Is a rough surface aerodynamically 'slipperier ' than a smooth one?

[Peter.]

I've seen it suggested a few times that a rough surface allows air to flow over it with less friction than a smooth one, but I can't help thinking that if it were true then planes, boats etc would not have nice shiny surfaces on them.

What is the truth of the matter?

[baldysm]

Not a physicist, but rough makes no sense to me to reduce friction. Airplanes take pains to recess rivets and screws in the body to reduce drag.

A rough surface would create turmoil with the air molecules, thereby increasing drag or friction.

[flathead4]

There might be a unique set of conditions where this could be demonstrated in a lab, but it probably wouldn't scale or it would be otherwise impractical for planes, train and autos. If it was true and practical at the macroscopic level, it would have been discovered a 1000 years ago and be in wide use.

Tom

[vpt]

From what I understand both are kind of needed. Trailing edges of "stuff" does better if it has a rough finish to break away the air/water from the vessel while the front side normally works better if smooth to flow the air/water over the vessel.

But looking at something like a canoe that has no trailing edge it works better to be smooth front to back so the water just flows around the vessel leaving the vessel at the rear most point that has almost no edge to create turbulence.

A plane is built almost the same way with minimal trailing flats/edges.

If they could build a golf ball in the shape of the rain drop they wouldn't need the dimples.

[Toolguy]

Golf Ball

Golf balls have dimples to decrease drag, but that may only work on a spherical surface - I don't know what the parameters are. Evan probably does though.

[Peter.]

I thought golf ball dimples were to provide lift from backspin somehow?

[rohart]

A smooth surface will in almost all cases allow a thinner boundary layer than a rough.

What is much more important is the shape of the object that is moving through the air. Where you don't want smoothness, and you want rough, but on a massive scale, is at the back of a car. Fast back cars are correctly named - a flat back lets the boundary layer break away properly. A large eddy is then entrained behind the car, and this air will be at relatively high pressure. But if the car tapers, like the old duck-arsed sports cars, then the boundary layer may never break away, and there will be low pressure, almost suction, at the back of the car.

With motorcycles, the airflow more or less breaks away as soon as it flows past the rider's body, so the analysis doesn't apply. I guess aircraft have tapered rears to avoid vibration from the oscillations in the turbulence, and to reduce noise.

Caveat = I'm very rusty on all this.

[loose nut]

"Somewhere" on TV I watch a show that had a stick on clear plastic that had very small "bumps" on it that could be applied to the surface of a car body etc. to reduce drag and give better gas mileage. The bumps break the air flow off of the skin of the body so that there is less surface/air contact and then less drag. It seemed to work OK, there may be an end to smooth shiny cars in our future.

[Peter.]

Funny that as one of my main reasons for asking is that a few years ago at a landspeed meet I saw that a guy had sprayed a textured 'grainy' coating all over his bike, in a bid to make it go faster.

[flathead4]

I thought golf ball dimples were to provide lift from backspin somehow?

You did not define "rough" and "smooth." I was thinking sandpaper and glass, respectively. What were you thinking?

Tom

[Lew Hartswick]

More than you ever wanted to know about that subject can be found
in the field of sail plane wing design. The idea is to separate the flow
at the appropriate location on the top of the wing. So the front is a
smoothe as possible and at the right place a "turbulator" strip is
located. This reduces the DRAG caused by the flow on the rear part
of the wing.
That is where a lot of the "misconception" occurs. You need to get
into Reynolds numbers etc to really get a handle on it.
And another thing to bear in mind: Characteristics of fluid flow DO NOT
scale. :-)
...Lew...

[danlb]

It was hardly scientific, but the Mythbusters did an episode where they applied a thick coating of something like clay to a car and sculpted dimples that were pretty big. The dimples were scaled to car size.

http://dsc.discovery.com/videos/myth...-minimyth.html


The results were surprising. The car got significantly better gas mileage. Thy do use a reasonable way to determine gas usage, BTW.

I think it had to do with creating a laminar flow that was better than the original body. That could be totally wrong.

Dan

[gizmo2]

+1 on the Mythbusters show. Also I hear sharks are a bit lumpy on the surface, so maybe Ma Nature has this figured out better than we do.

[Alistair Hosie]

Sharks aren't lumpy but very rough on the epidermal regions.In other words skin like sandpaper. Also skate fish is rough and jagged.I have always been taught the smoother a surface is the less drag. Alistair

[vpt]

I can't see the mythbusters video. Did they get rid of the door, hood, trunk gaps? Did the weight of the clay lower the car? Did they change the shape of the car at all besides the dimples? It all makes a big difference. Did they try a smooth front and dimpled back? Was the car waxed for the first text? A waxed/clean car will get better mileage than a dirty rough car.