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The Biomimicry Column

How one engineer's birdwatching made Japan's bullet train better

<p>A high-speed rail operator needed its trains to be faster and quieter. Its manager turned to owls and kingfishers for inspiration.</p>

What is the connection between an engineer going bird watching and his saving millions of dollars for his company?

Or, what does catching flies have to do with preventing plane crashes? How will locust swarms change the nature of our highways? Can a mold do a better job of plotting our mass transit systems than a team of engineers and planners?

The common thread in all these scenarios: Deep observation and analysis of the natural world can lead to amazingly creative innovations. I will write about all these things in this series on transportation, but first let’s take a look at a how a couple of interesting birds inspired a sleek design.

Eiji Nakatsu was the general manager of the technical development department for the so-called “bullet” trains of Japan, famed for their speed and safety record. After attending a 1990 lecture on birds by an aviation engineer, Nakatsu, who is also an engineer, realized studying the flight of birds could bring his train, and us, into the future.

The Sanyo and Kyushu Shinkansen Lines, operated by Japan Railway West, connect western Japan’s two biggest cities, Osaka and Fukuoka, and are an extension of the older Tokaido Line from Tokyo to Osaka.

The 515-kilometer Tokaido Shinkansen is the world’s busiest high-speed-rail line, having moved 4.9 billion passengers from its opening in 1964 (for the Toyko Olympiad) to 2010. Indeed, more people move by train in Japan  -- an estimated 64 million a day -- than anywhere else in the world.

Making his trains faster was one of Nakatsu’s goals, but to do that, he needed to first make them quieter. The trains ran through dense neighborhoods and many tunnels. The loudest noise came from the connections to overhead wires (pantographs), and the emergence of the trains from the tunnels on the line. This dynamic was so forceful that it was creating sonic booms heard by residents 400 meters away.

In the case of the pantograph noise, air rushing over the struts and linkages in the mechanism was forming into so-called Karman vortices, also known as a Karman vortex street, and this turbulence was causing most of the noise. Karman vortices are created at all scales, from islands in the ocean to car aerials, and are manifested wherever a single bluff body separates the flow of a fluid. Alternate and opposite eddies swirl downstream of the obstruction, swinging back and forth as the force of one dominates and then the other.

This turbulence is a major consideration in the design of any lone tower or vertical mast, and various ways have been devised to counteract it. Placing a leeward fin on a cylinder is an example. Vortex streets are a basic dynamic and indeed, some animals, such as bees, are thought to take advantage of it in their flight.

Nakatsu became intrigued with the noise-dampening feather parts (fimbriae) of the owl. These comprise a comb-like array of serrations grown on the leading edge of the primary wing feathers. The fimbriae serve to break down the air rushing over the wing foil into micro-turbulences, and this muffles the sound that typically occurs in wings without this feature. He set his team to testing prototype shapes that mimicked these forms.

In 1994, a new “wing-graph” replaced the traditional pantograph and was a great success. The train could now run at 320 km/hr and meet the stringent 70dBa noise standard set by the government.

The sonic boom problem was much more complex than the pantograph noise. Whenever a train sped into a tunnel, it generated atmospheric pressure waves that reached the tunnel exit at the speed of sound. Like a piston in a cylinder, the train was forcing the fluid air out of the other end of the tunnel. The air exited in low-frequency waves (under 20Hz) that produced a large boom and aerodynamic vibrations.

This problem was particularly troublesome because it was tied to both the geometry of the tunnel and the speed of the train. The micro pressure of the wave was in proportion to the ratio of the cross-section of the trainset to that of the tunnel. Moreover, every unit increase in speed was producing an increase in pressure to the power of three.

The design team would have to find a way to redesign the shape of the train to go faster without creating the boom. The key was in preventing the pressure wave buildup by reducing the cross-sectional area of the train and redesigning its nose.

A discussion with a junior engineer prompted Nakatsu to once again search for the answer in nature. The young engineer observed that the test train seemed to “shrink” when it was traveling through the tunnel. Nakatsu reasoned that it must be due to a sudden change in air resistance, from open sky to closed tunnel, and wondered if there was an organism that was adapted to such conditions.

From his birdwatching experiences, Makatsu remembered the kingfisher, a bird that dives at high speed from one fluid (air) to another that is 800 times denser (water) with barely a splash. He surmised the shape of its bill was what allowed the bird to cut so cleanly into the water.

Next page:  What turning the train's nose into a bird's beak accomplished

The JR West team analyzed the bill of the kingfisher and found it had just the streamlined shape that modeling at the University of Kyushu had predicted as one that was optimum: a rotational parabolic body. Both the upper and lower beaks of the bird have triangular cross-sections with the sides of the triangles being curved. Together, they form a squashed diamond shape.

Informed by these parameters, the design team set about to test various nose shapes in a to-scale model tunnel and measure the pressure waves generated. They shot bullets of various shapes into a pipe, from the more traditional bullet nose to a shape modeled after the kingfisher. Further tests compared model solids dropped into water in order to record the splash. Concurrently these same shapes were run in simulations on a space research supercomputer. A train nose very similar to the kingfisher was then selected.

All the tests confirmed the kingfisher bill was, indeed, the most efficient of all those tested, besting all alternates by a wide margin. Refined prototypes were built and ultimately made to full-scale for test runs on the tracks. It was at this point that Nakatsu became convinced that nature had much to teach about efficient forms.

The design reduced the sonic boom effect, and allowed the train to run at higher speeds and still adhere to the standard noise level of 70 dBa. It also reaped further benefits immediately.

The new Shinkansen 500 had 30 percent less air resistance than its predecessor. Energy consumption was reduced proportionally. A measured actual train run (maximum 270 km/hr) showed a 13 percent reduction in the power that had been needed by the predecessor 300 series.

On March 22, 1997, JR-West put the 500-Series Shinkansen electric train into commercial service. The train was able to run at 300km/h at its maximum, a world speed record at the time, and meet the stringent noise standard. Traveling time between Shin-Osaka and Hakata had, as the company had challenged, been shortened, from 2 hours and 32 minutes taken by the conventional 300-Series "Nozomi" train to 2 hours and 17 minutes.

From the study of natural forms had come some basic geometry that could be analyzed, tested, and reverse-engineered for an adapted improvement in the shape of a vehicle. This more efficient form reduced the turbulence that caused the initial problem of noise, and this, in turn, reduced the power needed to move passengers.

Less power needed; less fuel consumed; less money spent. The Shinkasen 500 story is an object lesson in how we can look to nature to make our designs faster, cleaner, and -- oh, yes, quieter.

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