Fin lift explanation ?
Patrick Johnson
I'm looking for a nice, simple way to explain fin lift to someone. The
question is: since the fin is symmetric, the usually Bernoulli diagrams
for an airplane wing don't work. Why does the fin generate lift ?
My explanation is that the pressure on the leeward side of the fin is
higher, since it is pushing harder agains the water flow on that side. I
claim that this causes a pressure differential that causes the lift.
Is that right ? Is there a better way to explain it ?
Thanks,
Patrick
Volker Wedemeier
On 22 May 1996, Patrick Johnson wrote:
> I'm looking for a nice, simple way to explain fin lift to someone. The
> question is: since the fin is symmetric, the usually Bernoulli diagrams
> for an airplane wing don't work. Why does the fin generate lift ?
Simple: because of the angle of attack!
Your board never points exactly in the direction that you travel but
always slightly upwind. Thus you have an angle of attack between the
water flow and the center line of the fin which creates the lift. Even a
wooden board creates lift with an angle of attack. The nice shape of the
fin is more or less just for reducing drag.
Well, there are some assymmetrical fins on the market, for them it's a
different story.
Cheers,
Volker
Matthew Davey
Patrick Johnson <pjoh...@thejohnsons.com> writes:
>I'm looking for a nice, simple way to explain fin lift to someone. The
>question is: since the fin is symmetric, the usually Bernoulli diagrams
>for an airplane wing don't work. Why does the fin generate lift ?
The fin is not parallel to the direction of flow of water over the fin.
In ascii:
- water flow ------------------>
----------> _fin____-------
-----> ----
-------------> ^ ^ ^
----------------------> | | |
| | |
Wind
Hopefully you can see that there is (in the usual aerofoil way) slower water
flow over the leeward edge, hence higher pressure, so this provides upwind
lift.
Humbly,
--
Matt Davey If everything is coming your way, you're in the
mcd...@stp.dias.ie wrong lane.
WWW Homepage: http://www.maths.tcd.ie/~mcdavey
David Casey
Patrick Johnson (pjoh...@thejohnsons.com) wrote:
: I'm looking for a nice, simple way to explain fin lift to someone. The
: question is: since the fin is symmetric, the usually Bernoulli diagrams
: for an airplane wing don't work. Why does the fin generate lift ?
: My explanation is that the pressure on the leeward side of the fin is
: higher, since it is pushing harder agains the water flow on that side. I
: claim that this causes a pressure differential that causes the lift.
: Is that right ? Is there a better way to explain it ?
: Thanks,
: Patrick
Patrick,
Drag your hand in the water at 20+ mph and feel the pressure of the water
trying to lift it out.
-David
Peter Somlo
Patrick Johnson <pjoh...@thejohnsons.com> wrote:
>I'm looking for a nice, simple way to explain fin lift to someone. The
>question is: since the fin is symmetric, the usually Bernoulli diagrams
>for an airplane wing don't work. Why does the fin generate lift ?
>My explanation is that the pressure on the leeward side of the fin is
>higher, since it is pushing harder agains the water flow on that side. I
>claim that this causes a pressure differential that causes the lift.
>Is that right ? Is there a better way to explain it ?
>Thanks,
>Patrick
Patrick, in this context, the `lift' is meant in the horizontal
direction (not up).
Cheers, Pete
--
Dr Peter I Somlo FIEEE | M1: "Every coin has 3 sides - at least"
Microwave Consultant | M2: "The wind ain't gonna blow from where it
tel/fax: 61-2-451-2478 | ought'a, it'l blow from where it can"
Mobile: 041-926-3168 | http://www.zeta.org.au/~somlo
Patrick Johnson
In article <4o8j8l$7...@gidora.kralizec.net.au> Peter Somlo,
so...@zeta.org.au writes:
>Patrick, in this context, the `lift' is meant in the horizontal
>direction (not up).
Thanks Pete. Yes, I'm aware of the direction of the lift. The
disagreement I'm having boils down to this: I claim that the fin
develops lift (in the horizontal, upwind direction), because the fluid
(water) flows faster on the upwind side. Although the fin is
symmetrical, the nonzero angle of attack causes the flow to travel around
it at different speeds on either side, producing exactly the same kind of
lift as an airplane wing.
The "other" point of view maintains that an uncambered foil cannot
develop the same kind of lift as a cambered airplane wing because the
symmetric fin will not allow the fluid to flow at different speeds on
either side. I say this is baloney.
Ian Knight
>
> The "other" point of view maintains that an uncambered foil cannot
> develop the same kind of lift as a cambered airplane wing because the
> symmetric fin will not allow the fluid to flow at different speeds on
> either side. I say this is baloney.
Agreed asymetric foils and symetric foils develop lift by means of
the same basic principle. Both types of foils bend or accelerate water
leeward. The opposite reaction is the resultant force on the fin.
Asymetric foils are a refinement to allow a better lift to drag ratio
but have an obvious compromise.
Bernoullis principle should be used carefully when viscoscity is
important but from a Bernoulli perspective the water flows faster
around the "upwind" side of the fin. The upwind side of the fin is
at a lower water pressure, if it wasn't the fin wouldn't work.
Fluid will accelerate as it moves into a region of lower pressure.
Is that a chicken and egg explanation?
Cheers Ian
GilW
I think that acrobatic airplanes use symmetric airfoils instead of the
cambered airfoils we associate with airplanes because they have to fly
upside down as well as right side up. The keels and rudders of sailboats
have been usually symmetrical for thousands of years. When you change the
angle of attack of the symmetric foil with respect to the air or water or
other fluid through which it travels, you convert it from symmetric to
assymetric as far as the molecules of fluid which flow past it. Some of
the molecules travel a shorter distance slowly resulting in greater
pressure and other molecules travel a greater distance over the other
surface. The greater distance traveled means greater speed and curiously
enough lower pressure. The difference in pressure between the high speed
side and the low speed side generate the lift that the formerly symmetric
foil now generates.
Don't think about it too much. The water is born with the knowledge of
what to do. All we have to do is learn how to jibe!
Gil Woolley
Patrick Johnson
Caution: the following may be nerdy.
In the days since I first asked for help in understanding and explaining
fin lift, I've received lots of really good information. I thought it
would be useful to summarize what I've learned. (Special thanks to the
patient and knowledgeable Scott Ward.) I should point out that this is
just MY understanding of the situation, and some of this is disputed by
...
uh ... people who don't understand it fully. Ahem.
In the beginning, there was this guy named Bernoulli (nothing to do with
Grateful Dead concerts -- you can check out this Bernoulli at
http://www-groups.dcs.st-and.ac.uk/~history/Mathematicians/Bernoulli_Da
iel.
html)
You can read about his theorem, along with cool diagrams and interesting
vocabulary courtesy of NASA, at
http://www.nas.nasa.gov/nas/products/education/tutorials/jmccabe/aerody
amic
s-fluidflow1.html
Anyway, his theorem says that the pressure of a fluid is inversely
proportional to the square of its velocity. In other words, fluid moving
twice as fast exerts 1/4 as much pressure. You can see this effect by
blowing over the top of a strip of paper. The paper will pull upwards
because the faster air on top has lower pressure than the slower air on
the
bottom.
One way to use this theorem is in building a cambered airplane wing. If
the wing is flatter on one side, the fluid (air) passing over that side
doesn't have to go as fast as it does to get around the other, more convex
side. The slower air has greater pressure and pushes the wing up,
allowing
you to fly to Aruba, assuming you put the flat side on the bottom. But
that's not the whole story. Different shapes have different lift and drag
characteristics. And you might want to work with, say, water instead of
air. When you work on general fluid and shape problems, you call the
wings
"foils."
The shape of the foil is important. How much it should bulge ? Where
should the bulge be located ? Etc. The system works best if the fluid is
flowing smoothly over the surface of the foil (known to the techies as
laminar flow). And it's a good idea to reduce the drag (since it's just
wasted energy, similar to friction). A lot of heavy math and big
computers
go into figuring this out, so the government helps out. The National
Advisory Committee for Aeronautics (NACA) has done a lot of work on foil
shapes, and publishes data on shapes and stuff. You can check them out at
http://spacelink.msfc.nasa.gov/NASA.Overview/History.of.the.NACA.and.NA
A/NA
CA.Origins.1915-1930
and at http://www.larc.nasa.gov/naca/.
A foil does not have to be cambered. It can be symmetric, or uncambered,
and still generate lift according to the Bernoulli theorem, as long as you
have fluid passing more quickly on one side than the other. There is
nothing magical about camber. It is not the camber that causes the
pressure differential & associated lift, it is the way the fluid flows
over
the foil that causes the lift. Setting a symmetric foil at non-zero angle
of attack forces the fluid to negotiate different paths (over vs under the
foil) around the foil just as the camber does in a cambered foil at zero
angle of attack.
