Has anyone fitted laminar airflow generators to their mast ?

[Singularity]

Originally Posted by 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...

Thank you for this.

I'd add that the eddies are chaotic where, in a sailboat that is trying to extract energy from the wind, it's sort of hard to extract energy capable of performing work (other than blunt force) from turbulent air, so why would someone make it.

What I suggest keeps getting fundamentally missed here is that an airplane intrinsically is perpetually adding energy into the system, energy that started at least as early as the aircraft's fuel tank. This is really the hickup with respect to VGs for 99.9999% of sailboats...the only driving force a sailboat has to account for the VG drag is inertia of the boat itself (unless the sailboat is a perpetual motion machine).

[Dave_S]

How about a flap on each side of the mast to seal the gap and smooth the transition from mast to sail ?

[Dockhead]

Originally Posted by Singularity

Thank you for this.

I'd add that the eddies are chaotic where, in a sailboat that is trying to extract energy from the wind, it's sort of hard to extract energy capable of performing work (other than blunt force) from turbulent air, so why would someone make it.

The whole principle of vortex generators/turbulators is to disrupt the laminar flow in order to energize the boundary layer. You guys are getting hung up on the word "turbulence" -- the turbulence produced by vortex generators/turbulators is of a small magnitude -- it's not binary. This relatively small amount of turbulence (which mixes the layers and disrupts laminar flow) produces a net gain because it delays separation of the air flow from the airfoil, and prevents a relatively large amount of turbulence further back in the air stream when the slow boundary layers collides with the faster layers.

Originally Posted by Singularity

What I suggest keeps getting fundamentally missed here is that an airplane intrinsically is perpetually adding energy into the system, energy that started at least as early as the aircraft's fuel tank. This is really the hickup with respect to VGs for 99.9999% of sailboats...the only driving force a sailboat has to account for the VG drag is inertia of the boat itself (unless the sailboat is a perpetual motion machine).

Not indeed -- inertia has nothing to do with it. The driving force of a sailboat is the relative motion between air and water -- i.e., wind. A sailboat has foils stuck into the water and up into the air. The relative motion between the two fluids -- i.e. wind -- contains energy.



So sails and airplane wings work exactly the same way. Airplane wings also work in "wind", only it's wind derived from relative motion between atmosphere and aircraft artifically created from propulsion. But it's just the same wind from the point of view of physics.


The other difference is what is the lift used for -- with sailboats the lift produced by wind acting on the airfoils is used for propulsion. With airplanes the lift is used to lift the aircraft into the air and keep there.



So vortex generators/turbulators could have exactly the same applications with sails as they do with airplane wings. We are just as interested in energizing the boundary layers and delaying flow separation. Only we need such measures much less with sails because we have a great deal of control over the shape of our sails, unlike airplane wings which have a more or less fixed shape. But now that we see more and more wing sails, it will be interesting to see whether vortex generators/turbulators will find application.

[Dave_S]

Originally Posted by Dockhead

The whole principle of vortex generators/turbulators is to disrupt the laminar flow in order to energize the boundary layer. You guys are getting hung up on the word "turbulence" -- the turbulence produced by vortex generators/turbulators is of a small magnitude -- it's not binary. This relatively small amount of turbulence (which mixes the layers and disrupts laminar flow) produces a net gain because it delays separation of the air flow from the airfoil, and prevents a relatively large amount of turbulence further back in the air stream when the slow boundary layers collides with the faster layers.






Not indeed -- inertia has nothing to do with it. The driving force of a sailboat is the relative motion between air and water -- i.e., wind. A sailboat has foils stuck into the water and up into the air. The relative motion between the two fluids -- i.e. wind -- contains energy.



So sails and airplane wings work exactly the same way. Airplane wings also work in "wind", only it's wind derived from relative motion between atmosphere and aircraft artifically created from propulsion. But it's just the same wind from the point of view of physics.


The other difference is what is the lift used for -- with sailboats the lift produced by wind acting on the airfoils is used for propulsion. With airplanes the lift is used to lift the aircraft into the air and keep there.



So vortex generators/turbulators could have exactly the same applications with sails as they do with airplane wings. We are just as interested in energizing the boundary layers and delaying flow separation. Only we need such measures much less with sails because we have a great deal of control over the shape of our sails, unlike airplane wings which have a more or less fixed shape. But now that we see more and more wing sails, it will be interesting to see whether vortex generators/turbulators will find application.

