901 publications from this institution
The term “mass transfer” is used here in a specialized sense, namely the transport of a substance that is involved as a component (constituent, species) in a fluid mixture. An example is the transport of salt in saline water. As we shall see below,...
People like to say that we cannot witness evolution because it occurred over a timescale immensely greater than human experience. Here, we show that it is possible to witness, explain, and predict evolution by focusing on technology evolution. We review the evolution of heat transfer devices from the point of view of the constructal law of design and evolution in nature. This is a broad view of the evolution where organs (components) are not stand-alone devices but play an important role in the 'live' finite-size systems that incorporate them. The larger systems can be human made or natural beings in which overall organization evolved, and continues to evolve. We show that heat transfer technology evolves predictably toward compactness, heat transfer augmentation, lower friction, and lower pumping power expenditure. These features are often misunderstood as 'competitors' in the design process, while in fact they are 'collaborators' from the perspective of the whole system that moves mass over the surface of the earth. Constructal law predicts that the characteristic size of the organ shifts toward lower thermodynamic imperfection and smaller sizes. In time, complexity increases predictably as larger numbers of high-density heat flux components become interconnected in finite-size constructs. Copyright © 2014 John Wiley & Sons, Ltd.
Why do individuals come to live (to move) together, to organize? Here, we propose that organization is a reflection of the physics reality (bio and nonbio) that it takes less power (useful energy, fuel, food, and exergy) to move 1 unit of mass in bulk than to move 1 unit individually. The objective of this work is to establish the connection between energy engineering and social organization and to bring social organization under the big tent of physics. We illustrate the predictability of organization and its evolution with 2 simple models of movement on an area, one inanimate (river basins, generated by several rules of construction) and the other animate (distribution of heated water for use in human settlements). The 2 models lead to the same conclusion: The movement becomes more hierarchical as the size and complexity of the architecture increase. The distribution can be made more uniform (more equal) by imposing identical channels distributed uniformly over the area. The flow architecture becomes a grid instead of a tree; yet, even in designs with imposed equality, the hierarchy persists. This theoretical framework also reveals the physical meaning of innovation: It is a local design change that liberates the flow over the entire territory inhabited by the organized movers.
This paper reports the development of optimal vascularization for supplying self-healing smart materials with liquid that fills and seals the cracks that may occur throughout their volume. The vascularization consists of two-dimensional grids of interconnected orthogonal channels with two hydraulic diameters (D1, D2). The smallest square loop is designed to match the size (d) of the smallest crack. The network is sealed with respect to the outside and is filled with pressurized liquid. In this work, the crack site is modelled as a small spherical volume of diameter d. When a crack is formed, fluid flows from neighbouring channels to the crack site. This volume-to-point flow is optimized using two formulations: (1) incompressible liquid from steady constant-strength sources located in every node of the grid and from sources located equidistantly on the perimeter of the vascularized body of length scale L and (2) slightly compressible liquid from an initially pressurized grid discharging in time-dependent fashion into one crack site. The flow in every channel is laminar and fully developed. The objectives are (a) to minimize the global resistance to the flow from the grid to the crack site and (b) to minimize the time of discharge from the pressurized grid to the crack site. It is shown that methods (a) and (b) yield similar results. There is an optimal ratio of channel diameters D2/D1 < 1, and it decreases as the grid fineness (L/d) increases. The global flow resistance of the grid with optimized ratio of diameters is approximately half of the resistance of the corresponding grid with one channel size (D1 = D2). The optimized ratio of diameters and the minimized global resistance depend on how the grid intersects the crack site: this effect is minor and stresses the robustness of the vascularized design.
Numerical calculation from the full differential equations for convection in an unbounded region is expensive, and hence approximate solutions are important. For small values of the Rayleigh number Ra, perturbation methods are appropriate. At large values of Ra...
In this chapter, we turn our attention to processes of combined (simultaneous) heat and mass transfer that are driven by buoyancy. The density gradients that provide the driving buoyancy force are induced by the combined effects of temperature and species...
This paper reports the results of a numerical study of the time-dependent removal of contaminant from a two-dimensional enclosure with one inlet and one outlet. The contaminant is generated beginning with the time t = 0 by a concentrated source located inside the enclosure. The contaminant is removed by the through flow established between the inlet and outlet ports. The flows studied cover the laminar and turbulent regimes represented by 30 ⩽ Re ⩽ 3000, where Re is the Reynolds number based on the inlet width and mean velocity. The effectiveness of the contaminant removal scheme is documented in terms of the removal efficiency η r , the volume-averaged concentration of contaminant C̄, and the critical (clean up) time t c . The effects of Re, ventilation jet orientation and source location are reported. It is shown that the movement and distribution of contaminant is complex and depends strongly on the source location. It is also shown how the relative positioning of the ports and the source location influence the contaminant removal process. The optimal inlet/outlet configuration associated with each position of the concentrated source of contaminant is reported. Slower ventilation schemes can lead to lower contaminant levels when a short contaminant removal time is not a major requirement.
