bluuebunny/crossref_metadata_2025 huggingface dataset:
Mechanisms of Plant Invasions of North American and European Grasslands
Authors: Seastedt T.R. , Pyšek Petr
Published: March 2011

North American and European grasslands consist of relatively young communities that have evolved under human influences. These communities are uniformly sensitive to top-down controls and exhibit rapid changes in plant composition when the intensity and frequency of these controls are altered. These changes are intensifying due to the suite of global change factors, including the continued presence and introduction of new plant species. Establishment of nonnative plant species into grasslands requires resource opportunities generated by natural and human-induced disturbances and by niche differences of the new species. Persistence and spatial expansion require that the traits of the introduced species be compatible with a new regime of competitors, predators, pathogens, and symbionts. Plant traits of the invaders may then further facilitate the invasion process by preempting resources or by restructuring the soil microbial community and trophic food web in ways that directly or indirectly benefit the invading species.
Rapid Evolutionary Change and the Coexistence of Species
Authors: Lankau Richard A.
Published: March 2011

Understanding the forces that allow multiple species to coexist remains a central focus of community ecology. Building evidence that evolutionary changes frequently occur on timescales relevant for ecological dynamics suggests that a complete understanding of the maintenance of diversity is likely to require incorporation of evolutionary dynamics. Coexistence mechanisms can be classified into two groups on the basis of whether they reduce inherent differences in species performance (equalizing effects) or buffer against extinction by providing advantages to rare species (stabilizing effects). Theoretical and empirical evidence suggests that rapid evolution can increase equalizing and/or stabilizing effects in several ways. Directional evolutionary changes can result in new ecological conditions that are more equal or stable than the initial state. Additionally, coevolution between competitors or consumers can lead to cyclic dynamics that provide novel evolutionary equalizing or stabilizing effects.
Can Informed Voters Enforce Better Governance? Experiments in Low-Income Democracies
Authors: Pande Rohini
Published: March 2011

This article evaluates a body of recent work that uses field and natural experiments to answer the question of whether informed voters can enforce better governance. A common finding in the literature is that voter behavior is malleable and that information about the political process and politician performance improves electoral accountability. Limited availability of information thus provides one explanation for the persistence of low-quality politicians and the existence of identity politics and electoral malpractices in low-income democracies. Understanding how voters can gain access to credible sources of information and understanding how politicians react to improved information about their performance are promising avenues for future research.
Social Norms and Social Assets
Authors: Postlewaite Andrew
Published: March 2011

Social norms are often posited as an explanation of differences in the economic behavior and performance of societies that are difficult to explain by differences in endowments and technology. The successful integration of social norms into economic models requires an understanding of the basis of the differences in preferences that lead to different behaviors. I explore the difference between deep and reduced-form preferences and how both can differ across societies.
The Mechanism Design Approach to Student Assignment
Authors: Pathak Parag A.
Published: March 2011

The mechanism design approach to student assignment involves the theoretical, empirical, and experimental study of systems used to allocate students into schools around the world. Recent practical experience designing systems for student assignment has raised new theoretical questions for the theory of matching and assignment. This article reviews some of this recent literature, highlighting how issues from the field motivated theoretical developments and emphasizing how the dialogue may be a road map for other areas of applied mechanism design. Finally, it concludes with some open questions.
The Political Economy of Public Debt
Authors: Battaglini Marco
Published: March 2011

We survey recent theories of public debt that incorporate political decision making in rich dynamic environments. These theories provide a new framework with which to interpret empirical evidence and to assess institutional reforms that may help control political inefficiencies. We discuss the inefficiencies that lead to overaccumulation of debt and their implications for the long-run distribution of debt.
Robustness and Macroeconomic Policy
Authors: Barlevy Gadi
Published: March 2011

This review considers the design of macroeconomic policies in the face of uncertainty. In recent years, several economists have advocated that, when policy makers are uncertain about the environment they face and find it difficult to assign precise probabilities to the alternative scenarios that may characterize this environment, they should design policies to be robust in the sense that they minimize the worst-case loss these policies could ever impose. I review and evaluate the objections cited by critics of this approach. I argue further that, contrary to what some have inferred, concern about worst-case scenarios does not always lead to policies that respond more aggressively to incoming news than the optimal policy would respond absent any uncertainty.
News and Aggregate Demand Shocks
Authors: Lorenzoni Guido
Published: March 2011

In this review, I look at the recent literature on news as a source of economic fluctuations. The main question in this literature is: how does the aggregate economy respond to a shock that raises consumers' and firms' expectations about future productivity growth? I discuss how different papers have addressed this question, emphasizing the mechanisms at work under different specifications of preferences and technology, under different assumptions about nominal and real rigidities, and under different assumptions about the agents' information structure. I also briefly discuss some challenges faced by the empirical literature on the topic.
New Developments in Aggregation Economics
Authors: Chiappori Pierre André , Ekeland Ivar
Published: March 2011

