However, when the velocity is low at a Reynolds number of 5 > 40 the vortex may not be shed and form drag may contribute to vertical drag for a short time until the vortex decays when the Reynolds number drops below 5.
Vince,
Spolek says that since the length dimensions of the fly leg are so much larger than the width of the line the Reynolds numbers for a fly line are very high so I do not understand why you would make any reference to low Reynolds numbers when talking about drag effects on a fly line.
He gives a relevant Reynolds number of 10^6<Re<10^9 to compute the skin drag not values of Re<100.
Vertical drag only exists when there is a rotational component of velocity. That is the Magnus effect.
I don't understand how you could think that. Anyone who has put their hand out of a car window knows there is lift from form drag when you angle your hand up up or down.
As noted in that free-body diagram there will be a vertical component of the Dn (normal) form drag anytime the line is inclined as shown below.
That is the trigonometry that Dr. Gatti-Bono use to compute the lift for different loop shapes in her paper, and it had nothing to do with a vertical force from the Magnus affect.
This does not tally with James description of getting DN with low power low rotation casts
If you have waded through to the end of the dolphin nose thread you would know that drag has nothing to do with making DN loops. Thus there is nothing to "tally" in order to recitify those two observations since one (drag) has no effect on the other.
For the fourth time:
What was the source of the "juggernaut" force you mentioned in post 125. To get a normal drag force pointing NW does that mean the line is propagating with an incline of 45 degrees to the East?
Gordy
"Flyfishing: 200 years of tradition unencumbered by progress." Ralph Cutter
I, and others, get the same DN shape whether the loop is propagating forwards, dropping backwards due to gravity or just hanging in mid air with relatively little position change. Have a look as this video from Aitor:
Spolek says that since the length dimensions of the fly leg are so much larger than the width of the line the Reynolds numbers for a fly line are very high so I do not understand why you would make any reference to low Reynolds numbers when talking about drag effects on a fly line.
Because we are talking about the drag induced lift or conventional lift in the loop not the fly leg. In the loop the Reynolds number will be very much lower, tending towards zero at the bottom.
I don't understand how you could think that. Anyone who has put their hand out of a car window knows there is lift from form drag when you angle your hand up up or down.
Unfortunately this is incorrect and is one of the fallacies from poor high school physics I previously mentioned. There are many aerodynamic references for this but NASA squashes these with concise explanation. It is worth paging through some of the fallacies to see the variations on a theme:
If you look at their definitions of lift and drag:
Lift is a force generated by turning a flow. Since a force is a vector quantity (like the velocity), it has both a magnitude and a direction. The direction of the lift force is defined to be perpendicular to the initial flow direction. (The drag is defined to be along the flow direction.)
When you start to rotate the object, as per the Magnus effect, you rotate the drag vector 90 degrees to produce your drag induced lift.
That is the trigonometry that Dr. Gatti-Bono use to compute the lift for different loop shapes in her paper, and it had nothing to do with a vertical force from the Magnus affect.
It appears that you have misunderstood the Gatti-Bono and Perkins papers because you have not correctly defined your force axes. You have applied the rotated axes of the loop to a flat plate. I would recommend that once you have understood the NASA explanation for flat plate lift, you look again at the rotating cylinder explanation to understand how the axes are changed as it should lead you to the circulation theory of lift:
I, and others, get the same DN shape whether the loop is propagating forwards, dropping backwards due to gravity or just hanging in mid air with relatively little position change. Have a look as this video from Aitor:
Do you think the Magnus effect could explain these empirical observations?
Has anyone got a big vacuum chamber we can cast in? Anyone here work for NASA?
Thank you James
If I can get a vacuum chamber, how long can you hold your breath?
As mentioned in my discussions with Gordy, I think the Magnus effect is at play in high speed, high rotation casts. This explanation accords with the Perkins and Gatti-Bono papers. However, at low speed, low rotations I think that the lower quadrant of the loop may generate conventional lift.
It is worth paging through some of the fallacies to see the variations on a theme:
Interesting. Do you know if they still teach it incorrectly in school?
Cheers, Paul
Paul
I do not know what is being taught now, but there is an elegant quote that may explain why it has happened:
Many years ago, a famous aerodynamicist, Dr. Theodore Von Karman, instructed his assistant: "When you are talking to technically illiterate people you must resort to the plausible falsehood instead of the difficult truth."
