Aeronautical Engineering question...

At our local Science Centre, there is a demonstration of how the dimples on a golf ball decrease the wind resistance compared to a ball of the same size which is smooth. The fact that golfball maunfacturers spend gobs of dough developing new dimple patterns for different conditions and control, I have to believe that there is a strong effect on the aerodynamics from the dimples.

During the Olympics, I noticed a cycling helmet worn by a rider in the velodrome which had dimples running down the centre of the helmet. I assume that she was getting a benefit from the same principle as the golfball (perhaps to a lesser degree).

MY QUESTION: For automotive/aeronautical applications, would this not be a benefit? I realize that for the average car traveling at 30MPH, it may not make a significant difference, but for aircraft, and F1/Indy cars I would think that they should get better performance.

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If it were a benefit (compared to the cost of producing dimpled sheets of steel), they’d already be doing it. Cars, airplanes, and spacecraft, before they’re built, all have models put through rigorous wind tunnel tests. Before the models are even built, similar tests are run by computer. If it were the case, we’d all be driving dimpled cars and flying on dimpled planes by now.


The dimples are to turbulate the boundary layer, thereby keeping it attached to the surface and reducing drag from a recirculating, low-pressure wake area behind the object. The turbulent boundary layer adds a little drag, but not nearly as much as the wake would. The boundary layer is a very thin layer of air that remains attached to the object, while the relative motion of the object and the air occurs in shear at the outboard side of the boundary layer.

Turbulating the boundary layer, which normally wants to be laminar (smooth), is useful only when the part has a relatively blunt aft side which would allow a significant wake. Golf balls are one example, and form-fitting bicycle helmets would be another. If the object’s aft side tapers smoothly back gradually enough, the boundary layer can remain attached instead of tearing off, and turbulating it would simply add drag. The added length adds some friction drag, though - but engineering is all about finding optimum balances, anyway.

I’ve seen bicycle helmets with long, tapered aft sides that presumably are to eliminate wake. Racing car bodies (except for “stock” cars - don’t get me started) are carefully tailored in wind tunnels and with computer techniques to minimize drag and wake while optimizing downforce for added traction in corners.

An interesting and informed reply, LazarusLong42. Can I extend your argument a little?

[LL42 mode on]
Clever people already design things.
Things must be perfect already, or the clever people would change them.
Because you are disagreeing with the clever people, I’m not going to justify things to you .
[LL42 mode off]

This next bit would be so much better if I could provide the cite - but I saw this plane on the Discovery channel… I can’t remember (or find) which fighter they were discussing ( a couple of weeks ago in the UK ), but the plane had ‘dimples’ (some kind of low pressure air jets) in the wings to create a turbulent air flow near the wing surface.


IANAAE, and what I’m saying here is from a dimly remembered “Aerodynamics for complete idiots” course I took when trying to learn to fly hang-gliders. In other words, WAG alert ON:

If I have understood things correctly, the dimples help cause a controlled turbulence, preventing the airflow from becoming separated from the surface of the golf ball.

A lot of aircraft have small attachments on the wings to do exactly the same. (I have NO idea what they’re called - small angular brackets mounted on the wings’ upper surface. The term “vortex generator” surfaces in my mind - or was that from an old Flash Gordon comic ?)

The golf ball is tumbling through the air, making it necessary to have the dimples all over its surface - an aircraft doesn’t tumble. And if it does, drag reduction is probably not a priority.

The same goes for cars, locomotives etc. - when you know the direction of the airflow and has a free hand (more or less) when making your design, you can choose a shape where the airflow stays (more or less) laminar. Or you can decide exactly where to induce vortexes by mounting spoilers. A golf ball designer, OTOH, is somewhat constrained in his selection of possible shapes.

S. Norman, waiting for an Aero Engineer to tear the above apart.

When I took Fluid Mechanics in college, we talked about some airplane wings and other components having a “sandpaper-like” leading edge on some designs…probably where they foresee the boundary layer leaving the surface of the wing or whatever…

So you don’t necessarily need dimples…just a rough surface will induce turbulent flow.

IIRC, the good ole Buccaneer aircraft had the “artificial dimples” - also known as “BLC” for Boundary Layer Control.

This was a carrier-based aircraft (back when the RN had real carriers) and of course, every trick counts when you want the maximum amount of lift from an aircraft that has to be relatively small and have a relatively low stall speed.

S. Norman

This is one of the ‘black arts’ in aviation design.

If I recall my aero engineering correctly:

The ‘Boundary Layer’ is a thin layer of air next to a flying surface which starts with the air not moving at all right at the surface, and extends to the point where the air is moving at the free stream velocity. This is typically a few mm thick on an airplane.

