# I Magnus effect: Why smooth cylinders?

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1. Nov 28, 2017

### greypilgrim

Hi.

All technical implementations of the Magnus effect I can find on Google (such as ships) seem to use fairly smooth cylinders. Why? Shouldn't the efficiency increase with increasing friction between cylinder and fluid, for example with a rough surface or even attaching blades?

2. Nov 29, 2017

### Staff: Mentor

Here's some info on Baseball physics, click through the pages to see air drag and magnus force.

baseball physics:
http://farside.ph.utexas.edu/teaching/329/lectures/node41.html

air drag:
http://farside.ph.utexas.edu/teaching/329/lectures/node42.html

Magnus force:
http://farside.ph.utexas.edu/teaching/329/lectures/node43.html

various baseball pitches are described as well as their spin orientation with a knuckle ball having no spin.

The baseball stitching introduces drag which decreases with faster speed which then allows the magnus effect to show up.

3. Nov 29, 2017

The idea is that, without incurring much in the way of a drag penalty, you can induce a lift force due to the Magnus effect by spinning the object. Due to viscosity, the fluid is going to stick to the surface (velocity of zero relative to the surface) anyway, so roughness isn't going to buy you much. On the other hand, adding too much roughness counteracts the "small drag" benefit without really changing the Magnus force itself.

4. Nov 29, 2017

### rcgldr

In the case of table tennis balls, a rougher surface reduces both drag and Magnus effect. I don't know if this is something specific about the size, weight, relatively low density (the balls are hollow), ... , of table tennis balls. I assume the reduction in drag due to roughness is similar to using roughness and/or tubulators on gilder wings to reduce drag (reduce profile drag related to separation bubble, at the cost of some friction drag). Don't the dimples in golf balls have the same effect (reduced drag, reduced Magnus effect)?

5. Nov 30, 2017

### A.T.

Any useful measure of efficiency in this case has to account for the energy that you use to spin the cylinders. What happens with that when you attach blades to the cylinders?

6. Nov 30, 2017

### cjl

If this is the case, it is likely that the table tennis balls are small enough and traveling at a low enough velocity that they have a laminar boundary layer (if they have a smooth surface). Usually, this would be a good thing, as laminar boundary layers have less drag caused by fluid shear than turbulent boundary layers do. However, they also are more prone to flow separation, which means that the separation bubble behind the sphere is larger. Turbulent boundary layers delay separation, so for an object with a fairly non-streamlined shape, the reduction in drag due to the smaller separation bubble is usually much larger than the increase in drag caused by the turbulent boundary layer. This is also the reason a dimpled golf ball flies farther.

For something the scale of the cylinders used on ships, I'd expect the boundary layer to be turbulent anyways, so smoother will likely have less drag.