Smoke on the Airstream
Lift and drag are just two elements of aerodynamics forces that come into play inside a wind tunnel. For aircraft testing in particular, there are dozens of variables (like pitch, yaw, roll and many others), that can affect the outcome of experiments.
Other factors also come into play during testing no matter what the test subject might be. For example, the quality of the air in the tunnel is changeable and has a tremendous bearing on test results. In addition to carefully gauging the shape and speed of the object (or the wind blowing past the object) testers must consider the viscosity (or tackiness) and compressibility (bounciness) of the air during their experiments.
You don't normally think of air as a sticky substance, of course, but as air moves over an object, its molecules strike its surface and cling to it, if only for an instant. This creates a boundary layer, a layer of air next to the object that affects airflow, just as the object itself does. Altitude, temperature, and other variables can affect viscosity and compressibility, which in turn changes the boundary layer properties and drag, and the aerodynamics of the test object as a whole.
Figuring out just how all these conditions affect the test object requires a system of sensors and computers for logging sensor data. Pitot tubes are used to measure airflow velocity, but advanced tunnels deploy laser anemometers that detect wind speed by "seeing" airborne particles in the airstream. Pressure probes monitor air pressure and water vapor pressure sensors track humidity.
In addition to sensors, visual observations are also extremely useful, but to make airflow visible, scientists rely on various flow visualization techniques. They may fill the test section with colored smoke or a fine mist of liquid, such as water, to see how air moves over the model. They may apply thick, colored oils to the model to see how the wind pushes the oil along the model's surface.
High-speed video cameras may record the smoke or oils as they move to help scientists detect clues that aren't obvious to the unaided eye. In some cases, lasers are used to illuminate mist or smoke and reveal airflow details.
Wind tunnels offer endless configurations for testing limitless ideas and concepts. Keep reading, and you'll see the wildly imaginative tunnels that engineers build when they find the money to turn a breeze of an idea into a full-scale technological tempest.