The goal of this article is to provide a general characterization of the aggregate behavior of a group in a market environment. We allow for public and private consumption, intragroup production, and consumption externalities within a group; we only assume that the group always reaches Pareto-efficient decisions. We show that aggregation problems involve a simple mathematical structure: The aggregate demand of the group, considered as a vector field, can be decomposed into a sum of gradients. We briefly introduce exterior differential calculus as a tool to study this structure. We analyze two main issues. One is testability: What restrictions (if any) on the aggregate demand function characterize the efficient behavior of the group? The second issue relates to identifiability; we investigate the conditions under which it is possible to recover the underlying structure—namely, individual preferences, the decision process, and the resulting intragroup transfers—from the group's aggregate behavior.
The Gravity Model
Authors: Anderson James E.
Published: March 2011

Gravity has long been one of the most successful empirical models in economics. Incorporating deeper theoretical foundations of gravity into recent practice has led to a richer and more accurate estimation and interpretation of the spatial relations described by gravity. Wider acceptance has followed. Recent developments are reviewed here, and suggestions are made for promising future research.
Theories of Heterogeneous Firms and Trade
Authors: Redding Stephen J.
Published: March 2011

This article reviews the recent theoretical literature on heterogeneous firms and trade, which emphasizes firm selection into international markets and reallocations of resources across firms. We discuss the empirical challenges that motivated this research and its relationship to traditional trade theories. We examine the implications of firm heterogeneity for comparative advantage, market size, aggregate trade, the welfare gains from trade, and the relationship between trade and income distribution. Although a number of studies examine the endogenous response of firm productivity to trade liberalization, modeling internal firm organization and the origins of firm heterogeneity remain interesting areas of ongoing research.
Agroecology: A Review from a Global-Change Perspective
Authors: Tomich Thomas P. , Brodt Sonja , Ferris Howard , Galt Ryan , Horwath William R. , Kebreab Ermias , Leveau Johan H.J. , Liptzin Daniel , Lubell Mark , Merel Pierre , Michelmore Richard , Rosenstock Todd , Scow Kate , Six Johan , Williams Neal , Yang Louie
Published: March 2011

This review by a multidisciplinary team maps key components and emerging connections within the intellectual landscape of agroecology. We attempt to extend and preview agroecology as a discipline in which agriculture can be conceptualized within the context of global change and studied as a coupled system involving a wide range of social and natural processes. This intrinsic coupling, combined with powerful emerging drivers of change, presents challenges for the practice of agroecology and agriculture itself, as well as providing the framework for some of the most innovative research areas and the greatest potential for innovation for a sustainable future in agriculture. The objective of this review is to identify forward-looking scientific questions to enhance the relevance of agroecology for the key challenges of mitigating environmental impacts of agriculture while dramatically increasing global food production, improving livelihoods, and thereby reducing chronic hunger and malnutrition over the coming decades.
State of the World's Freshwater Ecosystems: Physical, Chemical, and Biological Changes
Authors: Carpenter Stephen R. , Stanley Emily H. , Vander Zanden M. Jake
Published: March 2011

Surface freshwaters—lakes, reservoirs, and rivers—are among the most extensively altered ecosystems on Earth. Transformations include changes in the morphology of rivers and lakes, hydrology, biogeochemistry of nutrients and toxic substances, ecosystem metabolism and the storage of carbon (C), loss of native species, expansion of invasive species, and disease emergence. Drivers are climate change, hydrologic flow modification, land-use change, chemical inputs, aquatic invasive species, and harvest. Drivers and responses interact, and their relationships must be disentangled to understand the causes and consequences of change as well as the correctives for adverse change in any given watershed. Beyond its importance in terms of drinking water, freshwater supports human well-being in many ways related to food and fiber production, hydration of other ecosystems used by humans, dilution and degradation of pollutants, and cultural values. A natural capital framework can be used to assess freshwater ecosystem services, competing uses for freshwaters, and the processes that underpin the long-term maintenance of freshwaters. Upper limits for human consumption of freshwaters have been proposed, and consumptive use may approach these limits by the mid-century.
State of the World's Nonfuel Mineral Resources: Supply, Demand, and Socio-Institutional Fundamentals
Authors: Poulton Mary M. , Jagers Sverker C. , Linde Stefan , Van Zyl Dirk , Danielson Luke J. , Matti Simon
Published: March 2011

Current material supply-demand imbalances are driven by situational rather than physical scarcities, resulting in a growing interest among government, civil society, and industry groups to consider not only the availability of mineral resources, but also the sustainability implications of their production. This, in turn, places increasing pressure on mining companies to broaden their concerns when planning new mining projects, covering its “social license to operate” by incorporating strategies for limiting negative socioenvironmental impacts alongside calculations of the project's economic viability as well as balancing a large number of potential stakeholders. Accordingly, understanding also the sociopolitical context of mineral development is crucial for the creation of sustainable practices within the mining industry. By applying a sustainable development framework, this article outlines the complex web of challenges associated with sustainable mineral extraction, ranging from technological and economic development to political and institutional concerns on how to govern and manage scarce resources in a globalized world.
Green Chemistry and Green Engineering: A Framework for Sustainable Technology Development
Authors: Mulvihill Martin J. , Beach Evan S. , Zimmerman Julie B. , Anastas Paul T.
Published: March 2011

