901 publications from this institution
This paper discusses experimental observations of the meandering fall of light-weight tissue paper ribbons. The photographs show that the ribbons assume a sinusoidal shape with a unique wavelength which scales with the thickness of the airstream entrained by the ribbon.
No abstract is provided for this article.
No abstract is provided for this article.
No abstract is provided for this article.
Convection in Porous Media, 4th Edition, provides a user-friendly introduction to the subject, covering a wide range of topics, such as fibrous insulation, geological strata, and catalytic reactors. The presentation is self-contained, requiring only routine mathematics and the basic elements of fluid mechanics and heat transfer. The book will be of use not only to researchers and practicing engineers as a review and reference, but also to graduate students and others entering the field. The new edition features approximately 1,750 new references and covers current research in nanofluids, cellular porous materials, strong heterogeneity, pulsating flow, and more.
This paper reports optimal bifurcation shapes (T and Y) in turbulent regime of tree-shaped flows. Unlike earlier studies of T and Y constructs, here the effect of pressure losses at the junction is taken into account, and the wall roughness and duct cross-sectional shapes are free to vary. The optimal ratio of duct cross-sectional areas (as a generalization of Murray's law), the optimal ratio of duct lengths, and the optimal angle between the branches of the Y are presented. These optimal geometrical features are reported as functions of the flow direction (splitting flow versus merging flow), wall roughness, duct cross-sectional shape, and svelteness. The svelteness, Sv, is a global property defined as the external length scale of the flow construct divided by the internal length scale. It is shown that the effect of junction pressure losses on the optimized architecture can be neglected when Sv3∕2 is greater than approximately 104. Two dimensionless terms are introduced and shown to be useful for the optimization of flow networks.
No abstract is provided for this article.
This is a fundamental theoretical and numerical study of the evolution of tree-shaped flow on one side of the heat transfer surface of a ‘dendritic’ heat exchanger. The flow is shaped by step changes in the dimensions of the available openings. The architecture evolves toward providing easier access to the stream. Two extremes of this flow architecture are treated theoretically, orifices in parallel and equidistant partitions, and slender two-dimensional channels (slits) in sequential blocks of channels. The orifice and channel sizes change stepwise in the downstream direction. Theoretically, the recommended step down ratio for orifice diameters is 0.707, and for the slit size is 0.595. The theoretical step down ratio for the lengths of parallel channels is 0.648. Numerical simulations conducted in a wide parametric domain illustrate the evolution of the flow configuration toward greater access. Remarkable is that the numerical results for the step down ratio in slit sizes covers the very narrow range 0.72–0.75. This study contributes fundamentals that can serve as reliable reference in future designs of dendritic heat exchangers spearheaded by constructal theory.
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This paper draws attention to the 1996 constructal theory of the generation of geometric form in flow systems. Flow architecture can be reasoned on the basis of principle: the maximization of global performance subject to finite-size constraints. One example is the generation of tree-shaped flow patterns, as optimized paths between one point (source, sink) and an infinity of points (area, volume), The optimized tree-flow architecture accounts for allometric laws, for example, the proportionality between metabolic rate and body size raised to the power 3/4, and the proportionality between breathing and heart beating times and body size raised to the power 1/4. Another example is the proportionality between the cruising speed of flying bodies (insects, birds, airplanes) and body mass raised to the power 1/6, The “thermodynamics law” status of the constructal principle is discussed. 1 Constructal theory versus biomimetics Geometric similarities and patterns abound in flow systems in engineering and in nature. For example, tree-shaped flows are everywhere, in computers, lungs, dendritic crystals, urban street patterns, and communication links, In a new book [1], I started from the design and optimization of engineered systems and developed a deterministic principle for the generation of geometric form in natural systems. In flow systems for fluid, heat, mass, electricity, goods, and traffic, better performance means improved access: minimal flow resistance, minimal travel time, minimal cost. This observation led to constructal theo~,
The focus of this paper is on the fundamentals of the transient mechanism responsible for the establishment of natural convection in a two-dimensional porous layer confined between isothermal vertical walls and insulated horizontal walls. In the first part of the paper, pure scaling arguments are used to identify the characteristic time scales of the phenomenon and, based on these scales, the evolution of the porous system to steady state. The type of steady state is shown to depend on the relative order of magnitude of the characteristic time scales. Distinct vertical boundary layers are shown to be possible if (L/H)Ra1/2>1, and distinct horizontal layers if (H/L)1/2 Ra1/4>1. In the second part of the paper, these and other scaling laws of the transient evolution to steady state are successfully tested against a series of transient numerical experiments conducted in the range H/L=0.2–5 and Ra=10–50.