In other words, if the fluid is hitting the uncambered foil at an angle,
it
will have a little farther to go around the trailing side than the leading
side. That means the trailing side will be under less pressure, and the
pressure differential will produce lift. An interesting picture of the
pressure coefficients on a foil at 10 degree angle of attack is at:
http://www.quicklink.com/~kstreit/conmap.html
Note that the flow in this picture is probably coming 10 degrees from the
upper left of the diagram.
Now, about windsurfing ... The uncambered foil in this case is the fin,
and
the fluid is the water. Let's start with the board perfectly flat and the
fin tip pointing straight down towards the center of the Earth. If you're
headed on a close reach, the water going around the upwind side of the fin
has to go a little farther than the water on the downwind side. So it
moves a little faster, develops a lower pressure, and the pressure
differential causes "lift" that points approximately in the upwind
direction. If you lose that lift, either because the flow around the fin
becomes turbulent rather than laminar, or because air bubbles around the
fin screw up all the math (ventilation), you spin out. It's important to
bear in mind that just because it's called "lift" does NOT mean it points
towards the sky. The direction of the lift is more or less perpendicular
to the fin, i.e. it points sideways. When the board is flat, the "lift"
is
pointing to windward.
If the board is heeled so that the windward rail is a little higher, then
"sideways" is no longer purely horizontal and part of the lift points "up"
towards the sky. This can effectively lighten the board a little.
Anyway, that's how I understand it.
Further reading:
"Fluid Mechanics" by White.
"Aerodynamics for Engineers" by Bertin and Smith,
"Introduction to Flight" by Anderson.
Experiments to try at home:
http://www.sasked.gov.sk.ca/docs/physics/u6e3phy.html
Patrick Johnson
Sorry. Looks like some long URLs were mangled. Here's another attempt:
About Daniel Bernoulli:
http://www-groups.dcs.st-and.ac.uk/~history/Mathematicians/
Bernoulli_Daniel.html
Diagrams and vocabulary:
http://www.nas.nasa.gov/nas/products/education/tutorials/
jmccabe/aerodynamics-fluidflow1.html
What is NACA?
http://spacelink.msfc.nasa.gov/NASA.Overview/
History.of.the.NACA.and.NASA/NACA.Origins.1915-1930
and ... http://www.larc.nasa.gov/naca/
Foil at 10 degree AOA:
http://www.quicklink.com/~kstreit/conmap.html
William Scofield
Well - here are a couple of more thoughts.
> I should point out that this is
> just MY understanding of the situation, and some of this is disputed by
> ...
> uh ... people who don't understand it fully. Ahem.
I feel there is absolutely no shame in being in the minority. A Ghandi
said, even if you are a minority of
one, the truth is still the truth. If and when the questions I have
raised are fully resolved by reason and
evidence, then perhaps the answers will be clear to all.
> Anyway, his theorem says that the pressure of a fluid is inversely
> proportional to the square of its velocity. In other words, fluid moving
> twice as fast exerts 1/4 as much pressure. You can see this effect by
> blowing over the top of a strip of paper. The paper will pull upwards
> because the faster air on top has lower pressure than the slower air on
> the
> bottom.
Hold on to this "paper" experiment. I'll come back to it in a minute.
> The shape of the foil is important. How much it should bulge ? Where
> should the bulge be located ?
I couldn't agree more. Wilbur and Orville worked hard figuring this
out, before anybody really knew what
caused a wing to produce lift. Bernoulli, after all, did not really
understand the wing.
> A foil does not have to be cambered. It can be symmetric, or uncambered,
> and still generate lift according to the Bernoulli theorem, as long as you
> have fluid passing more quickly on one side than the other. There is
> nothing magical about camber. It is not the camber that causes the
> pressure differential & associated lift, it is the way the fluid flows
> over
> the foil that causes the lift. Setting a symmetric foil at non-zero angle
> of attack forces the fluid to negotiate different paths (over vs under the
> foil) around the foil just as the camber does in a cambered foil at zero
> angle of attack.
>
> In other words, if the fluid is hitting the uncambered foil at an angle,
> it
> will have a little farther to go around the trailing side than the leading
> side. That means the trailing side will be under less pressure, and the
> pressure differential will produce lift.
Even if there is some element of validity to this argument, I do not
believe it fully or satisfactorily
explains what I called "angle of attack lift," or the FAA calls
"deflection."
Let's go back to your piece of paper. Make it as thin as you can. Hold
it out the car window by it's leading
and trailing edges. First, keep it level. Then begin raising the
leading edge, so the angle of attack
increases. Keep raising the angle of attack to 10 degress, 45 degrees,
80 degrees, and finally 90 degrees.
When the paper is level, the angle of attack is 0, and there is no
"lift." At 10 degrees, there is a slight
force pushing the paper up, and some additional drag trying to push the
paper backwards. At 45 degrees, there
is much more lift, and also much more drag. At 80 degrees, there is
still some lift, but a lot of drag. At
90 degrees, there is no lift, only drag.
Now do this in a wind tunnel with smoke, so that you can observe the
quality of flow. As you increase the
angle of attack from 0 to 45 degrees, the lift will increase
continuously. However, the air flow over the
upper (leeward) surface of the paper will become more and more
turbulent, as opposed to laminar.
Remember that I asked you to make the paper very thin. Call the
thickness one micron. Or call it one inch. I
don't think you will find that it makes much difference, the "lift" will
be the same. And when the paper is
one micron, I do not believe that the point of incidence on the leading
edge will vary enough to result in any
discernable difference in the distance travelled across the upper and
lower surfaces. Thus, no "Bernoulli
lift" will be generated. But still there is considerable lifting force
experienced. Something else must,
therefor, be going on here.
This is where I do not believe that your diagram is helpful - you have
drawn the leading edge unrealistically
large. But as I said above, even if - with a realistic foil - there is
"some" variation in distance that
results from angling a symmetrical foil, and even if "some" Bernoulli
lift is generated as a result, I do not
believe this fully explains the observed force.
Bernoulli lift will stop (or be reduced) when the air flow over the
upper (leeward) surface becomes turbulent,
and the wing stalls. Every pilot knows this to be a fact. Yet the
lifting force on the paper does not stop
when the flow over the upper surface becomes turbulent. The lifting
force increases, all the way up to 45
degrees. Wings stall at much lower angles of attack (I think your
designated "expert" Scott says about 20
degrees for wings, and only a few degrees for a fin in water). So how
come this lift continues to increase?
I suggest the answer is simple, and lies in what the FAA calls
"deflection." I have yet to hear another
explanation that makes sense and explains the observable facts.
Will
Patrick Johnson
Well, it looks like I got a few points off, but still a passing grade.
Thanks to Scott Ward for these corrections:
Laminar (smooth) flow is NOT required for lift production. Laminar flow
is "good" because it has lower drag, but you can also have lift
production from turbulent flow (in which the fluid is sort of swirling
around a little).
What destroys the lift is when the flow detaches from the foil, and
ceases to follow its outline. In this case, things get very funky and
youlose lift and spin out. Turbulent flow can actually operate at
higher angles of attack than laminar flow before separation of the
boundary layer occurs.
As Scott puts it:
"Lift generation is not a function of boundary layer state (laminar vs
turbulent). The ability of a boundary layer to negotiate adverse pressure
gradients IS a function of boundary layer state though. Turbulent
boundary
layers have higher mixing and more energy near the surface of the foil -
giving
them an advantage in negotiating the adverse pressure gradients - but
penalizing them with higher skin friction - and higher drag. Laminar
boundary
layers have lower skin friction (lower drag) but higher propensity to
separate.
Either way, the boundary layer state (laminar vs turbulent) is unrelated
to
the mechanism for lift generation."
Special bonus fact (again, from Scott):
"Another complicating factor for fins is cavitation.
Cavitation is a term used to describe situations when the local water
pressure
has been lowered to the point that it is less than the vapor pressure for
the
fluid at the local temperature - resulting in "boiling". If you are
whizzing
along on your board at high speed, you can actually push on your fin hard
enough to make it develop so much suction on the lee side (leeward in
terms of
the fins reference system) that the water begins to locally boil (produce
bubbles of water vapor). The boiling itself isn't a problem, but the
disturbance it introduces into the boundary layer makes the bounary layer
MUCH
less stable and much less able to cope with adverse pressure gradiants
that it
may see downstream. Cavitating flows are thus much more prone to
separation -
or spin out."
So your screaming reach can actually BOIL THE OCEAN ! Cooool ! (Well,
maybe not the whole ocean, which is fortunate).
The boiling is the result of lowered pressure rather than increased
temperature, which is kind of a mixed blessing. On the one hand, the
fish are probably very grateful, on the other, it might be cool to have
cooked lobsters floating to the surface in your wake.
Patrick Johnson
OK, now that we've gotten into this, there are a couple of additional
questions that have been asked:
1) What does the slot in a slotted fin do ?
