Has anyone fitted laminar airflow generators to their mast ?

[valhalla360]

Originally Posted by a64pilot

Nope I meant add energy, look it up, they energize the airflow, the turbulent energized airflow will stay attached to the surface far longer than laminar airflow will.

https://www.boldmethod.com/learn-to-...ex-generators/

The holy grail used to be laminar airflow, flush rivets and polished surfaces, but that isn’t always faster, sometimes now a rough surface is faster, have a thin layer of air be trapped against the surface and the air shear against itself as opposed to shearing against a polished surface is often less drag.
So far as meaningful drag reduction from aerodynamic shapes, at the speeds we are talking about here, it’s not likely to mean much.

Sometimes what works at 500 kts, doesn’t at 5 or even 20.

A good example is golf balls.

Originally, they were smooth but golfers found that old nicked up balls actually went further. Eventually they learned that it was the turbulent flow and that's why modern golf balls have dimples.

Of course, what reduces drag on a golf ball is not the same as trying to extract energy out of the wind with a sail.

[Dave_S]

Please correct me if I'm wrong or tell me if I have it right.

3 types of airflow.

• Laminar, nice while it lasts, low pressure, smooth flowing area at the front of the wing.
• Turbulent, laminar air that is slowing, caused by to much difference in speed through the 1" of boundary layer it starts to be slowed at the surface and then drawn back to the low pressure area of the laminar flow
• Seperated air, where the air slows too much and no longer follows the shape of the wing anymore because it has stalled and in part reverses

A VG protrudes into the clean air layer above the boundary layer and mixes fast flowing/energised (relative to the wing) air into the boundary layer which helps keep the air flowing over the wing and follows the wings profile longer.

I'm having some problem with seperated air, I imagined seperated air meant the air pulled apart, something close to a vacuum because you were trying to bend it too far, too fast. What I now think is meant by seperated air is stalled air moving on average close to the wing speed so offers no lift. Have I got it right now ?

[Dockhead]

Originally Posted by Singularity

I'd suggest that for practical purposes to consider airflow to be laminar before it runs into anything, so we don't exactly create laminar flow. Furthermore, what we'd like to do with our airfoil/sail is to keep the airflow laminar for as long as possible as turbulence lends to separation/drag.. . .

Actually the opposite of this. In laminar flow, air moves smoothly in layers (hence "laminar"). Vortex generators disrupt laminar flow and create turbulent flow. Turbulent flow contrary to what you posted does NOT separate from the airfoil as easily, or as suddenly. See the posts 4, 12, 13 and 15 above which explain the concepts a lot better than I ever could.


To the OP: The usual way to improve the aerodynamic transition from mast to sail is not with vortex generators (although I don't see why that wouldn't have SOME beneficial effect) but with a rotating mast as you see on America's Cup yachts and Open 60's and some performance catamarans, which has a huge beneficial effect. The transition from mast profile to sail is less of a problem than the mis-alignment of the mast profile with the airfoil, see here:





From: https://sailwildling.com/2014/09/13/...rotating-mast/, which is good reading on the subject.

[Q Xopa]

Originally Posted by a64pilot

VG’s work by adding energy to the airflow, the energized airflow for whatever reason will stick to a surface with greater tenacity than non energized air will.

So where would you put the VG’s? On an aircraft wing they are not placed on the leading edge which is where the mast is, but they are placed pretty much at the max wing thickness which is close to the center of lift for a wing, on a sail boat that would be on the sail itself about 20% or so back from the leading edge.

If you look at Commercial airlines, you will almost never see full span VG’s, you will se some strategically placed to fix problems, they are a band aid.
VG’s are not magic, they have downsides, there is no free lunch.

Do the ultra high performance boats that have double surfaced wings for sails have VG’s? I have no idea myself, but speculate that if they do, maybe there is some gain from VG’s.

VG’s effects aren’t as well understood as you may suspect, most often especially on automobiles they are a styling feature more than anything else, a lot like carbon fiber pieces are.

I did some testing on VG’s effects on the spray pattern of agricultural aircraft with some interesting results.

I’d suspect that for VG’s to accomplish anything on a sail boat, you would have to have a rotating mast so that the relationship between the mast and sail wasn’t a constant variable, otherwise I’d suspect that VG’s on a mast would work at one specific wind angle and at all other angles they would just get in the way.

