# How Wind tunnels Work – Blockage factor, wall effects, scaled model, similarity number, moving floor

#### How Wind tunnels Work – Blockage factor, wall effects, scaled model, similarity number, moving floor

For more information, visit https://www.airshaper.com or email [email protected] ---------------------------------------------------------------------------------------- In this video, we explain the huge variations in size, speed and accuracy of low-speed aerodynamic wind tunnels. This is mainly due to variations in: - object size - object velocity - required accuracy Object size When you place an object in the wind tunnel, the air needs to curve around it and fit between the object and the walls of the wind tunnel. This creates 2 problems: - Blockage ratio: around the object, the cross-section through which the air can flow is reduced. This means the air will locally speed up compared to the reference velocity, which is typically set at the undisturbed beginning of the tunnel using a pitot tube. To compensate for this, you can calculate the blockage factor. Typically, it's recommended to keep this value below 5-10%. - Wall effects: a boundary layer of slower moving air is present close to the wall. In reality, this slower moving air is usually not present and can bias the results. Also, the walls can interfere with the flow & pressure patterns around the object if they're too close. In that regard, it can be very interesting to first run a CFD simulation to analyse the flow and see exactly how large this "zone of disturbed air" is to know how far it should form the walls. One example are the vortex structures around wingtips. In some cases, you'll even see wind tunnels employing an open test section, partially to overcome the challenges of blockage ratio & wall effects. Object speed The velocity of the application can vary greatly, with a cyclist going 10 m/s and an airplane 250 m/s. Ideally, you select a wind tunnel with the wind speed that you need. In reality, there will be 2 main constraints: - Budget: an automotive tunnel can easily cost 5.000€ per hour. Building one also costs over 100 million euros in some cases. - Availability: tunnels are often overbooked and sometimes they just don't exist in the size/speed combination that you need. Scaled model testing can help in that case. But keep in mind that you'll need to respect certain similarity numbers - the most important one being the Reynolds number. If you want to keep the Reynolds the number the same when scaling down your model by a factor X (so length_scaled = length_full / X), then you need to scale up the velocity by that same factor X (so velocity_scaled = velocity_full * X). This can make you hit the speed limit or the wind tunnel or even the limit of compressible flow (around Mach 0.3). Accuracy The higher the flow quality or the more realistic the setup needs to be, the more tricks you need to pull, like cooling the air, working with a moving ground or even employing gases other than air at different pressures / temperatures.