Then there must be an advantage from adding VG's to a sail.

If I understand everything that has been written on my thread then we should be able to shape the sail more aggressively with them on and provide more lift than we otherwise could without them.

[Dockhead]

Originally Posted by Dave_S

Then there must be an advantage from adding VG's to a sail.

If I understand everything that has been written on my thread then we should be able to shape the sail more aggressively with them on and provide more lift than we otherwise could without them.

Yes, it could be. But there is much less need for them since we can shape the sail to mitigate flow separation. We have a lot of tools in our toolbox which airplane pilots don't have.

Biggest aerodynamic gain would be from rotating the mast. Nothing we could do with vortex generators/turbulators compares to what a rotating mast gives. The mis-aligned mast section is a huge aerodynamic hit, which basically takes the first 1/3 or even 1/2 of the sail out of action.

This is the whole problem which roach addresses -- adding more sail area to that part of the sail which isn't taken out of action by the mis-aligned mast section.

And it goes without saying -- good sails which can be effectively shaped. That's basic and HUGE, much more important than all of this stuff. Laminate sails make a HUGE difference.

[Rothblum]

Originally Posted by tkeithlu

Ahmen, A64, although I suspect that you meant "add velocity" rather than "add energy." ...

I think what A64 really meant was to transfer energy from outside the boundary layer to the inside. The extra energy close to the boundary is what mitigates boundary layer separation (stall).

[billdomb]

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. ...

Then why the turbulence generators on some airplane wings. Sometimes, it appears, a little turbulence keeps the laminar flow layer attached longer.

Counterintuitive.

[Ken Fry]

Originally Posted by StoneCrab

Vortex generators are placed in laminar flow and create turbulent flow. This may seem counterintuitive, but the way I think of it is they actually "add energy" to the airflow in the form of swirl. This swirly turbulent airflow is able to remain attached to the surface for longer.

Well put. "Attached" flow is not the same as laminar flow.

In Marchaj's book "Aerohydrodynamics of Sailing" from many decades ago, there is a humorous picture of a sailboat with vortex generators, etc. This stuff was studied at several universities at the time, with the idea of application to sailboats.

I've built a few rigid-wing-powered boats, and even in them, vortex generators are not helpful enough to justify the added complexity and the need for repositioning them for each angle of attack, slat/flap configuration, Reynolds number, etc. In boats that can sail at multiples of windspeed, L/D ratios are critical, so that is where VG's might seem to make sense (but don't)*. (And that where rigid wings make sense, too.)

But even then, people do not use VGs (on C class cats, or my boats, or Yellow Pages Endeavour, for example) because windspeeds and angles of attack are always changing in sailing. (This is unlike in flying, where the airspeed the wing sees is the same on every landing, regardless of windspeed over ground. So every landing uses full flaps and slats etc. )

In conventional sailing rigs, (and especially on boats not capable of multiples -- 2x - 3x -- of wind speed) there is no practical use for vortex generators.

In my Windrocket, (rigid wing) in 10 knots of wind, full flaps and about 10 degrees angle of attack were required to get moving and accelerate quickly. Once the boat was moving at 30 knots, (still 10 knots true) however, the angle of attack was very small, (near zero) because 9 times the lift was available for a given angle of attack. Once the boat accelerated, the need for high lift devices disappeared, and the need became 1. righting moment, 2. control and 3. drag reduction. For the latter, a clean wing without flaps, slats, vortex generators etc, is hard to beat. (Thus sailplanes -- where L/D is also critical -- have very clean wings.)

Where one would be most likely to see VGs in sailing is in boats that are optimized for a particular wind speed and boatspeed/windspeed multiple: namely on record setting boats like Yellow Pages Endeavour. And there, the application would be only to a boat with otherwise optimized aerodynamics in the form of a rigid wing. Using them with soft sails makes as much sense as using soft sails on a Boeing 747 would.


* because VGs are mainly useful at high lift conditions, but speed records are set in low lift conditions (for somewhat the same reasons that airliners do not cruise with their flaps and slats deployed.) When a fast boat is generating its own wind, you don't need more lift, you just need less drag.

