The wind tunnel test in fluid mechanics refers to an aerodynamic experiment method in which an aircraft or other object model is placed in a wind tunnel to study the gas flow and its interaction with the model to understand the aerodynamic characteristics of the actual aircraft or other objects;

Test classification

Aerodynamic experiments are divided into two categories: physical experiments and model experiments. Physical experiments, such as aircraft flight experiments and live missile launch experiments, will not cause simulation distortion problems such as models and environments, and have always been the ultimate means of identifying the aerodynamic performance of aircraft and calibrating other experimental results. Such experiments are expensive and difficult to control. , and it is impossible to carry out in the initial stage of product development, so the aerodynamic experiments are generally multi-fingered model experiments. Aerodynamic experiments can be divided into 3 categories according to the way in which the air (or other gas) and the model (or real object) produce relative motion: 1. The air moves and the model does not move, such as wind tunnel experiments. ②The air is still, and the object or model is moving, such as flight experiment, model free flight experiment (experiment with powered or unpowered aircraft model flying in the air), rocket sled experiment (carried by a rocket-propelled sled that travels at high speed on the track model for experiment), jib experiment (the jib machine carries the model to rotate and conduct experiments), etc. ③ Both the air and the model are moving, such as the free flight experiment in the wind tunnel (the experiment is carried out relative to the wind tunnel airflow projection model), and the spin experiment (the model is put into the updraft of the tail cyclone wind tunnel, and the experiment is carried out to make it enter the spin state) Wait. When carrying out model experiments, the similarity between the model flow field and the real flow field should be ensured, that is, in addition to ensuring that the model is geometrically similar to the real object, the similarity criteria related to the two flow fields, such as Reynolds number, Mach number, general The Ronte number and so on correspond to the same (see Hydrodynamics Similarity Criteria).
 
In fact, under the conditions of general model experiments (such as wind tunnel experiments), it is difficult to ensure that all these similarity criteria are equal, and the main similarity criteria can only be made equal or reach the self-calibration range according to specific circumstances. For example, experiments involving viscosity or resistance should have equal Reynolds numbers; for experiments involving compressible flow, Mach numbers must be equal, and so on. The experimental error caused by the equality of the similarity criteria, which should be satisfied but not satisfied, can sometimes be eliminated by data correction, such as Reynolds number correction. The disturbance of the flow field by the cave walls and model supports should also be corrected. Aerodynamic experiments mainly measure airflow parameters, observe flow phenomena and states, and determine the aerodynamic forces acting on the model. The experimental results are generally organized into dimensionless similarity numbers in order to generalize from the model to the real thing.

Wind tunnels are generally referred to as wind tunnel tests. Simply put, according to the principle of relativity of motion, the model or object of the aircraft is fixed in the artificial environment on the ground, and the airflow is artificially created to simulate various complex flight states in the air and obtain test data. This is the "green channel" for the development and production of modern aircraft, missiles and rockets. Simply put, a wind tunnel is to artificially create a "sky" on the ground. As for why our country's wind tunnels are built deep in the mountains, it is due to historical reasons.
In the wind tunnel test, when the model aerodynamic force measured by the balance is converted to the airflow coordinate system, the model aerodynamic coefficient error will be introduced due to the error of the model angle of attack measurement, and this error can account for the total aerodynamic coefficient under some conditions. 25% of the error. Therefore, accurate angle of attack measurement technology is the basis for obtaining high-precision aerodynamic characteristic test data. The advanced index of wind tunnel test data accuracy requires that the drag coefficient error of the model does not exceed 0.0001 when the Mach number Ma is in the range of 0.4 to 0.9, which requires that the measurement error of the model angle of attack cannot exceed 0.01°.

Test advantages 
Although the wind tunnel experiment has limitations, it has the following four advantages: 1. It can control the experimental conditions more accurately, such as airflow speed, pressure, temperature, etc.; 2. It is more convenient to install, operate and use the test instrument; ③The experimental items and contents are varied, and the accuracy of the experimental results is high; ④The experiment is relatively safe, and has high efficiency and low cost. Therefore, wind tunnel experiments are widely used in the research of aerodynamics, the development of various aircraft, as well as in industrial aerodynamics and other fields related to airflow or wind.