What A64 said.

Just to add to his description.

It's about energising the boundry layer right next to an airfoil.

Flowing air is a 'fluid' which has viscosity (stickiness).

So the boundry layer dosent flow so well, it sticks to the airfoil instead of flowing over it which creates the lift we want.

So VGs energise this boundry layer, to get it moving.

But as stated they are also forcing the air a different direction so is some drag. Hopefully less drag than the extra lift they generate.

Some of the top end glider guys sand their Leading edges rough to do the same.

A shark skin is rough, and Whale flippers have 'turbicles' (leading edge nodules for the same reason. Mother nature has had millions of years of R&D (extinct) mistakes to work out what works ok.

[Dave_S]

I'm quite disappointed that my boat was originally built with a carbon fibre rotating mast. The PO replaced it with a fixed aluminium one after the original was damaged. The new one is 1.6m taller though.

It becomes obvious that air flow on to the sail is important when I leave the screacher up using the jib I loose a few degrees of pointing from bad air. I'm guessing with bad air off the mast there is at least the same losses.

It's also interesting that I get better upwind performance if there is some visable crush on the front of the main from the air passing between the jib and main. It maybe because it controls the air from mast to main ?


P.S. why have I got a grumpy face at the beginning of my topic ?

[Dave_S]

On a sail pointing high, the back half of the sail has lift appossing the direction of intended travel. Should we be trying harder to cause the air to seperate ?

[Dockhead]

Originally Posted by Dave_S

On a sail pointing high, the back half of the sail has lift appossing the direction of intended travel. Should we be trying harder to cause the air to seperate ?

Lift "opposing the direction of travel"? Think about that for a second. Lift is pulling, not pushing.

[Singularity]

Originally Posted by Dockhead

Actually the opposite of this. In laminar flow, air moves smoothly in layers (hence "laminar"). Vortex generators disrupt laminar flow and create turbulent flow. Turbulent flow contrary to what you posted does NOT separate from the airfoil as easily, or as suddenly. See the posts 4, 12, 13 and 15 above which explain the concepts a lot better than I ever could...

Turbulent flow above the boundary layer is outright draggy, full-stop. This is antithetical to the endeavor of creating a laminar airfoil (i.e. one that keeps the lamina...lamina for as long as possible) in the first place, but, such turbulent flow can be useful in specific aviation settings that really don't apply here.

In a sailing setting, each lamina has kinetic energy to be stolen and repurposed, while frankly there's a fair amount of inappropriate blending here of aviation foil theory and sailboats (as Valhalla intimated). No sailboat that anyone here owns will ever have a "laminar" airfoil, while for context it could be recognized that practically no small plane has a laminar airfoil.

The article below describes VG theory that applies practically nothing to any sailboat that doesn't have a Rolex sticker on the side and a helmsman with his/her eye 24/7/365 on the foil and wind gauge:

https://phys.org/news/2012-09-scient...se-vortex.html
vortex generators serve remarkably well at minimizing the boundary layer's drag by delaying its transition from a low-friction laminar flow to a high-friction turbulent flow. Their study is published in a recent issue of Physical Review Letters.As the researchers explain, an object's boundary layer starts out as laminar, or smooth and orderly. As the object continues to fly through the air, small disturbances create instabilities and, above a critical value, the laminar flow regime transitions to a turbulent one. The transition can easily result in an order of magnitude increase in skin-friction drag on an aircraft.

To demonstrate the effectiveness of using vortex generators to delay the transition to turbulence, the researchers attached miniature vortex generators (MVGs) to a flat plate and placed the plate in the wind tunnel at the Royal Institute of Technology (KTH) in Stockholm. They found that, under certain flow conditions, the MVGs could significantly delay the transition from laminar to turbulent flow.
"This is the first study ever that convincingly shows transition to turbulence delay in a realistic flow configuration," coauthor Alessandro Talamelli of KTH Mechanics and the University of Bologna told Phys.org. "We show that the disturbance energy inside a boundary layer can be reduced by three orders of magnitude by making use of appropriately designed MVGs. This is an important result in the quest to accomplish skin-friction drag reduction."

