Firstly, when I cast I only see dolphin-nosed loops when I'm using near minimum power i.e. they are more pronounced when the loop speed is slow (and also only when the loop is tight) - why would this be?
I agree.
Btw can you form a dolphin nose on purpose and then form almost the same loop without one on purpose, too?
Firstly, when I cast I only see dolphin-nosed loops when I'm using near minimum power i.e. they are more pronounced when the loop speed is slow (and also only when the loop is tight) - why would this be?
Also, when I cast the loop on the side I don't see it wanting to wander off to the right (when casting on the right hand side). If there was lift generated in the way you suggest then surely the cast would deviate? (A bit like when the chap in the film throws the ball with a side action).
James
I used "think" and "believe" because outside of getting you in a wind tunnel I could not do the maths on this one. No disrespect intended to the better mathematicians on the board but you cannot do Computational Fluid Dynamics on spreadsheet.
This is an interesting use of the effect in a 12 mph wind:
The Magnus effect is caused by drag, so there only needs to be sufficient rotational speeds to move the low pressure area around before it equalises. So a low power cast does not preclude this happening. Have a play with the Applet here:
James9118 wrote:To get rid of the DNL I'd be thinking I'd have to cast faster, is there another way?
I have tried to learn to cast it on purpose. But am not really there yet. I think it is not only speed but also the relationship between the relative speed in relation to mass distribution on the line.
I have seen slow, pretty tight loops without a DN.
B
The trouble, as I see it, with the Magnus effect as presented is that the 'top-spinning' cylinder or ball dives downwards as shown in the video (as any ping-pong or tennis player will confirm). So why would a fly-loop (which, to me, is equivalent to top-spin) exhibit lift according to this mechanism?
Pictorially, you would have a small lifting force pointing roughly North West and a comparative juggernaut coming from the West.
Vince,
I would think that increasing the velocity would increase the tension in the line and thus it would be more resistant to changing its shape due to drag forces.
However Alejandro showed you can get the DN shape by pulling a length of bead chain around a small diameter door knob, so that loop shape may have more to do with rotation forces than it does with drag forces.
Why is your lifting force pointing NW rather than N? As derived by Dr. Gatti-Bono lift is generally assumed to be a force pointing in the positive y direction.
What is producing the "juggernaut" force in your scenario?
Gordy
"Flyfishing: 200 years of tradition unencumbered by progress." Ralph Cutter
The trouble, as I see it, with the Magnus effect as presented is that the 'top-spinning' cylinder or ball dives downwards as shown in the video (as any ping-pong or tennis player will confirm). So why would a fly-loop (which, to me, is equivalent to top-spin) exhibit lift according to this mechanism?
James
You are doing the same as Bernd and seeing the loop as being equivalent to the cylinder. This is not necessarily the case, which is why I included the links to the skybow and rotor kite. As you can see from the Wiki quote below, the object does not have to be round; or in the case of the aerofoil, it is the air circulating and not the object.
The Magnus effect is the phenomenon whereby a spinning object flying in a fluid creates a whirlpool of fluid around itself, and experiences a force perpendicular to the line of motion. The overall behaviour is similar to that around an airfoil with a circulation which is generated by the mechanical rotation, rather than by aerofoil action.
However, aerodynamicists are split over lift generation between Bernoulli and Magnus. This simple explanation gives 2 views but introduces the bound vortex, which is where I think we are:
A cylinder (as in the line itself not the loop) will generate trailing edge vortices. As the loop rotates, so will the vortices with the low pressure area moving around the fly loop until the vortices detach or decay.
Vortex generated lift is a movable feast and has been an academic bunfight for a long time. if you want to get a flavour have a look at d'Alemberts paradox.
gordonjudd wrote:Vince,
I would think that increasing the velocity would increase the tension in the line and thus it would be more resistant to changing its shape due to drag forces.
However Alejandro showed you can get the DN shape by pulling a length of bead chain around a small diameter door knob, so that loop shape may have more to do with rotation forces than it does with drag forces.
Why is your lifting force pointing NW rather than N? As derived by Dr. Gatti-Bono lift is generally assumed to be a force pointing in the positive y direction.
What is producing the "juggernaut" force in your scenario?
Gordy
Gordy
I have not seen the Alejandro bead chain experiment but it sounds contrary to the experience James has of generating the DN shape, in that Alejandro used high momentum, James does not. Also it would appear easy to generate DN loops provided you can put enough momentum into the cast.
Looking at the Gatti-Bono paper she starts with 4 loop shapes then derives the lift component for each before concluding that although the DN shape generates the most lift:
How to generate a climbing loop through control of the casting stroke is left as a considerable exercise to the reader.