2) What does a forefin do ?
The claim is that they reduce spinout. I would assume that they do this
by reducing the tendancy for flow separation, or making it easier to
reattach flow, or both. But what is the mechanism ? How do they work ?
And ... do they really work, or are they placebos ?
Is it related to hindering the ventilation process ?
Thanks,
Patrick
john miller
I can't stay out of this any longer.
From my textbook, An Invitation to Fly, "... the two most important
theoretical explanations for the fact of flight: Bernoulli's principle
and Newton's Third Law of Motion."
Two things give a wing lift, and they are separate.
Blowing on a piece of paper and watching it rise is a demonstration of
Bernoulli's principle: the moving, lower pressure air "sucks" the paper
up. (okay, so the high pressure really pushes it up) Old fashioned
perfume atomizers demonstrate this also.
Sticking your flat hand out of a moving car window and porpoising it up
and down by slightly tilting it in the wind is a demonstration of
Newton's 3rd law. (someone called it 'deflection')
All control surfaces use both priciples. The main wing uses a lot of
Bernoulli's to keep the heavy aircraft in the air, but when you move the
ailerons, Newton's 3rd law changes the lift of the two wings relative to
each other to roll the airplane. (both wings are still producing a lot of
upward lift, though)
Looking at the size of my fin, and having sailed back to shore after
losing fins, I tend to believe that most of my lift is created by
Newton's 3rd Law (deflection) and by the rail of my board. The biggest
difference I notice when I spin-out (ventilate, see below) is my fin
doesn't keep my board going straight (loss of lateral resistance).
I can argue that with no fin lift, I can beat upwind using lift from the
apparent wind (which is off-angle of the true wind) and simply the
lateral resistance of the fin in the water. I can demonstate this using a
sail on a skateboard. The wheels are not airfoils and certainly create no
lift, but give me a lot of lateral resistance. And skateboard
land-sailers go upwind much better than windsurfers!
Spin-out of a windsurfing fin:
I know of 3 causes:
1. Cavitation. This is the water vaporizing on the low pressure side of
the fin. Submarines and battleships get this on their propellors, and
windsurfers attempting to break the speed record (around 50 mph) may also
cavitate. You won't convince me that recreational windsurfers cavitate
without a bunch of diagrams, tables, and equations.
2. Ventilation. This is an air bubble sucking onto the low pressure side
of the fin. The results are the same as cavitation, ie, loss of lift.
This is commonly reffered to as cavitation by windsurfers.
3. Stall. This is the same as an airplane wing stalling, that is, your
angle of attack is too great. This can happen at any speed. Stalls are
recovered by decreasing your angle of attack.
These three causes of spin-out were discussed in a well written article
some years ago in Windsurfing magazine (I think the one that went away).
And remember, 87.4% of people who quote statistics don't know what
they're talking about.
Just my opinion....
jm
NLW TFW NM
Who cares HOW they work, except designers.
They work.
They also cost us some top end speed potential on smooth water, but
they're not needed on smooth water. They usually make their lesser speed
potential up with their extra control in the rough, especially if
maneuvers and jumps are de rigeur.
Mike \m/
Never Leave Wind To Find Wind
Patrick Johnson
Got the answer. Courtesy of Scott Ward:
Both of the devices you ask about are basically intended to delay spin
out due
to boundary layer separation. They accomplish this by adding energy into
the
boundary layer, enabling it to negotiate a more severe adverse pressure
gradient than otherwise possible in the course of its transit from the
suction
peak to the trailing edge (on the suction or leeward, in terms of the fin
reference system, side).
1) What does the slot in a slotted fin do ?
The slot allows high pressure flow from the fin-windward side to be
ducted over
to the fin-leeward side. The high pressure flow accelerates as it passes
through the slot and mixes with the boundary layer that it is injected
into on
the fin-leeward side. The mixing adds kinetic energy to the fin-leeward
side
boundary layer (probably also causes transition to turbulent flow if the
lee
side flow is not already turbulent) which helps the boundary layer to
negotiate
the recompression that it has to go through in order to get back to the
trailing edge and still remain attached.
2) What does a forefin do ?
The forefin functions like a "strake" located ahead of a wing on an
aircraft.
If you will recall our earlier discussions, there is a vortex (swirling
tornado-like flow structure formed by fluid from the pressure side of the
fin
trying to get around the tip to the suction side) formed at the tip of a
lifting fin. The forefin is a small lifting fin. The strength of the tip
vortex is a function the fin lift - more lift => stronger vortex.
Forefins/strakes are used to generate a vortex that washes over the
downstream
primary lifting surface (main fin) in order to add kinetic energy to the
boundary layer on that surface, inhibiting its inclination to separate.
Both (1) and (2) will have a beneficial effect in that they delay the
onset of
spin-out due to classical separation but will have little or no effect in
delaying spin-out due to ventilation or cavitation. On the down side,
both
(1) and (2) are not "clean" aerodynamically and add alot of drag.
Peter Somlo
IMHO the expression `fin lift' should be discarded; it has misled
already 10^6-s of people.
The purpose of the fin (and of the centerboard) is to provide a keel,
i.e. directionality. The `keel' should ideally have zero axial drag
and infinite lateral resistance. -- Forget the `lift'.
Cheers, Peter
Shredddi
Slots just allow the water to reattach to the low preasure side of the fin
during spin out. Push a board (as in wood) sideways through the water,
the water has to flow around the sides of the board. Put a big slot in
the board and the water pours through the slot! Slots do work but they
are slower than regular fins. That's why they they are used on B&J fins
not race fins. For fins do the same thing but are extremely slow!
That's my 2 cents.
Patrick Johnson
In article <31AE37...@telepath.com> john miller,
Actually, the more I learn about this, the more I am convinced that they
are the same. The situation was explained to me thusly:
"Your "Bernoulli lift" and "angle of attack lift" are EXACTLY the same
thing. When you stick your hand out of the car window and angle it so
as to feel some lifting force, the flow on the underside of your hand is
somewhat decelerated from the free stream velocity (i.e. the speed the
car is traveling) and the flow on the upper side is locally somewhat
accelerated. This causes a pressure differential across your hand and -
a lifting force. If you hold your hand
perpendicular to the wind you will feel a greater force pushing back.
This is because you are causing a large region of flow on the upwind side
of your hand to be massively decelerated - thereby elevating the local
pressure. At the same time, the pressure in the wake of separated flow
downstream of the back of your hand is lowered slightly (compared to the
free-stream atmospheric pressure). Its the pressure differences across
your hand (and your fin) that cause the net aerodynamic forces.
"One of the beauties of physics is that its laws do not contradict each
other. Newtons first law is indeed valid in the case of a lifting
surface, just as Benoulli's law is at the same time! The Bernoulli
equation describes how the pressure differences are created in the flow.
Obviously, pressure differences across the airfoil result in a net force.
Newtons law comes in now. It tells us that if there is a net force in
one direction - like the lift on a wing,
there must be some force created in another direction to keep the system
(wing + air) in ballance. The opposing force is downwash. The lifting
foil causes the mass of air flowing over it to be somewhat redirected
with a velocity component in the direction opposite the one that the
lifting force acts in. The amount of force required to slightly
redirect the air flow is equivalent to that produced by the pressure
differential about the foil. This keeps the system in ballance -
physically speaking."
As far as spinout goes, the cause seems to be the flow detachment from
the fin, which causes loss of lift. The flow detachment can result from
the 3 factors you noted. I have also heard that cavitation can occur at
about 22 knots, well within the speed range of bozos like me.
Regards,
Patrick
so...@adnc.com
Patrick Johnson <pjoh...@thejohnsons.com> wrote:
>I'm looking for a nice, simple way to explain fin lift to someone. The
>question is: since the fin is symmetric, the usually Bernoulli diagrams
>for an airplane wing don't work. Why does the fin generate lift ?
>My explanation is that the pressure on the leeward side of the fin is
>higher, since it is pushing harder agains the water flow on that side. I
>claim that this causes a pressure differential that causes the lift.
>Is that right ? Is there a better way to explain it ?
>Thanks,
>Patrick
Patrick, symmetric airfoils (airfoils in which the camber line is the
same as the chord line) *do* produce lift at positive angles of
attack. No lift is produced at zero angle of attack in a symmetric
airfoil.
A while back, I asked the same question as you about fin lift. One of
the rec.windsurfers explained to me that if you see a video shot of a
planing windsurfer from above, you will notice the board "crabs"
slightly into the wind - resulting there is a positive angle of attack
between the board centerline (fin chord line) and the velocity vector
(direction of board travel) - hence, fin lift which counteracts
lateral force placed on the fin by the windsurfer.
This explanation satisfied my curiosity.
Mike
Sonic
Patrick Johnson
>the fin. Submarines and battleships get this on their propellors, and
>windsurfers attempting to break the speed record (around 50 mph) may also
>cavitate. You won't convince me that recreational windsurfers cavitate
>without a bunch of diagrams, tables, and equations.
