Pressure is measured by small devices called pressure taps that are placed at various locations on the surface of the model. Forces are recorded by sensors in the structures that support the model in the test section. The direction that air flows around the model can be seen by the way tufts , small yarn-like strands attached to the model, flap around. Sometimes smoke is blown into the test section to make it easier to see how the air is flowing. From these different kinds of measurements, a great deal can be learned about the model being tested.
Wind tunnels vary in size according to their function.
Some of the smallest wind tunnels have test sections that are only a few inches large and therefore can only be used with tiny models. Other wind tunnels have test sections that are several feet big. The largest wind tunnel in the world is at the National Full-Scale Aerodynamics Complex at NASA Ames Research Center . Wind tunnels aren't just used to test airplanes. Anything that has air blowing around or past it can be tested in a wind tunnel.