Green chemistry and engineering seek to maximize efficiency and minimize health and environmental hazards throughout the chemical production process. This review demonstrates how green chemistry principles and metrics can influence the entire life cycle of a chemical from design through disposal. After reviewing essential metrics and recent advances in the field within this context, we consider the case of nanotechnology. As an emerging field, nanotechnology provides an instructive framework to consider the influence and application of green chemistry. Interdisciplinary innovation guides both fields, and both seek to transform the nature of technology. The applications and implications of emerging green technology are discussed, and future opportunities for interdisciplinary collaborations are highlighted.
Religion and Environment
Authors: Jenkins Willis , Chapple Christopher Key
Published: March 2011

Understanding the interaction of human and environmental systems requires understanding the religious dimensions to the integration of ecology and society. Research on the significance of religion to environmental problems and of ecological ideas to religion has emerged into a robust interdisciplinary field. One sign of its vitality lies in the methodological arguments over how to conceptualize and assess that significance. Another lies in the diversity of research projects, which appear within most religious traditions, from many geographical contexts, and in several different disciplines. This article introduces major approaches to the field and key questions raised, and then briefly assesses recent work in three broad areas of tradition.
Water and Energy Interactions
Authors: McMahon James E. , Price Sarah K.
Published: March 2011

Human habitations require energy and water, which are increasingly interdependent. Energy systems have changed from using water for mechanical energy to building dams to provide irrigation water for agriculture and hydroelectricity. Large volumes of water are required to cool thermal electricity-generating stations—whether coal, natural gas, nuclear, or solar powered. Changes in cooling technology are reducing water withdrawals while increasing water consumption. Water produced from fossil fuel production represents environmental challenges and supply opportunities. Some renewable energy sources, such as wind turbines and photovoltaics, have far lower water requirements. Increasing development of biofuels creates a direct connection between water and energy systems. Energy, mostly for pumps, is necessary for supplying potable water and treating wastewater. Pumping from deeper underground as well as removing more contaminants (e.g., medicines, agricultural chemicals) and salt requires more energy. Water and wastewater treatment can dominate electricity demand in municipalities. Water reuse requires energy for treatment and pumping. Life cycle assessments and integrated resource planning strive to account for the total impacts.
Energy Intensity of Agriculture and Food Systems
Authors: Pelletier Nathan , Audsley Eric , Brodt Sonja , Garnett Tara , Henriksson Patrik , Kendall Alissa , Kramer Klaas Jan , Murphy David , Nemecek Thomas , Troell Max
Published: March 2011

The relationships between energy use in food systems, food system productivity, and energy resource constraints are complex. Moreover, ongoing changes in food production and consumption norms concurrent with urbanization, globalization, and demographic changes underscore the importance of energy use in food systems as a food security concern. Here, we review the current state of knowledge with respect to the energy intensity of agriculture and food systems. We highlight key drivers and trends in food system energy use along with opportunities for and constraints on improved efficiencies. In particular, we point toward a current dearth of research with respect to the energy performance of food systems in developing countries and provide a cautionary note vis-à-vis increasing food system energy dependencies in the light of energy price volatility and concerns as to long-term fossil energy availabilities.
Global Climate Forcing by Criteria Air Pollutants
Authors: Unger Nadine
Published: March 2011

Ambient air pollution has significant impacts on global climate change in complex ways, involving both warming and cooling, and causes an estimated one million deaths every year. Modeling studies and observations from a suite of platforms, including those that are space based, have revealed that air pollution is a widespread global phenomenon. The net effect of air pollution is a global cooling that is masking 50% of the committed greenhouse gas (GHG) warming from the Industrial Revolution. Aggressive air pollution abatement and climate stabilization strategies that reduce cooling pollutants may lead to a short-term warming surge that is unsafe for ecosystems and the human population, imposing complex trade-offs in policy making. Conversely, selective reduction of warming air pollutants to mitigate near-term climate change may offer opportunities for synergistic policy development. Reducing and preventing the accumulation of fossil-fuel carbon dioxide (CO2) in the atmosphere is the only sustainable way to protect climate safety in the long term. Here, the current understanding of air pollution effects on global climate change is reviewed, including assessment by individual pollutant, precursor emission, economic sector, and policy-relevant scenarios.
Rediscovering the Macroeconomic Roots of Financial Stability Policy: Journey, Challenges, and a Way Forward
Authors: Borio Claudio
Published: March 2011

