1. boundary effect or boundary interference
In real flight, the static atmosphere is borderless. However, in the wind tunnel, the airflow is bounded, and the existence of the boundary limits the boundary.
The nearby streamline bends, which makes the wind tunnel flow field different from the real flight flow field. Its influence is collectively called boundary effect or boundary interference. The way to overcome this problem is to make the test section of the wind tunnel as large as possible (the total size of the wind tunnel is also increased accordingly), and to limit or reduce the scale of the model to reduce the influence of boundary interference. However, this will greatly increase the cost and driving power of the wind tunnel. If the model scale is too small, the Reynolds number will also decrease. In recent years, a technology called self-correcting wind tunnel has been developed. The wall of the wind tunnel test section is elastic and adjustable. During the test, the real shape of the streamline equivalent to the wall is roughly and quickly calculated by computer, so that the wall of the test section is close to it, thus basically eliminating the boundary interference.
2. Support interference
In the wind tunnel experiment, it is necessary to support the model in the airflow with brackets. The existence of support causes interference to the model flow field, which is called support interference. Although the influence of support can be corrected by experimental methods, it is difficult to correct it cleanly. Recently, a technology called magnetic levitation model is being developed. A controllable magnetic field is generated in the test section, and the model is suspended in the airflow by magnetic force.
3. The impact of not meeting the similarity standard
The theoretical basis of wind tunnel experiment is similarity principle. The similarity principle requires that all similarity criteria be met between the wind tunnel flow field and the real flight flow field, or the number of similarity criteria corresponding to the two flow fields is equal. Wind tunnel test is difficult to meet completely. The most common main similarity criterion is that the Reynolds number of subsonic wind tunnel is not enough. Take Boeing 737 as an example. It flies at cruising altitude (9000m) and cruising speed (927km/h) with Reynolds number of 2.4× 107. However, it is only about 1.4× 65438 when tested in the 3m sonic wind tunnel with the wind speed of100 m/s. The methods to improve the Reynolds number of the wind tunnel mainly include:
The scale of (1) model and wind tunnel will increase, and the cost and driving power of wind tunnel will also increase greatly. As mentioned above, Russia's full-scale wind tunnel.
(2) Increase the air density or pressure. There are many pressure-type high Reynolds number wind tunnels, and the working pressure varies from several to ten atmospheres. China is also developing this kind of high Reynolds number wind tunnel.
(3) Reduce the gas temperature. If 90K(- 1830C) nitrogen is used as working medium, the Reynolds number is more than 9 times that of normal temperature air at the same scale and speed. Several low temperature and high Reynolds number wind tunnels have been built in the world. China has also developed a low-temperature wind tunnel, but the scale is still relatively small.