901 publications from this institution
Flying animals morph and flex their wings during their flight. Their wings morph with the turbulent flow created around them. The wings of modern airplanes do not have this ability. In this study we show that the ability to flex the wings leads to greater stability (higher flutter speed), and that this is due to the more uniform distribution of stresses in the flexing wing. This way the flexing wing becomes the lightest per unit of flapping force, or the strongest per unit of weight.
Why are the rankings of universities not changing? Why is the demographic composition of top universities the same? In this review, these questions are addressed based on physics. Although size matters, higher ranks do not correlate with bigger sizes. The higher ranks belong to universities that have more authors who receive more citations. Citations are a record of how ideas spread from the source to the whole globe, in accordance with the physics of the logistics S-curve phenomenon. The spreading occurs in three periods – slow, fast, slow – and the population served by each idea during its lifetime depends on the size of the first big channel that carries the idea. An idea from a famous university has a larger spreading territory around it than an idea from a lesser-known university. Creativity is key: rankings come from visibility through citations, and, in turn, visibility for an author is aided by the higher visibility of the university. The demographic composition of the top universities is the same: for instance, the percentage of female authors and authors of East Asian origin among the 200 most cited authors does not vary significantly over the 20 highest ranked universities.
Heat and fluid flow in fluid-saturated porous media has become increas ingly more attractive to researchers and thus it has become a very pro ductive field for many researchers and practical engineers in very diverse range of fields. The great interest in the topic stems from its widespread number of different practical applications in modern industries and in many environmental issues, such as nuclear waste management, build ing thermal insulators, geothermal power plants, grain storage, etc. In building sciences and thermal insulation engineering, an appreciable in sulating effect has been derived by placing porous material in the gap between the cavity walls and multishield structures of nuclear reactors between the pressure vessel and the reactor. Geophysical applications include modeling of the spread of pollutants (e. g. radioactive mater ial), water movements in geothermal reservoirs, enhanced recovery of petroleum reservoirs, etc. These, and many other, important practical applications have resulted in a rapid expansion of research in the general area of porous media and thus generated a vast amount of both theor etical and experimental research work. It has attracted the attention of industrialists, engineers and scientists from many varying disciplines, such as applied mathematics, chemical, civil, environmental, mechanical and nuclear engineering, geothermal physics, food science, medicine, etc. This book contains some of the contributions to the NATO Advanced Study Institute on Emerging Technologies and Techniques in Porous Media that was held in Neptun-Olimp, Constanta, Black Sea, Romania on 9-20 June, 2003.
This paper addresses the question of how to cool a stack of parallel, heat-generating boards when the flow is impeded by electromagnetic screens placed upstream and downstream of the stack. Four separate designs are considered: (1) forced convection cooling of a stack with board-to-board spacing selected to minimize the stack-coolant thermal resistance; (2) forced convection cooling of stack with fixed board-to-board spacing; (3) natural convection cooling of a vertical stack with spacing selected to minimize the overall thermal resistance; and (4) natural convection cooling of a vertical stack with fixed spacing. The optimal spacings in designs (1) and (3) are determined by intersecting the known asymptotic solutions for stacks with small spacings and stacks with large spacings. The results of parts (1) and (2) are extended to applications where the stack is cooled by immersion in a free stream. We show that the effect of the screen is controlled by a single dimensionless group, which is identified for each class of designs; namely, forced versus natural convection, and high- versus low-screen Reynolds number. Engineering results are reported for the design of screens made of wire meshes, or perforated plate with square (sharp) edges.
This paper describes an experimental study of transient natural convection heating and cooling of a 30 cm diameter horizontal cylinder filled with water or water-glycerol solutions. The Rayleigh number range covered by these experiments is 8 × 107 −3 × 109. The flow regime is turbulent. The overall Nusselt numbers measured when the cylinder is filled only partially with water agree with the other measurements and low-Ra correlations, provided the average heat flux is based on the wetted portion of the cylinder surface. Qualitative descriptions of the turbulent circulation during heating experiments are also reported.
The generation of heat in high-density electronics is concentrated in spots, not uniform. Here we account for this fundamental aspect as a phenomenon of heat conduction in a flat body, from a concentrated heat source to surface dissipation of heat to an external flow. To facilitate conduction through the body, the heat source is connected to the rest of the body (the blade) with a vein of considerably higher thermal conductivity. The architecture resembles a botanical leaf, in which the heat source deposits the heat current at the root of the vein, and the upper and lower surfaces of the leaf dissipate the heat current to the surrounding fluid flow. The optimal dimensions of this leaflike architecture are determined analytically and numerically: the blade length, width, and thickness, the vein cross-sectional area, and the global thermal conductance of the entire multiscale body. The optimized features are nondimensionalized and reported as power laws that show explicitly the design effect of external convection and the thermal conductivities of the vein and the blade.
