Vortices as source of aerodynamic force?

[Sailor Al]

If I had a half-decent secondary school physics lab with an air table and air track, I could easily show that there is no physical "lift", only a component of the body force in the direction you want to define it in. I'd have Sailor Al and the other students make all kinds of sails and "lifting" surfaces and we would test, and track, and analyze each and every one. We would get down to the "philosophy" of the physics of "lift". But I can't do that for people on the internet when they choose to keep their mind closed.

Now you're singing from my songbook, just a little off key!
The aviation industry generally accepts that lift is the component of the aerofoil force perpendicular to the airflow*. In aviation, it is logical to resolve the aerofoil force relative to the airflow since, in cruising flight, it coincides with the vertical force of gravity, and the primary purpose of a wing is to support the weight of the plane! In that scenario, the component parallel with the airflow is drag, which has to be oppose by the thrust of the engines to maintain the required airspeed (and incidentally transport the passengers/freight to their destination!).


My issue with the use of the term lift in relation to sails is that it is illogical to resolve the aerofoil force around the airflow. The primary purpose of the sails is to provide thrust in the direction of travel (C/L + leeway). It is more appropriate to resolve the aerofoil force into thrust and ....and there's the problem: what do we call it? sideways force, leeway/heel, FLat, ... there's no generally accepted term.
But we must stop talking about lift.

*OK, I know, is it relative to the airflow, or the chord of the wing, or the centreline of the plane... that just adds to the whole confusion. Let's stop talking about wings altogether!

 

[jehardiman]

Mr Halsey, understanding you state you are a retired Aeronautical Engineer, and as I mentioned that Vmg is tricky, I will not believe that you disrespected everyone on this forum and deliberately committed a most egregious violation of engineering ethics by attempting to deliberately misrepresent. Vmg in naval architecture, sailing, and navigation is defined as the component of velocity directly toward an object and generally, unless specified, with respect to sailing vessels Vmg is understood as to windward. In my previous comments we were discussing Vmg to windward. Indeed Marchaj works this case for Eq 1.12 on the bottom of the previous text page ( page 139 in my edition) so perhaps you just missed it. As Marchaj states in your example Eq 1.14, the ratio he calculates is to leeward, which generally would be considered negative Vmg, i.e. a Vmg/Vt ratio of -2.57.
Be that as it may, you still cannot travel faster than the energy exchange between the wind and the vessel.
Let us repeat my philosophical example with the numbers above.
Vt = 10 knt, AOB = 142, SOG = 32.6 knt. Same as before I'm going to sweep a mass of wind sailing for an hour. However, because I'm now moving in the same directions as the wind, I have to figure out how deep that mass of wind is.
25.7 = d+10 > d = 15.7.
What is my change in position relative to the wind that provided the energy in the wind direction only ..... vector math: wind vector + vessel vector ...((wind end)-(wind start))+ ((vessel end)-(vessel start))
Wind start = -7.85
wind end = 7.85
vessel start = 0
vessel end = - 25.7
((7.85)-(-7.85))+((-25.7)-(0)) = (15.7)+(-25.7) = -10
So in that entire hour, the energy transferred was equal to the dot product of body drag* and the distance it was blown DDW by the wind. So you can't outsail the energy that the wind is putting in. While Marchaj only deals with the forces in the section you referenced, there is no engineering doubt that those forces are caused by energy transfer. I would direct your attention to full discussion and Fig. 1.60 where he shows this graphically. The reason I specifically mentioned ice boats is because they have the most "efficient" (side force to drag) "hulls" on practical sailing vessels (air tracks don't count).

(Edit: * should be "net aero force on the sail", see Post #49)

 

[jehardiman]

So he's confusing circulation and vorticity in flow fields with real vortices and real flow in fluids? Do I have that right?
McLean's video would seem to support that theory.
If so isn't it a pretty fundamental mistake that an associate professor in the school of engineering should not be making?

This thread has kind of drifted past this topic now, but I have been looking to quote an old post I remembered related to the original topic (Vortices as source of aerodynamic force?).

Leo Lazauskas said:
My source of bewilderment stems from a commonly seen statement that:
"molecular viscosity is a property of water, whereas eddy viscosity is a property of the flow".

Big whirls have little whirls,
Which feed on their velocity.
Little whirls have smaller whirls,
And so on to viscosity.

L. F. Richardson: English mathematician, physicist, and meteorologist who pioneered modern mathematical techniques of weather forecasting

Have you seen "Vortex Flow in Nature and Technology" (Hans Lugt, Wiley & Sons, 1983, ISBN 0-471-86925-2)? There is a fairly good discussion on turbulence and how it is formed and it effects. In the end though, I think each hydrodynamicist needs to come to his own understanding on the interplay of the energy transfer between viscosity and flow because different needs will cause the boundary to be a moving point.

Edit: X-post with the two above items.

