When NASA Langley Research Center engineers wanted to put one of their aircraft models through a wind tunnel test, they needed a way to visualize how wind flowed over the design. So they sprayed fluorescent oil onto the model, which is 5.8 percent of the final craft's size. You can see the results here.
The tests took place at Langley Research Center's Subsonic Wind Tunnel, which is used for testing aircraft that can take off and land without long runways. The tunnel works well for testing stability, aerodynamics, acoustics and basic airflow, according a NASA description.