ALBERT

Notes: The preparation of skeletal catalysts is considered, with particular reference to the preparation of skeletal nickel and skeletal copper. Skeletal catalysts have physical properties that are highly desired for some industrial processes, and the means of controlling these properties to optimize performance is reviewed. The preparation can be affected by the initial alloy composition, the quenching process, the leaching procedure, aging and by the addition of promoters. These processes may affect either or both the chemical and physical characteristics of the catalysts. Preparation procedures need to be adjusted for individual reactions if optimal performance is required. The effect of the preparation conditions on the structure and catalytic performance of these systems is reviewed.

Notes: We review here the theory of the early stages of oxidation of the (110) surface of Ni1x Alx, based on ab initio calculations using a plane-wave pseudopotential method. The clean surface and several oxidized surfaces have been investigated, with oxygen coverages up to 2ML of oxygen (1ML = 3 O atoms per 2 surface Al atoms). The theory to date is a description in terms of equilibrium thermodynamics, with a comparison of the free energies of several surfaces of different composition, implemented at the atomic scale. Three environmental parameters are singled out as control variables in this treatment, namely the alloy composition x (assumed to be near 0.5), the temperature T and the partial pressure of oxygen pO2. With certain reasonable approximations an analytic formula for the surface energy ?? is derived in terms of these variables and some constants that are calculated ab initio together with others that are derived from experimental thermodynamic tables. At oxygen pressures just above the threshold for bulk oxidation of NiAl, the calculations explain the observed formation of a thin film of alumina in place of NiAl surface layers, with the consequent dissolution of Ni into the bulk. Ab initio calculations illustrate how the energetics of supplying Al to the surface depends on bulk stoichiometry, which alters the relative stability of different surface oxidation states so as to favour oxidation more if the alloy is Al-rich than if it is Ni-rich.

Notes: There is a pressing need for cleaner fuels (free or aromatics and of minimal sulfur content) or ones that convert chemical energy directly to electricity, silently and without production of noxious oxides and particulates; chemical, petrochemical and pharmaceutical processes that may be conducted in a one-step, solvent-free manner and that use air as the preferred oxidant; and industrial processes that minimize consumption of energy, production of waste, or the use of corrosive, explosive, volatile, and nonbiodegradable materials. All these needs and other desiderata, such as the in situ production and containment of aggressive and hazardous reagents, and the avoidance of use of ecologically harmful elements, may be achieved by designing the appropriate heterogeneous inorganic catalyst, which ideally should be cheap, readily preparable and fully characterizable, preferably under in situ reaction conditions. A range of nanoporous and nanoparticle catalysts that meet most of the stringent demands of sustainable development and responsible (clean) technology is described. Specific examples that are highlighted include the production of adipic acid (precursor of polyamides and urethanes) without the use of concentrated nitric acid nor the production of greenhouse gases such as nitrous oxide; the production of caprolactam (precursor of nylon) without the use of oleum and hydroxylamine sulfate; and the terminal oxyfunctionalization of linear alkanes in air. The topic of biocatalysis and sustainable development is also briefly discussed for the epoxidation of terpenes and fatty acid methyl esters; for the generation of polymers, polylactides, and polyesters; and for the production of 1,3-propanediol from corn.

Notes: This review covers the synthesis, characterization, and physico-chemical properties of microporous and mesoporous aluminophosphates and silicoaluminophosphates molecular sieves. Particular emphasis is given to the materials that have found applications as acid catalysts. We consider the evolution of the synthesis procedures from the first discoveries to the current methodologies and give perspectives for new possible synthesis strategies. Emphasis is given to the use of specially prepared precursors/reactants designed for the use as molecular sieves. Experimental (especially MAS-NMR and FTIR spectroscopy) and theoretical approaches to the description of the Si insertion into the ALPO framework and to the acidic properties of SAPOs and MeAPSOs materials are discussed.

