The Triple or Check Haul

[Paul Arden]

This is a very long time ago 2002 maybe http://www.sexyloops.com/picofday/sexyl ... ops7.shtml but the loop certainly travels backwards following the Snap. There must be some in other videos I’ve made because I’ve seen it many times. That POS sequence was part of the Instructor video where it was very clear.

Cheers, Paul

[gordonjudd]

I didn't ask about density, I asked about viscosity, don't you know the difference?

James,
There is no viscosity term in the drag equations that I use.

The kinematic viscosity for air does impact the Reynolds number, which in turn can impact the drag coefficient, but generally the Reynolds number is high enough that Cn=1 and Ct=.015 are generally used as the normal and tangential drag coefficients for the line.

The kinematic viscosity value I use to compute the Reynolds number is 1.460×10−5 m2 /s.

Gordy

[Dirk le Roux]

All works till the PB starts, then the trends differ: single stage process, limited rise in speed, negative loop travel velocity possible.

Maybe the observed fall exacerbates the split in two stages, it is not obvious to me, but I’m pretty sure I miss something. In real life the effect on line speed is sensible otherwise a short check would not be effective.

Maybe this plot of the same cast could help.

Gordy's (consolidated) smoothing was likely too aggressive - the time instance when the pullback actually starts is very clear on the footage (after the rod tip has been very still for ages), and is where I have inserted the first vertical dotted line.

Variances occur between different the fly and the fly leg markers' measured velocities, which 1) distorts the fly leg's (calculated) consolidated velocity representation, with resultant distortions of calculated loop travel and propagation values; and 2) indicates that some dynamics likely have occurred in the perpendicular plane.

Cheers,
Dirk

[Merlin]

Ok let's take items in order:

James,

I do not use air viscosity explicitely, it comes along with Cx & Reynolds.


All

Thanks for illustrating the loop travel with large PB , snaps and flips. If a loop can go back it is under large motions, and gravity likely. We can assume that loop travel speed can be slightly negative depending on conditions. In the case of Gordy's cast, the PB is not so huge even if it is large.

I had another run with the tiny loop model and I have added skin drag on the fly leg (fairly simple) because Gordy told me that markers create a lot of drag along his testing experience. Here is a run:

Gordy PB1 simulation Sdrag.JPG (48.13 KiB) Viewed 3419 times

It is not that bad at the beginning, loop travel velocity and loop tangential velocity diverge just after the start of the PB (that is in line with Dirk's analyzis at least), but anyway the loop travel velocity does not stay close to zero and diverge with the PB. It is clear that some I would say "inertial" condition is not met.

So there is still theoritical work to do on constitutive equations to find out where the model fails. I can get a "virtual peg" by casting a piece of running line by hand and make the loop stall with a gentle PB.

Merlin

[Dirk le Roux]

the loop travel velocity does not stay close to zero and diverge with the PB. It is clear that some I would say "inertial" condition is not met.

Hi Merlin

That run's loop travel goes negative because the fly leg response to the pullback is unrealistically low and slow. Anyway, that's where I would look for the problem.

Cheers,
Dirk

[Merlin]

Thanks Dirk

I think I have identified the issue, now I need some time to write a correct spreadsheet and test it.

We may believe that there are just two unknowns, leg speeds, but there are three in fact, hence the need for a third equation.

Merlin

[Merlin]

Looks better now:

Gordy PB simulation with correct mechanism.JPG (46.5 KiB) Viewed 3375 times

Merlin

[gordonjudd]

Merlin,
Here are the comparative speed curves that include the y velocity variations in that video to arrive at speed variations for the different legs and loop.

The thick red line is the expected loop speed based on the sqrt(sum of squares) of the .5(v1 +v2) values in the x and y directions. That expected loop speed value does not go negative as it did before just using the x velocity values.


Gordy

[Dirk le Roux]

That expected loop speed value does not go negative as it did before just using the x velocity values.

Hi Gordy

It is a good thing that your re-worked curves now yield a non-negative loop travel velocity, in line with observation. However, if we are still talking about the same cast, the time of the pullback start is still off, and the fly leg velocity trends are still not in line with the footage.

Cheers,
Dirk

[gordonjudd]

It is a good thing that your re-worked curves now yield a non-negative loop travel velocity,

Dirk,
These are speed values not velocity.

By definition the speed cannot go negative because we are taking the sqrt(sum of squares) to arrive at a speed value.

These markers are moving in different directions and thus just taking the difference of the x velocity values is not sufficient since it leaves out the contribution of the y velocity changes.

By the same token velocity is a vector quantity so you can't just replace the velocity values in the .5*(v1+v2) equation for the expected loop velocity with the speed values.

Because Merlin's model is one dimensional I am not sure how we could validate it with 2-D data. Your thoughts on the matter are most welcome.
Gordy