901 publications from this institution
This month in Tokyo we are seeing athletic competition at its best. Amazing individuals bring to the playing field the fruits of hard training and dedication, and the beauty of fair play. The spectators admire the performances and feel inspired. They
Natural porous structures are heterogeneous with multiple scales that are distributed nonuniformly. Few large pores (fissures, channels, and cracks) are accompanied by numerous finer channels. Can this type of flow architecture be attributed to a principle of maximization of global flow access? Features similar to those of multiscale porous structures are exhibited by tree-shaped flow structures. Trees have been deduced from the maximization of flow access between a point and a volume, a point and an area, and a point and a curve (e.g., circle). In this paper we invoke the same principle and consider fundamentally the question of how to bathe with minimal flow resistance a microchannel structure that globally behaves as a porous medium. We develop completely multiscale configurations that guide the flow from one side of the porous structure to the other (line to line and plane to plane) and show analytically the advantages of tree structures over the usual stacks of parallel microchannels. The “porous medium” that has tree-shaped labyrinths is heterogeneous, with multiple scales that are distributed nonuniformly. These features justify comparisons with the design of natural porous structures.
Workflow management techniques are aimed at supporting business processes across organizational boundaries. Current techniques lack the formal tools to model and analyze workflows on a large scale, and current WFMSs do not have the ability to react to the response of another WFMS dynamically. Using Petri nets as the modeling tool, we present the concept of a standard workflow structure, and a set of standard workflow blocks are designed. We prove that modeling a workflow process in standard workflow structures can guarantee the soundness of a workflow network as defined by W.M.P. van der Aalst (1998). We also present a method of decomposing a large-scale workflow network into sub-networks. From our analysis and proof, we draw the very good conclusion that a workflow network composed of self-loop-connected sub-networks maintains soundness under proper structural conditions. The method presented is not only a design language for the specification of complex workflows, but also a powerful analysis technique to verify the workflow procedures.
This study illustrates the thermodynamic information that is being routinely overlooked by a pure fluid mechanics analysis. The integral analysis of the two-dimensional turbulent jet is used as an example. It is shown that even when the temperature difference between the nozzle fluid and the reservoir fluid is zero, the jet region is nonisothermal. At every position along the jet, the temperature rise becomes maximum when the entrainment coefficient assumes a value that is comparable with the value determined experimentally. Furthermore, it appears that the natural shape of the velocity and temperature profiles of the jet is the one that minimizes the total entropy generation rate. This study suggests that an important relationship exists between the empirical components of the pure-fluid-mechanics integral treatment, and the temperature and entropy generation extrema unveiled by the thermodynamic analysis.
Our nature is continuous and morphing, not infinitesimal, discrete, and still. With the law of evolution, physics accounts for form and the time direction in evolving macroscopic structures everywhere.
By a porous medium we mean a material consisting of a solid matrix with an interconnected void. We suppose that the solid matrix is either rigid (the usual situation) or it undergoes small deformation. The interconnectedness of the void (the pores) allows the flow of...
This paper extends to three dimensions and to convective heat transfer the constructal method of minimizing the thermal resistance between a volume and one point. In the first part of the paper, the heat flow mechanism is conduction, and the heat generating volume is occupied by low conductivity material (k 0) and high conductivity inserts (k p). At the elemental-volume level the inserts are shaped as constant-thickness disks. In the first assembly the disks are mounted on a common stem of k p material. The internal and external geometric aspect ratios of the elemental volume and the first assembly are optimized numerically subject to volume constraints. In the second part of the paper the interstitial spaces once occupied by k 0 material are bathed by forced convection. The k p inserts function as ‘fin material’, and the first assembly becomes a fin bush with cylindrical symmetry. The geometry of the first assembly is optimized subject to total volume and solid volume constraints. The optimal number of circular fins, the optimal external shape of the assembly, the optimal ratio of the central stem diameter divided by the circular fin thickness, and the maximized global conductance of the assembly are reported as functions of the external flow (pressure drop number) and volume fraction of fin material. It is also shown that the optimization sequence can be shortened (albeit with approximate results) by adopting at the first-assembly level the internal geometric aspect ratios that were optimized independently at the elemental-volume level.
Thermodynamics is brief, simple, unambiguous and improving.Yet, confusion reigns in the field.The word "entropy" is pasted on almost any new thing, without any respect for its proper definition in thermodynamics.Every author bows to his own maximum or minimum principle, even when it contradicts English, not just thermodynamics.Minimizing resistance cannot be the same as maximizing resistance.Minimizing entropy generation cannot be the same as maximizing entropy generation.Because of the word "entropy", many believe that entropy generation minimization and maximization are covered by the second law, which is incorrect, twice.Because for an isolated system (or an adiabatic closed system) the second law states that the system entropy inventory increases during changes inside the system, many believe that the second law accounts for organization, evolution, and the arrow of time.This too is incorrect.It is time for a reality check, and this means to take a look at nature, at the physics, at the science of all the natural things that "happen".Here then is a review of the few, the noble, the laws with which in science we cover the few distinct phenomena that nature is made of.