Notes: The diffusion-multiple approachĐ??the creation of composition gradients and intermetallic phases by long-term annealing of junctions of three or more phases/alloysĐ??enables the generation of a large number of phases and compositions for efficient mapping of phase diagrams, phase properties, and kinetics. With an efficiency much higher than that of a conventional approach, many critical materials data, which otherwise would be too time-consuming and expensive to acquire, can be obtained and employed to accelerate alloy design. The critical data for structural materials design include phase diagrams, diffusion coefficients, precipitation kinetics, solution-strengthening effects, and precipitation-strengthening effects. All these data can be obtained from diffusion multiples. The combination of the diffusion-multiple approach with the CALPHAD approach can impact computational design of structural materials. The diffusion-multiple approach can also be applied to combinatorial screening of functional materials having unique physical, chemical, or other properties when micro-scale measurement techniques are available for these properties. Examples are shown to illustrate the progress made to date on applying the diffusion-multiple approach to accelerated design/discovery of materials.

Notes: Renewable energy resources, of which wind energy is prominent, are part of the solution to the global energy problem. Wind turbine and the rotorblade concepts are reviewed, and loadings by wind and gravity as important factors for the fatigue performance of the materials are considered. Wood and composites are discussed as candidates for rotorblades. The fibers and matrices for composites are described, and their high stiffness, low density, and good fatigue performance are emphasized. Manufacturing technologies for composites are presented and evaluated with respect to advantages, problems, and industrial potential. The important technologies of today are prepreg (pre-impregnated) technology and resin infusion technology. The mechanical properties of fiber composite materials are discussed, with a focus on fatigue performance. Damage and materials degradation during fatigue are described. Testing procedures for documentation of properties are reviewed, and fatigue loading histories are discussed, together with methods for data handling and statistical analysis of (large) amounts of test data. Future challenges for materials in the field of wind turbines are presented, with a focus on thermoplastic composites, new structural materials concepts, new structural design aspects, structural health monitoring, and the coming trends and markets for wind energy.

Notes: This review illustrates how catalytically active molecular structures are created on oxide surfaces by attachment of metal complexes with subsequent structural transformation and molecular imprinting on the surfaces. Also discussed is how the designed surfaces are characterized in situ by physical techniques including conventional and time-resolved X-ray absorption fine structure. The structural transformation and molecular imprinting for attached metal complexes can provide a new class of catalytic materials with a high complexity for selective catalysis including shape-selective and asymmetric catalysis.

Notes: This paper describes the use of extended X-ray absorption fine structure spectroscopy (EXAFS) to examine the structure of molten salts and ionic liquids and species dissolved in them. The EXAFS theory is briefly described as are the methods by which EXAFS of these systems can be studied. A range of applications have used EXAFS to investigate the structure of metallic species in ionic liquids from extraction studies to catalysts. The area of structural investigations of ionic liquids is still being developed, although growing rapidly, whereas the structure of molten salts has been studied using EXAFS in more detail.

Notes: In the nanoscience era, the properties of many exciting new materials and devices will depend on the details of their composition down to the level of single atoms. Thus the characterization of the structure and electronic properties of matter at the atomic scale is becoming ever more vital for economic and technological as well as for scientific reasons. The combination of atomic-resolution Z-contrast scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) represents a powerful method to link the atomic and electronic structure to macroscopic properties, allowing materials, nanoscale systems, and interfaces to be probed in unprecedented detail. Z-contrast STEM uses electrons that have been scattered to large angles for imaging. The relative intensity of each atomic column is roughly proportional to Z2, where Z is the atomic number. Recent developments in correcting the aberrations of the lenses in the electron microscope have pushed the achievable spatial resolution and the sensitivity for imaging and spectroscopy in the STEM into the sub-A??ngstrom (sub-A??) regime, providing a new level of insight into the structure/property relations of complex materials. Images acquired with an aberration-corrected instrument show greatly improved contrast. The signal-to-noise ratio is sufficiently high to allow sensitivity even to single atoms in both imaging and spectroscopy. This is a key achievement because the detection and measurement of the response of individual atoms has become a challenging issue to provide new insight into many fields, such as catalysis, ceramic materials, complex oxide interfaces, or grain boundaries. In this article, the state-of-the-art for the characterization of all of these different types of materials by means of aberration-corrected STEM and EELS are reviewed.

