Fluid Mechanics: 

Definition: Fluid Mechanics encompasses the study of all types of fluids under static, kinematic and dynamic conditions. The combination of experiments, the mathematical analysis of hydrodynamics and the new theories is known as ‘Fluid Mechanics’.

Fluid: A fluid is defined as a material which will continue to deform with the application of shear force however small the force may be. Generally matter exists in three phases namely (i) Solid (ii) Liquid and (iii) Gas (includes vapour). The last two together are also called by the common term fluids. 

(i) Solids: In solids atoms/molecules are closely spaced and the attractive (cohesive) forces between atoms/molecules is high. The shape is maintained by the cohesive forces binding the atoms.When an external force is applied on a solid component, slight rearrangement in atomic positions balances the force. Depending upon the nature of force the solid may elongate or shorten or bend. When the applied force is removed the atoms move back to the original position and the former shape is regained. Only when the forces exceed a certain value (yield), a small deformation called plastic deformation will be retained as the atoms are unable to move to their original positions. When the force exceeds a still higher value (ultimate), the cohesive forces are not adequate to resist the applied force and the component will break.

(ii) Liquids: In liquids the inter molecular distances are longer and the cohesive forces are of smaller in  magnitude. The molecules are not bound rigidly as in solids and can move randomly. However, the cohesive forces are large enough to hold the molecules together below a free surface that forms in the container. Liquids will continue to deform when a shear or tangential force is applied. The deformation continues as long as the force exists. In fluids the rate of deformation controls the force (not deformation as in solids). More popularly it is stated that a fluid (liquid) cannot withstand applied shear force and will continue to deform. When at rest liquids will assume the shape of the container forming a free surface at the top.

(iii) Gases: In gases the distance between molecules is much larger compared to atomic dimensions and the cohesive force between atoms/molecules is low. So gas molecules move freely and fill the full volume of the container. If the container is open the molecules will diffuse to the outside. Gases also cannot withstand shear. The rate of deformation  is proportional to the applied force as in the case of liquids.

Vapour is gaseous state near the evaporation temperature. The state in which a material exists depends on the pressure and temperature. For example, steel at atmospheric temperature exists in the solid state. At higher temperatures it can be liquefied. At still higher temperatures it will exist as a vapour. A fourth state of matter is its existence as charged particles or ions known as plasma. This is encountered in MHD power generation.

COMPRESSIBLE AND INCOMPRESSIBLE FLUIDS:

If the density of a fluid varies significantly due to moderate changes in pressure or temperature, then the fluid is called compressible fluid. Generally gases and vapours under normal conditions can be classified as compressible fluids. In these phases the distance between atoms or molecules is large and cohesive forces are small. So increase in pressure or temperature will change the density by a significant value.

If the change in density of a fluid is small due to changes in temperature and or pressure, then the fluid is called incompressible fluid. All liquids are classified under this category.

When the change in pressure and temperature is small, gases and vapours are treated as incompressible fluids. For certain applications like propagation of pressure disturbances, liquids should be considered as compressible.
CONTINUUM: 

As gas molecules are far apart from each other and as there is empty space between molecules doubt arises as to whether a gas volume can be considered as a continuous matter like a solid for situations similar to application of forces. 

Under normal pressure and temperature levels, gases are considered as a continuum (i.e., as if no empty spaces exist between atoms). The test for continuum is to measure properties like density by sampling at different locations and also reducing the sampling volume to low levels. If the property is constant irrespective of the location and size of sample volume, then the gas body can be considered as a continuum for purposes of mechanics (application of force, consideration of acceleration, velocity etc.) and for the gas volume to be considered as a single body or entity. This is a very important test for the application of all laws of mechanics to a gas volume as a whole. When the pressure is extremely low, and when there are only few molecules in a cubic metre of volume, then the laws of mechanics should be applied to the molecules as entities and not to the gas body as a whole.

How to make solar power 24/7

MIT team designs concentrated solar thermal system that could store heat in vats of molten salts, supplying constant power.