So she has not tackled the loop generation.
However, I do not see any contradiction between the Gatti Bono paper and the discussion I was having with James. We are looking at 2 different questions, I was discussing James DN loops with low force, low loop rotation casts. Her paper appears to look at high force, high rotation casts. I would agree that for the G-B examples your contention is correct and borne out by practical experience that:
increasing the velocity would increase the tension in the line and thus it would be more resistant to changing its shape due to drag forces.
This may not necessarily be true for James cast and was why I wanted to see some video.
Where I see common cause with G-B is with the drag induced lift. The form drag she calculates is primarily due to vortices induced in the trailing edge of the line:
If a static loop was held in a wind tunnel, there would be a crescent moon shaped area of low pressure behind the loop, with the lowest pressure centred on the area of the loop presenting the largest surface area. Once the loop is rotating that low pressure area will be pulled around the loop. If the speed of rotation is low, then the vortices are likely to be shed early, potentially causing the DN shape. At higher rotational speeds the vortices shed later perhaps on the belly of the line as described by G-B.
As I mentioned to James, it is a complex subject that has divided the aerodynamisists but have a look at the vortex theorems and bathtub experiment below:
A useful exercise is to consider all of the sources of form drag:
Fast cast (relative velocity)
average cast into a headwind (relative velocity again)
large surface area - fat line
rough line (also allows the vortices to stay bound longer)
Looking at the Gatti-Bono paper she starts with 4 loop shapes then derives the lift component for each before concluding that although the DN shape generates the most lift:
Vince,
I don't think Dr. Gatti-Bono looked at a DN loop shape in her paper. A DN has a hump and a dip in the fly leg just behind the front of the loop as shown below. That DN shape was produced by pulling a line around a coffee mug, and you can see it was produced almost immediately, so I think it was the result of rotation forces rather than drag forces as I used to.
The fly leg was straight in her climbing loop shape, so it would not be considered a DN shape.
I had to go to a very small diameter to get a DN shape in a length of bead chain as shown here so the linear mass density of the line has a big effect on whether or not you get the DN shape.
When you talk about vortices are you talking about the Magnus affect airflow you see in spinning objects like a golf ball? The loop in a fly line is rotating but it is not spinning so I do not see that it will have any lift coming from the Magnus affect.
What was the source of the "juggernuat" force you mentioned in your post.
Gordy
"Flyfishing: 200 years of tradition unencumbered by progress." Ralph Cutter
I like the chain bead experiment and would like to understand how you put it together but how does rotation explain James cast?
Firstly, when I cast I only see dolphin-nosed loops when I'm using near minimum power i.e. they are more pronounced when the loop speed is slow (and also only when the loop is tight) - why would this be?
I can see now that the DN shape and the Gatti-Bono highest lift shape is different but both have an angle of attack that should generate lift.
I had to go to a very small diameter to get a DN shape in a length of bead chain as shown here so the linear mass density of the line has a big effect on whether or not you get the DN shape.
Have you been able to repeat this for a fly line? I am not sure that I am following the logic of this statement, are you saying that the mass had to be low?
When you talk about vortices are you talking about the Magnus affect airflow you see in spinning objects like a golf ball? The loop in a fly line is rotating but it is not spinning so I do not see that it will have any lift coming from the Magnus affect.
That is a simplistic view of Magnus effect which is why I expanded the discussion in my last post to James:
Another way a swimmer produces lift is explained by the Magnus effect. The Magnus effect is how a fluid flows around a rotating cylinder, the forearm for example. The fluid flowing past the forearm appears as if it is rotating around the arm, this is called a bound vortex. Moving the arm with the vortex through moving water decreases the pressure above the arm and increases the pressure below the arm, Bernoulli's principle. In this way the arm produces lift in a similar way as the hand. However, if the swimmer loses the vortex for some reason, no lift will take place. This also explains the curved path of a golf ball.
The bound vortex on an aerofoil is discussed here:
That is a simplistic view of Magnus effect
and
or the mathematical treatment here:
Vince,
Simple or not, doesn't the fact that they say:
In addition, these popular pseudo-explanations often don't mention circulation at all, which is crucial to the production of lift by an airfoil, just as it is for a spinning cylinder.
Imply that you need to spin a smooth cylinder in order to get any lift from the Magnus effect?
Dr. Perkins (and everyone else) has looked at the form drag and skin drag effects of fly lines using the standard drag shown here. so I do not think it is necessary to add the complexities of the Magnus effect to explain why form drag can provide lift on a angled section of fly line.
Have you been able to repeat this for a fly line?
Yes. An example of The DN shape I got for a floating line is shown here.
I did not get much of a dip behind the loop in a sinking line that had a higher linear mass density as shown here.
