.
Once the overspray escapes from the tension of green water forces, it goes off on its own independent tangent. But somewhere just below that breakaway point the flow is much more toward the tail of the board then one might think by looking at the overspray and extrapolating linearly backwards. The breakaway is exponential to the extreme.
Somewhere in the green water is where the amount and mass is sufficient to support the riders downforce. Wherever this downforce is, is where the split is centered and the spreading is taking place from there. I think the center of the upforce and of the split is well below the green water edge seen on the bottom of a board in turn.
Thanks Bill. That is some serious food for thought. I wasnt going to post this so soon but you got my juices flowin so here is a sketch of what I observed with my test model. One on the left is going straight like your drawing, one on the right is during an average bottom turn in everyday waves. I think we are mostly in agreement but the one thing where Im fuzzy with is the part about water mass under the board supporting the rider's downward force. I cant get my head aroung that cuz of planening and lift. Its really fuzzy.
I should note that along that dashed line, the transition zone, is also a lot of fuzz. But its potential alignment with the outside fin might explain the acceleration one can feel sometimes/often during bottom turns. Im thinking of the word 'release'. Thanks for helping me stay a little more civil :)
A.
B.
C.
D.
In image A, the lines represent the upward flow of the water and wave. It is a cross section with the point of view looking down the line on a right.
In image B, I have inserted the crude representation (no contours, not fins, square rails, but all of this is irrelevant because I want to understand the splitting of the flow by the hull) of the cross section of the board in a position similar to that when the surfer is traveling “down the line”. The lines that disappear directly into the board are actually turning back toward the tail, and toward the observer (given that the point of view is that of the cross section of a right hand wave), the ones that are turning to the left are moving more off of the outside rail, and the ones turning to the right are reconnecting with the vertical flow of the wave.
In image C, the board is transitioning to a more “on the rail” position, and the split of the flow moves closer to the rail.
In image D, the board is fully on the rail, like during a bottom turn, and the split of the flow is even closer to the rail.
I think that the important thing to understand is that the hull is changing the direction of the flow of the water (this is what create lift), toward the back of the board and off of the rails, and one of the ideas that BB (and others, maybe) is trying to convey is that the hull is changing the direction of the flow, prior to it encountering the fins. This redirection of the flow lowers the AOA of the flow relative to the fins. Please correct me if I am wrong.
Note for Crafy,
These images are an attempt for me to clarify the model and and understand description of the splitting of the flow. Any misunderstandings or misrepresentations are unintended. Please correct me if this representation is wrong.
Aloha Crafty
I understand about the fuzzy part. I will try to explain further soon
Your welcome
[/quote]
Nice Job Obproud!
I will comment more when I get time. But consider this in the meantime. The water hits the inside fin on the inside side so in “D”, if I understand your drawing properly, the split will still have to take place further from the rail then you have drawn.
We’ll talk more soon
I understand about the fuzzy part. I will try to explain further soon
Ok thanks. Bring it ;)
Re the sketches as they compare to obprouds: my mental frame and how I conducted my test is of a board/rider SKIMMING FORWARD (left sketch) then almost instantaneously going into an average bottom turn in the flat bottom portion of the wave. Basically this is how I surf most of the time so I have a bias.
Ob's skectches are excellent, but in my mind it is showing somewhat an opposite scenario of mine. His is showing more DOWNWARD PLOWING thru the water on the face of the wave. This would have a strong braking affect and result in decelleration. Mine is representing far less plowing and more skimming at speed with more of a knife-like water splitting result. It is analgous to taking a spoon and knife to a pool and running crude tests. The spoon will plow, the knife will knife. To be clear, Im not saying Ob's drawing is incorrect, just different common scenarios found while surfing. Somewhere in the fuzzy middle is probably where BB's point of view is. Flipping Ob's drawings around would probably be a good representation of a solid TOP TURN.
IT IS REALLY IMPORTANT, one cannot lose sight of the fact that at speed, there is a lot more skimming and less plowing and the predominant flow is rearward during bottom turns. Rearward is the path of least resistance. Flow is significantly governed by speed and skim/plow.
My good bud has a sweet new pool and its summer so water temp is 92F! I may redo my test, make some improvements to the test board and try to account for skim and plow.
Looking forward to BBs take - hopefully with sketches. Cheers
EDIT: forgot to mention that an important consideration is the thickness of the sheet of spray coming off the board. Water is a dense heavy medium. It takes a lot of work to move it. The thickness of the spray sheet has a lot to do with what we are discussing. IMO, the sheet thickess of an average shortboarder during an average bottom turn is 1/4" or less. And IMOpinion, thin sheets represents less plowing.
