Publications

This paper reports a series of numerical simulations and a scale analysis of the penetrative convection occurring along the unevenly heated horizontal wall of a semi-infinite porous medium. Modeling the problem as two-dimensional, and assuming that the horizontal wall temperature varies between alternating hot and cold spots, it is found that the natural circulation consists of a row of counter-rotating cells situated near the wall. Each cell penetrates vertically into the porous medium to a distance approximately equal to λ Ra 1 2 λ , where λ is the distance between a hot spot and the adjacent cold spot, and Ra gl is the Darcymodified Rayleigh number based on λ and on the temperature difference between each spot and the porous medium situated far enough from the wall. The ability of each cell to convect heat between two adjacent spots increases with the Rayleigh number. The results of numerical simulations in the Ra λ range 1–100 are found to support a number of scaling laws derived based on pure scaling arguments.

This article reviews the role of nature in the development of wheels. The natural emergence of the wheel design can be predicted by using the constructal law in two ways. First, consider the evolution of the wheels made by humans. Second, imagine the horizontal movement of a terrestrial animal as a rolling body. Nature evolved not only the design of wheel-like movement, but also the design for changing speeds. The designs developed by humans are latecomers to this long evolutionary sequence. They come from the same natural tendency to move on Earth more easily, to go with the flow. The evolutionary designs of nature have arrived at wheel-like locomotion and at changes in body movement that result in changing speeds.

The constructal law states that every flow system evolves in time so that it develops the flow architecture that maximizes flow access under the constraints posed to the flow. Earlier applications of the constructal law recommended it as a self-standing law that is distinct from the second law of thermodynamics. In this paper, we develop a model of heat transport on the earth surface that accounts for the solar and terrestrial radiation as the heat source and heat sink and with natural convection loops as the transport mechanism. In the first part of the paper, the constructal law is invoked to optimize the latitude of the boundary between the Hadley and the Ferrel cells, and the boundary between the Ferrel and the Polar cells. The average temperature of the earth surface, the convective conductance in the horizontal direction as well as other parameters defining the latitudinal circulation also match the observed values. In the second part of the paper, the constructal law is invoked in the analysis of atmospheric circulation at the diurnal scale. Here the heat transport is optimized against the Ekman number. Even though this second optimization is based on very different variables than in the first part of the paper, it produces practically the same results for the earth surface temperature and the other variables. The earth averaged temperature difference between day and night was found to be approximately 7K, which matches the observed value. The accumulation of coincidences between theoretical predictions and natural flow configuration adds weight to the claim that the constructal law is a law of nature.

The energy crisis of the 1970s and the continuing emphasis on efficiency (the conservation of fuel resources) has led to a complete overhaul of the way in which power systems are analyzed and improved thermodynamically. The new methodology is exergy analysis and its...

This paper reports the solution to the fundamental problem of how to maximize the mechanical power extracted from a hot single-phase stream when the total heat transfer area bathed by the stream is constrained. It is shown that the optimization has two degrees of freedom: the shape of the stream temperature distribution as a function of the length (x) traveled along the heat transfer surface, and the position of this distribution on the absolute temperature scale. The optimal stream temperature distribution is exponential in x, and so is the temperature distribution along the hot end of the system that converts the heat transfer into mechanical power. At any x, the temperature difference across the heat exchanger is proportional to the local absolute temperature. Similar conclusions are reached for the cold end heat exchanger, when the power system rejects heat to a cold single-phase stream. It is shown that the optimal solution can be implemented in practice by using two counterflow heat exchangers. Each counterflow is imbalanced to a degree recommended by thermodynamic optimization. The effect of the sizes and capacity rates of the two heat exchangers is documented.

This paper focuses on the phenomenon of melting and lubrication by the sliding contact between a phase-change material and a smooth flat slider. The first part of the study considers the limit in which the melting is due primarily to “direct heating,” that is, to the temperature difference between the solid slider and the melting point of the phase-change material. It is shown that in this limit the relative motion gap has a uniform thickness and that the friction factor decreases as both the normal force and the temperature difference increase. The second part considers the limit where the melting is caused mainly by the frictional heating of the liquid formed in the relative motion gap. This gap turns out to have a converging-diverging shape that varies with the parameters of the problem. As the normal force increases, a larger fraction of the melt is pushed out through the upstream opening of the relative motion gap. Means for calculating the melting speed, the friction factor, and the temperature rise along the slider surface are developed.