Notes: The objective of this review is to critically assess the different approaches developed in recent years to understand the dynamics of open flows such as mixing layers, jets, wakes, separation bubbles, boundary layers, and so on. These complex flows develop in extended domains in which fluid particles are continuously advected downstream. They behave either as noise amplifiers or as oscillators, both of which exhibit strong nonlinearities ( Huerre & Monkewitz 1990 ). The local approach is inherently weakly nonparallel and it assumes that the basic flow varies on a long length scale compared to the wavelength of the instability waves. The dynamics of the flow is then considered as a superposition of linear or nonlinear instability waves that, at leading order, behave at each streamwise station as if the flow were homogeneous in the streamwise direction. In the fully global context, the basic flow and the instabilities do not have to be characterized by widely separated length scales, and the dynamics is then viewed as the result of the interactions between Global modes living in the entire physical domain with the streamwise direction as an eigendirection. This second approach is more and more resorted to as a result of increased computational capability. The earlier review of Huerre & Monkewitz (1990) emphasized how local linear theory can account for the noise amplifier behavior as well as for the onset of a Global mode. The present survey first adopts the opposite point of view by demonstrating how fully global theory accounts for the noise amplifier behavior of open flows. From such a perspective, there is strong emphasis on the very peculiar nonorthogonality of linear Global modes, which in turn allows a novel interpretation of recent numerical simulations and experimental observations. The nonorthogonality of linear Global modes also imposes severe constraints on the extension of linear global theory to the fully nonlinear re??gime. When the flow is weakly nonparallel, this limitation is so severe that the linear Global mode theory is of little help. It is then much more appropriate to develop a fully nonlinear formulation involving the presence of a front separating the base state region from the bifurcated state region.

Notes: We review the fluid mechanics and rheology of dense suspensions, emphasizing investigations of microstructure and total stress. "Dense" or "highly concentrated" suspensions are those in which the average particle separation distance is less than the particle radius. For these suspensions, multiple-body interactions as well as two-body lubrication play a significant role and the rheology is non-Newtonian. We include investigations of multimodal suspensions, but not those of suspensions with dominant nonhydrodynamic interactions. We consider results from both physical experiments and computer simulations and explore scaling theories and the development of constitutive equations.

Notes: George Gabriel Stokes died just over 100 years ago, and it has been more than 150 years since he published his great 1847 paper on water waves. The work of Stokes' precursors, which informed his early publications of 1842Đ??50, is described in the previous volume of the Annual Review of Fluid Mechanics ( Craik 2004 ). Here I examine Stokes' papers and letters concerning water waves.

Notes: Chaotic advection and, more generally, ideas from dynamical systems, have been fruitfully applied to a diverse, and varied, collection of mixing and transport problems arising in engineering applications over the past 20 years. Indeed, the "dynamical systems approach" was developed, and tested, to the point where it can now be considered a standard tool for understanding mixing and transport issues in many disciplines. This success for engineering-type flows motivated an effort to apply this approach to transport and mixing problems in geophysical flows. However, there are fundamental difficulties arising in this endeavor that must be properly understood and overcome. Central to this approach is that the starting point for analysis is a velocity field (i.e., the "dynamical system"). In many engineering applications this can be obtained sufficiently accurately, either analytically or computationally, so that it describes particle trajectories for the actual flow. However, in geophysical flows (and we concentrate here almost exclusively on oceanographic flows), the wide range of dynamically significant time and length scales makes the justification of any velocity field, in the sense of reproducing particle trajectories for the actual flow, a much more difficult matter. Nevertheless, the case for this approach is compelling due to the advances in observational capabilities in oceanography (e.g., drifter deployments, remote sensing capabilities, satellite imagery, etc.), which reveal space-time structures that are highly suggestive of the structures one visualizes in the global, geometrical study of dynamical systems theory. This has been pursued in recent years through a combination of laboratory studies, kinematic models, and dynamically consistent models that have all been compared with observational data. During the course of these studies it has become apparent that a new type of dynamical system is necessary to consider in these studies (i.e., a finite time, aperiodically time-dependent velocity field defined as a data set), which requires the development of new analytical and computational tools, as well as the necessity to discard some of the standard ideas and results from dynamical systems theory. In this article we review a number of the key developments to date in this young, but rapidly developing, area at the interface between geophysical fluid dynamics and applied and computational mathematics. We also describe the wealth of new directions for research that this approach unlocks.

