But in the hands of the base's scientific researchers, the ubiquitous wind is combed into a stream of air flow of various rules, various intensities and various "shapes."

The reporter happened to be in a hurry, a free-spin improvement test for a certain aircraft model was being conducted in a vertical wind tunnel.

What is tail spin? It refers to the phenomenon that the aircraft descends rapidly while rotating while continuing to stall. Before people can fully understand it, there are only one consequence of tail spins: machine destruction and death. Data show that from 1966 to 1973, the United States lost hundreds of F-4 aircraft due to a tailspin accident.

In the control center, the watchman turned on the light button and the huge motor started to rotate. The reporter involuntarily covered his ears with both hands in order to withstand the "thunderous roar". Unexpectedly, the loud noise in the imagination did not arrive, only the shallow singing and groaning through the air. 30 meters / second, 50 meters / second ... The wind speed has reached the extreme. The reporter stood next to the test section with good sound insulation, but did not experience the mood of "a strong wind rises".

Do you know what the concept of 50m / s wind speed is? Better than Hurricane! The attendant told reporters that if you put people in the test section, you can experience the feeling of being blown by the wind and flying in the wind.

China's first vertical wind tunnel has formed a strong test capability. The person in charge told the reporter: In addition to completing most of the conventional test projects in the existing horizontal wind tunnel, this type of wind tunnel can also complete the aerodynamic performance evaluation of the aircraft's tail spin, the return satellite and the manned spacecraft recovery process. Wait.

Wind Tunnel China Wind Tunnel Lab

Wind Tunnel China Aerodynamic Development and Research Center

The China Aerodynamic Development and Research Center in the western Sichuan mountainous region is equipped with the largest wind tunnel group in Asia. It has completed more than 500,000 wind tunnel tests and won 1,403 scientific and technological progress achievement awards at all levels. It is China's largest and strongest comprehensive strength. National aerodynamic test, research and development center, its comprehensive test capabilities among the world's leading.

The center has successively built 52 low-speed wind tunnels and sub-, trans-, super-, and hypersonic wind tunnels, and has 8 "world-class" wind tunnel equipment. It has built a computer system with a peak computing speed of 10 trillion times per second. Three methods are available for testing, numerical calculation and model flight test, which can perform from low speed to 24 times the speed of sound, from underwater, ground to 94 kilometers high altitude, covering aerodynamics such as aerodynamics, aerodynamics, aerophysics, aerophysics, etc. test.

China Aerodynamic Development and Research Center has made significant contributions to the development of China's weapons and equipment and the construction of the national economy. From the "J-10", "Jiaolong" fighter aircraft and the "Shenzhou" series of spacecraft to the magnetic levitation and "Harmony" high-speed trains; from the Oriental Pearl Tower up to more than 300 meters to the Hangzhou Bay Bridge across the sea of more than 30 kilometers , All have conducted wind tunnel tests here. So far, the base has accumulated a total of 44 national science and technology achievement awards.

XNJD-3 Southwest Jiaotong University XNJD-3 wind tunnel

In 2008, as the key construction project of the 15th and 211th Southwest Jiaotong University, the construction of the XNJD-3 wind tunnel greatly promoted the development of the wind engineering discipline and civil engineering discipline of the Southwest Jiaotong University. XNJD-3 wind tunnel is currently the world's largest boundary layer wind tunnel. The size of the test section is 22.5m (width) x 4.5m (height) x 36m (length). The cross-section size ranks first in the world. The wind speed range is 1.0 to 16.5. m / s, the main technical indicators have reached the world advanced level.

Wind Tunnel Jilin Wind Tunnel Lab

The Automotive Wind Tunnel Laboratory of Jilin University is the first automotive wind tunnel laboratory in China. It belongs to the Institute of Automotive Aerodynamics, College of Automotive Engineering, Jilin University. The automotive wind tunnel laboratory was established in 1999 and started construction in 2002. The laboratory building was completed and put into use in 2003. The laboratory is currently the only wind tunnel laboratory in China that is positioned to conduct professional automotive aerodynamic test research.

Wind Tunnel Laboratory of Shantou University

The wind tunnel laboratory of Shantou University is the only construction industrial wind tunnel laboratory in ordinary colleges and universities in Guangdong Province. In November 1996, it passed the formal acceptance of a group of experts in the field of structural wind engineering. The main test section of the wind tunnel is 3 meters wide, 2 meters high, and 20 meters long, with a maximum wind speed of 45 meters per second. The laboratory is equipped with the current advanced level of testing equipment, and is one of the earliest research units in the domestic similar wind tunnel to use imported high-speed electronic scanning valves and imported high-frequency bases.

Wind Tunnel Laboratory of Beijing Jiaotong University

The wind tunnel laboratory of Beijing Jiaotong University is a double-return closed-wind tunnel with advanced electronic pressure measurement system, control system and structural vibration measurement system. It belongs to the national 985 Project advantage discipline innovation platform. Can be used for building, bridge pressure and wind environment tests, and other industrial aerodynamic tests. The wind tunnel has a plane size of 41.0m × 18.8m. It will be officially put into use at the end of 2010. The size of the high-speed test section is 3.0m × 2.0m × 15.0m, and the size of the low-speed test section is 5.2m × 2.5m × 14.0m. The maximum test wind speed in the high-speed test section is 40m / s. ^[2]