With the caveat that my jibes _still_ stink, even after reading this ...
here are some calculations:
Date: Sat, 01 Jun 1996 13:02:34 +0100
From: "Mr. Marco Trucchi" <m.tr...@ic.ac.uk>
Subject: Calculations on cavitation
Hello! Here I put some of the calculations that brought me to think that
cavitation on a windsurf fin is possible even at low speed, if the fin
lateral area isn't enough.
All the calculations are based on the pressure coefficient Cp, defined in
aeronautics as:
Cp = ( p - p0 ) / ( .5 rho V0^2 )
where
p : local pressure on the wing
p0 : pressure of the free air stream, far from the wing
rho : mass density of the air
V0 : velocity of the free air stream, far from the wing
(i.e. the aircraft speed)
The Cp represents the value of the pressure along a profile, changing
from point to point, seen in non-dimensional form. The Cp is often used
in aeronautics, and it's tabulated for a great
quantity of different wings. As it is a pure number (it has no physical
dimension being the ratio between two pressure terms), its values are the
same for the same form of the wing, without considering the dimensions.
Thus it's valid for a windsurf fin, with the same shape of a particular
tabulated wing.
I considered a particular elliptic fin I once used on my Mistral Screamer
277. My fin was quite small for the sail I used, and I had problems of
spin-out. After many calculations, I found how much was the angle of
attack on the fin, while sailing very fast. Searching on the Cp tables, I
found the the minimum value for a shape like my fin's, in the suction
peak, in those conditions, was -1.6 . Then I considered the following
values (all in SI units):
p ) as I was searching for the values at which the pressure is so low
that water can evaporate, I took the vapour pressure of the water at a
temperature of about 15 Celsius degrees. It is about 2000 Pa.
p0 ) classic atmospheric pressure, about 10^5 Pa.
rho ) density of non salted water, 1000 Kg/m^3
V0 was the variable I wanted to find
Well, if you try you find
V0 = 11.07 m/s
which is the windsurf speed at which the suction peak on the fin is so
high (i.e. the pressure is so low) that water at that temperature can
evaporate. It's quite a low speed, isn't it? I know that the values I
took can't be exact, but they are not so different from reality, so I
think that V0 is really about 11 m/s. This doesn't mean that I spin-out
as soon as I reach that speed. The vapour bubble on the fin surface is
an instability for the stream, that can then separate and cause spin-out.
If the speed is higher, the vapour bubble is bigger and the probability
is higher.
Of course, in my explanations the only thing you must believe is the
value of the Cp. Leaving Cp as a variable, we can say that the V0 of
first cavitation (in m/s) is:
V0 = SQRT ( -196 / Cp )
Well, you can search (for example in the NACA data sheets) for the values
of Cp. Then you'll believe!!! For a thick profile, at a big angle of
attack, Cp can be -3.0 ! It means that V0=8.08 m/s! Only! But of course,
having a big angle of attack means that the fin lateral area is too small
for the conditions you are sailing in. Let's try to explain this:
suppose you use in the same conditions two different fins, of the same
shape but with different lateral areas. As the conditions are the same,
the two fins have to provide the same lateral force. Then the smaller
needs a higher angle of attack, i.e. a bigger leeway angle. This means
that the suction peak on it is higher, and the pressure on its suction
side is lower: on the smaller fin it's easier to reach conditions of
danger for cavitation.
Well , I think it's enough. I'd love to receive comments about this.
Anyway, HAVE FUN!!! Bye bye!
Marco Trucchi ----> m.tr...@ic.ac.uk
NLW TFW NM
You guys must have less wind than we do lately!
Patrick Johnson
In article <4oq6o3$9...@newsbf02.news.aol.com> NLW TFW NM,
Yeah ... yesterday we measured a peak wind gust of about 3.5. Not even
enough for the 11.0 !
Somebody forgot to pay the wind bill ...
Volker Wedemeier
Yes, obviously!
Here in Germany we haven't had any good wind for a whole year now. Last
year march and the first half of april were great, but from then on there
hasn't been any good wind. Well, we've had the occasional windy day, but
of course always during the week. Well, yes, there was one good 5.6 weekend
last fall, but that was about it. Nothing like the other years before.
Even fall and winter sucked where it usually blows the roofs off the houses
for a week or so each year.
Even Fuerteventura wasn't as good as usual this march. Only 3 windy days
out of 7.
And then those stupid metereologists said that with the green house
effect we would get more and stronger winds. I even started wasting
engery and burning as much fossile fuel as possible since I heard that.
:-))))
But obviously they were as far off with that prediction as they usually are
with their wind forecasts. :-(((((
Wind-sufferin'
Volker
william l kleb
In article <4ohk4m$e...@shore.shore.net> Patrick Johnson writes:
>
> As Scott puts it:
>
> "Another complicating factor for fins is cavitation.
> Cavitation is a term used to describe situations when the local water
> pressure
> has been lowered to the point that it is less than the vapor pressure for
> the
> fluid at the local temperature - resulting in "boiling".
cavitation does not happen for windsurfing fins. see richard caldwell's
george washington university '89 master's thesis, "The direct solution
for sailboard skeg ventilation prevention," or his aiaa paper 89-0843,
"A low-drag solution for high-performance sailboard spinout."
ventilation is the culprit (air being sucked down from the surface).
(after graduating from here, he went on to market RACE fins with his
design method. more recently, the VooDoo fin used NASA technology to
create a fin with advantageous variable camber and twist.)
bil
---
9.4 (285cm) astro rock / 8.8 (265cm) electric rock / 8.4 (260cm) kleb-custom
183/6.3 (83kg/190cm) white/gray-primer black-dog-on-the-roof 77 jeep wagoneer
Gastra, UP, Ezzy wave + sup G, & no-name UP http://ab00.larc.nasa.gov/~kleb/
3.3 4.0 4.5,5.2 6.0 7.0 tidewater virginia/hatteras
Keith R. Jurena
Just for clarification, the wetted surfaces of a board, well trimmed and
planing, are in a turbulent state. This turbulent boundary layer is
usually very thin and therefore, has little drag. Have you ever noticed
the surface of a good fin isn't polished? It is finished with 400 grit
sandpaper for a purpose.. to reduce drag by thinning the boundary layer.
The only way to do this is to add turbulence on a very small scale.
Otherwise the flow over a polished surface would be laminar, only for
brief times and limited conditions. Then all hell would break loose and
a big time turbulent flow would develop.
Savuti
This talk of pressure difference, NASA, wings, foil shape, acrobatic
airplanes and such is all very interesting. But totally inapplicable to
your fin. Your fin is in water. Water is an imcompressible fluid. It
does not develop lift in the same way as air (which is compressible). Lift
in water is purely a function of angle of attack.
Bernoulli's Law applies to compressible fluids only.
Hope this simplifies things. Windsurfing should be fun, not complicated.
Brett
Don Heffernan
I've been reading this thread with interest, but now I question all my
learning. Well, all you engineers, what is it? Does Bernoulli apply in water
(i.e., is the flow sucking the fin up) or are we just getting pushed up by the
pressure on the fin caused by angle of attack? If the latter, does that
confirm the master's thesis guy's conclusion that ventillation, not
cavitation, causes spinout?
Don Heffernan (H) don...@access.digex.net
Washington, DC http://www.access.digex.net/~donheff/donheffernan.html
Shredddi
If a windsurfing fin can not cavitate, then please explain what a spinout
is...
Keith R. Jurena
On Mon, 3 Jun 1996, Don Heffernan wrote:
> In article <0000126C...@prostar.com> sav...@prostar.com (Savuti) writes:
>
> >This talk of pressure difference, NASA, wings, foil shape, acrobatic
> >airplanes and such is all very interesting. But totally inapplicable to
> >your fin. Your fin is in water. Water is an imcompressible fluid. It
> >does not develop lift in the same way as air (which is compressible). Lift
> >in water is purely a function of angle of attack.
>
> >Bernoulli's Law applies to compressible fluids only.
WRONG! WRONG! WRONG!
Bernoulli's Law applies ONLY TO NON-COMPRESSIBLE FLOWS! A compressible flow
in air is a supersonic flow.
>
> >Hope this simplifies things. Windsurfing should be fun, not complicated.
>
>I've been reading this thread with interest, but now I question all my
>learning. Well, all you engineers, what is it? Does Bernoulli apply in water
>(i.e., is the flow sucking the fin up) or are we just getting pushed up by the
>pressure on the fin caused by angle of attack? If the latter, does that
>confirm the master's thesis guy's conclusion that ventillation, not
>cavitation, causes spinout?
>
As violently stated above, Bernoulli's Eq. applies to non-compressible flow.
This is basic fluid mechanics (or should I be in grad school?) for the
mechanical engineer. I have little fluid bench data for my hypothesis
but I believe the highly turbulent flow over the tail of the board could
cause the dissolved gas in the water to come out of solution.