Notes: The gas-lift technique comprises the injection of gas bubbles in vertical oil wells to increase production. It is based on a reduction of the tubing gravitational pressure gradient. Several fluid-flow phenomena influencing such vertical gas-liquid flows are discussed. These effects include the radial distribution of void fraction and of gas and liquid velocity, flow regime changes, and system stability problems. Associated consequences for gas-lift performance and related optimization approaches are also discussed.

Notes: What mechanisms of flow control do animals use to enhance hydrodynamic performance? Animals are capable of manipulating flow around the body and appendages both passively and actively. Passive mechanisms rely on structural and morphological components of the body (i.e., humpback whale tubercles, riblets). Active flow control mechanisms use appendage or body musculature to directly generate wake flow structures or stiffen fins against external hydrodynamic loads. Fish can actively control fin curvature, displacement, and area. The vortex wake shed by the tail differs between eel-like fishes and fishes with a discrete narrowing of the body in front of the tail, and three-dimensional effects may play a major role in determining wake structure in most fishes.

Notes: Over the past four decades, the combination of in situ and remote sensing observations has demonstrated that long nonlinear internal solitary-like waves are ubiquitous features of coastal oceans. The following provides an overview of the properties of steady internal solitary waves and the transient processes of wave generation and evolution, primarily from the point of view of weakly nonlinear theory, of which the Korteweg-de Vries equation is the most frequently used example. However, the oceanographically important processes of wave instability and breaking, generally inaccessible with these models, are also discussed. Furthermore, observations often show strongly nonlinear waves whose properties can only be explained with fully nonlinear models.

Notes: Electrophoretic separation of a mixture of chemical species is a fundamental technique of great usefulness in biology, health care, and forensics. In capillary electrophoresis (which has evolved from its predecessor, slab-gel electrophoresis), the sample migrates through a single microcapillary instead of through the network of pores in a gel. A fundamental design problem is to minimize dispersion in the separation direction. Molecular diffusion is inevitable and sets a theoretical limit on the best separation that can be achieved. But in practice, there are a number of effects arising out of the interplay between fluid flow, chemistry, thermal effects, and electric fields that result in enhanced dispersion. This paper reviews the subject of fluid flow in such capillary microchannels and examines the various causes of enhanced dispersion that limit the efficiency of separation.

Notes: Race car performance depends on elements such as the engine, tires, suspension, road, aerodynamics, and of course the driver. In recent years, however, vehicle aerodynamics gained increased attention, mainly due to the utilization of the negative lift (downforce) principle, yielding several important performance improvements. This review briefly explains the significance of the aerodynamic downforce and how it improves race car performance. After this short introduction various methods to generate downforce such as inverted wings, diffusers, and vortex generators are discussed. Due to the complex geometry of these vehicles, the aerodynamic interaction between the various body components is significant, resulting in vortex flows and lifting surface shapes unlike traditional airplane wings. Typical design tools such as wind tunnel testing, computational fluid dynamics, and track testing, and their relevance to race car development, are discussed as well. In spite of the tremendous progress of these design tools (due to better instrumentation, communication, and computational power), the fluid dynamic phenomenon is still highly nonlinear, and predicting the effect of a particular modification is not always trouble free. Several examples covering a wide range of vehicle shapes (e.g., from stock cars to open-wheel race cars) are presented to demonstrate this nonlinear nature of the flow field.

Notes: Large-eddy simulation (LES) of turbulent combustion is a relatively new research field. Much research has been carried out over the past years, but to realize the full predictive potential of combustion LES, many fundamental questions still have to be addressed, and common practices of LES of nonreacting flows revisited. The focus of the present review is to highlight the fundamental differences between Reynolds-averaged Navier-Stokes (RANS) and LES combustion models for nonpremixed and premixed turbulent combustion, to identify some of the open questions and modeling issues for LES, and to provide future perspectives.

Notes: Efficiently and accurately solving the equations governing fluid flow in oil reservoirs is very challenging because of the complex geological environment and the intricate properties of crude oil and gas at high pressure. We present these challenges and review successful and promising solution approaches. We discuss in detail the modeling of fluid flow in reservoirs with strongly varying rock properties. This requires subgrid-scale models that accurately represent the flow physics due to fine-scale fluctuations. A second focus is on the complex multiphase, multicomponent systems that describe miscible gas injection processes for enhanced oil recovery and CO2 sequestration.

