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What's the Most Aerodynamic Shape?

06/13/2008 11:28 AM

If you were to come up with a shape that in wind tunnel testing (real world) would have the least drag, what would it look like? That is what is the most aerodynamic shape?

I have always had it in my mind that a rain drop shape was the most aerodynamic, but is this true? I have never heard any proof stating this, it was just something I was told or heard somewhere.

If it is a rain drop shape, what proof exists?

I do have some constraints, in that the shape must have some volume, it can not be a sheet of paper. The shape should be such that it works best in mid air so no ground effects need apply.

Shapes I commonly think of when I think aerodynamics are car shapes, rocket shapes, and golf ball shapes.

Additionally does size make a difference, is their a shape that is universally the best?

This is just a question for discussion and curiosity, I am not designing anything.

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#76

Re: What's the Most Aerodynamic Shape?

06/18/2008 1:52 PM

Most of these postings are flawed because they cite examples of engineered shapes that have functional requirements beyond minimizing aerodynamic drag.

Golf balls have to be a shape that can be struck by a golf club and meet the limitations of golf rules. Row boats have to carry rowers. Airplanes have to lift engines and payloads and they have to be stable and maneuverable in flight. Bombs have to carry explosives and a trigger inside. All of these shapes compromise drag to some degree to perform their primary functions.

Even the example in nature, the rain drop, is a shape that is influenced by cohesive forces and other physical properties.

The original question asked which shape minimized drag in an ariflow, and had no other requirements (except to have a measurable volume). So forget the engineered examples. The shape that answers his question can be determined by aerodynamic theory and the design should start on a blank sheet of paper.

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#77
In reply to #76

Re: What's the Most Aerodynamic Shape?

06/18/2008 1:58 PM

I agree, as I said in my first post (number 2) that the ideal shape should be a shape that tends towards having no volume!!

John.

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#78
In reply to #77

Re: What's the Most Aerodynamic Shape?

06/18/2008 2:36 PM

Yup. The question of scale relative to air molecules is also interesting.

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#80
In reply to #76

Re: What's the Most Aerodynamic Shape?

06/18/2008 3:28 PM

I suspect that one need not be so literal-minded, particularly because the original post is presented in a colloquial, non-technical sense.

I seriously doubt that the original poster was thinking terms of protons or single atoms being the object to be enclosed by this aerodynamic shape. Without information to the contrary, I think we could reasonably conclude that the original poster might be thinking in terms of the things we see around us that are "streamlined": sailplane fuselages, airplane fuselages, tip tanks on airplane wings, torpedoes, bombs, etc. If that is what he has in mind, then spinning a 0025 section into a torpedo shape (as in a Tallboy bomb) is hard to beat.

Of course, if we answer more theoretically, and say that first you have to have an idea of the Reynolds number, compressibility, etc. then we would not want to advise starting with a clean sheet of paper. Better we should advise to start with a good 3D CFD program integrated into a good 3D CAD. Flomerics can supply this for $30,000 per seat. I suspect the original poster will not make that sort of investment, given that he said he was just curious. To give him the answer that he should start with schooling in aerodynamics and a clean sheet of paper cannot be a very satisfying answer for him can it?

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#82
In reply to #80

Re: What's the Most Aerodynamic Shape?

06/18/2008 6:20 PM

GA again....

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#83
In reply to #82

Re: What's the Most Aerodynamic Shape?

06/18/2008 6:52 PM

Danke

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#84
In reply to #80

Re: What's the Most Aerodynamic Shape?

06/18/2008 9:05 PM

Being literal minded is usually a good approach when an engineering questions is raised.

And since I can read minds as well as anyone else on this forum, I have re-read the original poster, and I still believe that he is inquiring about a theoretical shape, free of constraints other than a minimized drag coefficient.

He is posing a theoretical aerodynamic question, so why should looking to aerodynamic theory be such a reach? And why should I assume he wants a dumbed-down suggestion? How about a well thought out answer, stated in simple terms?