No abstract is provided for this article.
In hyperthermia cancer treatments, a crucial problem is keeping the temperature of the normal tissue surrounding the tumor below a certain threshold so as not to cause damage to the tissue. Thus, obtaining a temperature field of the entire treatment region is important to control the process. In this study we develop a model and a numerical method for obtaining an optimal temperature distribution in a triple-layered skin structure embedded with multi-level blood vessels. The heat is induced by electromagnetic (EM) radiation. The dimensions and blood flow of multi-level blood vessels are determined based on the constructal theory.
Two parallel fluid streams in counterflow and in close thermal contact convect an energy current longitudinally, in the direction of the warmer stream. This paper describes the flow of convective heat currents through two identical tree networks superimposed, and oriented in counterflow. The convective currents form a single tree. The fluid flow is laminar through straight tubes. The ratios of successive tube diameters and tube lengths are deduced from the minimization of flow resistance subject to two constraints, total volume and total tube volume. It is shown that the total heat current convected by the double tree is proportional to the total volume raised to power 3/4. The conclusion is that the resistance to the loss of body heat governs the relation between metabolic rate and body size in all animals.
The second law aspects of heat transfer by forced convection are illustrated in terms of four fundamental flow configurations: pipe flow, boundary layer over flat plate, single cylinder in cross-flow, flow in the entrance region of a flat rectangular duct. The interplay between irreversibility due to heat transfer along finite temperature gradients and, on the other hand, irreversibility due to viscous effects is analyzed in detail. The spatial distribution of irreversibility, entropy generation profiles or maps, and those flow features acting as strong sources of irreversibility are presented. It is shown how the flow geometric parameters may be selected in order to minimize the irreversibility associated with a specific convective heat transfer process.
This lecture outlines the basis for the entropy generation minimization method, and a series of key applications in power generation, refrigeration, and energy conservation. The lecture begins with a review of the concept of irreversibility, entropy generation, or exergy destruction. The proportionality between exergy destruction and entropy generation is used in the search for improved thermodynamic performance subject to finite-size constraints and specified environmental conditions. Examples are drawn from refrigeration, energy storage systems for sensible heat and latent heat, solar energy, and the generation of maximum power by using a stream of hot gas. It is shown that the physical structure of the system springs out of the process of global thermodynamic optimization subject to global constraints. This principle generates structure not only in engineering but also in physics and biology (constructal theory).
The developments that Geoffrey West and James Brown review in their article “Life’s Universal Scaling Laws” (Physics Today, September 2004, page 36) are important, but they fit into a much greater theoretical framework. West and coauthors’ first paper appeared in April 1997. 1 1. G. B. West, J. H. Brown, B. J. Enquist, Science 276, 122 (1997) https://doi.org/10.1126/science.276.5309.122. I published the basic idea behind their approach to the modeling of tree flows one year earlier 2 2. A. Bejan, J. Adv. Transportation 30, 85 (1996) https://doi.org/10.1002/atr.5670300207. as part of the constructal theory of organization in nature. Constructal theory, reviewed in my 1997 book Advanced Engineering Thermodynamics , 2nd edition (Wiley) and more recently, 3 3. A. Bejan, Shape and Structure: From Engineering to Nature, Cambridge U. Press, New York (2000). is now a growing field, with articles appearing regularly, including in physics journals. 4 4. L. Ghodoossi, N. Egrican, J. Appl. Phys. 93, 4922 (2003) https://doi.org/10.1063/1.1562008 S. M. Senn, D. Poulikakos, J. Appl. Phys. 96, 842 (2004) https://doi.org/10.1063/1.1757028. Constructal theory is about the generation of shape and structure in nonequilibrium thermodynamic systems—flow systems—in general. Simply put, the constructal law states, “For a finite-size flow system to persist in time (to live), it must evolve in such a way that it provides easier access to the currents that flow through it.” 2,3 2. A. Bejan, J. Adv. Transportation 30, 85 (1996) https://doi.org/10.1002/atr.5670300207.3. A. Bejan, Shape and Structure: From Engineering to Nature, Cambridge U. Press, New York (2000). The constructal law is not about what flows—fluid, energy, momentum, goods, or people—but about how the flow system generates its architecture. The three key assumptions that West and coauthors stated in 1997 and that West and Brown reviewed in their Physics Today article are present in constructal theory, not as convenient assumptions to make a model work, but as invocations of the constructal law. In particular, a space-filling tree architecture can be deduced from the constructal law. Constructal trees are not fractal objects.In a constructal tree there are at least two flow modes. The slow mode, which describes low-conductivity, low-permeability, high-unit-cost processes such as diffusion or walking, corresponds to interstices in the tree architecture. The fast mode, which describes high-conductivity, high-permeability, low-unit-cost processes such as flowing water and moving vehicular traffic, corresponds to channels in the architecture. Interstices and channels optimally connect to form a “tree,” in