Showing papers in "Annual Review of Fluid Mechanics in 1991"

Particle-Imaging Techniques for Experimental Fluid Mechanics

Abstract: An important achievement of modern experimental fluid mechanics is the invention and development of techniques for the measurement of whole, instantaneous fields of scalars and vectors. These techniques include tomographic interferometry (Hesselink 1988) and planar laser-induced fluorescence for scalars (Hassa et al 1987), and nuclear-magnetic-resonance imaging (Lee et al 1987), planar laser-induced fluorescence, laser-speckle velocimetry, particle-tracking velocimetry, molecular-tracking velocimetry (Miles et al 1989), and particle-image velocimetry for velocity fields. Reviews of these methods can be found in articles by Lauterborn & Vogel (1984), Adrian (1986a), Hesselink (1988), and Dudderar et al (1988), in books written by Merzkirch (1987) and edited by Chiang & Reid (1988) and Gad-el-Hak (1989).

Coherent Motions in the Turbulent Boundary Layer

Abstract: The role of coherent structures in the production and dissipation of turbulence in a boundary layer is characterized, summarizing the results of recent investigations. Coherent motion is defined as a three-dimensional region of flow where at least one fundamental variable exhibits significant correlation with itself or with another variable over a space or time range significantly larger than the smallest local scales of the flow. Sections are then devoted to flow-visualization experiments, statistical analyses, numerical simulation techniques, the history of coherent-structure studies, vortices and vortical structures, conceptual models, and predictive models. Diagrams and graphs are provided.

Fractals and multifractals in fluid turbulence

Abstract: The theory of . . . lines and surfaces, for example, has long been recognized as an important branch of geometry, but in treatises on motion it was regarded as lying as much outside of the subject as the four rules of arithmetic or the binomial theorem . . . . geometry itself is part of the science of motion, and [thatl it treats, not of the relations between figures already existing in space, but of the process by which these figures are generated by the motion . . . . This method of regarding geometrical figures seems to imply that the idea of motion underlies the idea of form . . . .

Turbulent mixing in stratified fluids

Abstract: ion layer in the oceanic benthic boundary layer against the stratification of the lutocline, and the growth of the planetary boundary layer (PBL) due to the turbulent mixing at the tropopause. Such mixing processes are of importance in studying and controlling various biological activities and the dispersion of pollutants in the environment. Pertinent engineering situations include mixing at the thermocline due to the dis­ charge of buoyant jets from power plants [e.g. ocean thermal-energy con­ version (OTEC)]; water-qua lity control in stratified, multi port reservoirs; control of mixing during the operation of solar ponds; and mixing of methane gas in mine shafts. Mixing mechanisms in stratified fluids depend on the nature of the external forcing and the background

The Theory of Hurricanes

Abstract: The hurricane remains one of the outstanding enigmas of fluid dynamics. This is so, in part, because the phenomenon is comparatively difficult to observe and because no laboratory analogue has been discovered. To this it must be added that hurricanes have received surprisingly little attention from the theoretically inclined fluid dynamicist, perhaps owing to an understandable tendency to avoid problems that involve complex thermodynamics and lack laboratory analogues. Yet hurricanes involve a rich spectrum of fluid-dynamical processes, including rotating, stratified flow dynamics, boundary layers, convection, and air-sea interaction; as such, they provide a wealth of interesting and consequential research problems. This article reviews recent developments in the theory of hurricanes and delineates the important remaining scientific challenges.

Numerical simulation of transition in wall-bounded shear flows

Abstract: The current status of numerical simulation techniques for the transition to turbulence in incompressible channel and boundary-layer flows is surveyed, and typical results are presented graphically. The focus is on direct numerical simulations based on the full nonlinear time-dependent Navier-Stokes equations without empirical closure assumptions for prescribed initial and boundary conditions. Topics addressed include the vibrating ribbon problem, space and time discretization, initial and boundary conditions, alternative methods based on the triple-deck approximation, two-dimensional channel and boundary-layer flows, three-dimensional boundary layers, wave packets and turbulent spots, compressible flows, transition control, and transition modeling.

Symmetry and Symmetry-Breaking Bifurcations in Fluid Dynamics

Abstract: The recognition that fluid-dynamical models can yield solutions with less symmetry than the governing equations is not new. Jacobi's discovery that a rotating fluid mass could have equilibrium configurations lacking rotational symmetry is a famous nineteenth-century example. In modern terminology, Jacobi's asymmetric equilibria appear through a symmetry­ breaking bifurcation from a family of symmetric equilibria as the angular momentum (the "bifurcation parameter") increases above a critical value (the "bifurcation point"). Chandrasekhar (1969) gives a brief historical account of this discovery. In this example, as in many others, the presence of symmetry breaking was discovered by solving specific model equations. In contrast to this specificity, it is widely recognized that symmetry-breaking bifurcations are of frequent occurrence in a variety of nonlinear, nonequilibrium physical settings-fluids, chemical reactions, plasmas, and biological systems, to

Exact Solutions of the Steady-State Navier-Stokes Equations

Abstract: 1. The solutions represent fundamental fluid-dynamic flows. Also, owing to the uniform validity of exact solutions, the basic phenomena described by the Navier-Stokes equations can be more closely studied. 2. The exact solutions serve as standards for checking the accuracies of the many approximate methods, whether they are numerical, asymp­ totic, or empirical. Current advances in computer technology make the complete numerical integration of the Navier-Stokes equations more feasible. However, the accuracy of the results can only be ascertained by a comparison with an exact solution.

