"Golf ball" aerodynamics... Why its not used?

Correa Neto

Philosopher
Correa Neto
Joined
Aug 4, 2003
Messages
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Some time ago I saw a "Mythbusters" episode where they tested the aerodynamics of cars with and without a golfball-like surface. Well, it seems their results indicate the dimples can indeed reduce drag and save fuel.

I was left with the question... If this thing is real, why not use it? A say, 5% fuel economy would be a nice thing for the transport industry.

A quick Googling presented some answers like

"Only blunt objects can take advantage"
So, use it in trains and trucks...

"Works only for small objects"
Cars are not small objects...

Seems I am missing something. What's it? Can cripple lift in airplanes? Would look ugly? Expensive to build? Mythbusters did something wrong in their test?
 
Correa Neto said:
Some time ago I saw a "Mythbusters" episode where they tested the aerodynamics of cars with and without a golfball-like surface. Well, it seems their results indicate the dimples can indeed reduce drag and save fuel.

I was left with the question... If this thing is real, why not use it? A say, 5% fuel economy would be a nice thing for the transport industry.

A quick Googling presented some answers like

"Only blunt objects can take advantage"
So, use it in trains and trucks...

"Works only for small objects"
Cars are not small objects...

Seems I am missing something. What's it? Can cripple lift in airplanes? Would look ugly? Expensive to build? Mythbusters did something wrong in their test?
Click to expand...

Here's a good explanation of how dimples work:
http://www.aerospaceweb.org/question/aerodynamics/q0215.shtml

The issue isn't the size, but the shape. But it's not as simple as blunt vs. smooth either. A streamlined body won't benefit because its wake is already tiny. But that doesn't mean a brick will, because its wake might remain large even with dimples.
 
OK, thanks for the link. I had forgotten about vortex generator and leading edge extensions in airplanes.

But still, if their tests were OK, trains, trucks and some cars could take advantage of the effect, probably because they are not streamlined to the point of generating a small volume of separated flow at their trailing edges.
 
Nowhere Man said:
So, if I take a ball-peen hammer to my car...? ;)

Fred
Click to expand...

Many years ago saw a reasonably new BMW in Munich with ball-peen sized dimples over the roof and bonnet. Assumed it was a vandal attack until I saw a second one and on inquiring...

steve s said:
We had baseball-sized hail over the weekend. A lot of people now have dimpled cars. Maybe they shouldn't take them to the body shop.

Steve S
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... there had been a hailstorm of golf ball size hail.

The Germans missed a trick in not claiming it was for aerodynamics. :)
 
Acleron said:
Many years ago saw a reasonably new BMW in Munich with ball-peen sized dimples over the roof and bonnet. Assumed it was a vandal attack until I saw a second one and on inquiring...


... there had been a hailstorm of golf ball size hail.
Click to expand...
1993? I lived in Germany at the time, around 100 miles from Munich. I was in that hailstorm while driving. Scary. I tried to find shelter under a bridge, but by the time I arrived all places were taken. :mad:
 
So I guess it's the same concept as frayed owl feather trailing edges? They quiet the owl's flight.

I wonder if scale comes in to play? A 1 1/2" golf ball is different than a car with a 3 feet by 6 feet blunt butt. But golf balls need to be totally covered because they spin in flight, but the effect is only at the trailing edge? On a car the 'fray' might need to be a saw-tooth like spoiler about 6 inches? Vertical or horizontical? Or in between?
 
ddt said:
1993? I lived in Germany at the time, around 100 miles from Munich. I was in that hailstorm while driving. Scary. I tried to find shelter under a bridge, but by the time I arrived all places were taken. :mad:
Click to expand...

Yes, that sounds about right, was one the BMWs yours perchance? :eek:
 
casebro said:
So I guess it's the same concept as frayed owl feather trailing edges? They quiet the owl's flight.

I wonder if scale comes in to play? A 1 1/2" golf ball is different than a car with a 3 feet by 6 feet blunt butt. But golf balls need to be totally covered because they spin in flight, but the effect is only at the trailing edge? On a car the 'fray' might need to be a saw-tooth like spoiler about 6 inches? Vertical or horizontical? Or in between?
Click to expand...
While I was watching the episode, my first guess was "no way its going to work, it has to do with object size, shape and fluid viscosity". Well, I was left with a "WFT" face at the end, since it worked for a real car. Now, if we remember vortex generators are used at airplanes' wings, we see big objects can be benefitted. The issue, as stated in the material Ziggurat linked to, seems to be related to the object shape being capable or not to create gradual adverse pressure gradient.

