Angle of Attack (AOA)

General Discussion
21

So the water is moving up the face. If the wave is 6’ from trough to crest then the water moved 6’ up the face. How else would it get to the top of the wave if it didn’t move there?

The question is significance. It seems like the bigger your fins the more significant (to the surfboard and rider) this movement would be. With bigger fins you catch more of this energy. Or with less toe you amplify this energy.

Maybe it only changes the AoA by a few degrees. How many degrees is the standard toe in of a fin? Changing the toe in of a fin by a few degrees has some significance, no?

22

Hey Craftee - How dare you highjack… Ha!

One thing I understand about AOA and stalling, as opposed to the drag factor, is the “stall” as I’ve experienced and explained it is, once the fins AOA hits the stall point you chance can’t turn any sharper.

And, as for the fin design question, the addition of the MVG is a way addressing stalling with out adding any significant drag.

Gotta say, I clicked on that link, and the turbulence coming off that outside fin is dramatic.

23

Ha! Got me. I can’t argue it the water doesn’t move up in space, but only as a result of the energy passing though it in relation to the changing bottom, hence the heave and suck at at place like Teahopu (for got the spelling already) as it come out of relatively deep water and hit the wall of the reef.

I will concede - I’m talking AOA and stalling in relationship to turning the board through water through which the board is moving. As for the relationship to the AOA, lift, stalling while going down the line, and the relationship to the water moving up the face and the board holding fighting against gravity - I haven’t been as concerned with toe, and AOA, as I have been with cant, as I had a board I made for small - mushy waves, with lots of cant, and it worked great in smaller/mushier waves, but when I rode it in bigger hollower surf, the lift from all that cant caused that sucker to ride up, up and away.

So, if the question is about down the line stuff… I’ll shut up now… Ha!

Except to say, it makes me want to try 4-way fins on my rails some day.

24

Guys…

OB. Craftee, TaylorO, Durbs. ALL OF YOU have a piece of the answer. Some of you have a bigger piece than others. The really neat thing about this thread, is that you are driving each other think the process through. I had my first awakening to this topic in 1960, at the hands of Phil Edwards. I’m really looking forward to this discussion at the workshop. I’ll share some discussion content that I’ve had with aerodynamicists, and hydrodynamicists, as it relates to AOA.

25

I’ve had some discussion with Dr. Fish.

We traded emails and information a few years back.

There is a picture of him

with his wing (or was it a humpback whale flipper)

in a very recent National Geographic Magazine.

Last time I checked he was teamed up with Dr. Lauder at Harvard on some kind of new enterprising research. Their work is usually available to read somewhere on the Internet.

Googlesurfing is in order.

Meanwhile, I’m dieing to learn what Phil Edwards told Bill Thrailkill.

26

This topic of water movement on a breaking wave (as opposed to an open ocean swell) and how it affects a range of design parameters appears almost on a rythmic basis on Sways.

My personal favourite was the infamous thread started by the redoubtable K Casey which led to the “duel” between Bill Barnfield and Roy Stewart. Roy got banned but alot of valuable discussion was collateral damage.

It seems to me from personal observation that the standard scientific understanding is woefully inaccurate at describing the situation during the actual breaking part of a waves life cycle, especially a long period swell coming out of deep water onto a shallow bottom contour.

Those who have surfed these kinds of waves will notice that as the swell approaches the reef the water will drain off the reef towards the wave and becomes part of the wave.

THIS WATER IS PHYSICALLY MOVING.

Throw a floating object on the reef as a wave approaches and it will rapidly move towards the wave and then up the face.

This physical water movement accelerates as it moves up the face of a steeply breaking powerful reef wave. It almost creates a standing wave type effect.

I’ve no idea what the velocities are but they are significant.

It seems to me anyone who would hide behind the classic scientific understanding that the water particles don’t actually move has never actually observed a wave like Pipeline, Waimea or Chopes (to name the most obvious) in full cry.

i think riding waves like this that this upward flow is a highly significant factor.

As to how it affect the AOA of fins I have no idea…but I’m sure someone does.

Steve

27

Terminal velocity From Wikipedia, the free encyclopedia • Ten things you may not know about Wikipedia • Jump to: navigation, search For other uses, see Terminal velocity (disambiguation).

An object reaches terminal velocity when the downward force of gravity (Fg)equals the upward force of drag (Fd). (Teahupoo, Waimea, Pipe would seem to be examples of the propagation upflow increasing drag on a rider trying to get down the face such that the drag reduces positive velocity toward the bottom, sometimes making the rider’s progress toward the bottom negative–this would have effects at other attitudes/vectors too–definitely advantageous when climbing and perhaps when maintaining a high line trim through a steep face/tunnel)

The net force on the body is then zero, and the result is that the (downward) velocity of the object remains constant. (Terminal velocity and trim given a set glide angle being the same. Pilots know this.)

