Smoke is entered into the wind tunnel, in front of the test object or the part of it that want to be studied, to make the air flow visible to the eye.
A smoke wand, hand held or attached to some traversing system, is generally used. Smoke can be emitted from only one point that can be arbitrary moved in the wind tunnel or from a "fork", giving a 2D or 3D pattern of smoke trails across the model.
Using a sheet laser together with the smoke, cross sections of the smoke trail can be illuminated to give a good picture of the structure of a vortex or even an estimate of the size of a separation bubble for example.
The optical access in STARS low speed tunnels; LT1 and LT5 is excellent thanks to the large windows, which makes it easy to observe and record the visualization.