Does a sail act as a turbine blade or a propeller blade?

FWIW, "absolutes are for tyrants" is becuase tyrants are absolute rulers and are the only ones who can make absolute laws, not that you or the poll is tyrantical.

Anyway, you seem to be caught up in the idea that that a propeller must cause a constant increase stream velocity (a vector it is important to remember). In truth that is not the case, though for most propulsor theory it is. Rather a propulsor must increase the energy in the fluid to generate a useful force, not necessarly increase any free stream velocity relative to the direction of travel (there are some cute toys based on some tricks to that).

Lets take Rule 42 as an example. You are sitting in a el Toro on a windless day. You sheet out the sail slowly and then pull back rapidly, the boat moves forward and you get protested under 42.2 and DSQ'd. Why? When the sheet came back it pushed (compressed) the mass air aft of the sail and pulled (suctioned by having the same mass now have to fill a larger volume) at the mass in front of it. Remember when I said there is no massless inviscid flow? The mass of air in direct contact with the sail now has to move, and moving a mass from rest requires an acceleration and a consequently a force. The sail does work on the air and consequently a force is generated on the sail which is transferd by the rig to the hull, which is free to move in the horizontal plane, and subsequently to the water as the hull does move (you pulling on the sail also pushes on the hull but that is cancled buy the hull pushing back so it is a null effect)---the sail was used as a propulsor.

Now getting back to what happens in the air, you have added energy to the fluid by creating two zones of del energy, the high and low pressure areas. This energy is dispersed to the surrounding air at the speed of sound (kenimatic viscosity limited). As the fluid must maintain a constant pressure limited only the ability of monemtum to disperse it, it uses a pressure wave and a viscious shear flow to distribute energy in the form of knetic energy to a greater and greater volume of mass until finaly the velocity of the infinte mass of air has been given an infintessimal velocity, thereby rasing the energy in the entire fluid. Conversely, the exact same thing is occuring in the water as the hull moves forward, except there is much less compressability and much more mass and viscious effect.

Edit to add:

If you want me to describe sallying a large boat I can, but it easily can be shown to be similiar to the discussion above.

 

No, I'm not caught up with any "absolute". I'm giving examples that I find relevent. For the sake of this discussion, I've as comforable with your definition : "a propulsor must increase the energy in the fluid to generate a useful force" as I am with my "accelerated relative to the free stream" definition -- we'll use yours.

Let's not. No offense, but while it may not be explicitly excluded from the OP wording, we're not interested in an obscure sailing example of taking a sail and flapping it like a wing. While everything you say about that scenario may be true, it doesn't relate to the OP.

Let's stick to steady state scenarios -- traditional points of sail, fixed and steady wind, fixed boat course, fixed sail position, etc.

Is there a point of sail and sail position that will always result in the sail acting to "increase the energy in the (air)"? And is there an inversion position?

As poorly worded or as "foolish" as you may find the poll, the above is at the core OP intent.

I'm guessing it should be really easy for someone of your knowledge to educate me as to the steady state scenarios that determine these transient states to which you referred.

JB

 

jehardiman, I'll ask a more precise question through pictures:

Could you tell me for the following examples, whether the sail on the boat is acting as a "propulsor" by our previously established definition, or the inverse:

First a downwind example with a VMG at or near 1.0 TWS


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and then an upwind example where I chose VMG of .5 TWS:



Thanks

JB

PS: I do apologize for my crappy drafting skills

 

Yes, I can....a boat traditionally being carried downstream by a steady current on a windless day, however I expect that you will apparently find fault with that and require the additional condition of static water and flowing air instead of the reverse...

As the "steady state" system you want is a closed contrivence (as much as the situation above) which can only lose energy to entropy, overall energy in the air stream will always be reduced, though axis systems can be contrived to show an apparent increase in energy along one of those axis. The Laws of Thermodynamics prevent anything else.

Edit: X-post with your drawings:

Both situations result in loss of energy from the air to the water and loss of overall energy in the system to entropy

 

Me:

So If I understand your response, you're saying that when a boat sails upwind, its sail is acting as a propeller and "increasing the energy in the air"?

Is that an accurate representation of your position?

Final answer?

JB

 

The final answer will always be "it depends".

When a boat is sailing in steady state with still air and moving water, the "keel" is extracting energy from the water and transfering it to the sail to propel the boat in the desired direction, so the sail functions as a "propeller" to you. It uses the air to generate a force in the direction you want to go, like the rudder on an airplane or a the wing of a glider.

