Talking about what something isn’t is kind of a terrible way to define it. So let’s take a step back and talk about what a wind tunnel is.
In its most basic form, a wind tunnel is a testing facility with two critical components:
- A mechanism for moving air at a specific, known speed, and
- An enclosed space through which to accelerate that air, where the item being tested is located
For many tunnels the mechanism to move air is a giant propeller (often called a fan). The fan pulls air through the actual tunnel portion of the wind tunnel.
The segment where the item under test—or “test article”—is located is called the test section. This is the region we care most about, because whatever enters the test section is the flow conditions that our test article will actually experience.
There are two main types of tunnels:
- Open-circuit tunnels are essentially just a test section with a fan behind the test article. Air is pulled into the test section, flows over the test article, and then is pushed out the back by the fan.
- Closed-circuit tunnels form a loop that keeps the circulated air captive in the system. Quite often these are a rectangular loop with the fan on the opposite side of the loop from the test section, to reduce how much the spiraling airflow interacts with the test article.
Most tunnels are closed-circuit because it’s more efficient—an open-circuit tunnel has to accelerate “fresh” airflow the entire time it’s running, while closed-circuit tunnels are just pushing around air that’s already got some momentum. You can only get so fast with an open-circuit tunnel due to this, so often they are built small and used as educational and research tools at universities.
Another benefit of closed-circuit tunnels is they let you control more variables during your testing because the entire circuit becomes its own isolated environment.
Some tunnels can partially depressurize the air inside to simulate flying at higher altitudes, where air density (and thus pressure) are lower.
Some tunnels can control the air temperature and bring it down to near-freezing temps! This lets us test the effect of ice buildup on wings and other surfaces. Ice on aircraft significantly changes the aerodynamics of the vehicle, almost exclusively for the worse, so this sort of testing is incredibly valuable for keeping air transportation safe.
There are all sorts of other unique tunnels too, for testing as many cases and solving as many problems as we can possibly come up with.
As cool as all these different testing options are though, how do they actually help us? What does testing in a wind tunnel give us that’s so great?
The short answer: lots and lots of data.