Is that really how you think the water is flowing in a wave?
If so, I am extremely dissapointed.
[quote="$1"] Is that really how you think the water is flowing in a wave?
If so, I am extremely dissapointed.
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Cudos to you. I've been waiting for someone to pick up on that.
[quote="$1"] ...there is some backward flow in the wave trough, as the upward swelling of the wave pulls water from in front of the wave ... [/quote]
EXAMPLE:: Water draining off of a reef, as a wave approaches.
[quote="$1"] ...there is some backward flow in the wave trough, as the upward swelling of the wave pulls water from in front of the wave ... [/quote]
EXAMPLE: Water draining off of a reef, as a wave approaches.
OK, cool… I think we shoud disregard that image, it is very misleading.
The wave is moving in that direction, but the flow isn’t, much. (different waves are different, as a low slope bathymetry will direct the flow less vertical, and more forward than a high slope bathymetry)
The forward component is mostly from the successive upward progression of the flow.
In the wave face, I actually think, that there is some backward flow in the wave trough, as the upward swelling of the wave pulls water from in front of the wave to fill the space that the wave occupies.
No, its not. And I never stated as much.
The actual flow on the wave face is up and forward. Here, as mentioned in my original post (and just about every subsequent post), I'm referring to the “forward” component of the flow – actually, I've be pretty careful to qualify which component of the flow I was addressing.
Does this mean that the upward component doesn't produce torques on the board? No. But the original point of the thread was about the consequences of the nature of the 'forward' component of flow in a shoaling wave – how there is a gradient of deceleration from bottom to top -i.e. the top moving (forward) faster than the bottom. And later in the thread, I also pointed out this 'forward' flow differential (or gradient) is not only a vertical one but that it changes transversely, from curl to shoulder. That is, the gradient is greater in the curl region, and less so out on the shoulder.
kc
Great post kcasey. There was an article in Scientific American called "Ocean Waves" that pretty much illustrates what you described. It's in an anthology called the Physics of Everyday Phenomena, published in 1954. Awesome Physics! There is always a thrill when you get insight into (just a peak) what Nature is doing, kinda like catching a wave!
Flow, here, as it relates to the motion of a mass of fluid is, well 'a mass of fluid in motion' – that's pretty much the definition of flow. The up and forward flow in a wave doesn't really look like your average flow, say coming out of a water-tap or garden hose, but it is a flow – its water in motion.
Flow has a direction, and direction can be resolved in any number of arbitrary ways. For example, if you're pushing something up a hill, you're pushing both horizontally and vertically. This is the basis for the lift/drag analysis commonly used in aerodynamics – one component of the net force, the lift, is orthogonal (90 deg.) to the direction of the flow and the other, the drag, is parallel to it [the direction of flow]. All I've done here is broken out the forward component to focus on it. Sort of like discussing drag on a submerged hydrofoil in a flow, without mentioning the lift component.
The role of bottom topography plays a pretty important role in how a wave will break – crumbly vs 'throwy' etc. It also impacts the apparent wavelength between shoaling waveforms, or the length between two crests of shoaling waves. The speed of the waveform itself is a function of depth – the shallower the water the more attenuation, or the more the waveform is slowed down. So, for two waveforms in a set say, the leading wave is slowing down faster than the wave following it as the two head into shallower water. This will distort that sort of perfect symmetry you see in deep water waves, where the trough is seemingly shared equally by two crests. As the waveform continues to shoal, (and I describe this in my initial post) faster traveling layers tend to slide up upon slower layers. And the backside of the wave may appear to have less of an extent.
[I'm not completely sure what you meant by "In the wave face, I actually think, that there is some backward flow in the wave trough...". The flow on the face is up and forward. If your describing that 'forward' component of flow as a "backward flow" in the sense that its direction is towards the trough which lies in front of the waveform, then I guess that's one way to put it.]
As for the diagram, okay, it could done better, but it was in response to OTaylor's post. I was kind of hoping to avoid diagrams and pictures. But, mea culpa, it probably wouldn't have hurt to have include a few – I can be lazy.
kc
This might help put out some fires…
Larry makes an excellent point. All this ‘stuff’ is pretty well known - I sure as hell didn’t discover it. But its application in surfing has been limited, if discussion of it is any indicator. I’ve actually never come across any discussion of it as it pertains to surfing. When is the last time you heard some announcer at a contest describe some guy in a tube as ‘oh yeah, he/she is slowing down just right.’ or ‘he/she has got that deceleration wired, baby!’
As it pertains to breaking waves, sure, but not surfing - which, doesn’t rule out that discussions on the topic haven’t taken place, either written for a general/professional audience, or more informally. Sure, you’ll hear phrases like ‘sucky’ wave to describe conditions at a given break on a given day, but not as a more general phenomena which is always present.
I can see how it might not be ‘top of mind’ stuff. But it does have a role in how waves are caught, right down to how rails and board bottoms behave… fins too. (This also applies to that inertial effect I described in my initial post.) Well, that’s my opinion.
kc
The flow on the face of the wave face is upwards and forwards, that on its backside, downwards and backwards.
Consider two waves one in front of the other traveling in the open ocean. Somewhere between the two the water is neither moving upwards and forwards nor downwards and backwards. You can view this region, as the dividing line between the two waves - call it the transitional zone. This argument is analogous to the following. Consider the stress in a surfboard that is being weighted such that the bottom is under tension (longitudinally). In such a case the deck will be under compression (longitudinally). But somewhere between the bottom and the deck there is plane that must be neither in tension nor in compression.
Where the water begins to move upwards and forwards is the face of the wave. Where this begins will depend on any number of factors, one of which is whether it is shoaling, and in that case, one contributing factor is bottom topography. That is, where the (above defined) transition zone appears will vary.
As a wave approaches an abrupt rise in bottom topography (a reef, for example), it may or may not be shoaling to any great degree. But because of the abrupt change, as it moves closer to the reef attenuation will also be rapid an abrupt. This will have apparent effect of pushing the (above defined) transitional zone ever closer to the approaching wave.
The problem with the above description is that it might not appear to explain the behavior of the 'first' wave in a set. Or perhaps real problem comes down to the definition of a wave. Sure one can point one out at the beach, or while on a boat, but what is it that is being pointed out? The crest? The tough? And if the crest, how far does that extend in the transverse direction. Putting one's finger on where a wave begins and ends, in any direction other than the vertical is often impossible. Waves are not really well defined objects, but a series of events that tend to end in an asymptotic manner, that is, they sort of just fade-out at their 'boundaries'. You can model parts of them as objects with well defined behaviors, but once you do you run the risk of the model failing to account for 'all' observed behavior. In which case you have to move to a broader treatment – wave fields.
I'm still not completely sure of the point of your posts, actually I may have of completely misunderstood them. But, after giving it some thought, the above is what came to mind.
kc
[quote="$1"] I'm still not completely sure of the point of your posts...I may have... misunderstood them. [/quote]
You did.
And the flow of the water in a wave, is different than the forward progression of the wave. In a open ocean wave, there is very little flow, the energy is proagating through the water medium, but the water doesn't move much. When it encounters the bottom, the wave energy is deflected up at an angle, and that forces some of the water to be displaced upward (up is the only place for it to go... it is easier for it to displace the air above, rather than the water in front, so that is what happens).
[quote="$1"]
Flow has a direction, and direction can be resolved in any number of arbitrary ways. For example, if you're pushing something up a hill, you're pushing both horizontally and vertically. This is the basis for the lift/drag analysis commonly used in aerodynamics – one component of the net force, the lift, is orthogonal (90 deg.) to the direction of the flow and the other, the drag, is parallel to it [the direction of flow]. All I've done here is broken out the forward component to focus on it. Sort of like discussing drag on a submerged hydrofoil in a flow, without mentioning the lift component.