[Ken Fry]

Originally Posted by Dave_S

As I understand it the lift comes at 90° to the surface of the sail.

That's incorrect, but a common misconception.

Lift and drag are aerodynamic terms that start to make little sense when used in the ways sailors casually use them.

Lift is always perpendicular to the free stream airflow, by definition.
Drag is always in line the free stream airflow, by definition.
Apparent wind = free stream airflow.

So when a wing is tested in a wind tunnel, and the angle of attack is increased from 0 degrees to 10 degrees, the lift is still always measured straight up, and drag is always measured parallel to the wind tunnel airflow.

A person could measure pressures along the surface of an airfoil, and find that there is a force at each point along the airfoil that acts perpendicular to the surface at each point. That force near the front of the airfoil would seem to pull the airfoil forward. Near the rear of the airfoil, such force would tend to pull the airfoil back. (CFD actually looks at things in roughly this this way.) But how cumbersome it would be to add up say 100 different vectors, to find out the "lift" in the direction of interest. So, the convention is as I stated above. If you get the angles wrong, then every thing else falls apart, and reliable vector sums become impossible.

The justification of a rigid wing for a fast boat is simply: improved lift over drag. None of that makes sense if one is led to believe that lift acts in all sorts of different arbitrary directions.

In determining the speed potential of a boat, one draws up a lot of vector diagrams, and one finds that as boat speed increases, the aero drag increasingly aligns with the boat's motion through the water, because the apparent wind increasingly aligns with the boat's motion, as well. Lift acts increasingly closer to perpendicular with the boat's motion through the water. The total lift (perpendicular to the apparent wind) has a small vector component pushing the boat forward, and a large vector component pushing the boat sideways (causing leeway and heeling).

Underwater there are similar vectors in play at the keel or centerboard, and with lift and drag both increasing with the square of speed.

[Ken Fry]

Originally Posted by StoneCrab

As for a sailboat, if I were him I would look up if anyone else has done it. If it is actually helpful, I would think racers would already be doing it. I don't see the harm is testing it out, but I don't think he would see a tangible benefit. Since you aren't worried about stall on your sails, the only benefit would be a decrease in drag. If possible the VGs would work best about 1/4 of the way behind the leading edge of the sail (instead of on the mast).

This whole post is excellent, and, if it were read by others, would clear up many misconceptions. It correctly characterizes VGs as being of use in energizing the boundary layer to delay full stall to some higher angle of attack. As he points out in the part quoted above, sailors do not sail with the sails stalled. (The exception is a for a spinnaker.) He also points out that if one wanted to delay stall on a sail, placing the VGs about 25% back would be a good starting point. (But why would you want to delay stall on a sail that is not trimmed to stall?) Placed on the mast, they would have no beneficial effect.

He implicitly makes clear that "attached" is the opposite of "separated" and that "laminar" is roughly the opposite of "turbulent". VGs do not contribute to making flow laminar: that is the opposite of their function. They make flow vortical, a special sort of turbulence. If the OP is looking for more laminar flow, VGs won't help. And they would have no beneficial effect on a sail that is not trimmed to near stall angles of attack. And even then they would need to be mounted to the sail cloth.

The only recent sailboats that operated (when at very low sailing speeds) at very high coefficients of lift (i.e somewhere near stall angles of attack) are the C class cats like Cogito. Cogito's wing could operate at something like a Cl of 3 (or more, per some claims). A sail operates at something like CL =1, or .8 if it is new and flat. Cogito had the same sort of high lift thinking that is applied to Boeings, and under certain limited conditions could have benefitted from VGs -- but only when the boat was going slow, when the high CL could be used. As soon as Cogito was going a little faster, then the wing would be flattened, and would operate at a CL where VGs are of no use.

My Windrocket wing was simpler that Cogito's, and rarely operated at its maximum CL of about 1.8. It might have been a candidate for VGs if I were using it as a tow boat, but as it was, the time spent at high CL was very brief -- a few seconds in 10 knots true, before the boat was going fast enough to back off on CL.

Marchaj's book is the classic on such matters as they apply to sailing. Abbott and Doenhoff is the classic airfoil book. Both worth reading for those so inclined.

