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The Triple or Check Haul

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gordonjudd
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Re: The Triple or Check Haul

#61

Post by gordonjudd » Sun Aug 30, 2020 6:45 pm

• In the paper about drag affecting loop, the expression of the horizontal drag force is wrong since it does not contain the skin drag contribution (skin drag forces are not symmetrical).
Daniel,
Would the skin drag forces be symmetrical (and hence cancel out in the horizontal direction) in a loop frame where the tangential velocity around the loop is constant? I think that is the assumption Lingard made when he developed the equation for the drag loss on a circular loop that Gatti-Bono used in her paper on calculating the fly velocity history.

I could not find the .gif example that compared her expected fly velocity history for a level line with measured data, but in essence her values were an overlay for the WWE ode model that I use.

You can see the values produced by the WEE model (green curve) closely tracked the measured velocities (blue curve) for this example. You might want to see how your tension model fits with this measured tethered cast data as a further check of your model. No guarantees but, with any luck I should be able to find the relevant data (from six years ago) that you would need.

Image

In the few cases I have measured using level lines the Gatti-Bono equations fit the measured data quite well

Gordy

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gordonjudd
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Re: The Triple or Check Haul

#62

Post by gordonjudd » Sun Aug 30, 2020 9:55 pm

If you don't mind, I'd rather use your data from the file you sent me some days ago,
Dirk,
That data came from markers that had different slopes in the marker paths and I also made some sign errors in the stitching process. Those errors produce steps that raise hell with the spline smoothing routine I use and produce the wiggles you are showing. I hope those stitching errors were fixed in the retro loop 2 data that I sent you.

Can you use the scale controls on Tracker to plot your data on the same scales as mine so it would be easier to compare our smoothed velocity and acceleration values? I think we should work from smoothed velocity and acceleration curves that are reasonably the same before we start talking about how the values and timing for another derivative (jerk) compare.

If it would help I can produce jerk curves as well, but as you know small ripples in the acceleration curves can produce huge ripples in the jerk curves.

Thanks for taking the time to do a deep dive into this data. I hope that we can come up with similar velocity and acceleration (maybe even jerk) curves and then make observations that are base on matching data.

Gordy

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Merlin
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Re: The Triple or Check Haul

#63

Post by Merlin » Mon Aug 31, 2020 5:15 am

What kind of experience are you after Daniel?
Hi Lasse

Many thanks for your kind offer.

I need to check whether or not casting a light shooting head can fail: incomplete rollover, ground hitting before end, etc. The reason is linked to the skin drag coefficient which may be too high at this time in my model (figure taken from litterature). Available videos are made with markers that deeply influence the drag on the line, that is clear, but I need a correct order of magnitude to work with. Maybe you have a 25 to 30 feet head weighting around 150 grains to check how an untethered cast (using thin braided running line for example) behaves. If the untethered line rolls over easily, for sure the skin drag coefficient I use is on the high side since the model says it should fail.

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

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Merlin
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Re: The Triple or Check Haul

#64

Post by Merlin » Mon Aug 31, 2020 5:17 am

Would the skin drag forces be symmetrical (and hence cancel out in the horizontal direction) in a loop frame where the tangential velocity around the loop is constant?
Gordy, one cannot compute that in the loop frame. In a tethered cast the bottom point of the loop has no speed (zero skin drag) and the top point of the loop has fly leg speed (highest skin drag).

The question is not in the WEE model which uses Lingard expression of work energy for drag forces. I found the same expression when integrating the true velocities, and with that same methodology I got the expression of drag forces in a general case (Vrod leg different from zero). The issue is in the controversial paper about “lift”. In that case the expression used for front drag is wrong. Nothing more than that, the WEE model is fine, the lift calculation is not.

If you found a match for a level line with the WEE model years ago, I should find the same using the same input data with the tension model.

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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Re: The Triple or Check Haul

#65

Post by gordonjudd » Mon Aug 31, 2020 2:31 pm

Gordy, one cannot compute that in the loop frame.
Merlin,
I should have said "Would the skin drag forces be symmetrical (and hence cancel out in the vertical direction). Lingard did get a contribution from skin drag in the horizontal direction. However it is much smaller than the form drag value.

I thought you should get the same results for an earth frame or a loop frame as long as the loop was moving in a straight line with constant velocity so it was also an inertial frame. Live and learn.

