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 ...
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
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.
Hmm ...
I am intrigued by something: what is the rotational RPM of these rotors, particularly in the case of the ship?
Intuitively to me, it seems like a boundary layer (https://en.wikipedia.org/wiki/Boundary_layer) effect, much like a tesla turbine (https://en.wikipedia.org/wiki/Tesla_turbine), causing the air to move quicker on the side perpendicular to (& rotating away from) the incoming breeze.
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'.
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 ...
https://youtu.be/XsGin7CFaF8
Quote from: Hardy Heinlin 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 ...
Yes, that's right - 'different' is probably clearer than 'changes' so I've amended my post slightly for clarity.
Regards, Steve.
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 ...