Publications

Urban heat dome flow, which is also referred to as urban heat island circulation, is important for urban ventilation and pollutant transport between adjacent cities when the background wind is weak or absent. A “dome-shaped” profile can form at the upper boundary of the urban heat island circulation. The horizontal extent of the heat dome is an important parameter for estimating the size of the area it influences. This study reviews the existing data on the horizontal extent of the urban heat dome flow, as determined by using either field measurements or numerical simulations. A simple energy balance model is applied to obtain the maximum horizontal extent of a single heat dome over the urban area, which is found to be approximately 1.5 to 3.5 times the diameter of the city’s urban area at night. A linearized model is also re-analysed to calculate the horizontal extent of the urban heat dome flow. This analysis supports the results from the energy balance model. During daytime, the horizontal extent of the urban heat dome flow is found to be about 2.0 to 3.3 times the urban area’s diameter, as influenced by the convective turbulent plumes in the rural area.

The variety of priority queueing systems with random switchover times is suggested in this paper. Such systems represent generalized models for a wide class of phenomena which involve queueing and prioritization and are considered in QoS and CoS network problems. The classiflcation of such systems is given and methods of their analysis are discussed. Specialists in QoS and CoS technologies may flnd such models adequate and appropriate for the network tra‐c analysis. 1991 Mathematics Subject Classication: Primary 90B22; Secondary 68M22, 68M10.

This paper reviews the recent progress made on the study of melting in the presence of natural convection in a porous medium saturated with a phase-change material. The basic geometries that have been considered are the confined porous medium that is heated from the...

This is a review of recent analytical and numerical work on the generation and flow of methane gas through a layer of porous medium impregnated with solid clathrate hydrates. The porous layer is depressurized suddenly on its lower plane and the phase-change front advances under the influence of heat conduction and convection. The first part of the chapter describes a simplified analytical solution based on a unidirectional phase-change model in which the conduction in the gas-filled region behind the front is neglected. The chapter continues with numerical results for the evolution of the unidirectional phase-change process. Both methods lead to the conclusion that the rate of gas flow through the depressurized (bottom) plane of the layer decreases approximately as t −1/2. Further numerical modeling shows that the presence of a vertical geothermal gradient has a significant effect on the rate of gas generation. Numerical results for phase change and gas generation in a porous sediment with non-uniform porosity and permeability are also reported.

Freedom, safety, and ease of movement are innate human urges attributed to conscience along with many other preferences such as attractiveness (beauty), economy, and life. This article addresses the physics basis of the innate urge to have freedom. It unveils the connection between animal freedom and the universal (constructal) tendency toward easier movement and greater access in all evolutionary systems throughout nature (animate & inanimate). The demonstration is made with a model of lack of freedom in animal movement: a man who walks his dog on a leash. When two animals are coerced to move at the same speed, their combined effort (the spent power) is greater than when they move freely, and independently. When the speed of the couple is dictated by the big body (man), the big one walks freely, and the small one must run. Participants in organized movement (life, society) are not equal. All participants move with less effort when they are not coerced to move the same way. The implications of this part of physics (nature) are numerous and help unify the animal realm with the design and evolution of human society. If you want diversity, give the population freedom, not prescriptions.

