Dimples

[aerohead]

This seems to be a subject we can never escape from and I climbed down into the deep dungeon of aerodynamics and present the following to anyone interested in "golf-balling" there vehicle in the quest for lower drag.

The data is from the Third Reich:the research was reported by Heinrich,"Wind-Tunnel Investigation of a Wing With Bombs,ZWB UM 4574 (FGZ 1944),as presented by S.F.Hoerner,Chapter 8,page 132-133,"Aerodynamic Drag",1951. Here's the gist of it.

A 2,500 kg bomb,with Cd o.04 increases to Cd0.14 with addition of tail fins,stiffening-ring,and propeller fuse. If a "nose ring" is attached to the nose of bomb to facilitate water entry for ship bombing,the Cd jumps to 0.20 ( a 42 % increase in drag ).

All values are for sub-sonic,super-critical Reynold's Number,full turbulent boundary layer flow. If you've seen fluids texts and photos from the U.S.Navy' David Taylor Tow Basin facility in Maryland,and seen underwater photos of (very smooth)bowling balls with sand glued to the leading edge,or a "ring" attached at same location,to trip the boundary layer into turbulent flow( a good thing!),then from the "bomb" example,you may appreciate the significance of "tripping" a tripped boundary layer and the added drag it could induce.

My advice is: Don't go there!

[aerohead]

One more entry and I'll leave you all to ponder.

This is from a textbook entitled Technical Aerodynamics,the source of info,National Advisory Committee on Aeronautics ( now NASA ). paraphasing," roughening the leading edge of a wing with a discontinuity as small as 1/2 a thousandth of an inch can have a negative impact on wing performance, do to the accompanying turbulence."

Imagine the nose of your car as the leading edge of the wing.

[Intrigued]

I've heard all along that dimples don't help wings and such. I wonder what it is that makes them help a golf ball in the first place. Everything just happens to be sized right???

[NeilBlanchard]

Hi,

It may be that it is the fact that the golf ball is spinning?

[mobilerik]

Here's a great link with lots of resources on this topic:

Google Search: "golf ball dimples"

[pasadena_commut]

Originally Posted by aerohead

The data is from the Third Reich

Better data from a couple of billion years of evolution.

Surely there would be a competitive advantage for any bird, fish, dolphin or whale that could reduce the drag along its body by a simple surface treatment. Yet there is a remarkable paucity of dimpled bodies. Some fish have other than smooth surfaces, but usually they are bumpy or even spiked, and this appears on slow moving species. Fish are interesting in that there is a sort of Z like pattern from the edges of their scales, but whatever effect that would have had is largely offset by a thin layer of slime, which acts to fill in these discontinuities. Sharks seem to do just fine with skin much like sand paper.

On a somewhat larger scale, humpback whales have knobs on the leading edge of their fins that do appear to reduce drag. Other than that, biology seems to favor simple streamlining over complex surface treatments.

[Bicycle Bob]

Shark skin feels like sandpaper, but it resembles an LP record more, with sharp-topped micro-grooves aligned with the flow. A similar finish on boats is banned in the Olympics. Dolphins seem to suppress turbulence by sensing it on their skin, and twitching it accordingly. The humps on whale fins are used to initiate a series of vortex streets under conditions when a normal foil would stall.

[Cd]

I agree with you guys, but I'm puzzled as to why that Lexus used dimples on the LS430 underbelly.
They aired a commercial bragging about how the dimples work to smooth airflow.
I just assumed that the dimples were done as a Lexus P.R. stunt, with no real benefit.

However, one day I took a peek underneath a newer model VW Passat / Jetta ( ? ) and was surprised to see that a large portion of its underbelly was dimpled. ( each dimple measuring around 2" diameter )

Why would VW go through the trouble of having the dimples there if no one ever saw them except the mechanics ?

VW does a lot of aero work on their cars. ( which is odd because for years their cars were so boxy )

[aerohead]

Originally Posted by Intrigued

I've heard all along that dimples don't help wings and such. I wonder what it is that makes them help a golf ball in the first place. Everything just happens to be sized right???

In the aero sticky at the top of the aero forum page,I posted info for Reynolds Number effects and boundary layer phenomena.From the material you will understand that the small size of the golf ball benefits from the dimples,not larger structures.

[aerohead]

Originally Posted by Cd

I agree with you guys, but I'm puzzled as to why that Lexus used dimples on the LS430 underbelly.
They aired a commercial bragging about how the dimples work to smooth airflow.
I just assumed that the dimples were done as a Lexus P.R. stunt, with no real benefit.

However, one day I took a peek underneath a newer model VW Passat / Jetta ( ? ) and was surprised to see that a large portion of its underbelly was dimpled. ( each dimple measuring around 2" diameter )

Why would VW go through the trouble of having the dimples there if no one ever saw them except the mechanics ?

VW does a lot of aero work on their cars. ( which is odd because for years their cars were so boxy )

We covered this one in another post.Basically the dimpling you see is actually part of a corrugation to stiffen the floor.By deforming the sheetmetal,thinner (lighter) metal can be used.Lexus sandwiches sound-deadening material to their's.------- The stiffening from the "dimpling" changes the resonant frequency of the floor so it doesn't"boom" when exposed to certain resonant frequencies or harmonics of those frequencies.It has nothing to do with aerodynamics other than the fact that the underfloor is relatively flat.