Publications

In the present study, we show that the fastest runners and swimmers are becoming not only faster but also heavier, taller and more slender. During the past century, the world record speeds for 100 m-freestyle and 100 m-dash have increased with body mass (M) raised to the power 1/6, in accordance with the constructal scaling of animal locomotion. The world records also show that the speeds have increased in proportion with body heights (H) raised to the power 1/2, in accordance with animal locomotion scaling. If the athlete's body is modeled with two length scales (H, body width L), the (M, H) data can be used to calculate the slenderness of the body, H/L. The world records show that the body slenderness is increasing very slowly over time.

Here we show the fundamental tradeoffs that underpin the design of a distributed energy system with two objectives: the production and distribution of electric power driven by solar heating, and desalinated water produced by consuming solar power. We show analytically that larger solar power plants and desalination plants are more efficient than smaller plants. This phenomenon of economies of scale is countered by the greater losses associated with larger distribution networks. From this conflict emerges the proper allocation of nodes of production of power and water on a territory. We show that as the individual needs of power and water increase in time, the sizes of solar plants and desalination plants increase, and so does the size of the territory served by each power plant. At the same time, the territory served by each desalination plant decreases, and this means that the number of desalination plants allocated to one power plant increases.

. This review article shows that the occurrence of macroscopic flow configuration is a universal natural phenomenon that can be explained and predicted on the basis of a principle of physics (the constructal law): "For a flow system to persist in time (to survive) it must evolve in such a way that it provides easier and easier access to the currents that flow through it". The examples given in this article come from natural inanimate flow systems with configuration: duct cross-sections, open channel cross-sections, tree-shaped flow architectures, and turbulent flow structure (e.g., eddies, laminar lengths before transition). Other examples that are treated in the literature, and which support the constructal law, are the wedge-shape of turbulent shear layers, jets and plumes, the frequency of vortex shedding, Bénard convection in fluids and fluid-saturated porous media, dendritic solidification, the coalescence of solid parcels suspended in a flow, global atmospheric and oceanic circulation and climate, and virtually all architectural features of animal design. The constructal law stresses the importance of reserving a place for pure theory in research, and for constantly searching for new physics – new summarizing principles that are general, hence useful.

This paper shows that the power output of various power plant configurations can be maximized by properly dividing the fixed inventory of heat exchange equipment among the heat transfer components of each plant. This conclusion is built over a sequence of fundamental power-maximization problems: the solar power plant with total area constraint, the solar power plant with storage by melting, the power plant with two heat exchangers and total area constraint, the combined-cycle power plant, the power plant viewed as an insulation between its heat source and heat sink, and the power plant with by-pass heat leak to the ambient. New diagrams are reported for the power plant at maximum power, and the refrigeration plant at maximum refrigeration load.

This is a theoretical, numerical and experimental study of how to select the spacing (S) between horizontal cylinders in an array with laminar natural convection, such that the total heat transfer (q) between the array and the ambient is maximized. The volume occupied by the array (height H, width W, cylinder length L) and the cylinder diameter (D) are arbitrary but fixed, while the spacing (or number of cylinders in the array) varies. The optimal spacing and maximum heat transfer results predicted theoretically are developed into accurate and well tested correlations by means of numerical simulations and experimental measurements. The recommended correlations are Sopt/D = 2.72(H/D) 1 3 RaD − 1 4 +0.263 and q ̃ max = 0.448[( H D ) 1 3 Ra D − 1 4 ]− 1 6 where q ̃ max is the dimensionless maximum overall thermal conductance, q ̃ max = q max D2/[HLWk(T w - T∞)]. The optimal spacing is relatively insensitive to whether the cylinders are isothermal or with uniform heat flux.

A Carnot engine operating on a closed cycle and having both external and internal irreversibilities is analyzed. The internal irreversibilities are caused by losses generate by finite piston speed and the external irreversibilities are caused by heat transfer through a temperature difference. The irreversible Carnot cycle is displayed on a original T-S property coordinates in a manner that accurately illustrates the lost work (Exergy losses) due to the irreversibilities (internal and external). A method for calculating the effect of the piston speed on the internal irreversibilities of Carnot cycle is developed and an example of the results is shown for a range of values of cycle high temperature (2000 K). Using the results of this example, the optimal Carnot engine efficiency is determined as a function of piston speed. The example is extended to include the determination of the optimal system temperature for Maximum Carnot engine power over a range of piston speeds.

No abstract is provided for this article.

This paper is a fundamental study of the effect of junction losses on the optimized geometry of tree-shaped flows. Several classes of flows are investigated systematically in a T-shaped construct with fixed internal and external size: laminar with non-negligible entrance and junction losses, and turbulent in tubes with smooth and rough walls. It is shown that in all cases junction losses have a sizeable effect on optimized geometry when Sv 2 <10, where the svelteness Sv is a global property of the entire flow system: Sv =external length scale/internal length scale. The relationship between the global Sv and the slenderness of individual channels is discussed. The study shows that, in general, the duct slenderness decreases as the tree architecture becomes finer and more complex. In conclusion, miniaturization pushes flow architectures not only toward the smaller, finer and more complex, but also toward the domain in which junction losses must be taken into account in the optimization of geometry.

