Oil Flow Image of Separation Bubble Courtesy of Bryan D. McGranahan and Prof. Michael S. Selig (UIUC)

Smoke Flow Image of a Separation Bubble Courtesy of Greg Cole and Prof. Mueller (University of Notre Dame)

Background

Boundary layer flow over most wing surfaces normally transitions from laminar to turbulent at some point. This transition is often accompanied by a separation of the laminar flow from the wing surface and a reattachment later of turbulent flow that remains attached better than laminar flow, but increases drag. This separation bubble adds to the effective thickness of the wing, adding to the form drag of the wing if the bubble is tall enough.

The images on the right beautifully illustrate the structure of a classic separation bubble. The green image is of a wing surface, illuminated with a fluorescent oil mist, that reveals how the flow moves in contact with the surface. The black and white image is a side view of a bubble that clearly shows the features mentioned above.

Turbulator tapes are often used to eliminate the separation bubble, and the additional form drag it causes, by tripping the laminar flow to turbulent flow at some point before the separation becomes high enough to cause significant additional form drag. The cost of this is turbulence, energy, added to the flow which must come from the glider itself. It's not a bad deal if the form drag eliminated is significantly more than the additional drag produced by the turbulator strip.

In his booklet Performance Enhancement of Modern Sailplanes (1991), pages 2-8 and 2-9, Peter Masak gives the magic position for turbulators on Standard Cirrus wings as 67% of chord line. In the spring of 2001, I put turbulator tape on #60 at that position. Naturally, I did not run before and after tests. I just had the feeling that my glider flew better. Then, in November, I decided to run oil flow tests to see what was going on. The results are shown in the image on the left. Notice the turbulator behind the bubble. At this point I consulted with Dick Johnson who's first impression was that the separation bubble was premature. However, after running a simulated wind tunnel analysis, he concluded that the separation should be around .45c, far ahead of the recommended turbulator position. This was consistent with my oil flow images. I was much relieved and decided to replace the old turbulators with properly installed ones that winter, fully intending to do additional oil flows to verify that they worked.

But, once again, I installed new turbulators and flew without proper testing. The ship flew great and more often than not I would gain on my L-NAV glide slopes. I was sure it was right. Then I had to return the wings to standard configuration for testing the Sinha Deturbulator. This time I ran before and after drag tests and was shocked to learn that the turbulator had no effect one way or the other at any airspeed! (See the data.) Not understanding oil flows and the structure of a bubble, I had taken the thickest collection of oil to be the entire bubble, and placed the turbulator inside the highest part of the bubble. The image below of the lower wing shows the bubble separation and reattachment points (brown lines), the .67c position (orange broken line) and the location of my last turbulator tape (green broken line).

Where to Put the Turbulator

Click for test results

Common wisdom says to locate the leading edge of the turbulator "1/2 inch in front of the bubble." But the bubble does not have a definite leading edge. It begins separating very early and gradually moves away from the surface. Also, the separation and reattachment points move with airspeed, as indicated in the wing tunnel simulations.

What are the penalties for getting it wrong? If the turbulator is too far aft, then at high speeds (bubble forward on lower surface) the bubble may form over or ahead of the tape, resulting in no effect or additional drag without benefits. If the turbulator is too far forward, then the transition to turbulent flow (no bubble) occurs farther forward than necessary and there is a net reduction in the laminar flow area on the wing.

Wondering what happens when the bubble encroaches on the turbulator, I advised Davis Horton, a high school student in Oxford, Mississippi, in a science project that ran oil flows on a winglet mounted on the side of my truck. The results did not appear to produce the condition described above in which my turbulator was able to hide under the separated flow with no significant effect on the wing. Evidently, the chord of the winglet was so short that the separation of the flow above the surface was not great enough to clear the turbulator, so the turbulator tripped the flow at every point within the original bubble. Had the turbulator been thinner, I believe it would have had no effect at some point within the bubble, just as I saw on the wing of #60.

The ideal place for the turbulator is the aft-most position in the early separation zone that destroys the tallest part of the bubble. If an oil flow shows no collection of oil, then the tape is working. The thicker the turbulator tape, the farther back it can be placed and still work. I believe this is a good strategy. Use thick tape and place it farther aft. That way, you maximize the laminar/separated flow area at the front of the wing and pay no penalty for using thick tape.

Lower Surface

Oil flow images and reports of wind tunnel tests suggest that no advantage can be gained from using turbulators on the outer panel of the lower surface. The inner panel, however, may be a different story. There is a strong bubble, but, as shown in the wing drawing above, the separation zone comes close to the leading edge of the wing. In this case, the suggestion above to use thick tape and place it as far aft as possible is the best solution. Rather than recommend a place for the turbulator, I suggest that you run an oil flow test to find the sweet spot. At inner and outer span stations on the inner wing panel, place short strips (4 inches should work) of turbulator at different chord positions to find the aft-most location that works. Be sure to put oil in places where no turbulator is located. This will verify the presence of the bubble and assist in making adjustments for subsequent flights. See the Oil Flow Page for measurements of the approximate flow separation points.

Upper Surface

Oil flow images of the upper surface suggest that turbulators could be very useful there. The bubble moves less on the upper surface with changes in airspeed around the 70 kt point and, as indicated by drag rake tests (on the right), the suction side creates most of the profile drag, especially at slow speeds. If the drag from the separation bubble is responsible for much of this, then real gains could be achieved with turbulators on the top surface. As above, I suggest testing to find the aft-most position that works. See the Oil Flow Page for measurements of the approximate flow separation points.