Here we see why humans unwittingly build fires that look the same: edifices of fuel, as tall as they are wide. The pile of fuel is permeable, air invades it by natural convection and drives the combustion. I show that the hottest pile of burning fuel occurs when the height of the pile is roughly the same as its base diameter. Future studies may address the shape effect of wind, material type and packing. Key is why humans of all eras have been relying on this design of fire “unwittingly”. The reason is that the heat flow from fire facilitates the movement and spreading of human mass on the globe.
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 patterns of heatlines reported in this paper illustrate for the first time the true path of convective heat transfer through a saturated porous medium. Heatline patterns are reported for the following fundamental configurations: the boundary layer near an isothermal wall, the boundary layer near a wall with uniform heat flux, and the two-dimensional porous layer confined by two parallel plates. Emphasis is placed on the convection features that are being visualized for the first time by the heatline method, i.e., not by traditional methods such as the use of isotherms. It is shown that the heatlines of a flow in which the wall serves as heat sink are unlike the heatlines of the same flow with a wall that serves as heat source. The seepage flow with slip at the boundary is visualized by heatlines that leave a hot wall at an angle. The wall heat-flux distribution is visualized by the density of the heatlines that intersect the wall. The heatline pattern in fully developed flow of a pure fluid through a parallel-plate channel is also reported in order to emphasize that the pure-fluid pattern is not exactly the same as the pattern in the corresponding two-dimensional space filled with seepage flow through a porous medium.
No abstract is provided for this article.
This paper illustrates the morphing of flow architecture toward greater performance in a counterflow heat exchanger. The architecture consists of two plenums with a core of counterflow channels between them. Each stream enters one plenum and then flows in a channel that travels the core and crosses the second plenum. The volume of the heat exchanger is fixed while the volume fraction occupied by each plenum is variable. Performance is driven by two objectives, simultaneously: low flow resistance and low thermal resistance. The analytical and numerical results show that the overall flow resistance is the lowest when the core is absent, and each plenum occupies half of the available volume and is oriented in counterflow with the other plenum. In this configuration, the thermal resistance also reaches its lowest value. These conclusions hold for fully developed laminar flow and turbulent flow through the core. The curve for effectiveness vs number of heat transfer units (N tu) is steeper (when N tu <1) than the classical curves for counterflow and crossflow.
This paper describes several fundamental trade offs that govern the optimization of cooling techniques for heat generating electric devices. Five basic cooling configurations are optimized. It is shown that in forced air cooling above room temperature the fan power requirement is minimum when the heat transfer contact area is optimized to a value that is of the order of 102 A f, where A f is the flow cross-sectional area. This optimal contact area is independent of whether the coolant is mixed or unmixed inside the heat generating enclosure. In air cooling with natural updraft, the heat generation rate (or amount of electronics, or overall thermal conductance) is maximum when the heat transfer contact area is of order 102 A f. In forced convection cooling at cryogenic temperatures, the refrigerator power requirement has minima with respect to the cold-end (refrigeration load) temperature and the heat transfer contact area. The optimal heat transfer area is again of the order of 102 A f.
This paper reviews recent progress on constructal theory and design. The emphasis is on the development of multi-scale, nonuniformly distributed flow structures that offer increased compactness (e.g., heat transfer density). Examples are counterflow heat exchangers with tree-shaped hot and cold streams, and tree architectures on a disc. Every flow system has a property called svelteness (Sv), which is the ratio between its external (global) length scale and its internal length scale ( V 1 / 3 ), where V is the volume occupied by all the ducts. Emphasis is placed on the development of simple strategies for decreasing the computational cost required by the development of such structures. The generation of multi-scale flow configurations is a process that can be projected on a diagram having global performance on the abscissa and degrees of freedom on the ordinate. This process rules the development (evolution) of all flow configurations for systems with global objective, global constraints and freedom to morph.