The recent financial crisis has triggered a major rethink of analytical approaches and policy toward financial stability. The crisis has encouraged a sharper focus on systemic risk, the inclusion of a financial sector in macroeconomic models, a shift from a microprudential to a macroprudential orientation in regulation and supervision, and questions about whether price stability is a sufficient criterion to guide monetary policy. In the process, it has led to a rediscovery of the macroeconomic roots of financial instability. This review argues that this development is welcome but has not gone far enough. To substantiate this conclusion, the review documents this analytical and policy journey before suggesting a way forward.
Finance and Governance in Developing Economies
Authors: Morck Randall
Published: March 2011

Classic Big Push industrialization envisions state planners coordinating economic activity to internalize a range of externalities that otherwise lock in a low-income equilibrium, but runs afoul of well-known government failure problems. Successful Big Push coordination may occur instead when a large business group, acting in its controlling shareholder's self-interest, coordinates the establishment and expansion of businesses in diverse sectors. Where business groups play this role, many basic axioms of Anglo-American corporate governance, including the advocacy of shareholder value maximization and contestable corporate control, must be qualified.
Inflation-Indexed Bonds and the Expectations Hypothesis
Authors: Pflueger Carolin E. , Viceira Luis M.
Published: March 2011

This review empirically analyzes the expectations hypothesis (EH) in inflation-indexed (or real) bonds and in nominal bonds in the United States and in the United Kingdom. We strongly reject the EH in inflation-indexed bonds, and also confirm and update the existing evidence rejecting the EH in nominal bonds. This rejection implies that the risk premium on both real and nominal bonds varies predictably over time. We also find strong evidence that the spread between the nominal and the real bond risk premium, or the breakeven inflation risk premium, also varies over time. We argue that the time variation in real bond risk premia most likely reflects both a changing real interest rate risk premium and a changing liquidity risk premium, and that the variability in the nominal bond risk premia reflects a changing inflation risk premium. We estimate significant time series variability in the magnitude and sign of bond risk premia.
Equilibrium in the Initial Public Offerings Market
Authors: Ritter Jay R.
Published: March 2011

In this review, I criticize the ability of popular asymmetric information-based models to explain the magnitude of the underpricing of initial public offerings (IPOs) that is observed. I suggest that the quantitative magnitude of underpricing can be explained with a market structure in which underwriters want to underprice excessively, issuers are focused on services bundled with underwriting rather than on maximizing the offer proceeds, and there is limited competition between underwriters. Since the technology bubble burst in 2000, U.S. IPO volume has been low. Although regulatory burdens undoubtedly account for some of the decline, I suggest that much of the decline may be due to a structural shift that has lessened the profitability of small independent companies relative to their value as part of a larger, more established organization that can realize economies of scope. I also discuss the long-run performance literature. My interpretation of the evidence is that except for the smallest companies going public, IPOs have long-run returns that are similar to those on seasoned stocks with the same characteristics.
Performance Measurement of Mutual Funds, Hedge Funds, and Institutional Accounts
Authors: Wermers Russ
Published: March 2011

This review describes several important recent advances in the measurement of the performance of actively managed portfolios. For returns-based performance evaluation, we discuss several innovations, such as conditional performance evaluation, Bayesian approaches, and a new multiple-testing approach—the false-discovery rate. For portfolio holdings–based performance evaluation, our discussion ranges from extensions of the standard Daniel, Grinblatt, Titman, and Wermers (DGTW) stock return adjustment procedure to conditional holdings-based approaches. Applications of these approaches in the mutual fund, hedge fund, and institutional account universes are presented.
Payment Systems
Authors: McAndrews James , Nosal Ed , Rocheteau Guillaume
Published: March 2011

Modern payment instruments can be complex. Yet, many of these can be interpreted as a form of money or credit, which are rather primitive instruments. We use a simple model of a monetary economy to provide an overview of some of the fundamental questions in the literature on payments. Why do agents pay? What are the frictions that prevent or limit the use of credit arrangements? Why is fiat money valued? Why do money and credit coexist? Our simple model can address these basic and important questions, and can be extended to address a variety of issues related to payments.
Predictability of Returns and Cash Flows
Authors: Koijen Ralph S.J. , Van Nieuwerburgh Stijn
Published: March 2011

We review the literature on return and cash-flow growth predictability from the perspective of the present-value identity. We focus predominantly on recent work. Our emphasis is on U.S. aggregate stock return predictability, but we also discuss evidence from other asset classes and countries.
Financial Intermediary Balance Sheet Management
Authors: Adrian Tobias , Shin Hyun Song
Published: March 2011

