In the aviation field, the term tuft refers to a strip of yarn or string of varying length (typically ~6" long) attached to an aircraft surface in a grid pattern and imaged during flight. The motion of these tufts during flight can be observed and recorded in order to locate flow features such as boundary layer separation and reattachment. Tufting can be considered a technique for flow visualization, used in aeronautics flight testing to study air flow direction, strength, and boundary layer properties.
The world's largest bed of tufts (61 feet by 61 feet) was created at NASA Ames Research Center to study air flow fields involving a helicopter's rotor disk.