As the object accelerates (usually downwards due to gravity), the drag force acting on the object increases. At a particular speed, the drag force produced will equal the object’s weight (mg). Eventually, it plummets at a constant speed called terminal velocity (also called settling velocity). Terminal velocity varies directly with the ratio of drag to weight. More drag means a lower terminal velocity, while increased weight means a higher terminal velocity. An object moving downward at greater than terminal velocity (for example because it was affected by a downward force or it fell from a thinner part of the atmosphere or it changed shape) will slow until it reaches terminal velocity.

For example, the terminal velocity of a skydiver in a free-fall position with a semi-closed parachute is about 195 km/h (120 mph or 55m/s)[1]. This velocity is the asymptotic limiting value of the acceleration process, since the effective forces on the body more and more closely balance each other as the terminal velocity is approached. In this example, a speed of 50% of terminal velocity is reached after only about 3 seconds, while it takes 8 seconds to reach 90%, 15 seconds to reach 99% and so on.

Higher speeds can be attained if the skydiver pulls in his limbs (see also freeflying). In this case, the terminal velocity increases to about 320 km/h (200 mph or 89 m/s)[1], which is also the terminal velocity of the peregrine falcon diving down on its prey[2], and a typical 150 g bullet travelling in the downward vertical direction which is returning to earth having been fired upwards or perhaps just dropped from a tower, which has a terminal velocity of about 300 feet per second (90 m/s) according to a 1920 U.S. Army Ordnance study[3].

Competition speed skydivers fly in the head down position reaching even higher speeds. The current world record is 614 mph (988 km/h) by Joseph Kittinger, set at high altitude where the lesser density of the atmosphere decreased drag[1].

An object falling on Earth will fall 9.80 meters per second faster every second (9.8 m/s²). The reason an object reaches a terminal velocity is that the drag force resisting motion is directly proportional to the square of its speed. At low speeds, the drag is much less than the gravitational force and so the object accelerates. As it accelerates, the drag increases, until it equals the weight. Drag also depends on the projected area. This is why things with a large projected area, such as parachutes, have a lower terminal velocity than small objects such as cannon balls.

Mathematically, terminal velocity, without considering the buoyancy effects, is given by (see derivation)

where Vt = terminal velocity,m = mass of the falling object,g = gravitational acceleration,Cd = drag coefficient,ρ = density of the fluid through which the object is falling, andA = projected area of the object.

Also relevant: http://www.grc.nasa.gov/WWW/K-12/airplane/right2.html:

Lift Generated in a Moving Fluid

For a body immersed in a moving fluid, the fluid remains in contact with the surface of the body. If the body is shaped, moved, or inclined in such a way as to produce a net deflection or turning of the flow, the local velocity is changed in magnitude, direction, or both. Changing the velocity creates a net force on the body. It is very important to note that the turning of the fluid occurs because the molecules of the fluid stay in contact with the solid body since the molecules are free to move. Any part of the solid body can deflect a flow. Parts facing the oncoming flow are said to be windward, and parts facing away from the flow are said to be leeward. Both windward and leeward parts deflect a flow. Ignoring the leeward deflection leads to a popular incorrect theory of lift.

28

Yes, Janklow. I like that.

Surfing is about controlling and leveraging a a continuous falling motion by changing the body english of the board/rider and leveraging surfboard design elements such as fin variables (i.e. size, shape, toe, cant, foil), and hull variables (rail shape, rocker, bottom contours). The surfer and board are falling down the wave, while the water moving up the wave is pushing the rider back up (and forward). Similar to how a sky diver can change the velocity at which he falls by changing his body shape the surfer can change the velocity at which he falls by manipulating his body english and changing how the fins and hull interact with the water moving up the wave. The surfer can position the fins and hull so that they are more perpendicular to the flow and slow his fall, or the surfer position the fins and hull so that they are more parallel to the flow and increase the velocity of his fall. If the surfer chooses the latter he must use the velocity that he has acquired to project back up the wave where he can again continue to fall back down the wave.

The surfer can position the fins and hull such to change the direction and distance of the fall. The more perpendicular to the flow that the surfer positions the fins and hull the more horizontal the direction of travel and the further the distance traveled. The more parallel to the flow that the fins and hull are posistioned the less horizontal the travel and the less the distance traveled.

The surfboard designer can change the angle(s) at which the the surfer positions the fins and hull with regard to flow by manipulating fin and hull variables. Toe-in is one of these variables.

29

Outstanding but the upflow moving the board “forward” (I took it to mean down the line) thing is better stated: the upflow is a deflecting/resisting force relative to a falling body and not necessary to the overall dynamic act, although it does increase or reduce the board’s relative speed and thus reduces (edit: also spelled “increases”) or reduces the forces you explain.

30

Hi Steve - I’m sorry if it sounds like I’m “hiding” behind classic science. I guess I thought the general question, was at first about turning, then going “down the line,” now, if it’s about places like the ones you mentioned, it seems like a more specific question.