A swarm is a temporary structure formed when several thousand honey bees leave their hive and settle on some object such as the branch of a tree. They remain in this position until a suitable site for a new home is located by the scout bees. A continuum model based on heat conduction and heat generation is used to predict temperature profiles in swarms. Since internal convection is neglected, the model is applicable only at low values of the ambient temperature T a . Guided by the experimental observations of Heinrich (1981a–c, J. Exp. Biol. 91, 25–55; Science 212, 565–566; Sci. Am. 244, 147–160), the analysis is carried out mainly for non-spherical swarms. The effective thermal conductivity is estimated using the data of Heinrich (1981a, J. Exp. Biol. 91, 25–55) for dead bees. For T a = 5 and 9°C, results based on a modified version of the heat generation function due to Southwick (1991, The Behaviour and Physiology of Bees, pp. 28–47. C.A.B. International, London) are in reasonable agreement with measurements. Results obtained with the heat generation function of Myerscough (1993, J. Theor. Biol. 162, 381–393) are qualitatively similar to those obtained with Southwick's function, but the error is more in the former case. The results suggest that the bees near the periphery generate more heat than those near the core, in accord with the conjecture of Heinrich (1981c, Sci. Am. 244, 147–160). On the other hand, for T a = 5°C, the heat generation function of Omholt and Lønvik (1986, J. Theor. Biol. 120, 447–456) leads to a trivial steady state where the entire swarm is at the ambient temperature. Therefore an acceptable heat generation function must result in a steady state which is both non-trivial and stable with respect to small perturbations. Omholt and Lønvik's function satisfies the first requirement, but not the second. For T a = 15°C, there is a considerable difference between predicted and measured values, probably due to the neglect of internal convection in the model.

Objective Data in electronic health records (EHRs) is being increasingly leveraged for secondary uses, ranging from biomedical association studies to comparative effectiveness. To perform studies at scale and transfer knowledge from one institution to another in a meaningful way, we need to harmonize the phenotypes in such systems. Traditionally, this has been accomplished through expert specification of phenotypes via standardized terminologies, such as billing codes. However, this approach may be biased by the experience and expectations of the experts, as well as the vocabulary used to describe such patients. The goal of this work is to develop a data-driven strategy to (1) infer phenotypic topics within patient populations and (2) assess the degree to which such topics facilitate a mapping across populations in disparate healthcare systems. Methods We adapt a generative topic modeling strategy, based on latent Dirichlet allocation, to infer phenotypic topics. We utilize a variance analysis to assess the projection of a patient population from one healthcare system onto the topics learned from another system. The consistency of learned phenotypic topics was evaluated using (1) the similarity of topics, (2) the stability of a patient population across topics, and (3) the transferability of a topic across sites. We evaluated our approaches using four months of inpatient data from two geographically distinct healthcare systems: (1) Northwestern Memorial Hospital (NMH) and (2) Vanderbilt University Medical Center (VUMC). Results The method learned 25 phenotypic topics from each healthcare system. The average cosine similarity between matched topics across the two sites was 0.39, a remarkably high value given the very high dimensionality of the feature space. The average stability of VUMC and NMH patients across the topics of two sites was 0.988 and 0.812, respectively, as measured by the Pearson correlation coefficient. Also the VUMC and NMH topics have smaller variance of characterizing patient population of two sites than standard clinical terminologies (e.g., ICD9), suggesting they may be more reliably transferred across hospital systems. Conclusions Phenotypic topics learned from EHR data can be more stable and transferable than billing codes for characterizing the general status of a patient population. This suggests that EHR-based research may be able to leverage such phenotypic topics as variables when pooling patient populations in predictive models.

Technical Briefs The Optimal Spacing for Cylinders in Crossflow Forced Convection A. Bejan A. Bejan Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC 27708-0300 Search for other works by this author on: This Site PubMed Google Scholar Author and Article Information A. Bejan Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC 27708-0300 J. Heat Transfer. Aug 1995, 117(3): 767-770 (4 pages) https://doi.org/10.1115/1.2822645 Published Online: August 1, 1995 Article history Received: January 1, 1994 Revised: October 1, 1994 Online: December 5, 2007

In this paper, we show how to use constructal design to distribute solar chimney power production on available land area most efficiently. The solar chimney design is used as an example of solar-driven power plant. We found that the power generated per unit of land area is proportional to the length scale of the power plant, as well as to the chimney height, roof radius, and chimney radius. It was also found that the chimney height, roof radius, and chimney radius cannot increase independently and indefinitely. Because of the flow resistances associated with distributing the power over a territory, the land area for each power plant must be finite and allocated optimally. Several patterns of the multi-scale plants on a square area are explored. The global performance of such patterns is greater when more land area is allocated to the largest plant. This performance depends comparatively less on the total land area covered by all power plants.

No abstract is provided for this article.

No abstract is provided for this article.

This paper reports a theoretical and experimental study of the fundamental mechanism responsible for transition in natural convection plume flow. Theoretically, it is argued that the transition occurs when the time of viscous penetration normal to the plume becomes comparable with the minimum time period with which the plume can fluctuate as an unstable inviscid stream. It is also argued that at transition the plume wavelength must always scale with the local plume diameter. The experimental part of the study focused on transition in the axisymmetric air plume above a point heat source. Smoke visualization of the plume shape at transition led to extensive observations that support strongly the transition mechanism proposed theoretically. The transitional plume is seen to meander in a plane (two-dimensionally) and with a wavelength which scales with the plume diameter. If excited externally by many such wavelengths, the plume has the property to select the natural wavelength proposed theoretically. The equivalence between the present transition mechanism and the transition predicted by the buckling theory is discussed.

No abstract is provided for this article.