Conservation Of Angular Momentum (COAM)

[Walter]

I'll let someone else make a cast that matches the model. That's beyond my capabilities I think. (At least here on earth where gravity is a factor in casting a heavy line. )

Cheers,
Graeme

It’s unfortunate that the digitization of your cast is so noisy but, even if it wasn’t, one cast is a very limited dataset for analysis purposes. The number of variables is bigger than a former US president’s vocabulary other than to say it’s “really yuge”.

Fwiw, you’re casting is impressive.

[Graeme H]

Thanks Walter.

[VGB]

I've checked the Graeme's video and I'm not surprised that it's difficult to reproduce results with your analytical model - because of upward directed trajectory and the fly leg has a significant wave / tailing tendency, the "measurements" of the velocity look very noisy to me. The requirements are straight fly/rod legs parallel to the ground. Not really suited to verify your model for predicting of the the loop size.

Graeme’s cast is as close as you can practically get to the semi circular model, Gordy’s cast is nowhere near as I have said throughout this thread. Chapeau to Graeme for getting close, the trial requirement and aim to draw conclusions about COAM based on loop size was unachievable. The problem was not the real world’s failure to match the model, it has bigger fish to fry.

Noise is a fact of practical testing and you have to learn to work with it. In this instance, I used greybeard experience but there are various filters and data smoothing techniques that data scientists such as Walter could employ.

The number of variables is bigger than a former US president’s vocabulary other than to say it’s “really yuge”.

The Conservation of Casting Common sense dictates that any answer that doesn’t start with “It depends” is invalid and should be disregarded.

If you want some explanation about dynamic sag in a rod leg (by opposition to static sag in a catenary)

I understand sag is changing throughout the cast because the horizontal and vertical forces acting on the rod leg are not constant. The rate of change of these forces can have a dramatic effect on the cast which is being seen by the rapid acceleration of the fly leg and can be experienced by the caster through their hands. I’m not sure that you capture this in your models but I’m not inclined to open a new thread unless I see a practical need to do so.

There are a number of disadvantages with simulation but one in particular that can be viewed as a risk as described by Stewart Robinson in his book “Simulation - The practice of model development and use”

Overconfidence. There is a danger that anything produced on a computer is seen to be right. With simulation this is further exacerbated with the use of an animated display, giving an appearance of reality. When interpreting the results from a simulation, consideration must be given to the validity of the underlying model and the assumptions and simplifications that have been made.

I think if Paul changed COAM for AM, he is probably right but don’t tell him, or he may get the book prize

Regards

Vince

[Walter]

The problem with data cleaning, besides being outside the scope of this topic, is that it is meant to remove incorrect or corrupted data from the dataset. The issues in this data set come from frame rate and measurement error inherent in the data, i.e. every data point has some degree of corruption built into it. The measurement error is just too large. It’s impossible, without using personal bias, to decide which data points are “correct” and which aren’t. Curve fitting will likely have the same issue. You could apply “greybeard” experience to try and get some meaningful data but I doubt if two different greybeards starting with that dataset would come up generate the same final dataset. Again, there would be too much personal bias imo.

If individual casts are going to have any meaningful input then a better camera would be needed but a single cast has issues as well. In this case Graeme’s casting skill is able to make a cast that many others simply could not. Even if he could repeat the cast to a high degree of accuracy no two casts are going to be the same so I don’t see video stacking isn’t an answer. I’m afraid we are left with taking a lot of casts with a better camera to get useful data. We could possibly train an ai model…

I sympathize with Daniel’s dilemma of trying to show how his model compares to reality but, unfortunately, this particular data set isn’t suitable for that. This is a case where I would trust the output from a model in progress to the dataset depicting a real cast.

[Graeme H]

Hi All,

There seems to be a lot of reliance on having the data cleaned before showing it. I understand that, but in my day job (making geological models) I tend to make the model I use "fit for purpose", meaning they serve one purpose. When the model is used for something outside the scope I designed it for, things can go wrong.

The mark that I posted a chart of earlier was not the main line mark I had planned to digitise, so I did a rough digitising effort with the aim of showing the data at a much broader scale, where small deviations are not shown as violent deviations in line speed. By posting at a scale that was not envisaged during digitising, people seem to have read more into the chart than they should have. Please look at the scales on a chart of velocity before jumping to conclusions about the data: a jump from 5.5 to 5.7 m/s looks huge when the scale has been enhanced. In reality, it's just noise.

