Showing papers in "Annual Review of Fluid Mechanics in 2011"

Actuators for Active Flow Control

Abstract: Actuators are transducers that convert an electrical signal to a desired physical quantity. Active flow control actuators modify a flow by providing an electronically controllable disturbance. The field of active flow control has witnessed explosive growth in the variety of actuators, which is a testament to both the importance and challenges associated with actuator design. This review provides a framework for the discussion of actuator specifications, characteristics, selection, design, and classification for aeronautical applications. Actuator fundamentals are discussed, and various popular actuator types used in low-to-moderate speed flows are then described, including fluidic, moving object/surface, and plasma actuators. We attempt to highlight the strengths and inevitable drawbacks of each and highlight potential future research directions.

High–Reynolds Number Wall Turbulence

Abstract: We review wall-bounded turbulent flows, particularly high–Reynolds number, zero–pressure gradient boundary layers, and fully developed pipe and channel flows. It is apparent that the approach to an asymptotically high–Reynolds number state is slow, but at a sufficiently high Reynolds number the log law remains a fundamental part of the mean flow description. With regard to the coherent motions, very-large-scale motions or superstructures exist at all Reynolds numbers, but they become increasingly important with Reynolds number in terms of their energy content and their interaction with the smaller scales near the wall. There is accumulating evidence that certain features are flow specific, such as the constants in the log law and the behavior of the very large scales and their interaction with the large scales (consisting of vortex packets). Moreover, the refined attached-eddy hypothesis continues to provide an important theoretical framework for the structure of wall-bounded turbulent flows.

Global Linear Instability

Abstract: This article reviews linear instability analysis of flows over or through complex two-dimensional (2D) and 3D geometries. In the three decades since it first appeared in the literature, global instability analysis, based on the solution of the multidimensional eigenvalue and/or initial value problem, is continuously broadening both in scope and in depth. To date it has dealt successfully with a wide range of applications arising in aerospace engineering, physiological flows, food processing, and nuclear-reactor safety. In recent years, nonmodal analysis has complemented the more traditional modal approach and increased knowledge of flow instability physics. Recent highlights delivered by the application of either modal or nonmodal global analysis are briefly discussed. A conscious effort is made to demystify both the tools currently utilized and the jargon employed to describe them, demonstrating the simplicity of the analysis. Hopefully this will provide new impulses for the creation of next-generation algorithms capable of coping with the main open research areas in which step-change progress can be expected by the application of the theory: instability analysis of fully inhomogeneous, 3D flows and control thereof.

Transition and Stability of High-Speed Boundary Layers

Abstract: This article reviews stability and laminar-turbulent transition in high-speed boundary-layer flows, emphasizing qualitative features of the disturbance spectrum leading to new mechanisms of receptivity and instability. It is shown that the extension of subsonic and low-supersonic stability concepts and transition prediction methods to hypersonic speeds is not straightforward. The discussion focuses on theoretical models providing insights into the physics of instability and helping make proper decisions on transition control strategies.

Numerical Methods for High-Speed Flows

Abstract: We review numerical methods for direct numerical simulation (DNS) and large-eddy simulation (LES) of turbulent compressible flow in the presence of shock waves. Ideal numerical methods should be accurate and free from numerical dissipation in smooth parts of the flow, and at the same time they must robustly capture shock waves without significant Gibbs ringing, which may lead to nonlinear instability. Adapting to these conflicting goals leads to the design of strongly nonlinear numerical schemes that depend on the geometrical properties of the solution. For low-dissipation methods for smooth flows, numerical stability can be based on physical conservation principles for kinetic energy and/or entropy. Shock-capturing requires the addition of artificial dissipation, in more or less explicit form, as a surrogate for physical viscosity, to obtain nonoscillatory transitions. Methods suitable for both smooth and shocked flows are discussed, and the potential for hybridization is highlighted. Examples of the application of advanced algorithms to DNS/LES of turbulent, compressible flows are presented.

