How Wind tunnels Work – Contraction, Test Section, Diffuser, Fan, Turning Vanes and Settling Chamber

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In this first video of our series on wind tunnels, we’ll discuss the basic layout.

History
Wind tunnels were invented in the late 19th century and even the Wright Brothers used a simple wind tunnel to study different shapes for their Wright Flyer, the first plane. The principle of a wind tunnel is quite simple: Use a single or multiple fans to force air across an object to study its aerodynamic properties.

Layout
In an open return tunnel, the air is drawn from the environment in which the tunnel is located. This type of wind tunnel is typically cheaper to construct and doesn’t suffer too much from smoke or exhaust gases accumulating inside. It is however quite expensive to operate, as it constantly needs to accelerate “new” air from standstill. It’s also more challenging to achieve a good flow quality as the air needs to curve into the inlet. And the open exit makes it quite noisy as well.

Then there’s the closed return tunnel, in which the exit air is fed back to the inlet. This design is much more energy efficient as the fan only needs to overcome the losses and it’s quieter too and it has the potential for a higher flow quality. Downsides are a higher construction cost and because the air circulates, it can heat up quite a lot and smoke & exhaust gases can accumulate.

Goals
Whichever return type you go for, it’s very important to have good flow characteristics at the test section. Typically, the goal is to have a low turbulence intensity, a uniform flow velocity and a thin boundary layer.

Components
Let’s have a look at how the typical components of a tunnel are set up to achieve this: first, the fan pushes the air forward. Then, in the corners, turning vanes are added to help straighten the flow and reduce pressure losses.

After the last corner, the air hits the settling chamber, where it first passes through a honeycomb structure. This is basically a set of tubes to force the air to move in a parallel way to reduce the swirl. After exiting the honeycomb, the air goes through a number of screens, which are composed of thin wires interwoven to form square or rectangular meshes. These screens impose a static pressure drop which is proportional to the velocity squared, penalizing speeds above average and boosting speeds below average. This helps to reduce variations in longitudinal flow velocity. They also break up large eddeys into smaller ones, which decay faster.
To reduce the pressure drop in the entire circuit and settling chamber, the cross-section is quite large and thus the flow velocity relatively low. To speed up the air and further improve flow uniformity, after the settling chamber the air is then sent through a contraction just before it enters the test section. After the test section, it is slowed down again in the expanding diffuser before it enters the fan again.
So that was it for our first video on how a wind tunnel works. Stay tuned for more as we discuss sizing challenges and measurement techniques!

Видео How Wind tunnels Work – Contraction, Test Section, Diffuser, Fan, Turning Vanes and Settling Chamber канала AirShaper