The first wind tunnels were just ducts with fans at one end. These tunnels made choppy, uneven air, so engineers steadily worked to improve airflow by tweaking tunnel layouts. Modern tunnels provide much smoother airflow thanks to a fundamental design that incorporates five basic sections: the settling chamber, contraction cone, test section, diffuser and drive section.
Air is a swirling, chaotic mess as it enters the tunnel. The settling chamber does exactly what its name implies: It helps to settle and straighten the air, often through the use of panels with honeycomb-shaped holes or even a mesh screen. The air is then immediately forced through the contraction cone, a constricted space that greatly increases airflow velocity.
Engineers place their scaled models in the test section, which is where sensors record data and scientists make visual observations. The air subsequently flows into the diffuser, which has a conical shape that widens, and thus, smoothly slows the air's velocity without causing turbulence in the test section.
The drive section houses the axial fan that creates high-speed airflow. This fan is always placed downstream of the test section, at the end of the tunnel, rather than at the entrance. This setup allows the fan to pull air into a smooth stream instead of pushing it, which would result in much choppier airflow.
Most wind tunnels are just long, straight boxes, or open-circuit (open-return) tunnels. However, some are built in closed circuits (or closed return), which are basically ovals that send the air around and around the same path, like a racetrack, using vanes and honeycomb panels to precisely guide and direct the flow.
The walls of the tunnel are exceedingly smooth because any imperfections could act as speed bumps and cause turbulence. Most wind tunnels are also moderately sized and small enough to fit into a university science lab, which means that test objects must be scaled down to fit into the tunnel. These scale models might be entire airplanes in miniature, built (at great expense) with exacting precision. Or they might just be a single part of an airplane wing or other product.
Engineers mount models into the test section using different methods, but usually, the models are kept stationary using wires or metal poles, which are placed behind the model to avoid causing disruptions in the airflow They may attach sensors to the model that record wind velocity, temperature, air pressure and other variables.
Keep reading to learn more about how wind tunnels help scientists piece together more complicated aerodynamics puzzles and how their findings spur technological advances.