Flapping and Bending Bodies Interacting with Fluid Flows

Abstract: The flapping or bending of a flexible planar structure in a surrounding fluid flow, which includes the flapping of flags and the self-streamlining of flexible bodies, constitutes a central problem in the field of fluid-body interactions. Here we review recent, highly detailed experiments that reveal new nonlinear phenomena in these systems, as well as advances in theoretical understanding, resulting in large part from the rapid development of new simulation methods that fully capture the mutual coupling of fluids and flexible solids.

Collective Hydrodynamics of Swimming Microorganisms: Living Fluids

Abstract: Experimental observations indicate that, at sufficiently high cell densities, swimming bacteria exhibit coordinated motions on length scales (10 to 100 μm) that are large compared with the size of an individual cell but too small to yield significant gravitational or inertial effects. We discuss simulations of hydrodynamically interacting self-propelled particles as well as stability analyses and numerical solutions of averaged equations of motion for low Reynolds number swimmers. It has been found that spontaneous motions can arise in such systems from the coupling between the stresses the bacteria induce in the fluid as they swim and the rotation of the bacteria due to the resulting fluid velocity disturbances.

Mammalian Sperm Motility: Observation and Theory

Abstract: Mammalian spermatozoa motility is a subject of growing importance because of rising human infertility and the possibility of improving animal breeding. We highlight opportunities for fluid and continuum dynamics to provide novel insights concerning the mechanics of these specialized cells, especially during their remarkable journey to the egg. The biological structure of the motile sperm appendage, the flagellum, is described and placed in the context of the mechanics underlying the migration of mammalian sperm through the numerous environments of the female reproductive tract. This process demands certain specific changes to flagellar movement and motility for which further mechanical insight would be valuable, although this requires improved modeling capabilities, particularly to increase our understanding of sperm progression in vivo. We summarize current theoretical studies, highlighting the synergistic combination of imaging and theory in exploring sperm motility, and discuss the challenges for future observational and theoretical studies in understanding the underlying mechanics.

Pulse Wave Propagation in the Arterial Tree

Abstract: The beating heart creates blood pressure and flow pulsations that propagate as waves through the arterial tree that are reflected at transitions in arterial geometry and elasticity. Waves carry information about the matter in which they propagate. Therefore, modeling of arterial wave propagation extends our knowledge about the functioning of the cardiovascular system and provides a means to diagnose disorders and predict the outcome of medical interventions. In this review we focus on the physical and mathematical modeling of pulse wave propagation, based on general fluid dynamical principles. In addition we present potential applications in cardiovascular research and clinical practice. Models of short- and long-term adaptation of the arterial system and methods that deal with uncertainties in personalized model parameters and boundary conditions are briefly discussed, as they are believed to be major topics for further study and will boost the significance of arterial pulse wave modeling even more.

Lagrangian Dynamics and Models of the Velocity Gradient Tensor in Turbulent Flows

Abstract: Many fundamental and intrinsic properties of small-scale motions in turbulence can be described using the velocity gradient tensor. This tensor encodes interesting geometric and statistical information such as the alignment of vorticity with respect to the strain-rate eigenvectors, rate of deformation and shapes of fluid material volumes, non-Gaussian statistics, and intermittency. In the inertial range of turbulence, similar properties can be described using the coarse-grained or filtered velocity gradient tensor. In this article we review various models that aim at understanding these phenomena using a small number of ordinary differential equations, written either as a low-dimensional dynamical system or as a set of stochastic differential equations. Typically these describe the Lagrangian evolution of the velocity gradient tensor elements following fluid particles and require models for the pressure Hessian and viscous effects. Sample results from various models are shown, and open challenges are high...

