Tufts are small lengths of cotton or wool which are used to determine the flow direction. These are a simple and cheap way to evaluate qualitatively what the flow is doing around a model placed in a wind tunnel. Generally, there are two types of tufts.

Type 1: Tufts are attached directly to the model using tape or a small dab of glue

Type 2: Tufts attached to some other device or structure which is inserted into the wind tunnel

To the left are two videos. The top video shows type 1 tufts on a wing reacting to an upstream obstruction. The tufts begin to "dance" when in the recirculation zone behind the hand. This behaviour is also observed when the wing is completely stalled. The bottom video shows a type 2 tuft attached to a rod. Trod can place the tuft anywhere the user wishes, which is useful for traversing an experiment. In this case, the tuft begins to swirl once it enters the region of separated flow due to the leading edge vortex system of the aircraft. Unfortunately while the tuft is swirling, it is very difficult to see. Another type 2 tuft arrangement is a grid where tufts are attached to a mesh type structure and placed downstream of the model or in a region of interest.

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The following considerations need to be made when installing tufts:

1. Make sure that the material of the tuft you are using is soft and not rigid. You want to be able to see the tuft behave naturally in the airflow

2. Tufts should generally only be up to a few centimetres long on most models. Keep in mind that the length of the tuft will also determine the spanwise and chordwise spacing available. Once tufts begin to move, you want to avoid having them tangle with each other

3. When attaching the tufts try to be minimally invasive with the glue or tape. You do not want to be observing the effect of the flow due to the tape or glue, but rather from the shape of the model.

As with all other wind tunnel tests, a test matrix should be determined prior to the tunnel campaign. The matrix should include the flow conditions, model orientation etc. As tufts do not require data to be recorded, a method of recording observations needs to be made. This can be in the form of writing notes or taking photos. The test procedure is ver simple:

1. Orient the model/set up the model for a test

2. Note observations

3. Proceed to next test

The images to the left show tufts used for examining the lip separation for an intake at 10 degrees AoA (top) and 15 degrees AoA (bottom). The severity of the intake separation is seen to increase with AoA.