David L. Chandler, MIT News Office

The biggest hurdle to widespread implementation of solar power is the fact that the sun doesn't shine constantly in any given place, so backup power systems are needed for nights and cloudy days. But a novel system designed by researchers at MIT could finally overcome that problem, delivering steady power 24/7. 

The diagram above shows the idealized arrangement of a vat of molten salt used to store solar heat, located at the base of a gently-sloping hillside that could be covered with an array of steerable mirrors all guided to focus sunlight down onto the vat. Image: Courtesy of Alexander Slocum et al. 

The basic concept is one that has been the subject of much research: using a large array of mirrors to focus sunlight on a central tower. This approach delivers high temperatures to heat a substance such as molten salt, which could then heat water and turn a generating turbine. But such tower-based concentrated solar power (CSP) systems require expensive pumps and plumbing to transport molten salt and transfer heat, making them difficult to successfully commercialize — and they generally only work when the sun is shining. 

Instead, Alexander Slocum and a team of researchers at MIT have created a system that combines heating and storage in a single tank, which would be mounted on the ground instead of in a tower. The heavily insulated tank would admit concentrated sunlight through a narrow opening at its top, and would feature a movable horizontal plate to separate the heated salt on top from the colder salt below. (Salts are generally used in such systems because of their high capacity for absorbing heat and their wide range of useful operating temperatures.) As the salt heated over the course of a sunny day, this barrier would gradually move lower in the tank, accommodating the increasing volume of hot salt. Water circulating around the tank would get heated by the salt, turning to steam to drive a turbine whenever the power is needed. 

The plan, detailed in a paper published in the journal Solar Energy, would use an array of mirrors spread across a hillside, aimed to focus sunlight on the top of the tank of salt below. The system could be "cheap, with a minimum number of parts," says Slocum, the Pappalardo Professor of Mechanical Engineering at MIT and lead author of the paper. Reflecting the system's 24/7 power capability, it is called CSPonD (for Concentrated Solar Power on Demand). 

The new system could also be more durable than existing CSP systems whose heat-absorbing receivers cool down at night or on cloudy days. "It's the swings in temperature that cause [metal] fatigue and failure," Slocum says. The traditional way to address temperature swings, he says: "You have to way oversize" the system's components. "That adds cost and reduces efficiency." 

The team analyzed two potential sites for CSPonD on hillsides near White Sands, N.M., and China Lake, Calif. By beaming concentrated sunlight toward large tanks of sodium-potassium nitrate salt — each measuring 25 meters across and five meters deep — two installations could each provide 20 megawatts of electricity 24/7, which is enough to supply about 20,000 homes. The systems could store enough heat, accumulated over 10 sunny days, to continue generating power through one full cloudy day. 

While exact costs are difficult to estimate at this early stage of research, an analysis using standard software developed by the U.S. Department of Energy suggests costs between seven and 33 cents per kilowatt-hour. At the lower end, that rate could be competitive with conventional power sources. 

The team has carried out small-scale tests of CSPonD's performance, but its members say larger tests will be needed to refine the engineering design for a full-scale powerplant. They hope to produce a 20- to 100-kilowatt demonstration system to test the performance of their tank, which in operation would reach temperatures in excess of 500 degrees Celsius. 

The biggest challenge, Slocum says, is that "it's going to take a company with long-term vision to say, 'Let's try something really different and fundamentally simple that really could make a difference.'" 

Most of the individual elements of the proposed system — with the exception of mirror arrays positioned on hillsides — have been suggested or tested before, Slocum says. What this team has done is essentially an "assemblage and simplification of known elements," Slocum says. "We did not have to invent any new physics, and we're not using anything that's not already proven" in other applications. 

Gershon Grossman, who holds the Sherman-Gilbert Chair in Energy at the Technion-Israel Institute of Technology, says this approach "includes several innovative CSP concepts." But, he adds, "the main advantage of this system is its ability to deliver power continuously, unlike other CSP systems, which are affected by clouds. This work is innovative and is expected to make a significant contribution" to the industry, he says. 

