6.1.1 Wool or silk tufts
Wool, silk or nylon tufts 5 f 15 mm long are glued at one end on the surface of the model and the other end is left free. If the position of the tufts has to be often changed, scotch tape can be used to fix the tufts to the model. The presence of the tufts can be a trigger for a premature transition so the tufts should be placed gradually from the trailing edge, checking whether the inclusion of new rows of tufts affects the downstream conditions.
If the tufts are short enough not to interfere with the external field, it is easy to interpret their behavior: the presence of a turbulent boundary layer is highlighted by a lively movement of the tufts, the separation of the boundary layer is highlighted by the tendency of the tufts to move violently and to rise from the wall pointing upstream (Figure 6.1 b).
Vortices downstream of a delta wing visualized with tufts mounted on a grid across the test chamber
In order to visualize the external flow, the tufts must be supported by a grid perpendicular to the stream: in Figure 6.2, the free vortices downstream of a delta wing at incidence are shown.