In the more common steady state case of still water and moving air , the sail is extracting energy from the air and transfering it to the keel to propel the boat in the desired direction, so the sail functions as a "turbine". It pulls energy from the air, but cannot direct that energy, similar to a kite whos string is cut. To work aganist the wind, a keel is needed.

Without a keel in the water, the boat would only drift down wind. Conversely, with out a sail, the boat would only drift down current. To sail a course, the boat needs both a sail and a keel, as to which is extracting energy and which is outputting energy depends of the point of reference (water, air, or boat), the relative speeds of the three items to each other, and the relative geometrical relationships between the sail and the keel. See the first part of Aero-hydrodynamics of Sailing.

 

You appear to be contradicting yourself:

First you say relative to the drawings:

Then you say:

There's no "depends" in your above answer. Your above answer has the sails in both pictured scenarios acting as the inverse of propulsors and thus acting as turbines.


JB

 

I not asking you to go into the details JE, but is there someway to instrument a sail to know when it's acting as a prop or a turbine? Could we perhaps 'smoke' it and film the results and know? Some other way perhaps?

JB

 

This is a cross-post issue. As you put no current in your drawings, I assumed you wanted still water. In that case my original statement stands. If you accept the possible case of moving water and still air, then it depends.

I'd have to think about it, but most likely an experimental result would need to be a net vector force determination of both hull/keel and sail which could be difficult. But is that really useful? As I started this discussion off with, wether it is work in or work out is irrelevent to the equations, they are the same in both cases, only the input relationships would vary.

 

Ahh, ok. That clears up the contradition. Thanks.

Me:

jehardiman:

Well, if your "moving water and still air" vs "moving air and still water" assertions are true, it would be more useful than you could ever imagine.



I've always believed that through even simple visual means one could tell if a blade is acting as a turbine or prop, but if your above assertion is true, then I'm thinking I must be wrong.

As a test of my above belief, let me try this on you:

Here's another image I created. You can imagine the below blades in one of two scenarios:

1: Top view of a windmill/fan with the axle running the same direction as the wind.

2: Top view of a set of vertical blades with a constraint such that they can only slide to the left and right.

For the purposes of this discussion, either one you pick will work for me.




If we set the above blades out in the wind per the above drawing, can we agree that:

A: If the blades move to the left in the steady state wind, they are acting as turbine blades and each blade is "removing energy from the air"?

B: If the blades move to the right in the steady state wind, each blade is resisting the force of the wind, is acting as propeller blade and is "increasing the energy in the air"? (this of course requires some other force to be acting on the blades other than the direct force of the wind)

Do you agree?

JB

 

Yes - I've pointed that out to him as well. He doesn't care what the O.P. means nor what it says. He's starting with a conclusion and coming up with bizarre transient cases and such to try and shore up what's left of his losing argument.

I am plenty familiar with your level of knowledge on the topic that I can assure you he won't be teaching you anything. If he's half as smart as he thinks he is, he may let you teach him something however.

I claimed that I did have the time and inclination to correct his flawed understanding of fluid dynamics, but I now have to confess... I can't possibly keep pace with his rate of making errors.

jehardiman decided to show up and insult me without the first clue as to who I am, what I do, or what level of understanding I have of the subject.

I doubt the discussion you have with him will hold much interest beyond the sociological level. Good luck.

 

Not really, waterwheels have been used for centuries to extract power from moving water (indeed many low head hydroelectric plants use propeller-like turbines) and the need for large side force to drag to go forward in a conventional sailing craft is well documented. There are even power plants, boats, and toys that use the wave orbital velocity for power generation. There is nothing mystic going on here that hasn't been better explained by half a dozen sailing theory books or other fluid mechanic texts.

Ok, assume simple frictionless model with no initial transverse velocity. If you constrain the axis of movement to perpendicular to the wind and transverse forces are only generated wind velocity (i.e. no input forces) then airfoils will always move to the left decreasing the total energy in the flow, because in the absence of any other force in the transverse direction, the foil cannot move to the right and is easy to show on any high-school physics air track with a fan and some simple setup. It is important to note that the foils will not continue to accelerate to the left, they will eventually reach a transverse velocity where the net force on them is all in the direction of V and no work is generated out of the system given the constraint in the flow direction, but the drag of the foil extracts energy from the flow in the form of viscous entropy.