Conventional discussions of balance sheet management by nonfinancial firms take the set of positive net present value (NPV) projects as given, which in turn determines the size of the assets of the firm. The focus is on the composition of equity and debt in funding such assets. In contrast, the balance sheet management of financial intermediaries reveals that it is equity that behaves like the predetermined variable, and the asset size of the bank or financial intermediary is determined by the degree of leverage that is permitted by market conditions. The relative stickiness of equity reveals possible non-pecuniary benefits to bank owners so that they are reluctant to raise new equity, even during boom periods when equity raising is associated with less stigma, and hence smaller discounts. We explore the empirical evidence for both market-based financial intermediaries such as the Wall Street investment banks, as well as the commercial bank subsidiaries of the large U.S. bank holding companies (BHCs). We further explore the aggregate consequences of such behavior by the banking sector for the propagation of the financial cycle and securitization.
Electrorheological Fluids: Mechanisms, Dynamics, and Microfluidics Applications
Authors: Sheng Ping , Wen Weijia
Published: March 2011

Electrorheological (ER) fluids, consisting of solid particles dispersed in an insulating liquid, display the special characteristic of electric-field-induced rheological variations. Nearly six decades after their discovery, ER fluids have emerged as materials of increasing scientific fascination and practical importance. This review traces the mechanisms responsible for these fluids' ER response and their attendant theoretical underpinnings. In particular, ER fluids are divided into two different types, dielectric electrorheological (DER) and giant electrorheological (GER), which reflect the underlying electric susceptibility arising from the induced dielectric polarization and the orientational polarization of molecular dipoles, respectively. The formulation of a continuum ER hydrodynamics is described in some detail. As an electric-mechanical interface, ER fluids have broad application potential in electrifying the control of mechanical devices. This review focuses on their applications in microfluidic chips, in which GER fluids have enabled a variety of digitally controlled functionalities.
Cascades in Wall-Bounded Turbulence
Authors: Jiménez Javier
Published: March 2011

The kinematics and dynamics of wall-bounded turbulence are surveyed, with emphasis on the multiscale processes associated with the logarithmic layer and with its interactions with the wall. It is shown that the logarithmic law reflects a momentum cascade and that its structure agrees reasonably well with Townsend's (1961) model of a self-similar family of attached eddies, each of which contains, on average, a sweep-ejection pair, a segment of a large velocity streak, and disorganized vorticity. Those logarithmic eddies are themselves turbulent objects and can be studied in minimal simulation boxes that are much larger than those in the buffer layer. It is argued that, near the wall, the logarithmic eddies are probably the same as the vortex packets identified by experiments, but that their dynamics does not appear to be especially linked to the buffer layer. Further from the wall, they align into longer superstreaks, although the mechanism remains unclear.
Dynamics of the Tear Film
Authors: Braun Richard J.
Published: March 2011

This review discusses the current understanding of tear-film physiology and mathematical models for some of its dynamics. First, a brief introduction to the tear film and the ocular surface is given. Next, mathematical models for the tear film are discussed, with an emphasis on models that describe the formation and relaxation of the tear film from blinking. Finally, future issues in tear film modeling are presented.
Nanoscale Electrokinetics and Microvortices: How Microhydrodynamics Affects Nanofluidic Ion Flux
Authors: Chang Hsueh-Chia , Yossifon Gilad , Demekhin Evgeny A.
Published: March 2011

When a direct current (DC) electric field is applied across an ion-selective nanoporous membrane or a nanochannel with an overlapping Debye layer, a surprising microvortex instability occurs on the side of the membrane/channel through which counterions enter. Despite its micro and nano length scales, this instability exhibits all the hallmarks of other classical hydrodynamic instabilities—a subharmonic cascade, a wide-band fluctuation spectrum, and a coherent structure dominated by spatiotemporal dynamics. Moreover, the resulting convection enhances the ion flux into the ion-selective medium and gives rise to an overlimiting-current bifurcation in the current-voltage relationship. This hydrodynamically driven nonequilibrium ion flux does not seem to have any equivalent in cell membrane ion channels. Yet, by introducing asymmetric entrances to provide different polarized regions and/or viscous arrest of the vortex instability, one can fabricate a hydrodynamic nanofluidic diode. With other modifications, hysteretic, excitable, and oscillatory ion flux dynamics could also be elicited—all with strong hydrodynamic features.
Fluid Mechanics of the Eye
Authors: Siggers Jennifer H. , Ethier C. Ross
Published: March 2011

Fluid mechanical processes are an intrinsic part of several aspects of the physiology and pathology of the human eye. In this article we describe selected phenomena that are amenable to particularly interesting mathematical, experimental, or numerical analyses. We initially focus on glaucoma, a condition often associated with raised intraocular pressure. The mechanics in this disease is by no means fully understood, but we present some of the modeling work that provides a partial explanation. We next focus on other features of the dynamics of the two specialized ocular fluids: the aqueous and vitreous humors. With regard to the aqueous humor, we discuss problems concerning the transport of heat and proteins and the hydration of the cornea. With regard to the vitreous humor, we discuss the possibility of flow, which occurs primarily as a result of saccades or motions of the eyeball. Finally, we describe a model of the degradation of Bruch's membrane in the retina.
Hydrodynamic Techniques to Enhance Membrane Filtration
Authors: Jaffrin Michel Y.
Published: March 2011