OK, so not much effective in wings, but in trucks and trains, I guess its possible the dimples could work and save fuel. I can't believe transport companies would close their eyes to say, 5% fuel economy. So, what am I missing?

Trucks and trains are not smooth enough to take advantage, for they have too many bumps and steps? Ok, I can imagine this could be the case, but what about those high-speed trains and last-generation aerodynamic trucks?

There's a catch somewhere... The trains are too long and their trailing edges create a comparatively small volume with separated flow? Or something was wrong with Mythbusters' experiment?
 
Correa Neto said:
While I was watching the episode, my first guess was "no way its going to work, it has to do with object size, shape and fluid viscosity". Well, I was left with a "WFT" face at the end, since it worked for a real car. Now, if we remember vortex generators are used at airplanes' wings, we see big objects can be benefitted. The issue, as stated in the material Ziggurat linked to, seems to be related to the object shape being capable or not to create gradual adverse pressure gradient.

OK, so not much effective in wings, but in trucks and trains, I guess its possible the dimples could work and save fuel. I can't believe transport companies would close their eyes to say, 5% fuel economy. So, what am I missing?

Trucks and trains are not smooth enough to take advantage, for they have too many bumps and steps? Ok, I can imagine this could be the case, but what about those high-speed trains and last-generation aerodynamic trucks?

There's a catch somewhere... The trains are too long and their trailing edges create a comparatively small volume with separated flow? Or something was wrong with Mythbusters' experiment?
Click to expand...
Seems to me that it could quite possibly work with trains, just like it worked with the Mythbuster's car. The problem, especially in cars I think, is in forming the sheet metal parts. Seems like getting a piece of sheet metal to curve just so isn't such a big issue, and getting it to curve in two directions isn't so hard either - but you bash it full of bubble-shaped dimples, and it's going to be much harder to get it into the shape that you want, for the door panel or whatever.
 
Didn't someone build a plane with dimples...and each little dimple had a tube to suck in air? It allowed greater stability for forward-swept wings, but was very impractical.
 
Correa Neto said:
"Only blunt objects can take advantage"
So, use it in trains and trucks...
Click to expand...

That's somewhat missing the point. Dimpling only helps when both:
1) The flow is at the right speed to be laminar
2) Laminar flow causes boundary layer separation, leading to a large wake
3) Turbulent flow inhibits boundary layer separation, leading to a smaller wake

For something like a truck, the flow is probably already turbulent at speeds where air resistance is a concern, and in any case no amount of turbulence is going to help with the wake caused by the back being square.

Trains are already pretty streamlined, as the cars are close enough together that they act more like a single object. A single wake at the end of 50 cars isn't that big a deal. You could do something similar with cars by tailgating, but obviously that's a terrible idea for safety.

Golf balls are really pretty special in that they travel at the right speed for boundary layer separation to really be important, and since they have to be balls they can't just be made into a more streamlined shape.
 
Beerina said:
Didn't someone build a plane with dimples...and each little dimple had a tube to suck in air? It allowed greater stability for forward-swept wings, but was very impractical.
Click to expand...


I recall a wing that had tiny holes in the surface to suck in air and improve flow over the wing, but I'm pretty sure it didn't have dimples.

ETA: Here's an article about it...
http://www.nasa.gov/centers/dryden/news/FactSheets/FS-023-DFRC.html

(about 2/3rds of the way down the page)
The metal surface of the experimental wing panel was perforated with about 10 million nearly microscopic laser-cut holes. Through the tiny holes, a suction system embedded in the wing (and mechanized by a turbocompressor in the fuselage) drew off a very small portion of the boundary layer of air just above the wing's surface, thereby expanding the laminar flow across the wing. The flight engineer used a control panel located in the aft cockpit of the airplane to fine-tune the amount of airflow sucked through the holes. This procedure permitted investigation of the effect of suction volume on the area distribution of laminar flow. Researchers believe that laminar flow conditions can reduce aerodynamic drag (friction) and help reduce operating costs by reducing fuel consumption.
Click to expand...