There was no dip to be seen in the experiment with a piece of bead chain for the small diameter you get from a coffee cup as shown here.
I am not sure that I am following the logic of this statement, are you saying that the mass had to be low?
No, I am saying the tendency to form a DN shape behind the loop depends on the linear mass density of the line (rho_l not mass) and the diameter of the loop. Since the line tension at the top of the loop is around rho_l*v_fly.^2/4 I would expect the loop velocity is another important variable in determining the rotational forces.
For the third time:
What was the source of the "juggernaut" force you mentioned in post 127.
Gordy
"Flyfishing: 200 years of tradition unencumbered by progress." Ralph Cutter
Imply that you need to spin a smooth cylinder in order to get any lift from the Magnus effect?
You need to read the links I provided. The forces were calculated from a rotating cylinder and is used to simplify the explanation. It is the relative circulation flow around an object that causes the lift by Magnus effect. You can spin the cylinder or rotate the airflow around the cylinder, the effect is the same. Circulation is what causes lift in an aerofoil, there are a lot of high school physics teachers peddling the wrong explanation.
Dr. Perkins (and everyone else) has looked at the form drag and skin drag effects of fly lines using the standard drag shown here. so I do not think it is necessary to add the complexities of the Magnus effect to explain why form drag can provide lift on a angled section of fly line.
You do need to understand how drag is turned into lift. Gatti Bono stated:
The source of this additional lift is the contribution of form drag on the ‘‘belly’’ of the fly line that has a positive angle of attack.
but did not discuss how the drag force was turned at 90 degrees to its origin. Magnus effect gives you that explanation.
That is a great video but how is your floating line generating lift from drag? It also appears that the lift is acting on the loop as well as the belly of the line.
I do not want to get drawn into a 2nd thread about Rho_L, lets keep that for the other thread.
No, I am saying the tendency to form a DN shape behind the loop depends on the linear mass density of the line (rho_l not mass) and the diameter of the loop. Since the line tension at the top of the loop is around rho_l*v_fly.^2/4 I would expect the loop velocity is another important variable in determining the rotational forces.
What dependencies are you seeing? Low Rho_L values and small diameter loops? high or low velocities in the loop? Why do you not consider line diameter as that has a direct bearing on drag?
For what its worth, I think that the loop velocity is one of the factors that determines where the lift is seen and loop size may be the cause of circulation. I'd like to see your floating line experiment in a wind tunnel.
You can spin the cylinder or rotate the airflow around the cylinder, the effect is the same.
Vince,
So what would rotate the airflow when our airflow comes from the relative velocity of the horizontally directed line in dead air? I assume that a net rotation of the airflow could come from a side wind, but there was no side wind in Dr. Gatti-Bono's calculations that showed how lift is produced in a an inclined piece of fly line due to drag.
and loop size may be the cause of circulation.
What do you mean by circulation?
but did not discuss how the drag force was turned at 90 degrees to its origin.
I do not know what you mean by turning the drag force 90 degrees to its origin, but on an inclined piece of line the form drag will have components in both the horizontal and vertical direction as described in the drag induced lift thread.
Why do you not consider line diameter as that has a direct bearing on drag?
Drag is what causes lift, but I was referring to factors that impact whether or not you see the DN shape in a propagating loop.
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.
Gordy
"Flyfishing: 200 years of tradition unencumbered by progress." Ralph Cutter
So what would rotate the airflow when our airflow comes from the relative velocity of the horizontally directed line in dead air?
If you mean that the loop is going forward through still air, it would still produce vortices because of the relative airflow velocity.
I do not know what you mean by turning the drag force 90 degrees to its origin, but on an inclined piece of line the form drag will have components in both the horizontal and vertical direction as described in the drag induced lift thread.
Vertical drag only exists when there is a rotational component of velocity. That is the Magnus effect.
I have been through your link to your Drag Induced lift topic. Within Perkins I noted his observation:
The results show that at higher speeds, larger values of skin friction can “cancel” (or even overpower) gravity for a short period of time. This might help explain why tight loops, which are also necessarily launched at higher speed, might actually stay “aloft” longer even when launched purely in the horizontal direction.
This does not tally with James description of getting DN with low power low rotation casts. As I showed earlier, the form drag is caused by vortices. At high velocities, the vortices will be shed downstream and Perkins statement holds true:
The first term in the integrand captures the contribution of skin friction while the second term captures the contribution of form drag. The integral of the second term vanishes and therefore form drag makes no contribution to the net drag force in the vertical direction.
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. This is what I think is happening in James cast. Magnus effect shows this happening as the lift force angle changes with the speed of rotation.
This video is quite an entertaining way of describing what is happening:
