Laminar flow vs. turbulant flow and visocity
10-07-2005, 04:19 PM
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Laminar flow vs. turbulant flow and visocity
Could someone please explain to me how turbulant flow over an airfoil has a higher visocity (it tends to stay attached to the airfoil better) than laminar flow over the same airfoil. Just when I think that laminar flow is more desirable than turbulent flow due to the fact that laminar flow produces less drag, I find out that laminar flow tends to seperate from the airfoil more easily than turbulent flow. Of course, flow seperation equals loss of lift. [sm=confused.gif][sm=spinnyeyes.gif][sm=confused.gif]
10-07-2005, 05:42 PM
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RE: Laminar flow vs. turbulant flow and visocity
without any tech speak--
the turbulant flow (done right on a craft which can take advantage of it) simply broadens the usuable lift -letting the craft go to a bit higher angle before the stall hits.
It does add drag and on some aircraft is of NO value
It's a little like saying flaps add lift-
well they do sorta - they actually change the lift profile of the wing
So -- a craft which is designed to fly at best efficiency and carry passengers and loads from point A to point B- may very well take advantage of turbulating devices . And they do
A high powered /low weight aerobatic craft has no need for this type device. It would be counter productive.
the turbulant flow (done right on a craft which can take advantage of it) simply broadens the usuable lift -letting the craft go to a bit higher angle before the stall hits.
It does add drag and on some aircraft is of NO value
It's a little like saying flaps add lift-
well they do sorta - they actually change the lift profile of the wing
So -- a craft which is designed to fly at best efficiency and carry passengers and loads from point A to point B- may very well take advantage of turbulating devices . And they do
A high powered /low weight aerobatic craft has no need for this type device. It would be counter productive.
10-07-2005, 11:41 PM
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RE: Laminar flow vs. turbulant flow and visocity
The way that I understand it is that a thin turbulent layer has more energy in it and that energy acts as a glue to hold the turbulent layer onto the wing and to hold down the laminar or near laminar flow in contact with it. Of course it's not a hard division as the turbulent layer has more of an interference transition band between the two air masses.
It also most helps at lower airspeeds and higher lift coefficients. At lower Cl's the laminar flow is less likely to separate anyway.
I've use turbulators on two sailplanes with D tube sheeted leading edges and in both cases the turbulators really helped the lower end of the speed spectrum. They also affected the upper end but not by much, just enough to notice. But it definetly shows that once you turbulate the boundry layer or turbulate it sooner on the airfoil than it would normally form it does offer a little more drag. But the benifit on the lower speed end far more than made up for it.
It also most helps at lower airspeeds and higher lift coefficients. At lower Cl's the laminar flow is less likely to separate anyway.
I've use turbulators on two sailplanes with D tube sheeted leading edges and in both cases the turbulators really helped the lower end of the speed spectrum. They also affected the upper end but not by much, just enough to notice. But it definetly shows that once you turbulate the boundry layer or turbulate it sooner on the airfoil than it would normally form it does offer a little more drag. But the benifit on the lower speed end far more than made up for it.
10-10-2005, 09:53 AM
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RE: Laminar flow vs. turbulant flow and visocity
ORIGINAL: spirit pilot
Could someone please explain to me how turbulant flow over an airfoil has a higher visocity (it tends to stay attached to the airfoil better) than laminar flow over the same airfoil. Just when I think that laminar flow is more desirable than turbulent flow due to the fact that laminar flow produces less drag, I find out that laminar flow tends to seperate from the airfoil more easily than turbulent flow. Of course, flow seperation equals loss of lift. [sm=confused.gif][sm=spinnyeyes.gif][sm=confused.gif]
Could someone please explain to me how turbulant flow over an airfoil has a higher visocity (it tends to stay attached to the airfoil better) than laminar flow over the same airfoil. Just when I think that laminar flow is more desirable than turbulent flow due to the fact that laminar flow produces less drag, I find out that laminar flow tends to seperate from the airfoil more easily than turbulent flow. Of course, flow seperation equals loss of lift. [sm=confused.gif][sm=spinnyeyes.gif][sm=confused.gif]
It's a trade-off. Turbulent flow on the surface of the wing increases drag, if everything else about the flow is the same. Separation also increases drag (and decreases lift), if everything else is the same. Turbulent flow also tends to delay or prevent separation, and usually this is a net win. Many wings designed for laminar flow use a turbulator to force the flow to become turbulent before it has a chance to separate. This gives the best of both worlds: laminar flow from the leading edge as far back as possible, and then little or no separation.
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10-10-2005, 10:27 AM
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RE: Laminar flow vs. turbulant flow and visocity
Another good example of a trade off. A standard dimpled golf ball will go farther than a smooth golf ball if both are hit just as hard. The dimples provide the energy needed for the air to stay attached longer. The dimpled ball has a smaller wake region behind it and less overall drag than the smooth ball.
10-10-2005, 05:10 PM
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RE: Laminar flow vs. turbulant flow and visocity
Okay, so without a windtunnel to show where the sepereation point is on an airfoil at a given airspeed, are there any guidelines to follow in locating turbulators?
10-11-2005, 01:11 AM
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RE: Laminar flow vs. turbulant flow and visocity
Selig used 10, 15 and 20% chord position turbulators at various times in his wind tunnel work.
The ideal spot is to put it just ahead of where the laminar separation bubble forms. To play with it you could start at about 15% and see if there is a notable difference in your low speed range performance. A glide that hangs in there better and notably flatter glide angle are signs that you corrected a separation bubble issue. Now move it back until you notice bad things happening in the slow end of the speed range. Move it back forward until it's just fixed.
With the two successful models I did I put two on. Once at about 10% back and one at around 20% back. As mentioned I found that the high speed end was affected but only just enough that I was able to notice. I stopped experiementing and just flew after that. Even with the very slight degradation in the high speed end of things I was still ecstatic about how the two models were able to penetrate at higher speeds so I didn't bother trying to fine tune the placement.
The ideal spot is to put it just ahead of where the laminar separation bubble forms. To play with it you could start at about 15% and see if there is a notable difference in your low speed range performance. A glide that hangs in there better and notably flatter glide angle are signs that you corrected a separation bubble issue. Now move it back until you notice bad things happening in the slow end of the speed range. Move it back forward until it's just fixed.
With the two successful models I did I put two on. Once at about 10% back and one at around 20% back. As mentioned I found that the high speed end was affected but only just enough that I was able to notice. I stopped experiementing and just flew after that. Even with the very slight degradation in the high speed end of things I was still ecstatic about how the two models were able to penetrate at higher speeds so I didn't bother trying to fine tune the placement.