Steve S
 
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Dilb said:
That's somewhat missing the point. Dimpling only helps when both:
1) The flow is at the right speed to be laminar
2) Laminar flow causes boundary layer separation, leading to a large wake
3) Turbulent flow inhibits boundary layer separation, leading to a smaller wake

For something like a truck, the flow is probably already turbulent at speeds where air resistance is a concern, and in any case no amount of turbulence is going to help with the wake caused by the back being square.

Trains are already pretty streamlined, as the cars are close enough together that they act more like a single object. A single wake at the end of 50 cars isn't that big a deal. You could do something similar with cars by tailgating, but obviously that's a terrible idea for safety.

Golf balls are really pretty special in that they travel at the right speed for boundary layer separation to really be important, and since they have to be balls they can't just be made into a more streamlined shape.
Click to expand...

So, for trains, its probably not usefull since they are long objects, thus generating a comparatively small volume of turbulence at the trailing edge.

Still, we're left with trucks. I recall seeing some pics of last-gen highly aerodynamic trucks. Concepts maybe? Can't remember their backs, but maybe these vehicles could get some advantages from the aerodynamic pov. Now, if this would make them still usefull vehicles (say, load/unload operations, maneauvering, etc.). I also imagine these streamlined trucks would pay only in trucks used for long-range transport in autobahns, for example.
 
Car and truck manufacturers already have a lot to win on fuel economy by improving streamlining. They don't due to esthetics and practicality in production and use.

Now I don't see dimpling being impractical in use... except for cleaning and puddles forming on any somewhat horizontal surface. Okay, I see some impracticalities in use. It'd also increase production cost. And it'd be somewhat of a gamble to assume enough people wouldn't balk at the esthetics.
 
Golf balls are very small and light, which makes each dimple proportionally larger in comparison to it, and they travel a couple hundred miles per hour. Why in the world would anyone expect the same aerodynamic effects to apply equally well to both?

Also, the back end of a truck's trailer is where the doors are, so that's where the opening/closing/locking mechanisms go. And they have to be flat, rectangular, and vertical in order to park at an enclosed dock so the interior of the trailer temporarily becomes like an extra room/extension from the building it's parked at, so they'll never be rounded, which does away with any hope of aerodynamics or golf ball analogies.
 
Buckminster's Dyna and bulging doors

Two things-
-Why isn't anyone talking about Buckminster Fullers Dynamaxion Car that was essentially shaped like a drop of water falling? That seems the best shape to me for as many of our ground vehicles as possible.
-As far as the back end of a semi-trailer needing to be flat. The doors don't need to be flat to fit against the sealing donut because the frame of the door fits against it, or inside it, so an aerodynamic bulge should be acceptable.
 
mrcharles said:
Two things-
-Why isn't anyone talking about Buckminster Fullers Dynamaxion Car that was essentially shaped like a drop of water falling? That seems the best shape to me for as many of our ground vehicles as possible.
Click to expand...

Well, there was the Aptera:
aptera-1.jpg


But that sort of shape is fairly low in interior volume, which introduces some large economic inefficiencies.

-As far as the back end of a semi-trailer needing to be flat. The doors don't need to be flat to fit against the sealing donut because the frame of the door fits against it, or inside it, so an aerodynamic bulge should be acceptable.
Click to expand...

They do if you want them to be able to open up all the way once docked. Plus, of course, you can't add that onto containers, since they need to stack tightly.
 
Well, just build the cargo doors at the truck's side. Faster loading and unloading ops, I would dare to say. But this certainly involves a big change in operational infrastructure.

ETA- Just remembered that there are pleny of side-loading trucks around already.

Now, I must say that after thinking a bit about this, much to my surprise I found a rather irrational resistance to the aesthetics of a car with dimples (assuming it would save me say, 5% fuel and its price tag would not ruin the effect)... Funny incoherent mind mine is... I am willing to pay more for a fuel with a lower carbon sig, such as ethanol, but the call of the aesthetics would influence my decision on buying a more economic car.
 
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Correa Neto said:
Now, I must say that after thinking a bit about this, much to my surprise I found a rather irrational resistance to the aesthetics of a car with dimples (assuming it would save me say, 5% fuel and its price tag would not ruin the effect)...
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Really? I thought the car in the Mythbusters episode was absolutely adorable, and would definitely buy something like that if it was available. (Though preferably not covered in actual clay.) Heck, I'd be tempted even if it didn't get better mileage.
 