The important part was "I think each hydrodynamicist needs to come to his own understanding on the interplay of the energy transfer between viscosity and flow"
There is more from Leo in that thread that makes interesting reading on the effects of turbulence.
resistance test in wave conditions https://www.boatdesign.net/threads/resistance-test-in-wave-conditions.37181/#post-450193

 

[jehardiman]

My issue with the use of the term lift in relation to sails is that it is illogical to resolve the aerofoil force around the airflow. The primary purpose of the sails is to provide thrust in the direction of travel (C/L + leeway). It is more appropriate to resolve the aerofoil force into thrust and ....and there's the problem: what do we call it? sideways force, leeway/heel, FLat, ... there's no generally accepted term.
But we must stop talking about lift.

Ok, first some terms for the point of this discussion (and yes I know I previously used "body force" when I should have used "net aero force on the sail" but I wanted to emphasize that I was talking about the forces on the vessel/body and not all the forces in the system):
Body force - Wikipedia https://en.wikipedia.org/wiki/Body_force
Surface force - Wikipedia https://en.wikipedia.org/wiki/Surface_force
Net force - Wikipedia https://en.wikipedia.org/wiki/Net_force FWIW in Naval Architecture, the typical origin of the vessel reference axis is the center of mass, right hand rule, X-fwd, Z-down. (One of the big differences between airplanes and submarines is that take-offs = landings, but dives do not always = surfaces).

To be precise, the term you want to use is the "net aero surface force". This is the force on the whole body (sails and air exposed hull surfaces including atmosphere communing internal compartments if any) from the static and dynamic pressure of the air which includes the body forces working on the air. This is not the "net force" which is the summation of all forces on the vessel. The net force is what causes the vessel to move, both in translation and rotation. Net force is a function of both the "sail" (aero) and the "hull" (aero and hydro) surface and body forces. Historically, the net aero surface force has been divided into a "lift" vector perpendicular to the relative "flow" vector and a "drag" vector parallel to the "flow" vector. Since the "flow" vector is a relative term, it gets a lot of people into trouble. The "flow" vector include the true wind speed vector and the net body velocity vector. In my first example the sensed "flow' vector was 18.5 knots from 022.5 vice the real 10 knots from 000. Likewise in the second example the sensed "flow" was 25.5 knots from 128.

 

[Doug Halsey]

Mr Halsey, understanding you state you are a retired Aeronautical Engineer, and as I mentioned that Vmg is tricky, I will not believe that you disrespected everyone on this forum and deliberately committed a most egregious violation of engineering ethics by attempting to deliberately misrepresent. Vmg in naval architecture, sailing, and navigation is defined as the component of velocity directly toward an object and generally, unless specified, with respect to sailing vessels Vmg is understood as to windward. In my previous comments we were discussing Vmg to windward. Indeed Marchaj works this case for Eq 1.12 on the bottom of the previous text page ( page 139 in my edition) so perhaps you just missed it. As Marchaj states in your example Eq 1.14, the ratio he calculates is to leeward, which generally would be considered negative Vmg, i.e. a Vmg/Vt ratio of -2.57.
Be that as it may, you still cannot travel faster than the energy exchange between the wind and the vessel.
Let us repeat my philosophical example with the numbers above.
Vt = 10 knt, AOB = 142, SOG = 32.6 knt. Same as before I'm going to sweep a mass of wind sailing for an hour. However, because I'm now moving in the same directions as the wind, I have to figure out how deep that mass of wind is.
25.7 = d+10 > d = 15.7.
What is my change in position relative to the wind that provided the energy in the wind direction only ..... vector math: wind vector + vessel vector ...((wind end)-(wind start))+ ((vessel end)-(vessel start))
Wind start = -7.85
wind end = 7.85
vessel start = 0
vessel end = - 25.7
((7.85)-(-7.85))+((-25.7)-(0)) = (15.7)+(-25.7) = -10
So in that entire hour, the energy transferred was equal to the dot product of body drag* and the distance it was blown DDW by the wind. So you can't outsail the energy that the wind is putting in. While Marchaj only deals with the forces in the section you referenced, there is no engineering doubt that those forces are caused by energy transfer. I would direct your attention to full discussion and Fig. 1.60 where he shows this graphically. The reason I specifically mentioned ice boats is because they have the most "efficient" (side force to drag) "hulls" on practical sailing vessels (air tracks don't count).

(Edit: * should be "net aero force on the sail", see Post #49)

When I first glanced at my phone this morning (in a hurry doing other things), I saw the word "disrespect" and decided to put off reading the rest of the post until I had more time. I figured you were probably upset (and rightfully so) that I had agreed with Sailor Al that what you wrote in post #35 was crazy. For that I feel like you do deserve an apology, and I am truly sorry.

Now that I've read the post quoted here though, I'm mystified how you can take the ambiguity in use of the term VMG and infer any disrespect, egregious violation of engineering ethics, or deliberate misrepresentation from it.

I probably should have been more careful to point out that I was mainly thinking about VMG downwind (even if you weren't), and how boats' having that VMG>Vt is in conflict with what you were saying, but compared to the earlier remark that post #35 was crazy stuff, I can't see getting so upset about it.