If you had a simi-circular shaped wire traveling on a horizontal path (what we see for the shape of the loop traveling in an earth frame) would the skin drag cancel out in the vertical direction but have a net negative drag in the horizontal direction?

In an earth frame a point on the line will follow a cycloid path and I can see that should have a net positive drag value in the y direction. Is that cycloid path what you are using to make your skin drag calculations and thus you get net vertical drag value due to skin drag in the y direction? Is the horizontal drag for the cycloid path the same as it would be for a semi-circular shape?

Sorry for all the questions. As you can see I am still trying to get my head around which shape should used.

Gordy

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Merlin
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Re: The Triple or Check Haul

#66

Post by Merlin » Mon Aug 31, 2020 2:47 pm

Gordy

I use the actual speed (along the cycloid path) and I project it on the tangent to the loop and on the normal to the loop. Since they are perpendicular I can switch for drag forces and recombine afterwards (a necessity since speed has to be squared for forces). One corresponds to skin drag and the other one to front drag.
Before integrating I project each force on horizontal. Then I integrate two force components for that direction, the horizontal drag has two terms, one linked to front drag, the other one to skin drag. I can repeat the process for vertical forces although it is not needed for a 1D model.

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

Dirk le Roux
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Re: The Triple or Check Haul

#67

Post by Dirk le Roux » Tue Sep 01, 2020 12:16 pm

gordonjudd wrote:
Sun Aug 30, 2020 9:55 pm
Those errors produce steps that raise hell with the spline smoothing routine I use and produce the wiggles you are showing. I hope those stitching errors were fixed in the retro loop 2 data that I sent you.
Hi Gordy

Though the loop 2 data is smoother, I suspect that your persistence with jumps between markers on the same tracks still contributed to slightly convoluted outcomes.

Here is a compilation of all the leg velocity data I have from you so far:

jerk periods - retro loop - Gordy's data_all.jpg

Once again the pink strips are the jerk zones. The red dashed line is my measured data, with the difference this time that I posted the rod tip’s profile and not the marker on the fly leg’s.

As you can see, there is generally a time agreement between velocity slope changes (jerks) in both of our data, negative slope changes by the rod leg matched by positive slope changes on the fly leg. The major jerks especially, at around 0.47s and 0.6s, consistently correspond between our data.
gordonjudd wrote:
Sun Aug 30, 2020 9:55 pm
Can you use the scale controls on Tracker to plot your data on the same scales as mine so it would be easier to compare our smoothed velocity and acceleration values? I think we should work from smoothed velocity and acceleration curves that are reasonably the same before we start talking about how the values and timing for another derivative (jerk) compare.

If it would help I can produce jerk curves as well, but as you know small ripples in the acceleration curves can produce huge ripples in the jerk curves.
I think the velocity overlays I posted are easy enough to read. As you can see, the velocity profiles match reasonably the same.

I don’t think it is worth to smooth for producing jerk curves, due to the issues with the methodology previously pointed out. And as you said, with every further derivative, the data becomes increasingly wobbly. Instead, the casual reader could learn to read acceleration and jerk in velocity profiles:
Constant velocity slope = constant acceleration. Upward/downward slope indicates the direction of acceleration. Slope steepness indicates acceleration intensity.

Change in velocity slope = change in acceleration (jerk). Whether the concavity of such a slope change points up or down tells us the jerk vector direction. A more acute slope change angle indicates a more intense jerk.

Here is the previous chart from my data, now with the rod tip curve (as referenced above) added in red and the old rod leg marker (blue) as a comparison.

jerk periods - retro loop - Dirk's data_incl_rod_tip.jpg

All the best,
Dirk

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Lasse Karlsson
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Re: The Triple or Check Haul

#68

Post by Lasse Karlsson » Tue Sep 01, 2020 8:06 pm

Merlin wrote:
Mon Aug 31, 2020 5:15 am
What kind of experience are you after Daniel?
Hi Lasse

Many thanks for your kind offer.

I need to check whether or not casting a light shooting head can fail: incomplete rollover, ground hitting before end, etc. The reason is linked to the skin drag coefficient which may be too high at this time in my model (figure taken from litterature). Available videos are made with markers that deeply influence the drag on the line, that is clear, but I need a correct order of magnitude to work with. Maybe you have a 25 to 30 feet head weighting around 150 grains to check how an untethered cast (using thin braided running line for example) behaves. If the untethered line rolls over easily, for sure the skin drag coefficient I use is on the high side since the model says it should fail.