This article reviews the use of various techniques for membrane filtration, such as Dean and Taylor vortices, pulsatile flows, and dynamic filtration, which can generate high shear rates more efficiently than cross-flow filtration. In dynamic filtration, shear rates are generated not by a pump, but by moving parts or by vibrations. The most successful application of Taylor vortices has been plasma collection from donors in transfusion centers by microfiltration (MF), using small rotating cylindrical filters. Industrial dynamic filtration modules consist of metal disks with vanes or blades rotating between circular flat membranes or rotating ceramic membrane disks. These systems can be operated at high rotation speeds in order to produce very high permeate fluxes, or they can be operated at low speeds and save energy as compared with cross-flow filtration for the same flux. Vibrating modules (i.e., vibratory shear-enhanced processing) consist of a stack of circular membranes oscillating around a vertical shaft at its resonant frequency. While instabilities created by Dean vortices and pulsatile flows are mostly efficient in laminar flow and in MF and ultrafiltration, the benefits of high shear dynamic filtration are even more impressive in nanofiltration and reverse osmosis, as the reduction in concentration polarization not only increases permeate flux as compared with cross-flow filtration, but also decreases microsolute transmission.
The Interaction of Jets with Crossflow
Authors: Mahesh Krishnan
Published: March 2011

It is common for jets of fluid to interact with crossflow. This article reviews our understanding of the physical behavior of this important class of flow in the incompressible and compressible regimes. Experiments have significantly increased in sophistication over the past few decades, and recent experiments provide data on turbulence quantities and scalar mixing. Quantitative data at high speeds are less common, and visualization still forms an important component in estimating penetration and mixing. Simulations have progressed from the Reynolds-averaged methodology to large-eddy and hybrid methodologies. There is a general consensus on the qualitative structure of the flow at low speeds; however, the flow structure at low-velocity ratios (jet speed/crossflow speed) might be fundamentally different from the common notion of shear-layer vortices, counter-rotating vortex pairs, wakes, and horseshoe vortices. Fluid in the near field is strongly accelerated, which affects the jet trajectory, entrainment, and mixing behavior. At low speeds, mixing depends more on Reynolds number than the jet trajectory or spatial extent does. Turbulence kinetic energy budgets are discussed that reveal the considerable nonequilibrium nature of the flow and the consequent challenges posed to time-averaged prediction methodologies. The parameter space at high speeds is fairly large, and even experimentally derived correlations for trajectories show significant scatter. Coherent motions in high-speed jets are seen to entrain large amounts of crossflow fluid but do not mix effectively in the near field.
Nonstandard Inkjets
Authors: Basaran Osman A. , Gao Haijing , Bhat Pradeep P.
Published: March 2011

Inkjet printers eject drops from microscopic nozzles and deposit them on substrates. For a number of years after its initial development, inkjet printing remained a method for visualizing computer output and printing documents. Beginning in the late 1990s, a number of researchers realized that inkjet printers could be employed as robotic pipettes to create microarrays, manufacture three-dimensional parts and spherical particles, print electrical devices, and facilitate combinatorial chemistry. Although most inks are low-viscosity Newtonian fluids, liquids in new applications are complex fluids. At the same time that these new applications were emerging, the replacement of traditional photography by digital imaging and the quest for ever-faster printing speeds resulted in the development of novel printing methods. Whereas most previous reviews of the field have focused on evaluations of well-known printing methods, this review instead presents a critical analysis from a fluid mechanics perspective of the recent developments in nonstandard printing techniques and the increasingly widespread use of nonstandard inks of complex fluids.
Physics and Computation of Aero-Optics
Authors: Wang Meng , Mani Ali , Gordeyev Stanislav
Published: March 2011

This article provides a critical review of aero-optics with an emphasis on recent developments in computational predictions and the physical mechanisms of flow-induced optical distortions. Following a brief introduction of the fundamental theory and key concepts, computational techniques for aberrating flow fields and optical propagation are discussed along with a brief survey of wave-front sensors used in experimental measurements. New physical understanding generated through numerical and experimental investigations is highlighted for a number of important aero-optical flows, including turbulent boundary layers, separated shear layers, and flow over optical turrets. Approaches for mitigating aero-optical effects are briefly discussed.
Modern Challenges Facing Turbomachinery Aeroacoustics
Authors: Peake Nigel , Parry Anthony B.
Published: March 2011