It is deeply entrenched and you would be surprised at the number of pilots that do not properly understand it. The FAA handbook contains a chapter on Magnus effect but it is a difficult concept to get your heard around. This is worth a look:
As a theory, its been about for nearly a century but the maths are difficult. I think that what has happened is that physics teachers have taken Bernoulli and stretched it a bit to explain an aerofoil. For basic principles of explaining static and dynamic pressure effects it works fine. However, it falls apart when you start using it to do analysis, that's when you find that an aircraft cannot fly upside down and a Boeing 737 cannot fly at all.
As an aside, I used to work with a scientist with a PhD in aerodynamics who would not fly. He never told me why.
Just an observation but I cast farther when I have a slight tail-wind.
The last few pages seem to say that aerodynamic lift plays a part in my line staying off the ground. To me that seems a little counter-intuitive because that seems to suggest we should be casting into a headwind and enjoying all that free lift?
Magnus,
I understand we have different effects being in balance during the cast. Depending on the conditions one may more and more outway another one.
If we cast against a wall the lift will probably not keep the line from being stopped at all.
Greets
Bernd
Just an observation but I cast farther when I have a slight tail-wind.
The last few pages seem to say that aerodynamic lift plays a part in my line staying off the ground. To me that seems a little counter-intuitive because that seems to suggest we should be casting into a headwind and enjoying all that free lift?
Magnus
Having lift does not guarantee a long distance cast, it may just give you a long duration cast that lands at your feet.
There are many aerodynamic references for this but NASA squashes these with concise explanation. It is worth paging through some of the fallacies to see the variations on a theme:
Vince,
The NASA site says:
Lift occurs when a moving flow of gas is turned by a solid object. The flow is turned in one direction, and the lift is generated in the opposite direction, according to Newton's Third Law of action and reaction. I.e. the lift comes from changing the momentum of the air flowing around the body.
Here is a link that describes how lift is produced by the drag on a buff body.
As shown in that video the lift that you get from the drag that you get by putting your hand out of a car window comes from changing the direction (one of the components of momentum) of the airflow as show in a frame grab from that video.
Dr. Gatti-Bono has computed the tangential and normal forces that you get from that momentum change as:
So are you saying that the drag equations that Dr. Gatti-Bono (and everyone else) has used to compute the drag forces are wrong because the real lift can only come from the Magnus effect?
As noted many times before the Magnus effect only comes into play when a buff body is spinning, and a fly line is not spinning. Look at how much lift you get in the applet you referenced when the rotation rate is zero.
If you think the lift can only come from the Magnus effect, then how would you use it to compute the lift on an inclined piece of fly line?
Until you can make a convincing case for using your Magus effect theory and come up with some testable values for it, I am going with Dr. Gatti-Bono's approach that makes use of time-tested concepts for computing the drag and lift effects on moving buff bodies.
Gordy
"Flyfishing: 200 years of tradition unencumbered by progress." Ralph Cutter
To me that seems a little counter-intuitive because that seems to suggest we should be casting into a headwind and enjoying all that free lift?
Magnus,
Lift is not "free". It is also associated with a drag force that increases with the attack angle. Thus as Vince said the lift will allow the line to stay in the air longer, but it does so at the expense of added drag.
As you may remember, some of the factors involved with that complicated tradeoff is discussed in this thread.
Gordy
"Flyfishing: 200 years of tradition unencumbered by progress." Ralph Cutter
Under normal conditions, NASA do not say that lift and drag are the same thing. The kite explanation below is based upon flat plate lifting theory look at the axes:
Whichever way you cook it, your flat plate drag axis is in the wrong place.
Dr. Gatti-Bono has computed the tangential and normal forces that you get from that momentum change as:
She has rotated the axes to account for the rotation of the loop against the flowstream.
If you think the lift can only come from the Magnus effect, then how would you use it to compute the lift on an inclined piece of fly line?
I did not say that.
Until you can make a convincing case for using your Magus effect theory and come up with some testable values for it, I am going with Dr. Gatti-Bono's approach that makes use of time-tested concepts for computing the drag and lift effects on moving buff bodies.
I used to think that shape was related to drag in some way until Alejandro did his bead chain experiment. That is why I now think it has more to do with forces that come from rotation effects.
Your last 2 quotes appear to be mutually exclusive, do you believe in drag or rotation?
The Gatti-Bono and Perkins analysis are using time tested theories derived from the derivations and practical experiments of Robins, Magnus, Euler, Navier and Stokes. They make perfect sense to me.
Anyone here work for NASA?
