morecy
Posts: 205
Joined: Fri May 19, 2000 4:07 am

Dimples

Wed Nov 17, 2004 2:28 pm

On PBS tonight I watched a show on the dam-busting bombs developed during WW-II. Anyway, they explained that during the initial development of this bomb, a dimpled surface (much like a golf ball) was used because of it's superior aerodynamics... hence why golf balls are dimpled. My question is; why isn't this concept used on modern airliners?
 
cancidas
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RE: Dimples

Wed Nov 17, 2004 2:38 pm

machining a smooth surface is easier than machining a dimpled surface in sheet metal. also, if i remember correctly the dimpled surfaces only enhance aerodynamics for round objects. that's if i remember, but i was asleep in most of my aerodynamics classes.

[Edited 2004-11-17 06:39:31]
"...cannot the kingdom of salvation take me home."
 
QantasA332
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RE: Dimples

Wed Nov 17, 2004 3:52 pm

Before I answer your question, Morecy, I'll expand a bit on what exactly the dimple principle is. Settle in.  Big grin

Basically, there are three primary types of drag acting on an aircraft: induced drag, skin friction drag, and pressure drag. It is pressure drag that is the main factor involved in the dimple design's existence. Pressure drag is primarily the result of a moving body's wake. Depending on how soon the airflow separates as it passes over an object - that is, how far along the object the flow travels before no longer following the contour of the object - the size of the wake will be larger or smaller. A larger wake equates to more pressure drag (put simply, there is a larger region of stagnant air behind the body meaning the airflow pushing on the front of the body has less impeding its production of drag) and vice versa.

Now, imagine a sphere. Because its height/diameter is large in comparison with its length, it is what's known as a "bluff body." Bluff bodies such as a sphere have disproportionately large wakes, and as a result they have disproportionately high pressure drag. (This is compared to both their own skin friction drag and a not-bluff solid's pressure drag). Obviously, then, overall drag on a sphere (or other bluff body) can be dramatically reduced if pressure drag is reduced. That is, pressure drag is what you want to specifically target and minimize.

Enter dimples. Dimples turbulate the airflow over an object, thus increasing the flow's kinetic energy. This acts to delay flow separation, which then leads to a smaller wake, which in turn leads to less pressure drag. And this solves the bluff body problem! Because bluff bodies have such high pressure drag compared to their skin friction drag, what little extra of the latter drag is created by dimples is more than offset by the drastic reduction of the former drag. So a golf ball - the classic example of a bluff body - will travel farther with dimples than without, and that is of course why they have come to carry these dimples.

Now, to finally answer your question: 'normal' aircraft are very simply not bluff bodies. Dimples would create more skin friction drag than they would reduce pressure drag, defeating their purpose.

I've probably told you more than you needed to know, but hopefully that was at least fairly understandable, and not too boring!  Big grin

Cheers,
QantasA332
 
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Starlionblue
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Joined: Fri Feb 27, 2004 9:54 pm

RE: Dimples

Wed Nov 17, 2004 8:13 pm

Great explanation Qantas332! I had always wondered about that.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
c172heavy
Posts: 105
Joined: Sat Aug 07, 2004 9:39 am

RE: Dimples

Thu Nov 18, 2004 2:41 am

Yeah, I'm the wiser because someone stayed awake through their aerodynamics class! But if I may, I have a follow-up question; why does the regenerated air stick to the objects surface? If I'm understanding correctly, the dimples act like chines/vortex generators on an aircraft surface, and I can't grasp why this turbulent air helps delay boundary layer breakaway.
"How's that working out for ya?....Bein' clever?"
 
morecy
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RE: Dimples

Thu Nov 18, 2004 2:58 am

Thanks for the replies !!!!
 
Staffan
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RE: Dimples

Thu Nov 18, 2004 5:51 am

C172heavy,
The fluid particles in a tubulent boundary layer have more momentum than the ones in a laminar one. That is what prevents the early separation.
In other words, the more kinetic energy the particles have, the longer they can keep flowing against the adverse pressure gradient.

Staffan
 
QantasA332
Posts: 1473
Joined: Sat Dec 13, 2003 5:47 pm

RE: Dimples

Thu Nov 18, 2004 5:51 am

But if I may, I have a follow-up question; why does the regenerated air stick to the objects surface? If I'm understanding correctly, the dimples act like chines/vortex generators on an aircraft surface, and I can't grasp why this turbulent air helps delay boundary layer breakaway.

Just remember that turbulent airflow has an inherently higher kinetic energy than laminar flow. Because of this high kinetic energy, turbulent flow passing over an object basically just has more energy than laminar flow to combat adverse pressure gradient and boundary layer friction on the surface, and as a result it travels further than laminar flow before succumbing to the aforementioned adverse effects.

I'm trying to think of a good example, but I can't really come up with one. Hopefully the above explanation was sufficient.

Cheers,
QantasA332
 
Logan22L
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Joined: Fri Jul 16, 2004 5:59 am

RE: Dimples

Fri Nov 19, 2004 1:42 am

QantasA332: Wow, when I clicked on your profile, I expected to see age: 36-45; occupation: aeronautical engineer. Thanks for enlightening me; your post was excellent.

Logan
"The deeper you go, the higher you fly. The higher you fly, the deeper you go."
 
cancidas
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RE: Dimples

Fri Nov 19, 2004 1:27 pm

hey forgive me for sleeping in aerodynamics class. always fell the morning after rugby practices that usually ran till like 2100. then there was always the kegger too! Big grin
"...cannot the kingdom of salvation take me home."

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