These other posts are fun to ponder, but they don't answer the question. A few have been on target and did not require $30,00 software - just college level aerodynamics and some thought.

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#91
In reply to #84

Re: What's the Most Aerodynamic Shape?

06/19/2008 5:01 AM

I simply do not agree with your words here, nothing could be farther from the truth in reality when you wrote:-

And since I can read minds as well as anyone else on this forum,(Not as good as most may I say at this point ....!) I have re-read the original poster, and I still believe that he is inquiring about a theoretical shape, free of constraints other than a minimized drag coefficient.

He is posing a theoretical aerodynamic question......

Here are some excerpts from the original posters original question showing quite clearly that he wants a "Real World" (he even wrote that!!) answer, not just theory and impractical suggestions. Please go back and re-read what you re-read again for final proof! I cite the following:-

If you were to come up with a shape that in wind tunnel testing (real world) would have the least drag, what would it look like? That is what is the most aerodynamic shape?

I have always had it in my mind that a rain drop shape was the most aerodynamic, but is this true? I have never heard any proof stating this, it was just something I was told or heard somewhere.

If it is a rain drop shape, what proof exists?

I do have some constraints, in that the shape must have some volume, it can not be a sheet of paper. The shape should be such that it works best in mid air so no ground effects need apply.

So please re-read the original question, take your time over it and then you may find a reason or two to alter your opinions!!!

Speed reading has a brought a lot of failures and misunderstandings into this world, try and not be one of those people who do that, it never helps......

Speed readers (at least good ones!) learn how to shift down a gear to make sure that they FULLY and CORRECTLY understand what they are reading when it is important to do so.

They also learn when and where to do this.....otherwise they would make serious mistakes in understanding.......It tells me that you most probably went to college or studied at some point, and are probably around 35 years old give or take 10 years. Am I anywhere near? (I have not looked at your profile when trying to guess your age, just your comments, so I might be way off!!!)

I learnt many years ago in the computer world the hard way, when NOT to speed read!!!

Have a great day.

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#93
In reply to #84

Re: What's the Most Aerodynamic Shape?

06/19/2008 12:14 PM

Being literal minded is usually a good approach when an engineering questions is raised.

Perhaps you are quite young and inexperienced, but as the years go by, (or if you decide to learn a little about communication theory) you will find that being literal-minded is often not the best approach, no matter what the subject. (Even the term "literal-minded" has connotations beyond its literal definition: it can suggest a certain dim-wittedness or rigidity of thought, for example. I suspect that you did not intend that meaning.)

And since I can read minds as well as anyone else on this forum,

You'd have to provide evidence of that. It seems that you might very well be less able to read minds than others. There is a large body of work on associating thoughts with brain electrical activity patterns, and it is not unreasonable to assume that two brains, brought close together, might influence one another. We see this all the time in sensitive electronic equipment. If one person were able to read the mind of another, it would be reasonable to assume that a good deal of practice and perhaps training would be required. Have you had such practice and training?

The original poster, very early on, wrote that he did not mean raindrop (which most people realize are not well streamlined) but instead the classic teardrop shape. Of course teardrops are not "literally" shaped in the "classic tear drop shape." Once they have "dropped" they quickly become approximately spherical. However, many of us, using our ability to read for meaning (i.e non-literally) were able to conclude that he probably had the sort of "teardrop shape" in mind that is associated with torpedoes, etc. A teardrop, even when clinging to a face, does not have that shape in any literal sense.

Being literal-minded can be a handicap, and if you find that you suffer from such a handicap, please don't take offence. Perhaps you are engaged in some endeavor in which you can cope quite well.

In this forum, at least half the questions posed are impossible to answer if read in a strictly literal sense: many have errors in syntax that render them nonsensical if read literally.