which the resistance across interstices is balanced against the resistance along channels.Because West’s three assumptions are consequences of constructal theory, every successful derivation of an allometric law that West and coworkers make is an affirmation of the validity of that theory.In return, every successful invocation of the constructal law in domains far removed from the living flow systems of West and coauthors is an indication not only that their model is correct but that it is an integral part of a theoretical framework that unites biology with physics and engineering. In addition to allometric laws, constructal theory covers many phenomena that fall well outside the biological scaling reviewed by West and Brown. Some examples are turbulent flow structure in various flow configurations, cracks in shrinking solids, the structure of animal hair, refraction in geographical economics, flight, atmospheric and oceanic circulation, and the structure of power plants and refrigeration plants—that is, the architecture of “human-plus-machine” species.West and Brown conclude by asking, Is all life organized by a few fundamental principles? According to constructal theory, the answer is yes.REFERENCESSection:ChooseTop of pageREFERENCES <<CITING ARTICLES1. G. B. West, J. H. Brown, B. J. Enquist, Science 276, 122 (1997) https://doi.org/10.1126/science.276.5309.122. Google ScholarCrossref, ISI2. A. Bejan, J. Adv. Transportation 30, 85 (1996) https://doi.org/10.1002/atr.5670300207. Google ScholarCrossref, ISI3. A. Bejan, Shape and Structure: From Engineering to Nature, Cambridge U. Press, New York (2000). Google Scholar4. L. Ghodoossi, N. Egrican, J. Appl. Phys. 93, 4922 (2003) https://doi.org/10.1063/1.1562008 , Google ScholarCrossref, ISI S. M. Senn, D. Poulikakos, J. Appl. Phys. 96, 842 (2004) https://doi.org/10.1063/1.1757028. , Google ScholarCrossref, ISI© 2005 American Institute of Physics.
This is a review of recent work on how the physics of evolution accounts for the origin, changes and future of social systems. Evolution means the changes (spatial, temporal) that happen with discernible direction in time. Changes are everywhere because physical features of ‘freedom to change’ are in every moving and flowing system. With freedom comes evolution, and with evolution come all these visible things, movement, change, configurations, diversity, multiple scales, and the universality of social organization phenomena such as human settlements, hierarchy, economics, economies of scale and diminishing returns.
Vascular flow architectures are proposed for controlling the temperature of walls that are subjected suddenly to intense heating from one side. After a short delay, single-phase coolant starts flowing from the other side, and fights off the heating effect. The time-dependent behavior of such vascularized composites is studied and optimized based on full numerical simulations of transient conjugate heat transfer. The focus is on the hot-spot temperatures that build up inside the composite – their evolution, spatial migration, and highest levels. Of interest are vasculature designs that keep the hot-spot temperatures below the safe level associated with long-term operation with steady heating from one side and steady coolant flow from the other side. It is shown that when the driving pressure difference is fixed, the approach to the steady-state temperature is the shortest when the dendrites have an optimal (finite) number of bifurcation levels. The allowable delay time is approximately the same as the duration that the hot-spot temperature reaches the steady-state hot-spot temperature in the absence of coolant.
No abstract is provided for this article.
Here we show that constructal-law physics unifies the design of animate and inanimate movement by requiring that larger bodies move farther and their movement on the landscape last longer. The life span of mammals must scale as the body mass (M) raised to the power 1/4 and the distance traveled during the lifetime must increase with body size. The same size effect on life span and distance traveled holds for the other flows that move mass on earth: atmospheric and oceanic jets and plumes, river basins, animals and human operated vehicles. The physics is the same for all flow systems on the landscape: the scaling rules of "design" are expressions of the natural tendency of all flow systems to generate designs that facilitate flow access. This natural tendency is the constructal law of design and evolution in nature. Larger bodies are more efficient movers of mass on the landscape.
Nature shows that fluids bathe the animal body as trees matched canopy to canopy. The entering streams invade the body as river deltas and the reconstituted streams sweep and exit the body as river basins. Why should this be so? Why is animal vascularization not based on arrays of parallel channels, as in modern heat exchangers? In this paper, we rely on constructal theory to show that the flow architecture that provides greatest access from point to volume and from volume to point is the three-dimensional compounding of trees matched canopy to canopy. This three-dimensional tree architecture is deduced, not assumed. Its flow performance is evaluated at every step relative to the performance of equivalent architectures with parallel channels. This paper also shows that the dendritic design must become more complex (with more levels of branching) as the volume inhabited by the flow design increases. The transition from designs with p branching levels to p+1 levels occurs abruptly as the available flow volume increases. This fundamental development has implications not only in evolutionary animal design but also in animal tissue modeling and the design of new vascular (smart) materials with volumetric functionalities such as self-cooling and self-healing.