Mohamed Gad-el-HakProfessor Mohamed Gad-el-Hak is an engineering scientist, globally well-known in the field of classical physics and the subfields of mechanics, biomechanics, fluid mechanics, turbulence, flow control, microelectromechanical systems (MEMS), microfluidics, and nanotechnology. He was born on Feb. 11, 1945 in Tanta, Egypt, a city in the heart of the Nile Delta, 94 km north of Cairo. Gad-el-Hak's elementary, secondary, and tertiary public education took place in Cairo. He received a B.Sc. in mechanical engineering from Ain Shams University in 1966, where he graduated summa cum laude and ranked first in his class. Gad-el-Hak moved to U.S. in 1968 to start his graduate studies. He received a Ph.D. Degree in fluid mechanics in 1973 from the Johns Hopkins University under the tutelage of Professor Stanley Corrsin. (Corrsin's graduate advisors at Caltech were Hans W. Liepmann and Theodore von Kárrmán. The latter's doctoral advisor at Göttingen was Ludwig Prandtl.) Gad-el-Hak's doctoral thesis is entitled “Experiments on the Nearly Isotropic Turbulence Behind a Jet-Grid.”After obtaining his Ph.D., Mohamed Gad-el-Hak joined the department of aerospace engineering at the University of Southern California, and afterward the department of engineering science and systems at the University of Virginia. After that, he was a senior research scientist and program manager at Flow Research Company in Seattle, Washington, where he managed different hydrodynamic and aerodynamic research projects. After this 10-year industrial experience, Gad-el-Hak returned to academia, joining the department of aerospace and mechanical engineering at the University of Notre Dame.In 2002, Gad-el-Hak accepted a position at Virginia Commonwealth University as chair of mechanical engineering and the Inez Caudill Eminent Professor of Biomedical Engineering. During his tenure as department chair, Gad-el-Hak initiated a unique graduate and undergraduate program in nuclear engineering. The unit is now known as the department of mechanical and nuclear engineering.Other academic appointments include visiting professorship at Institut de Mécanique de Grenoble, Université de-Poitiers, Friedrich-Alexander-Universität Erlangen–Nürnberg, Technische Universität München, Technische Universität Berlin, Brandenburgische Technische Universität Cottbus, Université de Valenciennes, and Peking University. Nonacademic appointments include the Naval Undersea Warfare Center in Newport, RI, and Forschungszentrum Rossendorf in Dresden, Germany.The present authors and numerous others describe Gad-el-Hak as original, pioneering, visionary, creative, indefatigable, nonconformist, having breadth and depth, and ahead of the curve. (The subject passionately opposed to the label “stable genius”.) Dr. Gad-el-Hak's teaching is innovative, rigorous, and does not appeal to the lowest common denominator, and his research is always leading edge. One of Gad-el-Hak's traits is his ability to invent novel measuring techniques where none existed. He has done that with the laser-induced fluorescence, compliant coating deformation, wind-waves characterizations, micropumping, and superhydrophobic surface's longevity, amount of entrapped air, and microscale thickness, among others.Dr. Gad-el-Hak holds two patents: one for a drag-reducing method for airplanes and underwater vehicles, and the other for a lift control device for delta wings. He has additionally disclosed seven patents. Gad-el-Hak has authored/edited 20 books [1–11], and authored 140 journal articles, 52 essays in magazines and newspapers, 63 book chapters, 223 conference papers, and 28 book reviews. He presented over 310 invited lectures in every continent except Antarctica.As of April 2020, Gad-el-Hak's papers have been cited more than 14,600 times in the technical literature, and his h-index is 53, i10-index is 142, and i100-index is 28 (source Google Scholar). Two of Gad-el-Hak's books have been translated into Chinese [9,10], and several of his articles/essays have been translated into Arabic, Chinese, the Czech language, French, German, Japanese, Spanish, and Turkish. Gad-el-Hak is the author of the book Flow Control: Passive, Active, and Reactive Flow Management, and editor of the books Frontiers in Experimental Fluid Mechanics, Advances in Fluid Mechanics Measurements, Flow Control: Fundamentals and Practices, The MEMS Handbook (first and second editions), Transition and Turbulence Control, and Large-Scale Disasters: Prediction, Control and Mitigation.Recent work on large-scale disasters by Gad-el-Hak [11] resulted in the establishment of a universal metric by which the severity of all natural and manmade disasters is measured. That book was the first in the U.S. and second in the world to view large-scale disasters from the physical point of view, in contrast to the social, logistical, or medical viewpoint.Professor Gad-el-Hak's scholarship has been featured in NPR, PBS, Nature magazine, Newsweek, and The New York Times. Additional to working in the broad field of mechanics, he penned essays and op/ed's on global warming [12]; energy crisis [13]; proliferation of scholarly publications [14]; massive open online courses (MOOC) [15]; university governance [16]; STEM and the humanities [17]; engineering education [18]; and societal values of basic research [19].In 1981, Dr. Gad-el-Hak authored the first archival paper to describe the laser-induced fluorescence flow visualization technique [20]. The novelty lies in the ability to generate a very thin sheet of laser light as to be able to see one plane at a time, and the use of extremely small amounts of fluorescent dye as not to make the fluid's interior opaque. Among the technique's advantages are its high signal-to-noise ratio and its ability to dissect the flow field, as a CAT scan would to solid objects. Laser-induced fluorescence is now routinely used in numerous laboratories around the world, for both gas and liquid flows. He also identified the mechanism by which a turbulent region grows into a laminar, vortical flow [20]. The efficient growth by destabilization mechanism is an order of magnitude more effective than the conventional entrainment process in which a turbulent region incorporates/engulfs the surrounding irrotational flow.The article “Reynolds Number Effects in Wall-Bounded Turbulent Flows” by Gad-el-Hak, published in Applied Mechanics Reviews, marked a paradigm shift in the approach to this subject [21]. Funding programs by the U.S. Defense Advanced Research Projects Agency (DARPA), Office of Naval Research (ONR), and Air Force Office of Scientific Research (AFOSR) were inspired by this seminal paper. He introduced