Purpose Computational abnormalities (e.g., lesion models) for use in medical imaging simulation studies are frequently generated using data collected from clinical images. Although this approach allows for highly‐customizable lesion detectability studies on clinical computed tomography (CT) data, the ground‐truth lesion models produced with this method do not provide a sufficiently realistic lesion morphology for use with current anthropomorphic simulation studies. This work is intended to demonstrate that the new anatomically‐informed lesion model presented here is not inferior to the previous lesion model under CT imaging, and can therefore provide a more biologically‐informed model for use with simulated CT imaging studies. Methods The lesion model was simulated initially from a seed cell with 10 μm diameter placed in an anatomical location within segmented lung CT and was allowed to reproduce locally within the available solid angle in discrete time‐intervals (corresponding to synchronous cell cycles) up to a size of ∼200 μm in diameter. Daughter cells of generation G were allowed also to reproduce on the next available time‐step given sufficient space. At lesion sizes beyond 200 μm in diameter, the health of subregions of cells were tracked with a Markov chain technique, indicating which regions were likely to continue growing, which were likely stable, and which were likely to develop necrosis given their proximity to anatomical features and other lesion cells. For lesion sizes beyond 500 μm, the lesion was represented with three nested, triangulated surfaces (corresponding to proliferating, dormant, and necrotic regions), indicating how discrete volumes of the lesion were behaving at a particular time. Lesions were then assigned smoothly‐varying material properties based on their cellular level health in each region, resulting in a multi‐material lesion model. The lesions produced with this model were then voxelized and placed into lung CT images for comparison with both prior work and clinical data. This model was subject to an observer study in which cardiothoracic imaging radiologists assessed the realism of both clinical and synthetic lesions in CT images. Results The useable outputs of this work were voxel‐ or surface‐based, validated, computational lesions, at a scale clearly visible on clinical CT (3–4 cm). Analysis of the observer study results indicated that the computationally‐generated lesions were indistinguishable from clinical lesions (AUC = 0.49, 95% CI = [0.36, 0.61]) and non‐inferior to an earlier image‐based lesion model—indicating the advantage of the model for use in both hybrid CT images and in simulated CT imaging of the lungs. Conclusions Results indicated the non‐inferiority of this model as compared to previous methods, indicating the utility of the model for use in both hybrid CT images and in simulated CT imaging.

In this book we have tried to provide a user-friendly introduction to the topic of convection in porous media. We have assumed that the reader is conversant with the basic elements of fluid mechanics and heat transfer, but otherwise the book is self-contained. Only routine classical mathematics is employed. We hope that the book will be useful both as a review (for reference) and as a tutorial work (suitable as a textbook in a graduate course or seminar). This book brings into perspective the voluminous research that has been performed during the last two decades. The field has recently exploded because of worldwide concern with issues such as energy self-sufficiency and pollution of the environment. Areas of application include the insulation of buildings and equipment, energy storage and recovery, geothermal reservoirs, nuclear waste disposal, chemical reactor engineering, and the storage of heat-generating materials such as grain and coal. Geophysical applications range from the flow of groundwater around hot intrusions to the stability of snow against avalanches.

Myelofibrosis (MF) is a devastating myeloproliferative neoplasm (MPN) hallmarked by marrow fibrosis, extramedullary hematopoiesis, vascular thromboembolism, and ~50% incidence of JAK2V617F. MF is difficult to study in large EHR datasets due to clinical heterogeneity and unreliable ICD coding. The Synthetic Derivative is a cloned and de-identified research EHR with 2.9 million unique patients linked to BioVU, a DNA biorepository. To develop phenotype-genotype associations, we created an algorithm to classify MF, using NLP with negation detection of MF keywords, medications, and ICD coding. To enrich our cohort, we developed JAKextractor, an algorithm to identify patients tested clinically for JAK2V617F across all 248,000 BioVU patients. For MF identification, we trained a supervised learning algorithm to learn decision rules that encode counts of MF-specific ICD codes, medications, text mentions, as well as the assertion status of MF and JAK2 mentions in patient notes. Experiments were evaluated using a 10-fold cross validation scheme. JAKextractor used pattern matching to extract the status (WT vs MUT) of each JAK2 text mention. Machine learning predicted a JAK2V617F patient based on the information extracted in the previous step from patient notes. We subsequently genotyped banked DNA on an enriched subset of MF cases via a Illumina® TruSight myeloid NGS panel to validate JAKextractor. The top performing MF algorithm combined all sources of clinical information and achieved an F1-measure (F1) of 96% and identified 309 MF patients in BioVU. The extracted decision rule for predicting an MF patient was [JAK2V617F ^ ICD>1] v [JAK2WT ^ ICD>1 ^ TXT>3]. ICD is necessary but not sufficient to predict MF identification. Utilizing only ICD counts created a detrimentally lower F1 of 88% (P<0.001). Our MF cohort had a mean age at onset (60.3±12.6), last visit age (63.1±12.1), and JAK2V617F (46.1%). The mean age of MF onset was higher with JAK2V617F (64) compared to JAK2WT (57) (P<0.001). Survival was no different between JAK2V617F and JAK2WT MF cases via log-rank test (P=0.11) with median survival 108 months. 131 MF cases were genotyped with JAK2V617F in 71/131 (54.2%) compared to 66/131 (50.4%) via JAKextractor. Mean JAK2V617F allelic frequency was 0.569 with detection ranging 0.069-0.976. Ten cases displayed disagreement between JAKextractor and NGS. There were 2 FP and 4 FN JAKextractor predictions 6/131(4.6%); 2 true NGS failures, 1 incomplete chart and 1 loss of JAK2V617F over time. NGS detected JAKV617F on MF patients who had not been previously tested 7/131 (5.3%). Our results demonstrated successful identification of MF and JAK2V617F within an EHR. We established the feasibility of creating a MPN database with retrospective genotyping of biobanked DNA. We plan for scaled implementation of similar algorithms across all myeloid disease within BioVU with the ability to retrospectively genotype each case. Citation Format: Cosmin A. Bejan, Andrew Sochacki, Shilin Zhao, Yaomin Xu, Michael Savona. Identification of myelofibrosis from electronic health records with novel algorithms and JAKextractor [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 5303.

