901 publications from this institution
Science holds that the arrow of time in nature is imprinted on one-way (irreversible) phenomena and is accounted for by the second law of thermodynamics. Here I show that the arrow of time is painted much more visibly on another self-standing phenomenon: the occurrence and change (evolution in time) of flow organization throughout nature, animate and inanimate. This other time arrow has been present in science but not recognized as such since the birth of thermodynamics. It is Maxwell's demon. Translated in macroscopic terms, this is the physics of the phenomenon of design, which is the universal natural tendency of flow systems to evolve into configurations that provide progressively greater access over time and is summarized as the constructal law of design and evolution in nature. Knowledge is the ability to effect design changes that facilitate human flows on the landscape. Knowledge too flows.
In this paper we report experimentally the hydraulic and thermal behavior of a balanced counterflow heat exchanger in which each stream flows through a tree-shaped structure covering a circular area. The tree structure is the same on both sides of the heat exchanger: they have three channels reaching/leaving the center, and three branching levels (i.e., 24 channels on the periphery of the circular area). On the hot side, fluid is pumped from the center to the periphery. On the cold side, fluid is pumped from the periphery to the center, and leaves the heat exchanger as a single stream. Two experimental apparatuses were built and tested. In the first design, the body of the heat exchanger was made out of plexiglass and a peripheral plenum was used to collect or distribute the working fluid to the tree structure. The measurements showed that the use of a plenum generates undesirable volumetric flow asymmetries. These lessons led to a second design, which has two major improvements: (i) the heat exchanger core was made out of aluminum and (ii) individual ports (inlets/outlets) were used for each of the peripheral channels. The hydraulic results show a relation between the appearance of volumetric flow rate asymmetries and the bifurcation angles throughout the dendritic structure. The heat transfer results are also discussed.
The recorded history of technology and science and art shows that the evolutionary path of design is toward a greater number of dimensions, more degrees of freedom, and greater performance, efficiency and economy. Until now, designs have evolved in steps from one to two and three dimensions. The future will bring one more step, to four dimensions: three-dimensional objects that morph in time in accord with their time-changing environmental conditions. This concept is illustrated with the thermo-fluid design and time-behavior of a volume filled with parallel plates cooled by forced convection. When the pressure difference that drives the flow varies stepwise in time, the plate to plate spacing for maximum heat transfer density must change. If the structure is free to morph to maintain its optimal spacings in step with its changing environment, then the time-integrated performance of the morphing object is maximum. If the structure is rigid (sized optimally for one flow condition), its performance is inferior. The general significance and applicability of this future of design activity is discussed.
This book presents the diverse and rapidly expanding field of Entropy Generation Minimization (EGM), the method of thermodynamic optimization of real devices. The underlying principles of the EGM method - also referred to as "thermodynamic optimization," "thermodynamic design," and "finite time thermodynamics" - are thoroughly discussed, and the me
No abstract is provided for this article.
This paper describes the heat transfer and fluid flow through a confined porous layer heated and cooled along one of the vertical side walls. In the first part of the study, consideration is given to the case when the side-heating effect is positioned above the side-cooling effect. Numerical experiments and scale analysis show that the natural circulation in this first configuration is either one of incomplete vertical penetration, or one of incomplete horizontal penetration. Scale analysis shows that the heat transfer and fluid flow scales differ substantially from one type of penetrative convection to the other, in agreement with the trends revealed by numerical simulations. When the heated portion of the side wall is located below the cooled portion, the flow spreads throughout the porous medium. Scale analysis shows that if the Rayleigh number is sufficiently high, the two mirror-image cells of this flow make contact with the vertical side wall and with each other via slender boundary layer regions. The parametric domain in which the conclusions of this study are valid is outlined on the H/L-Ra H plane, where H/L is the height/length ratio and Ra H is the Darcy-modified Rayleigh number based on porous system height.
This paper shows that the emergence of body organs is predictable as an integral part of the design for moving animal mass more easily on Earth, in accord with the constructal law of design in nature: For a finite-size open system to persist in time (to live), it must evolve such that it provides easier access to the imposed (global) currents that flow through it. Every organ destroys useful energy in two ways: Internally by thermodynamic irreversibilities and by having to be carried. From the constructal law follows the necessity of characteristic-size organs and the emergence of solid columns (legs) to facilitate the flow of stresses. This natural “wheel” endows the body with rolling (falling-forward) locomotion, with predicted speeds that agree with the observed speeds in the body mass range of 10−6–103 kg. The constructal law also accounts for animal design features for changing speeds. Skeletons (bones and legs) are solid organs that emerge in accordance with the constructal-law design of moving animal mass: More and stronger material emerges along the lines of highest stresses. A connection between animal wheel movement and swimming, water waves, and tsunamis is also made.
No abstract is provided for this article.
This review draws attention to an emerging body of work that relies on global thermodynamic optimization in the pursuit of flow system architecture. Exergy analysis establishes the theoretical performance limit. Thermodynamic optimization (or entropy generation minimization) brings the design as closely as permissible to the theoretical limit. The design is destined to remain imperfect because of constraints (finite sizes, times, and costs). Improvements are registered by spreading the imperfection (e.g., flow resistances) through the system. Resistances compete against each other and must be optimized together. Optimal spreading means spatial distribution, geometric form, topology, and geography. System architecture springs out of constrained global optimization. The principle is illustrated by simple examples: the optimization of dimensions, spacings, and the distribution (allocation) of heat transfer surface to the two heat exchangers of a power plant. Similar opportunities for deducing flow architecture exist in more complex systems for power and refrigeration. Examples show that the complete structure of heat exchangers for environmental control systems of aircraft can be derived based on this principle.
Exergy analysis is applied to sudden thermal interaction between a molten metal (m m) and water (m w). The melt experiences cooling, solidification and subcooling. The water is pressurized at constant volume. It is found that the exergetic efficiency is of the order of the mass ratio m m/m w when this ratio is much smaller than 1. This result is relatively insensitive to the definition of the dead state and to whether the melt is initially superheated. The reported exergetic efficiency is an upper bound because of the irreversibility of the expansive processes that occur after the initial thermal interaction. We use the model to account for irreversibility due to flow of the mixture through the cracks of the mixing vessel.
This book offers a unifying view of nature, evolution, freedom, complexity, diversity, organization, hierarchy, direction of change in time, and their manifestations in science, society and engineering. It begins with familiar designs that are around us and inside of us.
The objective of the Journal of Mechanics is to provide an international forum to foster exchange of ideas among mechanics communities in different parts of world.
The objective of the Journal of Mechanics is to provide an international forum to foster exchange of ideas among mechanics communities in different parts of world.
ComplexityComplexity is a lot like freedomFreedom: everybody knows, but very few can tell what it is. Complex is often confused with complicated and random. ComplexityComplexity, organizationOrganization, and evolutionEvolution in natureNature are most powerful and...
What is evolution and why does it exist in the biological, geophysical and technological realms - in short, everywhere? Why is there a time direction - a time arrow - in the changes we know are happening every moment and everywhere? Why is the present different than the past? These are questions of physics, about everything, not just biology. The answer is that nothing lives, flows and moves unless it is driven by power. Physics sheds light on the natural engines that produce the power destroyed by the flows, and on the free morphing that leads to flow architectures naturally and universally. There is a unifying tendency across all domains to evolve into flow configurations that provide greater access for movement. This tendency is expressed as the constructal law of evolutionary flow organization everywhere. Here I illustrate how this law of physics accounts for and unites the life and evolution phenomena throughout nature, animate and inanimate.
