aerodynamic drag

[steve everard]

Just be told that a rough surface on a race car body would be better than a waxed smooth surface for aerodynamics, apparently is common sense in the world of physics, example being a sharks rough skin !!!!!!!!!!! What say you ????
Steve

[Neil_Roberts]

Take a look at any sailplane, or any bird. Sssssmoooooth.

For small, blunt shapes, such as a golf ball, surface roughness trips the aerodynamic boundary layer to turbulent in the short length available. A turbulent boundary layer will remain attached farther around the downstream side of the ball than a laminar boundary layer would. So, the wake of the ball will be smaller and its drag will be lower. That is one of the few exceptions where surface roughness is beneficial.

[DK540]

It's been tried a few times, and laminar flow is the key. But there has not been any definitive answer on fuel efficiency or speed. However, worth noting that FIA said, NO! to any dimples on any surface of any F1 car, so.... ??

[alangbaker]

Just be told that a rough surface on a race car body would be better than a waxed smooth surface for aerodynamics, apparently is common sense in the world of physics, example being a sharks rough skin !!!!!!!!!!! What say you ????
Steve

Mythbusters

[Frank C]

Google "riblets" for about a 10% decrease in turbulent boundary layer skin friction drag. See the book by Mohammed Gad el Hak, "Flow Control."
Edit:
Mike Walsh at NASA Langley was the originator of the idea, and NASA's description of them is at:
http://www.nasa.gov/centers/langley/...iblets_prt.htm

Riblets were shown on ESPN in 1987 when Dennis Connor's "Stars and Stripes" used them to win the America's Cup in Australia. The 3M stick-on riblet film was applied to the ship at night so that the Australians wouldn't see. There was no data, but the crew reported that the ship "felt faster." There have been debates about whether the dermal denticles of sharks produce the same effect. See research by Bechert. Speedo claimed that their swimming suits, outlawed after the China Olympics, used the effect. The flow has to be aligned with the riblet grooves to within about 15 degrees. It is questionable whether this condition was met on the swimsuits, and other mechanisms, including body compression and smoothing, were involved. Shark speed also has been attributed to embedded cavity drag reduction - flow over the open cavities between denticles having lower drag than flow over solid surfaces.
- Frank C

[Evl]

Google "riblets"

Oh man, now I'm hungry!

[R. Pare]

US Navy document on riblets:

http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA182242

The benefit of a roughened surface depends greatly on the geometry of the roughness, and precise orientation to the flow direction.

Basically, the answer to the original questions is No, smooth is not always best.

[Christopher Crowe]

Dimples
It's been tried a few times, and laminar flow is the key. But there has not been any definitive answer on fuel efficiency or speed. However, worth noting that FIA said, NO! to any dimples on any surface of any F1 car, so.... ??

I've got to guess that if their were no sanctions against it, we'd probably see dimpled surfaces on things like mirrors and other protrusions where a conventional aero-shape can't be employed.

It'd be fun to see the (down-stream) air-flow-disturbence difference between a dimpled mirror housing and a non-dimpled one. Anyone with a CFD program curious enough to run the simulation?

Thanks,

Chris

PS: Or maybe a tiny "riblet strip" around the rear-most and fattest part of the mirror-housing...

PPS: Got to agree: "Riblets" WOULD look good on a restaruant's menu.

[Frank C]

Dimples are unlikely to do any good on mirrors for the separation is governed by geometry - where the surface ends.
- Frank C

[LAJ]

Unless you are on the pole of a race or close.I think you better spend more time on other things or you will NEVER GET CLOSE!!!
Lee
http://metroevents.com/jordaneng/index.htm

[Christopher Crowe]

For small, blunt shapes, such as a golf ball, surface roughness trips the aerodynamic boundary layer to turbulent in the short length available. A turbulent boundary layer will remain attached farther around the downstream side of the ball than a laminar boundary layer would. So, the wake of the ball will be smaller and its drag will be lower. That is one of the few exceptions where surface roughness is beneficial.

And a mirror is small and blunt, for sure. I know the discussion is essentially academic... but if I were running an Indy car at the Speedway, for instance, I'd be curious about the little gain that might be had in this area. Running at 225 mph, could you lose a quarter degree of rear wing A of A if you chopped up the air coming off that damned intrusive mirror housing?

Dunno!

Chris

[Rick Ross]

Chris,

The dimples help the keep the flow attached to the rear side of the golf ball for a greater distance, thus lowering the overall drag. The flow will still eventually seperate from the ball surface.....it just takes a little longer with the dimples.

A race car mirror housing is a completely different situation. You can think of the mirror housing as one half of a golf ball, with the mirrored part attached to the flat rear surface. The flow on the housing seperates as soon as it encounters the flat rear surface (where the actual mirror is), and dimples will have no effect on this seperation location. The flow is going to seperate immediately when it encounters the 90 degree angle at the rear edge of the housing......with or withour any dimples.

So while dimples may reduce drag on other parts of a car, they would be of little benefit on a typical race car mirrror. I would expect them to be of much more benefit in locations of adverse pressure gradient where you might wish to delay flow seperation for as long as possible.

[Christopher Crowe]

But I'm still curious. First, a golf ball is spinning, so not much commonality with a mirror.

In the case of a mirror, wouldn't the turbulence created by dimples (or vortex generateraters or "riblets") reduce the (downstream) effect of this stark obstruction?

I.e., isn't seriously "excited" air going to be more likely to regain a laminar flow (downstream) than the great big turbulating wave the mirror's blunt shape would likely create?

Not trying to be rhetorical, my question is an honest one.

Thanks,

Chris

[Rick Ross]

Chris,

Very small objects like golf balls tend to have very low Reynolds numbers, meaning that the flow is dominated by viscous effects (friction). As I understand it, the dimples help the boundary layer transition from laminar to turbulent flow along the very short front part of the golf ball, and I assume that they add energy to the turbulent boundary layer flow. The turbulent boundary layer then remains attached for a greater distance along the rear half of the golf ball as compared to the case of a laminar BL. The delayed flow seperation due to the dimples causes a significant decrease in the total drag of the ball. The skin friction drag may be increased slightly, but there is a major reduction in the pressure ("wake") drag.

So the real benefit of the dimples is realized only on the back half of the golf ball, where the dimples allow the flow to remain attached longer. On a race car mirror, there is no back half -- the flow will seperate immediately when the flat aft mirror face is encountered.

I'm sure there are some fluid dynamics experts on Apexspeed that can explain this much better than I can, and hopefully point out any errors in my post.

[Angus]

For better or worse I got my Bachelors degree in this sort of stuff.

Rick's explanation is spot on and he put it into words much better than I could.

[racerdad2]

Years ago, high performance sailplanes had thousands of small holes on the upper surface that bled air from leading edge inlets for reduced drag... A novel idea In the eighties, when our Twin Astir had a glide ratio around 33/1. Now, gliders are upwards of 55/1, totally slick, thin profiles with winglets. Kinda like comparing my Reynard to the new VD's... Perhaps an air in take on the leading edge of the mirrors to ease the separation on the backside ?