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Smoking out the Vortex

By Bruce Gilbaugh

August 25, 1999

On this test we are trying to visualize turbulent air, called a vortex, which creates both good and bad air for aircraft. Vortices can be good, and engineers can use them to help create the lift that airplanes need to stay in the air. But they can also be bad and cause parts of the aircraft to fail. If the vortex that is created goes into the engine it can cause the engine to stop operating, and if it beats on a part like the vertical tail it can fall off.

To visualize the vortex for this test we will inject smoke into the wind tunnel while it is running. Burning pharmaceutical grade mineral oil makes smoke with very fine particles. As the fine particles go across the model some of them will be drawn into the vortices that exist. We may not be able to see these with the naked eye. We are going to take a very thin sheet of light created by a laser and shine it across the model. The reflection of light by the smoke particles will be captured with solid state cameras that can see the laser light. Then we can see the size, shape, and location of the vortex. The thin sheet of light gives a cross section of the vortex, and then we will sweep the model to get lots of cross section images.

The camera is a solid state charge coupled device. They are only black and white and are used in low light situations, similar to surveillance cameras. They are sensitive to infrared light. The laser will project infrared light that uses optical components to create a thin sheet of light. The video is a standard NTSC format. The resulting data is the video record and can be viewed on video recorders.

In this photo taken at NASA Dryden Flight Research Center in 1989 you can see the F-18 HARV smoke and tuft flow visualization. The angle of attack here is 20 degrees.
Here you can see vortex visualization being done in a wind tunnel for an FA-117 test. This picture was taken in the 40x80 wind tunnel at NASA's Full-scale Aeronautics Complex at Ames Research Center. Smoke is introduced into the airstream and appears brown in color at the left of the nose area. An argon-ion laser producing a green sheet of light is used to illuminate the vortex as the smoke is entrained in it.
A second picture which I've included shows a "slice" of the vortex further back on the wing where it is joined by a second vortex being shed on top the wing (to the left in the picture) near the right hand outboard wing tip. In the first photo tunnel lighting has been turned off to provide a dark visualization area. In this second photo tunnel lighting has been left on and you can see the vortex beginning to burst behind the green vortex slice. Notice also that it appears that the vortex is headed right for the vertical control surface or tail. This is the sort of thing design engineers worry about because it could cause premature failure. Testing like this helps them control the vortecies. Go to this website for details on the second picture. http://ails.arc.nasa.gov/Images/WindTunnels/AC94-0480-29.html

This is experimental because we have not used the very small smoke particles in this tunnel before. We have used water vapor before, but water has much larger particles. My customer for this data is Mina Cappuccio, and the Boeing Aircraft researchers will be interested in this data too.

The canard on this model is a vortex generator. This data will show where the generated vortex goes. We have also done a video with pink string. The length and size of the string can impact the results with that technique. Smoke has an advantage in that it has less impact on the off body flow field.

 
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