The pressure difference required for phase change (boiling, if you must)
is easily found in the steam tables. The pressure on the lee side of the
fin is the density of the water (998 kg/m^3) multiplied by the velocity
squared divided by 2. If the stagnation flow on the windward side is
assumed to be zero, yes water will boil. But for this to happen, the
angle of attack must be around 30 degrees. Talk about side slipping!
On a beam reach, the board would appear to be making good upwind
purchases. The drag would be too much.
Looking at the thermodynamics, boiling any appreciable quantity of water
would shed power and hit the surfer like a brake. Aside from that, a
spin due to phase change would not migrate to the lee side and cause any
effect, since it would immediately condense upon reaching the higher
pressure side.
This is why I believe spin-out is a combination of dissolved gas theory
(my contribution to this fray) and linear/lateral ventilation.
The non-condensable gas migrates to the lee side, lowering the lateral
resistance and causes the "stepped on a bananna peel" feeling.
I ommitted the numbers to conserve sanity........Windsurfing should be fun.
"Flame all you want, I'll learn more!"
Keith Jurena
very "short" student of Mechanical Engineering
boardhead, radical off-road cyclist
Warp Power User
"just say no to Windoze (95)"
Tom von Alten
In rec.windsurfing, Shredddi writes:
: If a windsurfing fin can not cavitate, then please explain what a spinout
: is...
Ok.
rec.windsurfing FAQ - SPINOUT
29 Sept 1994 Tom von Alten <al...@boi.hp.com>
There you are, sailing along, powered up in some conditions a little beyond
where you've been before, you hit a little chop and then
-- WHAT THE @%#*?! Did I break off my fin?! --
You're still sort of on course, but the board is pointing about 45 degrees
closer to the wind, and blasting along sideways. You're still on a plane,
but you're not sure you should be.
Whoa. Now that you've stopped and jumped in the water and turned your board
over, you can see that the fin is still there, and just fine. (Or not, in
which case you need to look for the "busted fin" FAQ.) What happened?
You've just experienced SPINOUT.
That nice, smooth flow of water on either side of the fin that was providing
the (sideways) lift to keep you on course, and heading a little upwind is
not so nice and smooth when this happens. Your fin has "stalled," and at the
new angle of attack, 45 degrees or so, there's hardly any lift, but plenty of
drag. What's more, it's a long way back to that smooth flow!
There are three causes of spinout, and even more ways to cure it. The actor
in the scene described above has come to be known as "ventilation" and is the
way most of us first encounter spinout; after going airborne off a wave, we
lose it on re-entry. If air can get at the root of the fin, it can be drawn
into the low-pressure flow on the windward side and lead to a stall. The
turbulence resulting from chop and the landing helps this happen, as does
putting the fin back in the water at the wrong angle - it needs to be pretty
close to "on course."
The other term that gets tossed around is "cavitation," which is low pressure
boiling. It's a known and well-studied problem for propellors, and for
hydrofoils at speeds above 40-45 knots. You probably weren't going that
fast, were you? Whatever the threshold, any nicks, dings and other surface
imperfections will bring it closer, and increase speed-robbing drag as well.
The third cause is a fin that doesn't match the conditions. Too small a fin
for that big sail will do you in, as will too big a fin when the wind is
blowing "small sails." (The latter problem is from loss of control as the
vertical component of lift from your fin wants to make your board fly.)
How do you keep it from happening? Keeping those dings tuned out of your fin
is certainly important, and better fairing (smoothing of the transition)
between the fin and the board can help. Technique has a lot to do with it,
too. Will Estes <wes...@usc.com> suggests the fix I use:
1) As soon as you spin out, try to remove all weight from the
back foot. I find that in most cases this alone will allow the
board to correct on its own. Usually the spin-out is initiated
by too much weight on the back foot, and once the spin-out
begins, weight on the back foot keeps it going.
As you get better, you will find yourself becoming very familiar
with the sensation of the fin just as it is about to begin to
spin out. There is a moment before the spin out when you can
feel the fin lose force against the water. If you act quickly to
remove weight from the fin just at the moment when the force of
the fin is lost, the spinout will correct before it even happens.
2) Learn to grab the board with the back foot and literally yank
it toward you (to windward). The idea is to force the fin back
to the direction you're going and re-establish smooth flow around it.
The side force that you apply to your board and fin is what "drives" the fin.
If you drive it too hard, or when it's not completely in the water (or if
you don't have enough fin to start with) it stalls. In the worst case,
you have to get out of the straps and move your weight closer to the
mast to get it off the fin.
Paul Billings <p...@maui.com> describes a more agressive technique:
...You must absorb the bumps with your legs. When going over the top of
the chop, let the sideways pressure off a bit (don't push so hard with
your back leg).
If it happens, pull HARD with your back leg and push with the
front. Actually it's more of a jerk than a pull.
And another approach is to just do a little chop hop to get the
fin out of the water and situated properly while in the air. This
assumes you can land without spinout, however. :-)
There are plenty of experiments in fin design going on, and you can certainly
join in that fun. Hydrofoil designers have been working around cavitation
for more than 20 years, and with the sailing speed record topping 50 knots,
it's certainly a problem for windsurfing's "leading edge."
If your designer hasn't made any gross mistakes (or found the Holy Grail),
fin SIZE is the key parameter you need to pay attention to. If you can
control your sail in the conditions you're in, but you spinout easily, or
can't point as high as you want to, you need MORE fin. If the board is
getting squirrely, trying to fly on its own, leaving you overpowered and out
of control, you need LESS fin (and maybe less board, too).
REFERENCES:
_Fundamentals of Fluid Mechanics_, 2d ed., Munson, Young, Okiishi, 1994
A recent, and excellent, engineering textbook
_Surf_ magazine (German), May 1992, and August 1993
Watertunnel testing of fins; good photos, but not much test data
_Life in Moving Fluids: They Physical Biology of Flow_, Vogel, 1981
An excellent introduction to fluid dynamics principles, without the
full mathematical emphasis in an engineering text. Interesting
biological examples, too.
_Hovercraft and Hydrofoils_, McLeavy, 1976, p. 143-145
These were the rage back then. High speed foil overview
_The 40-knot sailboat_, Bernard Smith, 1964 (!)
Interesting comparison with the Yellow Pages Endeavor
Thanks to the many rec.windsurfers who helped me improve this FAQ!
-- Tom von Alten
Ian Knight
sav...@prostar.com (Savuti) wrote:
>
> This talk of pressure difference, NASA, wings, foil shape, acrobatic
> airplanes and such is all very interesting. But totally inapplicable to
> your fin. Your fin is in water. Water is an imcompressible fluid. It
> does not develop lift in the same way as air (which is compressible). Lift
> in water is purely a function of angle of attack.
>
> Bernoulli's Law applies to compressible fluids only.
>
>
Physics is fun.
Air behaves as an incompressible fluid for Mach numbers less than 0.2.
The speed of sound is 340 m/sec so that works out at 78 m/sec or
132 knots. So the airflow about a sail is not compressed. Well
to such a small extent that you don't have to worry about it in
calculations.
Bernoulli's principle works in both air and water.
Bernoulli's principle is nothing more than an extension of f= ma to
a fluid particle. F = ma can be integrated to produce the well
known conservation of energy rule. ie
Force times distance a particle travels
equals
the change in kinetic energy of the particle
ie the change in the particles 1/2 m v^2. ( note the resemblence
to Bernoulli's equation already )
the force on a fluid particle is proportional to the pressure
gradient.
So the force times distance term in the conservation of energy
equation works out to be just the difference in pressure at the
two points being considered when applying Bernoulli's equation.
So whether you say the lift is generated by a Bernoulli effect
or a defection it's all the same ...F= ma.
cheers Ian
hugh
While i am an engineer, i couldn't claim to be an aero-hydrodynamics expert.
However, i did read a text once which claimed that cavitation could lead to spin-out
on a windsurfer if you were doing about 70 knots. Below that speed cavitation
wouldn't happen since the pressure drop would never get big enough.
At a normal speed the causes of spin-out were ventilation and stall.
As for compressible/incompressible fluids and Bernoulli's Law - I'm just off to look
it up.
Hugh
Volker Wedemeier
On 3 Jun 1996, Savuti wrote:
> This talk of pressure difference, NASA, wings, foil shape, acrobatic
> airplanes and such is all very interesting. But totally inapplicable to
> your fin. Your fin is in water. Water is an imcompressible fluid. It
> does not develop lift in the same way as air (which is compressible). Lift
> in water is purely a function of angle of attack.
>
> Bernoulli's Law applies to compressible fluids only.
>
> Hope this simplifies things. Windsurfing should be fun, not complicated.