This paper reports the results of a fundamental study of the transient removal of a contaminant from a two-dimensional enclosure with one inlet and one outlet. The evolution of the flow and concentration fields are simulated numerically using Jones and Launder's low Reynolds number k-ε model. The Reynolds number is varied over the range 5–5000, where Re is based on the jet inlet width. The effectiveness of this forced convection mass transfer process is documented quantitatively in terms of a ventilation efficiency and a critical concentration decay time. The relationship between these quantitative performance parameters, the Reynolds number and the ventilation jet orientation, is reported. It is shown that significant gains in ventilation efficiency (or a shortening of the critical concentration decay time) can be made by properly orienting and positioning the inlet and outlet ports relative to each other and to the enclosure. The numerical ventilation efficiency results are summarized by a compact analytical expression based on a theoretical two-zone model of the enclosure flow.
The power-law correlation between frequency and rank is a physics characteristic of natural fl ow systems (river basins, demography) in accordance with the constructal law.Here we show that this robust characteristic is a consequence of their design as area-point and point-area fl ow systems that morph freely and compete for access on an area.The emergence of hierarchical design is illustrated with the rankings of university basketball programs.Although basketball rankings have the same character as the university rankings, there is no correlation between the two rankings.Academic excellence and basketball excellence are two different fl ow architectures on the same area, like the fl ow of humanity (demography) and the fl ow of water (river basins).Together, they show how the evolution of sports allows us to witness biological evolution in our life time.
The fundamental features of time-dependent heat transfer between two bodies making contact across a thin film of a lubricating fluid are described. The fluid is gradually squeezed out of the contact region as the two bodies are pressed against each other. Equations are obtained which contain a dimensionless time group and a dimensionless number governing the complete fluid mechanics of the thin film. A general solution is presented for the film thickness history. 9 refs.
The roofs of houses look similar in profile, especially in villages and old settlements. Why? The question is about the common angle of inclination. The answer comes from heat transfer by natural convection. Inspired by the evolution of the sapiens toward more power from the adoption of artifacts, we show that the existence of common roof shapes can be reasoned from the needs of those who live under the roof. Is there a shape that, while reducing the heat loss to the ambient, reduces the human effort (gathering firewood, etc.)? We consider two classes of roof shapes, Λ and cone, and two roof sizes, small and large (respectively, two flow regimes, laminar and turbulent). In laminar natural convection, the common profile of the Λ and the cone looks shallow, with height/base ratios comparable with 1/4. In turbulent flow, the Λ and the cone look like an equilateral triangle. These findings reinforce the evolutionary record of human civilization toward economy of effort and longer life. They also present an opportunity for the future ‘energy design’ of buildings: the external shape has a significant effect on the heat loss from the building.
No abstract is provided for this article.
Here we report the transient cooling performance of a body vascularized with tree-shaped channels supplied with coolant, which flows from one side of the body to the other. The vasculature consists of trees that alternate with upside down trees. Heat is generated volumetrically through the body at t=0. A time delay td separates the start of the flow of coolant from the start of heating. Three-dimensional simulations of conduction and convection in the solid-fluid composite show the formation and evolution of hot spots in the material. If the delay is not short enough, the maximum temperature of the body overshoots the maximum allowable level. The paper shows how to design the cooling delay time such that the hot-spot temperature does not exceed the safe level represented by the maximum temperature in the limit of steady state operation. The critical delay time is determined as a function of the applied pressure difference and the complexity of the dendritic flow architecture.
The first industrial application of superconducting materials appears to be their use as the field windings of rotating electrical machines. Research is being carried on throughout the world on this application, and many reports have appeared in the literature on the...
Here, I review the physics meaning of optimization, knowledge and design evolution, and why these concepts and human activities are profoundly useful for human life. A law of physics is a concise statement that summarizes a phenomenon that occurs in nature. A phenomenon is a fact, circumstance, or experience that is apparent to the human senses and can be described. The design in nature phenomenon facilitates access for everything that flows, evolves, spreads, and is collected: river basins, atmospheric and ocean currents, animal life and migration, and technology (the evolution of the “human-and-machine species,” wealth, life). This phenomenon is summarized by the constructal law: the occurrence and evolution of designs in nature, its time direction. Based on its record, the constructal law accounts for the design phenomenon and also for all the phenomena that have been described individually (ad-hoc) with end-design (destiny) statements of “optimality” (min, max). Most notably, the constructal law accounts for contradictory end-design statements such as minimum entropy production and maximum entropy production, and minimum flow resistance and maximum flow resistance.
The fundamental question in heat transfer engineering is to determine the relationship between the heat transfer rate and the driving temperature difference. In nature, many saturated porous media interact thermally with one another, and with solid surfaces that...
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...
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,...
The choice among available energy sources for a given task requires technical and economic tradeoffs on the part of the individual investor. From the national perspective, however, the effectiveness with which available energy sources are utilized may well become an overriding consideration. End-use matching is a procedure for introducing solar energy into the national energy infrastructure. The result of end-use matching is an identification of the most cost-effective combination of process energy needs, solar collector technology, geographic location, and economics by matching currently available solar system hardware with particular industrial processes and their locations. End-use matching is not intended to be a design tool for a specific plant, but rather a planning tool for determining where and why general applications solar systems appear economically viable in the near future. This paper discusses the end-use matching methodology, and illustrates first and second law thermodynamics analyses applied to a solar system producing process steam.
