Showing papers in "Annual Review of Fluid Mechanics in 1989"

Colloid Transport by Interfacial Forces

Abstract: In a historical context the interface between two phases has played only a minor role in the physics of fluid dynamics. It is of course true that boundary conditions at interfaces, usually imposed as continuity of ve­ locity and stress, determine the velocity field of a given flow; however, this is a more or less passive use of the interface that allows one to ignore the structure of the transition between two phases. When an interface has been assigned a more active role in flow processes, it generally has been assumed that one parameter, the interfacial (surface) tension, accounts for all mech­ anical phenomena (Young et al. 1 959, Levich & Krylov 1969). In these studies, kinematic effects of the interface were not considered, and the "no-slip" condition on the velocity at interfaces was retained. The basic message of this article is that the interface is a region of small but finite thickness, and that dynamical processes occurring within this region lead not only to interfacial stresses but also to an apparent "slip velocity" that, on a macroscopic length scale, appears to be a violation of the no-slip condition. The existence of a slip velocity at solid/fluid interfaces opens a class of flow problems not generally recognized by the fluid-dynamics community. Three previous articles in this series deal with flow caused by interactions between interfaces and external fields such as electrical potential, tem­ perature, and solute concentration. Melcher & Taylor ( 1969) and Levich & Krylov (1969) consider fluid/fluid interfaces where stresses produced at the interface by the external field dictate the flow. Saville ( 1977), on the other hand, discusses the action of an electric field on a charged solid/fluid interface and reviews the currently accepted model for electrophoretic

Turbulent Boundary-Layer Separation

Abstract: This article summarizes our present understanding of the physical behavior of two-dimensional turbulent separated flows, which occur due to adverse pressure gradients around streamlined and bluff bodies. The physical behavior of turbulence is flow dependent, so detailed experimental infor­ mation is needed for understanding such flows and modeling their physics for calculation methods. An earlier review (Simpson 1 985) discussed in much detail prior experimental and computational work, and this was followed by an updated review of calculation methods only (Simpson 1 987). Here additional recent references are added to those cited in the two other works. By separation, we mean the entire process of departure or breakaway, or the breakdown of boundary-layer flow. An abrupt thickening of the rotational-flow region next to a wall and significant values of the normal­ to-wall velocity component must accompany breakaway, or otherwise this region would not have any significant interaction with the free-stream flow. This unwanted interaction causes a reduction in the performance of the flow device of interest (e.g. a loss of lift on an airfoil or a loss of pressure rise in a diffuser). It is too narrow a view to use vanishing surface shearing stress or flow reversal as the criterion for separation. Only in steady two-dimensional flow do these conditions usually accompany separation. In unsteady two­ dimensional flow the surface shear stress can change sign with flow reversal without the occurrence of breakaway_ Conversely, the breakdown of the boundary-layer concept can occur before any flow reversal is encountered. In three-dimensional flow the rotational layer can depart without the

Stability of three-dimensional boundary layers

Abstract: The computational modeling of the transition process characteristic of flows over swept wings is discussed. Specifically, the crossflow instability and crossflow/Tollmien-Schlichting wave interactions are analyzed through the numerical solution of the full three-dimensional Navier-Stokes equations including unsteadiness, curvature, and sweep. This approach is chosen because of the complexity of the problem and because it appears that regular stability theory is insufficient to explain the discrepancies between experiments and between theory and experiment. The leading edge region of a swept wing will be considered in a three-dimensional spatial simulation with random disturbances as the initial conditions.

Boundary-Layer Receptivity to Long-Wave Free-Stream Disturbances

Abstract: The present treatment of the early stages of boundary layer-transition phenomena, where the unsteady motion is of small amplitude and can be accordingly treated as a small perturbation of an appropriate mean flow, elaborates the Heinrich et al. (1988) discussion of the role played by this 'receptivity' stage: in which the unsteady flow exhibits the same harmonic time-dependence as the externally-imposed forcing. Freestream disturbance wavelengths are noted to often be much longer than the Tollmien-Schlichting wavelength. Attention is given to the variety of wavelength-reduction mechanisms able to couple the long-wavelength, freestream disturbances to the comparatively short Tollmien-Schlichting waves.