In this review we describe current scientific and technological issues in the quest to reduce aeroengine noise, in the face of predicted rapid increases in the volume of air traffic, on the one hand, and increasingly strict environmental regulation, on the other. Alongside conventional ducted turbofan designs, new open-rotor contra-rotating power plants are currently under development, which present their own noise challenges. The key sources of tonal and broadband noise, and the way in which noise propagates away from the source, are surveyed in both cases. We also consider in detail two key aspects underpinning the flow physics that continue to receive considerable attention, namely the acoustics of swirling flow and unsteady flow-blade interactions. Finally, we describe possible innovations in open-rotor engine design for low noise.
The Turbulent Flows of Supercritical Fluids with Heat Transfer
Authors: Yoo Jung Yul
Published: March 2011

Serious heat-transfer deterioration may occur in fluids at supercritical pressure owing to the effects of buoyancy, flow acceleration, and significant variations in thermophysical properties. Although there have been numerous experimental studies on this subject, no single heat-transfer correlation has been found to be capable of describing this phenomenon accurately. Relatively few experimental studies have been carried out on the fluid mechanics of supercritical flows because of the technical difficulties of dealing with turbulent flows and high heat fluxes simultaneously. Conversely, many computational fluid dynamics studies have examined a number of low-Reynolds-number turbulence models. However, none have reproduced the buoyancy production of turbulence reasonably well because of limitations with the use of a constant turbulent Prandtl number. Direct numerical simulations may provide more insight into the physics of fluids at supercritical pressure within a limited range of flow and heat-transfer conditions.
Two-Dimensional Turbulence
Authors: Boffetta Guido , Ecke Robert E.
Published: March 2011

In physical systems, a reduction in dimensionality often leads to exciting new phenomena. Here we discuss the novel effects arising from the consideration of fluid turbulence confined to two spatial dimensions. The additional conservation constraint on squared vorticity relative to three-dimensional (3D) turbulence leads to the dual-cascade scenario of Kraichnan and Batchelor with an inverse energy cascade to larger scales and a direct enstrophy cascade to smaller scales. Specific theoretical predictions of spectra, structure functions, probability distributions, and mechanisms are presented, and major experimental and numerical comparisons are reviewed. The introduction of 3D perturbations does not destroy the main features of the cascade picture, implying that 2D turbulence phenomenology establishes the general picture of turbulent fluid flows when one spatial direction is heavily constrained by geometry or by applied body forces. Such flows are common in geophysical and planetary contexts, are beautiful to observe, and reflect the impact of dimensionality on fluid turbulence.
Wake-Induced Oscillatory Paths of Bodies Freely Rising or Falling in Fluids
Authors: Ern Patricia , Risso Frédéric , Fabre David , Magnaudet Jacques
Published: March 2011

Leaves falling in air and bubbles rising in water provide daily examples of nonstraight paths associated with the buoyancy-driven motion of a body in a fluid. Such paths are relevant to a large variety of applicative fields such as mechanical engineering, aerodynamics, meteorology, and the biomechanics of plants and insect flight. Although the problem has attracted attention for ages, it is only recently that the tremendous progress in the development of experimental and computational techniques and the emergence of new theoretical concepts have led to a better understanding of the underlying physical mechanisms. This review attempts to bring together the main recent experimental, computational, and theoretical advances obtained on this fascinating subject. To this end it describes the first steps of the transition in the wake of a fixed body and its connection with the onset and development of the path instability of moving bodies. Then it analyzes the kinematics and dynamics of various types of bodies along typical nonstraight paths and how the corresponding information can be used to build low-dimensional predictive models.
A Genetic Approach to the Transcriptional Regulation ofHoxGene Clusters
Authors: Tschopp Patrick , Duboule Denis
Published: March 2011

The evolution of vertebrate genomes was accompanied by an astounding increase in the complexity of their regulatory modalities. Genetic redundancy resulting from large-scale genome duplications at the base of the chordate tree was repeatedly exploited by the functional redeployment of paralogous genes via innovations in their regulatory circuits. As a paradigm of such regulatory evolution, we have extensively studied those control mechanisms at work in-cis over vertebrate Hox gene clusters. Here, we review the portfolio of genetic strategies that have been developed to tackle the intricate relationship between genomic topography and the transcriptional activities in this gene family, and we describe some of the mechanistic insights we gained by using the HoxD cluster as an example. We discuss the high heuristic value of this system in our general understanding of how changes in transcriptional regulation can diversify gene function and thereby fuel morphological evolution.
Brassinosteroid Signaling Network and Regulation of Photomorphogenesis
Authors: Wang Zhi-Yong , Bai Ming-Yi , Oh Eunkyoo , Zhu Jia-Ying
Published: March 2011

