Author Topic: Dimples  (Read 2856 times)

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oscar11

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Dimples
« on: September 11, 2015, 08:20:11 PM »
If you added dimples , like on a golf ball, to a windmill blade I'm thinking it would increase production by, reducing the weight of the blade, and lowering the drag. It's difficult to find any numbers on how much energy goes to drag.

dnix71

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Re: Dimples
« Reply #1 on: September 11, 2015, 09:05:15 PM »
http://www.j3-cub.com/forum/showthread.php?t=20798

People who fly model planes have tried dimple tape and report it doesn't work. Note the comment about it being dependent on the Reynolds Number https://en.wikipedia.org/wiki/Reynolds_number

Model props are usually thin, homemade mill props are not. Even small differences in shape affect the Reynolds Number.

http://www.aircraftspruce.com/menus/ap/aerodynamics_vortex.html  Vortilator tape is sold here.

Most of the action is at the tip. Birds flight feathers have a big effect on their ability to fly. https://en.wikipedia.org/wiki/Flight_feather

Vortex generators do work https://en.wikipedia.org/wiki/Vortex_generator but those are placed on fixed wings, not props.

oscar11

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Re: Dimples
« Reply #2 on: September 12, 2015, 03:05:45 PM »
Thank you for the information. It's going to take me awhile to wrap my head around the reynolds numbers. Maybe it doesn't work on an airplane because of the added weight. I seem to remember a show where they used clay to make dimples on a car and it increased gas mileage. Maybe the super mileage car or the inside of a hydro pipe.

dnix71

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Re: Dimples
« Reply #3 on: September 12, 2015, 10:36:48 PM »
On a fixed wing at speeds much less than the speed of sound you get lift all the way across the wing. But there are things behind the wing on a fixed wing aircraft (struts, the tail, etc.) Keeping the turbulence away from things behind the wing increases efficiency.

On a rotating wing the thrust goes up with the 4th power of the diameter, but only the square of the rpm. All the action is near the tip and size matters a lot. On a windmill you have a tail most of the time, but keeping turbulence from the tips away from the tail isn't usually an issue.

On a commercial plane prop, the prop tip speed may approach the speed of sound. Feathering something moving that speed would weaken it at the very place with the highest thrust forces. Birds have edge feathers but don't fly near Mach speeds, so it helps them.

https://en.wikipedia.org/wiki/Wingtip_device Wingtip devices on fixed wing aircraft help recover some energy lost to turbulence but your wing is rotating.

Helicopters main rotors have wingtip devices https://en.wikipedia.org/wiki/BERP_rotor But those are designed with other things in mind because helicopters operating near the ground or stationary have different issues maintaining stability than when operating in clear air or in motion.

My guess is for a homemade mill there is nothing to gain by changing where the trailing vortex is because there is nothing back behind the prop chord (3/4 of the length out) for it to impinge on.


oscar11

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Re: Dimples
« Reply #4 on: September 13, 2015, 06:53:16 PM »
The action is at the tip because its torque I think. Maybe you want the dimples under the wing to reduce the resistance of the laminar flow to increase the speed to increase the pressure difference but it doesn't matter anyway. Apparently windmill blades are 95 percent efficent but maybe in the name of pure research, laminate like those swimsuits the Olympics had to ban or ionize the wing? Lots of good info on windpower. org.

oscar11

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Re: Dimples
« Reply #5 on: September 15, 2015, 08:26:41 PM »
According to other power blades are 35% efficent but according to Best windmill turbine design on youtube (2:55) they are 95% efficent. Who's right?

joestue

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Re: Dimples
« Reply #6 on: September 15, 2015, 11:49:57 PM »
In theory you could get to 95% of the Betz limit, so that's probably what they are talking about. but i don't know if anyone has gotten that high.

there are certain ways of measuring a steam turbine's efficiency, and some of them are better than 90% efficient, but the overall thermodynamic efficiency of the whole cycle may be only 30%, but of the recoverable energy, 95% of it was captured.
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oscar11

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Re: Dimples
« Reply #7 on: September 16, 2015, 06:49:53 AM »
Thanks! I'm trying to see strictly from the blades to the axle. It's so hard because there's so many variables. Two exact same windmills at the same sight will vary in output and then they start affecting each other.