40 Fun Facts About Fluid Mechanics

Which physicist explained in 1926 why stirred tea leaves collect at the centre of the cup?

Albert Einstein solved the tea leaf paradox in a paper about how rivers erode their banks. Friction slows the liquid near the bottom, setting up a secondary flow that sweeps the leaves inward.

Oobleck, a cornstarch goo that stiffens when hit, takes its name from a book by which author?

Dr. Seuss invented the word for his 1949 book Bartholomew and the Oobleck, in which a sticky green substance falls from the sky. The real mixture acts like a solid under a sudden punch.

Which sticky sweetener burst from a Boston storage tank in 1919 and swept the streets as a deadly wave?

Molasses weighing about 13,000 tons surged through the North End at an estimated 35 miles per hour, killing 21 people. The tank that burst stood 50 feet tall.

Which shear-thinning hazard of adventure films is too dense for a human body to sink into fully?

Quicksand is about twice as dense as the human body, so a trapped person cannot sink much beyond the waist. The real dangers are exposure and a rising tide.

Which bubble effect lets the pistol shrimp stun prey and also wears away fast-spinning ship propellers?

Cavitation occurs when low pressure opens vapour bubbles that then implode with a shock wave. The shrimp's snapping claw fires a bubble at about 90 km/h, and its collapse even gives off a flash of light.

Which consequence of Earth's rotation spins hurricanes but is far too weak to steer a draining sink?

The Coriolis effect only builds up over large distances and long times, which is why it shapes cyclones. In a bathtub it is negligibly small beside the many other influences on the water.

Which feature, repeated hundreds of times on a golf ball, makes it fly farther than a smooth one?

Dimples keep the air clinging to the ball for longer, which narrows the low-pressure wake trailing behind it. Most modern balls carry 300 to 500 of them.

Which Renaissance artist's Codex Leicester, later bought by Bill Gates, is full of notes on flowing water?

Leonardo da Vinci wrote in it about how river water moves around obstacles placed in its way. Bill Gates paid about 30.8 million dollars for the notebook in 1994, then a record for a manuscript.

Which Greek thinker reputedly cried 'Eureka' after noticing the water rise when stepping into a bath?

Archimedes is said to have run naked into the streets after realising that displaced water could measure a crown's volume. His treatise On Floating Bodies set out the law of buoyancy.

Which condiment is a shear-thinning liquid that turns runnier once its bottle is shaken or thumped?

Ketchup is a non-Newtonian fluid: stress lines up the particles suspended in it so that it thins, and that order lasts only while the stress is applied.

Which American test pilot first flew faster than sound in level flight, piloting the Bell X-1?

Chuck Yeager broke the sound barrier over California's Mojave Desert on October 14, 1947, in an aircraft christened Glamorous Glennis. It was later put on permanent display at the Smithsonian.

Which ratio, named after an Austrian physicist, compares an aircraft's speed with the local speed of sound?

The Mach number honours Ernst Mach, who photographed the invisible shock waves around supersonic bullets in the 1880s. Aeronautical engineer Jakob Ackeret proposed the name in 1929.

Which French thinker, whose name is the SI unit of pressure, had a barometer carried up a mountain in 1648?

Blaise Pascal showed that pressure applied to a confined fluid is passed on undiminished in every direction. The automobile lifts seen in service stations rely on that principle.

Which property of a liquid explains why syrup pours far more slowly than water from a tilted jug?

Viscosity matches the everyday idea of a liquid's thickness. It falls as a liquid warms, which is why heated syrup runs freely, yet in gases it rises with temperature.

Which effect lets water striders and even steel razor blades rest on a pond without sinking?

Surface tension makes the top of the water act like a stretched skin, holding up objects that are denser than water itself. The same pull shapes raindrops and soap bubbles.

Which fluid phenomenon did Richard Feynman call the most important unsolved problem in classical physics?

Turbulence fills a flow with unsteady swirls of many sizes, the large ones breaking down into ever smaller ones.

Which effect draws spilled liquid up into a paper towel and melted wax up a candle wick?

Capillary action moves a liquid through narrow spaces without any help from gravity. Sports fabrics that wick sweat away from the skin are said to rely on the same effect.

Which aerodynamic force do racing cyclists cut by tucking in close behind the rider in front?

Drag acts against the motion of anything moving through air or water, and a rider in another's slipstream meets far less of it. That is why a road race's main pack, the peloton, bunches so tightly.

Which testing device did the Wright brothers use in 1901 to study airflow while developing the Wright Flyer?

Wind tunnel testing was three decades old by then: Francis Wenham had built the first enclosed one in 1871. Earlier experimenters had spun their models on a rotating arm instead.

Which Swiss scientist's 1738 book Hydrodynamica linked faster-moving fluid to lower pressure?

Daniel Bernoulli came from a quarrelsome family of mathematicians: his father Johann allegedly lifted key ideas from the book for his own Hydraulica, then backdated it to look like the earlier work.

Which Italian, who succeeded Galileo as mathematician to the Medici grand duke, invented the barometer?