Slocum emphasizes that this approach is not intended to replace other ways of harvesting solar energy, but rather to provide another alternative that may be best in certain situations and locations. Playing on the familiar saying about rising tides, he adds, "A rising sun can illuminate all energy harvesters." 

Source: MIT News

Smell-a-Vision

A proof-of-concept design shows that including a system to generate smells to accompany TV images is "quite doable." Karen Hopkin reports...

How many times has it happened to you: you’re sitting around watching a rerun of Friends and you think: Man, if only I could catch a whiff of that hazelnut mocchaccino they’re all pretending to drink. Well, me neither. 

But engineers have now developed a programmable, odor-emitting device that, like it or not, brings us one step closer to realizing the dream of smell-a-vision. Their design is described in the journal Angewandte Chemie. [Hyunsu Kim et al, An X–Y Addressable Matrix Odor-Releasing System Using an On–Off Switchable Device]

TV tickles us with sight and sound. Why not smells? All you’d need is a box that would sit near the set and generate fragrances that match the images on screen: a woman’s perfume or a hot apple pie. But how big would the device have to be to generate thousands of odors? 

To keep the dimensions down, the scientists envision using a 100-by-100 grid, so that just 200 on/off switches could unleash 10,000 stored bouquets. Which they say makes the whole thing “quite doable.”

The question then becomes: why would anyone want to do it? Presumably there are some things that are best left unsmelled. Because a sound cue is perfectly sufficient to tell us where Archie Bunker or Al Bundy has just been.

[The above text is an exact transcript of this podcast.]

Modern-day sea level rise skyrocketing

Increase began with the Industrial Revolution

By Janet Ralof

This North Carolina house was filmed for the movie Nights in Rodanthe while researchers nearby studied the history of sea level rise in the area. The team found that seas have risen much faster in the last 150 years or so than in the previous two millennia.
Credit: A. Kemp/Yale University. 

Sea levels began rising precipitously in the late 19th century and have since tripled the rate of climb seen at any time in at least two millennia, a detailed analysis of North Carolina marsh sediments shows.

“This clearly shows the recent trend is not part of a natural cycle,” says Ken Miller of Rutgers University in Piscataway, New Jersey, who was not associated with the analysis.

Andrew Kemp of the University of Pennsylvania and his colleagues spent five years plumbing salt marsh sediments that had remained largely undisturbed for millennia. Kemp, now at Yale, and his team drilled cores at two sites, unearthing the microscopic remains of single-celled shelled organisms known as foraminifera.

Foraminifera vary in their salt tolerance. So as sea level changed over millennia, so did the mix of species living at any given site, explains University of Pennsylvania coauthor Benjamin Horton. Knowing the modern-day distribution of foraminifera at various water depths along the modern-day coast, the researchers could infer past sea levels at the two core sites from the abundance of different species in successive sediment layers. Radioisotope dating showed that the sediments recorded 2,100 years of sea level history, the researchers report online June 20 in the Proceedings of the National Academy of Sciences.

“We know what sea level has done, in a broad sense, going back 20,000 years,” Miller says. But detailed records of what’s happened over the past 2,000 years have been spotty, he says. 

The cores show that sea level at the North Carolina sites was largely unchanging from 100 B.C. until A.D. 950. Then sea level underwent a four-century rise averaging 0.6 millimeters per year. Sea level didn’t rise again until after 1865. Since then, it’s been climbing an average of 2.1 millimeters annually. And at least for the last 80 years, Horton says, “the fit with North Carolina tide gauge data is one to one: It’s perfect.”

The results validate the use of general equations relating past temperatures and sea level changes to predict sea level rise as the climate continues to warm, says Aslak Grinsted of the University of Copenhagen’s Centre for Ice and Climate.

“What’s great about this new record is that it’s really high resolution and continuous,” Grinsted says, “and quite consistent with records all around the world.”

