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Author Topic: Flettner rotor  (Read 263 times)

Hardy Heinlin

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Flettner rotor
« on: Tue, 20 Jul 2021 10:18 »
How does the air around a Flettner rotor "know" the rotor's rotation energy? The rotor's surface is smooth. Where's the friction that draws the air around the cylinder at the respective rotation rate?

https://en.wikipedia.org/wiki/Flettner_rotor


I-Iardy



There are more details under https://en.wikipedia.org/wiki/Magnus_effect
And it mentions "skin friction" just to say that friction doesn't matter ...
« Last edit: Tue, 20 Jul 2021 10:36 by Hardy Heinlin »

Timo

  • Join date: Jun 2021
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Re: Flettner rotor
« Reply #1 on: Tue, 20 Jul 2021 19:15 »
I think the rotor surface is not really smooth, respectively the effect depends on the surface roughness. The rougher the surface (to a certain degree) the more the laminar airflow is affected / interrupted asymmetrically and the bigger the Magnus effect is. Maybe it‘s all about „micro-roughness“.

in former times, I experimented a lot with R/C gliders and even a thin layer of hairspray on a smooth wing affected the laminar airflow and e.g. stall characteristics.

BTW, Magnus effect is even an interesting phenomenon in aviation:
https://youtu.be/hlmvHfIAszo

Timo


United744

  • Join date: Oct 2014
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Re: Flettner rotor
« Reply #2 on: Wed, 21 Jul 2021 00:10 »
Rotating bodies experience some interesting physics, not least, relativistic effects. Putting precision timers near rotating masses measurably alters the local time dimension as a consequence of the increase in local gravity caused by the rotating object. https://en.wikipedia.org/wiki/Gravitational_time_dilation

This is also interesting (and why we no longer rotate rockets when launching them): https://en.wikipedia.org/wiki/Explorer_1

As for the Flettner rotor, you need to look at the surface from the perspective of air molecuies. The surface is positively mountainous, and so this causes the air at the surface to be moved sideways, this of course extends to the "macro" scale, causing the larger effect seen.

Hardy Heinlin

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Re: Flettner rotor
« Reply #3 on: Wed, 21 Jul 2021 09:53 »
Hmm ...

United744

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Re: Flettner rotor
« Reply #4 on: Wed, 21 Jul 2021 14:13 »
I am intrigued by something: what is the rotational RPM of these rotors, particularly in the case of the ship?

Mark

  • Join date: Nov 2014
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Re: Flettner rotor
« Reply #5 on: Thu, 22 Jul 2021 09:52 »
Intuitively to me, it seems like a boundary layer effect, much like a tesla turbine, causing the air to move quicker on the side perpendicular to (& rotating away from) the incoming breeze.

Steve Hose

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Re: Flettner rotor
« Reply #6 on: Thu, 22 Jul 2021 16:43 »
Imagine a non-rotating cylinder moving through a fluid (liquid or gas) - the fluid molecules bounce off the cylinder imparting a force on the cylinder pushing it in the opposite direction - this is the regular drag effect.

Imagine the same situation but now with the cylinder rotating. As the fluid molecules bounce off the cylinder, the direction of their deflection is different than before due to the cylinder's rotation, because the rotation adds a new component vector tangential to the direction of rotation. The accumulation of this effect results in a net force that is perpendicular to the direction of the motion through the fluid and the cylinder.

You can see this same effect applying sidespin when kicking a soccer ball - it's horizontal path is curved through the air, i.e. you can 'bend it like Beckham'.
« Last edit: Thu, 22 Jul 2021 23:17 by Steve Hose »

Hardy Heinlin

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Re: Flettner rotor
« Reply #7 on: Thu, 22 Jul 2021 17:13 »
Ah! The different collision vectors along the arc of the rotating cylinder. Yes, that sounds plausible ...

Before we can even talk about boundary layers or friction, the fluid must come to the surface. And it does that by collisions. Collisions at many different angles. Asymmetric. "Different" because the cylinder is rotating ...

jcomm

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Re: Flettner rotor
« Reply #8 on: Thu, 22 Jul 2021 22:49 »

Steve Hose

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Re: Flettner rotor
« Reply #9 on: Thu, 22 Jul 2021 23:17 »
Ah! The different collision vectors along the arc of the rotating cylinder. Yes, that sounds plausible ...

Before we can even talk about boundary layers or friction, the fluid must come to the surface. And it does that by collisions. Collisions at many different angles. Asymmetric. "Different" because the cylinder is rotating ...

Yes, that's right - 'different' is probably clearer than 'changes' so I've amended my post slightly for clarity.

Regards, Steve.

jcomm

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Re: Flettner rotor
« Reply #10 on: Fri, 23 Jul 2021 08:06 »
And a more "formal" way of seeing the - "collisions at many angles" - intuitive / practical interpretation :-)

https://www.hindawi.com/journals/ijrm/2016/3458750/


Steve is right, but he has a flaw in his exposition - it's not Beckam's sidespin.. it's Ronaldo's ...
« Last edit: Fri, 23 Jul 2021 09:40 by jcomm »