Correa Neto said:
Well, just build the cargo doors at the truck's side. Faster loading and unloading ops, I would dare to say. But this certainly involves a big change in operational infrastructure.
Click to expand...

But it's not actually faster loading and unloading unless the entire side of the trailer opens (which has its own problems), because you can't go straight in and out. If you're driving a forklift, the turn in a confined space slows you down.
 
I've seen a number of just-in-time delivery road trucks that can open the whole side of the trailer; I remember that some had doors made of canvas while some had metal doors that could slide upwards.

Sure, they do require a different loading/unloading area layout, but I can imagine advantages.

I also see lots of trucks with similar layouts delivering beer in cities.

xtifr, humans are strange...
 
Correa Neto said:
I've seen a number of just-in-time delivery road trucks that can open the whole side of the trailer; I remember that some had doors made of canvas while some had metal doors that could slide upwards.

Sure, they do require a different loading/unloading area layout, but I can imagine advantages.

I also see lots of trucks with similar layouts delivering beer in cities.

xtifr, humans are strange...
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We mostly had tautliners for pick up and delivery when I worked in a factory. Nice and easy to load and unload from the sides.

Similar to these.
 
If I recall the episode correctly, (and I may not) they only showed the measurements being made in one direction on a straight course. There aren't many roads that are perfectly flat. It wouldn't take much slope for the additional weight of the clay to affect the measurement.
 
Here's a practical consideration... Wouldn't all those dimples tend to attract dirt, dust, standing water, and the like? When it snowed, it would be a real pain to clear the snow or ice from those things with conventional snow-brushes.
 
Bikewer said:
Here's a practical consideration... Wouldn't all those dimples tend to attract dirt, dust, standing water, and the like? When it snowed, it would be a real pain to clear the snow or ice from those things with conventional snow-brushes.
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Golf balls have dimples all over because they don't have a front or a back
In a practical application of turbulation on a car, you probably wouldn't use dimples all over.
.
On the front half of the car, where the increasing thickness tends to keep the airflow from separating, you'd want to keep it smooth, so the air stays laminar, which is lower drag.
Then, just before the laminar flow separated into a bubble, you'd "trip" it into turbulent flow with a discontinuity. It doesn't take much.

I think the Mythbusters asked the wrong question. You don't need to "fix" a laminar bubble unless you have a laminar bubble.
Painting a little used motor oil on the car and driving at speed for awhile will show where the attached flow is laminar, where the flow is turbulent, and whether there's a bubble at the transition. But that's probably not as good a visual as a car covered in clay with dimples in it.
 
Bikewer said:
Here's a practical consideration... Wouldn't all those dimples tend to attract dirt, dust, standing water, and the like? When it snowed, it would be a real pain to clear the snow or ice from those things with conventional snow-brushes.
Click to expand...
One of the problems engineers have to deal with in design of vehicles is "Water collection" points.\
The dimples would collect water, dirt, bugs, and other debris--and would therefore lose effectiveness, and be starting points for rust...
 
Delvo said:
Golf balls are very small and light, which makes each dimple proportionally larger in comparison to it, and they travel a couple hundred miles per hour.
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I don't think so. I doubt that even Tiger Woods gets 200 mph with his golf balls. Most golfers would be doing well to get much over 120 mph or so.

Delvo said:
Why in the world would anyone expect the same aerodynamic effects to apply equally well to both?
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Why in the world would anyone expect the aerodynamic effects to apply differently without evidence otherwise?
 
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TjW said:
If I recall the episode correctly, (and I may not) they only showed the measurements being made in one direction on a straight course. There aren't many roads that are perfectly flat. It wouldn't take much slope for the additional weight of the clay to affect the measurement.
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I don't understand what point you're trying to make here. They compared the fuel efficiency of the car with clay and no dimples to the fuel efficiency of the car with clay and dimples. Same weight, just surface configuration different. You wouldn't need any slope for the addition of a ton of clay to change your fuel efficiency.
 
TjW said:
Okay. Then I didn't remember correctly.
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What may have been confusing you is the fact that they originally tested the car with no clay on it. Then they covered it with a smooth layer of clay, and then finally with the dimples. All the clay that was removed to make the dimples was put into a milk crate and tossed into the back seat.

FYI, here is the location where they tested that episode...

37°59'4.16"N 121°58'57.60"W

Steve S
 
I've never been able to take Mythbusters seriously because a lot of their stuff is sloppily done without proper test conditions
 
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