Courtsey: Science News

Discover Interview The Dark Hunter

Physicist Elena Aprile is certain that dark matter exists. She just hasn’t found it yet.

by Fred Guterl

Dark matter sounds like some physicist’s tall tale: There’s this invisible matter, see, and it has this powerful gravitational effect on galaxies. That’s why we know it exists. In fact, it outweighs ordinary matter by about five to one. Problem is, dark matter doesn’t reflect or absorb light, so we can’t see it. Oh, and it rarely interacts with conventional atoms, so we can’t feel it, either. However, we know it makes up a huge part of the universe, so we keep looking for it.

As mind-bending (and perhaps logic-challenging) as these ideas may seem, a lot of physicists are searching for this elusive matter. Elena Aprile is one of the leading lights in this dark business. She heads a prominent dark matter experiment called Xenon, which is based 5,000 feet underground in Italy’s Laboratori Nazionali del Gran Sasso, one of the world’s largest subterranean physics labs. Aprile, who is also a codirector of Columbia University’s Astrophysics Lab, started the project in 2007 with a detector called Xenon10. Since then she has upgraded to the more sensitive Xenon100. “I feel proud to have one of the best instruments in the field for detecting dark matter,” Aprile says of Xenon100. But the huge questions remain: What is dark matter, and how close are scientists to finding it? Aprile recently updated DISCOVER on how things are going in her search for the missing majority of the universe.

Seriously—what is dark matter? 

The best answer is that we have no idea. We know dark matter is there. We’ve known it for more than 70 years. There was a 1933 paper by the Swiss astronomer Fritz Zwicky showing that visible matter is only a small fraction of the universe. Just 18 percent of the matter in the universe is composed of the stuff we know. The remaining 82 percent is what we call dark matter. Other discoveries in astronomy have since reinforced this view that something is missing. We know dark matter is there, but only from its gravitational effects. For example, the presence of dark matter helps explain why our galaxy is stable. The Milky Way is a disk that rotates like a merry-go-round. The question is, what keeps it from flying apart? Gravity, of course, but there is not enough visible matter in the galaxy to account for the amount of gravity needed to hold it together. That’s why we know that there must be other matter there that we can’t see.

What is dark matter composed of? 

We think it’s made of a type of particle that doesn’t like to interact with normal matter [protons, neutrons, and other types of particles] very often. And it’s very heavy, very massive. Perhaps as heavy as an entire lead atom or even heavier. It’s probably a relic particle from the Big Bang, a member of a family of particles that we’ve named weakly interacting massive particles, or WIMPs.

How do we know dark matter consists of some new kind of particle? 

Actually, we might be on the wrong track in thinking that dark matter is composed of a fundamentally new type of particle. That’s why we call it “the WIMP miracle.” The so-called standard model of particle physics, which lays out the way physicists think the universe works, has deficiencies. A lot of things, a lot of data, don’t fit. We have theories, such as supersymmetry and extra dimensions, that have been put forward to explain the things that are missing from the standard model or that don’t fit with the data we get. Some of the particles predicted by those theories are natural candidates to be dark matter because they have all the right characteristics. A particle called the neu­tralino, for instance, is a type of WIMP that’s a perfect candidate for dark matter in part because it doesn’t interact with other particles much, and that would explain why nobody has yet detected it.

If WIMPS don’t interact much with other particles, how can you find them? 

The way we go about this search is to wait for a particle of dark matter to come into contact with our device, which is basically a pot of liquid xenon [an element that is used, in gas form, in the very bright headlights of many new cars] sandwiched between two detectors. We use xenon because it is one of the heaviest elements—meaning that each atom contains a lot of protons and neutrons—and that increases the odds that dark matter will interact with it. Whenever that happens, whenever a WIMP gets stuck in there, the xenon displays some remarkable properties. There will be a flash or scintillation of ultraviolet light. You can’t see it with the naked eye, so to detect this light, we have 178 extremely sensitive one-pixel cameras, called photomultipliers, above and below the liquid-xenon-filled detector. We also look for an ionization signal: If a dark matter particle rubs against a xenon atom, there will be electrons liberated and a charge produced. Those electrons drift upward through the liquid xenon to a positively charged anode [electric terminal], which produces a second flash of light that the cameras will detect.