the concept of selective/ targeted/opposition/reactive control [22] to achieve drag reduction, lift enhancement, mixing augmentation, and noise suppression in wall-bounded flows. This patented closed-loop control is now researched intensively around the world. Entire scientific conferences and funding programs are dedicated to reactive (in contrast to active) control.Dr. Gad-el-Hak conducted the groundbreaking experiments that detailed the fluid–compliant surface interactions in turbulent boundary layers [23–27]. He also introduced a noninvasive technique to probe the coating's instability waves. The laser-based probe has a spatial resolution of 1 μm and temporal resolution of several kHz. He was among the first group of aerodynamicists in the U.S. to work on the “supermaneuverability” research program [28], a word coined by the German aerodynamicist Wolfgang Herbst. The DARPA/AFOSR unsteady aerodynamics program formed the foundation of the millions of unmanned aerial vehicles flying today.Gad-el-Hak's paper in ASME Journal of Fluids Engineering, “A Novel Pump for MEMS Applications” [29], presented a new method for pumping fluids in microelectromechanical systems (MEMS) applications at very low Reynolds numbers. Inertial pumps do not work at low Reynolds numbers. The only type that worked for MEMS, prior to introducing Gad-el-Hak's rotary pump, was of the reciprocating variety. His most cited paper was published in the ASME Journal of Fluids Engineering, “The Fluid Mechanics of Microdevices—The Freeman Scholar Lecture” [30]. He is the first scientist to place the fledgling field of microfluidics on firm physical ground. Whole books, courses, and funding programs sprang worldwide as a result of this single paper. As of April 2020, that article has been cited over 1480 times (source Google Scholar).Dr. Gad-el-Hak's analytical, numerical, and experimental research on the characterization of superhydrophobic coatings resulted in better understanding of the fledgling field. Twenty journal publications, including two invited review papers [31,32], resulted from his three-year effort. Two measurement techniques were introduced for the first time: (i) an in situ, noninvasive probe to assess the longevity of such coatings and (ii) a method to measure the coating's thickness down to the micron scale as well as the amount of entrapped air in the coating's micropockets [31–34]. Additionally, using electrospinning, Gad-el-Hak advanced a cost-effective fabrication technique that can produce superhydrophobic surfaces with both random and aligned fibrous micro/nanostructures. He also developed an alternative method for engineering affordable superhydrophobic surfaces by randomly depositing size-controlled hydrophobic aerogel particles on a substrate. Dr. Gad-el-Hak advanced theoretical and experimental strategies to characterize the micro/nanostructure of the surface morphology. His research showed that the laboratory-scale surfaces could be scaled-up for marine applications.To commemorate the 90th Anniversary of the ASME Fluids Engineering Division, Professor Mohamed Gad-el-Hak delivered the plenary talk “Nine Decades of Fluid Mechanics” at the ASME FEDSM 2016 meeting, July 10–14, 2016, Washington, DC. His talk was published in October 2016 in a special issue of the ASME Journal of Fluids Engineering. In this review paper [35], Gad-el-Hak covered the progress in fluid mechanics during the period 1926–2016. He described selected experimental, theoretical, and numerical advances, and elucidated the grand benefits of inventing the computer and the laser to the field of fluid dynamics.Dr. Gad-el-Hak's research covers a remarkable range of Reynolds, Mach, and Knudsen numbers. His recent work on hypersonic flows identified a new principle for aerodynamic heating [36,37]. The resulting five journal papers were independently highlighted in 2018 by two prestigious publications: American Institute of Physics' SciLight, and Oxford's National Science Review.In his role as an engineering professor, Gad-el-Hak developed a unique writing course for advanced undergraduate and beginning graduate students in science and engineering [38]. The class is writing intensive and, through assessments, has proven to be more effective than similar classes taught by English and communications faculty. The semester-long course has also been offered as a short course in other universities. His superbly penned essay for The Chronicle of Higher Education, “We Must Stop the Avalanche of Low-Quality Research” [39], was chosen in 2011 by the British Science Council to be part of a standardized English examination.Professor Gad-el-Hak has served as editor of eight international journals including Applied Mechanics Reviews, AIAA Journal, and Bulletin of the Polish Academy of Sciences. Gad-el-Hak is also a contributing editor for McGraw-Hill's Year Book of Science and Technology, for Springer-Verlag's Lecture Notes in Engineering and Lecture Notes in Physics, and for CRC Press's Mechanical Engineering Series. He serve as a consultant to the United Nations, the governments of 12 countries, and many industrial and academic concerns. Professor Gad-el-Hak has been a member of many advisory panels for the U.S. Department of Defense (DOD), Department of Energy (DOE), National Aeronautics and Space Administration (NASA), and National Science Foundation (NSF).In testament to Dr. Gad-el-Hak's successes, he has been the elected Fellow of the American Academy of Mechanics (AAM) [40], American Association for the Advancement of Science (AAAS), American Institute of Physics (AIP), American Physical Society (APS), and American Society of Mechanical Engineers (ASME). In addition, Gad-el-Hak is the recipient of numerous international awards, for example, the 14th Freeman Scholar Award by the American Society of Mechanical Engineers [41], the Japanese Government Research Award for Foreign Scholars, and the German Alexander von Humboldt Prize. He was designated an ASME Distinguished Lecturer as well as inducted into the Johns Hopkins Society of Scholars in 2002. In 2016, Dr. Gad-el-Hak was awarded an ASME Medal for seminal contributions to the discipline of fluids engineering. In the same year, he also received an ASME certificate of appreciation in testimony of the high regard of his associates and the deep appreciation of the society for his valued services in advancing the engineering profession.On the occasion of his 75th birthday, and on behalf of his friends, colleagues, and students all over the globe, the present authors wish Professor Mohamed Gad-el-Hak a continuous active life in happiness and good health, and a very happy birthday!

