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Excellent FP on modeling

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Paul Arden
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Re: Excellent FP on modeling

#11

Post by Paul Arden » Thu Jul 18, 2019 3:57 pm

Models are very interesting. As many of you will know (and some of you were very much involved :pirate:) we came up with a fly casting definitions model. The one that we ended up defining around was certainly not the first model. It was approximately the sixth or seventh. The first models were defined around the loop, then there was rod movements, acceleration or line speed (quite different of course) and finally we came around to intent. The basic simplification was the intent to reposition the rod, intent to reposition the line, intent to form a casting loop. And of course the IFF had another model altogether, based around six motions and acceleration.

Anyway in order for us to do this, it wasn't enough to hold one model in our heads, with its respective definitions; it was necessary to compartmentalise it so that we cold simultaneously hold another model, with its respective definitions, also in our head. It was fun, but I noticed in the discussions that we had, that some people simply couldn't extract themselves from the model. It was almost like it was hard-wired into their brain. I was quite fortunate of course, because at one point in my life I took rather a lot of acid and so had already learned to compartmentalise many things.

Cheers, Paul
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Will
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Re: Excellent FP on modeling

#12

Post by Will » Thu Jul 18, 2019 9:04 pm

AstroTurf James? 🤔😁
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Re: Excellent FP on modeling

#13

Post by Torsten » Thu Jul 18, 2019 10:08 pm

Thanks James, some thoughts from me:

"All models are wrong, but some are useful."

In my opinion you should quote the origin:
"[..] The first record of Box saying "all models are wrong" is in a 1976 paper published in the Journal of the American Statistical Association.[1] The 1976 paper contains the aphorism twice. The two sections of the paper that contain the aphorism are copied below. [..]"
https://en.wikipedia.org/wiki/All_models_are_wrong

.. because not every reader knows this.
In general I'd agree with many of your statements; but I don't think this aphorism should be taken too literally.

--

Information modelling was part of my education, maybe good time to brush up on that. Models are used everywhere in daily life, I'm reading some misconceptions about models; I've translated an overview from the German philosopher Herbert Stachowiak (he has witten a standard book about modelling):

A model is a simplified representation of reality. The simplification can be carried out on sensually, especially optically perceptible objects or in theories. According to Herbert Stachowiak, it is characterized by at least three features:

1. Mapping
A model is always a model of something - namely the depiction or representation of a natural or artificial original, whereby the original itself can also be a model.

2. Reduction
In general, a model does not capture all attributes of the original, but only those that appear relevant to the model creator or model user.

3. Pragmatism
Models are not clearly associated with their originals. They fulfil their replacement function
a) for certain subjects (for whom?)
b) within certain time intervals (when?)
c) with restriction to certain mental or actual operations (why?).

..

Important modelling principles:

Differentiation
Non-consideration of irrelevant objects
Reduction
Omitting object details
Decomposition
Decomposition, breakdown into individual segments
Aggregation
Combining, combining segments to form a unit
Abstraction
Concept and class formation

-> The goal is not to copy the reality 1:1.
E.g. reduction: You don't want to include everything into a model! Especially not every parameter needs to have a significant influence, but this has to be quantified.

--

Grunde: the description of the mindsets looks quite unusual for me. It's said that scientists are the the theoreticians and engineers are the practical people; but I'm sure you're referring between the lines to the situation here.

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Graeme H
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Re: Excellent FP on modeling

#14

Post by Graeme H » Thu Jul 18, 2019 11:22 pm

Grunde: the description of the mindsets looks quite unusual for me. It's said that scientists are the the theoreticians and engineers are the practical people; but I'm sure you're referring between the lines to the situation here.
I think Grunde constructed an aggregate model to illustrate his point. As such, he was wrong but useful.

:)

Cheers, Graeme
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Merlin
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Re: Excellent FP on modeling

#15

Post by Merlin » Fri Jul 19, 2019 5:09 am

Interesting to see that modeling has become an obsession on the Board. We do not face a model problem; we face a problem of understanding the mechanisms involved in the flight of the line. If we understand something someday then someone may be able to build a relevant model.

To make it short (see post #2 in the “Thank you” thread), a model is just a tool and must be fit for purpose. In my case they are built to allow comparing situations, identify trends which can be understood (in the sense of physics), and not to make accurate predictions. So they have limitations and I have to do with them. I do not really care about the category they may belong to (academic, engineering), they must stay practical and must not be “black boxes”.

Today we do not know why sometimes a fly line seems to stay aloft until the end of its rollover and then falls down. Is it purely an effect of trajectory and launch conditions? Is it an aerodynamic effect? Is it due to inertial effects of the fly leg as suggests Graeme’s interpretation of his video as the loop nose makes a down up motion? Is the assumption of the impossibility for a string to transmit moment a bit too restrictive since a fly line as some stiffness? Nobody really knows and can back up his opinion with explicit physics (I mean something like equations of schemes), and not only an interpretation of what he can see. It could well be a combination of several things, and good luck to explicit the mechanisms. The tension at the loop is also a good candidate for explaining most of what happens to a fly line, but let me know when you have the demonstration that it holds both legs aloft. At least it is an all purpose answer to any difficult question. Another question: what is the influence of line taper on this issue?

Controversy came up with publications mentioning an aerodynamic effect which was not backed up by experiments. Thanks to Graeme’s videos we have better analysis of actual situations and a “lift” example of a loop nose. Building new experiments is necessary to improve understanding or suggest mechanisms, but here again we must be careful. If one wants to investigate aerodynamic effects then there must be a significant speed level for the line, if not I could eventually show that an airplane cannot take off if I limit the thrust of its engines.

Forget models or spreadsheets, we have none able to reproduce a line flight; our problem is to get the basic mechanisms and select among the options we are thinking about. We may be disappointed at the end, I’m not optimistic about an issue for this problem.

Merlin
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Re: Excellent FP on modeling

#16

Post by Paul Arden » Fri Jul 19, 2019 5:52 am

Hi Merlin,

I was of the opinion that the loop nose stayed on course until LSP and then fell according to gravity. So I was definitely of the view that there was a force involved. However it was when looking for something else (counterflex and its effect) in my long carry videos that I noticed that the loop wasn’t doing what I expected and appeared to be falling. And so that is why I have since revised my opinion and have looked for another reason why some loops do appear to unroll first before falling, and one reason could of course be the flyline’s upward angle at the beginning of the cast in relation to the forward trajectory - ie inertia.

I think what is necessary now is more videos. Particularly since the video where I watched the loop fall is exactly the video where if anything it should maintain height. Of course if we are to discover that lift or DOG is significant then all the better. However I think some healthy skepticism is in order until such time as it is shown one way or another.

I will say that vertical loops certainly fly better than horizontal ones. However there are other possible reasons for that - in particular it’s hard to keep the legs parallel. What is sorely missing from our conversations is forward loops originating from high backcasts. No doubt Graeme and I can rectify this next week. I also think we need to drop a full line and measure it’s free-fall time to confirm Noel’s equations.

We need to solve this and I find the process very interesting. Particularly so because it’s not a 2 week discussion but actually goes back many years through different Board memberships/communities. This community I think is in the strongest possible position to solve many of these unresolved questions, both because of the great depth of knowledge it brings as well as the tools at our disposal. (High-speed cameras 20 years ago cost tens of thousands of pounds. Now almost every smart phone has one).

So for me, I think we are in the best possible position to finally work this out. And if we work together then I believe we will.

Cheers, Paul
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Re: Excellent FP on modeling

#17

Post by James9118 » Fri Jul 19, 2019 9:53 am

Torsten wrote:
Thu Jul 18, 2019 10:08 pm
-> The goal is not to copy the reality 1:1.
E.g. reduction: You don't want to include everything into a model! Especially not every parameter needs to have a significant influence, but this has to be quantified.
Torsten,

I agree. However if you're going to produce a simple model then you have to have the primary physics engine selected correctly. In my FP example of a thrown ball the equations derived from F=ma would give a pretty good estimation of the drag affected trajectory.

Now, unfortunately in the case of the 'drag induced lift' paper by Gatti-Bono, she has chosen the wrong primary causation. So in effect she's taken a minor (or negligible) parameter, i.e. aerodynamic lift, and built the whole model around it. As such she's neglected the overriding factor - the line's trajectory.

I'm sure she's an intelligent woman and if someone early on would have asked her - 'Caroline - why don't my side casts veer off in the direction of the rod-leg' I'm confident she'd have instantly recognised the error in her assumptions. However, the peer review team almost certainly wouldn't contain any fly-fishing physicists who would know that side-casting was even a 'thing'.

It appears that you, Gordy and Merlin cannot see this flaw also. You treat trajectory as a minor parameter because you've already 'fudged' your ideas/models not to need it. You then look for confirmation in the most obscure of places, rather than looking at actual fly casts.

Can I suggest you take a long hard stare at Bernd's Dangly Bits (BDB). Now I know Bernd isn't a physicist but his picture of the progression of a fly cast is insightful and absolutely matches reality - as it should because he drew it after watching what happens with real fly-casts. What he saw was a very complex trajectory with the fly line going up, down, forward and to the side. In it you'll see why the fly leg lifts.

Regards, James.

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Re: Excellent FP on modeling

#18

Post by grunde » Fri Jul 19, 2019 10:33 am

Graeme H wrote:
Thu Jul 18, 2019 11:22 pm
I think Grunde constructed an aggregate model to illustrate his point. As such, he was wrong but useful.

:)
:whistle: :pirate:

James’ last post also makes a lot of sense.
"Essentially, all models are wrong, but some are useful."
George E. P. Box

Always question the assumptions!

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Merlin
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Re: Excellent FP on modeling

#19

Post by Merlin » Fri Jul 19, 2019 10:42 am

James

Thanks to tell me in which post I support Gatti Bono and her lift theroy. It would be nice not to categorize people bluntly, and mix everything up. The fact that one computes the lift on a loop (or tries to) does not mean he supports the theory in question. Reread my post above please.

We agree about the trajectory issue, if you can remember. :)

The string shooter is another story, in that case one can speak of an aerodynamic effect, it is proven despite skepticsm. The limitation for interpretation is the fact that most of that effect comes from the bottom of the loop of the string, which is the rod leg in the fly cast case, and has no speed. Consequently the string shooter is not the proof that an aerodynamic force holds the fly line aloft. The question being today to explain that phenomenon which might have different sources, who knows? An aerodynamic force is not the prime option.

Merlin
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Charles Ritz, A Flyfisher's Life

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Merlin
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Re: Excellent FP on modeling

#20

Post by Merlin » Fri Jul 19, 2019 10:45 am

Grunde

A careful study made by Dirk on one's of Graeme's cast does show something similar to what Bernd explains with his scheme. Interesting isn't it? But that does not give the clue. Graeme made an interpretation, and the challenge is now to demonstrate that it can be true, or not.

Merlin
Fly rods are like women, they won't play if they're maltreated
Charles Ritz, A Flyfisher's Life

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