Even today, with pretty good 3D CFD available, the wind tunnel is still the gold standard for measuring drag, and flipping through a copy of Abbott and Doenhoff (which is loaded with real-world wind tunnel data) can save many hours of rediscovering the wheel through CFD or wind tunnel time.

I find little in the other answers that is fundamentally "flawed". They offer a range of perspectives on the matter. If the poster wanted a textbook answer, then he'd go to a library, right? But if you have a "well thought out answer, stated in simple terms," I'd love to read it. Your first post only hints that the answer lies in a clean sheet of paper and aerodynamic theory. Had the Wright brothers worked only with theory and without hundreds of experiments in their wind tunnel, we would not remember them -- others would have beat them to the punch. (No, I am not referring to pugilism.)

My guess is that the sort of answer the OP was looking for is closer to that found in this link, which was provided by one of the other respondents. This link fails, however, to suggest a thickness ratio for the airfoil -- but there is wind tunnel data that does suggest an appropriate thickness ratio.

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#90

Re: What's the Most Aerodynamic Shape?

06/19/2008 3:23 AM

It must be the red shape...

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#92

Re: What's the Most Aerodynamic Shape?

06/19/2008 9:56 AM

This is an interesting question.

Raindrops are a big factor in weather. With the very significant interest that exists in weather, raindrops have been studied in great detail, theoretically, in situ and in wind tunnels. Wind tunnel work, frequently cited, was done by Pruppacher and Beard.

A basic description of the raindrop shape citing their work is easily accessible in the abstract for this document:

"A New Model for the Equilibrium Shape of Raindrops;" by Kenneth V. Beard and Catherine Chuang; Department of Atmospheric Sciences, University of Illinois at Urbana-Champaign, IL 61801; Journal of the Atmospheric Sciences; Article: pp. 1509–1524 | Abstract | PDF (1.42M)

It states, in part: "The model yields the peculiar asymmetric shape of raindrops: a singly curved surface with a flattened base and a maximum curvature just below the major axis." Basically, small raindrops are spherical. As they get larger they become a flattened sphere. Neither is the most aerodynamic shape around.

One must suspect you were thinking of a "teardrop" shape, which is used ubiquitously for efficiency in aerodynamic applications. However, that shape is bit like the Supreme Court's view of obscenity. While the shape is not mathematically defined, we all know it when we see it. In any case, even if it became defined there is no reason to believe it to be the "best."

In aero, by the way, size makes a difference.

That puts most of your questions "to bed." As to "what is the most aerodynamic shape?" that is a more complex issue. Peak efficiency is very dependent on application. Evolutionary algorithms are often used to find a likely peak. This is because there are many characteristics to be considered and outcomes are usually unpredictable my any other means.

Finally the rumor, it was once said that the lowest drag shape was a sphere with "dimples" – two of them – one directly opposite the other. It makes no particular sense to me and I have never seen evidence to support it, theoretical or empirical. Just a thought.

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#94
In reply to #92

Re: What's the Most Aerodynamic Shape?

06/19/2008 12:49 PM

v1sor,

"Peak efficiency is very dependent on application."

Seems that we are seeking peak efficiency for a shape that is not compromised by application, as the only requirement is wind tunnel testing. Would you agree?


I remain skeptical of the raindrop. It is clearly shaped by equilibrium, but I wonder if that implies peak aerodynamic efficiency.

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#95
In reply to #94

Re: What's the Most Aerodynamic Shape?

06/19/2008 3:09 PM

I remain skeptical of the raindrop. It is clearly shaped by equilibrium, but I wonder if that implies peak aerodynamic efficiency.

It seems very improbable that it would imply peak aerodynamic efficiency. Consider a parachute. Fortunately they only take their "natural" low drag shape on the rarest of occasions -- if that were not the case, skydiving would not be considered sport so much as slaughter. Instead, the shape is determined by the structure of the device and the forces acting on it, and a parachute normally takes a high drag shape.