Well, in principle you are right about the difference between air and water,
_but_ unless you get close to the speed
of sound (somewhere around 2/3 Mach), the compressibility of the air
doesn't make much difference. Until you reach that speed, the air simply
isn't compressed much and thus acts just like an incompressible fluid.
So you can apply most of the principles for subsonic flight directly to
foils in water.
And then, Bernoulli's Law only strictly applies to _incompressible_ fluids
not compressible ones!!! (In Bernoulli's Law you have the density rho as a
constant. In a compressible medium rho wouldn't be constant, but
it would depend on the local pressure.)
Volker
Patrick Johnson
Bernoulli's equation describes how pressure differences are related to
velocity differences in
incompressible flows.
For all intents and purposes, we can assume both air and water to be
incompressible flows, and hence covered by Bernoulli's equations. Oh, I
know ... you can compress air, ... but this does NOT generally happen
with air flowing around a wing. In these cases, Bernoulli's calculations
can be used with sufficient accuracy. If you want to dig a little
deeper, try looking for Bernoulli and flow in Alta Vista, or your
favorite search engine. (I've learned much about this in the past few
weeks through web surfing, and a few very patient, knowledgeable people
who have answered many of my dumb questions via EMail.)
Windsurfing IS fun, and for some of us propeller-heads, it's fun to learn
about the technology behind parts of it. Now I know a little bit more
about spinout, lift, etc. Not that it helps my jibes any, but it
probably doesn't _hurt_ my sailing to have a little better feeling for
"what's under the hood."
Bernoulli MOST DEFINITELY applies in water. And in air. Of this I am
certain.
With respect to the cavitation vs. ventilation debate, I'm less certain.
Marco's calculations look about right to me (which admittedly is not
setting the bar real high), but several other folks sound equally certain
that it can't be true. I'm inclined to believe that it does happen, at
conceivable amateur speeds (i.e. even bozos like me), but probably not as
often as ventilation. I'd be interested in whether someone can poke a
hole in Marco's math.
Note that the cavitation we're talking about here is on a very small
scale. Tiny bubbles of water vapor form on the low pressure side of the
fin and make the flow around it less stable. It's not necessary to have
clouds of steam rising up behind you (although that could be kind of
cool).
Happy sailing,
Patrick
william l kleb
In article <Pine.SOL.3.91.960603...@lonestar.jpl.utsa.edu> "Keith R. Jurena" <kju...@lonestar.jpl.utsa.edu> writes:
>
> Just for clarification
[i don't think it worked]
> [...snip...] This turbulent boundary layer is
> usually very thin and therefore, has little drag.
it seems that you are implying that a turbulent boundary layer
is thinner than an equivalent reynolds-number laminar boundary layer.
this is entirely untrue: the turbulent boundary layer is thicker.
> [using surface roughness] to reduce drag by thinning the boundary layer.
again, not true, surface roughness usually promotes transition to
turbulent flow and a turbulent boundary layer is thicker and has
higher drag than an equivalent laminar boundary layer. as with the golf ball
dimple thing, the only reason you want a turbulent boundary layer
rather than a laminar one is if you are getting a significant pressure
drag due to a large separated region, and by reducing that region
you more than offset the drag increase by going to turbulent flow.
in other words, if there is a large region of "dead" air
or water just milling around in the back of the fin due to flow
separation, a turbulent boundary layer tends to diminish or entirely
eliminate it since it contains more energy near the surface than
a laminar boundary layer. the increased energy near the surface
means that it can withstand a stronger pressure rise before it will
separate from the surface. this is the reason golf balls have dimples,
reducing the extent of the separated region reduces drag much more than
the increase due to the now turbulent, boundary layer.
> [...snip..] Then all hell would break loose and
> a big time turbulent flow would develop.
i thought you said turbulent flow was good?
Keith R. Jurena
On 4 Jun 1996, Ian Knight wrote:
> sav...@prostar.com (Savuti) wrote:
> >
> > This talk of pressure difference, NASA, wings, foil shape, acrobatic
> > airplanes and such is all very interesting. But totally inapplicable to
> > your fin. Your fin is in water. Water is an imcompressible fluid. It
> > does not develop lift in the same way as air (which is compressible). Lift
> > in water is purely a function of angle of attack.
> >
> > Bernoulli's Law applies to compressible fluids only.
> >
> > Hope this simplifies things. Windsurfing should be fun, not complicated.
> >
> > Brett
>
> Physics is fun.
>
> Air behaves as an incompressible fluid for Mach numbers less than 0.2.
> The speed of sound is 340 m/sec so that works out at 78 m/sec or
Wait one second.
The speed of sound at sea level, 25 degrees C is 340 m/s.
This is 661 knots or 760.6 miles/hour.
I really don't think anyone could find winds, both real and apparant, of
such magnitude.
In water, with its much greater density, the speed of sound is much greater
(sorry folks, the ideal gas equation doesn't apply here). You will never
exceed it so don't worry.
william l kleb
In article <4p0219$9...@newsbf02.news.aol.com> shre...@aol.com (Shredddi) writes:
>
> If a windsurfing fin can not cavitate, then please explain what a spinout
> is...
for really, really high performance situations, cavitation is possible,
however for nearly all other sailing conditions around 30 kts
ventilation is the culprit:
laminar separation bubbles form around the mid-chord of the fin starting
about a 1/10th of the span down from the board. when you hit a piece of
chop, the air disturbance near the root of the fin (next to the board)
allows air to rush into the separated area.
check out the explanation found in the Decavitator project:
http://lancet.mit.edu/decavitator/Basics.html
or the surface-peircing propeller page:
http://www.well.com/user/pk/SPAprofboat.html
Patrick Johnson
There is a good discussion / definition of cavitation and ventilation at:
http://www.aoe.vt.edu/aoe3054/ch11/ch11p03.html
Even though they are talking about hydrofoils, does the following sound
familiar ?
Ventilation: Any surface-piercing foil system tends to suffer from air
entrainment. This is generally known as ventilation and also called
entry. Ventilation by a hydrofoil is usually an unmitigated nuisance. It
arises from the fact that the hydrodynamic pressure over the top of the
foil is considerably below atmospheric pressure, so that any air that is
offered the chance of being sucked into this region will rush in
immediately. The result is a sudden and severe loss of lift. In bad cases
the effect is as if the foil had broken off. If the supply of air is
limited, or if it rushes in for only a brief moment of time, a single
transient bubble forms on the dorsal surface of the foil and is shed into
the slipstream. The effect is as if the ship had run over a rut. A much
more serious situation occurs when air can enter continuously. This has
often posed a problem in the development of a foil mounted on a hollow
strut, or a variable-incidence foil driven by a push-pull rod, because
these arrangements tend to provide a path for the air. It is endemic in
all surface-piercing foils by their very nature, because air can leak in
down the surface of the foil itself. Low fences or screens running
chordwise on struts and foils are used to stop this leakage.
Volker Wedemeier
On 4 Jun 1996, hugh wrote:
> While i am an engineer, i couldn't claim to be an aero-hydrodynamics expert.
>
> However, i did read a text once which claimed that cavitation could lead to spin-out
> on a windsurfer if you were doing about 70 knots. Below that speed cavitation
> wouldn't happen since the pressure drop would never get big enough.
Hmm, I really don't quite beleive this. The german 'surf' magazine did a
fin test about a year ago. They put several fins into a water channel and
examined the development of spinouts, the lift to drag ratio on different
sorts of fins and things like that. They showed a picture of a fin with
several notches in the trailing edge in the water channel and you could
see bubbles attached to those notches. This definately was cavitation.
Also they said litterally that cavitaion can be a cause for spin out.
They did not say at which speeds this happenes or what water speed they
used for the picture, but I really don't beleive that they used an
unrealistically high speed as 70 knots for the tests. (Well, they
obviously really did the tests and they don't have a reason to lie about
cavitaion or no cavitation.)
One more point to take into consideration in this discussion is
that the cavitaion threshold depends very much on the quality of the
water. In sea water with many cavitation cores (plankton cells, mud
particles, small salt crystals and so forth) cavitation occurs _much_
earlier than in clean fresh water (at least, that's what our professor
told us in our fluid dynamics course). Also when you have notches in the
fin, the local water speed in the resulting turbulence can be much
higher than the speed at which you travel.
> At a normal speed the causes of spin-out were ventilation and stall.
Well, I agree that ventilation and stall certainly are the most
frequent reasons for spin out in everyday windsurfer life.
Volker
Glenn Ramsey
In article <Pine.SOL.3.91.960603...@lonestar.jpl.utsa.edu>,
kju...@lonestar.jpl.utsa.edu ("Keith R. Jurena") wrote:
>
> This is why I believe spin-out is a combination of dissolved gas theory
> (my contribution to this fray) and linear/lateral ventilation.
> The non-condensable gas migrates to the lee side, lowering the lateral
> resistance and causes the "stepped on a bananna peel" feeling.
