Vortex generators?

Ok, I understand how vortex generators disturb the airflow to allow the wing to stall at a lower speed, but there are lots of claims of better L/D for high performance planes like some jets. It seems like add bugs to your wing? Wouldn't those vortex generators reduce performance at higher speeds on a laminar flow wing? Aren't all jets using a laminar flow wing these days? I think I'm getting separation around my wing root and want to do some testing on our 2-32. Thoughts?

Boggs

On Monday, September 5, 2016 at 9:29:12 AM UTC-4, Waveguru wrote:
Ok, I understand how vortex generators disturb the airflow to allow the wing to stall at a lower speed, but there are lots of claims of better L/D for high performance planes like some jets. It seems like add bugs to your wing? Wouldn't those vortex generators reduce performance at higher speeds on a laminar flow wing? Aren't all jets using a laminar flow wing these days? I think I'm getting separation around my wing root and want to do some testing on our 2-32. Thoughts?

Boggs

Possibly some of these aircraft have laminar separation bubbles that are made better by tripping the flow directly to turbulent.
UH

On Monday, September 5, 2016 at 10:22:02 AM UTC-4, wrote:
Possibly some of these aircraft have laminar separation bubbles
that are made better by tripping the flow directly to turbulent.

As I think I understand it...

A turbulator is used to transition laminar to turbulent flow to
avoid laminar separation; the turbulent flow causing less drag
than a big bubble. Turbulators can be a relatively thin tape, or
blow holes...

A vortex generator is a lot taller, and is used to pull
higher-speed flow down to the much slower boundary layer,
and create a vortex that inhibits span-wise flow in the
slower boundary layer. That can be used to prevent separation
in some cases; either in laminar or turbulent flow.
It also be used to aid in cooling - there was a great article
recently (KitPlanes ? Sport Aviation?) about a rear-engined
canard that needs tall turbulators to get air into the
aft cooling scoops.

Gary - start with some thin tell-tales and a GoPro to look
at the flow around the top wing surface at the root, aft
of the spar.

Hank - Could Gary be seeing the same issue as the 20's
experience - double-bad-adverse-gradient at fuselage
junction? You install turbulators on the 20 top surface
at the root for this, no?

Interesting stuff....
Best Regards, Dave

On Monday, September 5, 2016 at 11:05:05 AM UTC-4, Dave Nadler wrote:
On Monday, September 5, 2016 at 10:22:02 AM UTC-4, wrote:
Possibly some of these aircraft have laminar separation bubbles
that are made better by tripping the flow directly to turbulent.

As I think I understand it...

A turbulator is used to transition laminar to turbulent flow to
avoid laminar separation; the turbulent flow causing less drag
than a big bubble. Turbulators can be a relatively thin tape, or
blow holes...

A vortex generator is a lot taller, and is used to pull
higher-speed flow down to the much slower boundary layer,
and create a vortex that inhibits span-wise flow in the
slower boundary layer. That can be used to prevent separation
in some cases; either in laminar or turbulent flow.
It also be used to aid in cooling - there was a great article
recently (KitPlanes ? Sport Aviation?) about a rear-engined
canard that needs tall turbulators to get air into the
aft cooling scoops.

Gary - start with some thin tell-tales and a GoPro to look
at the flow around the top wing surface at the root, aft
of the spar.

Hank - Could Gary be seeing the same issue as the 20's
experience - double-bad-adverse-gradient at fuselage
junction? You install turbulators on the 20 top surface
at the root for this, no?

Interesting stuff....
Best Regards, Dave

Dave- In a word- yes. The simple construction geometry leaves a square hollow corner where the wing meets the fuselage. Due to the pressures in that area, sooner or later the air can't follow the contour, especially at high angles of attack in slow flight. The "standard" cure for this is a fillet to provide a contour the air can follow. Putting a turbulator/vortex generator ahead of the separation can entrain more air into the boundary layer there and possibly reduce the effect somewhat. I once put some fillets on the root of a 1-26 and reduced the separation enough to lower the stall speed about 4 mph if I remember correctly. It is worth noting that this is a touchy area on certificated aircraft.
UH

I figured the first thing I'd do is tape some yarn to the wing and film it, but doesn't having the tape and yarn on the airfoil effect what's going on too? I am NOT pouring dirty oil on my wing! (unless I REALLY have to) Yuck.