[Dockhead]

Originally Posted by Singularity

Turbulent flow above the boundary layer is outright draggy, full-stop. This is antithetical to the endeavor of creating a laminar airfoil (i.e. one that keeps the lamina...lamina for as long as possible) in the first place, but, such turbulent flow can be useful in specific aviation settings that really don't apply here.

In a sailing setting, each lamina has kinetic energy to be stolen and repurposed, while frankly there's a fair amount of inappropriate blending here of aviation foil theory and sailboats (as Valhalla intimated). No sailboat that anyone here owns will ever have a "laminar" airfoil, while for context it could be recognized that practically no small plane has a laminar airfoil.

The article below describes VG theory that applies practically nothing to any sailboat that doesn't have a Rolex sticker on the side and a helmsman with his/her eye 24/7/365 on the foil and wind gauge:

https://phys.org/news/2012-09-scient...se-vortex.html
vortex generators serve remarkably well at minimizing the boundary layer's drag by delaying its transition from a low-friction laminar flow to a high-friction turbulent flow. Their study is published in a recent issue of Physical Review Letters.As the researchers explain, an object's boundary layer starts out as laminar, or smooth and orderly. As the object continues to fly through the air, small disturbances create instabilities and, above a critical value, the laminar flow regime transitions to a turbulent one. The transition can easily result in an order of magnitude increase in skin-friction drag on an aircraft.

To demonstrate the effectiveness of using vortex generators to delay the transition to turbulence, the researchers attached miniature vortex generators (MVGs) to a flat plate and placed the plate in the wind tunnel at the Royal Institute of Technology (KTH) in Stockholm. They found that, under certain flow conditions, the MVGs could significantly delay the transition from laminar to turbulent flow.
"This is the first study ever that convincingly shows transition to turbulence delay in a realistic flow configuration," coauthor Alessandro Talamelli of KTH Mechanics and the University of Bologna told Phys.org. "We show that the disturbance energy inside a boundary layer can be reduced by three orders of magnitude by making use of appropriately designed MVGs. This is an important result in the quest to accomplish skin-friction drag reduction."

That's all true, but does not address the point -- vortex generators (or maybe better call them turbulators, to avoid confusion with wingtip vortex generators), as was explained in posts 4, 12, 13 and 15 above by people far more knowledgeable than you or me (this is not stuff you can learn in a few minutes of Googling), do not indeed "keep the air flow laminar", as you posted -- they do the opposite of that, they induce turbulence in the boundary layer which makes the air flow LESS laminar, which helps keep the air flow attached and increases lift, at the expense of some drag. Overall drag may be reduced despite higher drag in the boundary layer, because the reduced separation of the airflow may reduces that different turbulance which occurs beyond the boundary layer when the flow separates. But the main point is what a64 said -- which is that turbulence in the boundary layer results in greater lift and reduced flow separation. Turbulence, not laminar flow. Laminar flow is the enemy, not the friend here. We are breaking up the laminar flow so that higher energy air can reach the airfoil.

Another way in which making the airflow LESS laminar reduces overall drag is because when the slow boundary layer meets the faster layers of air behind the wing, the difference in speed creates a wake and turbulence which causes drag. So making the flow over the wing less laminar, mixing the air of the boundary layer with the air of the next layer, results in less difference of speed to cause a wake.

I do not claim more than superficial knowledge of this field, but I have done some reading and I have a good friend who is an aerodynamic engineer whom I taught to sail in exchange for his observations on the aerodynamic principles at work. The way he explained it to me was like this: In laminar flow, the boundary layer moves much more slowly over the airfoil while other layers slide smoothly against each other and against the boundary layer. The slowness of the boundary layer means less lift. Turbulators (or rough finishing of the leading edge of the wing) introduce turbulence in the boundary layer which causes the boundary layer air to mix with the next layers, imparting "energy" (as A64 said) into the boundary layer, or rather, speeding up the boundary layer in relation to the airfoil. I think that is probably a gross oversimplification, but maybe one of our experts on here could correct anything which is misstated.

I actually discussed the OP's exact question with my friend, and he was of the opinion that there could be some benefit of installing turbulators on the mast. But as he looked at the sail and mast while beating to windward, he said "could you possibly just rotate the mast so that the profile lines up with the airfoil"? I laughed and told him he was not the first genius to think of that, and told him about Open 60's with their rotating masts.