>
> I ommitted the numbers to conserve sanity........Windsurfing should be fun.
>
Couldn't possibly be because the foil has stalled then? ;-)
--
Glenn Ramsey
gle...@es.co.nz
Mark James Day
1)
>IMHO the expression `fin lift' should be discarded; it has misled
>already 10^6-s of people.
>The purpose of the fin (and of the centerboard) is to provide a keel,
>i.e. directionality. The `keel' should ideally have zero axial drag
>and infinite lateral resistance. -- Forget the `lift'.
2)
>This turbulent boundary layer is usually very thin and therefore, has
>little drag.
3)
>This talk of pressure difference, NASA, wings, foil shape, acrobatic
>airplanes and such is all very interesting. But totally inapplicable to
>your fin. Your fin is in water. Water is an imcompressible fluid. It
>does not develop lift in the same way as air (which is compressible). Lift
>in water is purely a function of angle of attack.
>Bernoulli's Law applies to compressible fluids only.
I think the inaccuracies of the three separate comments above have
been dealt with sufficiently in the last few days (thanks to Bil Kleb
in particular) so I won't jump into the fray myself. I just wanted to
say thank you to the original posters, on behalf of the entire
department of fluid mechanics at Stanford University, for providing
such amusing material for the noteboard outside my office.
And on the "Windsurfing should be fun, not complicated" comment, I
have found that my technical background in fluid mechanics has made
the sport far more interesting and has helped significantly with my
tuning both equipment and sailing technique for better
performance. Indeed, windsurfing was one of the reasons why I have
ended up studying this subject at the PhD level. I would strongly
encourage people who are interested to stop by their local library or
bookstore and take a look at books on sailing theory; there is little
need for the technical approach that is taken in most fluid mechanics
texts to explain, at a basic level, the physical mechanisms that
govern the performance of both the fin and sail. I know I have seen
such books out there but I don't recall the authors - can anyone
suggest a few references?
Mark
--
Mark J. Day <d...@stanford.edu> (415) 723-4503 (LAB)
http://www.stanford.edu/~day (415) 723-4548 (FAX)
Wilbert Knol
shre...@aol.com (Shredddi) wrote:
>If a windsurfing fin can not cavitate, then please explain what a spinout
>is...
A result of stall (water flow detaching itself from the foil) or air
attaching itself to the foil thus preventing an affective flow of
water over it. Windsurfing fins don't cavitate.
Wilbert
Wellington, NZ.
so...@adnc.com
d...@leland.Stanford.EDU (Mark James Day) wrote:
>encourage people who are interested to stop by their local library or
>bookstore and take a look at books on sailing theory; there is little
>need for the technical approach that is taken in most fluid mechanics
>texts to explain, at a basic level, the physical mechanisms that
>govern the performance of both the fin and sail. I know I have seen
>such books out there but I don't recall the authors - can anyone
>suggest a few references?
>Mark
The Art and Science of Sails, Tom Whidden, St. Martin's Press, 1990
has a very good writeup of aerodynamics basics for sailboats,
including sail, keel and rudder interaction. While targeted for
sailboats, it certainly applies to windsurfing. There is a good
chapter on board sails too.
Mike
Sonic
Patrick Johnson
In article <4p7jnb$3...@firebird17.Stanford.EDU> Mark James Day,
d...@leland.Stanford.EDU writes:
>explain, at a basic level, the physical mechanisms that
>govern the performance of both the fin and sail. I know I have seen
>such books out there but I don't recall the authors - can anyone
>suggest a few references?
"The Aero-hydrodynamics of Sailing" by C.A. Marchaj
ISBN 0-87742-993-6 Int'l Marine Publishing
_Theory of Flight_, Richard von Mises, Dover, ISBN 0-486-60541-8
I haven't read these, but they have been recommended to me.
Ian Knight
so I won't jump into the fray myself. I just wanted to
> say thank you to the original posters, on behalf of the entire
> department of fluid mechanics at Stanford University, for providing
> such amusing material for the noteboard outside my office.
>
>
You must have some original insights on the subject of fin lift
to add.
For instance the subject of Bernoulli and compressible flow was
raised and I can't see why the Bernoulli principal shouldn't work
incompressible or not? Do you have a minimal maths explanation?
No one will paste your answer on the Stanford notice board.
cheers Ian
Keith Knight
so Bill
who do you think you are
where do you think you work
NASA or something?
Tom von Alten
In rec.windsurfing, Mark James Day writes:
: ...so I won't jump into the fray myself. I just wanted to
: say thank you to the original posters, on behalf of the entire
: department of fluid mechanics at Stanford University, for providing
Well, it's easy to be smug when you haven't taken the risk of making
a contribution.
I didn't hear the answer, but I was surprised to hear the following challenge
posed on the radio show "What Do You Know?" this weekend:
Using only common household objects, illustrate Bernoulli's
principle.
We'll even give you plenty of time to compose your reply, Mr. Day - go
ahead, give it a shot.
_____________
Tom von Alten email: al...@boi.hp.com
LSJU MSE '90
Mark James Day
Ian Knight wrote:
>Come on Mark, you can't say that WITHOUT jumping into the fray.
>You must have some original insights on the subject of fin lift
>to add.
This is not original in any way (since the basic principles of fluid
mechanics are not up for debate and have been discussed by others) but
here is another explanation, presented in a very basic way on the
subject of spin out.
When the fin is at a small angle of attack the flow on both sides will
closely follow its shape. The accelerated flow around the top side of
the fin is accompanied by a drop in pressure (a la Bernoulli's) and a
pressure difference across the fin develops. The pressure difference
multiplied by area gives you a lift force. As the angle is increased,
this lift will also increase (linearly, if everything else is held
constant). However, a critical angle is eventually reached were the
flow can no longer follow the shape of the fin (the physical mechanism
is caused by a combination of the boundary layer profile and the
adverse pressure gradient). In simple terms, it cannot "turn the
corner" at the top of the fin and the flow looks schematically like
this:
/---->---->---->-------
--->----->---- \
FLOW --->----->------ \ <-- Fin WAKE REGION
--->----->---->--- \
--->----->---->---- \
--->----->---->-----_____--->--->---
If the flow was still "attached" then it would follow down the back
side of the fin, having accelerated around the tip and therefore
dropped in pressure. But in the case shown above, the "separated" flow
hasn't significantly changed in velocity so the pressure difference is
much smaller across the fin - translation: lift drops in a big way and
you spin out.
That is the physical mechanism, now comes the personal viewpoint: with
the efficiency and materials of contemporary designs, I would suspect
that fins do not stall without the help of some form of ventilation
i.e. introduction of air to the water that surrounds the fin. The
reason is simple: the lift scales on density (it also scales on the
square of velocity and on the area) so any introduction of air
(roughly 1000 times less dense than water) will result in a drop in
fin lift and it will demand that the fin go to a higher angle of
attack before a force balance is maintained. Eventually the fin will
hit the maximum described above. Air can find its way into the region
of the fin many different ways: for example, it can be trapped under
your board after a chop hop or simply via agitation of the water
(from wind or waves).
>For instance the subject of Bernoulli and compressible flow was
>raised and I can't see why the Bernoulli principal shouldn't work
>incompressible or not? Do you have a minimal maths explanation?
Actually there is a form of the Bernoulli equation that
applies to compressible flow (as a matter of semantics is it not
usually identified as Bernoulli's). The far more common and useful
form of the equation is found by making the *assumption* that the flow
is incompressible. The other key assumption that is always made is for
inviscid flow (flow without any effects of viscosity). Finally, the
commonly used form of the equation usually also incorporates the
simplification of steady flow (where velocities are not a function
of time), but it does not have to do so.
Mark James Day
Tom von Alten wrote:
>In rec.windsurfing, Mark James Day writes:
>: ...so I won't jump into the fray myself. I just wanted to
>: say thank you to the original posters, on behalf of the entire
>: department of fluid mechanics at Stanford University, for providing
>: such amusing material for the noteboard outside my office....
>Well, it's easy to be smug when you haven't taken the risk of making
>a contribution.
Yes, it was smug because I was a little put off by how authoritative
some people were sounding (see #2 and #3 below). And I didn't jump
full scale into the fray until today (see my last post) due to exams,
which finished for me on Friday, and some great wind we had in the Bay
Area over the weekend.
Let me summarize just to make sure that the record is set straight
(most of this has been covered by others already). The following three
excerpts came from different posts over the last two weeks:
1)
>IMHO the expression `fin lift' should be discarded; it has misled
>already 10^6-s of people.
>The purpose of the fin (and of the centerboard) is to provide a keel,
>i.e. directionality. The `keel' should ideally have zero axial drag
>and infinite lateral resistance. -- Forget the `lift'.
No. It is all based on the principle of lift as it applies equally to
any type of airfoil (plane wing, windsurfing fin or sail).
2)
>This turbulent boundary layer is usually very thin and therefore, has
>little drag.