Planetary Magnetic Fields and Fluid Dynamos

Abstract: The magnetic fields of the planets, including the Earth, are generated by dynamo action in their fluid cores. Numerical models of this process have been developed that solve the fundamental magnetohydrodynamic equations driven by convection in a rotating spherical shell. New results from these theoretical models are compared with observations of the geomagnetic field and magnetic data gathered from space missions. The mechanism by which a magnetic field is created is examined. The effects of rotation and magnetic field on the convection are of paramount importance in the simulations. A wide range of simulations with different convection models, varying boundary conditions, and parameter values have been performed over the past 10 years. The effects of these differences are assessed. Numerical considerations mean that all dynamo simulations use much enhanced values of the diffusivities. We consider to what extent this affects results and show how the asymptotic behavior at low diffusion is starting to be inferred using scaling laws. The results of specific models relating to individual planets are reviewed.

Aerodynamic Aspects of Wind Energy Conversion

Abstract: This article reviews the most important aerodynamic research topics in the field of wind energy. Wind turbine aerodynamics concerns the modeling and prediction of aerodynamic forces, such as performance predictions of wind farms, and the design of specific parts of wind turbines, such as rotor-blade geometry. The basics of the blade-element momentum theory are presented along with guidelines for the construction of airfoil data. Various theories for aerodynamically optimum rotors are discussed, and recent results on classical models are presented. State-of-the-art advanced numerical simulation tools for wind turbine rotors and wakes are reviewed, including rotor predictions as well as models for simulating wind turbine wakes and flows in wind farms.

Fluctuations and Instability in Sedimentation

Abstract: This review concentrates on the fluctuations of the velocities of sedimenting spheres, and on the structural instability of a suspension of settling fibers. For many years, theoretical estimates and numerical simulations predicted the fluctuations of the velocities of spheres to increase with the size of the container, whereas experiments found no such variation. Two ideas have increased our understanding. First, the correlation length of the velocity fluctuations was found experimentally to be 20 interparticle separations. Second, in dilute suspensions, a vertical variation in the concentration due to the spreading of the front with the clear fluid can inhibit the velocity fluctuations. In a very dilute regime, a homogeneous suspension of fibers suffers a spontaneous instability in which fast descending fiber-rich columns are separated by rising fiber-sparse columns. In a semidilute regime, the settling is hindered, more so than for spheres.

Shock-Bubble Interactions

Abstract: When a shock wave propagates through a medium of nonuniform thermodynamic properties, several processes occur simultaneously that alter the geometry of the shock wave and the thermodynamic state of the medium. These include shock compression and acceleration of the medium, refraction of the shock, and vorticity generation within the medium. The interaction of a shock wave with a cylinder or a sphere (both referred to as a bubble in this review) is the simplest configuration in which all these processes take place and can be studied in detail. Shock acceleration leads to an initial compression and distortion of the bubble, followed by the formation of a vortex pair in the two-dimensional (2D) case and a vortex ring in the 3D case. At later times, for appropriate combinations of the incident shock strength and density contrast between the bubble and ambient materials, secondary vortices are formed, mass is stripped away from the original bubble, and mixing of the bubble and ambient fluids occurs.

Fluid Mechanics of Papermaking

Abstract: Papermaking is to a large extent a multiphase flow process in which the structure of the material and many of the relevant properties of the final product are determined by the interaction between water and the wood fibers. The dominant feature of a suspension composed of wood fibers and water is its inherent propensity to form bundles of mechanically entangled fibers, known as fiber flocs. However, the phenomena apparent throughout the papermaking process are not unique but in fact have a generic fluid dynamical nature.

Fish Swimming and Bird/Insect Flight

Abstract: This expository review is devoted to fish swimming and bird/insect flight. (a) The simple waving motion of an elongated flexible ribbon plate of constant width propagating a wave distally down the plate to swim forward in a fluid, initially at rest, is first considered to provide a fundamental concept on energy conservation. It is generalized to include variations in body width and thickness, with appended dorsal, ventral and caudal fins shedding vortices to closely simulate fish swimming, for which a nonlinear theory is presented for large-amplitude propulsion. (b) For bird flight, the pioneering studies on oscillatory rigid wings are discussed with delineating a fully nonlinear unsteady theory for a two-dimensional flexible wing with arbitrary variations in shape and trajectory to provide a comparative study with experiments. (c) For insect flight, recent advances are reviewed by items on aerodynamic theory and modeling, computational methods, and experiments, for forward and hovering flights with producing leading-edge vortex to yield unsteady high lift. (d) Prospects are explored on extracting prevailing intrinsic flow energy by fish and bird to enhance thrust for propulsion. (e) The mechanical and biological principles are drawn together for unified studies on the energetics in deriving metabolic power for animal locomotion, leading to the surprising discovery that the hydrodynamic viscous drag on swimming fish is largely associated with laminar boundary layers, thus drawing valid and sound evidences for a resounding resolution to the long-standing fish-swim paradox proclaimed by Gray (1936, 1968).