The boundary layer gets weaker as you move back on the airfoil, until finally the flow of air detaches from the wing completely and goes turbulent. On a typical high-lift airfoil, this happens about 30% of the way back from the leading edge of the wing. On a ‘laminar flow’ airfoil, the separation point is typically about 50% of the way back.

‘Vortex Generators’ are little metal spades that stick up into the airflow and cause turbulence which tends to re-energize the boundary layer and keep the airflow from detaching, thus giving slightly more lift at high angles of attack.

Placing vortex generators on a wing is often a matter of trial-and-error, which is why I referred to it as an art. There are vortex generator kits available for many aircraft, and some come from the factory with VG’s installed. Some aircraft claim a decrease in stalling speed by as much as 5 mph or so when vortex generators are installed. Because they are primarily used to maintain lift at high angles of attack, you most often see them on aircraft that are designed to land slowly, like bush planes.

There are other tricks for maintaining lamilar flow. Some aircraft are built with slots in the wing. At high angles of attack, air flows from the bottom of the wing through the slot on top, creating a kind of second mini-wing behind the leading edge. I’ve heard of suction devices with pinholes on the surface of the wing. ‘Stall Fences’ keep air from moving spanwise across the wing, improving efficiency and controllability at high AOA.

**[ensuring understanding]**Okay, I was going on the assumption that dimples leading the way were doing the work, but in fact that is ass-backwards. The trailing dimples are decreasing the air pressure behind the ball and allowing the energy to be retained in the forward flight…or something akin to that.

So this is not (particularly) necessary for aircraft/racecars because the trailing edges are engineered to taper to reduce the amount of drag experienced.
[/ensuring understanding]

Would there be any benefit to doing this on vehicles which cannot be tailored to such a degree? Think of the fat ass on the Plymouth Prowler…I’m guessing that there’s a decent wake created behind it. Could there be any appreciable difference if the rear end were dimpled?

Another question prompted by Jeels response:
Would a smoothly dimpled leading edge aid in preventing icing of wings? I guessing that the ice is formed within the boundary layer where the air is calm(er). (Or does the force of the water/ice pellets negate any benefit?)

OK, I no longer have the vocabulary to dazzle you with an articulate description of the processes at work, nor do I have the inclination to find any citations so bear with me. (Not to mention I need to look like I’m working right now)

First, it seems that there are no errors in any of the general explanations give so far, just some gross over-simplifications and sloppy use of terminology. For the purposes of the OP they are fine.

A real easy explanation is that the dimples do indeed create a relatively thin layer of turbulence (I wouldn’t go so far as to call it a controled turbulent layer) on the surface of the golf ball. It indeed does have the net effect of lower overall drag on the golf ball. A reasonable way to justify this is by saying that the air moving around the ball has a lower coefficient of friction with a stationary (relative to the ball) layer of air than it does with the surface of the ball itself. There are several other factors such as detached flow, vorticies, and revolution which play into the give and take that is engineering, but this is the fundamental that its based on.

An aside, I didn’t see the helmet you refer to in the Olympics, but I wonder if the dimples down the center were for ventilation and not specifically aerodynamics.

To answer your question, as others have stated its essentially a question of optimizing the results. There no doubt are applications other than golf balls where dimples or other methods of creating a turbulent layer are utilized. The dimples however are not ideal, and are more or less a patch on a greater problem. Dimples by nature can’t eliminate drag, only reduce it. In many cases the result is more efficient than a smooth surface, but with advanced materials, our advancing understanding of flow characteristics, and excellent dynamic flow measuring methods this is becoming increasily less likely to be true.

Elvis, um, turbulate???

Maybe I’ll check in after work and touch on some more detailed attempts at explaining.

Not entirely, the dimlples in the front do indeed slightly lower the incident drag on the ball, and there still is some seperation behind the ball. There is a contribution on both ends (as well as the sides), but there isn’t a solution on either end.

The big boost that the dimples give a golf ball in flight is by creating lift. The drag reduction is only a minor component in comparison when you are discussing overall distance. The lift is created by the backspin imparted by the lofted club face (unfortunately for the 15 handicapper, slice and hook as well). As the ball rotates
the dimples carry that turbulent layer into the oncoming flow of air (and with the aft flow on the top) creating increased air pressure on the lower half of the ball (and reduced above). This is the dynamic which dramatically increases the performance compared to a smooth ball. This lift is what causes a golf ball to not exhibit the traditional parabolic flight path that Physics 101 teaches. The reduction in drag notwithstanding.

So don’t over estimate the principles we’re discussing here, and since the Plymoth Prowler probably isn’t going to be backspinning through the sky anytime soon I’m going to ignore that effect for now.

