Beefmoney From United States of America, joined Oct 2000, 1113 posts, RR: 4 Posted (11 years 9 months 3 weeks 3 days 23 hours ago) and read 18504 times:
Hi all, Last week I was out playing golf and as I was sitting in the cart and studying the dimples on my golfball, I remembered watching a TV show where they said that the dimples on a golfball are used to increase lift and decrease drag on the surface of the ball. Then I thought, why couldnt this dimple pattern be used on an aircraft wing or body to decrease drag?
I asked my father who is a former CFI and he said that he really had no clue, and that I should ask it here, so if anyone could give me an answer I would appreciate it.
Si02y From Singapore, joined Nov 2001, 23 posts, RR: 0
Reply 1, posted (11 years 9 months 3 weeks 3 days 22 hours ago) and read 18460 times:
A properly struck golf ball leaves the club face with back spin. The presence of dimples can be seen to enhance the "circulation" (fluid mechanics potential flow theory), thus resulting in better lift characteristics or alternatively limit the boundary layer growth.
Some aircraft do use the same principles to enhance lift, though not with dimples, but with vortex generators, as can be seen on the leading edge of the 777 wing. These protrusions on the upper wing surface, generate vortices that reduce flow separation on the suction side of the wing, delay and reduce the chord-wise boundary layer growth rate.
Dimples are easier to produce for a small golf ball, but can you imagine the enormity of the task to dimple an aircraft fuselage. Also, apart from reasons for ease of manufacture, there is also the need to address repair schemes resulting from bird strike, impact damage from vehicles (air-tugs, cherry-pickers, cargo loaders, catering trucks etc (people who work in airlines or airports see this all the time!). Flat panels are for these reasons the most viable solution.
Jetguy From , joined Dec 1969, posts, RR:
Reply 2, posted (11 years 9 months 3 weeks 3 days 21 hours ago) and read 18410 times:
A dimple pattern was used on certain aircraft tires during WWII. I'm not sure what the reasoning was for sure, but I think that the theory behind it was that if they could get the tires to start rotating prior to touchdown it would save some tire wear.
Ikarus From United Kingdom, joined Jan 2001, 3524 posts, RR: 2
Reply 3, posted (11 years 9 months 3 weeks 3 days 20 hours ago) and read 18392 times:
Dimples on golf balls. Invented by one of my professors...
The main reasoning for dimples on golf balls is about bluff body aerodynamics and boundary layers. A boundary layer is the air immediately around the object in an airflow - at the surface of the body, the air moves with the body, so the relative airflow velocity is zero (relative to the body!). From there, it increases, so that a few millimetres from the surface, the airflow velocity is almost identical to the airflow velocity infinitely far away from the body.
Now there are two main types of boundary layers: turbulent and laminar. In a laminar layer, the air moves mainly parallel to each other, shearing, but not intermingling. In a turbulent layer (or any turbulent flow), there are significant random movements of air packets (eddies) in all directions, also cross-flow. The end result is that a turbulent boundary layer has much more momentum close to the surface, but takes much more distance (from the body) for the flow to reach the free stream velocity.
Now here comes the crucial part: What does a golf ball do? Imagine it, for a moment, as a circular cylinder, in an airflow. Basically, in 2D, a circle placed in oncoming airstream. What happens to the streamline, the air? It moves around the cylinder. And theoretically, behind the cylinder, it moves back together - potential flow (i.e. ideal, unrealistic) flow does.
But what does real airflow do? It begins to move around the cylinder. If the cylinder has a scale of angles painted on it, with zero facing directly into the airflow, then at 90 degrees, the air has been squeezed together the most, and should be the fastest. So there the pressure is lowest. (Using Bernoulli: Along a stream line, total pressure is constant - total pressure being static pressure plus 1/2 times density times velocity squared. So faster air has lower static pressure). That means up to that point, the air is accelerating, and the pressure gradient moves from higher pressure to lower pressure, pulling the flow (and the boundary layer) along.
But behind that location, at 180 degrees, the flow should - in theory - be back to normal. I.e. back to a higher pressure. So now the flow is fighting against the pressure - it is moving from a low pressure zone to a high pressure zone, decelerating. What does that mean for the boundary layer? At the surface boundary, the velocity is zero, as stated. But a tiny distance from the surface, say half a millimetre, for example, the air is moving at some velocity. So there is a velocity gradient in the boundary layer. If the pressure gradient fights against the velocity gradient, it shifts the velocity gradient, until the velocity changes direction - i.e. the flow actually moves backwards, against the airflow, near the surface! This leads to separation. Separation means a highly chaotic, energetic airflow behind our circular cylinder, and if the air suddenly has all that energy, it needs to come from somewhere. It comes from the force it takes to hold the cylinder still in the airflow - it is drag.
So what is the big difference between turbulent and laminar boundary layers? Turbulent boundary layers have a much higher velocity gradient near the surface, because they have much more momentum (due to the random crossflows). So the adverse pressure gradient needs to fight a lot longer until it manages to turn the boundary layer around and cause separation. So a laminar flow separates near to 90 degrees (actually, even slightly ahead of the 90 degree location in reality), but the turbulent flow clings on to the surface until, say 130 degrees, meaning the separated (drag-inducing) region is much smaller.
If you put dimples on your cylinder, this simulated a rougher surface, and causes the flow to be turbulent. So the most basic and essential result is less drag.