Notes: Mammalian fertilization requires the coordinated activity of motile spermatozoa, muscular contractions of the uterus and oviduct, as well as ciliary beating. These elastic structures generate forces that drive fluid motion, but their configurations are, in turn, determined by the fluid dynamics. We review the basic fluid mechanical aspects of reproduction, including flagellar/ciliary beating and peristalsis. We report on recent biological studies that have shed light on the relative importance of the mechanical ingredients of reproduction. In particular, we examine sperm motility in the reproductive tract, ovum pickup and transport in the oviduct, as well as sperm-egg interactions. We review recent advances in understanding the internal mechanics of cilia and flagella, flagellar surface interaction, sperm motility in complex fluids, and the role of fluid dynamics in embryo transfer. We outline promising computational fluid dynamics frameworks that may be used to investigate these complex, fluid-structure interactions.

Notes: This article provides a critical review of computational techniques for flow-noise prediction and the underlying theories. Hybrid approaches, in which the turbulent noise source field is computed and/or modeled separately from the far-field calculation, are afforded particular attention. Numerical methods and modern flow simulation techniques are discussed in terms of their suitability and accuracy for flow-noise calculations. Other topics highlighted include some important formulation and computational issues in the application of aeroacoustic theories, generalized acoustic analogies with better accounts of flow-sound interaction, and recent computational investigations of noise-control strategies. The review ends with an analysis of major challenges and key areas for improvement in order to advance the state of the art of computational aeroacoustics.

Notes: The challenges in understanding hypersonic flight are discussed and critical hypersonic aerothermodynamics issues are reviewed. The ability of current analytical methods, numerical methods, ground testing capabilities, and flight testing approaches to predict hypersonic flow are evaluated. The areas where aerothermodynamic shortcomings restrict our ability to design and analyze hypersonic vehicles are discussed, and prospects for future capabilities are reviewed. Considerable work still needs to be done before our understanding of hypersonic flow will allow for the accurate prediction of vehicle flight characteristics throughout the flight envelope from launch to orbital insertion.

Notes: We review mathematical models of confined bubbles, emphasizing physical mechanisms as expressed in simple geometries. Molecular interactions between liquid, gas, and the confining solid are all important and are described through the disjoining pressure concept. Methods for finding static shapes are considered. The static solution is a springboard for discussing pressure-driven and surface-tension-driven flows, both of which involve viscous effects and macroscopic films entrained near apparent contact lines. We next discuss vapor bubbles produced by thermal effects. Vaporization localized near contact lines and condensation distributed in colder parts of the interface lead to steady vapor bubbles. Their size is determined through global constraints. Unsteady vapor bubbles are discussed and we end with thoughts on open problems.

Notes: The review deals with drop impacts on thin liquid layers and dry surfaces. The impacts resulting in crown formation are referred to as splashing. Crowns and their propagation are discussed in detail, as well as some additional kindred, albeit nonsplashing, phenomena like drop spreading and deposition, receding (recoil), jetting, fingering, and rebound. The review begins with an explanation of various practical motivations feeding the interest in the fascinating phenomena of drop impact, and the above-mentioned topics are then considered in their experimental, theoretical, and computational aspects.

Notes: It has recently become possible to measure the anisotropic distribution of interfaces in polycrystals and composites. Because classical theories for grain growth and coarsening assume isotropic interface properties, they are incapable of explaining how these distributions arise. The purpose of this paper is to review the results of recent experiments, simulations, and theories that document the effects of anisotropy on the capillarity-driven evolution of granular systems. The results suggest that meaningful predictions of evolving microstructural characteristics can be made using models that incorporate the anisotropy of the interfacial energy and mobility.

Notes: This review is an introduction to the problem of determining the nature of the diffraction spectra of ordered solids. After explicit computation examples of Bragg diffraction spectra of quasicrystals (bulk, surface) in kinematical and dynamical (high-resolution electron microscopy) situations, we show on simple toy models that long-range order does not necessarily lead to Bragg peaks which, in turn, are not necessarily the signature of the unique long-range periodic order.