Nice.
I think the 'thruster hypothesis' that has been proposed suggests that the changed flow (I called it shedding flow in a prior post) increased the AOA on the lateral fins. A thrust (propulsion in the direction of motion of the surfboard) being a result of that increase AOA. If that's not the case, boy did I misunderstand the thrust (pun intended) of the 'thruster hypothesis'.
Back to your diagrams – couple of points on terminology. In the figure below (yours, but mutilated by me) I've drawn in the stagnation line, and labeled the spray root. Stagnation line and spray root are the terms you will see in the literature on planing. The stagnation line is important because it tends to be associated with a peak in the pressure/force intensity profile. The spray root, because it tells you how much of the flow is actually being turned around and sent off in another direction. This flow suffers a virtual reversal in its direction of motion, and on a change in momentum basis, tends to produce a lot of force, though it may appear to involve only a relatively small volume of water.
Obviously, how much goes in either direction (at the stagnation line) is a function of the orientation and geometry of the board -i.e. of the bottom surface.(And getting the balance right is always a 'call' on such diagrams.)
The 'thruster hypothesis' as presented in this thread, suggests that there is a further change in the flow, post its initial collision with the bottom surface, or in its mixing with flow that already has collided with the bottom. Further, that this change in direction is from a 'high pressure' region (perhaps located at the stagnation line?) to regions of 'lower pressure' at the rails, tail, etc.
By the way, such gradients in pressure don't guarantee such changes in flow, nor will they always produce a flow. For example, the pressure is greater with increasing depth in a water tank, but that somehow that doesn't result in an upward flow of the water. Here, as with virtually a fluids in motion, the governing parameter is the momentum of the flow itself. To change motion requires a force, not just the 'opportunity' to spread out for instance. That's not to say however, that under some circumstances such an opportunity wouldn't be taken advantage of by a fluid.
The argument [thruster hypothesis] also suggests that the tail section is progressively displacing more water, I believe because of the nature of the boards geometry and the geometry of the wave face -i.e. that 'bite' the board appears to take out of the wave. At least I think that is what is being referred to when the term displacement has been used. And that this progressive displacement also contributes to this shedding flow.
By the way, the board (and fins) moving transversely will experience a flow which is some combination of the flow up and forward which is attributed to the wave, and that which is in a direction opposite to its motion. If you add in the proposed shedding flow, I guess some combination involving that too. You seem to have hinted at as much, that is, suggested more than than the 2-D limitations of any such diagram, in your comments.
If for the moment we exclude the contribution of the shedding flow(?), the net flow will contribute to the force of planing. The contribution to the propulsion from the shedding flow still involves changing momentum and all, but as proposed it is quite a unique mechanism. If true, you might have expected to see it employed elsewhere -i.e. not only on surfboards. In fact, toe and cant may be employed elsewhere, but I not sure the claim is that of additional propulsion. But that is not an argument against the hypothesis. In some of the above posts further qualifications on when its operating tend to such a unique and possibly limited application to surfing.
As for the value of the 'thruster' fin configuration during turns etc. Its likely that with any reasonable fin configuration under the right flow conditions, you will be able to produce a component of the forces developed in the direction of motion of the object to which the fins are attached, there is nothing new to this. What the thruster hypothesis suggests that is new, is that this propulsion is there even when you are not necessarily turning, but develops as a result of the 'shedding' flow which happens under the board. And the conditions which give rise to that kind of shedding have been suggested by others in their posts.
Nice diagrams, … it's a bitch though isn't it... pictures may say a thousand words, but every f@#king one of them can wind up being taken literally.
kc
A quick update/improvement showing the board AT SPEED RIDDEN FLAT skimming across flat water before leaning into bottom turn, then second, a FRACTION OF A SECOND later at the initiation of a bottom turn, then third, one second or so into a knifeing bottom turn still moving forward at speed and perhaps gaining some speed due to thrust. My bottom turns are fairly quick so this is the way I think. In larger waves this process is extended in time but there is even more skimming across and knifeing affect. It is also convinient to note that without fins, knifeing or plowing would not be possible or a least much more difficult. To me this would suggest that without fins, at speed a TYPICAL board would mostly skim and have very little ability to knife or plow. So by default, a board at speed with fins is skimming more and plowing less and the thickness and direction of the spray sheet would be a good indicator.
deleted
Thanks.
The black dashed line in my original drawing, the transition zone, has air and water on either side. Since flow follows the path of least resistance, some of the the water in this transition zone would more easily displace towards less dense air - in the form of sheet spray. The red lines in my drawing depict "relative flow" due to forward movement of the board, combined with some plowing which splays mostly rearward flow, out to the sides.
Better drawing showing PATH OF LEAST RESISTANCE with red lines that match my original drawing's red lines above. It is important to note that the red line does not move perfectly left to right, but mostly towards your face (looking into the boards tail/rear, board moving rapidly away from you) at an angle with a strong rearward component. It is also vital to understand that any plowing occurs right next to the board's bottom as this drawing more accurately shows. Ok, gotta run and try to make some MONEY now which is really scarce these days.
Actually, my point to obproud related to the definitions found in the literature and their physical significance. As for your diagrams and terminology, go for it, …have a ball?
kc
I must say , I like where this is headed:
Obprouds drawings do a very good job of showing the various possible flow paths as the wave rises up and traverses the board.
The view direction (end-view) compares well to BB’s latest piece of art . Now that Crafty has weighed in with his observations, its interesting to see how similar and different the two perceptions are.
KC has provided a simple glossery of terms that relate well to the concepts you are each trying to convey.
You are not obliged to use them, but their relevance appears central to this part of the discussion.
As others have said, its hard to move forward until there is some common agreement on the basics
By a ‘show of hands’ the relative position of the stagnation point for each concept appears to be a good metric for comparison..
If I may add to KCs definition, the stagnation point also represents the position on the board where the flow actually decelerates to zero prior to making a rapid change in direction. Left , Right or Straight back are all possible depending on the conditions.
Having the flow stop and then turn sounds simple enough, but that’s how the fluid imparts a force on the board. Again, KC said it first (or at least most recently). Force is the result of a change in momentum. More specifically, the rate of change, or how fast you change the velocity of the fluid.
Slowing the fluid down produces a resultant force as well as altering its direction.
When the flow splits in more or less equal parts as proposed by BB the resultant force remains aligned with the initial flow to conveniently balance the gravity vector of the rider in his diagram.
The lovely pic provided by NuclearFishin appears to confirm this. The rider is hard on the rail. Lots of water moving outward as well as water that must be allowing the inside fin to bite. All this and still supporting the weight of the rider. (similar to obproud image C)
In crafty’s pic, he has chosen the location of the stagnation point to the far left or at the theoretical water-line for the board. (opproud, post image A- before image B) The end view shows crafty's board is aligned more parallel to the wave face than the BB diagram indicates.
The water approaches from the left and exits to the right . Very little of the total mass flow is redirected left. So very little force can be generated in that direction.
The result should be same vertical up force with a net lateral force that must be resolved somewhere else in the system.
To be honest crafy’s concept seems to be the easiest to visualize. ..In my mind, anyway
It fits well with the top view of a planning board moving straight ahead and goes well with just about any description of planning hulls.
But craftys experiment ,as I understand it, had no method to simulate the up welling of the wave.
Further more, I suspect the ‘test model’ required little speed to actually get on plane as there was no downward force simulated by a rider.
If the board were aligned at a more oblique angle to the direction of travel with the need for greater lift forces due to the “ballast” provided by a rider, I wonder if the crafty model would begin to simulate the BB concept.
Thanks,
- another bill
To be honest crafy’s concept seems to be the easiest to visualize. ..In my mind, anyway
Thanks but to be honest its feeling inadequate so I decided to update it. Btw, the board is skimming on top and slicing thru at speed. So I think there is little to no stagnation. Im also resistant to accept the term 'plowing'. Plowing tends to create a strong negative bias towards pushing a lot of water, which is not the case - if it were wouldnt move very fast at all. Shallow displacement is a far better description than plowing. When Im surfing I dont feel any plowing, I feel skimming, planening, slicing and carving. But no plowing. Plowing means slow. I dont like slow surfing so I usually do something else if I cant surf at least modestly fast.
Well I guess there is plowing and stagnation but not much during a good fast bottom turn which has been my point of reference. Just wanted to clarify that. There is lots of plowing and some stagnation during very hard turns like gauging top turns where you almost come to a stop/stall.
I really like this representation of all three points of view. I think it would be helpful to discuss how the flow changes durring different turns (top turn, bottom turn, trim), and have flow images of all three POV’s for each type of turn.
I think that it is apparant how the rail line and rocker influence the angle of the flow, and the AOA of flow that the fins are presented with.
Is the dotted line in Crafty’s images the stagnation line?
In this context it's hard to think of a scenario where the would be no stagnation line or spray root. Whenever there is an angle-of-attack both will present to some degree. The dashed line in the “C: End Result of A & B Combined” would be a good candidate
kc