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Area 51 of Fluid Dynamics

Area 51


Grays paradox
In 1936 Gray brought to light a problem that had never occurred to scientists before. It had been well-known for centuries that dolphins exhibited extraordinary speed in the water. Accounts of these playful animals racing ships were available in the early 20th century (Kramer 1960). The question that Gray focused upon was how they could attain such speeds. With a rudimentary estimate of the physiological power of a dolphin Gray was able to arrive at the conclusion that this power was mathematically insufficient to achieve the speeds mariners had observed dolphins to reach (1)

d'Alembert's paradox
 is a contradiction reached in 1752 by French mathematician Jean le Rond d'Alembert. D'Alembert proved that — for incompressible and inviscid potential flow — the drag force is zero on a body moving with constant velocity relative to the fluid. Zero drag is in direct contradiction to the observation of substantial drag on bodies moving relative to fluids, such as air and water; especially at high velocities corresponding with high Reynolds numbers. (1)

von Neumann paradox
Numerous experimental investigations on the reflection of plane shock waves over straight wedges indicated that there is a domain, frequently referred to as the weak shock wave domain, inside which the resulted wave configurations resemble the wave configuration of a Mach reflection although the classical three-shock theory does not provide an analytical solution. This paradox is known in the literature as the von Neumann paradox. (1)

Myths and Urban Legends:

Pickup truck with tailgate up and down?

When is the flow resistence minimized?
Interesting question, suprising answer - suprising because counter-intuitive:
"One of the more striking features of the pickup truck flow is the downwash on the symmetry plane behind the tailgate, and the formation of two smaller recirculating flow regions on both sides of the symmetry plane."
Experimental Investigation of the Near Wake of a Pick-up Truck (2003)
Fluent Newsletter: Streamlining Pickup Trucks
Mythbusters: Tailgate Up or Down Revisited: Mesh tailgate, hardcover, tailgate removal

Dimpled car myths
Dimpled golf balls have been around for a long time but dimpled cars? Surpising though "preliminary" results of this quasi-scientific test: dimpled car is according to Discovery Channel Mythbusters 11% more fuel efficient. 

Enigmas and Mysteries:

Laminar to Turbulent Transition  Pipe Dream
The puzzle of why fluid motion along a pipe is observed to become turbulent as the flow rate is increased remains the outstanding challenge of hydrodynamic stability theory, despite more than a century of research.  The issue is both of deep scientific and engineering interest since most pipe flows are turbulent in practice, even at modest flow rates.  All theoretical work indicates that the flow is linearly stable ie infinitesimal disturbances decay as they propagate along the pipe and the flow will remain laminar. (1)

Throbbing heart oil drops enigma
Drop oil and detergent in a bowl of water and the droplets will beat like hearts. These small droplets can rhythmically expand and contract for almost half an hour, a phenomenon that was first formally described in 1975.

The mystery of nutty rotating seeds
If you live in a temperate deciduous climate, you probably know what a maple tree is. And if you’ve been in a maple forest during spring or fall—or, really, any time of year—you’ve probably seen maple seeds. They look a bit like badminton birdies, only flattened: a heavy, solid "nut" at the bottom, with a single "wing" above. The wing helps the seed fly relatively long distances (for a plant)—up to a few kilometers in some cases. (1)

Fast swimming sharks and slow riblets
A fundamental point in fluid mechanics is that rough surfaces increase drag, and sharkskin is considered rough. If such a rough surface reduces drag, that doesnt seem to make sense. (1, 2)

Quotations Citations:
Insightfull Interesting Humorus

“... as Sir Cyril Hinshelwood has observed ... fluid dynamicists were divided into hydraulic engineers who observed things that could not be explained and mathematicians who explained things that could not be observed.” - James Lighthill

"I have the result, but I do not yet know how to get it." - Gauss (1777-1855)

"In science, one tries to tell people, in such a way as to be understood by everyone, something that no one ever knew before. But in poetry, it's the exact opposite." - attributed to Dirac (1902 - 1984)

"An expert is a man who has made all the mistakes, which can be made, in a very narrow field." - Bohr (1885 - 1962)

"The purpose of computing is insight, not numbers" (1961). "The purpose of computing numbers is not yet in sight" (1997). - Hamming (1915-1997)

"We often think that when we have completed our study on `one', we know all about `two', since `two is one and one'. We forget that we still have to make a study of `and'." - Eddington (1882-1944)

"Th'invention all admir'd, and each, how he to be th'inventor miss'd; so easy it seem'd, once found, which yet unfound most would have thought impossible." - Milton (1608-1674)

"I would rather have today's algorithms on yesterday's computers than vice versa." - Toint (contemporary)

"A couple of months in the laboratory can frequently save a couple of hours in the library." - Westheimer (contemporary)

http://worldscibooks.com/etextbook/p546/p546_chap01.pdf Lighthill Bio
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