Wind tunnel: the cradle of aircraft ■ Zhang Yuan, Zhou Zhiyu, Liu Jie According to Xinhua news agency, on August 22 this year, the Chinese Academy of Sciences publicly disclosed for the first time the construction of jf-22 high overspeed shock wind tunnel and the good news that it is expected to be completed in 2022. So, what is a wind tunnel? What is the relationship between the development of cutting-edge aircraft and wind tunnel? What impact will wind tunnel have on a country's aerospace industry? See the interpretation. The above figure is a schematic diagram of the wind tunnel. Integrate high-end technology If you think the wind tunnel is a hole, you are wrong. Wind tunnels are usually long coiled thick pipes. According to the information publicly disclosed by the Chinese Academy of Sciences, the main body of jf-12 shock tunnel, the previous generation of jf-22 high overspeed shock tunnel, is a long metal pipe above the ground, with a length of 265 meters, ranking first in the length of shock tunnel in the world. In the pipeline, airflow with different flow rates, different densities and even different temperatures is generated by manual control, that is, wind. Whether it's a flying bird or a roaring train, usually an object is moving. Let's observe the interaction between it and the air. The wind tunnel goes the opposite way, fixing the object and blowing it to simulate the airflow movement in the natural environment. In this way, a special space can be formed on the ground, and the object still presents the motion effect when it is not moving. Wind tunnel has become the most widely used tool for aerodynamic research and test. Researchers blow the model or object repeatedly in the wind tunnel to observe and measure the effects of different airflow on it, so as to provide scheme basis for its structural design and performance improvement. The principle of wind tunnel sounds simple, but it is not easy to build a wind tunnel. It involves more than 20 professional fields such as aerodynamics, materials science, electromechanical, acoustics and so on. For example, the wind tunnel can be divided into low-speed wind tunnel, high-speed wind tunnel and hypersonic wind tunnel according to the different air velocity. Many countries have low-speed wind tunnels, but it is very difficult to reach supersonic or even hypersonic speeds. Another example is the simulation problem. The wind comes and goes without trace, but in the wind tunnel, how can the free wind be accurately controlled into regular and "shaped" airflow? How to simulate unbounded nature in a bounded wind tunnel? How to reduce the influence of additional experimental equipment such as support? This requires the integration of power, power, electromechanical, control, measurement, model making and other technologies. Blow out sophisticated weapons In real life, wind tunnel plays a role in many aspects. Large wind power generation fan blades, high-speed trains, television towers up to hundreds of meters, sea crossing bridges of tens of kilometers, etc. their models must be "blown" in the wind tunnel before construction. At this year's Tokyo Olympic Games, China's athletes also contributed to the low-speed wind tunnel laboratory in winning gold medals in swimming, rowing and other events. In the military field, large high-end wind tunnels are an important strategic resource of a country. Fighter planes, spacecraft, ships, rockets, missiles, man-made satellites, etc. must pass the wind tunnel in the process of R & D. Therefore, the wind tunnel is also called "the cradle of aircraft". In the process of developing a new aircraft, researchers use wind tunnel experiments to understand the aircraft structure, obtain complete aerodynamic performance data, calculate kinematic performance and verify aerodynamic layout, and then carry out follow-up work such as improving shape design, reducing drag and noise, and strengthening equipment performance, so as to achieve twice the result with half the effort. In 1900, the Wright brothers built wind tunnels twice to carry out experiments, and the world's first aircraft came out. Today, high mobility weapons have to undergo thousands of hours of wind tunnel experiments to "refine into steel". The U.S. F-22 Raptor fighter has experienced 10 years of wind tunnel experiments. Its 23 models have conducted 75 high-speed and low-speed wind tunnel experiments in 15 wind tunnels for a total of about 44000 hours before finally determining the aerodynamic shape. Industry insiders said that without advanced supporting wind tunnels, it is difficult to develop cross generation weapons. "Dongkai" new future Near space usually refers to the airspace 20 ~ 100km away from the ground, which is between the highest flight altitude of aircraft and the lowest flight altitude of space orbiter. Militarily, this space is a new highland for integrated air and space operations, which is of great strategic significance. After entering the supersonic era, space hypersonic flight has become a necessary option for the development of international aerospace field. Mach number is the ratio of velocity to sound speed. When the ratio reaches 5 or 6, hypersonic speed is considered to be reached. How fast is this? For example, a hypersonic aircraft flying at Mach 5 to Mach 10 can reach anywhere in the world in an hour or two. The world's martial arts, only fast not broken. The development of hypersonic vehicles will lead to the era of rapid and accurate strike weapons. Hypersonic weapons have incomparable advantages over traditional weapons, such as global fast arrival, long-distance precision attack, difficult interception and great power, just like fast, accurate and ruthless sharp knives. In recent years, world powers have stepped up the layout of "first hand chess" in the field of hypersonic technology. Such as Russian "dagger" and "zircon" hypersonic missiles, as well as American tactical boost gliding weapons and hypersonic cruise missiles. However, the research and development of high dynamic near space vehicle has been for decades, and there are still few successful cases so far. The main reason is that the flight environment in near space is very different from that in troposphere and stratosphere. In this process, complex effects such as "molecular vibration excitation", "real gas effect" and "rarefied gas effect" are easy to occur. At the same time, as the speed of the aircraft accelerates, the temperature rises sharply after friction with the air, and complex thermochemical processes occur in the surrounding air to form an ionic state. The aircraft will be like swimming in the mud. The new complex phenomenon caused by hypersonic flight subverts the similarity simulation criterion of traditional wind tunnel experiments. Therefore, a new generation of wind tunnel, which can reflect the ability of "unique thermochemical reaction mechanism of hypersonic flow", was born in the urgent call. At present, the United States, Russia, Europe, Japan and other aerospace powers have built different types of hypersonic wind tunnels, providing powerful simulation means for various launch vehicles, space vehicles, aerospace aircraft, hypersonic vehicles, hypersonic weapons, medium and long-range ballistic missiles, etc. (outlook new era)