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Re: Why is wind against current a problem?

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For the last eleven years we have spent 5 or 6 months in the Bahamas crossing over from Florida and returning to the U.S. somewhere between West Palm and Cape Fear. For planning purposes we listen to Chris Parker and check the NWS Gulf and Tropical North Atlantic Briefing daily. I recommend that if you can listen to Chris Parker on HF in the morning, do so. He describes the Gulf Stream crossing conditions daily including the "why". You could also check out Gulf and Tropical Atlantic WX Briefing Package where you can see the wave and wind fields and develop an idea of their properties, extent, and interrelation and also ftp://tgftp.nws.noaa.gov/data/foreca.../am/amz671.txt for a south Florida Gulf Stream wave forecast. It will take two weeks or so to see a whole cycle of the weather.

Bill
Bill, I don't doubt that the phenomenon exists. I just want to understand WHY it exists!
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Re: Why is wind against current a problem?

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Originally Posted by MastUndSchotbruch View Post
Let's do what Einstein did and do a Gedankenexperiment (thought experiment). Let's imagine a pot of water without a lid on top of a train
Your example of a pan of water on a train has a very small characteristic length (size of the pan), making it very different from the middle of the sea. It's really irrelevant to unbounded seawater with wavelengths and inertial forces that are millions of times larger.

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The laws of motion apply in all frames of reference, accelerated or not. These additional 'ficticious forces' are simply ways to describe the dynamics of the system in a simple way.
The laws of motion are differential equations that describe the dynamics of the system, basically a 4-dimensional force balance (3-D space + time). If you adopt an accelerating frame of reference, there must be additional terms (commonly referred to as fictitious forces) or the equations will not get the correct answer. Do not belittle the importance of these additional terms with words like "simple" and "pretend". The additional terms are essential, and if you don't include them you'll get a wrong answer (unless dimensional analysis proves that they're negligible). And I guarantee that when trying to model chaotic phenomena like ocean waves and turbulence, they are anything but simple.

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Yup, that is exactly what I meant when I said 'no Reynolds number'.
This is still wrong. Reynolds number is the ratio of inertial forces to viscous forces. You have both in this situation. Inertia is huge because of the nearly infinite length scale. But you cannot neglect viscous forces either, because it is viscous drag of the wind against the water surface that transfers momentum across the boundary layers from air to water. If you assume no viscous forces, the equations of motion would predict that the wind has no effect in stirring up the water, which is clearly incorrect.

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My question remains, why is it that this does not seem to be the case in the practical situation of wind against current in the ocean, or even a river?
You keep repeating this question, and the answer is hiding in plain view in my posts. If you consider motion of air relative to a molecule of water in the ocean (which is the Lagrangian frame of reference that you are proposing), your frame of reference is accelerating, so there are additional nonlinear terms in the equations of motion besides the simple difference between air and water speed. Those additional nonlinear terms are what explains the chaotic waves, eddies, and currents that can propagate over thousands of miles and cause the ocean to continue churning for many days after the wind dies down. If you don't accept these additional nonlinear factors, you will continue asking the same question without getting a correct answer.

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Last edited by TakeFive; 12-13-2018 at 09:25 AM.
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Re: Why is wind against current a problem?

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Your example of a pan of water on a train has a very small characteristic length (size of the pan), making it very different from the middle of the sea. It's really irrelevant to unbounded seawater with wavelengths and inertial forces that are millions of times larger.
Ehm, no. Nowhere did I say anything about the size of the pan, or rely on it at any point in the argument. Remember, we are doing a Gedankenexperiment. Imagine the pan being a thousand miles wide, or a million if you want.

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Originally Posted by TakeFive View Post
The laws of motion are differential equations that describe the dynamics of the system, basically a 4-dimensional force balance (3-D space + time). If you adopt an accelerating frame of reference, there must be additional terms (commonly referred to as fictitious forces) or the equations will not get the correct answer. Do not belittle the importance of these additional terms with words like "simple" and "pretend". The additional terms are essential, and if you don't include them you'll get a wrong answer (unless dimensional analysis proves that they're negligible). And I guarantee that when trying to model chaotic phenomena like ocean waves and turbulence, they are anything but simple.
Yes, the laws of motion are 4 dimensional PDEs, and they describe everything there is to know. When interpreting the results you get from them, it is often helpful for us to introduce imaginary 'forces' to get an intuitiive understanding (I don't belittle that, it is helpful). But that does not mean there actual forces. Everything is described in the equations, there is nothing 'additional.'

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Originally Posted by TakeFive View Post
This is still wrong. Reynolds number is the ratio of inertial forces to viscous forces. You have both in this situation. Inertia is huge because of the nearly infinite length scale. But you cannot neglect viscous forces either, because it is viscous drag of the wind against the water surface that transfers momentum across the boundary layers from air to water. If you assume no viscous forces, the equations of motion would predict that the wind has no effect in stirring up the water, which is clearly incorrect.
Sorry, this is wrong. The Reynold's number has no meaning if there is no characteristic length in the system, like the diameter of a tube, the size of an airplane wing etc. The derivation on the wikipedia page is pretty nice and correct, see
https://en.wikipedia.org/wiki/Reynolds_number. It is incorrect that 'inertia is huge because of the nearly infinite length scale.' If you do not have a characteristic length the whole concept is meaningless and this statement is simply wrong.

That has nothing to do with neglecting inertia: the forces due to inertia acting on a local scale do not become infinite because the system is infinite.

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Originally Posted by TakeFive View Post
You keep repeating this question, and the answer is hiding in plain view in my posts. If you consider motion of air relative to a molecule of water in the ocean (which is the Lagrangian frame of reference that you are proposing), your frame of reference is accelerating, so there are additional nonlinear terms in the equations of motion besides the simple difference between air and water speed. Those additional nonlinear terms are what explains the chaotic waves, eddies, and currents that can propagate over thousands of miles and cause the ocean to continue churning for many days after the wind dies down. If you don't accept these additional nonlinear factors, you will continue asking the same question without getting a correct answer.
Sorry for repeating the question, I wanted to keep the discussion on track. If that is annoying, I won't do it again.

But as I said before, the equations of motion describe the whole physics. Period. Of course there is turbulence but that does not make the system violate relativity. Again, if by blowing water over air you could determine ABSOLUTE motion relative to some fixed reference frame, the system would violate (special) relativity. As we know since Michelson and Morley (and explained by Einstein), this is incorrect. You can introduce as many nonlinear term in the description as you want, but none of that will violate relativity.
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Re: Why is wind against current a problem?

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Originally Posted by MastUndSchotbruch View Post
Ehm, no. Nowhere did I say anything about the size of the pan, or rely on it at any point in the argument. Remember, we are doing a Gedankenexperiment. Imagine the pan being a thousand miles wide, or a million if you want.



Yes, the laws of motion are 4 dimensional PDEs, and they describe everything there is to know. When interpreting the results you get from them, it is often helpful for us to introduce imaginary 'forces' to get an intuitiive understanding (I don't belittle that, it is helpful). But that does not mean there actual forces. Everything is described in the equations, there is nothing 'additional.'



Sorry, this is wrong. The Reynold's number has no meaning if there is no characteristic length in the system, like the diameter of a tube, the size of an airplane wing etc. The derivation on the wikipedia page is pretty nice and correct, see
https://en.wikipedia.org/wiki/Reynolds_number. It is incorrect that 'inertia is huge because of the nearly infinite length scale.' If you do not have a characteristic length the whole concept is meaningless and this statement is simply wrong.

That has nothing to do with neglecting inertia: the forces due to inertia acting on a local scale do not become infinite because the system is infinite.



Sorry for repeating the question, I wanted to keep the discussion on track. If that is annoying, I won't do it again.

But as I said before, the equations of motion describe the whole physics. Period. Of course there is turbulence but that does not make the system violate relativity. Again, if by blowing water over air you could determine ABSOLUTE motion relative to some fixed reference frame, the system would violate (special) relativity. As we know since Michelson and Morley (and explained by Einstein), this is incorrect. You can introduce as many nonlinear term in the description as you want, but none of that will violate relativity.
You did say something about the size of the pan. You asserted that there are “tiny wavelets”, which I agree would exist in a small pan. If the pan is a thousand miles wide (and deep) motion would be dominated by large waves and eddies, as predicted by a large Reynolds number.

There is a characteristic length scale in the ocean. It’s the depth, which is very large but not infinite. Flow is dominated by eddies and waves, as with high Reynolds number flows.

Your continued invocation of Einsteinian relativity is an unnecessary complication. Velocities are nowhere near the speed of light, and gravity is 1G. Newtonian mechanics is sufficient to model this phenomenon, but it must include the nonlinear inertial terms, which is why the simple difference between air and water current do not fully describe the wave motion. Due to inertia, the relative direction causes a different behavior for 10 kt relative co-directional flow than 10 kt counter-directuinal flow. None of this violates relativity, or any other laws of physics.

Even Einstein’s Special Relativity requires selecting a non-accelerating reference frame, or the laws of physics are different (additional terms for fictitious forces):

Quote:
Albert Einstein, in his theory of special relativity, determined that the laws of physics are the same for all non-accelerating observers
https://www.space.com/17661-theory-g...elativity.html

Your suggestion of setting the frame of reference moving with a single molecule of water would be an accelerating frame of reference due to the chaotic motion of the water.

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Re: Why is wind against current a problem?

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Then why are sailors warned about Northern winds in the (North-setting) Gulf Stream? They should be warned about wave trains coming from New England or Greenland or something. But that is not the case.
When asking a question about the *physics* of steepened waves, you must get the physical terms exactly right in order to get a meaningful answer; when asking a question about practical *sea conditions*, you will hear people use the terms of weather instead. At the time scale in which people create weather reports, and hear weather reports, and cast off, and get out to sea, the wind conditions are highly predictive of wave conditions. Hence its good enough to use rules of thumb (like the one talking about "northerly a component of wind near the Gulf Stream").

BTW, although they don't say "coming from Greenland", a good marine report will indeed mention the height, period, and direction of large swells.
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Re: Why is wind against current a problem?

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You did say something about the size of the plan. You asserted that there are “tiny wavelets”, which I agree would exist in a small pan. If the pan is a thousand miles wide (and deep) motion would be dominated by large waves and eddies, as predicted by a large Reynolds number.

There is a characteristic lenght scale in the ocean. It’s the depth, which is very large but not infinite. Flow is dominated by eddies and waves, as with high Reynolds number flows.

Your continued invocation of Einsteinian relativity is an unnecessary complication. Velocities are nowhere near the speed of light, and gravity is 1G. Newtonian mechanics is sufficient to model this phenomenon, but it must include the nonlinear inertial terms, which is why the simple difference between air and water current do not fully describe the wave motion. Due to inertia, the relarive direction gets a different behavior for 10 kt relative co-directional flow than 10 kt counter-directuinal flow. None of this violates relativity, or any other laws of physics.

Even Einstein’s Special Relativity requires selecting a non-accelerating reference frame, or the laws of physics are different (additional terns for fictitious forces):



https://www.space.com/17661-theory-g...elativity.html

Your suggestion of setting the frame of reference moving with a single molecule of water would be an accelerating frame of reference due to the chaotic motion of the water.
You are right in your first point, I did refer to the size of the container. I should not post at 2 am, did not read my own text (wiping some egg from my face )

Unfortunately this is a very busy time for me and I can't reply now in detail. It may take until after the holidays that I will find the time. I agree with you that special relativity is likely not needed to understand this (though I don't agree with your statement that SR is valid only in non-accelerated frames, this is a common misconception). But the laws of physics are the same in all frames of reference, accelerating or not, and there are no 'forces' that are not described by the equations of motion. Everything else is a violation of relativity, even Galilean.
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Re: Why is wind against current a problem?

But... if you are at beach and watch the waves breaking, wind DOES make a lot of difference.

With onshore winds, roughly the same direction of the wave trains, the waves tend to break earlier, less steep and crumbling from the top, releasing energy more gradually. With offshore winds, the waves tend to get steeper and break harder, forming tubes and releasing a lot of energy in one go.

So, the effect of the wind on the waves is somewhat similar to the effect of the current.



Cheers,

Ismael
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Re: Why is wind against current a problem?

Been away for a bit. Wow, this is still not settled. Maybe its because there is more to the earth, wind and sea than molecules and momentum. Im happy to just go with that, and know that the condition exists.
Mic drop.
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Been away for a bit. Wow, this is still not settled. Maybe its because there is more to the earth, wind and sea than molecules and momentum. Im happy to just go with that, and know that the condition exists.
Mic drop.
Yes, nothing has changed. This horse is still dead.
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Re: Why is wind against current a problem?

Sorry for being quiet for a week. I have now some time to reply to your posting.

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Originally Posted by TakeFive View Post
You did say something about the size of the pan. You asserted that there are “tiny wavelets”, which I agree would exist in a small pan. If the pan is a thousand miles wide (and deep) motion would be dominated by large waves and eddies, as predicted by a large Reynolds number.

There is a characteristic length scale in the ocean. It’s the depth, which is very large but not infinite. Flow is dominated by eddies and waves, as with high Reynolds number flows.
First, again my apologies for not reading my own text. You are right of course that I implicitly referred to the size of the pan ('tiny wavelets'). But I believe it does not make a difference, see further down.

As for your second argument, I would argue depth is NOT a characteristic length scale in the ocean as far as wind-driven waves are concerned. The depth is so large that it is in fact infinite as far as these waves are concerned because they do not interact with the bottom. In other words, the waves would have identical behavior if the ocean were infinitely deep as if it were, say, 1Km deep. In other words, you could not distinguish at the surface between the two conditions.

You are of course right that there is turbulence in the wave layer but this is not captured by the standard Reynolds number which is relative to the dimension of some fixed object, like the diameter of a pipe or an air foil. I do not know enough hydrodynamics to give a precise definition what it is in the case of ocean waves but I believe it is something like the distance between the ocean surface and the boundary layer between turbulent and non-turbulent flow (perhaps the van Dorn book that you recommended addresses this; I have received it but it came just before I flew out and I am now on a different continent, so I can not consult it). This boundary layer is not a fixed object so the simple derivation of Re from the Wikipedia article does not apply. But in any case, the ocean floor can not be the 'object' used in the derivation of Re because then it would mean that the whole ocean is turbulent. Instead, turbulence occurs only in a very thin layer (on the order of 10m), everything below that is laminar.

Nevertheless, in spite of differences in the details, I think we agree in the main part, namely that there is turbulent flow at the top of the ocean. However, I believe this does not address the question of relative motion, see below.


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Your continued invocation of Einsteinian relativity is an unnecessary complication. Velocities are nowhere near the speed of light, and gravity is 1G. Newtonian mechanics is sufficient to model this phenomenon, but it must include the nonlinear inertial terms, which is why the simple difference between air and water current do not fully describe the wave motion. Due to inertia, the relative direction causes a different behavior for 10 kt relative co-directional flow than 10 kt counter-directuinal flow. None of this violates relativity, or any other laws of physics.

Even Einstein’s Special Relativity requires selecting a non-accelerating reference frame, or the laws of physics are different (additional terms for fictitious forces):

https://www.space.com/17661-theory-g...elativity.html

Your suggestion of setting the frame of reference moving with a single molecule of water would be an accelerating frame of reference due to the chaotic motion of the water.
OK, there is more to answering this than I want to type here. You are right that you don't need Einstein's theory of relativity. As far as 'Newtonian mechanics' it depends on what you understand by it. Newton seemed to believe that there is an absolute frame of reference (the 'fixed stars' that I referred to earlier) which he thought was necessary to explain, e.g. that if you rotate a bucket of water around its long axis the water will climb up the walls. In contrast, if you imagine that you yourself accelerate around the bucket, you might get dizzy but you will not see the water rise up the walls of the bucket. His explanation was that this difference arises because the bucket moves relative to this absolute frame of reference. To make this mathematically treatable, 'fictious forces' were introduced in the presence of acceleration (not sure if Newton did this already, I am not a historian of science). Many have argued that this makes not much sense (like Ernst Mach, and even before that) but it was Einstein who made everything crystal-clear.

The laws of physics are the same in all frames of reference, accelerating or not. It is a common misconception that special relativity only applies to non-accelerating frames of reference. The first sentence in the space.com site that you refer to, "In 1905, Albert Einstein determined that the laws of physics are the same for all non-accelerating observers" is correct but misleading: the laws of physics are also the same for all ACCELERATING observers. A few lines down in that article is a sentence that is clearly wrong: "Einstein then spent 10 years trying to include acceleration in the theory and published his theory of general relativity in 1915". This is nonsense, the difference between special and general relativity is not that the first does not include accelerating frame but that the first is valid in the special case of a flat space-time, ie in the absence of gravity (or constant gravity), and the latter does include gravity.

This issue of fictional forces is explained at https://en.wikipedia.org/wiki/Inerti...e_of_reference. I can't explain it shorter or better than they do. The most important sentence from that essay is this:



"In practical terms, the equivalence of inertial reference frames means that scientists within a box moving uniformly cannot determine their absolute velocity by any experiment. Otherwise, the differences would set up an absolute standard reference frame.[21][22]"
Ref. 21 is Einstein's book, ref 22 is a collection of lectures by one of my favorites, Dick Feynman (I had the pleasure of seeing him lecture in person, he was even more clear then than in the books that collect his lectures in writing). Neither is easy reading but Feynman starts at the basics and the book is available for free: https://nirstern.files.wordpress.com...asy-pieces.pdf.

The sentence in red is one of the foundations of physics, just like the conservation laws of mass/energy, momentum etc. No violation of them has ever been found so we take them as gospel. Therefore, imagine you have a mass of water large enough that you can neglect the effects of its boundaries (which otherwise would establish a frame of reference) with a given current (vector) of velocity v_c. Now you blow wind over it, say with a strength v_s in the same direction as the current. The relative velocity of the wind over the water is (v_s-v_c). This will create some kind of wave pattern. Now take the same ocean with current velocity 0 and wind velocity v_s-v_c (same values as before). The wave pattern (and anything else) must be IDENTICAL in the two cases. Because, if that were not the case, we could define an absolute frame of reference, e.g. as that one that generates the first wave pattern. This is impossible. Therefore, any complications that you introduce (nonlinear interactions, fictitious forces, whatever) that you introduce can not violate any of these fundamental laws. If they do, something is wrong with these additions.
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