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Upward Force from Form Drag

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Graeme H
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Upward Force from Form Drag

#291

Post by Graeme H » Wed Mar 28, 2018 9:15 am

Alas, the flying spaghetti experiment is still on hold due to the weather gods conspiring against me. We had the remnants of a tropical cyclone (Cyclone Marcus) come through last weekend and this weekend we've just got normal Perth high winds. I'm raring to go though, with a sacrificial line all marked up. (I'll make several casts, then cut 3m off the line and repeat until there's only a few metres left..)
Screen Shot 2018-03-28 at 4.37.06 pm.png
I'm still confused about the need for a moving frame of reference, but I hope to understand soon. Is this diagram a fair representation of your description posted in #285? I think you stated that the tension in the fly leg balances (equals) the tension in the rod leg. Are there any other forces involved? I can't find them mentioned in that post.
Screen Shot 2018-03-28 at 4.59.57 pm.png
Cheers,
Graeme
FFi CCI

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Lasse Karlsson
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Upward Force from Form Drag

#292

Post by Lasse Karlsson » Wed Mar 28, 2018 9:39 am

Paul Arden wrote:I don’t think a short carry is going to show anything Lasse. What I would like to see is the same cast as the one where I’m standing on a log. Personally I don’t think it’s possible but I’m open-minded to try!

Cheers, Paul
I still don't know what it is we would be looking for, so can't say if itll work or not.. And replicating your standing on a log, carrying 90 feet is a bit difficult to replicate, I agree :D only problem I see in theory, is that the line will fall the other way, so we have to change trajectory the other way compared to what we normally would do standing waving a stick around....

Cheers
Lasse
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Upward Force from Form Drag

#293

Post by guest » Wed Mar 28, 2018 3:06 pm

Merlin wrote:his dependence is illustrated with the “Reynolds number”, a dimensionless characteristic (Vince can elaborate on this if you wish, I think). Here is an example for a cylinder:
Paul

Simply put, the Reynolds number describes the relationship of how various size shapes perform in a fluid (air) at various speeds. I did explain this phenomenon a few years ago in a thread but the associated pictures have disappeared with Photobucket. I picked up these pictures from the NASA site https://www.grc.nasa.gov/www/k-12/airpl ... phere.html

As a rough rule of thumb, the left hand side of the Reynolds chart deals with very small objects and/or very slow speeds, the right hand side with larger and/or fast moving objects. If you are trying to derive a coefficient of drag, you should calculate the Reynolds number to obtain the correct figure for the size of object at a particular speed.

The coefficient of drag describes the characteristics of the aerodynamic force associated with the drag axis due to induced and parasitic drag. Parasitic drag is composed of form, skin and interference drag (for complex shapes), induced drag is a product of lift. There is also wave drag that occurs at transonic and supersonic speeds.

For a falling fly line, we should be looking about Point 2 on this graph and Point 4 for the loop face above about 10 m/S (from memory).
Re Chart.jpg
The Point 2 and Point 4 CD figures give an airflow as described in Pictures 2 and 4.
vortex.jpg
As explained by the text for Point 2:
A stable pair of vortices are formed on the downwind side of the cylinder. The flow is separated but steady and the vortices generate a high drag on the cylinder or sphere.
The attached vortices make the line appear to the airflow as being bigger than it really is, as you can see by the way the laminar airflow flows around the sphere. This reduces the terminal velocity of the falling line which may make it appear a bit more floaty than you might expect.

If you don't have wind tunnel data, the aerodynamicists use Navier-Stokes equations,which scares the pants off me:

https://www.grc.nasa.gov/www/k-12/airplane/nseqs.html

Vince
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Upward Force from Form Drag

#294

Post by guest » Wed Mar 28, 2018 3:08 pm

Paul Arden wrote:I don’t think a short carry is going to show anything Lasse. What I would like to see is the same cast as the one where I’m standing on a log. Personally I don’t think it’s possible but I’m open-minded to try!
If you do the cast on the log, but upside down from a bridge. My model predicts that you will fall off the log and it will land on you :p

Regards

Vince

PS Please can you video it?
Bright but shite

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George C
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Upward Force from Form Drag

#295

Post by George C » Wed Mar 28, 2018 3:36 pm

Merlin wrote:Hi George

Let’s see if we can progress step by step.

