Sculling

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caustic
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Re: Sculling

Postby caustic » Thu Aug 26, 2010 1:47 pm

abr wrote:
caustic wrote:http://riversportokc.org/row/about/rowing-terms

conveniently, this shows a great photo of an oar patently showing a lack of directional flow across it's convex surface. No linear flow = no lift. You'll notice that you can see that not only is water moving around each side, but also over the top! And although we can't see it, it is also flowing up from the bottom. Flow that is conducive towards a lifting force cannot all move together from all the edges like that.


Dude:

I'm not going to try to get at all the different things you've said that don't make any sense, but how can you possibly think it helps your case to get people to look at a photo of an oar that appears to be exactly perpendicular to the direction of travel, at the precise place where the proponents of lift agree that there is no lift?!

Try to find the same kind of shot, but near the catch or finish.


Tell you what, you list which things don't make sense, and I'll clarify.

Not a problem. I can get the oar to make a similar pattern at any angle, because I've already done it. You can to, actually. Square your blade in the water, at any angle you like, and just do a short pull through - a few inches is all you really need to move some water. You'll clearly see that two vortices rotating in opposite directions, form on the convex side of the blade. You cannot have any kind of net water flow across the blade that can generate lift when you have vortices forming on both the leading and[i]trailing edge of the blade.

That photo shows three things that have to have happened well before that angle - 1)the outer vortex, 2)the inner vortex, and 3)the large depression. None of those form instantly the blade hits the "stall" phase of the stroke. Thats just where they are [i]maximal
. They exist for almost the entire duration of the stroke. You can form them at any angle, at any phase, and with almost any pressure on the blade - their magnitude will be different, but they will always show up.

caustic
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Re: Sculling

Postby caustic » Thu Aug 26, 2010 2:08 pm

John Ewans wrote:I have always been intrigued by this.

While wing paddles appear to be well established in the kayak world
they apparently need a significant change of technique (more
exaggerated vertical movements) in order to generate sufficient flow
across their, quite radical, aerofoil sections.
I would say that lift would have a greater effect on a kyak paddle because their method of paddling is vastly different from that of a rowing stroke - there is little to no arc the blade transcribes - it's a very linear pull throuh, with a flare out to correct steering- the 'J' stroke. That flare out is very quick, and the blade will easily travel 1-2x the blade distance laterally. It could be enough to generate an appreciable amount of lift. However, even with a kyak paddle the vastly greater force is simply the fact that the blade is pushing water.

My questioning of whether there is a measurable similar effect in
rowing centre around the low speed the water must flow at over the
length of the blade and the fact that modern rowing blades are really
curved plates, rather than aerofoil sections. While I know that true
aerofoil sections are not necessary to generate lift if they are set
at an angle of attack to the fluid, this doesn't seem to the
mechanical system we have in rowing.


There have been many studies proving that a non-zero amount of lift can exist. However, I have only seen one numerical study that reasonably modeled the dynamic nature of the rowing stroke - i.e. they didn't just take the blade angle at discrete angles and wait for a linear flow to be established before measuring the lifting force. That computational model pretty much put to bed the notion that lift was a determining factor in a good catch, much less in keeping the blade anchored - It would peak briefly 1/2 way to the stall point, but due to the turbulence generated during stall and afterwards, was esentially zero.

http://cat.inist.fr/?aModele=afficheN&cpsidt=22038909

Links to an abstract of that study. To quote the final sentence, because that really shows the conclusion of the study: "Drag and lift coefficients calculated for the blade during a stroke show that the transient hydrodynamic behavior of the blade in motion differs substantially from the stationary case. "

To re-phrase, any kind of blade lift study done using static angle of attack to measure lifting force on the blade are not directly equivalent to the oar blade in it's dynamic motion, and thusly cannot be considered an accurate view of what's actually happening. And how to the oar makers market their product? Through doing static testing on their oar blades to wrongfully conclude that lift must be a big factor in rowing. This begs the question of oar manufacturers - If you can't accurately model what's going on in a real stroke, how can you with any credibility "design" a blade to make it more efficient?