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But the important thing about flow, with regard to fluid dynamics, is the direction and the velocity. If you disregard the actual angle of the flow you are left with useless information.
[quote="$1"]
The role of bottom topography plays a pretty important role in how a wave will break – crumbly vs 'throwy' etc. It also impacts the apparent wavelength between shoaling waveforms, or the length between two crests of shoaling waves. The speed of the waveform itself is a function of depth – the shallower the water the more attenuation, or the more the waveform is slowed down. So, for two waveforms in a set say, the leading wave is slowing down faster than the wave following it as the two head into shallower water. This will distort that sort of perfect symmetry you see in deep water waves, where the trough is seemingly shared equally by two crests. As the waveform continues to shoal, (and I describe this in my initial post) faster traveling layers tend to slide up upon slower layers. And the backside of the wave may appear to have less of an extent.
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The wave and the flow are different. The wave is the energy propagating through the water, the flow is the moving of the water. They are not the same.
[quote="$1"]
[I'm not completely sure what you meant by "In the wave face, I actually think, that there is some backward flow in the wave trough...". The flow on the face is up and forward. If your describing that 'forward' component of flow as a "backward flow" in the sense that its direction is towards the trough which lies in front of the waveform, then I guess that's one way to put it.]
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The water is moving up, and it pulls the water from in front of the wave. Like Bill T. said, this is what is happening on a low tide reef, when the wave sucks the water off of the reef as it approaches.
[quote="$1"]
As for the diagram, okay, it could done better, but it was in response to OTaylor's post. I was kind of hoping to avoid diagrams and pictures. But, mea culpa, it probably wouldn't have hurt to have include a few – I can be lazy.
kc
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If I remember correctly you are a good illustrator… I can’t understand (or at least try, since I could be wrong) this stuff without drawing pictures. 
.
The following is an animation from Longitudinal and Transverse Wave Motion of the motion associated with open ocean ocean waves.
Follow the yellow ball. When or where you consider the motion to begin or end is arbitrary (your choice), but I tend to take the beginning (and ending) of a complete cycle to be in the middle of the trough. This is of course an totally idealized animation. In the open ocean, there is in fact a small net forward motion (in the direction of propagation), but its generally considered to be insignificant -i.e. the 'net' flow in the direction of propagation is close to zero
As a wave starts to shoal, the faster (less attenuated) layers will start to slide up upon the slower (more attenuated layers). Regrettably, I couldn't find an animation of that, but perhaps its not that hard to visualize. Just imagine the yellow ball slowing down faster than the balls immediately to its left (that is, to your left when looking at the animation). They would bunch up, and some topple over it.
Energy is a term of convenience invented by physicists which allowed them to compare certain phenomena which seem to ultimately have the capacity to do work, like move a box. To my knowledge, it has never been observed or experience in a pure form, and it would be surprising if it ever was - because pure energy doesn’t exist.
Energy is property that is associated with other things, like the inherent energy of motion -i.e. the ability of a moving stone to dent the door of my car, here the ‘work’ is in denting my door. An ocean wave is an interplay between the kinetic energy of the water particles and their motion in the gravitational field. But as mentioned, kinetic energy is the energy associate with motion, and water in motion is a flow (by definition.)
Admittedly, to refer to a wave a sequence of flows is not usually done, but then most of the time people aren’t likely to be talking about surfing and the forces involved in surfing.
I hope that helps you understand my post.
kc
That animation is good… and we are on the same page.
You can see that even though the progression of the wave is forward, there is backward movement of water (flow), at the trough to about half way up the face, then it changes direction to forward toward the lip.
I am just pointing out that it is not completely correct to say “The flow on the face of the wave face is upwards and forwards”, since at the bottom half of the wave face it is upwards and backwards.
I was trying to quote “in response to TaylorO’s post…” but the quote stuff
No wonder they seemed like the vectors they were intended as to me…
So, I think you are saying this, but to be clear, is it one of your ascertions: the difference in speed towards shore in the verticle gradiant of the wave form is more responsible for how a given type of rail works - round = more push on the slower/lower side, so slower, hard = much less push on the lower edge by the slower water so the faster water on top pushes it out faster. Or something to that effect?
