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[Jeremy McCreary]

Over in Spoke wheel performance, I suggested that ortwin dress his metal Spartan finger top in a skin-tight shiny black latex fairing -- strictly to reduce drag, of course. Also suggested he call it "Mistress von Karman".

Well, too slow, buddy. This isn't what I had in mind then, but Mistress von Karman is way too good a top name to go to waste...







(You know you're too far gone as a topmaker when you start giving your tops names.)

[ta0]

With those spikes she does look like a femme fatale   

[Jeremy McCreary]

Expensive, too. Before they reappeared in some recent sets, those 48 mm red dish fairings were going for to up to $6 each on BrickLink.

Oh wait, Mistress von Karman is summoning me to the dungeon. Right away, Mistress! (Gotta go.)

[ortwin]

Yesterday YouTube informed me, that the video of my "Curtain-Ring-Top Nr. 4" is no longer age restricted. It was their mistake.
But with a video of the Mistress you would not get away that easy! That is why I am only posting this picture of Mr. Spartan in his highly transparent leisure suit.
And no, I won't answer further questions on the brand of the suit!

[Jeremy McCreary]

Great idea! This is what happens when you put unsupervised tops together in bins in the dark without proper protection...



Mr. Spartan's suit looks like a promising fairing. How did it work?

[ortwin]

Mr. Spartan's suit looks like a promising fairing. How did it work?

It could be applied easier then the other balloons I tried, but the results did still not show a clear difference in comparison to a naked Spartan.

[ta0]

That is why I am only posting this picture of Mr. Spartan in his highly transparent leisure suit.
And no, I won't answer further questions on the brand of the suit!

Well, Spartan and Trojan kind of go well together . . .

[Jeremy McCreary]

Beginning to get the feeling that removing obstacles to von Karman-like swirling flow isn't the answer. You have to shut down the centrifugal pump the flow entails.

How to do that without an external shroud?

[ortwin]

How about counteracting it with a flow directed in the opposite direction? A blow-top turned the wrong way? I don't have the feeling this would work, still I would check for the difference in spinning a blow top. Or build a Lego top where the flat wide spokes can be twisted slightly to give some fan action. Again the influence of the direction of the twirl could be analyzed.

[Jeremy McCreary]

How about counteracting it with a flow directed in the opposite direction? A blow-top turned the wrong way? I don't have the feeling this would work, still I would check for the difference in spinning a blow top. Or build a Lego top where the flat wide spokes can be twisted slightly to give some fan action. Again the influence of the direction of the twirl could be analyzed.

The spokes in this large, rare 68 mm Znap wheel are pitched so as to blow downward when spun clockwise. When I finally got hold of one years ago, thought I might use the fan action to reduce tip resistance by deweighting the contact. Never thought about disrupting centrifugal pump action.



No go then, but rechecked just now. The starter here turns in both directions at 3,600 RPM with no top loaded. Chucking this top drops max speed to about 3,100 RPM.

Same 130 s spin time in both directions.

Come to think of it, a blowing fan also consumes energy. But this top would probably spin a lot longer without the tread pattern.

[Iacopo]

Beginning to get the feeling that removing obstacles to von Karman-like swirling flow isn't the answer. You have to shut down the centrifugal pump the flow entails.

How to do that without an external shroud?

To make the top without the core like in the curtain ring tops of Ortwin is one way, I would like to make a top in that way one day.
The spokes should be very thin, it is not very easy to make a precision top in this way. 
Maybe the spokes could be directed not towards the center of the top but could be set obliquely, to better accommodate the flow of the air, (photo below). 



Another way could be to look for surfaces slippery for the air, (to reduce skin friction), but I am not sure if this is possible in our case.
I read that it has been found that the surfaces of feathers, (if memory serves), or the skin of the wings of bats, have less air drag than simple smooth surfaces.
There are aerodynamic fabrics, the textures of which reduce the air drag;
they are used by athletes in sports where reducing the air drag is an advantage, like skiing or cycling.
The problem is that, as far as I know, (which is very little), some speed is necessary for these fabrics to work properly, and probably spinning tops spin too slowly.  These things can be tested, in any case.


 

[ortwin]

No go then, but rechecked just now. The starter here turns in both directions at 3,600 RPM with no top loaded. Chucking this top drops max speed to about 3,100 RPM.

Same 130 s spin time in both directions.

.


 If the ground clearance is large, more then 3mm, that is to be expected, everything quite symmetric. As you and Iacopo found out by experiment, the ground effect needs small spacings. If you could redo the experiment with that in mind it would be great.

[ortwin]

To make the top without the core like in the curtain ring tops of Ortwin is one way, I would like to make a top in that way one day.
The spokes should be very thin, it is not very easy to make a precision top in this way. 

No! It is very easy to make spokes 1/10 of a mm or less in diameter that are strong enough for what we need. I try to make my point clear when I present the curtain ring top Nr. 6  .

[
Another way could be to look for surfaces slippery for the air, (to reduce skin friction), but I am not sure if this is possible in our case.
I read that it has been found that the surfaces of feathers, (if memory serves), or the skin of the wings of bats, have less air drag than simple smooth surfaces.
There are aerodynamic fabrics, the textures of which reduce the air drag;
they are used by athletes in sports where reducing the air drag is an advantage, like skiing or cycling.
The problem is that, as far as I know, (which is very little), some speed is necessary for these fabrics to work properly, and probably spinning tops spin too slowly.  These things can be tested, in any case.

The golf ball dimples and shark skin effect come to mind. Those things have been discussed a few years ago in this forum I found out by a search a few days ago. The last word of ta0 on this was, that those things are only for linear motion, not for the circular motion that is in question here. Is that correct ta0??

[ortwin]

Come to think of it, a blowing fan also consumes energy. But this top would probably spin a lot longer without the tread pattern.

Yes, a fan like rotor is surely to extreme. For a top I was envisioning something more  subtle, similar to the ridges on the  surface of this billetspin top.I wonder if with this a direction effect would be visible.



This other cheap top here I just ordered, I will keep you informed on its direction sensitivity once it arrives in about four weeks or so.

[Jeremy McCreary]

It's very dangerous to take aerodynamic strategies developed for bodies translating through the air and apply them to a top spinning in place without translation.

For one thing, a top with a smooth outer surface of revolution presents no leading edge to the surrounding air. Nor is there a trailing edge.

That geometry totally changes the induced air flow and largely eliminates the possibility of a boundary layer separation capable of generating pressure drag -- the very thing that many of said strategies were designed to address. At least not a pressure drag directly resisting spin rate.

Also, there may be no such thing as a drag crisis in a top. The dimples on a golf ball are to there to precipitate the drag crisis so as to match it to typical ball flight speeds. That works in part because the ball presents a fairly blunt leading edge to the oncoming air.