Drag reduction in nature

Abstract: Recent studies on the drag-reducing shapes, structures, and behaviors of swimming and flying animals are reviewed, with an emphasis on potential analogs in vehicle design. Consideration is given to form drag reduction (turbulent flow, vortex generation, mass transfer, and adaptations for body-intersection regions), skin-friction drag reduction (polymers, surfactants, and bubbles as surface 'additives'), reduction of the drag due to lift, drag-reduction studies on porpoises, and drag-reducing animal behavior (e.g., leaping out of the water by porpoises). The need for further research is stressed.

Coastal-Trapped Waves and Wind-Driven Currents Over the Continental Shelf

Abstract: The 1970s saw a remarkable development in the understanding of wind­ driven current variability over the continental shelf, allowing Allen (1980) to summarize many of the keystones of our current understanding of the subject. Since 1980, our understanding of such processes has become a good deal more sophisticated, especially in terms of coastal-trapped-wave theory. In fact, the point has been reached where models and observations are now often compared with a reasonable expectation of quantitative agreement. Along the way, some exciting new physical insights have also been gained. It thus seems timely to revisit the problem of wind-driven variability over the continental shelf, and therefore the present offering is intended to be a continuation of Allen's (1980) original fine review. The following material focuses mainly on variability having time scales longer than a day but not more than a few weeks. In all cases, attempts are made to emphasize the relation between theoretical concepts and observed behavior in the coastal ocean. The approach is to address "water­ column" models in which nonadiabatic effects, while often present, are not of overwhelming importance. Within this classification, deterministic problems expressed in terms of coastal-trapped-wave theory are treated

Flow Phenomena in Chemical Vapor Deposition of Thin Films

Abstract: The deposition of thin inorganic films from precursors in the gas phase onto a solid substrate is a key element in a wide range of technological applications, including the fabrication of microelectronic circuits, optical and magnetic recording media, optical devices, and high-performance culling and grinding tools. The deposited films range in thickness from a few nanometers, as in the active layer of a quantum-well optical device, to tens of microns for wear-resistant coatings. The thin films must be pro­ duced with controllable properties (e.g. purity, composition, thickness, adhesion, crystalline structure, and surface morphology). In addition, the deposition cannot have a significant impact on the microstructures already existing in the substrate. The tolerance limits on the properties of the films vary with the application, but stringent demands are characteristic in electronic and optical materials processing. In such applications, the requirements increase with the level of integration, the decrease in device

Lagrangian ocean studies

Abstract: This review concerns the use of quasi-Lagrangian current-following floats and drifters to observe the ocean and, therefore, involves the union of fluid mechanics and oceanography. Oceanography addresses specific fluid­ dynamical processes as they occur in the world ocean, and one focus of this review is on how free-drifting instruments have led to ocean discover­ ies. The other focus is on the interpretive tools developed to utilize the unique aspects of current followers. The review is addressed to non­ specialists interested in how the ocean is studied. It is intended as a guide to the literature of ocean current followers, emphasizing observations and methods rather than theories or fundamental dynamics. Compared with experimental fluid mechanics, oceanographic methods are limited, and to appreciate Lagrangian ocean studies it is important to understand why. First, the ocean is complex. The individual phenomena are not especially difficult to understand, but there are many going on at any one time. Energetic space scales span ten decades, from I mm to 10,000 km. Time scales range upward from 0.1 s without bound, making separation of mean and variability dubious. The largest scales make up the general circulation, a central topic of oceanography, but its signatures are usually masked by variability. Second, the ocean cannot be controlled; we can observe but not experiment. The system cannot be repeatedly perturbed, nor are observations long enough that pertinent factors repeat. Thus, dynamical inference is indirect, frequently statistical in form, and, because ocean dynamics are nonlinear, ambiguous. Third, the ocean is logistically difficult. The ocean's size and thc expense of operating in it make attended observations discouragingly sparse. The substantial engi­ neering required to make ocean instruments seaworthy also makes them