[Ken Fry]

Originally Posted by billdomb

Then why the turbulence generators on some airplane wings. Sometimes, it appears, a little turbulence keeps the laminar flow layer attached longer.

Counterintuitive.

VGs keep turbulent flow (not laminar flow) attached longer. See the very good post from the friend of aero student, with its diagram.

[jeepers]

Yes.... But No.
The perceived wing shape of a sail is not rigid like that of an airplane wing. It has little or no frame. The "convex" & "concave" type of shape, is created by air. It is fluid. It is neither symmetrical, semi symmetrical or under chambered. It is a sail... The semi wing like profile "concave shape" is first created by high pressure air in laminar flow on the windward side of the sail not exactly like an under cambered wing. But has vaguely similar effect in low air speeds is that it creates a minor lift on the lee side(which complements the windward pressure on the sail) With a large value by-product being drag. A source..https://youtu.be/3VTmx_JIf9E?t=105 if your interested to pursue it.. Also search; under cambered wing profiles. The amount of negative air pressure on the lee-ward side of the wing, will produce a vector with little resulting forward component.Mostly because the "effect" changes the "cause" the shape in a sail. Now if you could fill a "structured sail" (Maybe with "low pressure" compressed air..)Then it would be more of a wing with structure & controlled shape. A sail that had shape characteristics such as the 2 wing surfaces on such a large area. Could create lift & the need to control laminar flow. VG or Vortex Generators is such an Oxymoron term. It is a stupid as a Weatherman calling a "Coldfront" a Polar Vortex...

[Dave_S]

Originally Posted by Ken Fry

That's incorrect, but a common misconception.

Lift and drag are aerodynamic terms that start to make little sense when used in the ways sailors casually use them.

Lift is always perpendicular to the free stream airflow, by definition.
Drag is always in line the free stream airflow, by definition.
Apparent wind = free stream airflow.

So when a wing is tested in a wind tunnel, and the angle of attack is increased from 0 degrees to 10 degrees, the lift is still always measured straight up, and drag is always measured parallel to the wind tunnel airflow.

A person could measure pressures along the surface of an airfoil, and find that there is a force at each point along the airfoil that acts perpendicular to the surface at each point. That force near the front of the airfoil would seem to pull the airfoil forward. Near the rear of the airfoil, such force would tend to pull the airfoil back. (CFD actually looks at things in roughly this this way.) But how cumbersome it would be to add up say 100 different vectors, to find out the "lift" in the direction of interest. So, the convention is as I stated above. If you get the angles wrong, then every thing else falls apart, and reliable vector sums become impossible.

The justification of a rigid wing for a fast boat is simply: improved lift over drag. None of that makes sense if one is led to believe that lift acts in all sorts of different arbitrary directions.

In determining the speed potential of a boat, one draws up a lot of vector diagrams, and one finds that as boat speed increases, the aero drag increasingly aligns with the boat's motion through the water, because the apparent wind increasingly aligns with the boat's motion, as well. Lift acts increasingly closer to perpendicular with the boat's motion through the water. The total lift (perpendicular to the apparent wind) has a small vector component pushing the boat forward, and a large vector component pushing the boat sideways (causing leeway and heeling).

Underwater there are similar vectors in play at the keel or centerboard, and with lift and drag both increasing with the square of speed.

Thanks for your response.

"Lift is always perpendicular to the free stream airflow, by definition.
Drag is always in line the free stream airflow, by definition.
Apparent wind = free stream airflow."

I'm having trouble getting my head around this, if apparent wind is at 30° while tacking upwind then the sum of lift from the sail would be at 300° (90-0, 330-60 etc..) irrespective of, but assuming a workable trim.

Surely sail shape will change this? What if we have seperation on the leech where lift was pulling backwards, close hauled. Your statement doesn't allow for the sum of the lift angle change.

[Ken Fry]

Originally Posted by Dave_S

Thanks for your response.

"Lift is always perpendicular to the free stream airflow, by definition.
Drag is always in line the free stream airflow, by definition.
Apparent wind = free stream airflow."

I'm having trouble getting my head around this, if apparent wind is at 30° while tacking upwind then the sum of lift from the sail would be at 300° (90-0, 330-60 etc..) irrespective of, but assuming a workable trim.