That signal would tell you that a WIMP has finally made contact? 

Well, we can extract a wealth of information from those two signals, including the speed of the particle, the location of the interaction, and the type of particle it was—an electron, a neutron, or dark matter. The more gently the particle touches the xenon, the more likely that it’s a WIMP.

But you haven’t definitely found one yet? 

No. I mean, it constantly happens that you look at something and say, “Hey, what’s this?” and you think it could be dark matter. But we have always found an explanation for these events. Still, it’s important to consider the possibility that we might actually be looking right in the eye of dark matter.

How close are we to finding dark matter? Have others had hits, or possible hits? 

There was news in December (article; live-blog) that another group of researchers, the Cryogenic Dark Matter Search (CDMS), had detected dark matter in a mine in Minnesota, but they saw a very weak signal. They recorded two events that they cannot fully explain as background noise. One of the events is very close to the threshold of noise. It’s not a detection; the collaboration itself doesn’t call it that. It’s the hint of a detection. There was initially a lot of excitement, but that has died down.

As for our group, we have collected data for several months now with Xenon100 and will continue through the summer. This powerful detector has the lowest background noise ever measured for any dark matter detector, and it is the largest-scale detector in operation. If the signal CDMS found was truly from dark matter, we’ll easily be able to confirm it this year. At the same time, particle physicists are looking for dark matter with the Large Hadron Collider in Geneva. We’re hoping they can tell us more about these particles in the next few years.

What is it like to search for something that you may never find? 

It feels very exciting and almost like a duty. The fact that we don’t know if we will discover dark matter does not take away the necessity to try. The Italian particle physicist Carlo Rubbia, who was my doctoral thesis adviser at the University of Geneva, recently quoted Galileo at a conference on dark matter: Provando et riprovando—“Try and try again.” This is the basis of experimental science. We must try and try again to find the truth. If we stop because there is no guarantee that we will find anything, then we would never find anything again. In fact, in terms of dark matter, not finding anything is extremely important because it will make us find new roads to explore. We must keep searching for it with the best tools we have.

Lightning Unleashes Antimatter Storms

You don't have to go all the way to supernovas to find natural events powerful enough to generate gamma rays says Shannon Palus

The powerful blasts of particles and light energy known as gamma-ray bursts come from violent cosmic events in deep space, such as stellar explosions and black hole collisions. But smaller-scale bursts called terrestrial gamma-ray flashes (TGFs) can occur much closer to home, erupting thousands of times a year in association with lightning strikes during storms in Earth’s atmosphere. Two satellites originally designed to observe gamma rays from space recently caught the atmospheric flares in action, revealing that they emit far more energy than previously thought and release streams of antimatter particles, which bear a charge opposite that of their normal counterparts.

In a study of 130 TGFs recorded by 
the AGILE satellite, Italian Space Agency physicist Marco Tavani and colleagues report that the most energetic particles released carry four times as much energy as previous measurements detected, and hundreds of times as much as those produced by normal lightning strikes. In fact, Tavani describes a storm hurling photons into AGILE’s detectors as basically a giant particle accelerator in the sky. “It’s the equivalent of the Large Hadron Collider acting in the atmosphere for a fraction of a second,” he says. Next, Tavani plans to evaluate how TGFs might affect aircraft flying nearby.

Researchers working on another mission, NASA’s Fermi Gamma-ray Space Telescope, announced in January that about 10 percent of the particles fired off by TGFs consist of positrons—the positively charged antimatter twins of electrons. Because gamma rays can convert into electrons and positrons, physicists had predicted the anti­particles’ presence in the bursts, but until now they had never been directly observed. Astrophysicist Michael Briggs, a Fermi team member based at the University of Alabama in Huntsville, hopes such findings will aid in modeling how TGFs form. Currently, he says, scientists do not understand why some lightning strikes produce such mayhem while others do not.