Several scientists have used the approach of splitting the line into tiny pieces to write a full set of equations allowing estimating the behavior of the line, but you need sophisticate software to get the results (this corresponds to the graphic above). Your representations of little pieces flipping over each other make sense, but this does not involve vertical forces, it is just a roll over constrained by air with very little braking force (skin drag on the loop). In the illustration below, the red/blue curve represents the path of the red/blue point (cycloid) as the loop is moving forward on the right hand side. At start the blue point is at the top of the loop and the red one is behind, on the fly leg (not represented). The loop is the half right part of the circle (I used a circle for ease). When the red point hits the rod leg, the blue point is now behind him on the rod leg (not represented): the flip over has been done, no brake involved.
Flip over.JPG
Today I personally think that a true lift is a red herring. The fly leg follows its launching direction thanks to tension, the rod leg is under balance because of tension, and even if there is very little upward (aerodynamic) force on the loop, this one has its own “gyroscopic” momentum as long as there is tension. Nothing can “lift” the legs, and the fly leg is in balance with aerodynamic forces. In the video posted by Gordy, one can see that the angle of the fly leg with horizontal is pretty high at start (the line is lifted from water), and this angle reduces as the line is accelerated by the rod first, then by tension. When tension vanishes, and I have no idea of the exact limit, the whole line starts to fall down. As a professor told recently to Gordy, it is easier for the line to glide on air than to fall in air (effect of Reynolds number for specialists). So it flights first, and falls after.

Merlin
Thank you, Merlin. And apologies for the long and complex questions that follow. If you feel I am way off base and these questions don't contribute to the problem at hand then by all means please ignore them and only address those that might help advance the discussion. Thanks.

The concept of glide certainly is an interesting idea, but if it was the major issue wouldn't a shooting head released before the formation of a loop travel as far as or further than a line with a loop tethered on one end? Maybe it does but I'm skeptical. I think it would act more like Graeme's projectile, maybe with a little slower rate of fall. Imagine a fly cast in a vacuum. Without glide would it then fall at the rate of gravity or would it travel like it does in air? My bet is on the latter.

I keep coming back to my belief that both tension AND the loop are necessary to keep a fly leg aloft.
So what is going on in the loop to help keep the fly leg up? What is the contribution, if any, of "gyroscopic momentum".

In a nutshell, and at the expense of proving my stupidity..............I'm working on the assumption that tension and momentum are different manifestations of the same thing (i.e., energy) and, as such, they can interchange (or transfer that energy) through a medium via a wave?

Isn't this what happens in a fly loop? The fly leg has mass and velocity so it has momentum, the rod leg has mass and much less velocity so it has less momentum. Where did that energy go? Well I presume it is seen as tension (is this a swap between kinetic and potential energy?). So, if the energy in the fly leg is seen as momentum and the energy in the rod leg seen as tension, then (neglecting the small effect from skin drag) it seems to me that tension would be building only in the loop and "stored" in the rod leg (presumably this is why the line rebounds back at the end of a stopped cast). Tension in the fly leg (behind the boundary zone of the loop) would be mostly absent and the energy would be evident as momentum instead.

In your circle diagram, the red and blue dots on the top are moving forward, while on the bottom they are stopped (presuming line is not being shot). Doesn't this represent a "braking" force transmitted from the rod tip and seen as tension in the rod leg? In a simple flip-over situation the loop would just continue to flip (like a thrown wrench). But tension in the lower leg obviously prevents this. So, isn't a "braking" force (tension) necessary for the circles in your diagram to move forward?

I also note, in your diagram, that the dots move from top to bottom (or side-side, or bottom to top, depending on loop orientation, it shouldn't matter). As they do they travel in an arc that looks to me like the rod tip path just after release as a loop is formed. At one point Tobias(?) posted a thread about his ideas regarding the transfer of angular momentum from arm to rod to loop in a fly cast. Is that what we are seeing here? And is this what you mean by "Gyroscopic momentum"?
How big is this momentum? Is it tiny and just enough to keep the loop in plane, or does it contain energy sufficient to keep the loop/wave propagating throughout the cast? Is energy from "linear" momentum being constantly fed into the loop to maintain angular momentum? Why does a line with no taper or leader "kick over" so hard at the end of the cast? Where is all that energy during the cast and is it doing something to hold the fly leg up?

Finally, in my mind when I look at you diagram I envision the red and blue dots as opposite ends of the same molecule acting like a rod. As the blue dot enters the circle it is in line with the red dot, while in the circle the relationship becomes an arc, and at the end it is in line again. To form an arc the molecule has to bend and if it bends it is under internal tension/compression which must hold energy until it straightens again and releases that energy (or transfers it to the next molecule in line). Are these tiny arcs transferring momentum into tension back into momentum and so on down the line related to the angular momentum imparted (and maintained) during the cast at the moment the loop forms or are they the result of "linear momentum" colliding with and flipping over the slower object in front?