Evangelista Torricelli upended a tube of mercury in a bowl and saw the column fall only a short way, leaving a vacuum above it. He argued that the weight of the air was holding the rest up.

Which dimensionless quantity tells engineers whether pipe flow will stay smooth or break into chaotic eddies?

The Reynolds number weighs a fluid's inertia against its internal friction. Osborne Reynolds made it famous in 1883 by feeding a jet of dyed water into a pipe and watching the streak stay sharp or break up.

Which equations of fluid motion are the subject of a Millennium Prize Problem worth one million dollars?

The Navier–Stokes equations describe how liquids and gases move, yet nobody had proved that smooth solutions always exist when the Clay Mathematics Institute posted the prize in 2000.

Which phenomenon, named for a German physicist, makes a spinning football or baseball swerve in flight?

The Magnus effect is what bends a curveball, and it matters just as much in tennis, cricket, volleyball and table tennis. Heinrich Gustav Magnus was the physicist who investigated it.

Which gas-filled organ lets most bony fish hold a chosen depth without the effort of swimming?

The swim bladder is missing in sharks and rays, so most of them must keep swimming to hold their depth. Others store light oils instead, a float they cannot adjust as they rise or sink.

Which compartments does a submarine fill with water to dive and with air to return to the surface?

Ballast tanks made their first recorded appearance on a submarine in David Bushnell's Turtle, which in 1776 became the first submarine used in combat. Taking on water lets a boat submerge and blowing it out brings it back up.

Which steady speed does a skydiver reach when air resistance grows to balance the pull of gravity?

Terminal velocity for a skydiver falling belly-to-earth is about 55 metres per second. Pulling in the limbs presents less area to the air and pushes the figure higher.

Which floating instrument, a staple of brewing, shows a liquid's density by how deep it sinks?

A hydrometer sinks deeper in lighter liquids, and the level where the surface crosses its stem gives the reading. Versions made for testing the creaminess of milk are called lactometers.

Which term describes fluid moving in smooth parallel layers, the orderly opposite of chaotic, eddying motion?

Laminar flow shows up when shallow water slides slowly over a smooth barrier, looking clear and almost still. Speed the fluid up enough and the neat layers break apart into eddies.

Which phenomenon makes water droplets skitter across a very hot pan on a cushion of their own vapour?

The Leidenfrost effect sets in for water at roughly 193 °C, when an insulating film of vapour keeps each drop from boiling away quickly. Drops on such a pan last longer than drops on a cooler one.

Which phenomenon, named for an Italian physicist, creates the 'tears of wine' that run down the inside of a goblet?

The Marangoni effect is liquid flowing along a surface toward wherever that surface pulls hardest. In wine, patches with less alcohol pull more strongly than patches with more, which sets the liquid moving.

Which thin region of slowed fluid hugging a surface did Ludwig Prandtl first describe at a 1904 congress?

The boundary layer let engineers split a flow in two: a thin skin where friction rules, and everything outside it, where friction can be ignored. That shortcut made the equations of flight workable.

Which forward-facing probe on an airliner gauges airspeed from the pressure of the oncoming air?

The Pitot tube was invented by French engineer Henri Pitot, who devised it in 1732 when he was asked to measure the flow of the Seine. Boats use the same device to read their speed through the water.

Which element, chilled to near absolute zero, becomes a superfluid that creeps up its container's walls?

Helium turns superfluid about two degrees above absolute zero and then flows with no internal friction at all. It can run up the wall of an open cup, over the rim and down the outside.

Which science, named in 1920 from the Greek for 'flow', studies how soft materials like toothpaste deform?

Rheology was named by Eugene Bingham, a professor at Lafayette College, on a colleague's suggestion. It covers 'soft solids' that flow instead of springing back when a force is applied.

Which instability, named for two 19th-century physicists, shows up in Earth's clouds and around Jupiter's Red Spot?

The Kelvin–Helmholtz instability arises where two fluids, or two parts of one fluid, slide past each other at different speeds. Hermann von Helmholtz discovered it in 1868.

Which trail of alternating swirls behind an obstacle makes overhead wires 'sing' and car antennas vibrate?

A Kármán vortex street forms when a flow sheds swirls from alternate sides of a blunt object, pushing it sideways one way and then the other. It is named after the engineer Theodore von Kármán.

Which English engineer's trials with scale-model hulls led to the first ship test tank, at his Torquay home?

William Froude worked out a formula for turning small-scale test results into predictions for full-sized hulls, and it still carries his name. Full-scale Admiralty trials proved him right.

Which pressure-surge effect makes household pipes bang when a tap or valve is shut off too quickly?

Water hammer is a shock wave that travels back and forth along the plumbing after moving water is stopped suddenly. Hydroelectric stations face the same surge when a turbine's valve closes too fast.

Which sudden loss of lift strikes an aircraft wing tilted past its critical angle, typically about 15 degrees?

A stall happens when the airflow separates from the wing's upper surface. As lift falls the aircraft descends, which raises the angle further and costs still more lift.