Courtesy: Discover Magazine

Animals Spinning Their Wheels
Nature anticipated mankind in the development of one of civilization’s fundamental machines.
By Adrian Bejan

No topic is more “mechanical engineering” than the wheel. When the wheel appeared, the movement of humanity jumped to new dimensions, higher speeds, longer distances, and less effort per unit of mass moved through a distance. If engineering is the kitchen of civilization, then the wheel is the key ingredient. Today we take the wheel for granted, because it is everywhere. Older generations were more keenly aware of where we came from, and commemorated the wheel in the emblems of cities, business groups, trade unions, and engineering departments in universities. It is good that we maintain these images. The icon for “settings” on my iPhone consists of several wheels, even though there are no wheels inside. Along with complacency comes arrogance. “Everybody knows” that nature did not invent the wheel. The famous Harvard biologist Stephen Jay Gould wrote a book about natural history and gave it the title Kingdoms Without Wheels. The common wisdom is that humans invented the wheel and that it does not exist in nature. This idea places humans in a world distinct from and higher than all the other animals. Darwin must be rolling in his grave. The common wisdom is wrong. But first, here is a brief reminder of why the wheel was such a dramatic change in how humans move. The work, W, spent on sliding a mass, M, through a horizontal distance, L,  is equal to the weight, Mg, times L and a coefficient of friction, μ. With wheels placed between M and the ground, the work formula remained the same (W = μMgL) but the coefficient μ decreased considerably. The constructal-law evolution of rolling locomotion, from the ancient wheel to  the modern wheel. The highest allowable stresses are distributed more uniformly, and the wheel becomes lighter and less costly in terms of the useful energy destroyed in order to carry it. The time direction of this change, from high μ to low μ, is in accord with the constructal law of design and evolution in nature, which states that all flow systems (including human movement) persist in time by changing into configurations that flow more and more easily. Humans and their loads found an easier way to move on the map, just as river basins find better tree-shaped flow designs every year. Seepage in the wet mud is not eliminated by the birth of the river channel, because seepage continues to improve flow by finding new channels. Similarly, when humans got their stuff off the ground and rolled with it, sliding was not eliminated. It persists today, at speeds and scales small enough to be comparable with the movement that existed before the wheel. For example, when we stock the shelves in grocery stores, we slide cans or boxes into place. On top of the old design of movement that slid loads across the ground, a better one with reduced friction was added.An Evolutionary Design: The natural emergence of the wheel design can be predicted by using the constructal law in two ways. First, consider the evolution of the wheels made by humans. In the beginning, the wheel was a solid disk. The wheel and the ground made contact over a narrow strip on the rim. The stresses were distributed nonuniformly in the disk. The highest stresses were in the vicinity of the contact strip. Most of the wheel body was not stressed. Less material is needed when the maximum allowable stresses are distributed more uniformly through the loaded structure. When the design requires less material, it becomes lighter. A single column with uniform cross-section requires the least material to support a weight. The stresses in the column are distributed uniformly. The volume of the column is a tiny fraction of the volume of the solid disk. The column is a much lighter organ than the disk to carry on the vehicle, but one column is not enough to serve as a wheel. Three or more columns, a rigid rim, and a rigid track are required to prevent the body from falling. Fewer columns are lighter, and this constructal-law direction for easier movement in time is confirmed by the evolution of wheel technology from solid disk to spokes supporting a rim. Second, imagine the horizontal movement of a terrestrial animal as a rolling body. Imagine the human body, or the front or back half of a quadruped. The leg is a single column. Many bipedal creatures can stand still on one leg. There are many kinds of birds that sleep that way. To walk requires the forward movement—in essence, a falling forward from the one-legged stance.  