I'm not sure whether I am just misunderstanding what you are trying to say, or if I outright disagree, but for now I'm going to drop the subject.

 

[Sailor Al]

Yes, it's all become too silly and I'm signing off too.

 

[tspeer]

...
My issue with the use of the term lift in relation to sails is that it is illogical to resolve the aerofoil force around the airflow. The primary purpose of the sails is to provide thrust in the direction of travel (C/L + leeway). It is more appropriate to resolve the aerofoil force into thrust and ....and there's the problem: what do we call it? sideways force, leeway/heel, FLat, ... there's no generally accepted term.
But we must stop talking about lift....

Actually, there is a very good reason for considering aerodynamic lift in the context of yacht performance. Here is what the lift and drag vectors, both aerodynamic and hydrodynamic, look like:

If you are concerned with yacht performance, it does not matter what the leeway angle is. What matters is the velocity vector through the water. So it makes sense to take the apparent wind angle as the angle between the boat's course through the water and the apparent wind vector. This may seem like it's only a small modification of your point, in that now we're concerned with resolving the aerodynamic force in the direction of motion, opposing the hydrodynamic drag component, instead of resolving the aerodynamic force along the centerline of the hull.

The aero and hydrodynamic drag angles are defined as aero_drag_angle = arctan(aero_drag/aero_lift) and hydro_drag_angle = arctan(hydro_drag/hydro_lift). In the figure to come, gamma is the course through the water relative to the true wind direction (zero is heading straight into the wind).

A little rearranging of the vectors will show that the apparent wind angle, taken between the course through the water and the apparent wind vector, is equal to the sum of the aerodynamic and hydrodynamic drag angles. This comes directly from the wind triangle and the definition of lift and drag as being perpendicular and parallel to their respective fluid motions.

Apply the Law of Sines and you get the fundamental yacht performance equation. It is not necessary, nor even helpful, to resolve the lift and drag into drive and side force in order to calculate the performance. Lift and drag are what you want.

 

[jehardiman]

Actually, there is a very good reason for considering aerodynamic lift in the context of yacht performance. ...<snip>...

Tom, I wish I could have been as succinct and elegant when I made this comment:

Ok, the first concept a sailor needs to grasp is that there is no "lift", i.e. moving "up" an energy flow. There is only drag. And both the sails and the hull only provide drag. It is the vector summation of these two drag forces on the vessel as a whole that provides motion that appears to go "up wind", but in the energy sense it doesn't. This is easily provable by direct experiment. A sailor needs to know that the sails are useless without the hull, and the hull limits absolute performance regardless of what type of sail is used.

And kept this topic out of the black pit of energy transfer.
However this topic started with a post about the concept of how the wind is converted into the forces on a sail. As I said in a thread long ago; fluid-mechanical theory needs to explain everything from wings, to sails, to keels, to propellers, to eddy sheading in wakes, to why soup gets hot in a blender. The only way to do this is from an energy perspective...where is it coming from, where is it going to, and how is it getting transferred. In most cases we can get by with less than complete philosophical models. However, push too hard on the edges and it just falls over because it is only a paper and wish façade.

 

[Sailor Al]

Actually, there is a very good reason for considering aerodynamic lift in the context of yacht performance. Here is what the lift and drag vectors, both aerodynamic and hydrodynamic, look like

And that, dear readers is an example of exactly why we should stop talking about lift.
For aeronautical reasons, the net aero force has been resolved around the airflow:

If the net aero force is resolved around the direction of travel, the picture is so much clearer:

 

[Sailor Al]

I should have posted the simplified version:

 

[DCockey]

And that, dear readers is an example of exactly why we should stop talking about lift.

Who are "we"? If I find talking about lift to be useful in a particular context I will talk about lift in that context. Others are free to ignore any discussions involving the concept of lift.

 

[Sailor Al]

Who are "we"? If I find talking about lift to be useful in a particular context I will talk about lift in that context. Others are free to ignore any discussions involving the concept of lift.

In which particular sailing context do you find it useful to talk about lift?

 

[jehardiman]

Who are "we"? If I find talking about lift to be useful in a particular context I will talk about lift in that context. Others are free to ignore any discussions involving the concept of lift.

Yeah, I think that is the whole point. Everyone has their own concept of how to divide up the forces based upon their needs. Helle, I know I change terms and concepts based upon the angle I approach from.

 

[Sailor Al]

Yeah, I think that is the whole point. Everyone has their own concept of how to divide up the forces based upon their needs. Helle, I know I change terms and concepts based upon the angle I approach from.

Agreed, but again I ask, in what particular sailing context is it useful to resolve the net aero force into lift (and drag around the airflow)?

 

[Sailor Al]

Who are "we"?

"We" are the inquisitive sailors who want to learn and share ideas about making the boat go faster ( up the current leg) by talking about the how's and why's of sail and rig trimming and match racing tactics.