What Is a Wind Tunnel?
A wind tunnel is a test facility for measuring and analyzing the forces acting on a fixed model of a building, airplane, automobile, etc., and the flow of wind around the model. By sending an airflow around a model, it is possible to simulate flight conditions for airplanes and driving conditions for automobiles. By matching the Reynolds number, even if the model is smaller than the actual aircraft, it is possible to obtain almost the same experimental results as actual flight or driving.
Wind tunnels come in all sizes: In the test facility of JAXA (Japan’s national air and space agency), there is a low-speed wind tunnel, that is the largest in the aerospace field in Japan, measuring 5 to 6 meters in length and width, where the model is fixed. In the United States, there is a huge wind tunnel measuring about 24 meters in length and 37 meters in width.
Uses of Wind Tunnels
Uses of wind tunnels are extremely wide-ranging, including the use of measurement data for aircraft and rockets, for which fluid design is important, as well as for automobiles, railroads, and the design of high-rise buildings and bridges, for which consideration of wind effects is important.
In addition to basic measurements of forces acting on the model, such as lift and drag, and pressure on the model surface, wind tunnel experiments also utilize particle image velocimetry (PIV) to visualize airflow.
Wind tunnels consist of a blower, a nozzle section, a rectifying plate, a measurement section, and a diffuser section. The same is true for flow visualization using PIV and other methods.
The Principle of Wind Tunnels
The Principle of Wind tunnels is to predict the real-world wind (fluid) flow by measuring the actual wind flow by changing a large object to be analyzed into a small, similar-shaped model, subjecting it to actual wind and matching the Reynolds number under the right conditions. Wind tunnels are experimental facilities that use Reynolds’ law to measure and analyze the fluid effects of actual ambient wind.
By matching the Reynolds number Re, the flow of ambient fluid is equal when the geometry of the actual machine and the model are similar, which is called Reynolds’ law in fluid mechanics. The Reynolds number Re can be calculated by the following equation.
The inertia of the momentum of the entire fluid (velocity x length) ÷ physical quantity calculated by kinematic viscosity (dimensionless quantity)
For example, considering the case where a precise model of a car running is made 1/10th the size of the actual car, Reynolds’ law of similarity can be satisfied if the wind tunnel wind speed is set to 10 times the actual running speed. However, since kinematic viscosity varies with temperature, it is also important to adjust the temperature to match the kinematic viscosity of the actual run with the wind tunnel speed.
Types of Wind Tunnels
Wind tunnels can be broadly classified into two types of structures
1. Simple Blowout Type
The simple blowout type, also called an Eiffel-type wind tunnel, has the advantages of a simple structure and small installation space but has the disadvantage of requiring a large amount of power to provide airflow.
2. Circulating Flow Type
The circulatory flow type requires less power to generate air velocity and the flow is more stable, but it has the disadvantage that the temperature of the airflow rises significantly. The device itself tends to be large in scale. The Goettingen-type Wind tunnel is a well-known example.
More Information on Wind Tunnels
Application of CFD
CFD (Computational Fluid Dynamics) technology, which uses simulations to predict the results of wind tunnel tests, has evolved rapidly in recent years. Wind tunnels are used for scaled-down models compared to actual test rigs and buildings, but the prototyping cost and man-hours involved are still expensive.
CFD, on the other hand, involves the cost of installing PCs and software, but the subsequent operating costs can be reduced compared to Wind tunnel testing. However, to reach a level where CFD data alone is sufficient for designing without wind tunnel testing, it is essential to accumulate data and determine detailed parameters. The complementary relationship between CFD and Wind tunnel data verification is improving design accuracy and reducing man-hour costs at an ever-increasing pace.