Challenge: Explain the Physics of Wind Over Tide

[Extemporaneous]

As waves come ashore they are affected by the rising incline of the sea bottom due to the friction of the sea bottom.
I think a similar thing is happening (friction) except that it is now at the top of the water surface (instead of the bottom) and the cause of the friction is now the wind instead of the sea bottom.

??
Extemp.

[s/v Moondancer]

Too complicated guys, but Extemporaneous has it basically correct...

Just have 2 students bend over at 45 degrees and run headlong into each other... the result is that they will transfer energy to each other and become vertical with a big bang. If they then repeat the experiment with one walking way and the other one running after him then when they collide they will just fall down.

That is what the winds is doing to the tide driven waves...

[htraudes]

Same physics as violin bow coated with resin running over strings, or chalk running in wrong direction over blackboard - making that awful noise. As water surface at certain air-to-water speed gets rougher, the surface gets 'rubbed' and starts oscillating.

[htraudes]

I might add that the specific examples of unexpectedly high waves of a particularly agressive shape could well be a case of resonance - the mass of the water column meeting its 'own frequency' - as well as of the shape and material of the bottom.

Like with the violin, the movement of the bow, the thickness of the string and the size and material of the body work together to make not only certain frequencies and amplitudes but also 'smooth' and 'warm' sounds versus ' 'harsh' and 'agressive' sounds, the wave shape depends on similar conditions inside a water body that is rubbed by the air. Isn't that why you always see patterns on the water? They do not only vary with the wind but also with the depth, flow rate and viscosity of the water and the shape, depth and material of the bottom.

I don't think it is the shape of the bottom that you start seeing in the surface. It's not that simple. But the bottom does work like the body of the violin and affects the frequency, amplitude and shape of the waves.

[Lodesman]

Originally Posted by Rex Delay

Anyone know the physics involved?

Fluid dynamics


Paradix was sort of on the right track. Waves are vertical motion - think of water as being rows of columns; as a column of water descends due to gravity its energy is transferred to the adjacent column forcing it up, which then comes down due to gravity and so on. Wave size is a factor of wind strength, fetch and time. Now consider that the columns of water are now stacks of blocks. When the wind is still blowing it's like knocking the block on the top of the stack off onto the next column, so that its weight and momentum is added to the next column thus forcing it up higher - this is why time and/or fetch causes the waves to build. Surface currents are horizontal motion - back to the stack of blocks as you lift up the column of blocks, you now pull the bottom block out and the rest of the stack comes crashing down; this is what happens with waves over a current. If the bottom block goes the opposite direction of the block falling from the top (current opposite wind), then that falling block has a long steep drop, building up speed before it hits, making the seas steep and choppy. If the bottom block goes in the same direction as the top block (wind and current same) then the top block has a shorter drop before it lands on the bottom block, so it doesn't have as much vertical distance to accelerate and its impact is cushioned by the added block - or in other words, the wave is smoothed out.
Hope my example is clear.

[chris_gee]

Sorry not right. For a start it does not fit the observed phenomena. Waves against current are steep rather than breaking. Secondly although waves are often described as a pulse moving rather than water moving - the cork bobbing up and down, they also involve a circular movement of water as I said.

[Lodesman]

Chris,

I don't think you understand my explanation. I never said anything about breaking waves - I said steep and choppy. Breaking waves exemplify circular motion.

'Course, this is just my opinion.

[David M]

They are a type of Standing Wave. Waves build up energy from the wind. When this energy has built up so far that the energy can no longer accumulate by an increase in amplitude, then the waves break, dissipating this energy. This is why these waves have such high amplitudes and short periods. The energy is normally dissipated by allowing the waves to continue out to infinity or when they hit a shoreline. There is actually very little friction to cause partial attenuation which is why longer period waves can cross the entire Pacific Ocean.

When the current running in the opposite direction prevents the waves from dissipating their energy by continual movement, then the energy starts building and must be dissipated.

Think of yourself in traffic jam going zero miles per hours where the cars behind you are going 60 MPH and have no brakes. All the energy behind you is going to be dissipated when the cars start slamming in to each other. For cars the energy is dissipated as heat being created mostly by bending metal. For waves, the energy is dissipated as a breaking wave.

Standing waves are nothing more than a wave traffic jam. When the medium (the water) is running in the opposite direction from the waves, you get a stack up of energy which must be released as heat. A breaking wave releases this heat energy. Total energy in a system is never lost, its usually kept , lost or converted to another form of energy. For waves, its a heat conversion where the heat energy is released into the atmosphere.

[chris_gee]

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

[LtBrett]

Let me combine the previous two posts to give a direct answer. Each molecule of water moves approximately in a circle as a wave passes through it, per the diagrams in Chris's link. When wind acts in the opposite direction from the wave, all the molecules on the wave face feel resistance on the semicircle traveling into the wind, slowing them down. However, the water molecules behind the wave face don't feel the same resistance, causing the traffic jam David mentioned. Wave face slows, rest of wave doesn't. The water has to go somewhere--it can't go down or to the side, so it stacks up. Now apply the same logic to anything that slows the wave face relative to the back of the wave: tide, shallow water, wind, current. You get a steeper, higher wave than if the wave face weren't slowed.

Brett

[Bash]

Originally Posted by Liam Wald

This happens quite dramatically at the Golden Gate entrance to the San Francisco Bay when tidal currents of 5+knots oppose 30 knot winds. The waves that are created seem to just stand up and stay in place. They are quite steep and very close together. It was once explained to me as being caused by the friction of the wind on the water. I was told they were called overfalls.

We even have a name for this phenomenon: "ebb chop."

Talk to any surfer, and you'll get a simple answer for why the surfing is better in an offshore breeze. It's because the wind holds the waves up, dude.

[LakeSuperior]

In a broad ocean current like the Gulf Stream a wave doesn't know that it is in a moving river of water so it propagates along normally. Relative to a landmark however it looks like the waves are moving slowly.

Note that if the propagation speed of the wave is equal to the current speed it will look like a standing wave to an observer on shore.

When the wind blowing against the current and the backside of the waves (on shore wind for the dude) the relative velocity of the wind on the water molecules is increased exactly by the speed of the current. Because of this there is more energy transfered from the wind to the water when compared to a wave not in a current.

It also makes sense that the waves in a current are closer together because they have not moved over 1000s of miles of ocean, they are relatively newly formed. This appearance of the waves being close together is furthered because they are much higher due to the increased energy transfer from the wind.

So, waves made from wind against current are close together, very high and steep, and made in a short amount of time (I made this stuff up but it all seems to make sense .)

[S&S]

It's fluid dynamics. The simple explanation is that waves are the viscid coupling of the low viscosity fluid (air) and the higher viscosity fluid (Water). If the wind and tide are in opposiite directions the magnitudes of their velocities are added (opposing vectors) for the purposes of energy transfer.

[Maggie-K]

Friction at the interface of the air and water . The air moving faster than the water will accelerate the water in the direction of the wind. If a current is involved the surface water flow rate will vary compared with that of deeper water either complimenting or opposing it's flow. It opposing it will set up a further friction interface between the slower surface water and faster deep water creating turbulence , It's similar to weather interfaces between cold and warm fronts spinning off in fronts.

[swagman]

Hi Guys,

Maggies spot on.

It is the physics of friction if you are talking wind AGAINST tide as oppposed to wind OVER tide which was the original heading. That second circumstance will see flat seas. It is first scenario that sees the steep short seas.

So students - a definition of friction is 'A force that resist the relative motion or tendancy to such motion by two bodies or substances in contact'.

In this circumstance we have a body or water going one way - and a body of wind going another.

The mass of water at surface level slows down as a result of friction with the wind at sea level, and the greater the water flow or the greater the wind, so the greater the friction and the greater the effect.

As the surface water slows, so the mass of water below keeps going at speed. (As it happens, the water right down on the bottom also slows due to friction with the sea bed, but not the water at mid depth).

This under surface water overtakes the surface water and the surface level friction of the wind (aided by the lesser density at the surface) causes mid depth water mass to curl upward - where it gets further exposed to the actual wind forces, thereby creating waves.

It is just the same - but in reverse - as water reaching a shore and friction causing the sea to roll over as they come in.

So the surface seas peak up, the higher they peak the greater the resistance presented by the wind, and despite what someone else refuted above, they can end up breaking back against the flow of the water.

But it is friction that results in those short steep seas.

You'll see it anywhere you get a narrow channel or tidal flow against the wind. Happens twice a day off our home port in the Solent and boats still get lost and people still die, especially when the winds are strong and the tide is strong in the opposing direction. Locals know you leave it six hours for the tide to reverse and it flattens out completely - so standing off until the tide is right is common practice.

You'll of course see further complexities ( re fluid dynamics) when you get cold water meeting warmer water (like the Gulf Stream). Due to density one tries to dive below the other...........but that IS another story.

So wind OVER tide when both going same way gives flat water.
Wind AGAINST tide gives the steep seas - due to the laws of friction.

Good luck in getting it over to the students.

JOHN