Hexagonal Dimples as a way to improve aerodynamics

[Palionu]

Yes, I did a search and you're probably thinking, wtf are hexagonal dimples. Think golf ball.

I know that unless you have an unlimited budget you can't modify your car exclusively with these dimples and i seem to remember the backing of some refrigerators have a sheet of aluminum that has similar dimpling. I want to conduct some experiments by blocking my grills or improving aerodynamics in some way with dimpled material, does anyone have a cheap source for this kind of material?

Oh yes, sorry, I have to explain my reasoning. Well (taken from here)

Cylinders and spheres are very convenient and strong shapes when designing bicycle frames and components, but it turns out they are terribly un-aerodynamic. I know they look all smooth and round, how could the air not just flow right on by them? Part of what makes an object aerodynamic is the object’s ability to keep the air attached to the surface as long as possible. As soon as you have what is called flow separation from an object, drag increases. This is why deep rims and disk wheels are more aerodynamic, because the air stays on surface longer, and the flow separation takes place much further along the flow of that object. The same with those thick downtubes that almost all tri bikes have now. More surface area on certain parts of the frame mean less flow separation and improved aerodynamics.

It turns out that with a cylinder or sphere, that flow separation takes place very early as the air travels over the object. But, please dimples on that sphere, like a golf ball, and something changes. Those dimples increase turbulance, which normally you would want to eliminate in aerodynamic design. But this turbulence, or “dirty air” on a sphere has the effect that it actually speeds up the airflow and gives it more forward momentum. As a result, flow separation takes place much later in the flow over the sphere. Even though there is increased turbulence, the trade off is that the increased speeds in airflow has a net benefit on the aerodynamics of the sphere, and the air stays attached to the surface much longer.

It is also important to note this is why we don’t put dimples on just any shape or object for which we are trying to improve aerodynamics. If dimples improve aerodynamics on a ball, why don’t we dimple the wings of an airplane? Or make the bike frame itself dimpled? The reason is that those dimples increase turbulance, and on a wing or aerodynamically tapered bike frame, the flow separation is already fairly good. The net result of increasing turbulence on a shape that already has late flow separation is decreased aerodynamics, while the net aerodynamic result of turbulence on a sphere is positive, because the turbulence contributes towards delayed flow separation.

What would be the effects of aerodynamic dimpling on a car body? If a golf ball has a high CD on the trailing end then can dimpling decrease the cd of steep rear ends? Dimpling sports wear helped Lance Armstrong and the Japanese Women's Swim team, it helps reduce heat from high speed aircraft.. so, what exactly stops testing on cars? Again, anyone have an idea on cheap materials i can use to test with?

[NeilBlanchard]

Hi,

I think this is the same principle as zigzag tape -- which is a linear version of dimples.

[dichotomous]

you could always leave your car outside during a strong hailstorm, or drop marbles or ball bearings from high up on your car....

or, go to home depot, go to the cabinet area, get the little vinyl bumpers for drawers, stick them on the car.... check out my "getting my Si slippery" thread to see how I stuck them on mine, just monday even

[trebuchet03]

What's the Reynolds number for a dimpled golf ball, swimmer in water, cyclist?
What's the Reynolds number for a car?

If they're close - then these features may directly apply for the same reasons. The last paragraph is most telling

[hypermiler01]

The effect would be not even remotely close to the benefits of adding a Kamm back or boat tail.

[Palionu]

Originally Posted by dichotomous

you could always leave your car outside during a strong hailstorm, or drop marbles or ball bearings from high up on your car....

or, go to home depot, go to the cabinet area, get the little vinyl bumpers for drawers, stick them on the car.... check out my "getting my Si slippery" thread to see how I stuck them on mine, just monday even

Thanks for the idea, I especially like the idea of sticking those on my mirrors instead of a full mirror delete (I like the aesthetic nature of them ok?).

Before applying them, did you do the string test to ensure optimal placement?

[Palionu]

Originally Posted by trebuchet03

What's the Reynolds number for a dimpled golf ball, swimmer in water, cyclist?
What's the Reynolds number for a car?

If they're close - then these features may directly apply for the same reasons. The last paragraph is most telling

I stayed up until 1am last night doing research and I think the only benefit I can see from dimpling would be from testing.

I would suspect the rear section could gain the most and maybe the trailing edge of the mirrors, sides of the front bumper and a body pan could benefit from a lower drag coefficient, especially those areas which have a greater curve.

To test Reynolds we need to use the following formula:



Where:
r = atmospheric density
V¥ = velocity
l = reference length (in the case of a sphere, this variable is defined as the diameter)
m = viscosity (or friction) (the funny symbol for those not familiar with it)

I found another page which explains what golf ball dimpling does. Essentially the dimpling creates a turbulent air flow over the surface of the ball (like applying a layer of oil to a ball bearing) this creates a similar effect to an airplane wing which decreases trailing wind disturbance. If this holds true, what are the effects of 1/4 of a golf ball shaped area (the right and left front bumper area in some cars). This is clearly raising more questions than answers so the research continues.

[aerohead]

for the golf ball,the dimples help establish a premature transition to a turbulent boundary layer and the attendent lower drag coefficient which accompanies the turbulent boundary layer. The Reynold's Number environment in which automobiles inhabit guarantees a turbulent boundary layer and lower and stable drag coefficient at and above about 20-mph.Dimpling the leading edges of a car will not lower its drag.---------------------------" Dimpling"or adding turbulators near trailing edges can and may reduce drag in some cases however are never used in the forebody.

[SVOboy]

Originally Posted by aerohead

for the golf ball,the dimples help establish a premature transition to a turbulent boundary layer and the attendent lower drag coefficient which accompanies the turbulent boundary layer. The Reynold's Number environment in which automobiles inhabit guarantees a turbulent boundary layer and lower and stable drag coefficient at and above about 20-mph.Dimpling the leading edges of a car will not lower its drag.---------------------------" Dimpling"or adding turbulators near trailing edges can and may reduce drag in some cases however are never used in the forebody.

[Palionu]

Originally Posted by aerohead

for the golf ball,the dimples help establish a premature transition to a turbulent boundary layer and the attendent lower drag coefficient which accompanies the turbulent boundary layer. The Reynold's Number environment in which automobiles inhabit guarantees a turbulent boundary layer and lower and stable drag coefficient at and above about 20-mph.Dimpling the leading edges of a car will not lower its drag.---------------------------" Dimpling"or adding turbulators near trailing edges can and may reduce drag in some cases however are never used in the forebody.

Yes, I agree, doing a homer simpson on your car is not a good idea, especially around those flat areas. I remember one luxury car manufacturer has a dimpled bodypan that they said increases their fuel efficiency for the same model car by a certain % but that could have been a pitch. I like the idea of adding turbolators to the trailing edges and my front bumper = EEW so I might install some dimpling on the outside edges after i do the front grill delete. I like the idea of adding those silicone bumpies on the mirrors though. I lack the money to invest in a $200+ mirror delete (the cost is in the cameras).

For the rear area I read on a Smart Four-Two forum that aside from them complaining that their not so 'smart' cars aerodynamics were inhibiting his cars potential he went to a canadian semi-truck site and invested in some AirTabs. He installed 8 at the cost of $2.75 each and went from 34mpg to 42mpg... maybe another vortex generator option? Sorry, lost the link =/

Another site with various color options is here