The order of magnitude of the speed of falling forward is the same as the speed of falling down, namely V ~ (Rg)1/2, where the distance above the ground (R) is the body length scale, which is the same as the length of the leg. The body mass scale is equal to the density of the body times the length scale of the leg cubed. Coincidentally, because M ~ ρR3, where ρ is the body density, the speed of locomotion is also recognized as V ~ M1/6g1/2ρ–1/6, in agreement with the known characteristic speeds of all animals (runners, fliers, and swimmers), and with the world-record human speeds in running and swimming during the past 100 years. To maintain this horizontal speed, the body design requires a second column, which must also have the ability to absorb shocks and to elongate itself to reposition the body weight to its traveling height (R). That’s why legs are most efficient when they come in pairs. The second leg is brought forward in time to catch the body before it falls too far or accelerates too much toward the ground. This function is like the cooperation of the spokes of a wheel. But because it can be done by consistently cycling two spokes, which are paired legs in animals, that is the way nature does it. A third leg would continue the work of the first two, but it would increase by a factor of 3/2 the mass of the organs that the animal must carry along in order to have locomotion. Thus, the third beat of this rhythm is executed by the first leg, which swings outward, from behind the body, to take the position that the third leg would have occupied. This alternative is much lighter and faster, and (in accord with the constructal law) it is the natural design of rolling locomotion. The legs, as two columns swinging back and forth, perform the function of an entire wheel-rim-track assembly. They do it in record fashion—one wheel with just two spokes and with uniformly stressed material in each spoke. No wheel is stronger and lighter than this. The animal body is both wheel and vehicle for the animal mass that moves on the surface of the Earth. Changing Speeds: Nature evolved not only the design of wheel-like movement, but also the design for changing speeds. Larger bodies move faster—that is, as the average of speed over a lifetime. The cheetah may be able to outrun any other land animal in a short sprint, but like all cats, it spends much of its time sleeping and watching. The cheetah can reach running speeds exceeding 100 km per hour for a distance of about 500 meters. Then it must rest or die. The body mass of a cheetah is generally less than 40 kg. A medium-size man has at least twice that mass, and humans generally move more mass faster and farther than cheetahs over long periods of time. Considering the movement over a lifetime, humans are bigger, faster, and more economical vehicles of animal mass than cheetahs. Because bigger means faster, greater speed could be found by increasing the height of the body mass above the ground. Animal bodies have shapes with multiple scales. A simple body shape is the elongated body of a serpent. A quadruped’s body is much taller and more massive than a serpent’s, but it is also elongated: its length is greater than the height of its torso. Evolution toward higher speeds points toward designs that are taller. This agrees with the evolutionary design of animal locomotion: quadrupeds occurred after swimmers and crawlers, not the other way around. The animal becomes faster by orienting its longer dimension vertically, i.e. by making itself taller. The constructal-law direction is from long to tall, and this too agrees with the evolution of animal locomotion: bipedal locomotion evolved after quadrupedal locomotion. The vertical orientation increased the size of R. In the formula for speed, in which velocity correlates to the square root of R times g, a greater value of Rcontributes to higher speed. There are many examples of the animal design for changing speeds. A human has two speeds: walk and run. A horse has three speeds: walk, trot, and gallop. The human and the horse increase their speeds by increasing the height from which their centers of gravity fall during each locomotion cycle. From the walk to the gallop, the horse body movement changes abruptly such that the amplitude of the jump increases stepwise. The animal body with three different designs for movement (rhythm) is like one vehicle with one engine and a gearbox with three speeds. Engineering and Nature: The evolutionary designs of nature have arrived at wheel-like locomotion and at changes in body movement that result in changing speeds. The designs developed by humans are late comers to this long evolutionary sequence. They come from the same natural tendency to move on Earth more easily, to go with the flow. The human wheel and gearbox were not copied from nature. They are not fruits of biomimetics. These artifacts are part of our own evolutionary design for moving our mass on the landscape. Engineering makes a contribution to understanding design in nature—a contribution that the other sciences cannot make. Biologists and geophysicists argue that one cannot witness and test “evolution” because of the enormous time scale of the phenomenon. Engineers bring an important idea to the current debate of design and evolution in nature. Yes, we can witness and test evolution during our lifetime, by studying the evolution of our designs and technologies. These evolutionary designs illustrate the time direction of the constructal law, which unites animate and inanimate design phenomena. Note: Adrian Bejan is the J.A. Jones Distinguished Professor of Mechanical Engineering at Duke University. He can be reached at abejan@duke.edu. Courtesy: ASME NEWS ONLINE