However, if there was a transverse energy gradient such as an EM field, or gravity, then whether it moved left or right would depend on the effect of that field also. Think of your drawing as left up and right down. If the foil has weight, then there would be V below which it would move right (down) putting energy into the air, at exactly V it would not move right or left ("lift" = weight) doing 0 work, and above V it would move left (up) extracting energy from the air. This is just the same a kite on a string.

But this is a simple 2 DOF frictionless example: moving to 3D and 6 DOFs complicates the matter and we must get into multiple constraints.

 

If all that JB has learned from this discussion is that opinions you get on the internet are worth what you pay for them, then he has learned a significant thing. The best that I hope for is that he follows my original advice and get a good text and some real design experience and finds his own answer to his poll question.

The problem that we appear to have is that the original 4 lines of this thread allowed us to play in the whole sandbox and not be limited to classical theory and unrealistic constraints. There was nothing to restrict the sceanairos, and as an engineer that deals with some really off the wall hydrodynamic situations, I have to deal with those ragged edges.

If JB wanted to start from the classic steady state, invisicid, irrotational, isodense, static water point then, yes, it is easy to prove that a "sail" in the classic sense always extracts energy from the air and would be a "trubine" as defined. On the other hand, as any sailor that has significant sea time under sail in waves knows, it is obvious that there are situations where the interaction between the rig, the hull , and the environment, cause the rig to impart work into the air, which by the polls defination is a "propeller". And you even agree to that (post #17). Both cases are contrived, so my statement that the poll is foolish stands.

Pity that when the first stone landed in my yard (post #31) and I tossed it back, it reflects poorly on both of us. I am sorry that you feel slighted, but your question about the windmill really was off if you had though about it. Of course no real wind is going to always be constant and depending on the situation AOA will be jumping all around. It would have been interesting to hear your views on some of the process of actually selecting modeling techniques to solve a real world problem. Come on, proposing to use disk theory on a sail (post #17 and 31)? Blade element theory would have been a much better choice, and energy methods an even better one. And the fact of the matter is that the development of the equations of state and the total energy equation are the same all flows (unless you want to dig into some of the weirder behaving fluids), only the limit constraints are different leading to different solutions. You do not reformulate the equations if del P is positive or negative, the math handles that. And steady state never exists in the real world, especially when dealing with two fluids at thier interface, so it would be interesting to see if you are familiar with Beck's and Newman's work of flow in waves these last few decades. The problem exists in all fluid modeling that some assumptions and limiting conditions must be made to allow viable solution. In the real world, only solving the steady state condition is often viable, but sometimes not. The question is to chose 20 hours of Cray time to run a Multi-Vortex solution on a regular body that only gives the loads for steady flow, or will the first principles panel model that handles non-steady flow give a better answer at slow speeds of advance in a seaway. Even RANS has coefficients to "calibrate" itself to the total energy equation that can be argued over given the starting Rn or the presence of large eddies.

FWIW, I've seen lots of discussions like this my job, and some real knock down drag out fights much bitter than this. Opinions about and solutions to hydrodynamic problems are as varied as the individuals who have to solve them. If two people independently solve the same problem and get within ~20% of each other, I have usually considered the problem solved. The problem exists when only one solution is proposed and the assumptions to that solution are flawed.

 

Actually, I think if we want to find the first projectile, we'd have to go back just a bit further to your very first post in this thread when you offered: "JB, I would suggest you hit the books more; a good thermo and fluids book with historical perspective will show you just how foolish your poll was."

If you really do think a civil discussion on the topic would be interesting it certainly isn't too late. Unfortunately, my schedule will have me only checking in on occassion in the short term. But I will say this. You've taken the question in entirely the wrong way. It's not intended to ask whether a specific lifting device (i.e. sail, wing, prop, keel, etc.) CAN be employed as a prop or turbine; nor is it intended to investigate transient cases of gusts, turbulence, etc. I also doubt the discussion takes us down the path of Navier-Stokes, Reynolds numbers, inviscid flow, etc. but I'm open minded to that possibility.

I know JB well enough, and have been involved in this discussion long enough, to know that he's asking whether a sail is behaving as the blade of a prop or the blade of a turbine at any given point of sail and steady state condition (e.g. downwind VMG faster than the wind).

 

Yes, really.

You're a hard man to keep on track ... I can't tell if that's a "Yes we can." or a "No we can't." as it related to my original question: 'Can we learn from instrumenting or examining a sail somehow whether at any given point it's adding energy to the air, or the inverse?"

When I first asked the question, you responded:

If you're still thinking about it, that's fair -- but it's a more relevent question than you might think and I'd greatly appreciate a "Yes" or "No".

JB