Merlin
No problem, I do have that kind of shootinghead, and a hunch says it will roll over easily. Will cast it and shoot it so you get some real world figures!


Cheers
Lasse
Your friendly neighbourhood flyslinger

http://www.karlssonflyfishing.com

***Bring Mark back!!!!!! ***

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Merlin
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Re: The Triple or Check Haul

#69

Post by Merlin » Wed Sep 02, 2020 9:46 am

Thanks lasse

There must be a lower limit in speed at launch time, if the cast is very fast "anything" can rollover IMHO.

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

Dirk le Roux
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Re: The Triple or Check Haul

#70

Post by Dirk le Roux » Thu Sep 24, 2020 2:47 pm

gordonjudd wrote:
Mon Sep 14, 2020 3:08 pm
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.
Dirk,
Not only is it readable, the jerk derived from the measured velocity profiles in that pull back cast can be calculated as shown below.
Image
Since the calculated correlation coefficient between those two jerk curves was only 12%, I don't think you can cherry pick a time range that fits with your centrifugal force theory while ignoring the 88% of the time when there is no correlation between the two jerk curves.

Invoking jerk as an explanation for the delay in the fly legs velocity increase relative to the start of the pull back is a derivative too far for me.

Gordy
Hi Gordy

I transferred the reply to the above post to this thread, which is where this specific difference arose.

I am sure you agree, that graph is gibberish. Progressive derivatives of noisy data progressively become a cacophony. We can, though, from a velocity profile read acceleration (slope, indicating direction and intensity of net force acting on the object) and changes in acceleration, which is jerk (change in slope, indicating a change in net force).

From experiments, we want to learn if and when, and if we’re lucky, by how much, a pullback action affects the fly leg’s velocity.

We may see at some point a change in the fly leg’s velocity slope (acceleration), indicating a change in net force acting on the fly leg. We may see at some later point that the initial negative (decelerating) fly leg velocity slope has incrementally been changed to positive (accelerating), indicating net force direction change. This latter observation is the criterion you propose as proof that a pullback action has affected the fly leg’s velocity.

One problem with that criterion is that, without a carbon-copy non-pullback reference cast to compare with, we cannot experimentally confirm whether a fly leg’s acceleration was due to a pullback action or not. Non-pullback casts’ fly leg velocity profiles often show acceleration toward the end of roll-out. In fact, the following figure was lifted from a cast without pullback:

fly leg velocity.jpg

I am sure you agree that there is no way to tell if, or when, a pullback was responsible for the acceleration that occurred.

Another problem with that criterion is that changing opposing or assisting forces (changing the net force) acting on an object can affect its velocity without ever achieving a reversal in net force direction and hence a reversal of acceleration/deceleration. Pullback effect may, while reducing a fly leg’s deceleration, remain insufficient to accelerate that fly leg. In such a case, do we say the pullback had zero effect?

If a change in the slope of a velocity profile is sharply defined, we can read from that a non-gradual net force shift, an explicit event in time. With a velocity profile such as this:

AC_1.jpg
AC_1.jpg (16.11 KiB) Viewed 396 times

Newton tells us that up to the kink, the object is accelerating at a constant rate, a constant positive net force acting on it in the direction of travel. Then, at the time indicated by the kink, the net force acting in the direction of travel increases and steepens the velocity slope (the acceleration). A positive change in the net force occurs during the kink.

And with a velocity profile such as this:

AC_2.jpg
AC_2.jpg (15.75 KiB) Viewed 396 times

Newton tells us a constant negative acceleration, a constant opposing net force, changes during the kink to constant less negative acceleration, less opposing net force. A positive change in the net force occurs during the kink. Thanks to Newton, we know that either the opposing force diminished or the assisting force increased at the kink.

From your experiments, both legs’ velocity profiles exhibit quite defined changes in slope, and these changes correspond in time. By lack of alternative observable events which could explain the fly leg’s velocity slope changes, we can deduce that events of increased force acting on the fly leg relate directly to events of increased rod leg pullback.

All the best,
Dirk

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