Enough for now.
George

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gordonjudd
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Upward Force from Form Drag

#296

Post by gordonjudd » Wed Mar 28, 2018 3:49 pm

Is this diagram a fair representation of your description posted in #285?
Graeme,
I am glad that you are willing to look at using a moving frame to analyze the expected tension in the different line sections that Merlin asked George to fill in back in post 263.

Here is the way I look at the loop propagation in a moving frame. This is like a snapshot in time so the x=0 point in the frame at the V_loop velocity would be changed as the loop propagated. Thus as the loop propagates it will change the length of line in the fly and rod legs as well as the line tension.
https://imageshack.com/a/img922/2135/Tpzp9e.jpg
If Vince does not like the use of moving frames, then he can try to compute the tension at the top and bottom of the loop in the earth frame where a point on the line is following a cycloid path as Merlin showed in post 283. Good luck.

Skin and form drag will also impact the nominal tension values, but I think this diagram is complicated enough to get an understanding of what tension values you could expect for tethered loop propagation.

I will be most interested to see your flying line results especially when you have much more line in the fly leg than you do in the rod leg. Under those conditions i would expect the larger drag force on the fly leg would cause the line to rotated in the opposite direction for a bit, before the loop falls apart.

Gordy

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gordonjudd
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Upward Force from Form Drag

#297

Post by gordonjudd » Wed Mar 28, 2018 4:42 pm

Here are some tracking paths for the different loops that Graeme produced in his video casting into a net.

This is a bit redundant, but I thought it might be informative to show where the tracking point was located for the different loops. Here is the track (white marks) for the initial loop. You can see it had a downward slope of around 5 degrees.
https://imageshack.com/a/img924/3595/VAIG0W.jpg

On the first cast the escaped loop came off the bottom of the net and thus the rod leg was above the fly leg. The red tracks for that inverted loop had a slope of -13 degrees.
https://imageshack.com/a/img923/7338/eLsFH4.jpg

One a later cast the loop escaped from the top of the loop and thus had a normal orientation where the fly leg was above the rod leg. The slope of those green tracks was also around -5 degrees.
https://imageshack.com/a/img923/2898/8XszqO.jpg

This is a sample of two, but it appears to me the inverted loop fell at a faster rate than did the escaped loop with a normal orientation.

Gordy

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Upward Force from Form Drag

#298

Post by guest » Wed Mar 28, 2018 4:45 pm

gordonjudd wrote: If Vince does not like the use of moving frames, then he can try to compute the tension at the top and bottom of the loop in the earth frame where a point on the line is following a cycloid path as Merlin showed in post 283. Good luck.

Skin and form drag will also impact the nominal tension values, but I think this diagram is complicated enough to get an understanding of what tension values you could expect for tethered loop propagation.
Gordy

I love the idea of moving frames, it means I can keep my tomatoes in the sun. However, using a moving frame to calculate a value for tension that has a basic assumption that the loop is a constant speed and direction will come in handy if I take up exoatmospheric fishing. Please can you advise on fly selection?

Vince
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Graeme H
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Upward Force from Form Drag

#299

Post by Graeme H » Wed Mar 28, 2018 11:21 pm

gordonjudd wrote:Here are some tracking paths for the different loops that Graeme produced in his video casting into a net.
Gordy,

The most remarkable thing about those two(?) sample casts is just how well I managed to duplicate the incoming cast! When I overlay the two images, it looks like I produced exactly the same track paths for both casts. The white markers occupied the same position in space at the same stage of the casts. Now that's what I call superb casting consistency! Would you care to comment on what's going on here?
Overlay.jpg
The second thing I would draw your attention to is that the white marks are not actually maintaining a straight line path, but a dropping (parabolic?) path as we might expect for a projectile under the influence of gravity. Of course, the red and green paths are not tracking the same marker that the white path was following: that marker crashed into the net.

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.

Image

Cheers,
Graeme
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gordonjudd
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Upward Force from Form Drag

#300

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

but at the boundaries of the frame.
Graeme,
What do you mean by the boundaries of the frame?

Everything is still in the moving frame; i.e, rod, rod leg, loop, and fly leg. It is just that their apparent velocities change.

Think of what you would see looking out of a car window that is moving at the same speed as the loop front.

The loop would appear to stay in the same location out of your car window.

Instead of the fly leg having a velocity of 2*V_loop the fly would appear to be coming towards the car at a velocity of V_loop.

Instead of the rod leg having zero velocity, it and the caster would appear to be moving away from the car at a velocity of -V_loop as shown in my annotations on Merlin's drawing.
Would you care to comment on what's going on here?
Do you want something more than:
but it appears to me the inverted loop fell at a faster rate than did the escaped loop with a normal orientation.
Gordy

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