In a real-world, dynamic stroke, the linear water flow required to generate an appreciable amount of lift on the convex side of the blade simply doesn't exist. There is so much pressure put on the water from pulling that it spills around the sides, top, and bottom of the blade, interrupting the linear flow and inhibiting lift generation.
My doubts were really crystallised though when my crew mate Charlie
Hamlin held a blade in his hand and floated the head perpendicular to
a landing stage. He then vigorously pulled and pushed it away from and
towards himself with no sign of any sideways movement. If lift is not
discernable in that situation why would there be any measurable lift
generated during the rowing stroke?

I (genuinely) await enlightenment!


John Ewans


This is really what's so aggravating - anyone here can easily observe, through very simple testing, very obvious patterns in their puddles that, if lift was being generated in any significant portion could not exist. Airplane wings do not generate the lift the do and have vortices on the leading edge of the wing, yet we are to just assume that oar blades will. It's completely nonsensical.

Splashy
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Re: Sculling

Postby Splashy » Thu Aug 26, 2010 7:54 pm

Let's consider the resolved components of motion parallel to the direction of motion of the shell (and count any component in the direction of the shell's travel as positive).

We know that for a significant part of the stroke, following the start of blade entry, the blade has a positive component of velocity in this direction. This has been shown so many times that you can't deny it.

For that part of the the stroke, the compenent of drag force on the blade must therefore be opposing the motion of the boat, meaning that we can do no useful work with drag for that part of the stroke.

So why do want to row long at the catch if we can't do any useful work. Or maybe we get a contribution from lift?

gdoyle
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Re: Sculling

Postby gdoyle » Thu Aug 26, 2010 9:23 pm

RSR is having a field day with this....somebody flip caustic over....he's done on one side.

http://groups.google.com/group/rec.spor ... 3dd53920f#

iwantmorepies
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Re: Sculling

Postby iwantmorepies » Fri Aug 27, 2010 8:26 am

AHHHHHHHH HA HA HAHAAHAHAHAHAHA!!!!!

RiggerJigger
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Re: Sculling

Postby RiggerJigger » Fri Aug 27, 2010 10:17 am

I just wanted to thank Caustic and Steven mm for all of their input. This is exactly the discussion I was hoping to arise when I originally posted the topic! Extremely informative.

caustic
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Re: Sculling

Postby caustic » Mon Aug 30, 2010 4:07 pm

gdoyle wrote:RSR is having a field day with this....somebody flip caustic over....he's done on one side.

http://groups.google.com/group/rec.spor ... 3dd53920f#


I love reading their straw men :). I like how Carl Douglas immediately chooses to not read anything I say, and invent things.

1)I never said that a brick doesn't create lift.
2)I never said that it would NOT travel further in a vacuum. I said that it would not travel appreciably further.
3)I never said that I don't think that some lift exists - anything traveling through a liquid medium, regardless of velocity or travel, will always create lift. My argument is that whatever lift does exist, is negligible and not appreciably contribution to the rowing stroke.

I'd be glad to quote the particular passages where I said these things.


Further, while they like to bag on me for my lack of understanding of hydrodynamics, I have actually seen a lot of research on this, from multiple sources, including both what has come out of the oar manufacturers (which is always short on how to duplicate their "results"), and the actual funded research on oar hydrodynamics which (like the link I posted previously) shows that oar LIFT is NOT a contributing factor to moving the boat. It's oar drag, which is very greatly measurable.

Usually, when folks who AREN'T out to make money selling oars start showing data that's in direct contrast to what oar makers are peddling, that at least should raise a red flag.

Splashy
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Re: Sculling

Postby Splashy » Mon Aug 30, 2010 6:10 pm

caustic wrote:Further, while they like to bag on me for my lack of understanding of hydrodynamics


Just forget hydrodynamics for a minute. Let's just use simple mechanics.

For the first, not insignificant part of the stroke, the blade is moving through the water towards the finish line. Ok, it's moving outwards in an arc as well, but it is moving towards the finish line.

So, drag can't possibly be doing anything to propel the boat/athlete system, because it is acting in the wrong direction.

So why don't we just row shorter and miss that bit of the stroke?

caustic
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Re: Sculling

Postby caustic » Mon Aug 30, 2010 6:17 pm

Splashy wrote:
caustic wrote:Further, while they like to bag on me for my lack of understanding of hydrodynamics


Just forget hydrodynamics for a minute. Let's just use simple mechanics.

For the first, not insignificant part of the stroke, the blade is moving through the water towards the finish line. Ok, it's moving outwards in an arc as well, but it is moving towards the finish line.