I’ve gone with the “spoon” example to a large degree. And, I use to ride a board with a hard tucked edge all the way up and I did not like how it rode in the pocket/off my front foot… It flet like it always wanted to push out of the face. It was good for sucky round tubes, but it was not easy to slow it down subtley off the front foot, and I’ve noticed difference in the turning of a board, especially down the face, when I’ve changed the hardness of the bottom of the rail in the tail section, plus the “realese” down the line speed difference.
I see it.
This is bad habit on my part. I’m just inclined to think about that region which is characterized by an upwards and forwards flow on a shoaling wave when thinking about surfing - a region which usually has a far greater extent during shoaling (or what are commonly seen as surf’able waves,-i.e the ‘wall’), than in open ocean waves.
So, I’d be interest to know how does this impacts the theme of the thread -i.e. the differential forward flow from bottom to top (in those regions that are characterized by an upwards and forward flow)?
Or in the lcc picture, do you see the rider in that zone? He’s pretty low on the face.
Or maybe, you have a different take on why a shoaling wave breaks?
Or, maybe you see this as strengthening the case for a given rail profile ‘holding’ or being held by suction onto the face, or perhaps ‘held’ onto the face by this flow?
Whatever the case, I do see what you’re referring to.
kc
I think that most surfing (bottom turns, trim) happens on the bottom half of the wave (although a lot of fun is at the top half).
I think rails are designed to work with the upward and backward flow (round and tucked creates suction, and control). I think that fins are flat on the inside, and toed and canted to account for that angle of attack. It is why concave is more responsive than vee…
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No. I do not see it as the primary mechanism. I was foolish to introduce it the way I did - in relative isolation. Your questioning it on in terms of magnitude alone made that clear to me.
Anyway, here’s my take.
When traveling transversely along the face, the waveside rail is subject to a transverse flow. The rail will interact in a manner similar to any other partially submerged object subject to a flow. Here, it will ‘plan’, or to put it another way, experience a force that seemingly wants to push it out of the flow.
But the rail is also subject to the forward component of the flow in the wave face, which will want to move it along in the direction of propagation. However, the rail (which is attached to the board, and effectively the rider) has a inertia. That is you will appear to sink a little into the face due to the fact that the flow has to accelerate you in its direction of propagation.
You might think that, given both of these forces are ‘pushing’ your rail in the same direction that just about any rail profile would pop out of the face, but you also have to factor in what the rider is doing. If left along, the board will eventually just start moving in the direction of propagation. That is, its requires active control to move transversely across the face.
Both these effects are impacted by the rail profile, and both, I believe play a larger role than the ‘differential’ nature of the forward flow.
But you do also have the gradient to contend with, which in effect wants to roll you (in the direction of propagation). Which is also impacted by the rail profile. But as you reasonably pointed out, the bottom will also be in play too.
A rail which presents a greater barrier, via its shape or the surface presented to the forward flow component will tend to experience more of a push and appear to sink less into the face, and conversely one that presents less of a barrier will appear to sink more into the face Hard rails of the type, I assume you’re referring to, tend to fall into the latter category. Whereas a rounded rail is more ‘accomodating’ allowing the board to appear to sink further. (I say ‘appear to sink’, because what is happening is that the wave is trying to overtake the rail in the direction of propagation.)
As for the impact of the differential flow, in the case of a rail which is less likely to sink, it will also be more likely to have a greater imbalance of torques, which are being generated by the action of the forward flow on the rail and also on the bottom. This effect is present, but plays a lesser role. Well, at least, the rider’s adjustment of his posture or stance is less dramatic in the case of a more accommodating rail profile in order to compensate for the torques that are set up, e.g. see the lcc picture.
Wow, do I regret making that mess of a post. I actually do appreciate your calling me out on that.
kc