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Old 04-11-2009, 07:28   #31
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I don't think surface friction explains it fully. A 10-knot wind over a two-knot opposing current does not give the same effect as a 12-knot wind over still water.
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Old 04-11-2009, 07:35   #32
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Friction

Technically, viscosity is to fluids (water and air) as friction is to solids.

Viscosity determines the boundary layer thickness(es) and shear forces for a given velocity differential between two fluids or a fluid and a solid. Maybe think of thick grease versus thin lubricating oil.
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Old 04-11-2009, 08:39   #33
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Originally Posted by chris_gee View Post
Try this link. It is not possible to put the diagrams here. There is a circular motion of water back from the trough to the face and up from the trough to the back. The current accentuates the build up on the face making it steeper. The short wavelength may arise from slower wavespeed because of shallow water, less fetch, and compression of the wavelength by the current.
Wave Motion
Nice link Chris. Might I point out a tiny flaw - in the first diagram labelled "Progression of Wave" it shows the wave direction as being left to right across the page and a particle making a clockwise circular path. We all agree that the water particles don't actually move forward by such a large degree, so the actual path of a particle, as a wave passes, will be from the peak, down the backside of the wave to the trough, then up the frontside of the wave to the successive peak - so its apparent circle would be counter-clockwise. This is just caused by pressure - the pressure of the oncoming wave causes the forward surge and the relative low pressure after the peak passes causes the backward ebb. A good example of this might be seen in a stadium wave (you know, when successive rows of people stand up with their hands in the air at a stadium to make the appearance of a wave). If you're crammed in shoulder to shoulder (fighting over single armrests) as the person to your right stands you naturally lean to the right scooping the vacated armrest, but you then stand up in the centre, then as you sit down the person to your left stands and you lean to your left; as he sits back down you're squished back to centre once again.

None of this contradicts my 'stack of blocks' explanation - in fact, the net forward motion described in the link is attributable to the wind knocking the top block off the stack.
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Old 04-11-2009, 08:48   #34
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Technically, viscosity is to fluids (water and air) as friction is to solids.
Viscosity is to fluid, as malleability is to solids. If I chuck a glob of grease on the side of my brick house and another glob on the window, the thickness of the grease, or its viscosity will hold the glob together, but the difference in friction means the one glob will stay put on the brick, but the other will slide down the glass.
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Old 04-11-2009, 10:31   #35
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Viscosity is to fluid, as malleability is to solids. If I chuck a glob of grease on the side of my brick house and another glob on the window, the thickness of the grease, or its viscosity will hold the glob together, but the difference in friction means the one glob will stay put on the brick, but the other will slide down the glass.
Lodesman, you are mixing apples, oranges, and peaches together here. Words like viscosity and friction have a specific technical meanings.

Dynamic fluid mechanics and mechanical wave theory needed to technically answer the post is saved for graduate school and it is a extremely difficult subject for many students. There is little chance of a definitive answer regarding the initial post given this forum. In fact, I am sure there are still many unanswered questions with regard to ocean wave propagation particularly in the surf zone.