This paper demonstrates the existence of an optimal on/off sequence for operating a household refrigerator that accumulates ice on its evaporator coils. Experimentally it is shown that the rate of ice formation is constant in time. The optimal (intermittent) regime of operation determined in this paper minimizes the power required by the refrigerator, maintains the prescribed temperature of the cold space, and removes intermittently the ice layer. The second part of the paper proves that a similar strategy can be used for dealing with fouling in heat exchangers, i.e. for maximizing the power output of a power plant that is diminished by the formation of scale on its heat exchanger surfaces. These optimal on/off sequences of operation for refrigerators and power plants are proven based on pure heat transfer and thermodynamics grounds. The refrigerator and power plant models employed are the simplest models possible. Consequently, the optimal on/off sequences demonstrated in this paper are fundamental features that will be present (and deserve to be identified and exploited) in the design of actual refrigerators and power plants, no matter how complicated these designs may be.
The cerebral vascular network has evolved in such a way so as to minimize transport time and energy expenditure. This is accomplished by a subtle combination of the optimal arrangement of arteries, arterioles and capillaries and the transport mechanisms of convection and diffusion. Elucidating the interaction between cerebral vascular architectonics and the latter physical mechanisms can catalyse progress in treating cerebral pathologies such as stroke, brain tumours, dementia and targeted drug delivery. Here, we show that brain microvascular organization is predicated on commensurate intracapillary oxygen convection and parenchymal diffusion times. Cross-species grey matter results for the rat, cat, rabbit and human reveal very good correlation between the cerebral capillary and tissue mean axial oxygen convective and diffusion time intervals. These findings agree with the constructal principle.
WhenDiscipline knowledge develops faster than languageLanguage, the new is often described in ad hoc terms that vary from one speaker to the next. In time, the languageLanguage catches up by zeroing in on the fewest new terms and rulesRules that govern the new...
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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 global property called sveltness (Sv), which is the ratio between its external (global) length scale and its internal length scale (V1/3), 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.
This paper reports the optimal board-to-board spacing and maximum total heat transfer rate from a stack of parallel boards cooled by laminar forced convection. The optimal spacing is proportional to the board length raised to the power 1/2, the property group (μα) 1 4 , and (ΔP)− 1 4 , where ΔP is the pressure head maintained across the stack. The maximum total heat transfer rate is proportional to (ΔP) 1 2 , the total thickness of the stack (H), and the maximum allowable temperature difference between the board and the coolant inlet. Board surfaces with uniform temperature and uniform heat flux are considered. It is shown that the surface thermal condition (uniform temperature vs uniform heat flux) has a minor effect on the optimal spacing and the maximum total heat transfer rate.
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.
This paper shows that the temporal and spatial structure of adsorption–desorption processes can be optimized for maximal packing of mass transfer into a fixed space, and for minimal overall pumping power. In the first part of the paper, simple models demonstrate that the periodicity of such processes can be optimized in three settings: fixed desorption time, variable desorption time, and variable species concentration in the gas space. The second part of the paper shows how the optimized time scales determine the dimensions of the smallest flow channels—the elemental system. The available space can be packed with elemental systems in a hierarchical (constructal) structure [Bejan, Shape and Structure, from Engineering to Nature, Cambridge University Press, 2000], in which elements are assembled into first constructs, first constructs are assembled into second constructs, etc. The robustness and nearly optimal performance of the optimized mass exchanger structure is discussed.
This paper shows that the intermittent operation of a defrosting vapour-compression-cycle refrigerator can be optimized with respect to: (1) the frequency of on/off operation, and (2) the way in which the supply of heat exchanger surface is divided between evaporator and condenser. The method used is that of thermodynamic optimization (or entropy generation minimization, or finite-time thermodynamics), in which heat transfer and thermodynamic aspects are accounted for simultaneously to produce a realistic description of the in-time operation of the installation. The optimal on/off frequency and surface allocation ratio are reported in non-dimensional charts, which show the effect of the heat exchanger size, cycle temperature ratio, defrosting time, compressor efficiency, and refrigerant type. The charts are based on the real properties of refrigerants R12 and R134a. The heat exchanger equipment constraint is that of fixed total heat transfer surface. It is shown that, under certain conditions, the optimization conclusions reached in this study are similar to those that would be obtained based on other constraints proposed in the literature.