Notes: Recent research trends of the preparation and characterization of highly efficient titanium oxideĐ??based photocatalysts are reviewed on the basis of studies done in our laboratory. Special attention is focused on the preparation and characterization of active sites of highly dispersed titanium oxide species within mesoporous matrices and on titanium oxideĐ??based binary catalysts such as Ti/Si and Ti/B oxides and unique visible light-responsive second-generation TiO2 photocatalysts prepared by an advanced metal-ion-implantation and RF magnetron sputtering deposition method. Highly dispersed titanium oxide species within mesoporous frameworks and titanium oxideĐ??based binary catalysts exhibit high and unique photocatalytic activities for various reactions, such as the direct decomposition of NOx. These species also have photo-induced superhydrophiric properties. It is of significance that visible light-responsive TiO2 photocatalysts can be applied to the decomposition of NOx in air and the degradation of pollutants in water, as well as to the splitting of water under UV and visible light irradiation.

Notes: Heterogeneous asymmetric catalysis remains a topic of intense research interest because control of enantioselectivity represents one of the major synthetic challenges today. Successful strategies to achieve high enantioselection have been developed using three key approaches: (a) adsorption of chiral modifiers onto an active metal surface, which is found to be particularly effective for enantioselective hydrogenation; (b) covalent tethering of homogeneous catalysts; and (c) electrostatic interaction between a negatively charged framework and a cation. Other approaches have been advocated, e.g., encapsulation, but in general these lead to lower enantioselectivity for these heterogeneous asymmetric systems when compared with the equivalent homogeneous catalyst. This review covers the strategies for achieving high enantioselection as well as recent interesting features concerning confinement effects, the inversion of enantioselectivity, and specific cases in which enantioselection increases with conversion, all being factors that can guide researchers to identifying improved systems.

Notes: The review highlights the state-of-the-art research regarding the application of modern in situ spectroscopic, microscopic, and electrochemical techniques to improve the understanding of the interaction of organic molecules with metal surfaces. We also consider the chemical and electrochemical processes that lead to a de-adhesion of polymers from metal surfaces. Spectroscopic techniques such as surface-enhanced infrared or Raman spectroscopy provide molecular understanding of organic molecules and water at buried metal surfaces. This information is complementary to adhesion studies by means of atomic force microscopy and de-adhesion studies of polymer layers from metals by means of a scanning Kelvin probe. Adhesion and de-adhesion mechanisms are discussed, especially those involving humid and corrosive environments, which are the predominant and most important for metal/polymer composites in engineering applications.

Notes: Chemical analysis at high spatial resolution is the domain of analytical transmission electron microscopy. Owing to rapid instrumental developments during the past decade, electron energy-loss spectroscopy offers now a spatial resolution close to 0.1 nm and an energy resolution close to 0.1 eV. This development has been accompanied by the introduction of numerous new techniques and methods for data acquisition and analysis, which are outlined in the present article. Recent results for a wide range of material systems are addressed. These comprise first-principles calculations, which have contributed to enormous progress in the calculation of near-edge fine structures, and fingerprinting methods, which are still important for the interpretation of experimental data.

Notes: Early work and recent advances in feedback control of combustion oscillations are described. The physics of combustion oscillations, most commonly caused by a coupling between acoustic waves and unsteady heat release, are discussed, and the concept of using feedback control to interrupt these interactions is introduced. Factors affecting practical implementation of feedback control, including sensors, actuators, and controller design are described, and the historical development of control strategy for combustion oscillations is reviewed. Finally, demonstrations of feedback control on full-scale combustion systems are described, and it is concluded that there is potential to apply more systematic controller designs at full scale.

Notes: Flow simulations are one of the archetypal multiscale problems. Simulations of turbulent and unsteady separated flows have to resolve a multitude of interacting scales, whereas molecular phenomena determine the structure of shocks and the validity of the no-slip boundary condition. Particle simulations of continuum and molecular phenomena can be formulated by following the motion of interacting particles that carry the physical properties of the flow. In this article we review Lagrangian, multiresolution, particle methods such as vortex methods and smooth particle hydrodynamics for the simulation of continuous flows and molecular dynamics for the simulation of flows at the atomistic scale. We review hybrid molecular-continuum simulations with an emphasis on the computational aspects of the problem. We identify the common computational characteristics of particle methods and discuss their properties that enable the formulation of a systematic framework for multiscale flow simulations.