In plants, the steroidal hormone brassinosteroid (BR) regulates numerous developmental processes, including photomorphogenesis. Genetic, proteomic, and genomic studies in Arabidopsis have illustrated a fully connected BR signal transduction pathway from the cell surface receptor kinase BRI1 to the BZR1 family of transcription factors. Genome-wide analyses of protein-DNA interactions have identified thousands of BZR1 target genes that link BR signaling to various cellular, metabolic, and developmental processes, as well as other signaling pathways. In controlling photomorphogenesis, BR signaling is highly integrated with the light, gibberellin, and auxin pathways through both direct interactions between signaling proteins and transcriptional regulation of key components of these pathways. BR signaling also cross talks with other receptor kinase pathways to modulate stomata development and innate immunity. The molecular connections in the BR signaling network demonstrate a robust steroid signaling system that has evolved in plants to orchestrate signal transduction, genome expression, metabolism, defense, and development.
Genomic Analysis at the Single-Cell Level
Authors: Kalisky Tomer , Blainey Paul , Quake Stephen R.
Published: March 2011

Studying complex biological systems such as a developing embryo, a tumor, or a microbial ecosystem often involves understanding the behavior and heterogeneity of the individual cells that constitute the system and their interactions. In this review, we discuss a variety of approaches to single-cell genomic analysis.
DNA Elimination in Ciliates: Transposon Domestication and Genome Surveillance
Authors: Chalker Douglas L. , Yao Meng-Chao
Published: March 2011

Ciliated protozoa extensively remodel their somatic genomes during nuclear development, fragmenting their chromosomes and removing large numbers of internal eliminated sequences (IESs). The sequences eliminated are unique and repetitive DNAs, including transposons. Recent studies have identified transposase proteins that appear to have been domesticated and are used by these cells to eliminate DNA not wanted in the somatic macronucleus. This DNA elimination process is guided by meiotically produced small RNAs, generated in the germline nucleus, that recognize homologous sequences leading to their removal. These scan RNAs are found in complexes with PIWI proteins. Before they search the developing genome for IESs to eliminate, they scan the parental somatic nucleus and are removed from the pool if they match homologous sequences in that previously reorganized genome. In Tetrahymena, the scan RNAs target heterochromatin modifications to mark IESs for elimination. This DNA elimination pathway in ciliates shares extensive similarity with piRNA-mediated silencing of metazoans and highlights the remarkable ability of homologous RNAs to shape developing genomes.
Genetics and Control of Tomato Fruit Ripening and Quality Attributes
Authors: Klee Harry J. , Giovannoni James J.
Published: March 2011

Tomato ripening is a highly coordinated developmental process that coincides with seed maturation. Regulated expression of thousands of genes controls fruit softening as well as accumulation of pigments, sugars, acids, and volatile compounds that increase attraction to animals. A combination of molecular tools and ripening-affected mutants has permitted researchers to establish a framework for the control of ripening. Tomato is a climacteric fruit, with an absolute requirement for the phytohormone ethylene to ripen. This dependence upon ethylene has established tomato fruit ripening as a model system for study of regulation of its synthesis and perception. In addition, several important ripening mutants, including rin, nor, and Cnr, have provided novel insights into the control of ripening processes. Here, we describe how ethylene and the transcription factors associated with the ripening process fit together into a network controlling ripening.
The Genetics of Hybrid Incompatibilities
Authors: Maheshwari Shamoni , Barbash Daniel A.
Published: March 2011

Incompatibilities in interspecific hybrids, such as sterility and lethality, are widely observed causes of reproductive isolation and thus contribute to speciation. Because hybrid incompatibilities are caused by divergence in each of the hybridizing species, they also reveal genomic changes occurring on short evolutionary time scales that have functional consequences. These changes include divergence in protein-coding gene sequence, structure, and location, as well as divergence in noncoding DNAs. The most important unresolved issue is understanding the evolutionary causes of the divergence within species that in turn leads to incompatibility between species. Surprisingly, much of this divergence does not appear to be driven by ecological adaptation but may instead result from responses to purely mutational mechanisms or to internal genetic conflicts.
Human Mitochondrial tRNAs: Biogenesis, Function, Structural Aspects, and Diseases
Authors: Suzuki Tsutomu , Nagao Asuteka , Suzuki Takeo
Published: March 2011

Mitochondria are eukaryotic organelles that generate most of the energy in the cell by oxidative phosphorylation (OXPHOS). Each mitochondrion contains multiple copies of a closed circular double-stranded DNA genome (mtDNA). Human (mammalian) mtDNA encodes 13 essential subunits of the inner membrane complex responsible for OXPHOS. These mRNAs are translated by the mitochondrial protein synthesis machinery, which uses the 22 species of mitochondrial tRNAs (mt tRNAs) encoded by mtDNA. The unique structural features of mt tRNAs distinguish them from cytoplasmic tRNAs bearing the canonical cloverleaf structure. The genes encoding mt tRNAs are highly susceptible to point mutations, which are a primary cause of mitochondrial dysfunction and are associated with a wide range of pathologies. A large number of nuclear factors involved in the biogenesis and function of mt tRNAs have been identified and characterized, including processing endonucleases, tRNA-modifying enzymes, and aminoacyl-tRNA synthetases. These nuclear factors are also targets of pathogenic mutations linked to various diseases, indicating the functional importance of mt tRNAs for mitochondrial activity.