To see what that actually means in Tracker, I've cleaned the data capture more carefully here, paying careful attention to both the chart and the line mark when I position the mark in tracker:

If I happen to miss the optimal position by a single pixel, this is the result in the chart when the scale is fine. Zoom into the pic and see where the red mark is on the fly line and look at the affect that has on the chart near 3.3 seconds:

In addition to that, there is a LOT of chatter in the video from the iPhone's capture. Sometimes I can fix it, sometimes I can't. You can see where I've had to skip placing marks in Tracker due to that chatter. This video is okay in some parts of the cast, but there are certain parts where frames are all over the place.

I realise these charts are not ideal. At least we can start to measure physical objects in motion without spending tens of thousands of dollars on the hobby.

Cheers,
Graeme

(BTW, I'm still cleaning this data ...)

[Graeme H]

And after spending another 40 minutes on cleaning this single mark in Tracker, carefully adjusting each point by fractions of pixels, this is the cleaned data for that mark on the line:

It's not a PhD data set and it's made by a person who is not deeply motivated with a phone and some free software. Beware of the limitations of someone else's work.

For those still interested, the transition between sinking line and running line leaves the loop, entering the rod leg at 3.57 seconds here.

Cheers,
Graeme

[VGB]

Hi Graeme

Thank you for your efforts and the data is more than sufficient for our purposes.

I asked you for the Birds Eye view because my impression when I cast was that the fly leg did not accelerate until the head/running line junction of my fly line was in the rod leg. This wasn’t what I expected to see from the model predictions, so I was pleased to see your Tracker data confirming my initial impressions. To me this means that unbalanced forces are acting on the system under study. Although it’s not COAM related, please could you confirm what the rod leg layout was when the significant acceleration started?

To throw a turd in the punch bowl, if you look at between 3.6 and 4.2 seconds there is no appreciable acceleration in the fly leg. The correct definition of COAM is that its rotational velocity remains constant unless it is acted upon by an external torque. This means that the loop can have external forces such as gravity acting upon it, provided that the net torque across the loop is zero. I interpret this to mean that forces acting on the system are balanced. Given the lack of fly leg acceleration, it is possible that COAM is at play in this period of time.

I can think of a couple of reasons that this could occur but please could you also let me know what the rod leg configuration is in this time period?

Regards

Vince

[Walter]

FYI, if anybody is interested I’ve done the full calculation for angular momentum of the loop in the fixed frame. As expected , a lot of stuff cancels out so we end up with a transformation of pi/2 so closer to 1.57 rather than the value of 2 that I had “intuited” previously.

As usual this is for a semicircular loop with level line and a tethered cast

The formulaic statement would be:

Given Al = angular momentum in the loop centric frame
Af = angular momentum in the fixed frame

Af = 2 Al/pi
Or Al = Af *pi/2 (that’s around 1.5*Af for the handwavers and 2*Af for us cro magnon handwavers)

The linear momentum of the fly leg in the fixed frame remains twice that of that in the loop centric frame.

[Graeme H]

Although it’s not COAM related, please could you confirm what the rod leg layout was when the significant acceleration started?

Hi Vince,

Here is an annotated screen shot of the point where acceleration of the fly leg kicks off in earnest (as best as I can tell - it is right on one of those shitty gaps in the frames)

t = 4.167 s

Here, the fly leg is red, the rod leg is yellow and the dashed bit is a guess for its location (it's not visible against the background.) The blue dot is the join between sinking and running line.

To throw a turd in the punch bowl, if you look at between 3.6 and 4.2 seconds there is no appreciable acceleration in the fly leg.

.... please could you also let me know what the rod leg configuration is in this time period?

Regards

Vince

Here's the same thing at 3.8 seconds, 0.37 seconds before the above screen shot:

t = 3.8 s

During the capture of these images for you Vince, I noticed that the join between sinking and running line has moved forward during this period, which is a source of slack that I didn't realise was there. That forward movement might simply be the rod leg becoming taut though.

Here's a video of that part of the cast with the progression of the marks in time on the right. Each jump is a frame on the original video.




Cheers,
Graeme

[VGB]

That’s outstanding, thank you Graeme. I think that nails the source of the external torque on the loop for this cast but I’ll have a few more looks at it, to see if there’s anything I missed.

I noticed that the join between sinking and running line has moved forward during this period, which is a source of slack that I didn't realise was there. That forward movement might simply be the rod leg becoming taut though.

That’s probably what I interpreted as being the impulse with the line snapping tight


Regards

Vince