In the case of the rain drop, the structure (determined largely by surface tension and intermolecular cohesiveness) is fairly easy to deform, but we've known for many decades that it does not deform into a streamlined shape. We also know that many shapes that are reasonably streamlined are also inherently unstable, an airplane wing being the most obvious example. So without evidence to the contrary, we'd expect that raindrops would tumble in flight and that the leading side would be flattened by simple momentum effects.

If raindrops were to naturally shape themselves to a streamlined shape, then why do soap bubbles not do likewise?

Rivers, given time, meander and become more resistant to flow. Water and sand, subjected to constant wind, take shapes that increase, rather than decrease, surface friction. More broadly, stuff in general tends toward greater disorder, rather than less, and we know from watching streamlines in wind tunnels that ordered flow is less draggy than disordered flow. So there is little to suggest that a streamlined shape would be naturally created by fluid flow.

Seems that we are seeking peak efficiency for a shape that is not compromised by application, as the only requirement is wind tunnel testing. Would you agree?

V1sor might, but I would not. Wind tunnels operate under quite limited conditions, and are physical devices into which a physical device to be tested must be introduced and mounted on a balance. The size of the balance dictates reasonable limits on model size and the strong influence of Reynolds number on drag effects also dictates a particular model size. Putting a model in a wind tunnel is, in itself, an application. This is pretty obvious for any test, and many wind tunnel tests do not reflect real world conditions at all (the classic example being wing section tests in which the model spans the entire tunnel: there are no real world wings of infinite span.) Accurate automotive studies require a tunnel with a moving floor in order to make the wind tunnel results correlate strongly with real world studies.

I doubt that there is any shape uncompromised by application. The dimples on a golf ball are effective at reducing drag at low Reynolds numbers but increase drag at high Reynolds numbers. So without an application (size and airspeed) in mind you can make no conclusions -- what applies at one scale does not apply at another.

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#96
In reply to #95

Re: What's the Most Aerodynamic Shape?

06/19/2008 8:20 PM

Let's address it in terms of velocity and nothing else. Generally speaking, at subsonic speeds the minimum drag is on some variety of a teardrop shape. Think USS Albacore, the first US teardrop-shaped submarine. Teardrop trailer:

Or the EA-6B Prowler jet aircraft.

(These pix were from wikipedia.)

But, if the application is transonic or supersonic then the lowest-drag shape is what is called a Sears-Haack Body.

Here is the Haack minimum drag bullet.

And, the V-2 cruise missiles of WWII.

Russian VA-111 Shkval Torpedo

Optimum aerodynamic shape (minimum drag) is driven by application.

So, just for clarity lets compare a teardrop and a raindrop

A drawing of a raindrop (from a very good site at http://en.wikiversity.org/wiki/Waves_in_composites_and_metamaterials/Rainbows )

Raindrop can be spherical and smashed spherical, but not teardrop which is the shape traditionally associated with a raindrop (but wrong.)

Now, is it clear that raindrop is not a teardrop, teardrop is not a defined shape and minimum drag shape is a matter of how the thing is used?

r/ Sam

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#98
In reply to #96

Re: What's the Most Aerodynamic Shape?

06/20/2008 1:55 PM

Generally speaking, at subsonic speeds the minimum drag is on some variety of a teardrop shape...if the application is transonic or supersonic then the lowest-drag shape is what is called a Sears-Haack Body.


I believe you've nailed the question. Can you suggest a link or reference regarding the teardrop shape, as with the Sears-Haack body? I suppose airfoil theory might cover the teardrop, but then again it is three dimensional.

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#99
In reply to #98

Teardrops and Drag

06/20/2008 8:51 PM

"Can you suggest a link or reference regarding the teardrop shape?"

OK. There is a real good discussion of golf balls and drag at http://www.aerospaceweb.org/question/aerodynamics/q0215.shtml Works its way up to comparative shapes (including teardrop) and aircraft. It is written by an aerospace engineer named Jeff Scott. A good piece.

