the many
and the
few

In a parade of machines, a young engineer saw the principle
that everything flows one way, from high to
low.

by Adrian Bejan

ME 101
is the number of the thermodynamics course in
mechanical engineering at Duke University. I cannot think of a better
course for "ME101." It was put on the books by the old professors, the
ones who had the vision to build Duke many decades ago. Today, I am sure,
thermodynamics would get a less memorable course number.

Times
change. Generations replace generations, but the principles remain.
Contrivances, gadgets, and fads are the opposite. They parade in front of
our eyes, but their impact on the "thin book" of fundamentals is
nil.

I was reminded of this recently. I had just finished teaching
thermodynamics for the semester when I visited two colleagues in Paris.
With them I use constructal theory to predict the global weather. They
work in a historic university called The National Conservatory of Arts and
Professions (CNAM), a few blocks north of the Pompidou Center. It is
historic for many ideas that persist today: evening classes for co-op
students, continuing education, and the first museum of technology in the
world.

A statue of French physicist Denis
Papin (1647-1712), one of the inventors of the steam engine.

CNAM has a 300-year-old edifice with towers, an inner court, statues,
clocks, and history that is chiseled in stone. In the front court, there
is the statue of Denis Papin and the first piston-and-cylinder machine
that expanded steam to produce work (1690). This was before the engine
builders of Britain, and 80 years before James Watt.

Around the
buildings, and around the ceilings of the oldest and most decorated
classrooms, are the names of professors and students who breathed at CNAM.
One name is Sadi Carnot. Without his work there would be no
thermodynamics, engineering, or standard of living as we know them
today.

Early in the 19th century, Sadi Carnot was a young engineer,
not unlike my students today. He had graduated from the École
Polytechnique. Tradition holds that he came to CNAM to contemplate, to
think in quiet about the army of contrivances that was invading France:
the steam engines. The industrial revolution was on the march. Britain had
industrialized itself in the 1700s. A century later, it was the turn of
the Continent to do the same.

Freedom
Through Steam

Why were the steam engines invading? Because their effect on
people's lives was good. It was dramatic. Engines were empowering people.
They were liberating serfs, slaves, and animals. They were facilitating
the movement of humanity all over the globe.

Sadi Carnot came to
CNAM to study the machines that were on display. They were built by many
hands-on engineers in Britain. These "many" fed the imagination of one
individual. In turn, today Sadi Carnot's mental viewing feeds the minds of
enormous numbers of builders of all sorts of machines throughout the
world.

The principle that Sadi Carnot saw in that parade of
machines is that everything flows one way, from high to low. Water flows
through a pipe from high pressure to low pressure. Heat flows from high
temperature to low temperature. This principle is known today as the
second law of thermodynamics, irreversibility, dissipation, inefficiency,
one way, water under the bridge, etc. Today, this is thermodynamics, the
science of everything that kicks and moves.

The new principle that
Carnot's visit at CNAM illustrates today is that one individual sustains
the crowd, and vice versa. The big river sustains the many tiny streams of
the river basin.

The reverse is equally true: The numerous sustain
the singular. The river basin, like the tree of the lung, connects an
entire area or volume to one point—flow resistances allocated to areas and
volumes, all over the world, so the whole world flows best. This principle
is the "constructal law"; to read more, see
www.constructal.org.

Both principles—the second law and the
constructal law—are in action. Their footprints persist, like the river
beds and the beaten tracks. The river and the caravan that do not follow
their beds and beaten tracks do not get far.

No flow system is an
island. No river exists without its wet plain. No human settlement thrives
without its farmland and open spaces. Everything that flowed to this day
to "survive" is in an optimal balance with the flows that surround it and
sustain it. The airflow to the alveolus is optimally matched to the blood
flow through the vascularized tissue, and vice
versa.

"Vascularized" is a good name for the energy systems that
thermodynamics covers. The tissues of energy flows, like the fabric of
society and all the tissues of biology, are optimized architectures. The
climbing to this high level of performance is the transdisciplinary
effort: the balance between seemingly unrelated flows, territories, and
disciplines. This balancing act—the optimal distribution of
imperfection—generates the very design of the process, power plant, city,
geography, and economics.

All things that flow follow
certain principles, whether it is the constructal design for maximum
flow access from the center to the edge of a circle, or the delta of
the River Lena in northern Siberia. The flow goes from high to low,
with hierarchy and multiple scales.

The need for considering the whole—the macroscopic system—is great and
universal. No matter how successful we are in discovering and
understanding small-scale phenomena and processes, we are forced to face
the challenge to assemble the invisible elements into palpable devices.
The invisible grains must be kept alive with flows, which connect them and
serve them. The challenge is to construct—that is, to connect and optimize
while assembling.

This challenge is becoming increasingly
difficult. While the smallest scales are becoming smaller, the number of
components and the complexity of the useful device (always macroscopic)
become greater.

A good example is the rush to nanotechnology.
Technology means more than the new physical phenomena that may appear on
the frontiers of progressively smaller scales. A technology is truly new
when it is made useful in the form of macroscopic devices that improve our
lives. Usefulness means that we must discover principles of constructing,
connecting, and packing multiscale flow systems into macroscopic
spaces.

And so I return to the start of this article. The new
gadgets are like the engines in the invasion contemplated by Carnot. It's
certain they will all be forgotten, unless there is a Sadi Carnot
watching, to see a pattern and immortalize it with a short page in the
thin book of principles. This happens only rarely, and when it does, it
illustrates again the constructal principle of "one sustains the crowd."

Adrian Bejan is the J.A. Jones Distinguished
Professor of
Mechanical Engineering at Duke University in Durham, N.C.
His research covers thermodynamics, natural convection, heat and mass
transfer, convection in porous media, and the constructal theory of
organization in nature.

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