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Analysing loop propagation

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Paul Arden
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Analysing loop propagation

#1

Post by Paul Arden » Sat Sep 12, 2020 12:23 pm

Hi gents,

Something I seem to recall reading here was that there are many ways to analyse loops. Such as momentum (/change) being one. Another would be to look at the energy in the cast. A third would be to consider the loop to be a wave.

Since it’s the same loop I assume that the result of the analysis would be the same using each method. I also remember reading that we choose the simplest method as opposed to wave propagation for example which for an untethered end is rather complex?

Do I have this right? If so does anyone use two methods to check the analysis? And finally is anything lost in one method that another method might help explain? Dolphin Nose for example.

Thanks guys. Sorry I’ve been pretty damned busy this last week but will organise these physics questions shortly. I do want to be around fully when we start - so as not to miss anything exciting :cool:

Thanks, Paul
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Graeme H
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Re: Analysing loop propagation

#2

Post by Graeme H » Sun Sep 13, 2020 1:04 pm

I'll go first, since I don't really care too much what others are thinking about my own theories ... :D

Much to the disgust of many of my friends here and on FB, I'm absolutely convinced that the loop itself is a traverse wave propagating through the line. My online friends have objections, to which I have answers that they don't find acceptable. Ce la vie.

Some of the things you yourself have observed can only be explained if the loop is viewed as a wave (e.g. the circular loop and what happens when there is excessive overhang) but there are many other "unexplained phenomena" that work out nicely when we look at it as a wave. For example, underpowered curve casts, the "sexy loop", "loop morphing", "checking" the shoot and how maintaining tension in the rod legs changes the cast can all be related to how transverse waves propagate through a medium under tension.

It seems the only bend in the line not considered by most people here to be a transverse wave is the loop, which seems bizarre to me. It's the single biggest bend in the line!

This is not merely of academic interest to me these days. I use my understanding of this when I teach students how they can improve their casting. I never mention waves at all, but I use methods of improving tension in the rod leg to get more efficient casts out of my students.

Have at it guys. Happy to discuss this now, since it will keep my mind off all the other shit happening in my life right now. (Mainly work bullshit.)

Cheers,
Graeme
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gordonjudd
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Re: Analysing loop propagation

#3

Post by gordonjudd » Sun Sep 13, 2020 4:29 pm

of course you can use a ODE solver for predicting the fly velocity. My model is somewhat similar but more complex as I'm using a system of partial differential equations and a solver for nonlinear differential systems. But this is another topic.
Paul,
I could not find the examples of the loop morphing that Torsten produced with the model he described above, but it was by far the best match I have seen to matching the loop shapes we get in casting.

A complex system of partial differential equations is probably what it will take to accurately model loop propagation. That model would also have to take the viscoelastic properties of the line into account as well so it is not going to be a trivial tasks to develop it.
I'm absolutely convinced that the loop itself is a traverse wave propagating through the line.
Graeme,
I would agree that the loop is a disturbance that is is propagating along a medium, but don't think it is a form of classical transverse wave propagation where the mass of the medium stays in place while transverse wave propagates through it.

I think a "fly wave" is a different form of a mechanical wave where the mass of the medium is being transported as the wave propagates along the medium.

We had a discussion of the characteristics of different types of waves in a thread some time ago, but a search for "mechanical waves" did not find it.

Gordy

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Graeme H
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Re: Analysing loop propagation

#4

Post by Graeme H » Mon Sep 14, 2020 1:10 am

Hi Gordy,

Please explain how a transverse wave can occur without the medium being displaced as the wave passes through it.

One easily observed transverse wave is a mend. It's pretty easy to see that the line lands on the water with the medium still displaced - if it isn't displaced, it was done incorrectly.

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Graeme
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Graeme H
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Re: Analysing loop propagation

#5

Post by Graeme H » Mon Sep 14, 2020 1:13 am

Oh, and if the objection is that we've moved line from behind to in front during a cast, that's only half the wave cycle. The full cycle (as seen in a pick up and laydown cast) is for the fly to return to its starting position through both a back cast AND a front cast, which returns the medium to its position in space. (Displacement, being a vector, is zero at the end of the cycle.)
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Re: Analysing loop propagation

#6

Post by Dirk le Roux » Mon Sep 14, 2020 4:23 am

Paul Arden wrote:
Sat Sep 12, 2020 12:23 pm
Since it’s the same loop I assume that the result of the analysis would be the same using each method.
Hi Paul

There may be a range of aspects that analysis considers, or not. When you speak of "result of the analysis", what question/s do you wish the result to answer?

Cheers,
Dirk

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Paul Arden
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Re: Analysing loop propagation

#7

Post by Paul Arden » Mon Sep 14, 2020 4:28 am

I don’t think back and forward casts constitute as one wave, Graeme, and is undoubtedly two. The point about it not displacing the line is for example a string tethered on both ends. The wave travels through the string and it returns to its starting point. Where it becomes difficult is when the end is untethered.

There is no question I think from anyone that it is a wave. A loop wave if you like - in the same way that a whip (and very similarly of course!) is a whip wave. As I understand it, it’s just bloody complicated equations and looking at momentum or energy allows us to study the phenomenon more easily.

So my question was not about what it is, my question was really about since we have three methods of analysis do we use one to check another or - the bit that I was mostly wondering about - do we actually lose something when we switch from one to another?

I live on waves and I find them totally fascinating. A boat can pass half a kilometre away from me, and much later it can knock me out of my hammock. The disturbance of waves through water is something quite amazing.

When I watch them form off the stern of my boat, there are many waves and it crossed my mind that perhaps the dolphin nose is a standing wave created by the main loop wave travelling through the line.

Cheers, Paul
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Re: Analysing loop propagation

#8

Post by Paul Arden » Mon Sep 14, 2020 4:32 am

Hi Paul

There may be a range of aspects that analysis considers, or not. When you speak of "result of the analysis", what question/s do you wish the result to answer?

Cheers,
Dirk
Excellent question Dirk!

Well one would be for example, the speed at which a loop propagates.

Thanks,
Paul
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Paul Arden
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Re: Analysing loop propagation

#9

Post by Paul Arden » Mon Sep 14, 2020 4:40 am

Edit
When I watch them form off the stern of my boat, there are many waves and it crossed my mind that perhaps the dolphin nose is a standing wave created by the main loop wave travelling through the line
“Standing wave” is wrong here. Some sort of secondary wave is more along the lines of what I was thinking.

Cheers, Paul
It's an exploration; bring a flyrod.

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Dirk le Roux
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Re: Analysing loop propagation

#10

Post by Dirk le Roux » Mon Sep 14, 2020 8:42 am

Paul Arden wrote:
Mon Sep 14, 2020 4:28 am
So my question was not about what it is, my question was really about since we have three methods of analysis do we use one to check another or - the bit that I was mostly wondering about - do we actually lose something when we switch from one to another?
Thanks for clarifying, Paul.

I do consider whether a result in terms of one approach makes sense in terms of the others. Though it is quick checking against those frameworks rather than recalculating. Sniffing for rats, if you will.

If our methodology is careful, and the data is handled in a non-destructive fashion, we should not lose anything by taking different approaches. Furthermore, by sharing on here, we will soon enough have someone point out an incoherence in terms of a different angle.

On a sort of flip side, I prefer to switch between approaches according to their strengths in usefulness (to me) toward practical understanding. For instance, when thinking about how a little tension aids controlling the final Svirgolato flip-over way at the front end of the cast, I am in wave mode. A jerk is readable in Gordy's pull-back cast rod leg velocity and at the same time an opposite direction jerk is readable in the fly leg velocity - centripetal force explains it for me.

All the best,
Dirk

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