The thickness of a boundary layer has nothing to do with its drag. The
frictional drag is only a function of the shear at the surface of the
airfoil. Pressure and induced drag are also independent of the
thickness of the boundary layer. On a loosely related note, turbulent
boundary layers are actually thicker than their laminar counterparts.
3)
>This talk of pressure difference, NASA, wings, foil shape, acrobatic
>airplanes and such is all very interesting. But totally inapplicable to
>your fin. Your fin is in water. Water is an imcompressible fluid. It
>does not develop lift in the same way as air (which is compressible). Lift
>in water is purely a function of angle of attack.
>Bernoulli's Law applies to compressible fluids only.
No. Fins develop lift in the same manner in both air and water, and
Bernoulli's equation (not Law or Principle) is used more commonly for
incompressible flows although there is a much less useful compressible
version of the equation. No form of the equation applies to flows with
friction, which brings me to your (Tom's) second point.
> Using only common household objects, illustrate Bernoulli's
> principle.
>We'll even give you plenty of time to compose your reply, Mr. Day - go
>ahead, give it a shot.
Sure I'll give it a shot.... I though of a few things but the basic
complication is that you can't measure anything (pressure or velocity)
in a household and that frictional effects will be present in all
flows. So I settled on this very simple example: hold a paper by its
edge slightly below your mouth so that it droops down in front of
you. Form a nozzle with your lips and blow hard. You will see the
paper rise up due to the drop in pressure on the upper side (caused by
the high velocity); this drop causes a pressure difference that, when
multiplied by the area of the paper, creates a lift force. This
demonstrates the basic principle involved in fin lift and shows the
relationship between velocity and pressure that Bernoulli's equation
describes mathematically. The common form of Bernoulli's also
incorporates the fluid elevation (head), but I am no camp counselor
and couldn't come up with an example that included it beyond the
obvious.
Hans Geittmann
> > Using only common household objects, illustrate Bernoulli's
> > principle.
>
> >We'll even give you plenty of time to compose your reply, Mr. Day - go
> >ahead, give it a shot.
>
somewhat flexible waist. Professor John Eaton, who Mark Day probably
sees daily, deserves credit for the idea, not me. If I can't remember
the details correctly, though, that's my fault entirely, the class was
at 8am.
Bend 90 degrees at the waist so you when you exhale you're blowing at
the floor. Put the paper plate up to your lips, holding the plate so
that the curvature is towards the floor and the center of the plate is
real close to your lips. Do the nozzle thing with your lips and blow as
hard as you can (let go of the plate when you start to blow). If all
goes correctly, the plate should actually stay close to your face,
momentarily 'defying' gravity (and the force of your breath on the
plate) until you run out of breath.
Why? Airflow out of your mouth hits the plate and turns 90 degrees to
flow out over the plate axially, horizontal to the floor, perpendicular
to the direction gravity is acting in. The axial flow over the top of
the plate creates a region of pressure lower than that on the bottom of
the plate, which is what Bernoulli tells us. The higher pressure below
acting on the surface of the plate keeps it suspended while you're
blowing.
--
Hans Geittmann h...@engr.sgi.com
Silicon Graphics, Inc. http://reality.sgi.com/hg
(415) 933-4543 voice (415) 933-4376 fax
Ian Knight
d...@leland.Stanford.EDU (Mark James Day) wrote:
> >This turbulent boundary layer is usually very thin and therefore, has
> >little drag.
>
> The thickness of a boundary layer has nothing to do with its drag.
No I tend to agree with the first statement ie a thin turbulent boundary
layer has little drag.
Drag equates to wasted energy. The rate of heat released into the
water is equal to the fin drag times the fin velocity.
The intermediate stage between heating the water is turbulence, so
surely if drag is greater the thickness of the turbulent layer
reflects this.
Alternatively a thicker turbulent layer is capable of transporting
faster water closer to the fin, hence increasing the shear at the
fins boundary layer.
I like to think of Bernoulli's principal in terms of an analogy.
I think of a fluid particle as a hocky puck. A fluid particle, say
1 cubic centimetre of fluid, moves as a constant speed if there is
no change in pressure the same as a hocky puck moves at constant
speed on flat ice.
If the hocky puck moves through a depression in the ice it will
speed up, depending on how deep the depression, but exit with
the same speed as it went in with, maybe in a different direction.
The fluid particle sees a low pressure region as if it were
the depression in the ice. It will speed up but come out
with the same speed it went in with ,also maybe in a different
direction.
That's all without friction, If there's friction in the ice
depression the puck will come out slower or maybe it will not
even mount the exit slope but reverse direction and get trapped.
If the shear of the fluid particle with it's neighbours is low,
friction will be low and the fluid particle will not slow down
after negotiating the pressure depression.
However if shear is high as it is if we visualise a fluid particle
close to the fin, friction will be high and the fluid particle may
not get out of the pressure depression but reverse direction and get
trapped blocking the path of following particles. IE spin out.
cheers Ian
Patrick Johnson
In article <4phauf$5...@hpbs2500.boi.hp.com> Tom von Alten,
There are some answers on the web, too:
http://storm.ph.utexas.edu/~phy-demo/fl_mechanics_index.html
http://www.sasked.gov.sk.ca/docs/physics/u6e3phy.html
http://www.csc.peachnet.edu/Schools/AS/NatSci/pratte/jmp6.html
http://buphy.bu.edu/~duffy/fluids.html
Mark James Day
In article <4plfjf$3...@hercules.its.csiro.au>,
Ian Knight <Ian.K...@cbr.for.csiro.au> wrote:
>d...@leland.Stanford.EDU (Mark James Day) wrote:
>
>> >This turbulent boundary layer is usually very thin and therefore, has
>> >little drag.
>>
>> The thickness of a boundary layer has nothing to do with its drag.
>
>No I tend to agree with the first statement ie a thin turbulent boundary
>layer has little drag.
>
>Drag equates to wasted energy. The rate of heat released into the
>water is equal to the fin drag times the fin velocity.
>
>The intermediate stage between heating the water is turbulence, so
>surely if drag is greater the thickness of the turbulent layer
>reflects this.
>
Yes, your argument does hold true and my statement was misleading. I
should have replaced "has nothing to do with" with "isn't the
important parameter" but I got a carried away with rhetoric. The
correct statement (and you can check any fluid mechanics text on this
one) is that frictional drag depends on the Reynolds number, the
universal parameter of all things good in fluid mechanics. For those
not familiar with it, the Reynolds number is a dimensionless group
formed by the product of density, velocity and a length scale divided
by the viscosity. This is the parameter that allows aircraft wing
section theory and results to be applied to your windsurfing fin; the
the ratio of density to viscosity is on the order of 30 times greater
for water compared to air, but if we match the Reynolds number of the
flows in the two different fluids then the same results can be
expected. This is the basic principle but in practice the issue
becomes more complicated.
So, more precisely, it is the combination of density, velocity,
viscosity and length scale that governs the frictional drag. I have
left the definition of the "length scale" ambiguous for a reason: as
long as you use something sensible the relationship will still
hold. Sensible choices include the various definitions of the boundary
layer thickness (velocity, displacement, or momentum based) or simply
the surface distance from the starting point of the boundary
layer. Different thicknesses work better in different situations, but
those are details best left to textbooks.
Thanks for the correction Ian and, by the way, as a Canadian (and one
who is still staggering over the current travesty of the Stanley Cup
and the *stupid* blue puck halo that Fox insists on using) who studies
fluid mechanics I got a kick out of the hockey puck analogy.
vie...@ibm.net
In <4pi28h$m...@firebird17.Stanford.EDU>, d...@leland.Stanford.EDU (Mark James Day) writes:
>Tom von Alten wrote:
Just came across your article and found it great. Maybe its the glass of wine,
But hey; I think your right on. I also studied aerodynamics in college and find
it interesting how people compare the fluid dynamics of water-sports with the aerodynamics
of my job (flying). All in all its just great that we are all interested this much in windsurfing to
make the comparisons, rather there right or wrong. I didn't get in on any other parts
of this article so I don't know whats gone on but the last part about bernullis principal
and a household object: how about when we turn on the faucet, granted alot of other
principals apply here but I was wondering if bernie might not be behind that increase in pressure
along with mister Newton. Sorry for the spellings (bernullis) etc but never one to sweat the small stuff.
Glenn Ramsey
In article <4q2iu4$2r...@news-s01.ny.us.ibm.net>,
vie...@ibm.net (you) wrote:
> and a household object: how about when we turn on the faucet, granted alot of other
> principals apply here but I was wondering if bernie might not be behind that increase in pressure
I remember having to solve this problem at University.
When you turn on a tap (faucet) that has the exit facing
downward, so that the water is coming out gently, the
diameter of the stream decreases as the stream gets further
from the tap. Bernoulli again.
g
--
Glenn Ramsey
gle...@es.co.nz