Constructal Law

The Shape of a Perfect Fire
A new Duke theory identifies the height-to-base ratio that
helped humanity master fire and migrate across the globe
By Ken Kingery, Duke University Pratt School of Engineering, June 8, 2015

From ancient Egyptians roasting a dripping cut of beef next to the Great Pyramid of Giza to a Boy Scout learning to build a log cabin fire in his backyard, everyone builds fires with the same general shape.

And now we know why.

In a study published in Nature Scientific Reports on June 8, 2015, Adrian Bejan, the J.A. Jones Professor of Mechanical Engineering at Duke University, shows that, all other variables being equal, the best fires are roughly as tall as they are wide. This is why, he argues, everyone has built fires that basically look the same since the dawn of time.

“Humans from all eras have been relying on this design,” said Bejan. “The reason is that this shape is the most efficient for air and heat flow. Our success in building fires in turn made it possible for humans to migrate and spread across the globe.  Heat flow from fire facilitates the movement and spreading of human mass on the globe, which is a direct prediction of the Constructal Law*.”

In 1996, Bejan penned the Constructal Law that postulates that movement—or “flow”— systems such as trees, rivers or air currents evolve into configurations that provide easier and easier access to flows. Now internationally recognized, the law is increasingly finding applications in improving design and maximizing efficiency of manmade systems.

Bejan continued, “Our bonfires are shaped as cones and pyramids, as tall as they are wide at the base. They look the same in all sizes, from the firewood in the chimney, to the tree logs and wooden benches in the center of the university campus after the big game. They look the same as the pile of charcoal we make to grill meat. And now we know why.”

http://www.pratt.duke.edu/news/fire

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*Constructal Law

Constructal law is a theory of the generation of design (configurations, patterns, geometry) in nature. According to this theory, natural design and the constructal law unite all animate and inanimate systems.  The constructal law was stated by AdrianBejan in 1996 as follows: "For a finite-size system to persist in time (to live), it must evolve in such a way that it provides easier access to the imposed currents that flow through it."  The constructal law is receiving increased acceptance within the scientific community.

"Constructal" is a word coined by Bejan to describe the natural tendency of flow systems (e.g. rivers, trees and branches, lungs, tectonic plates and engineered forms) to generate and evolve structures that increase flow access.

Introduction

The constructal law was proposed in 1996 as a summary of all design generation and evolution phenomena in nature, bio and non-bio. The constructal law represents three steps toward making “design in nature” a concept and law-based domain in science:

1. Life is flow: all flow systems are living systems, the animate and the inanimate.
2. Design generation and evolution is a phenomenon of physics.
3. Designs have the universal tendency to evolve in a certain direction in time.

The constructal law is proposed as a first principle of physics accounting for all design and evolution in nature. It holds that shape and structure arise to facilitate flow. The designs that happen spontaneously in nature reflect this tendency: they allow entities to flow more easily – to measurably move more current farther and faster per unit of useful energy consumed.  Rain drops, for example, coalesce and move together, generating rivulets, streams and the mighty river basins of the world because this design allows them to move more easily.

The constructal law asks the question: Why does this design arise at all? Why can't the water just seep through the ground? The constructal law provides this answer: Because the water flows better with design. The constructal law covers the tendency of nature to generate designs to facilitate flow.

http://en.wikipedia.org/wiki/Constructal_law