So, drag can't possibly be doing anything to propel the boat/athlete system, because it is acting in the wrong direction.

So why don't we just row shorter and miss that bit of the stroke?



Let's definitely use mechanics in the scenario. We're dealing with a lever. The longer your lever distance, the greater work you do, and thus the greater velocity you end up with. That's why a long stroke is critical - not for angles of attack of lift components, but for the very simple reason that if you pull longer, you have more time and distance with which to accelerate the boat.

Splashy
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Re: Sculling

Postby Splashy » Mon Aug 30, 2010 7:28 pm

caustic wrote:Let's definitely use mechanics in the scenario. We're dealing with a lever. The longer your lever distance, the greater work you do, and thus the greater velocity you end up with. That's why a long stroke is critical - not for angles of attack of lift components, but for the very simple reason that if you pull longer, you have more time and distance with which to accelerate the boat.


If the drag is in the wrong direction to provide acceleration to the boat/athlete system, for the first part of the stroke, having a long stroke doesn't do anything for you.

caustic
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Re: Sculling

Postby caustic » Mon Aug 30, 2010 7:37 pm

Splashy wrote:
caustic wrote:Let's definitely use mechanics in the scenario. We're dealing with a lever. The longer your lever distance, the greater work you do, and thus the greater velocity you end up with. That's why a long stroke is critical - not for angles of attack of lift components, but for the very simple reason that if you pull longer, you have more time and distance with which to accelerate the boat.


If the drag is in the wrong direction to provide acceleration to the boat/athlete system, for the first part of the stroke, having a long stroke doesn't do anything for you.



Drag doesn't exist to accelerate the boat, so much as to resist the slippage of teh blade, and allow the oar to lever against the mass of the water to move the boat.

With a foil, the primary force moving the foil is in-line with the velocity vector. With rowing, this is not the case. The primary accelerating force is actually orthogonal to the velocity vector. This changes the scenario quite a bit - it's like trying to fly a plane by having the engine pointing down while the wing is pointed horizontal - you're not going to get lift to elevate the plane that way.

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Re: Sculling

Postby Splashy » Tue Aug 31, 2010 4:23 am

caustic wrote:Drag doesn't exist to accelerate the boat, so much as to resist the slippage of teh blade, and allow the oar to lever against the mass of the water to move the boat.

With a foil, the primary force moving the foil is in-line with the velocity vector. With rowing, this is not the case. The primary accelerating force is actually orthogonal to the velocity vector. This changes the scenario quite a bit - it's like trying to fly a plane by having the engine pointing down while the wing is pointed horizontal - you're not going to get lift to elevate the plane that way.


Just forget about lift - you don't believe it is useful in rowing anyway.

So that leaves drag as the only resistance we've got to create an acceleration on the boat/athlete system.

For the first part of the stoke when the blade is still moving towards the finish line, the drag is in the wrong direction to give a resistance that you can lever against to accelerate the boat.

So why bother with that part of the stroke?

Unless, you want to say that the blade follows a different path through the water to the one that seems to be universally acknowledged?

Steven M-M
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Re: Sculling

Postby Steven M-M » Wed Sep 01, 2010 8:19 am

Thanks to Alistair on RSR: http://etheses.bham.ac.uk/793/1/Coppell10PhD.pdf

Lots of good stuff here.
Steven M-M

loblaw
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Re: Sculling

Postby loblaw » Sun Sep 05, 2010 12:41 pm

Steven M-M wrote:Thanks to Alistair on RSR: http://etheses.bham.ac.uk/793/1/Coppell10PhD.pdf

Lots of good stuff here.


Pretty hard to argue with that. I'm still not sure that the science of lift and fluid dynamics makes much difference in terms of how I row. There aren't many people who don't advocate a long quick catch regardless of what they believe. One thing that may have changed I suppose is the rigging as the sweep oars seem to be getting a bit shorter with a tighter span in an effort to get a bigger arc and take advantage of the physics of how the blade moves through the water at the ends of the stroke. When you think about it, this is essentially the reason why a 2x is faster than a 2-.

bloomp
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Re: Sculling

Postby bloomp » Sun Sep 05, 2010 5:06 pm

loblaw wrote: When you think about it, this is essentially the reason why a 2x is faster than a 2-.


Or maybe it's because sculling oars have ~35% more surface area (looking at Fat2s)... Maybe.


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