Boggs

On Monday, September 5, 2016 at 1:18:21 PM UTC-4, wrote:
On Monday, September 5, 2016 at 11:05:05 AM UTC-4, Dave Nadler wrote:
On Monday, September 5, 2016 at 10:22:02 AM UTC-4, wrote:
Possibly some of these aircraft have laminar separation bubbles
that are made better by tripping the flow directly to turbulent.

As I think I understand it...

A turbulator is used to transition laminar to turbulent flow to
avoid laminar separation; the turbulent flow causing less drag
than a big bubble. Turbulators can be a relatively thin tape, or
blow holes...

A vortex generator is a lot taller, and is used to pull
higher-speed flow down to the much slower boundary layer,
and create a vortex that inhibits span-wise flow in the
slower boundary layer. That can be used to prevent separation
in some cases; either in laminar or turbulent flow.
It also be used to aid in cooling - there was a great article
recently (KitPlanes ? Sport Aviation?) about a rear-engined
canard that needs tall turbulators to get air into the
aft cooling scoops.

Gary - start with some thin tell-tales and a GoPro to look
at the flow around the top wing surface at the root, aft
of the spar.

Hank - Could Gary be seeing the same issue as the 20's
experience - double-bad-adverse-gradient at fuselage
junction? You install turbulators on the 20 top surface
at the root for this, no?

Interesting stuff....
Best Regards, Dave

Dave- In a word- yes. The simple construction geometry leaves a square
hollow corner where the wing meets the fuselage. Due to the pressures
in that area, sooner or later the air can't follow the contour, especially
at high angles of attack in slow flight. The "standard" cure for this is
a fillet to provide a contour the air can follow.

Its the gradient that causes the separation. The fuselage is angled away
from the wing, the top of the wing is angled downwards. Combination reduces
the pressure too much too fast, hence separation. It may take a pretty big
fillet to fix, for example on Genesis.

Putting a turbulator/vortex generator ahead of the separation can entrain
more air into the boundary layer there...

Different animals though, no? A turbulator takes laminar flow and turbulates
it to make it stickier. But it doesn't really change the boundary layer,
thickness does it? Any aero experts out there? A vortex generator sticks
up to mix air outside the boundary layer, reducing the boundary layer
thickness in the resulting vortex area, right?

I once put some fillets on the root of a 1-26 and reduced the separation
enough to lower the stall speed about 4 mph if I remember correctly.

Cool! Of course you need a pitot-static and ASI better than factory
to really find out the speeds down at that end.

Anyway, all these solutions improve the situation at high CL but
add just add drag at low CL, right? Probably don't improve max LD
unless something is drastically wrong with the original design...

On Monday, September 5, 2016 at 2:50:02 PM UTC-4, Dave Nadler wrote:
On Monday, September 5, 2016 at 1:18:21 PM UTC-4, wrote:
On Monday, September 5, 2016 at 11:05:05 AM UTC-4, Dave Nadler wrote:
On Monday, September 5, 2016 at 10:22:02 AM UTC-4, wrote:
Possibly some of these aircraft have laminar separation bubbles
that are made better by tripping the flow directly to turbulent.

As I think I understand it...

A turbulator is used to transition laminar to turbulent flow to
avoid laminar separation; the turbulent flow causing less drag
than a big bubble. Turbulators can be a relatively thin tape, or
blow holes...

A vortex generator is a lot taller, and is used to pull
higher-speed flow down to the much slower boundary layer,
and create a vortex that inhibits span-wise flow in the
slower boundary layer. That can be used to prevent separation
in some cases; either in laminar or turbulent flow.
It also be used to aid in cooling - there was a great article
recently (KitPlanes ? Sport Aviation?) about a rear-engined
canard that needs tall turbulators to get air into the
aft cooling scoops.

Gary - start with some thin tell-tales and a GoPro to look
at the flow around the top wing surface at the root, aft
of the spar.

Hank - Could Gary be seeing the same issue as the 20's
experience - double-bad-adverse-gradient at fuselage
junction? You install turbulators on the 20 top surface
at the root for this, no?

Interesting stuff....
Best Regards, Dave

Dave- In a word- yes. The simple construction geometry leaves a square
hollow corner where the wing meets the fuselage. Due to the pressures
in that area, sooner or later the air can't follow the contour, especially
at high angles of attack in slow flight. The "standard" cure for this is
a fillet to provide a contour the air can follow.

Its the gradient that causes the separation. The fuselage is angled away
from the wing, the top of the wing is angled downwards. Combination reduces
the pressure too much too fast, hence separation. It may take a pretty big
fillet to fix, for example on Genesis.

Putting a turbulator/vortex generator ahead of the separation can entrain
more air into the boundary layer there...

Different animals though, no? A turbulator takes laminar flow and turbulates
it to make it stickier. But it doesn't really change the boundary layer,
thickness does it? Any aero experts out there? A vortex generator sticks
up to mix air outside the boundary layer, reducing the boundary layer
thickness in the resulting vortex area, right?

I once put some fillets on the root of a 1-26 and reduced the separation
enough to lower the stall speed about 4 mph if I remember correctly.

Cool! Of course you need a pitot-static and ASI better than factory
to really find out the speeds down at that end.

Anyway, all these solutions improve the situation at high CL but
add just add drag at low CL, right? Probably don't improve max LD
unless something is drastically wrong with the original design...

The contrast is more about degree than action, I think. Vortex generators are commonly larger and many are placed at angles to the local flow to disturb much more air than the smaller turbulators used to transition attached laminar flow into attached turbulent flow. Both stick up through the local layer into higher energy air and cause mixing. It's mostly about degree.
On big jets it is probably almost all intended to add energy to the boundary layer so that controls work better and air stays attached to high lift devices at very high C/L.
One interesting application I heard from Mike Opitz was the adding of turbulators forward of the hinge line of the rudder on a Grob Twin. He reported a remarkable improvement in feel and handling.
FWIW
UH

As both Dave and Hank have insinuated, turbulators and vortex generators generally have different functions. Turbulators are used to force transition from laminar to turbulent flow. This is done to eliminate (drag-producing) laminar separation bubbles that form for Reynolds numbers typical of sailplanes. Turbulators have heights less than the boundary-layer thickness. Conversely, vortex generators are usually used to alleviate turbulent separation. They act like tiny wings, generating a vortex that causes higher velocity flow farther from the surface to be directed closer to the surface, thus energizing the boundary layer. This reduces the tendency of the boundary layer to separate in the adverse pressure gradient toward the trailing edge of the wing, particularly at the high lift coefficients corresponding to low airspeeds.

To get to Boggs's original question, vortex generators may alleviate a separation in the wing-fuselage juncture. Tufting the region in question is certainly a good idea. The (yarn) tufts will have negligible effect provided they are attached downstream of transition. As Hank noted, the modification may run afoul of the law for a certificated aircraft. Alleviating a wing-root separation may also have unintended consequences, for example, by changing the separation progression along the span and, therefore, the stall behavior. There is, of course, a drag penalty due to the vortex generators that generally increases with increasing airspeed. As one aerodynamicist put it: "There's no free lunch."

I agree oil-flow visualization of a separated region should be avoided. While it may be enlightening for an aerodynamicist, the almost explosive scattering of the oil downstream will create quite a mess.

Wing-root fillets are a significant undertaking, often involving as much art as science. Be prepared for the inevitable research project.

Finally, to clarify a few misconceptions:
No transport aircraft employ laminar-flow airfoils---yet; some business jets do, fewer actually achieve laminar flow (e.g., the HondaJet).
Turbulators increase the boundary-layer thickness because they force the thin, laminar boundary layer to transition into the thicker, turbulent boundary layer. Turbulators do not make the boundary layer "stickier."
Vortex generators do not make the boundary layer thinner; they alter the boundary-layer velocity profile, making it "fuller."

Dan Somers/ID
Port Matilda, Pennsylvania

On Monday, September 5, 2016 at 4:15:09 PM UTC-4, wrote:
On Monday, September 5, 2016 at 2:50:02 PM UTC-4, Dave Nadler wrote:
On Monday, September 5, 2016 at 1:18:21 PM UTC-4, wrote:
On Monday, September 5, 2016 at 11:05:05 AM UTC-4, Dave Nadler wrote:
On Monday, September 5, 2016 at 10:22:02 AM UTC-4, wrote:
Possibly some of these aircraft have laminar separation bubbles
that are made better by tripping the flow directly to turbulent.

As I think I understand it...

A turbulator is used to transition laminar to turbulent flow to
avoid laminar separation; the turbulent flow causing less drag
than a big bubble. Turbulators can be a relatively thin tape, or
blow holes...

A vortex generator is a lot taller, and is used to pull
higher-speed flow down to the much slower boundary layer,
and create a vortex that inhibits span-wise flow in the
slower boundary layer. That can be used to prevent separation
in some cases; either in laminar or turbulent flow.
It also be used to aid in cooling - there was a great article
recently (KitPlanes ? Sport Aviation?) about a rear-engined
canard that needs tall turbulators to get air into the
aft cooling scoops.

Gary - start with some thin tell-tales and a GoPro to look
at the flow around the top wing surface at the root, aft
of the spar.

Hank - Could Gary be seeing the same issue as the 20's
experience - double-bad-adverse-gradient at fuselage
junction? You install turbulators on the 20 top surface
at the root for this, no?

Interesting stuff....
Best Regards, Dave

Dave- In a word- yes. The simple construction geometry leaves a square
hollow corner where the wing meets the fuselage. Due to the pressures
in that area, sooner or later the air can't follow the contour, especially
at high angles of attack in slow flight. The "standard" cure for this is
a fillet to provide a contour the air can follow.

Its the gradient that causes the separation. The fuselage is angled away
from the wing, the top of the wing is angled downwards. Combination reduces
the pressure too much too fast, hence separation. It may take a pretty big
fillet to fix, for example on Genesis.

Putting a turbulator/vortex generator ahead of the separation can entrain
more air into the boundary layer there...

Different animals though, no? A turbulator takes laminar flow and turbulates
it to make it stickier. But it doesn't really change the boundary layer,
thickness does it? Any aero experts out there? A vortex generator sticks
up to mix air outside the boundary layer, reducing the boundary layer
thickness in the resulting vortex area, right?

I once put some fillets on the root of a 1-26 and reduced the separation
enough to lower the stall speed about 4 mph if I remember correctly.

Cool! Of course you need a pitot-static and ASI better than factory
to really find out the speeds down at that end.

Anyway, all these solutions improve the situation at high CL but
add just add drag at low CL, right? Probably don't improve max LD
unless something is drastically wrong with the original design...

The contrast is more about degree than action, I think. Vortex generators are commonly larger and many are placed at angles to the local flow to disturb much more air than the smaller turbulators used to transition attached laminar flow into attached turbulent flow. Both stick up through the local layer into higher energy air and cause mixing. It's mostly about degree.
On big jets it is probably almost all intended to add energy to the boundary layer so that controls work better and air stays attached to high lift devices at very high C/L.
One interesting application I heard from Mike Opitz was the adding of turbulators forward of the hinge line of the rudder on a Grob Twin. He reported a remarkable improvement in feel and handling.
FWIW
UH

I wish this forum had a like button. Thank you all for this input. Now all I need is the double sticky tape...

Boggs

On Monday, September 5, 2016 at 5:20:34 PM UTC-4, wrote:
...Turbulators increase the boundary-layer thickness because they
force the thin, laminar boundary layer to transition into the thicker,
turbulent boundary layer.
Turbulators do not make the boundary layer "stickier."

Vortex generators do not make the boundary layer thinner; they alter
the boundary-layer velocity profile, making it "fuller."

Thanks Dan!
Best Regards, Dave