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Tension Discontinuity /DN

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Paul Arden
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Tension Discontinuity /DN

#1

Post by Paul Arden » Fri Jun 18, 2021 8:38 am

Hi Merlin,

I’ve been spending my afternoon chilling out and reading this http://www.sexyloops.com/flycasting/mer ... %20use.pdf

Can I ask about Tension Discontinuity. Is something that occurs at loop formation? Or is there some other some other factor involved here?

You know for a paper like this and with some of the readership here it might be very useful to explain every equation in words. Particularly for those of us who haven’t used momentum equations for 30 years or more. I know it’s a pain to make everything in long hand but I think that wound be helpful. I’ve found myself with pen and ink. I think it would go a long way to making it more accessible.

Also I think there are some key points here that could be highlighted. There are some gems that need emphasising. Anyway I’ll drop you an email on this.

So what are the causes of Tension Discontinuity? Lee once told me he reduced the DN effect by taping the line to the rod tip.

Thanks,
Paul
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Merlin
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Re: Tension Discontinuity /DN

#2

Post by Merlin » Fri Jun 18, 2021 3:59 pm

Hi Paul

The topic is pretty difficult. Some speak of a "singular" point within the loop which would correspond to a so called discontinuity in tension. I have to re read articles again. The condition for the singularity is described (I shall elaborate on it later), and there are specific parameters to match to get it. If not the "loop" of the string shooter is pear shaped. Discontinuity in tension is hard to figure out since the medium (a string) is continuous. I looked after examples on the web and found nothing. The condition given relates to actual tension level and inertial forces but as such it does not speak to us.

I can try to re write the article with more words than equations ( :) ) and describing the case of the singularity will be a challenge. I have to write it for me first and see if I follow the publication without interpretation. When I tried to get some explanations from the French University who published on a string shooter, I hardly got a response.

Merlin
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Re: Tension Discontinuity /DN

#3

Post by Torsten » Fri Jun 18, 2021 6:38 pm

Hi Paul,

in this case Daniel has used the simple 1-D fly line model which consists of a straight fly leg, a straight rod leg and a semicircular transition (the loop front). Other authors made the same assumptions, I think there is a paper from Perkins/Gatti-Bono with the same subject. I think the alternative approach from Daniel is interesting (directly from momentum). That's a bit tricky, because parts of the fly line are variable mass systems. You can also derive the equations of motion from the energy e.g. with the Lagrangian as starting point.

The disadvantage is the model itself, the assumption of a semicircular loop front is basically wrong - in the real world the line particles are not constrained to move along a circle. Also you can't describe non-straight rod/fly legs. For that you need more complex models and simulations with more degrees of freedom.

For tension variations I'd expect the first if there are wrinkles / wavy shapes in the fly leg. When I have more time to work further on the fly line simulation I'll try to visualize the tension somehow.

Torsten.

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Graeme H
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Re: Tension Discontinuity /DN

#4

Post by Graeme H » Sat Jun 19, 2021 4:04 am

Torsten wrote:
Fri Jun 18, 2021 6:38 pm
The disadvantage is the model itself, the assumption of a semicircular loop front is basically wrong - in the real world the line particles are not constrained to move along a circle.
The line particles never move along a circle even if the loop shape looks circular.
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Paul Arden
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Re: Tension Discontinuity /DN

#5

Post by Paul Arden » Sat Jun 19, 2021 8:33 am

Mostly a 1/4 circle or a semi-circle with a snap-cast. What fascinates me is when they rotate around a 1 1/4 circle. I wish I could learn to throw those on demand!

Cheers, Paul
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Re: Tension Discontinuity /DN

#6

Post by Graeme H » Sat Jun 19, 2021 1:56 pm

The shape of the loop doesn't describe the path a single point on the line takes through space. It's a fallacy to think the size of the loop influences the aerodynamics of the loop. Under normal circumstances, a loop is not pushing itself into wind. The particles of line enter a vertical loop, move forwards and downwards and stop once they hit the rod leg.

A single point doesn't even follow a 1/4 circle in a normal "semi-circular loop". It has a sort of "1/4 parabolic" path, shedding forward velocity rapidly. (This one was also shooting line so the point continues moving forward after it leaves the bottom of the loop. I'll find another video that doesn't do that later.)



And in a "sexy loop":

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Graeme H
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Re: Tension Discontinuity /DN

#7

Post by Graeme H » Sat Jun 19, 2021 2:25 pm

That other video, a tighter loop:

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Re: Tension Discontinuity /DN

#8

Post by Paul Arden » Sun Jun 20, 2021 9:19 am

Very nice videos Graeme - thanks!!
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Re: Tension Discontinuity /DN

#9

Post by Paul Arden » Sun Jun 20, 2021 9:32 am

What would happen if the rod leg was moved to here?
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Re: Tension Discontinuity /DN

#10

Post by Graeme H » Sun Jun 20, 2021 9:58 am

Are you asking me Paul?

If so and you're asking about the tracks of the points on the line, the answer is "the track would have less vertical displacement." It looks the same but the deceleration is happening over a shorter distance. (There are enough videos linked above to answer the question.)
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