The 'Dimple Effect"

[Ciro Pabón]

This is a subject that raises its head once a year or so.

I post just to clarify that you do not need an object to spin for the dimples to work.

The dimples do not work because they diminish drag, but because they create turbulent flow. The turbulent flow has more energy, so the flow separation behind the golf ball is smaller. They diminish what's called pressure drag, caused by a relative "void" behind the ball.



So, dimples are not that good for objects that have small flow separation, like wings or F1 cars. They work well in "blunt" objects, like golf ballls. Check this, taken from one of the threads mentioned by Belatti:



For wings and the such, you use vortex generators to diminish drag, like those serrated edges, taken from the same thread:

Serrated Gurney flap (the small vertical plate at the back edge of the wing)


Serrated windscreen (the metallic plate with holes in front of the steering wheel)


Sorry for the repetition, but I'm positive most people don't follow links in threads.

Finally, I think Junior is taking risks and spinning because he must be a little desperate. That's nothing to be ashamed of.

NOTE: if this thread "drags" on, I think it's better to incorporate it into the previous ones.

[snoro]

well it have already been applied to high end bicycle wheel, Zipp wheels have like 90% of their wheel range made with dimple.go check their website for more info on the advantage that it gives for a cyclist.

[pgj]

Even if dimples are not suitable for aerofoil surfaces, could it work on things like side-pods or even the underside of the floor?

[RacingManiac]

IIRC the idea is a turbulent flow reduces boundary layer thickness and makes the flow less stagnant and therefore stays attached longer and reduces drag. The same effect on the underbody is achieved through use of serrated edges off the bargeboard or just plain vortex generator like Ciro's pics above....in effect the principle is in use, just done differently....

[pgj]

@RacingManiac Thank you.

[tc9604]

So would tyres with tread experience this effect to some extent?

[Mystery Steve]

There might be a slight effect, but due to combination of ground effect and tire rotation it would likely be a negligible contribution (I assume, at least...). If you wanted to fill the wake behind the wheel, it would much easier to use a turning vane, or some other aerodynamic device.

[Giblet]

When I was a young boy I fell on a fireplace hearth and cracked my cheekbone, and it left me with a permanent dimple on my face.

Luckily it landed in the exact place a dimple is supposed to be.

Am I off topic?

[Mystery Steve]

Do you notice any asymmetrical aerodynamic effects when you go for a bike ride then?

[tc9604]

Thanks guys

[autogyro]

I know that there is a slightly greater increase in lift with the wheels still rotating than if they have been braked to stationary, when a Hawker Sea Fury takes off and as the undercarriage is retracted. There is a markedly greater sudden upward jerk.
The tyres on the main undercarriage usualy have grooves.
Work that one out aero guys.
Is it an aero effect on the rotating tyre surfaces, or perhaps the gear hitting the wheel well, increased air flow over the flaps or a pilots stick reaction to the change in control surface feed back, or something else.
Come on you guys, all that uni must be worth something!
Model it?

[mep]

It is caused by the magnus effect.
http://en.wikipedia.org/wiki/Magnus_effect
It’s the same principle who enables you to let a football fly on a curve or to lift a spinning golf ball. In fact is is not only the dimples who let a golf ball fly far, it's more the spin you gave them.

On a tire the lower surface is covered by the road so you get even more lift.
I don't think the groves play a big role there.

On an airplane you have a lot of drag with the wheels outboard what makes it more sensitive. The magnus effect should play a marginal role there.
Your wheel suspension will suffer under high structural what you don't like on a light optimised part.

[HungryHebbo]

But surely that would reduce the overall lift? Think of topsin and backspin in tennis. Topspin makes the ball dip...

[autogyro]

Dont forget, as the wheels retract on the Sea Fury, they move from vertical to horizontal and there is also the gyroscopic effect.

We can move on to light unpowered rotor autogyros with retracting gear if you like. I will be looking at one next Wednesday.
This one has teeter bearings in the rotor head rather than the helicopter collective/cyclic head. Single blade with self balancing A of I.

Question would be to model all the forces at work during take off, retract, cruise and landing.
Guess what: The CAA has not even got a definition of autogyro flight and Glasgow Uni who were paid to do a computer evaluation of the flight envelope for them got it completely wrong, so it is now an unknown technical area of aerodynamics.
The old Ministry of Aviation knew 40 years ago, just shows how backward the technology has progressed.

[HungryHebbo]

Dont forget, as the wheels retract on the Sea Fury, they move from vertical to horizontal and there is also the gyroscopic effect.

I was thinking about that too when I posted about the topspin. If I remember correctly, both wheels move inwards, so the port undercarriage comes up anti-clockwise, starboard clockwise. Unless I'm mistaken (probably am!) the forces this would induce should cancel out - the relative input force to each gyropscope (wheel) is equal and opposite, right?

We can move on to light unpowered rotor autogyros ... just shows how backward the technology has progressed.

I have no clue whatsoever about autogyro's, so not even going to attempt to comment on them!