NASA has a Beginner's Guide to Aerodynamics (BGA) site. Right hand column has a section titled "Drag" which includes an item on shape. It addresses the "airfoil" shape, which you will recognize as a teardrop.

Any introductory aero text book should address the matter of drag with a discussion of this ubiquitous shape. It is unlikely to include a standard to define the shape.

BTW: The US Geological Survey site has a nice discussion of raindrop shape and dynamics. Start at www.usgs.gov

Hope this helps.

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#100
In reply to #99

Re: Teardrops and Drag

06/27/2008 1:02 PM

Given the "airfoil" shape spun into three dimensions (then called axisymmetric or a "body of rotation") there is still the issue of slenderness ratio: to house a particular mass, is it better to make the shape fat or skinny? Above, I suggested that 25% thickness is a good compromise, and studies done at (I think) University of Birmingham, support this contention.

Tom Spear is an engineer with Boeing with a strong background in the theory and practice of aerodynamics and hydrodynamics (and he is an accomplished sailor -- where the two come into play). In this thread, he discusses some of the issues involved. One respondent claims 4.5 fineness ratio (22% thickness) is optimum. Tom replies that there is no single optimum, particular for the bulb on a keel, which operates at an angle of attack (which he does not specify -- but 4 degrees is typical). Tom mentions shapes as fat as 3:1 being effective, but for general purposes, in which the body is nearly aligned with the airflow, I think 4:1 is a good average, but Tom's analysis is well worth reading.

Of course, there is no substitute for a wind tunnel (or good 3D CFD). Once you are close, then you'd want to make models either side of what you feel is ideal, and test them. Relatively simple 2D CFD code can get quite close, provided you stay away from fully separated flow. But 3D code (and meshing) has to be quite good to serve as a virtual wind tunnel.

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#101
In reply to #100

Re: Teardrops and Drag

06/28/2008 2:50 AM

See what could be done with a real flying saucer - a round wing airplane durring the 40's!!!

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#115
In reply to #96

Re: What's the Most Aerodynamic Shape?

11/15/2009 8:34 PM

a "teardrop" is a shape in which a drop of water is hung from a needle or sharp object

a sears-haack body is just an altered "teardrop"

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#127
In reply to #96

Re: What's the Most Aerodynamic Shape?

09/18/2010 1:32 AM

I wouldn't have put the

in that set - it's somewhat different in attaining 'low drag'

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#102

Re: What's the Most Aerodynamic Shape?

06/28/2008 2:52 AM

Ask any zoo visitor... A monkey throwing its feces!!!

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#103

Re: What's the Most Aerodynamic Shape?

07/02/2008 2:43 PM

By looking at the replies, That is a good question,

To reduce even farther the microlaminars flows at the boundry layers, the next follow up question would be "What type of surface should this shape have?.

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#104

Re: What's the Most Aerodynamic Shape?

07/30/2008 11:05 PM

Aerodynamics as hydrodynamics vary with velocity.

A teardrop is subsonic and unpowered.

Aerodynamically , it only offers low resistance at low speeds.

Move through rain , or at speed , and you have to streamline it.

Aero/hydrodynamics are a little more interesting.

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#105

Re: What's the Most Aerodynamic Shape?

10/05/2008 12:25 AM

Busemann's Biplane

At supersonic speeds it has zero wave drag!!!

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#109

Re: What's the Most Aerodynamic Shape?

05/03/2009 10:33 PM

Hi guys I found your discussion because I'm asking the same question right now.

I'm studying in aeronautical conception /construction and I'm planning to build a new design of wing for a model test plane. The new aerodynamic model is based on a conventional wing design but with "tubercules" holes in the attack border.

Note-- This is not my model. It is a prototype for more efficient eoliens design in Canada right now.

My goal is to try this on an airplane. They say that reduce the turbulence and increase the portance. This is based on natural whales fins.

And to answer your initial question, the water shape is aerodynamicaly good. Not the best but good. What we have to consider is the speed of air too. At subsonic speed, the air reacts in other ways than supersonic. Thank you for posting.

