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I was reading this book that features a description of a shark:

It had fins at its sides, a triangular fin that rose from its back, a raked, aerodynamic tail, and eyes that were small, black, and empty.

(Emphasis mine.)

Now, since "aero-" means "air", and this tail travels through water rather than air > 99% of the time, it strikes me that aerodynamic may not be the best way to describe it?

Another example:

The submarine was beautifully built, with a smooth, aerodynamic design.


1. Is using "aerodynamic" here wrong, acceptable or good?

2. Is there a better word (or possibly phrase) used to describe it?

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    Is the word here meant as a kind of descriptive term for how good it looks, or a factual statement about the actual attribute (i.e. good at moving through water) – colmde Aug 11 '16 at 13:15
  • @colmde Both can be considered. – Revetahw Aug 11 '16 at 13:16
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    @Fiksdal if you consider both you get two different answers. I think you need to clarify who the intended audience is. If you're writing a pop culture fictional or dramatic account, then aerodynamic will suggest a sleek, streamlined appearance to most people. But if you are doing a documentary film, or are striving for technical accuracy, and your audience is one that will appreciate technical accuracy, then hydrodynamic is the more accurate term. – barbecue Aug 14 '16 at 14:14
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    @barbecue I've gotten several different answers, including all that you mention. I think they're all valid, and I don't think the question is too broad. – Revetahw Aug 14 '16 at 14:17
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Hydrodynamic is the right word. The notions of hydrodynamics and aerodynamics parallel each other: one is to air or gases as the other is to water or liquids. Here are the Merriam-Webster definitions:

Aerodynamics (M-W):
: a branch of dynamics that deals with the motion of air and other gaseous fluids and with the forces acting on bodies in motion relative to such fluids

Hydrodynamics (M-W):
: a branch of physics that deals with the motion of fluids and the forces acting on solid bodies immersed in fluids and in motion relative to them

And hydrodynamic has been used to mean the qualities that allow easy and fast motion through water. Here’s an example from the Daily Beast (my emphasis in all quotes):

These suits also made the body shape very smooth and hydrodynamic. Instead of the joint between a man’s body and the waist cord of his swimsuit adding extra drag, there was now a seamless, wrinkle-free, low-resistance outer shell skimming through the water.

Wordnick has a few more examples of usage, some of which are in the sense you want, such as (from the ImpactLab.net):

Like its natural archetype, the AquaPenguin from Festo has a hydrodynamic body contour.


Now, aerodynamic appears in some dictionaries with the relevant sense. See Oxford Learner’s Dictionary or The American Heritage® Science Dictionary (2002) via Dictionary.com:

Designed to reduce or minimize the drag caused by air as an object moves though it or by wind that strikes and flows around an object.

Merriam-Webster has the corresponding noun. On the other hand hydrodynamic does not appear with the precise parallel sense in any dictionary I have access to. I could find only the more general meaning. See several dictionaries in Dictionary.com or Merriam- Webster:

: of, relating to, or involving principles of hydrodynamics

The same is true, however, of aerodynamic in some dictionaries (see Dictionary.com), which give the general definition of aerodynamics — branch of dynamics dealing with gases — and simply list the adjective without defining it explicitly.

As to whether it is wrong to use aerodynamic for an underwater object, aerodynamic and aerodynamics are defined with respect to air or gases in every dictionary I could find, so I don’t think it appropriate to say that, say, a submarine is aerodynamic. Suppose the submarine is hydrodynamic and can be sustained in the air by some external force that does not interfere with its motion. Let’s assume for the sake of argument than the features that allow the submarine to move with minimum drag in water would also allow it to do so in the air — whether this is true or the comparison even makes sense I don’t know; physicists and engineers may want to weigh in on this. Then the statement the submarine is aerodynamic is true; but to my mind it only says that the submarine can move with minimum drag through the air, even though a well-informed person could tell from it that it would also move with minimum drag in water. That’s my two cents anyway; philosophers may want to weigh in on this.

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    The one thing I haven't seen anyone mention is that, as far as i can tell, aerodynamic has an extra meaning that refers merely to the aesthetic that grew out of aerodynamic design; that is, thing that fit our image of aerodynamic design. Can also mislead, e.g., i would imagine a full teardrop shape to be described as far more aerodynamic than a cropped one despite the latter only losing a fraction of a percent… – StarWeaver Aug 12 '16 at 15:00
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  1. Dictionary definitions of aerodynamic are (usually?) specific to air (boldface mine):

    the qualities of an object that affect how easily it is able to move through the air [Merriam–Webster]

    having a shape that reduces drag when moving through the air [Wiktionary]

    As air and water are both fluids, there are at best subtle differences between a shape optimised for moving through air and one optimised for moving through water (taking compression into account would be something that could cause such differences). Therefore a shark’s fin or a submarine are certainly more aerodynamic than your average shape. Calling them aerodynamic is thus technically correct.

    Of course, in your examples, the writer probably does not only want to be technically correct but also allude to the actual property of the shape being optimised to the medium. As laymen (i.e., most people) can hardly distinguish between optimised for air and optimised for water, it is only to be expected that this distinction is lost in colloquial usage – just as decimate is not specifically about destroying one tenth of something anymore.