This is essentially correct, but don’t assume that a dramatic taper is always the most efficient design either. One must also consider that in race cars down-force is frequently of paramount importance over drag reduction. No matter how aerodynamically clean the shape is, putting the power to the pavement is the real goal.

On that same point however, look at some of the recent models of minivans. I know the older Toyota Previa was an example, but I haven’t looked at any newer designs that close recently. These had what essentially was a rear spoiler mounted on the top of the rear door. They, however, were directed parallel to the rear of the vehicle as opposed to parallel to the roof. I can’t say with certainty unless i can perform a wind tunnel test, but I’d wager that these effectively created a turbulent layer along the rear of the vehicle. Similar to the effect you hypothesize with the dimples. The difference here is that a dimple is a very inefficient method to achieve this goal, but it is however well suited for a ball which has no determined front or rear. In this case, one assumes that a spoiler does the job with less parasite drag costs than would dimples.

The Physics of Golf, Chapter 8: The Aerodynamics of Golf has availible on the web from GolfWeb. It agrees with just about everything said here, but has a lot of history and includes diagrams for those of us who move our mouths a lot when we read the really big words.

Do the dimples help magnify the Coanda effect?

turbulate, v.: to make turbulent

You gotta problem widdat? If so, take it up with the authors of virtually every fluid dynamics text ever written in English. It’s an entirely standard usage, I assure you.

Now, do you have anything to add to this thread other than a long-winded recap of all the previous posts in it?

Yes, that’s what the Coanda effect is. The dimples keep the boundary layer attached further around the ball than would otherwise be the case.

For cars, if they want the airflow to be turbulent behind the car, they put a spoiler in the back. This spoils (hence the name) the airflow, reducint the suction effect on the back of the car.

Well, sort of. Being more or less round on top and more or less flat on the bottom, a car body will actually produce lift, which is not good for something you want to stay stuck to the ground. The spoiler kills the lift, increasing traction at the cost of a small drag increase due to wake turbulence. Further downforce can be obtained by using an actual “upside down” wing instead of a spoiler, but also at the cost of some drag. In fact, a Formula 1 or CART car will have a downforce of up to 3 G’s - they could stay stuck to an upside-down track.

Racing teams go to extreme lengths to find the optimum downforce settings for individual tracks to minimize lap times. A Formula 1 car will have an extreme amount of wing incidence angle for a slow, twisty track like Monaco, and nearly none for a fast track like Monza.

Hey stroke boy, I knew what you meant obviously, and I’ve seen the word used on a few remedial websites, usually in quotes (“turbulate”) as to imply it is a non-standard term. The fact is Merriam-Webster and the OED don’t recognize the word as such so you’re claim that its “standard” is pretty high handed…unless of course you hold the background to be setting conventions. And as to “virtually every fluid dynamics text”, gimme a cite. Title, Author, Edition and page number…chances are I have it on hand. Now, I was just giving you a hard time, but if you’d rather be a dickhead I’m more than happy to oblige.

I guess I shouldn’t be suprised that with your grasp of the english language you didn’t understand the rest of my post. I suppose since you posed a vague retort to the OP we should just lock up the thread, huh? You know, seeing as you didn’t actual bother to answer the question of why there aren’t dimples on automobiles and aircraft. But I guess you’re more interested in being a dickhead than anything else, sorry if the rest of us piss in your pool.

Oh, Omni, I think it was LazarusLong42 who posed a vague retort to the OP. Elvis has been pretty informative, aside from taking issue with you. I seem to recall hearing the word turbulate used way back in the dark years I was majoring in aeronautical engineering. Unfortunately, I’ve since given away my textbooks, so I can’t provide a cite.

As for racing, I remember hearing that when practicing at the F1 track at Indy for the first time, they were surprised to find that the benefits of cranking up the downforce for the twisty half outweighed the benefits of turning it down for the fast part. Damn, talk about a cool job, being an aerodynamics expert for an F1 team. Makes me almost wish I hadn’t changed majors.

Well, an “aerodynamics expert” on the track with an F1 team is typically just a very very good mechanic. AFAIK they don’t really bring the fluid dynamics and theory to the track, its pretty much finished once the car is designed and built. I’m sure that the extent of determining which downforce is omptimum is just a matter of running it around the track, timing and asking the driver how it feels, changing it and doing it again. Bacially trial and error, although to many this is engineering at its core. Still a damn fun job none the less.

As to the rest, no, I know exactly who I am critisizing, and generally, I agree, there wouldn’t be any need. But if he’s going to denigrate the value of my comments, he’d better have posted something awfully damn thorough and complete…he certainly didn’t.