However, I think it is a great problem and it is interesting to speculate on the phenomenology.
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Old 04-11-2009, 13:58   #36
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Thank you. That was a very clear explanation and I should be able to get it across to them with as much clarity.
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Old 04-11-2009, 15:36   #37
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The difficulty is that by experiment the water in a wave does move albeit in a circular pattern so it ends in much the same place while the wave form moves on. The source and experiments to back it up are in conflict with Lodesman's view that a left to right wave has an anti clockwise pattern.
While it is true that friction of wind on water or the fluid dynamics equivalent produces waves some of the explanations don't seem right.
Take 20 knots of wind against say a 3 knot current. The contrary current peaks say at the mid ebb. So not only is there usually not great fetch but there is only a limited time for the sea to build in these conditions.
It is difficult to estimate wavelength but in the conditions I frequently encounter it would be say 10m although it seems less. That gives a wavespeed of around 16 knots roughly. Wave height is said to be wavelength/7 ie 1.4m.
According to my source the circulatory motion is halved that is the radius of the circular movement and therefore the angular velocity at a depth of wavelength/9 so around a metre.
I calculate the velocity due to circular motion as about 6 knots directed back to the front of the wave from the preceding trough. That velocity in the top metre or so plus the current gives 9 knots going back, versus 3 deeper.so the top part is moving faster and piling up on the front of the approaching wave.
Considering the crest as one goes up the back the forward velocity due to circular motion gradually increases as it becomes more forward until at the crest you get 6-3 going forward so the apparent wind decreases to 20-3 =17. This is versus a relative wind with no current of 20 so it is not due to the wind effect which is less at the crest with a contrary current.
At the trough though the relative wind over water speed, assuming that the wave did not shelter it, would be 20 +9 29 giving some pushing off water forward steepening both trough and crest.
Long time since I did any physics so just my reading of it.
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Old 05-11-2009, 10:51   #38
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I think Paradix got it right. The water moves up and down in a typical wave. With the current moving the base, a sinusoidal wave becomes lopsided. It is kinda like sweeping the bottom out from underneath the wave, thus shorter, steeper waves. The gulf stream threw us around like a toy between Miami and Biminni with a Northwesterner. I should have taken alot of pictures, because it will be the only time I will be in waves that I could stand at the wheel, reach out and touch the middle of the wave. I will always have respect for ocean currents!
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Old 23-04-2010, 11:50   #39
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I just had one of my Navigation students ask me the physics of the phenomena of wind and tide in opposite directions causing short, steep waves. I cannot explain the physics but know it to be true. Anyone know the physics involved?
Don't know the physics involved, but there's a scene in "The Vikings" starring Tony Curtis and Kirk Douglas that exemplifies the issue ....

(just for grins)
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Old 23-04-2010, 13:33   #40
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FWIW...the effects of wind on water is, in part, a combination of the fact that air is compressible and that there is friction between the air and the surface of the water (much as there is friction on the skin of an aircraft which at great enough speeds, can superheat the skin).

Even a “steady wind” is not truly “steady” but, in fact, generally procession of pressure waves although the slight pressure differentials and their separation may not be enough to be detectible to ones senses. Never the less, the pressure oscillations can be enough to lower the water surface ever so slightly with the pressure increases; and raise it ever so slightly with the subsequent pressure drop. This effect generates “ripples”, wherein the angle of the water surface oscillates. Coincident with the foregoing is the effect of the aforementioned “skin friction” which imparts more or less energy to the water’s surface as the surface angle oscillates—which alternates the relative angle of attack of the wind. As the angle of attack increases with the emergence of a ripple, the air-flow at the surface is slowed, which also tends to raise its pressure, followed by a decreasing angle as the ripple passes, which, of course, allows a speed increase, which lowers the pressure. “Drag” also tends to impart a flow in the water surface in the same direction the wind is traveling (although coriolis effects do effect the direction of surface flows with large north-south components.) (As anyone living along a coastline will attest, the direction of the wind alone can make a difference in the height and fall of a tide on any given day.) These interactions form a feed-back loop until equilibrium is established between the energy imparted by the wind, and the energy consumed by the oscillations of the water surface.

Comes now a tide moving in opposition to the wind. As the body of water moves inimitably against the wind, the relative motion is increased and while the surface water tends to move with the wind, the body forces the surface forward against the wind, causing it to “pile up”, which of course, increases the angle of attack of the wind, which increases the energy imparted to the water, etc., etc., etc. Of course, the opposite effect is observed when a tide flows with the wind, hence one can frequently comfortably exit a pass with an outbound tide and following wind when would not attempt to do so with onshore winds save at slack water or on a flood.

Obviously there is more to the process including taking into account the circular flow of water imparted to the water itself as previously mentioned but the esoterica would become unduly laborious, no? And as Froude observed, the proof is in the pudding anyway.
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Old 26-10-2015, 08:09   #41
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Re: Challenge: Explain the Physics of Wind Over Tide

It's much simpler than suggested. Bear in mind that the effect happens just as well in deep water (so it's not caused by shallow water or coastal friction) and without wind. Yes, without wind the effect is the same: a wave-train (swell) encountering a head current locally (as opposed to everywhere) will cause a bend in the waves (as seen from above) because the waves in the counter current travel slower than those outside the current. The waves on the left and right now converge towards the center where their energy and height add up a noticeably larger wave. The opposite can also happen, where diversion creates smaller waves. I have posted more text illustrations here: Waves and current
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