What about the rotating golf ball? Well, any rotating cylinder - even without dimples - will generate lift (if it rotates inside an airflow!), because it induces vorticity. The most basic and simplified formula is Lift equals density times velocity times vorticity. So the dimples can enhance that effect a little, but they do not cause it.
There is one other interesting example: Some balls (for some sport I don't know) have a thick circular leather ridge around them. If you throw them in the air with the ridge at the right angle, it will act as a trip wire (i.e. cause turbulence) on one half of the ball, keeping the flow attached for longer and causing less drag, while on the opposite side, the ridge is so far back that the flow is laminar until the (early) separation point, causing more drag on that side. The end result is a moment on the ball, causing it to fly a curved path! (so you could, with skill, throw a ball into the air that begins climbing more steeply up before losing momentum and tumbling back down)
Now back to the original question: Dimples in golf balls, trip wires and other methods are there to induce turbulent flow and turbulent boundary layers. That makes sense for bluff bodies only. Because sleek, aerodynamic bodies (like an airplane wing for example) keep the flow attached even if it is laminar, by being as gradual as possible in joining the flow back together. And laminar airflow causes less skin friction drag than turbulent one. So an airplane would not benefit from dimples or trip wires.
Main points again:
* turbulence needs energy, causes drag.
* Separated regions are full of giant, strong turbulence.
* Small (dimple-induced) turbulence near the body surface, i.e. a turbulent boundary layer, causes flow to stay attached to the body for longer. So the separated region (with all the giant turbulenc) is smaller, and the energy required is smaller - you have less drag.
I hope this explanation makes any sense without diagrams. It's kinda hard to explain Fluid Dynamics without lots of pretty pictures...
SSTjumbo From , joined Dec 1969, posts, RR:
Reply 6, posted (11 years 9 months 3 weeks 3 days 6 hours ago) and read 18279 times:
...must I expose the tech_ops world to this picture to make a point?
See what happens when clever ideas go wrong?
No seriously, I don't know if this is right (help here, Ikarus?), but the way I've always seen it, not knowing if it's scientifically correct or not, is that the dimples grab more air, so when you impart enough backspin on the ball, the golf ball will hold the air underneath longer because the dimples are trying to throw it back forward and the air on top is moving faster because the dimples are trying to throw that air backwards, thus, brounelli's principle is in effect. the same kind of things will happen on other ball surfaces, like when you see a soccerball make a spectacular mid-air turn. On airplane wings, I would think that you don't have backspin on the wings, and the dimples would not only contribute to extra skin drag, but might create interference drag as well, plus, if it is put in practice on the fuselage, the monocoque would fail completely IMHO.
Ikarus From United Kingdom, joined Jan 2001, 3524 posts, RR: 2
Reply 7, posted (11 years 9 months 3 weeks 3 days 5 hours ago) and read 18265 times:
SSTJumbo: so when you impart enough backspin on the ball, the golf ball will hold the air underneath longer because the dimples are trying to throw it back forward and the air on top is moving faster because the dimples are trying to throw that air backwards, thus, brounelli's principle is in effect.
There's just one problem with that explanation: It works fine without dimples, too! Then, the surface of the ball accelerates the air on one side, and decelerates it on the other. In fact, there has been a ship driven not by sails, but by a giant rotating cylinder attached to the mast. (British designers... what more can I say!) Dimples may or may not improve on that behaviour, I'm not sure, but they aren't the cause.
Fundamentally, they are there to induce (boundary layer) turbulence and thereby prolong the distance for which the flow remains attached (by having more flow momentum close to the surface), and reduce the separated region.
On an aircraft wing, dimples would increase skin friction drag without reducing the separated region - provided the airfoil shape is well designed. So overall, their effect would be negative.
There is, however, an Airbus project flying around to oppose that conservative view: Sharkskin on wings (i.e. dimples). I have no idea what the results of that project are, but the theory behind it is more or less the same - they want to cause small separation and fast reattachment, hundreds of times, and hope that the airflow will separate even later than it already does on the airfoil. I'm sceptical about whether or not it works...
Ikarus From United Kingdom, joined Jan 2001, 3524 posts, RR: 2
Reply 11, posted (11 years 9 months 3 weeks 2 days 10 hours ago) and read 18153 times:
Boeing_nut: Perhaps I should check whether my professor invented the dimples, or merely investigated (or perhaps improved) them. Some fellow student told me he had invented them. But I never bothered to look it up. Look for Profs Harvey and Bearman, publications around 1976...
MD-90 From United States of America, joined Jan 2000, 8506 posts, RR: 12
Reply 15, posted (11 years 9 months 2 weeks 5 days 15 hours ago) and read 17871 times:
I dunno, it looks to me like Piper made the tails like that for strength, not for aerodynamic purposes.
Anway, one of the first things we looked at in my intro to aerospace engineering lab were pictures of a sphere that was perfectly smooth inside an airflow that had smoke particles added so that you could see the airflow. around the mid point of the ball, the air separated, and you could see the vortices and such behind the ball, creating drag, as the main airflow smoothly went around it and rejoined together behind the ball.
However, there was another sphere that had a small ridge abour 2/3 up the ball (towards the top and bottom). That ridge "tripped" the airflow and kept it attached to the backside of the ball for a ways, and dramatically reduced the size of the drag bucket (okay, I know it's not a real drag bucket, like when you refer to wing designs, but it sounds cool) behind the sphere. It wasn't a golf ball, but it worked on similiar principles (I think).