Another thing he told me which surprised me, was that the foil of the furling jib would make the sail work better, than if the leading edge was as thin as the headstay, as would be the case with a hanked-on jib. I don't remember how he explained that, but I couldn't believe it, and did some reading and saw that he was right.

I know A64 was involved with development of STOL aircraft; I bet he could tell us more than we ever wanted to know on this subject.

[Dave_S]

Originally Posted by Dockhead

Lift "opposing the direction of travel"? Think about that for a second. Lift is pulling, not pushing.

If we are sailing at 30° AWA, the boom is at 185° to the boat (5° windward of the centreline), when viewed from above most of the sail is behind it's thickest section, about 60+% of the sail is equal to or less than parallel to the direction the boat is headed, if you pull or apply lift on that part of the sail it is pulling you backwards.

I understand that the majority of the lift happens on the front 30% and it is enough to overcome the rest but can we sabotage the parts of the sail working against us.

From another thread I was talking about the leach tell tails flicking back and forth from streaming to behind the leeward side and guessed this was beneficial because it did disturb the lift on some of the counterproductive part of the sail.

[Dockhead]

Originally Posted by Dave_S

If we are sailing at 30° AWA, the boom is at 185° to the boat (5° windward of the centreline), when viewed from above most of the sail is behind it's thickest section, about 60+% of the sail is equal to or less than parallel to the direction the boat is headed, if you pull or apply lift on that part of the sail it is pulling you backwards.

I understand that the majority of the lift happens on the front 30% and it is enough to overcome the rest but can we sabotage the parts of the sail working against us.

From another thread I was talking about the leach tell tails flicking back and forth from streaming to behind the leeward side and guessed this was beneficial because it did disturb the lift on some of the counterproductive part of the sail.

The majority of lift comes from the part of the sail nearest the leech, not nearest the mast. That's why roach is so effective. Racers have a saying -- "the only purpose of the front part of the sail is to have a means of attaching the leech area of the sail to the mast." A well trimmed mainsail may even have the forward part of the sail slack and not drawing at all ("speed bubble"). The power is in the leech, at least upwind.



It doesn't work like you say -- lift pulls the LEEWARD side of the sail forward, all of it, unless you've raised the traveller too high, in which case it's DRAG, not lift, which acts on the windward side of after part of the sail.

[Dave_S]

Originally Posted by Dockhead

The majority of lift comes from the part of the sail nearest the leech, not nearest the mast. That's why roach is so effective. Racers have a saying -- "the only purpose of the front part of the sail is to have a means of attaching the leech area of the sail to the mast." A well trimmed mainsail may even have the forward part of the sail slack and not drawing at all ("speed bubble"). The power is in the leech, at least upwind.



It doesn't work like you say -- lift pulls the LEEWARD side of the sail forward, all of it, unless you've raised the traveller too high, in which case it's DRAG, not lift, which acts on the windward side of after part of the sail.

I'm not sure.

As I understand it the lift comes at 90° to the surface of the sail. If so, the angle of lift changes along the sail relative to the boat. If I imagine a line perpendicular to the sail radiating out from the leeward surface, all along the sail when I'm close hauled a significant amount of sail is pointing behind 90° to the direction of travel. It may not be 60% as I said, I was imagining the foot when I was thinking of it.

Looking at the image, the leech of my sail isn't flat like that it continues to curve around and the clew is to the windward side of the centreline of the boat.

I like to experiment so I think I have the setup right, it seems efficient and boat speed v wind speed seems good.

[Dockhead]

Originally Posted by Dave_S

I'm not sure.

As I understand it the lift comes at 90° to the surface of the sail. If so, the angle of lift changes along the sail relative to the boat. If I imagine a line perpendicular to the sail radiating out from the leeward surface, all along the sail when I'm close hauled a significant amount of sail is pointing behind 90° to the direction of travel. It may not be 60% as I said, I was imagining the foot when I was thinking of it.

Looking at the image, the leech of my sail isn't flat like that it continues to curve around and the clew is to the windward side of the centreline of the boat.

I like to experiment so I think I have the setup right, it seems efficient and boat speed v wind speed seems good.

