Showing papers in "Annual Review of Fluid Mechanics in 2005"

Immersed boundary methods

Abstract: The term “immersed boundary method” was first used in reference to a method developed by Peskin (1972) to simulate cardiac mechanics and associated blood flow. The distinguishing feature of this method was that the entire simulation was carried out on a Cartesian grid, which did not conform to the geometry of the heart, and a novel procedure was formulated for imposing the effect of the immersed boundary (IB) on the flow. Since Peskin introduced this method, numerous modifications and refinements have been proposed and a number of variants of this approach now exist. In addition, there is another class of methods, usually referred to as “Cartesian grid methods,” which were originally developed for simulating inviscid flows with complex embedded solid boundaries on Cartesian grids (Berger & Aftosmis 1998, Clarke et al. 1986, Zeeuw & Powell 1991). These methods have been extended to simulate unsteady viscous flows (Udaykumar et al. 1996, Ye et al. 1999) and thus have capabilities similar to those of IB methods. In this review, we use the term immersed boundary (IB) method to encompass all such methods that simulate viscous flows with immersed (or embedded) boundaries on grids that do not conform to the shape of these boundaries. Furthermore, this review focuses mainly on IB methods for flows with immersed solid boundaries. Application of these and related methods to problems with liquid-liquid and liquid-gas boundaries was covered in previous reviews by Anderson et al. (1998) and Scardovelli & Zaleski (1999). Consider the simulation of flow past a solid body shown in Figure 1a. The conventional approach to this would employ structured or unstructured grids that conform to the body. Generating these grids proceeds in two sequential steps. First, a surface grid covering the boundaries b is generated. This is then used as a boundary condition to generate a grid in the volume f occupied by the fluid. If a finite-difference method is employed on a structured grid, then the differential form of the governing equations is transformed to a curvilinear coordinate system aligned with the grid lines (Ferziger & Peric 1996). Because the grid conforms to the surface of the body, the transformed equations can then be discretized in the

Large-eddy simulation of turbulent combustion

Abstract: 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.

GLOBAL INSTABILITIES IN SPATIALLY DEVELOPING FLOWS: Non-Normality and Nonlinearity

Abstract: 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 regime. 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.

Microcirculation and hemorheology

Abstract: Major experimental and theoretical studies on microcirculation and hemorheology are reviewed with the focus on mechanics of blood flow and the vascular wall. Flow of the blood formed elements (red blood cells (RBCs), white blood cells or leukocytes (WBCs) and platelets) in individual arterioles, capillaries and venules, and in microvascular networks is discussed. Mechanical and rheological properties of the formed elements and their interactions with the vascular wall are reviewed. Short-term and long-term regulation of the microvasculature is discussed; the modes of regulation include metabolic, myogenic and shear-stress-dependent mechanisms as well as vascular adaptation such as angiogenesis and vascular remodeling.

Dissecting insect flight

Abstract: ▪ Abstract “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 (10–104). 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.

Principles of microfluidic actuation by modulation of surface stresses

Abstract: 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

Feedback control of combustion oscillations

Abstract: ▪ Abstract 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.

Multiscale flow simulations using particles

Abstract: ▪ Abstract 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.

The dynamical systems approach to lagrangian transport in oceanic flows

Abstract: ▪ Abstract 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 (ie, 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 (a

Modeling fluid flow in oil reservoirs

Abstract: ▪ Abstract 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.

The physics of tropical cyclone motion

Abstract: ▪ Abstract This article reviews our current understanding of the physical mechanisms governing the movement of a tropical cyclone. In a barotropic framework, a tropical cyclone is basically “steered” by the surrounding flow but its movement is modified by the Coriolis force (referred to as the beta effect) and the horizontal vorticity gradient of the surrounding flow. In the presence of vertical wind shear and latent heat release, a tropical cyclone tends to move toward an area with a maximum in the time tendency of potential vorticity, which is mainly contributed by two processes: (a) advection that depends on the structures of the vortex and the environment surrounding the vortex in terms of their flow speed and vorticity gradient (including the beta effect), and (b) heating that results from a coupling between the latent heat released in the clouds and the vertical wind shear.

Gravity-driven bubbly flows

Abstract: ▪ Abstract 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...

Bladerow interactions, transition, and high-lift aerofoils in low-pressure turbines

Abstract: ▪ Abstract The flow in turbomachines is unsteady due to the relative motion of the rows of blades. In the low-pressure turbine, the wakes from the upstream bladerows provide the dominant source of unsteadiness. Because much of the blade-surface boundary-layer flow is laminar, one of the most important consequences of this unsteadiness is the interaction of the wakes with the suction-side boundary layer of a downstream blade. This is important because the blade suction–side boundary layers are responsible for most of the loss of efficiency and because the combined effects of random (wake turbulence) and periodic disturbances (wake velocity defect and pressure fields) cause the otherwise laminar boundary layer to undergo transition and eventually become turbulent. This article summarizes the process of wake-induced boundary-layer transition in low-pressure turbines and the loss generation processes that result. Particular emphasis is placed on how the effects of wakes may be exploited to control loss genera...

George gabriel stokes on water wave theory

Abstract: ▪ Abstract 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 1842–50, 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.

Robert T. Jones, One of a Kind

Abstract: His contemporaries saw R.T. Jones as one of the notably creative aerodynamicists of the twentieth century. This essay reviews his remarkable life and career, including his years as a farm-country boy, college dropout, and fledgling airplane designer in Missouri, his time as an elevator operator and self-directed student in Washington, D.C., and his long professional career as an aerodynamicist at the Langley and Ames Aeronautical Laboratories and Stanford University. The focus in his career is on his fundamental discovery of the benefits of sweepback for the wings of high-speed airplanes. This includes speculation about his highly intuitive thought processes in arriving at his creative ideas. I also give an account of his work on blood flow and the mechanical heart, his avocational accomplishments as a maker of telescopes and violins, and his philosophical interest in human affairs.