Smoke and Tuft Techniques:
Aerodynamicists use wind tunnels to test models of proposed aircraft and engine components. During a test, the model is placed in the test section of the tunnel and air is made to flow past the model. In some wind tunnel tests, the aerodynamic forces on the model are measured. In some wind tunnel tests, the model is instrumented to provide diagnostic information about the flow of air around the model. In some wind tunnel tests, flow visualization techniques are used to provide diagnostic information about the flow around the model. Two of the oldest flow visualization techniques are the use of smoke and tufting.
The figure shows smoke flow and tufts being used on the NASA Dryden F-18 flight vehicle, but the techniques are used more often in wind tunnel testing. Smoke is used to visualize the flow that is away from the surface of the model. Smoke can be used to detect vortices and regions of separated flow. On the figure, smoke has been introduced at the corner of the fuselage and leading edge extension (LEX) to visualize the vortex generated by the LEX at angle of attack. In the picture we see that vortex is well established until the flow encounters the vertical stabilizer of the aircraft. Smoke has the advantage that is relatively inexpensive to produce. Smoke can be injected from the surface or dispersed with a hollow wand that can be moved through the flow field. The disadvantage of smoke is that it does not work well at higher speeds (greater than ~300 mph), the smoke must be introduced at the proper location without altering the flow, and the smoke can leave a residue in the tunnel or on the model, depending on the type of smoke employed.
Here’s a picture of a paper airplane in a small smoke tunnel. You can see the smoke swirl as it passes the leading edge of the wing on the far side of the tunnel. You can watch a movie of this experiment which shows the motion of the smoke over the wing. Thanks to Dwayne Hunt for the production of the movie clip.
Chemical methods for producing smoke include titanium tetrachloride and tin tetrachloride which re-act with damp air. However, both materials are corrosive. Anhydrous ammonia and hydrogen sulphide produce smoke, but they also produce odours and, with damp air, sulphuric acid. Steam and liquid nitrogen produce dense smoke with no ill effects. Light oils can also be burned to produce smoke with some residue.
Tufts are another old visualization technique that is used in both flight test and wind tunnel testing. Tufts are small lengths of string that are frayed on the ends. Popular materials for tufts include monofilament nylon, and polyester or cotton No. 60sewing threads. Tufts may be coated with fluorescent dyes to increase visibility for photography. The tufts are attached to the surface of the model using some adhesive such as tape or glue, and as the air flows over the model, the tufts are blown and point downstream. If the entire model is tufted, as shown on the photo, then regions of strong cross-flow, reverse flow, or flow separation are indicated by the direction of the tufts. Tufts can also indicate regions of unsteady flow when recorded by film or video.
Tufts are relatively cheap to produce, although they require time to apply to the model and must be firmly secured so that they are not blown off the model. Tufts must be cut to the proper length and weight so that they move with the flow, but do not alter the flow. Surface tufts only provide information about the surface flows in the lowest part of the boundary layer. Interpreting surface patterns to visualize the free stream flow features takes some skill and experience. Tufts can be mounted on wands to visualize vortices.
Dye can be used to mark and visualize particular regions of flow or individual fluid streamlines. To mark streamlines adjacent to a test body, dye is injected from small ports on the surface of the object.
Image A. from Flometrics, Solana Beach, Image B. contribute by Marco Ghisalberti. Tracer plume reveals flow structure above submerged canopy.
To mark streamlines within the fluid, dye can be released from a thin needle aligned to the local flow. In the latter case care must be taken to minimize disruption to the existing flow field. For example, the injection velocity should match the local velocity. The injection velocity can be controlled either with a syringe pump or a constant head reservoir, such as the Mariotte bottle described in the figure below.
Figure 1: A Mariotte bottle delivers tracer at constant rate, even as the reservoir is depleted. The bottle is sealed except for a vent tube. As the reservoir empties, air bubbles are drawn into the bottle to equalize the pressure. The pressure at the tip of the vent tube is maintained at atmospheric pressure. Evaluating Bernoulli’s Equation between the tip of the vent tube and the point of dye injection into the flume gives the velocity of the injected dye,
To ensure that the tracer faithfully follows the undisturbed flow field, the density of the tracer must match that of the experimental fluid. For example, consider a water channel experiment. Most dyes are heavier than water and must be mixed with alcohol to achieve neutral density. Remember that density is also a function of temperature, such that a neutrally buoyant dye prepared for a tank at room temperature will not be neutrally buoyant if the tank is refilled with fresh colder water. Finally, you will note that in most of the examples given below a blue dye is viewed against a white backdrop, because this offers the best contrast.
Dye Injection Examples: