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Upward Force from Form Drag

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Graeme H
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Upward Force from Form Drag

#71

Post by Graeme H » Tue Feb 20, 2018 11:25 am

I did "make it up", but the term is based on the standing waves we see in flowing water currents over rocks or into a pinch point. The wave in that case is traveling upstream at the same pace that the current is flowing down stream. The compression wave (dolphin nose) "wants" to travel back up the fly leg but with no viable medium to traveling in, it constantly forms and dissipates at the loop end of the fly leg.

By "viable medium", I mean a string does not act as a medium for compression waves because the string collapses under compression. It has some stiffness, but not enough to transmit a compression wave.

Cheers,
Graeme
FFi CCI

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gordonjudd
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Upward Force from Form Drag

#72

Post by gordonjudd » Tue Feb 20, 2018 4:48 pm

We know bead chain has stiffness
Trev,

I assume that stiffness is referring to bending stiffness (i.e. the product of its area moment (I) and Young's modulus (Ey) measured in N·m.^2) not the longitudinal extension it might have due to tension. That being the case, it seems to me that until you get to a ball on ball situation bead chain has no bending stiffness.

Try holding a 10 cm loop of bead chain vertically, and I don't think you will see the shape that Graeme pictured above for a fly line.

To my knowledge there is no paper for Dr. McMillen's analysis of falling loops in viseoelastic materials. We only know that his complicated computer model predicts a DN shape. I think this example was for a dropped yarn loop that had a small bending stiffness and small linear mass density.
Image


Gordy

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gordonjudd
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Upward Force from Form Drag

#73

Post by gordonjudd » Tue Feb 20, 2018 5:01 pm

My guess is that it is related to line stiffness.
Merlin,
I may have jumped to a conclusion regarding how you think bending stiffness affects a line's propensity to form a dolphin nose (DN) loop.

Do you think a DN loop is more likely to show up in a material with a higher bending stiffness that resists taking on a shape with a small bending radius or you are more likely to get a DN in a material with a smaller bending stiffness?

I was really surprised to see how narrow the DN loop was in Graeme's video. It looks to me the diameter of the loop formed when holding a length of line in the air was much larger than the narrow diameter in the video.
Image

Gordy

crunch
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Upward Force from Form Drag

#74

Post by crunch » Tue Feb 20, 2018 8:05 pm

When line loop crashes in Graemes great video there can not come the line loop acceleration when fly leg shortens like in this video

https://vimeo.com/175139942

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gordonjudd
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Upward Force from Form Drag

#75

Post by gordonjudd » Tue Feb 20, 2018 9:48 pm

there can not come the line loop acceleration when fly leg shortens like in this video
Esa,
I agree.

Since there is no rho_l*v_loop.^2 acceleration force to offset the drag forces on the line I would expect the velocity of the fly to be decreasing when the front of the fly leg crashes into the net to kill its momentum. Those retarding drag forces would cause the fly to decelerate as it heads towards the net.

Gordy

TrevH
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Upward Force from Form Drag

#76

Post by TrevH » Tue Feb 20, 2018 10:51 pm

Thanks, Gordy.

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Graeme H
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Upward Force from Form Drag

#77

Post by Graeme H » Wed Feb 21, 2018 1:49 am

crunch wrote:When line loop crashes in Graemes great video there can not come the line loop acceleration when fly leg shortens like in this video

https://vimeo.com/175139942
Hi Esa,

If you watch the end of the fly line in isolation in those casts, you can see the line itself does not accelerate after the initial "back" cast. The loop appears to accelerate, but the loop is simply the place where the fly leg hits the rod leg. While he pulls the rod leg out from under it, the loop appears stationary, but once the line within the rod leg is no longer moving, the loop just resumes its journey to the right of screen.

As a side note, I think there's a clue about the dolphin nose in there too. Watch between 48 and 52 seconds. It's one of the experiments I have in mind for filming: let the loop progress for a while, then apply sudden pullback. Does a DN form? In this video it does, but then they cut away too soon. :(

Cheers,
Graeme
FFi CCI

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Merlin
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Upward Force from Form Drag

#78

Post by Merlin » Wed Feb 21, 2018 3:52 pm

Gordy

if this is a viscoelasticity issue, stiffness (elasticity) is part of the phenomenon and the bended line may store and release some energy. The viscous nature means loss of energy, and I do not know how to interpret Dr Mc Millen comment. The line flight effectively flies above my head at some point, and I am much more comfortable with rod design.

Graeme's idea bout a couple of waves going in opposite direction is tempting, but where does the second wave comes from?

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

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gordonjudd
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Upward Force from Form Drag

#79

Post by gordonjudd » Wed Feb 21, 2018 4:49 pm

If you watch the end of the fly line in isolation in those casts, you can see the line itself does not accelerate after the initial "back" cast.
and
While he pulls the rod leg out from under it, the loop appears stationary, but once the line within the rod leg is no longer moving, the loop just resumes its journey to the right of screen.
Graeme,
I don't know if we are looking at the same part of the cast, but it appears to me the loop (and hence the fly velocity) does accelerate once the loop starts going forward again after the rod leg hits the water on his snap. The green dots in the overlay below track the front of the loop at two frame intervals as it is going to the right of the screen.
Image
It looks to me the space between the markers is increasing as the loop propagates to the right (it actually goes backward since the velocity of the rod leg is higher than the fly leg until the fly leg hits the water). That would indicate the velocity of the loop over the ground is accelerating.

Gordy

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gordonjudd
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Upward Force from Form Drag

#80

Post by gordonjudd » Wed Feb 21, 2018 5:01 pm

Graeme's idea bout a couple of waves going in opposite direction is tempting, but where does the second wave comes from?
Merlin,
If they were going in opposite directions down the line then I would expect one would past through the other. To stay with the loop I would think the second wave (source unknown) would would have to be traveling down the line at the same speed and direction the loop is.

That is similar to the hump that follows the loop in Chris Korich's cast shown below. Both of these waves are in the same high tension part of the line, and thus they tend to have the same velocity going down the line and thus appear to travel together in a lab frame.


In your reference to line stiffness being a primary factor in forming a DN loop, do you think the DN is more likely to form with a stiffer line or one with a smaller stiffness such as yarn?

Gordy

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