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Upward Force from Form Drag

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Upward Force from Form Drag

#311

Post by guest » Thu Mar 29, 2018 12:43 pm

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George C
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Upward Force from Form Drag

#312

Post by George C » Thu Mar 29, 2018 1:02 pm

VGB wrote:
Graeme H wrote: And back to the earlier question I posed, is this a fair summary of the forces acting at moving frame itself? That is, I'm not asking what is happening at the extremities of the line, but at the boundaries of the frame. This should be a "yes or no" answer. If no, please show other forces acting.

This diagram is how I understand the behaviour of a loop in a moving frame with no external forces, there’s no reason for there to be different results for tension in the rod and fly legs.

Regards

Vince
Hi Vince

The fly leg carries energy as momentum
The rod leg carries energy as tension
The loop is where the energy stored as momentum is being turned into energy stored as tension.

Shouldn't the tension in the fly leg immediately before it enters the loop (and starts to give up kinetic energy) be very low (and related only to skin drag, length of line behind it, straightness, etc)?

Doesn't tension in the fly leg have to be balanced along its entire length since it is all moving at the same velocity? Hence if it is low at the front it is low at the back (and vice versa)?

George

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Upward Force from Form Drag

#313

Post by guest » Thu Mar 29, 2018 1:10 pm

George

I think that you need to finish you conversation with Merlin about energy, momentum, forces and tension. You seem to be confused about some of the concepts.

regards

Vince
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George C
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Upward Force from Form Drag

#314

Post by George C » Thu Mar 29, 2018 1:18 pm

VGB wrote:George

I think that you need to finish you conversation with Merlin about energy, momentum, forces and tension. You seem to be confused about some of the concepts.

regards

Vince
Well said, Vince.
Thanks
Goerge

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Upward Force from Form Drag

#315

Post by Paul Arden » Thu Mar 29, 2018 1:34 pm

Thanks Merlin, that's a great post.

Cheers,
Paul
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Upward Force from Form Drag

#316

Post by gordonjudd » Thu Mar 29, 2018 4:20 pm

I understood that you have proposed a model that excludes any external forces from the rod tip, gravity and drag to produce a value for tension that is not representative of a real cast.
Vince,
Sometimes your comments make me think you really don't understand the Perkins and Gatti-Bono model (and others as well).

Perkins ODE model using a moving frame to compute the accelerations and tensions expected as the loop propagates does include external forces from the rod tip and the changing drag forces and produces fly velocity histories that are very close to what we see in practice. So I think is it very representative of what we see in a real cast.

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Upward Force from Form Drag

#317

Post by guest » Thu Mar 29, 2018 4:47 pm

gordonjudd wrote:
I understood that you have proposed a model that excludes any external forces from the rod tip, gravity and drag to produce a value for tension that is not representative of a real cast.
Vince,
Sometimes your comments make me think you really don't understand the Perkins and Gatti-Bono model (and others as well).

Perkins ODE model using a moving frame to compute the accelerations and tensions expected as the loop propagates does include external forces from the rod tip and the changing drag forces and produces fly velocity histories that are very close to what we see in practice. So I think is it very representative of what we see in a real cast.

Gordy

Gordy

Given your previous difficulties with Newtons laws, I suspect it's more a case of you either misinterpreting or misrepresenting Perkins work. Both Graeme and I have asked you at least twice to state your external forces for the diagram you posted. It looks like we will not be getting an answer soon:
gordonjudd wrote: https://imageshack.com/a/img922/2135/Tpzp9e.jpg
There is no rod, no fly drag, no drag on the loop face and no gravity. Had Perkins used that model as the foundation of his modelling as you seem to be claiming, he would have had some very strange results indeed. The Gatti-Bono extract that Merlin posted earlier gave all the information that was required:
Line tension.PNG
Line tension.PNG (48.88 KiB) Viewed 1367 times
Through sections A and B which comprise the casting stroke, there are large differentials in tension between the rod tip and fly while the large external force is applied. Once that large force is removed after loop formation about t=0.65 s, the tension at the rod tip and fly are more or less equal as I stated.

Vince
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Upward Force from Form Drag

#318

Post by gordonjudd » Thu Mar 29, 2018 5:15 pm

Are the FORCES acting on the loop as shown or not? If not, please explain what's missing.
Graeme,
No.

Compare your drawing (the tension value should be changed to rho_l*V_loop.^2) with the one below.
https://imageshack.com/a/img922/2135/Tpzp9e.jpg

Do you see a difference in the two drawings?

The moving frame (view from a car window) still includes the offsetting force from the rod tip, so that should be included in your drawing and you should note that the tension in the fly leg is near zero.

I get the feeling that you think the moving frame only applies to the loop (the area with the white background). In fact the moving frame includes everything, it just makes a difference in the velocities, and converts the cycloid path of a dm section of line in an earth frame to the circular path in the moving frame.

The tension in the line going around the loop (neglecting the added complications imposed by drag forces) can be computed using the same approach as that used to compute the tension in a rotating ring. Did you understand that analysis?

With drag the tension at the top of the loop is slightly less than rho_l*V_loop.^2 as described on the old board but those topics are still in limbo. The form and skin drag forces on the loop and fly leg (especially the form drag) would need to be included on a complete model, but I think that would just confuse the understanding of where the tension in the loop comes from. Again the affect of drag on the propagation of the loop is discussed on many topics on the old board.

There is little or no tension at the fly end of the line. Thus the fly leg will tend to accelerate (or decelerate) depending of the magnitude of the drag forces on that leg compared to the rho_l*V_loop.^2 tension that is associated with the change in the momentum of the line as its goes around the loop. That momentum change is due to the change in direction, not a change in velocity as we generally suppose.

The tension in the rod leg is nominally constant because of the offset force coming from the rod tip. I think we are coming to the conclusion that the tension in the rod leg helps support it (much like the sag you would expect for a uniform line under tension and supported at its two ends) but I still do not see what supports the loop end of the line aside from external drag forces.

Gordy

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Upward Force from Form Drag

#319

Post by gordonjudd » Thu Mar 29, 2018 5:41 pm

the tension at the rod tip and fly are more or less equal as I stated.
Vince,
That comment is not true and just shows that you do not understand the Perkins and Gatti-Bono model.

The tension at the rod tip and the bottom of the loop are nominally equal as you state, but the line tension at the fly is near zero since there is no line mass being accelerated behind that point. The tension in line at the fly comes from drag on the fly, and thus is very small compared to the rho_l*V_loop.^2 tension at the loop.

Why is the black line for the tension at the rod tip slightly increasing as the loop rolls out in the tension plot you referred to?

Gordy

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Upward Force from Form Drag

#320

Post by guest » Thu Mar 29, 2018 5:50 pm

gordonjudd wrote:
the tension at the rod tip and fly are more or less equal as I stated.
Vince,
That comment is not true and just shows that you do not understand the Perkins and Gatti-Bono model.

The tension at the rod tip and the bottom of the loop are nominally equal as you state
Gordy

Will you make your mind up, are you agreeing or disagreeing?

Vince
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