  2. Consider streamlined:

    contoured to reduce resistance to motion through a fluid (as air) [Merriam–Webster]

    Designed to offer little resistance to the flow of fluid, especially by having sleek, graceful lines. [Wiktionary]

  • There is a one huge practical difference in the design of "streamlined shapes" in air and in water, namely objects which operate on the surface of the water, not completely submerged in it. Also the compressibility of air can be ignored at speeds below about 150 mph / 250 kph - for example in designing most "streamlined" vehicles used for road and rail transport, and even some small aircraft. – alephzero Aug 11 '16 at 18:20
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Consider "low drag."

Drag (Oxford Online)

1.1 The longitudinal retarding force exerted by air or other fluid surrounding a moving object: "the coating reduces aerodynamic drag"

While "streamlined" can focus on the fluid appearance and "aerodynamic" and "hydrodynamic" can be interpreted to focus on what is causing the resistance, "low drag" only speaks to the measurable functional aspects.

5

Hydrodynamic is the phrase that must be used for underwater dynamics. According to Merriam-Webster, the definition of hydrodynamics is:

a branch of physics that deals with the motion of fluids and the forces acting on solid bodies immersed in fluids and in motion relative to them.

Now aerodynamics (Merriam-Webster) is completely related to fluid dynamics in air, and also describes "the qualities of an object that affect how easily it is able to move through the air", which is different from the way hydrodynamics is used.

Aquadynamics is the word which can be used for fluid dynamics especially for dynamics in water, and

aquadynamic (Wikitionary) is used to describe

having a shape that reduces drag when moving through water.

So for your context, aquadynamic is a suitable word to use.

  • +1 for aquadynamic - first word I thought of – Daenyth Aug 15 '16 at 12:35
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I will offer a counterpoint. Perhaps you're focused too much on the etymology and thus "literal" meaning of the word. Scientifically, as a body moves at high speeds, air and liquids exhibit the same properties. The qualities that make a body "aerodynamic" will also make it "hydrodynamic". Distinguishing between the two is splitting hairs and a lay person is more likely to be familiar with the term aerodynamic. Hence, the use of the term when describing an aquatic body is not incorrect at all.

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    Agreed. Oh, and people splitting hairs on SE?? Unheard of!! – Revetahw Aug 12 '16 at 14:38
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    Of course you're right that ærodynamic will be fine to use, however it's an oversimplification to say that air and liquids exhibit the same properties, even at high speeds (more to the point: at high Reynolds numbers). Because water is practically incompressible, it still behaves quite different in many regards. – leftaroundabout Aug 13 '16 at 0:23
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    @leftaroundabout, it is rather at low speeds, where air can be considered incompressible (below Mach 0.3..0.6), is where air and liquids "exhibit the same properties". In fact, it is often more accurate to test small-scale models in water rather than in a wind tunnel - yes, because the Reynolds number will be closer to reality. At low Re, aerodynamicists really don't distinguish between fluids and air, and can use the terms in question interchangeably (and more commonly as fluid dynamics). – Zeus Aug 14 '16 at 7:26
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    @Zeus: right indeed: as soon as you approach transsonic speeds, you can't compare water and air at all. But that's not what I meant with high speeds – I meant speeds just high enough that they're clearly in turbulent domain in both air and water. At lower speeds, the flow will more readily be dominated by viscous forces if you're in water than it will in air. But of course it depends a lot on the model scale etc. – really one should say, “ærodynamic and hydrodynamic are equivalent if the Reynolds number matches and you keep well below transsonic speed”. Not equivalent unconditionally. – leftaroundabout Aug 14 '16 at 9:59
  • Hence why I brought up what would be more common for a lay person. Of course in scientific circles, the differences are quite obvious ;) – Michael Brown Aug 15 '16 at 14:38
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Actually, liquid and gas dynamics are similar, but not the same. Liquids are generally highly incompressible, and that generates slightly :) different behaviors, for example at high speeds(supersonic, >Mach1 - and even lower), when air/gas tends to compress in front of the moving object. Anyway, "Hydro" comes from "water" like in "maintaining your hydration" for example.

So "Hydrodynamics" is the generally accepted term.

Of course, there are ways to be more catholic than the Pope, as previous answers generously demonstrate.

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    Hi Pandele, welcome to English Language & Usage. This site is a bit different from other Q&A sites: preference is given to detailed answers supported by evidence and/or links to sources. Evidence distinguishes the answer from mere opinion, and helps in building a library of detailed answers for future users. Your answer reads as a highly knowledgeable and relevant comment, but not necessarily as an answer offering a new solution to the question. If you think you might use our site again (and I hope you do!), please make sure you take the Tour. :-) – Reinstate Monica Aug 12 '16 at 9:09
  • I do agree that answers based on evidence are more valuable, but simple questions deserve simple, short & to the point answers. And yes my answer was intended to be a comment. I do not like the academic style of answering questions were you have to read a lot to get one quick simple answer. "Ask questions, get answers, no distractions". Yes it asks for detailed questions, so the problem is clearly understood, but let's not overdue it. Time & patience are a limited resource, let us use it wisely. – Pandele Florin Aug 23 '16 at 15:04
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Doubtless it is hydrodynamic as already pointed out. Bernoulli's Law, airfoil shaping and streamlining, flow losses in mechanics of either continuum have a counterpart in what is commonly termed as a fluid.

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