Hydraulics of rotating strait and sill flow

Abstract: The role of deep-water formation and spreading as an indicator and regulator of global climatic change has recently been emphasized in a number of investigations (e.g. Watts & Hayder 1983, Boyle & Keigwin 1987, Labeyrie et al 1987, Anderson & Lundberg 1988). Within this context, processes in the Atlantic Ocean have attracted the greatest attention, since both the Norwegian and the Weddell Seas (which constitute the world’s two major areas where deep water is formed during wintertime convcction) are located on the margins of this ocean basin (Wrist 1936). The cold and dense water that fills the deeper parts of the Norwegian Sea escapes southward through two main passages: the Denmark Strait, and the Faroe Bank Channel. The deep-water outflow through the Denmark Strait (between Greenland and Iceland) over a comparatively shallow sill of depth ~ 600 m was known from the pioneering investigations at the turn of the century; it was, however, not studied in a systematic fashion until the 1960s (Mann 1969, Worthington 1969, Ross 1984). The existence of the Faroe Bank Channel, with a sill depth of 850 m, was unknown to Helland-Hansen & Nansen (1909), who stated that "no bottom water with low temperatures can anywhere get out of the Norwegian

Mechanics of Fluid-Rock Systems

Abstract: Most of the Earth's mass is solid. The crust and mantle of the Earth extend down almost 3000 km and support seismic shear waves, a standard test for solidity in the earth sciences. The inner core, with a radius of 1200 km, is believed to be solid for the same reason. It is less often noted that in large regions, this solid mass is actually a two-phase or multiphase medium, containing varying amounts of fluid (usually liquid) interspersed throughout the granular solid. Processes in fluid-rock media play an essential role in many important geological phenomena. It is percolation through such a medium (a partial melt) that permits melts to escape prior to volcanic eruptions and other igneous emplacements. Nearer the Earth's surface, the migration of aqueous or hydrocarbon fluids in the crust is of interest in metamorphic and sedimentary geology as well as in hydrology and hydrocarbon exploration. Fluid migration at all levels within the Earth can profoundly influence heat flow and material transport. The layering of the Earth, including the existence and nature of the core and the stratification of the mantle and crust, is likely to be due in part to the behavior of fluid-rock systems. There are many different fluid-rock systems in geology, and this review considers only a subset of these. The primary emphasis is on two geologic environments where the coupling between fluid flow and rock deformation is important: high-temperature systems where a melt coexists with a solid matrix that can deform by creep as the melt migrates; and low-temperature systems where a fluid, usually aqueous, saturates a matrix that has varying degrees of cohesion and fracturing. We do not consider suspensions (e.g. crystals in a convecting magma chamber or rock fragments carried in the plume of a volcanic eruption). At the opposite extreme, we exclude solidlike systems where fluid is present but little or no fluid movement occurs (e.g. fluid inclusions in crystals). Minor consideration is given to systems where the fluid moves but the solid is essentially rigid (e.g. the flow of oil, water, and gas in hydrocarbon reservoirs). We also limit ourselves to processes where inertia is negligible: Both the microscopic motion of fluid through cracks and pores and the macroscopic motion of the solid matrix are assumed to be characterized by a small Reynolds number. For example, we do not consider seismic-wave propagation in poroelastic media. Even with these restrictions, more important geodynamic phenomena are covered than can be effectively reviewed in detail. Our goal is rather to identify common ground between studies of different geological environments, and to demonstrate to practitioners of fluid mechanics the interesting range of phenomena that fluid-rock systems exhibit. We have divided the main body of this review into three sections: theory, material properties, and modeling and applications. The latter section is necessarily eclectic but encompasses studies ranging from laboratory experiments on analogue systems to computer simulations of large-scale processes in the crust and mantle.

Mechanics of Gas-Liquid Flow in Packed-Bed Contactors

Abstract: Packed beds are typically tall cylindrical vessels filled with pieces of inert solid that have been dumped or stacked to leave considerable space for fluid between them (Leva 1 95 1 ). They are used to bring flowing gas and liquid into the intimate contact needed for rapid exchange of dissolved substances and, often, for chemical reaction as well. This use began nearly two centuries ago and is common in large-scale chemical processing; there are other applications too. Flow in packed beds is justly regarded as complex, irregular, even capricious, and scaleup from laboratory and pilot­ plant models to commercial sizes is notoriously risky (Ng & Chu 1 987). Despite this, packed beds have had relatively little fluid-mechanical study (Westerterp et a1 1 984, NRC Panel Report 1 986). This article reviews what appears most significant from what has been done. The point of view we take began to emerge in the mid-1970s in the course of research into how immiscible fluids flow together in the much finer, yet no less irregular, pore space of sedimentary rocks. The key idea was H. T. Davis's-that the way to approach macroscopic two-phase flow in porous media is with the statistical physics of network processes, then blooming in applications of so-called percolation theory (Larson et al 1977, 1981, Levine et al 1977). The basic ingredient was the essence of accumulated knowledge of the fluid mechanics of microscopic behavior in the individual passages, enlargements, and junctions of the pore space (Ng