My thoughts were/are most people are not surfing those types of places on a regular basis, so it is not as relevant. I freely admit I have not seen any of those places going off. I have seen them on film, and have seen similar local phenomena, and I fully agree with you, I think in those types of cases there is water moving, heck, I’ve felt it, and at that level I’m sure stuff goes on with the fins and AOA, and like yourself, I don’t know - haven’t given it much thought.

To me it starts to speak to the classic surfboard design “dilemma:” It’s all about compromise; speed v. control. And, rider, and waves, and…

I agree with Mark - I’d love to know what Bill ThrillKill heard… Bill- Share?

Just for fun - I saw a video about Kelly Slater’s world title run a while back, and one of the scenes was a paddle in contest at decent size “Chopes,” and Oh My God… a couple drops he made… the one where he “falls” off the ceiling… I’ll bet there was some fin-AOA stuff going on there.

31

Hey Ian,

There’s a scientist at Scripps named Grant Dean who studies the mathematics of waves. Maybe if you google his name you can come up with some stuff. I know he was interviewed on NPR a few years back. Waves are really, really complex when you get to the molecular level, which is the level on which they operate. Surfers on the other hand are pretty simple.

C

32

lennox76 wrote:

"… It seems to me from personal observation that the standard scientific understanding is woefully inaccurate at describing the situation during the actual breaking part of a waves life cycle, especially a long period swell coming out of deep water onto a shallow bottom contour.

Those who have surfed these kinds of waves will notice that as the swell approaches the reef the water will drain off the reef towards the wave and becomes part of the wave.

THIS WATER IS PHYSICALLY MOVING.

Throw a floating object on the reef as a wave approaches and it will rapidly move towards the wave and then up the face.

This physical water movement accelerates as it moves up the face of a steeply breaking powerful reef wave. It almost creates a standing wave type effect.

I’ve no idea what the velocities are but they are significant.

It seems to me anyone who would hide behind the classic scientific understanding that the water particles don’t actually move has never actually observed a wave like Pipeline, Waimea or Chopes (to name the most obvious) in full cry."

“Classic scientific understanding” does not claim that the water particles don’t move. What it does claim is that to a first approximation in deep water the net motion of the water particles averaged over one wave cycle is zero. In a higher approximation (i.e. more realistic) the math predicts that there is a actually a net motion of the water as well–but it is much weaker than the orbital motion or the motion of the wave form.

The motion of water parcels changes drastically as the wave moves into shoal water and begins to break. A pioneer in the modeling of breaking waves over a shoal bottom is Prof. S.T.Grilli of the University of Rhode Island. A number of years ago he presented an example of the development of a breaking wave as it moved into shallow water on his web site. Unfortunately, apparently it has subsequently been removed. However, a similar simulation (“A fully non-linear model for 3 dimensional overturning waves over an arbitrary bottom”, 2001) is presented on page 32 of the paper reproduced on that web page. Unfortunately, however, the illustration contains less detail than did the earlier presentation).

This paper can be found at:

http://www.oce.uri.edu/~grilli/paper3d.pdf

In this presentation the changes in the locations of several parcels of water positioned at the surface of the wave are shown with the passage of time. By measuring (or observing) the change in location after each time step, the average motion during each time step can be estimated. From the illustration on page 32, it is readily evident that there are both horizontal and vertical movements of the parcels of water in the face of the breaking wave that appear to me to do a good job of mimicing the actual motions of a real breaking wave. As I recall from a quick analysis of the wave motion illustrated in the original presentation, the total energy (potential and kinetic energy) of a parcel of water was preserved up to approximately the location on the wave face where the slope of the face became vertical. Once a parcel was above that location, it was subject to a significant onshore acceleration (which, combined the simultaneous downward acceleration associated with gravity, gives rise to the parabolic form of the pitching lip).

Overall, I’d say that in spite of the complexities of modeling breaking ocean waves (e.g. read the paper referenced above), science appears to be doing a pretty good job of reproducing the dominant features of this type of breaking wave.

33
Quote:

As I recall from a quick analysis of the wave motion illustrated in the original presentation, the total energy (potential and kinetic energy) of a parcel of water was preserved up to approximately the location on the wave face where the slope of the face became vertical. Once a parcel was above that location, it was subject to a significant onshore acceleration (which, combined the simultaneous downward acceleration associated with gravity, gives rise to the parabolic form of the pitching lip).

.

MTB , could you translate this into plain English for me and other dummies and maybe give a brief summary of what this means in terms of water flow up the face of a steeply shoaling wave.

I don’t have time to read the lit. but it seems to me it would be pretty easy to measure the velocities of surface water particles "up the face " at places like Chopes or Pipe by using small floating devices with electronic GPS in them.

Would be interesting.

Greenough calls this phenomenon of water drawing backwards of a reef or sandbar towards the breaking wave “bottom tension” and used it heavily to design his spoons.

Steve