Wool tufts are the cheapest flow visualisation there is
What tufts show, what they hide, and how to read a photograph of them.
Wool tufts are short lengths of yarn taped in a grid across a car's bodywork, and because each one is light enough to be pushed around by the air passing over it, a photograph of the whole grid taken while the car is moving shows, almost for free, which parts of the surface the airflow is following smoothly and which parts it has already abandoned.
Taping wool all over a car and photographing which strands lie flat is the cheapest wind tunnel anybody has, and the first time I saw a whole afternoon's worth of aerodynamic argument settled by a single blurry photograph, I stopped thinking of flow visualisation as something only a properly funded team could afford.
Streaming flat or flickering
Where the airflow is attached, following the body's shape and moving in one steady direction, a tuft streams flat and still along that direction. Where the flow has separated into the disorganised, swirling wake described elsewhere in this set, the tuft has no steady push to settle into, so it flickers, twitches or stands up off the surface, sometimes reversing direction as the air around it changes from moment to moment. It measures nothing numerically; its motion simply reveals whether the air just above it has a clear direction to follow. The yarn is chosen to be light and soft enough for a modest airflow to push it around, but heavy enough to ignore stray gusts that have nothing to do with the car.
A ribbon tied to a fence in a steady breeze shows the same thing. In open air it streams out flat and barely trembles, while the same ribbon tied downwind of a building corner or a row of trees twists, flaps and reverses with no settled pattern. A grid of tufts shows that same distinction across a whole panel at once.
A whole surface in one photograph
Because the grid can be photographed in a single pass, a team can survey an entire wing or the full length of a floor's edge for separation in one test run. For a small team without a wind tunnel or serious computing capacity, a rough but informative answer about where a shape is losing the flow comes from little more than yarn, tape and a camera with a clear view of the surface. Needing no calibration or booking, tufts stay a first resort even for teams that later gain better tools. Photographs from before and after a change show directly whether the modification delayed separation or just moved it elsewhere on the same surface.
Attachment is only part of the answer
A tuft shows whether the flow at the surface is attached, and nothing about how much force the surface is generating. It also cannot tell smooth laminar flow from a turbulent layer that is still attached, since both hold it flat, and it sees nothing of the flow a short distance out. Two designs can therefore look identical in a tuft photograph while producing different downforce or drag. A tuft test is a check for one specific failure, separation, and questions about force need some other measurement, even a simple scale under a wheel. A shape can give up a little tidy, attached flow in one place for more downforce elsewhere, so the tufts are one input to a decision and should not settle it alone.