Paddle leading and trailing vortices

[josko]

I notice that my Werner Ikelos typically leaves two vortices in the water at the start of the stroke. One comess off the leading edge of the blade, and the other off the trailing edge. My Stellar wing blade only leaves one vortex, off the trailing edge. With some practice and concentration, i can get the Ikelos leading edge vortex to disappear, or at least be much smaller than the trailing edge one, and the stroke 'feels good' when it does that. However, the trailing edge vortex grows as I put in more power with both the Ikelos and Stellar.

I'm assuming these vortices are just wasted energy, and the paddler should try to minimize them. It also might explain why the wing blade is more efficient - no wasted energy in the leading edge vortex.

I'm not sure whether I should be trying to minimize the size of the trailing edge vortex with either blade. Is it something arising out of a faulty stroke on my part, or is it an inherent feature of the blade design? Also, I notice that it grows proportionally to power applied - again, is this because my form degrades with power applied, or an inhwerent feature of blade design?

Thanks in advance.

[JohnHuth]

Is this on a forward stroke or a steering stroke? If so, how do you define "leading edge" and "trailing edge"?

In some cases, vortices create lift around the paddle, particularly for steering - so in that case the vortex is helping you by creating a rotational flow in the opposite direction giving you lift.

In any case, Kelvin's rotation theorem says that for any rotation created in one direction, you get equivalent rotation in the other in a fluid - it may just be that it circulates around the paddle and doesn't manifest itself as a vortex.

[josko]

I'm referring to the forward stroke, when I'm moving the paddle outward from the catch as close to the hull as possible, to the exit which is quite a distance away from the hull. So the paddle is moving sideways through the water, with the outer edge of the blade as the leading edge, and the inner as the trailing edge. During the stroke, the paddle is acting as a lifting airfoil (hydrofoil?), and I'm assuming that one wants to maximize lift forces generated.

I stumbled on this by being curious why my Ikelos generates two vortices, and my Stellar only one, for what seem like similar forward strokes. That led me to reading up on airfoil theory and potential flows, and by now I expect I've thoroughly over-complicated and confused the issue.

Another relaed issue I'm struggling with is the sudden 'caving in' of the wing blade when I try to apply lots of power, for instance when trying to accelerate on a wave. I've come to think I'm seeing blade stall, as I try to generate too much force for the lateral velocity (of the blade0), and the (potential) flow suddenly detaches., I don't quite have the wherewithal to visually observe the flow at this point, but expect the paddle generates a large leading-edge vortex at this point. I don't see this behavior with thye Ikelos since the flow is already detached during my typical stroke.

[JohnHuth]

If the flow detaches, you should be able to see turbulent behavior on the top edge of the wing blade. When it detaches, do you see as a growth of the trailing vortex, or the leading edge, as a detached bubble that grows? Yeah, in either case, it would be a stall. Typically a stall is when laminar flow breaks down on the upper surface and turbulent infiltrates.

I have to go out and play with my stroke to see which way the vortices swirl.

Edited August 26, 2013 by JohnHuth

[JohnHuth]

Josko -

OK, I think that what is going on is something like this - the two vortices form on the flatter blade because you mostly have turbulent flow on the forward side of the blade and the vortices are peeling off both sides of the blade, as I might expect.

With the wing blade, the point of getting lift is that you have a trailing vortex, and the top side of the blade (lift side) is in laminar flow. A consequence of the trailing vortex is that Kelvin's rotation theorem sets up a counter-rotational flow around the entire wing - this combines with the blade motion to produce a faster flow on the forward (upper) edge of the wing. This gives it lift, with less drag than the "flat" blade. So, ideally, you only want that trailing vortex.

The breakdown presumably happens when you have too great an attack angle for the velocity and you get turbulence that infiltrates - the asymmetry in the turbulence makes the blade unstable and it'll flip one way or the other. Depending on whether it's a detached bubble on the leading edge or the turbulence infiltrates from the trailing edge, the blade will twist one way or the other.

For the power strokes, you might consider changing the attack angle as you crank up the power (reduce it).

Edited August 26, 2013 by JohnHuth

[josko]

"For the power strokes, you might consider changing the attack angle as you crank up the power (reduce it). "

Yeah, I hear you. That's what I'm working on this week. It seems with the wing blade one neds to be a whole lot more careful when 'pouring on' power, than with a regular blade. But it can be made to work...

[JohnHuth]

The same thing that makes the wing blade more efficient, makes it more unstable.

[leong]

In any case, Kelvin's rotation theorem says that for any rotation created in one direction, you get equivalent rotation in the other in a fluid - it may just be that it circulates around the paddle and doesn't manifest itself as a vortex.

John,

Just started to read this thread. I agree with you but do you mean Kelvin’s Circulation Theorem (where circulation is the integral of velocity around a closed loop)? Oh boy, my head hurts; it’s been a long time since I studied fluid mechanics.

[Pintail]

Remember that you can never remove those vortices -- only diminish them somewhat. They will always be there, to a lesser or greater degree. As usual, it all boils down to compromise, doesn't it? No paddle is perfect for all conditions or uses (same as hull design) or all speeds.

As others have written, I suspect that it is up to you to modify your stroke to suit...?

I use an Ikelos, too, and generally love the fact that it is such a smooth paddle, which generates little in the way of (trailing-edge) vortex; <but> I can generate vortices easily with a Greenland paddle if I am not careful with entry angle.

Edited August 28, 2013 by Pintail

[JohnHuth]

Yes, Kelvin's circulation theorem. For some reason I keep calling it "rotation theorem", because I sort of view it as the fluid mechanics equivalent of conservation of angular momentum. It's just a weird mental association I have.

[JohnHuth]

Remember that you can never remove those vortices --

Actually for a wing, you should be able to remove the "forward" or leading edge vortex. You're left with a trailing vortex for sure, but the Kelvin *circulation* theorem (ahem) gives a rotation all the around the wing, which is how lift is generated.

[josko]

It's kind of amazing how hard it is to generate a leading-edge vortex on a wing blade, and how hard it is to get rid of in on a 'euro' blade. That's what got me thinking about all this. I'm starting to think that elimination of the leading edge vortex was the design criterion that led to a wing blade in th forst place.

[JohnHuth]

I'm attaching an illustration of what's going on with the Kelvin circulation theorem and wings. You get vortices peeling off the trailing edges, but the circulation theorem says that you have to get some counter-rotating flow. That counter-rotation goes all the way around the wing.

Now, if you add an overall velocity going from left to right, there is a higher velocity on top and a lower velocity on the bottom - this gives lift on the upper side. So, you don't really get a leading edge vortex.

For a flat blade, the sharp edges on either side with give you counter-rotationg vortices on both sides. The rounded leading part of the wing and sharp trailing edge gives rise to the overall flow around the wing.

[JohnHuth]

For the savants, I'm attaching a figure that shows what happens in a stall. The attack angle becomes too high. There's a pressure distribution on the upper surface of the wing. The "favorable" pressure gradient sucks the air over the wing in the direction of motion. The "adverse" gradient 'wants to' suck it back, but the inertia of the flow keeps it going and laminar over the wing.

If the attack angle gets too large, the adverse pressure gradient becomes too large and the vortices on the trailing edge get sucked in - the laminar flow becomes detached, and turbulence penetrates from the rear, which creates a stall. For the case of a paddle, it will twist.

[Pintail]

<Actually for a wing, you should be able to remove the "forward" or leading edge vortex>

I perfectly understand what you're driving at, John: I simply don't consider the "wing paddle" typical of paddles, as most of us understand them. My analogy is aviation-related, which is where my experience lies..