You've got the force vector right, but that says nothing about what part of the sail is generating it.


What I said is true -- just read up a little.

[Singularity]

Originally Posted by StoneCrab

No offense but the premise presented for the use of vortex generators is misunderstood.

I agree that the premise of the VGs was incorrect but, no offense, I suggest that you're confusing at least Dockhead (he spoke to someone).

A laminar airfoil is highly efficient but suffers from a sudden loss of lift when the ideal angle of attack is exceeded. The sudden air ripping away from the airfoil is accompanied by shock waves which are very expensive in terms of energy loss.

How is this loss of lift different than any other airfoil? I agree with the sudden part, but this is no different than many old 4 and 5-series NACA airfoils, while the laminar airfoil on my Ez's canard had very comfortable stall characteristics (while otherwise not bleeding appreciable energy).

The vortex generators disturb the boundary layer and provide a more gradual transition into a stall condition....

Yes, if appropriately positioned as A64 stated.

read that as giving the pilot some warning, rather than a sudden loss of control.

This is 2 statements: warning and control. VGs don't necessarily provide warning...if at all. Do you mean mushy feel? Indeed if the VGs are before a control surface, all else equal control ought to be enhanced (which A64 stipulated).

It also reduces the energy losses by reducing the shock waves.

What do you mean by this energy loss? Energy from where that goes where? At best I'm assuming that you're referring to drag at/after the trailing edge.

Non laminar wings can maintain some amount of lift well into a stall, providing a loss of the ability to maintain altitude, but generally with very little feeling of a loss of control.

Feeling of control is a function of wing planform, rigging, washout or not, etc. All airfoils maintain some amount of lift well into the stall, just that the window of mush might be smaller, but so what. It's the stall that makes the stall, right?

A laminar wing aircraft will stall violently without vortex generators. Since it is atypical for both wings to stall exactly at the same moment, this sudden and often violent loss of lift, and huge increase in drag, can provide some interesting gyrations.

How is this different than (typically thin profiled) non-laminar flow airfoils? Ever stalled a Taylorcraft with it's old 4-series airfoil?

Most commonly the sudden dropping of one wing. Normal pilot input to correct is opposite aileron which drops the outboard control surface on the falling wing further aggravating the stall of that wing. As that wing falls and slows, the opposite wing speeds up, gaining lift, flipping the plane on its back as the airplane enters a spin.

Again, not unique to laminar flow airfoils.

The aircraft discussion is more for entertainment, but it is relevant. While a mast may be aerodynamically dirty, an entirely laminar "wing" profile wouldn't be a solution either. You would experience sudden stalls if the sail trim wasn't maintained at all times.

A64 said the same above...the window where all this work applies to planes in a specific configuration of flight where pilots spend a few seconds per flight.

This would suck, so vortex generators would probably be added anyway. My conclusion is that a mast is one big vortex generator for the cloth sail. No matter what they tell you at the loft, a fabric sail will never be laminar.

A vortex generator is a vortex generator...it provides a very specific type of dirty air in a specific location to achieve a specific benefit. Agreed, though, that the mast overall is a benefit, and floppy sails never achieve laminar flow (using the language of the aviation world).

[zstine]

Vortex generator create a spiral air pattern, a vortex. It is not turbulent air by definition as turbulent air is unpredictable 'eddies' vice a tornado like flow. Turbulence from a non rotating mast transition to the sail is in no way the same thing.



Lift cannot pull the trailing edge or leech forward. Lift is a lower pressure on the surface and always acts normal to the sail.


With respect, dockhead is not correct about the placement of lift on a foil. The center of lift is typically 20 to 25% of cord from the leading edge... not near the mast, but closer to it than the roach/leech.



Stall, is turbulent eddies in a separated flow. It does not cause "shock waves"


VGs always add drag, but only add lift in specific conditions. You don't see them on boats because the added drag outweighs the benefits of added lift which is fleeting at best. Keep in mind that unlike aircraft, there is a flow velocity gradient from the water to top of mast, which is why we twist the sail. There is a significant 'pulsing' of velocity and hence angle of attack as the boat pitches in waves. So it is not really possible on a displacement hull to maintain the sail in the critical parameters which would benefit from a VG... now those foiling A75's may be another story.