Surfactant Effects on Bubble Motion and Bubbly Flows

Abstract: Small amounts of surfactant can drastically change bubble behavior. For example, a bubble in aqueous surfactant solution rises much slower than one in purified water. This phenomenon is explained by the so-called Marangoni effect caused by a nonuniform concentration distribution of surfactant along the bubble surface. In other words, a tangential shear stress appears on the bubble surface due to the surface tension variation caused by the surface concentration distribution, which results in the reduction of the rising velocity of the bubble. More interestingly, this Marangoni effect influences not only the rising velocity, but also the lateral migration in the presence of mean shear. Furthermore, these phenomena influence the multiscale nature of bubbly flows and cause a drastic change in the bubbly flow structure. In this article, we review the recent studies related to these interesting behaviors of bubbles caused by the surfactant adsorption/desorption on the bubble surface.

Aerobreakup of Newtonian and Viscoelastic Liquids

Abstract: In this review, we consider and unify all aspects of the dynamics of Newtonian and viscoelastic liquid drops in high-speed gas flows, including shock waves. The path to understanding is opened by novel, laser-induced fluorescence visualizations at spatial resolutions of up to 200 pixels for millimeter and exposure times as low as 5 ns. The central role of the competition between Rayleigh-Taylor and Kelvin-Helmholtz instabilities is assessed in the frame of rich aerodynamics, from low subsonic to supersonic, and the multitude of characteristic length scales and timescales at play with varying liquid properties. Acceleration and liquid redistribution (drop deformation) early in the evolution set the stage for this competition, and we insist on an interpretation of the drag coefficient that is physically meaningful. Two principal breakup regimes (patterns of bodily loss of coherence) are identified depending on whether the gas finds its way through the liquid mass, causing gross disintegration, or goes aroun...

Experimental Studies of Transition to Turbulence in a Pipe

Abstract: In his landmark paper of 1883, Reynolds addressed the question of why the fluid motion along a pipe changes from a laminar to a turbulent state at modest flow rates. His discoveries have remained a focus of hydrodynamic stability for the intervening 125 years, and the central puzzle of why the transition takes place at all remains unresolved. It is an enigma as all theoretical and numerical evidence suggests that the base state of fully developed flow, Hagen-Poiseuille flow, is linearly stable. The transition to turbulence is abrupt, mysterious, and largely dependent on the quality of the facility used in any experimental investigation. It is therefore not an example of transition via a sequence of instabilities or bifurcations in which considerable success has been achieved over the same period. Despite wide-ranging research activity that has uncovered many important pieces of the jigsaw, the central puzzle remains unresolved. The purpose of this review is to bring together the available experimental evidence and attempt to extract a set of accepted facts about this important problem.

Fluid-Structure Interaction in Internal Physiological Flows

Abstract: We provide a selective review of recent progress in the analysis of several physiological and physiologically inspired fluid-structure interaction problems, our aim being to explain the underlying physical mechanisms that cause the observed behaviors. Specifically, we discuss recent studies of self-excited oscillations in collapsible tubes, focusing primarily on studies of an idealized model system, the Starling resistor—a device used in most laboratory experiments. We next review studies of a particular physiological, flow-induced oscillation: vocal-fold oscillations during phonation. Finally, we discuss the closure and reopening of pulmonary airways, physiological fluid-structure interaction problems that also involve the airways' liquid lining.