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#112

Re: What's the Most Aerodynamic Shape?

11/15/2009 8:12 PM

the most aerodynamical shape would be something like a teardrop

the size of the teardrop wouldnt matter because the biggerscale the object gets the more drag is created

buteven less drag than the teardrop is a teardrop with the trailing edge cut off

this makes the air "fill" in the remaining trailing edge reducing the drag of the trailing edge, air over air doesnt equal drag

there is no way to reduce the drag of the leading edge, any shape with have greater than or equal to drag than a teardrop shape

this is coming from an aeronautical engineer so my point is valid

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#116

Re: What's the Most Aerodynamic Shape?

11/23/2009 3:15 PM

Look at fishses

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#118

Re: What's the Most Aerodynamic Shape?

01/16/2010 10:30 AM

I'm making a small battery car for a school project. I'm using plexiglas for the bottom but I'm not sure how to make it go fast! I made 5 cars so far and they all stink! I really need help. There is nothing on battery car making on google!

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#119
In reply to #118

Re: What's the Most Aerodynamic Shape?

01/16/2010 1:34 PM

Certainly nothing to do with aerodynamics (at the sort of speeds you're working with). Suggest you register as a CR4 member and post your question as a new thread.

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#123

Re: What's the Most Aerodynamic Shape?

09/17/2010 10:59 PM

better question would be... If you were to drop a 200lb object straight down through the atmosphere what shape would it need to get it to the ground the fastest? lets say starting 62miles above the earth surface.

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#124
In reply to #123

Re: What's the Most Aerodynamic Shape?

09/17/2010 11:26 PM

made solid all the way through

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#125

Re: What's the Most Aerodynamic Shape?

09/17/2010 11:28 PM

An object to carry a human to the ground...

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#130

Re: What's the Most Aerodynamic Shape?

02/12/2011 1:36 AM

The shape of a tear drop is the most aerodynamic. Look at an aeroplane wing. It is the shape of half a tear drop to give lift while providing minimum turbulence. As gravity pulls a teardrop towards the ground, air forces it into the shape of a teardrop. That is how I know that a tear drop is the most aerodynamic shape. I'm not an engineer or anything like that, but that's just what I think.

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#132

Re: What's the Most Aerodynamic Shape?

02/17/2011 10:40 PM

bnggghjghghghjghjggjh

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#133
In reply to #132

Re: What's the Most Aerodynamic Shape?

02/22/2011 2:13 PM

Is that some new computer language?

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#134
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Re: What's the Most Aerodynamic Shape?

02/22/2011 4:46 PM

First few terms of the DNA sequence of an imbecile.

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#135

Re: What's the Most Aerodynamic Shape?

09/18/2016 6:31 AM

HERE IS NO DEFINITE AERODYNAMIC SHAPE THE MOST SUITABLE SHAPES TO BE SELECTED FOR APPLICATION.

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#137

Re: What's the Most Aerodynamic Shape?

11/15/2016 4:24 PM

Most of the answers are way off. Also, to those dozens of people who keep saying a teardrop is the best shape, I want to say that A) as others have pointed out, water has strong cohesion so achieving an airfoil shape with a teardrop is impossible, and B) even if it could form any shape, it probably wouldn't form the optimal one since even in a zero skin friction scenario (aka the surface is not pushed back making the shape tailheavy) it is going to form a shape that has equal pressure, not minimum drag.

For the actual shape, as others have pointed out it will depend on lots of things like the flow regime, and how you define your object's "size." Before we can come to any sort of good answer we need to know the answer to both those questions. However, in the meantime I suggest we use frontal area as the size definition, and that we choose M<0.3 so no real compressibility issues, and Re>10^6 so we avoid the strange world of the low Re world. I don't have a definitive answer for you yet but this is a good start. I also wonder if the OP wanted the best 2D shape or the best 3D shape.

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