Surely sail shape will change this? What if we have seperation on the leech where lift was pulling backwards, close hauled. Your statement doesn't allow for the sum of the lift angle change.

Here's a pic, maybe:


When we put a wing or sail or keel in a wind (or water) tunnel, we usually span the entire tunnel so that all the flow goes around the foil in the nice streamlines we imagine, and not around the ends of the foil, the way it does in real life. (That spanwise flow is why airliners often have winglets to improve L/D.) That kind of flow modeling is called two dimensional: you can look at it on a two dimensional computer screen, or a piece of paper. The data obtained is called "section data"

In real life flow is three dimensional, with flow running out along the wing to the wingtip (or over the leach of a triangular sail). In real life, each "section" along the span (or up the mast) is different (unless you are flying a Piper Cherokee, with its "Hershey Bar" wings -- its sections are all pretty much the same). (In sails, the top 5 feet or so is near useless in section shape -- thus the popularity of square top sails.)

So, when we look at a wing lift-drag definition picture, it is as if we are looking straight down at a sail (or a wing on one of my boats) from above. A sail is like any of 100 other airfoil shapes -- just far less efficient than most that are found on airplanes, and far less adjustable (contrary to common misconception). The terms lift and drag are used in exactly the same ways for a sail or wing or keel. (Lift in a sailboat is, of course, in a different direction). Total aerodynamic force is decomposed into a lift component and a drag component, which are, always and forever, exactly perpendicular to one another. The utility of vector addition falls apart if this is not the case. But also, in both sailing and flying, lift (the good stuff) and drag (the bad stuff) are the subject of a great deal of study... and the terms and their definitions go back to before the Wright brothers.

There are an infinite number of directions of "liftlike" force acting on a wing surface. Add up all their magnitudes and directions, and you get total aerodynamic force -- a force that is rarely mentioned, because it is not useful in engineering an airfoil. What is useful to know is lift and drag.

So you are correct, first, that if the apparent wind is from 30 degrees, then lift acts a 300 degrees. Drag acts at 210 degrees. (Aero drag on the rest of the boat -- rig, cabin, etc, also acts at 210 degrees. ) Changing the shape of the sail does not change the direction of these forces; it changes their magnitudes and the ratio between them. Flattening a sail reduces lift and reduces drag. A baggy sail incurs an increase in drag but also an increase in lift, typically. In your example of separation near the leach, the drag force increases and remains at 210 degrees. The lift force may be greater of less, but remains at 300 degrees. What you are calling "lift" is total aerodynamic force, and its magnitude and direction changes with the length of the lift and drag vectors.

Sail shape (as compared to the "real" airfoil shapes that NACA and aircraft manufacturers spent - and spend - billions to develop) ranges from quite poor to awful. Plan form (what an aircraft wing looks like from above, or a sail looks like from abeam) ranges from poor to awful. So... gliders operate at 60:1 lift-drag ratios (even when throwing the fuselage drag in) whereas sails operate at about 6:1.

Most sailors have experienced mainsail trimming. Once you have the sail close to OK, and then play around with easing a little, you find that there is one setting that "works": just when the luffing goes away. Oversheeting increases lift a bit, but increases aero drag even more (and adds heel and underwater drag too) So if your rig does not have a lot of strings to tweak sail shape, your best coefficient of lift might be .8. Ease much from that, and you start to luff, so there is no good way of getting a CL of say .5. That's why sailboats traditionally have numerous sail sizes, reef points and any number of additional controls to make them better than totally useless. Compare that to Cogito, the wing of which can operate all day at a CL of .2 or .4 or .8 or 1.4 or 2.0 or 2.5, or 3.0 and can change instantly from one to another -- just as an airplane can do by deploying flaps and slats.

I'm rambling here. But back on point: You are essentially correct regarding there being a force that changes its direction with the angle and shape of a soft sail. That force is not called lift, however. It's called total aerodynamic force. Lift is always perpendicular to the apparent wind -- because that is its definition and because the two perpendicular forces (L and D) are very useful to know , because they determine how fast a sailboat can go and how efficiently an aircraft can fly. (In gliders, L/D and glide slope are essentially the same.)

If my picture does not show up it is from wikipedia: https://en.wikipedia.org/wiki/Lift_(force)