No abstract is provided for this article.

The floating constants in Weber's boundary layer solution for free convection in a differentially heated vertical porous slab are reevaluated with a new approach. This approach uses Weber's solution to calculate the net vertical heat flux which is equated to zero near the top and bottom ends of the enclosure. It is shown that the Nusselt numbers predicted with the new constants are in excellent agreement with experimental and numerical results.

WhySocial organization and innovation does social organizationOrganization happen by itself? Why does it evolve as the movement of the members of the societySociety increases? Why does the organizationOrganization become more hierarchical, with greater...

“Inequality” is a common observation about us, as members of society. In this article, we unify physics with economics by showing that the distribution of wealth is related proportionally to the movement of all the streams of a live society. The hierarchical distribution of wealth on the earth happens naturally. Hierarchy is unavoidable, with staying power, and difficult to efface. We illustrate this with two architectures, river basins and the movement of freight. The physical flow architecture that emerges is hierarchical on the surface of the earth and in everything that flows inside the live human bodies, the movement of humans and their belongings, and the engines that drive the movement. The nonuniform distribution of wealth becomes more accentuated as the economy becomes more developed, i.e., as its flow architecture becomes more complex for the purpose of covering smaller and smaller interstices of the overall (fixed) territory. It takes a relatively modest complexity for the nonuniformity in the distribution of wealth to be evident. This theory also predicts the Lorenz-type distribution of income inequality, which was adopted empirically for a century.

Recent developments in thermodynamic optimization are reviewed by focusing on the generation of optimal geometric form (shape, structure, topology) in flow systems. The flow configuration is free to vary. The principle that generates geometric form is the pursuit of maximum global performance (e.g., minimum flow resistance, minimum irreversibility) subject to global finiteness constraints (volume, weight, time). The resulting structures constructed in this manner have been named constructal designs. The thought that the same objective and constraints principle accounts for the optimally shaped flow paths that occur in natural systems (animate and inanimate) has been named constructal theory. Examples of large classes of applications are drawn from various sectors of mechanical and civil engineering: the distribution of heat transfer area in power plants, optimal sizing and shaping of flow channels and fins, optimal aspect ratios of heat exchanger core structures, aerodynamic and hydrodynamic shapes, tree-shaped assemblies of convective fins, treeshaped networks for fluid flow and other currents, optimal configurations for streams that undergo bifurcation or pairing, insulated pipe networks for the distribution of hot water and exergy over a fixed territory, and distribution networks for virtually everything that moves in society (goods, currency, information). The principle-based generation of flow geometry unites the thermodynamic optimization developments known in mechanical engineering with lesser known applications in civil engineering and social organization. This review extends thermodynamics, because it shows how thermodynamic principles of design optimization account for the development of optimal configurations in civil engineering and social organization.