Notes: Gravity-driven bubbly flows are a specific class of flows, where all action is provided by gravity. An industrial example is formed by the so-called bubble column: a vertical cylinder filled with liquid through which bubbles flow that are introduced at the bottom of the cylinder. On the bubble scale, gravity gives rise to buoyancy of individual bubbles. On larger scales, gravity acts on nonuniformities in the spatial bubble distribution present in the bubbly mixture. The gravity-induced flow and flow structures can increase the inhomogeneity of the bubble distribution, leading to a turbulent flow. In this flow, specific scales are identified: a large-scale circulation with the liquid flowing upward in the center of the column and downward close to the wall. On the intermediate scale there are vortical structures; eddies of liquid, with a size on the order of the diameter of the column, that stir the liquid and radially transport the bubbles. On the small scale there is the local stirring of the bubbles. We describe the ideas developed over time and identify some open questions. We discuss the experimental findings on the turbulence generated, the stability of the flow, axial dispersion, and the similarities between bubble columns and air lifts. Especially for higher gas fractions, many questions still lack accurate answers. The lateral lift force in bubble swarms and the structure of the turbulence in the bubbly mixture are important examples of inadequately understood physical phenomena, providing many challenges for fundamental and applied research on bubbly flows.

Notes: Development and optimization of multifunctional devices for fluidic manipulation of films, drops, and bubbles require detailed understanding of interfacial phenomena and microhydrodynamic flows. Systems are distinguished by a large surface to volume ratio and flow at small Reynolds, capillary, and Bond numbers are strongly influenced by boundary effects and therefore amenable to control by a variety of surface treatments and surface forces. We review the principles underlying common techniques for actuation of droplets and films on homogeneous, chemically patterned, and topologically textured surfaces by modulation of normal or shear stresses.

Notes: "What force does an insect wing generate?" Finding answers to this enduring question is an essential step toward our understanding of interactions of moving objects with fluids that enable most living species such as insects, birds, and fish to travel efficiently and us to follow similar suit with sails, oars, and airfoils. We give a brief history of research in insect flight and discuss recent findings in unsteady aerodynamics of flapping flight at intermediate range Reynolds numbers ( 10Đ??104 ). In particular, we examine the unsteady mechanisms in uniform and accelerated motions, forward and hovering flight, as well as passive flight of free-falling objects. The results obtained by "taking the insects apart" helped us to resolve previous puzzles about the force estimates in hovering insects, to ellucidate basic mechanisms essential to flapping flight, and to gain insights about the efficieny of flight.

Notes: We review recent advances in understanding, modeling, and controlling oscillations in the flow past a cavity. The fundamental mechanisms underlying cavity flow oscillations have been known for at least 40 years, but suppressing these oscillations in a reliable and robust way is still a challenge today. Interest in controlling the flow past a cavity is motivated by aerospace applications, but in addition, cavity flows provide an attractive canonical problem for exploring general flow control techniques. The focus is on recent advances in modeling these flows, and in controlling them, using both open-loop and closed-loop techniques. A relatively new perspective is that cavity oscillations may not always be self-sustained, but under some flow conditions may be lightly damped resonances, sustained by external disturbances such as boundary layer turbulence. Areas in which our understanding is incomplete, and which deserve further study, are discussed, in particular the effects of high-frequency open-loop forcing, fundamental limitations of feedback control for a given configuration of sensors and actuators, and the development of a feedback design methodology that respects the limited range of validity of the available dynamical models.

Notes: When the free surfaces of most solids approach their bulk melting temperatures from below, the molecular structure of the material gives way to a disordered structure with some attributes of both the solid and liquid phases. When the temperature is sufficiently close to that of bulk transition, the surface melts and literally flows as a viscous fluid. This phenomenon, called interfacial premelting, lies at the heart of the microscopic theory of melting of solid matter, and captures the interest of condensed matter physicists and physical chemists alike. The process is ubiquitous and responsible for a wide range of consequences in materials with biological, geophysical, and technological significance. Because such systems are often exposed to spatial or temporal variations in thermodynamic forcing, there are a host of fluid mechanical phenomena that result from this underlying melting behavior. The fluid dynamics of unfrozen surfaces holds clues for understanding the bulk behavior of polycrystalline materials, from Earth's mantle to the stratosphere and beyond. In this review we focus on the fluid dynamical consequences of the premelting of solids.