This paper develops a solution to the fundamental problem of how to collect and ‘channel’ to one point the heat generated volumetrically in a low conductivity volume of given size. The amount of high conductivity material that is available for building channels (high conductivity paths) through the volume is fixed. The total heat generation rate is also fixed. The solution is obtained as a sequence of optimization and organization steps. The sequence has a definite time direction, it begins with the smallest building block (elemental system) and proceeds toward larger building blocks (assemblies). Optimized in each assembly are the shape of the assembly and the width of the newest high conductivity path. It is shown that the paths form a tree-like network, in which every single geometric detail is determined theoretically. Furthermore, the tree network cannot be determined theoretically when the time direction is reversed, from large elements toward smaller elements. It is also shown that the present theory has far reaching implications in physics, biology and mathematics.

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 documents the sinuous flow of a vertical water column which impinges on a horizontal surface. It is shown experimentally that the solid obstacle buckles the column into a plane sinuous (static) shape whose wavelength scales with the local column diameter. The measured ratio (buckling wavelength)/(column diameter) is of order π/2, in agreement with the buckling theory of inviscid flow.

An analytical and numerical study is reported of steady-state natural convection in a two-dimensional porous layer heated from the side. Contrary to previous investigations of the phenomenon, which were all based on the Darcy flow model, a vector generalization of Forchheimer’s one-dimensional model is used in the present study, which is valid for all values of local Reynolds number based on pore size. A matched boundary layer solution of the type developed by Weber for Darcy flow is developed for the limit of large-pore Reynolds numbers (the ‘‘non-Darcy’’ limit). It is shown that the natural convection phenomenon in the non-Darcy limit is governed by a new dimensionless group, the Rayleigh number for the higher Reynolds number limit, Ra∞. Numerical experiments are reported in the range 1.6×105≤Ra∞≤1.6×109, in a porous layer with height/thickness ratio equal to 2, and with a high value of Darcy modified Rayleigh number (Ra=4000). The numerical experiments confirm the flow features and scales anticipated by the matched boundary layer solution for the non-Darcy limit. The experiments also document the transition from the well-known Darcy flow to the large-pore Reynolds-number limit treated in this paper.

This study identifies the most basic scales and regimes of the phenomenon of melting with natural convection in an enclosure heated from the side. In the first part of the study the method of scale analysis is used to show that the phenomenon consists of a sequence of four regimes: (a) the pure conduction regime, (b) the mixed regime in which the upper portion of the liquid gap is ruled by convection and the lower portion by conduction, (c) the convection regime and, finally, (d) the last or ‘shrinking solid’ regime. For the first three regimes the scaling theory predicts a Nusselt number vs time curve that has features similar to a van der Waals isotherm, in particular, a clear Nu minimum of order Ra 1 4 at a time SteFo of order Ra− 1 2 , where Ste is the liquid superheat Stefan number and Fo the Fourier number based on H. The corresponding average melting front position has an inflexion point at a time of order Ra− 1 2 . The theory shows further that during the fourth regime the solid disappears during a Ste Fo time interval of order Ra− 1 4 . The second part of the study consists of numerical experiments the purpose of which is to verify the correctness of the theory constructed in the first part. The numerical simulations are based on the quasi-stationary front approximation and the quasi-steady natural convection assumption. The parametric domain covered by these simulations is 0 ⩽ Ra ⩽ 108, 0 < SteFo < 0.2, Pr = 50 and H/L = 1, where L is the horizontal dimension of the enclosure and Ra the Rayleigh number based on H. Closed form correlations for both Nu and the melting front location time functions are developed by combining the theoretical and numerical conclusions of the study.

This paper documents the conjugate heat transfer through a wall with nonuniform thickness, which is lined on one side by a boundary layer. In the first part, variational calculus shows that the total heat transfer rate is minimized w hen the wall thickness decreases in an optimal manner in the direction of flow. The reductions in total he; t transfer rate are significant when the Biot number is smaller than 1. In the second part of the study, th e complete problem of a laminar forced convection boundary layer coupled with conduction through a variiible-thickness wall is solved numerically. Means for calculating the total heat transfer rate are reported graphically. It was again found that the total heat transfer rate decreases when the wall profile is tapered so that the wall thickness decreases in the direction of flow.

Here we develop vasculatures for smart materials with volumetric cooling capability under time-varying conditions. The objective is for the vascularized composite to survive without coolant flow such that its peak temperature does not overshoot the maximum allowable level. The transient performance of four vasculatures is reported: grids (G) and radial channels (R), and two flow directions, inlet in the center (I) and outlet in the center (O). Designs with outlet in the center offer short response times when the Be number is smaller than 109. Configurations with outlet in the center offer short heat removal times and small hot volume fractions.