Rarefied Gas Dynamics

Abstract: The current state of those aspects of rarefied gas dynamics research that appear to be most important to research planned over the next decade is evaluated. These aspects encompass assessments of computational rarefied-gas dynamics (CRGD) that will allows their use as surrogates for experiments, the development of hybris-flowfield computational techniques matching continuum computations with particle computations, and the validation of CRGD through the results of experimental studies of Knudsen layers in simple flows. The design of surfaces for the achievement of stable, low-momentum and thermal accommodation coefficients will be a major priority, together with theorization and experimentation on evaporation and condensation effects close to surfaces.

Turbulence Control in Wall Flows

Abstract: Most of the detailed turbulence-structure data available pertain only to the simplest cases, involving zero pressure-gradient boundary layers and free-shear layers, and indicate that each disparate geometry possesses its own set of dominant nonlinear instabilities. Various boundary/input conditions act to modify these instabilities for low input levels; for stronger inputs, the basic instability modes/structures sustaining the turbulence field may be altered. Steady-state inputs are noted to be extremely effective in altering turbulence structures, in the directions of either amplification or diminution.

Atmospheric dispersion of dense gases

Abstract: Public concern over the risks posed by the use of hazardous materials has grown markedly over the past decade. The dioxin release in Seveso (Italy) in 1976, that of methyl isocyanate in Bhopal (India) in 1984, and the liquefied petroleum gas explosions in Mexico City in the same year emphasized the possible scale of the tragedies that may accompany activi­ ties involving hazardous materials. The development of appropriate regulatory measures to achieve an ac­ ceptable balance between economic benefit and potential harm accom­ panying such activities requires quantitative assessment of the conse­ quences of the accidental release of material into the environment. It is commonly the case that hazardous industrial materials, be they flammable or toxic, produce a cloud, upon release into the atmosphere, that is denser than the environment. The information on dense-gas dispersion that is of interest to the hazards analyst is contained in the distribution of concentration as a function of the spatial coordinates and time. Very often, this information is required only in summary form, such as

Coherent Structures in Transitional and Turbulent Free Shear Flows

Abstract: The development of a quantitative understanding of large-scale coherent structures in shear flows has led to the recognition of their evolutionary features and 'rules' of nonlinear interaction among each other. These determinations, in conjunction with the study of fine-grained turbulence and mean motion, constitute a general view of hydrodynamical instabilities that has resulted in useful concepts for the achievement of shear-flow control. The dynamical role of longitudinal structures in free shear flows is presently exemplified by the axisymmetric and helical modes in a round jet, which resemble the two-dimensional and spanwise-periodic three-dimensional modes in a two-dimensional mean flow.

New Directions in Computational Fluid Dynamics

Abstract: Computational Fluid Dynamics (CFD) deals with the solution of fluid­ dynamic equations on digital computers and the related use of digital computers in fluid-dynamic research. It is used for basic studies of fluid dynamics, for engineering design of complex flow configurations, for understanding and predicting the interactions of chemistry with fluid flow for combustion and propulsion, for basic and applied research into the nature and properties of turbulence, for interpreting and analyzing experi­ mental data, and for extrapolation into parameter regimes that are rela­ tively inaccessible or very costly to study experimentally. Recent successes in computational fluid dynamics are based on inno­ vative numerical algorithms that discretize the continuum equations offluid dynamics into a large but finite number of algebraic or ordinary differential equations. These equations are then solved using currently available, large­ memory, high-speed digital computers. A number of books, conference proceedings, and review articles have recently been written on this subject. Among them, CFD in aerodynamics is considered in National Academy of Sciences/National Research Council ( 1986), reactive flow in Oran & Boris (1987), fluid mechanics and heat transfer in Anderson et al. (1984), strongly shocked flows in Woodward & Colella (1984), and spectral methods for subsonic flows in Canuto et al. ( 1988). Books treating opti-