Optical Particle Characterization in Flows

Abstract: Particle characterization in dispersed multiphase flows is important in quantifying transport processes both in fundamental and applied research: Examples include atomization and spray processes, cavitation and bubbly flows, and solid particle transport in gas and liquid carrier phases. Optical techniques of particle characterization are preferred owing to their nonintrusiveness, and they can yield information about size, velocity, composition, and to some extent the shape of individual particles. This review focuses on recent advances for measuring size, temperature, and the composition of particles, including several planar methods, various imaging techniques, laser-induced fluorescence, and the more recent use of femtosecond pulsed light sources. It emphasizes the main sources of uncertainty, the achievable accuracy, and the outlook for improvement of specific techniques and for specific applications. Some remarks are also directed toward the computational tools used to design and investigate the performance of optical particle diagnostic instruments.

Fluid Dynamics of Dissolved Polymer Molecules in Confined Geometries

Abstract: The past decade has seen a renaissance in the study of polymer solutions flowing in confined geometries, the renaissance driven in part by advances in visualization of large DNA molecules and the desire to manipulate DNA for genomic applications. This article summarizes the features of the fundamental polymer physics and fluid dynamics that are relevant to the flow of confined polymer solutions, then reviews the recent literature on the topic. Experiments have clarified and extended prior work showing that diffusion of confined flexible polymers is substantially altered by confinement and that, during flow, polymers exhibit substantial cross-stream migration. Simulation methods have been developed that have the capability of capturing both polymer and fluid motion in confined geometries and yield results that are in semiquantitative agreement with experiments in dilute solutions. Kinetic-theory treatments of simple polymer models have led to analytically tractable models that qualitatively encompass the k...

Discrete Conservation Properties of Unstructured Mesh Schemes

Abstract: Numerical methods with discrete conservation statements are useful because they cannot produce solutions that violate important physical constraints. A large number of numerical methods used in computational fluid dynamics (CFD) have either global or local conservation statements for some of the primary unknowns of the method. This review suggests that local conservation of primary unknowns often follows from global conservation of those quantities. Secondary conservation involves the conservation of derived quantities, such as kinetic energy, entropy, and vorticity, which are not directly unknowns of the numerical system. Secondary conservation can further improve physical fidelity of a numerical solution, but it is typically much harder to achieve. We consider current approaches to secondary conservation and techniques used outside of CFD that are potentially related. Finally, the review concludes with a discussion of how secondary conservation properties might be included automatically.

Scale Interactions in Magnetohydrodynamic Turbulence

Abstract: This article reviews recent studies of scale interactions in magnetohydrodynamic turbulence. The present-day increase of computing power, which allows for the exploration of different configurations of turbulence in conducting flows, and the development of shell-to-shell transfer functions, has led to detailed studies of interactions between the velocity and the magnetic field and between scales. In particular, processes such as induction and dynamo action, the damping of velocity fluctuations by the Lorentz force, and the development of anisotropies can be characterized at different scales. In this context we consider three different configurations often studied in the literature: mechanically forced turbulence, freely decaying turbulence, and turbulence in the presence of a uniform magnetic field. Each configuration is of interest for different geophysical and astrophysical applications. Local and nonlocal transfers are discussed for each case. Whereas the transfer between scales of solely kinetic or so...

Shear-Layer Instabilities: Particle Image Velocimetry Measurements and Implications for Acoustics

Abstract: The use of particle image velocimetry (PIV) to study the spatial and temporal features of unsteady fluid flow has increased dramatically in the past five to ten years. One particular application of PIV is to examine how shear-layer instabilities and turbulence lead to radiated sound. In this review, the basic operation of a PIV system is provided along with an introduction to the equations that relate unsteady fluid motion to sound. The references then illustrate how PIV is currently used in a number of canonical flow problems of interest in which the phenomena are dominated by shear-layer instabilities that lead to radiated noise. Specifically, cavity flows, flow over airfoils and cylinders, and finally jet flows are considered.