Subject says it all. Educate me. Thanks.
They are - sorta’
Plastic underside tray/shielding of my wife’s Audi is dimpled like a golf ball. I have to remove it to change the oil and I was studying it. They definitely shaped it for air flow. I assume they don’t “dimple” the top of the car for aesthetic reasons 
http://www.airliners.net/aviation-forums/tech_ops/read.main/51641
airplane nerds got you by 10 years
.
The Mythbusters tested dimples on a car a few years ago and saw some improvements. They don’t always do a great job at controlling variables, but the testing on that one seemed pretty sound. Here is a brief article on it…
http://www.mycarforum.com/blog/12-golf-ball-like-car-saves-fuel/?showentry=1079
Would you want to drive that car?
They are - sorta’
Plastic underside tray/shielding of my wife’s Audi is dimpled like a golf ball. I have to remove it to change the oil and I was studying it. They definitely shaped it for air flow. I assume they don’t “dimple” the top of the car for aesthetic reasons
Exactly what I came in to say, the trays on my A3 and on my Jetta sportwagen are dimpled.
The new electric golf appears to have flat trays but since there is no exhaust the entire underside is covered.
Subject says it all. Educate me. Thanks.
Because dimpling only works for a certain range of Reynolds number. Reynolds number is a scale-independent airspeed that allows you to make conclusions for any size object. The dimpling shifts the Cd curve for a small range of airspeeds.
Like this: http://imgur.com/eUMiAW4
Airplanes are way over to the right of the region where dimpling would help.
Because in many cases they can design in such a way they do not need to resort to boundary layer tripping methods to reduce drag. edit: (strictly speaking about cars here)
I had a truck that was dimpled. Hail storm. Couldn’t really notice an effect on my mileage.
saw this a couple years ago. A few months later my Brother-in-Law got a new job and it turned out this is his boss’ “car”.
Subject says it all. Educate me. Thanks.
Dimples are to make the airflow turbulent and stay with the surface which reduces pressure drag.
Airplanes go so fast the airflow is turbulent from the get go.
I believe because of the high speeds they’re going they’re effectively at very low yaw angles, so it’s more useful to optimize shape and reduce skin drag (which is more dominant at high speed) than to induce turbulent flow (and increase skin drag) from a suboptimal shape.
- expensive.
- ugly.
saw this a couple years ago. A few months later my Brother-in-Law got a new job and it turned out this is his boss’ “car”.
Corbin Sparrow?
Cars: aesthetics. Though the under-body panels of many cars are dimpled.
Planes: NASA experimented with a system whereby there were holes in the wings and one of the engines drew it’s intake air through said holes. There were significant benefits but the additional equipment necessary to regulate the size of the holes, along with the whole Rube-Goldberg nature of the project, led them to abandon the concept. I’ll dig up the white-paper.
If it was an issue of Reynolds numbers, then why are wheels dimpled and bike frames aren’t?
Thirty-nine years ago. Almost as old as triathlon itself.
As others have correctly posted: Reynold Number (however one poster said “airplanes are so fast they’re turbulent from the get go”: that’s dead wrong).
If the Reynold Number is low enough on an airplane, (or golf ball, or bike, or model aircraft, or bumblebee) “turbulating” the boundary layer can be beneficial. Have a look at many of the high performance sailplanes produced in the last 20 years. Blow holes or zig-zag tape can be found on the lower surface of the wing at about 70% MAC to aide in delaying separation. Back in my youth, many of my model sailplanes/gliders (which were very low Reynolds Number vehicles) had lateral stringers up to the spar to introduce higher energy air to the boundary layer ahead of separation. Factors affecting Reynolds Number are chord width and speed, so narrow chord and slow speed = low RN, meaning there might be a benefit to some form of boundary layer control on a bike frame for sure. Aerodynamically, a bike is pretty dirty though. There’s lots of stuff spinning away upstream, not to mention an airflow that is hardly ever uniform due to the earth’s own turbulent boundary layer. I’d wager in a sterile wind tunnel, you’d see measurable gains, but bike manufacturers likely don’t dimple their bikes mainly due to aesthetics and marketing. I was thinking of testing some zig-zag tape that I still have in a box somewhere from my sailplane racing days with Golden Cheetah to help measure the change, but it’s kind of low on my priority list.
Dave
The general concept of causing the airflow to become turbulent (higher energy) and preventing it from becoming detached can also be caused using other methods.
In aircraft leading edges there can be slat and slot arrangements. These are more complex than dimpling, but offer better control and optimization possibilities across a range of speeds and angles of attack, especially for airfoil shapes.
Another common system is given the fancy name “vortex generators” which are basically things sticking out of the surface to disrupt airflow.
Good question. Fun topic.
To steer the subject on a tangent a little, the management of the transition from laminar to turbulent flow has applications in cycling. Textures in speedsuits are designed to do just the same thing as dimples do to a golf ball.
I believe because of the high speeds they’re going they’re effectively at very low yaw angles, so it’s more useful to optimize shape and reduce skin drag (which is more dominant at high speed) than to induce turbulent flow (and increase skin drag) from a suboptimal shape.
Aero shaped objects have proportionally more skin drag than do bluff bodies. The extremes are a flat plate in two orientations:
- if the plate aligned is parallel to the flow, then the drag is all skin friction drag, while pressure drag is zero, however
- if the plate is perpendicular to the flow, the the drag is all pressure drag and no skin friction drag.