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...

This article is adapted from the Introduction of Design in Nature: How the Constructal Law Governs Evolution in Biology, Physics, Technology, and Social Organization, written by Adrian Bejan and J. Peder Zane. This law sweeps the entire mosaic of nature from inanimate rivers to animate designs, such as vascular tissues, locomotion, and social organization. The constructal law tears down the walls that have separated the disciplines of science by providing a new understanding of what it means to be alive. The constructal law defines life in physics terms, and it covers all live-system phenomena. The author believes that the constructal law also challenges another idea that has become dogma since Darwin—that there is no overarching direction to evolution. The constructal law, by contrast, predicts that evolution should occur because of the tendency of all flow systems to generate better and better designs for the currents that flow through them.

“Design in nature” is a topic of growing interest throughout science. The constructal law is the physics law of design generation and evolution in nature: “for a flow system to persist in time (to live), it must evolve such that it provides easier and easier access to its currents.” In this paper, we show how the constructal law accounts for the main features of the design of the biosphere: global movement of mass as the action of constructal engines (geophysical, animal, and human made) that dissipate their power into brakes, animal locomotion, vision, cognition, and hierarchy. The architecture and hierarchy of vegetation results from the constructal tendency to generate designs that facilitate the flow of water and “the flow of stresses” (i.e., mechanical strength per unit volume). Natural porous media have multiple scales because their flows are also configured as trees. The paper concludes with the oneness of design in nature, global design, and science and technology evolution—all as manifestations of the natural tendency captured by the constructal law and unified constructal theory of evolution.

This paper shows that the geometry of the heat flow path between a volume and one point can be optimized in two fundamentally different ways. In the “growth” method of the original constructal theory the structure is optimized starting from the smallest volume element of fixed size. Growth, or optimal numbers of constituents assembled into larger volumes, is one route to resistance minimization. In the “design” method the overall volume is fixed, and the designer works “inward” by optimizing the internal features of the heat flow path. The design method is new. It is shown analytically that the two methods produce comparable geometric results in which the high-conductivity channels form constructal tree networks, and where the low-conductivity material fills the interstices. For simplicity, it is assumed that the high-conductivity channels and their tributaries make 90-deg angles. In both approaches, the overall resistance decreases as the internal complexity of the conductive composite increases. In the growth method the number of constituents in each assembly can be optimized. In the design method, some of the constituent numbers cannot be optimized: these numbers assume the roles of weak parameters. The growth method is the simplest, and provides a useful approximation of the design and performance that can be achieved using the design method. Numerical solutions of the volume-to-point optimization problem confirm the results obtained analytically, and show that the geometric features of the optimal design are robust.

In a recent paper, Gobin and Benard considered the task of correlating the heat transfer data for melting in the presence of natural convection when the Pr value of the liquid phase is considerably smaller than 1. To correlate the low-Pr data is an important and timely task, especially in view of the voluminous work that has been dedicated to situations in which Pr is greater than 1. For the convection-dominated regime known also as quasi-stationary melting, Gobin and Benard correlated their low-Pr numerical data with the formula: Nu=0.29 Ra[sup 0.27]Pr[sup 0.18] They noted that this correlation does not agree with the Nu[approximately](RaPr)[sup 1/4] trend that might be expected from the single-phase natural convection scales for low Prandtl numbers. They concluded that: (1) The relevance of the group (RaPr) is not verified by their numerical results for convection-dominated melting, and (2) further work is required to determine the scaling laws that govern the transition from the initial (conduction) regime to the final (convection) regime of the process of melting by side heating. These two conclusions defined the work presented in this note. In it is reported that the correct scales of natural convection melting when the Prandtl number is small.more » Then construct a scaling-correct heat transfer correlation that spans the entire range of Prandtl numbers.« less

In the field of heat transfer, the method of thermodynamic optimization or entropy generation minimization (EGM) brings out the inherent competition between heat-transfer and fluid-flow irreversibilities in the optimization of devices subjected to overall...

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...