Showing papers in "Annual Review of Fluid Mechanics in 1987"

Viscous fingering in porous media

Abstract: Mecanisme de digitation visqueuse. Ecoulements de Hele Shaw. Deplacements non miscibles en cellules de Hele Shaw

A Description of Eddying Motions and Flow Patterns Using Critical-Point Concepts

Abstract: On montre l'utilite des concepts de point critique dans la comprehension des regimes d'ecoulement. Resume de la methode analytique et exemples d'application

Cavitation Bubbles Near Boundaries

Abstract: Revue de resultats experimentaux et theoriques concernant l'effondrement de bulles de cavitation pres de frontieres rigides, de surfaces libres, d'interfaces entre deux fluides de densites differentes et de materiaux composites

Turbulent secondary flows

Abstract: Revue des connaissances concernant les couches limites tridimensionnelles et autres couches de cisaillement ainsi que les ecoulements a tourbillons dans les turbomachines

Theory of Solute Transport by Groundwater

Abstract: The transport of solutes in groundwater flow has been studied with increas­ ing intensity in the last two decades as a result of growing concern about water quality and pollution. The intensification of groundwater exploi­ tation, on the one hand, and the increase in solute concentration in aquifers due to saltwater intrusion, leaking repositories, use of fertilizers, etc., on the other, have made this a subject of immediate and wide interest. The phenomenon of solute transport is quite complex, as it depends on several factors, such as the complicated geohydrological structures of aquifers, the nonuniformity and unsteadiness of flow, the physico-chemical interactions between solutes and matrix, and the mechanism of solute spreading. The field study of solute transport also faces serious difficulties. First, measurements must be carried out by drilling numerous observation wells and by monitoring the concentration, which is quite costly and time con­ suming. Second, the spreading of the solutes is a very slow process, and an experiment may last many years if one wishes to investigate the long­ range transport process. Under these circumstances, the theory plays an important role, being instrumental in interpreting field tests and in predicting the fate of solutes under new conditions. The aims of the theory are to identify the main factors that influence transport and to provide the mathematical tools that permit one to compute the spatial distribution and the time evolution of the solute concentration, given the flow conditions.

Dynamics of tornadic thunderstorms

Abstract: Tornadic thunderstorms are the most intense and most damaging type of convective storm. Whereas ordinary convective cells grow, produce rain, and then decay over a period of 40 rain to an hour, certain thunderstorms may develop into a nearly steady-state structure that persists for several hours, producing heavy rain, large hail, damaging surface winds, and tornadoes. Although tornadoes may arise in a variety of storm conditions, these long-lived storms produce tornadoes most frequently and generate virtually all of the most damaging ones. Prominent features of these tornadic thunderstorms, which are particularly common i the Great Plains and midwestern regions of the United States, are illustrated in an idealized schematic in Figure 1. Our understanding of tornadic storms has evolved gradually over the years as technological advances have been made in observing systems and computer models. The first indications that certain storms exhibit a special behavior came from early studies of data from upper-air soundings and surface stations. While most thunderstorms moved with the mean winds over the lower and middle troposphere (Byers & Braham 1949), certain large storms were found to propagate consistently to the right of the mean winds (Byers 1942, Newton & Katz 1958). Observing the shift in the wind direction as storms passed by surface stations, Byers (1942) and later Brooks (1949) surmised that these intense storms have cyclonic ci rculation (counterclockwise rotation about a vertical axis, viewed from above). Using data from scanning radar and ground observations, case studies of a severe thunderstorm near Wokingham, England (Browning Ludlam 1962), and another near Geary, Oklahoma (Browning & Donaldson 1963), revealed numerous distinctive features that were strikingly

Turbulent premixed flames

Abstract: Apres un rappel des equations fondamentales, on etudie la structure des flammes du titre pour differents regimes de combustion

Isolated Eddy Models in Geophysics

Abstract: Geophysical fluid flows often appear to be dominated by a strong, but localized, vortical structure that lasts for many circulation times even when relatively turbulent flows are impinging upon it. A prime example is Jupiter’s Great Red Spot, still swirling strongly some 300 years after our first observations. The Voyager photographs offer a vivid impression of the robustness of the Red Spot despite the shearing flows surrounding it and the small vortices impinging upon it. In the Earth’s ocean, we find similar long-lived structures---e.g. Gulf Stream rings--that can progress over thousands of kilometers. Satellite imagery shows that the rings exist in a highly variable eddy field, yet they retain their distinctive chemical and biological water characteristics over hundreds of revolutions. In the atmosphere, too, synoptic meteorologists often tend to characterize a "blocking" event as an isolated phenomenon. In these situations, a strong, nearly stationary disturbance diverts and weakens, or "blocks," the normal westerly flows and alters the storm tracks, often for several weeks. (Unfortunately, this phenomenon, in the fluid medium we are able to observe best, is the least clearly isolated!) Other synoptic-scale atmospheric vortices, such as hurricanes, are likewise localized but, because of the extremely active role of the thermodynamic proces,~es, may require quite different explanations. From such observations, we can extract a set of dynamical properties characterizing these flows :

Upstream Blocking and Airflow Over Mountains

Abstract: )obstacle of height h, then naive energy arguments (and common sense) indicate that if Nh/U is sufficiently large, fluid near the ground would be blocked on the upstream side and not flow over the obstacle. Casual observations and "folklore" have long indicated that this phenomenon is common near mountain ranges in the atmosphere. However, the nature and mechanics of how it occurs have only recently become clear. It is now known that upstream blocking in large-Reynolds­ number flows propagates as a wave phenomenon, generated by nonlinear effects over the topography. These waves may be linear or nonlinear depending on circumstances, and they propagate primarily as "columnar" motions, meaning that they permanently alter the density and horizontal velocity profiles as they pass through the fluid ahead of the obstacle. Blocking occurs when these changes reach sufficient amplitude. Since they alter the upstream conditions, the understanding of these upstream dis­ turbances caused by the obstacle is a prerequisite for calculating the steady­ state flow over an obstacle, regardless of the other details of the flow. These effects generally depend on the topography being approximately two-dimensional (2D) with sufficiently large height. They are common in geophysical situations such as fjords, estuaries, and in the atmosphere. Since blocking is primarily a two-dimensional stratified phenomenon, in this review we exclude the effects of rotation and are concerned with

Confined Vortices in Flow Machinery

Abstract: Local recirculation or extensive regions of reversed flow, periodic fluc­ tuations in pressure and velocity, and high levels of kinetic-energy dissi­ pation are the principal characteristics of (sufficiently intense) confined vortex flows. The present article represents a necessarily descriptive rather than analytical discussion of swirling-flow behavior in various practical devices where these effects may be either the desired result of design or unavoidable, possibly unforeseen, side effects. One of the most widespread technical applications of swirl is for flame stabilization in furnaces and combustion chambers, whereby fuel and air are mixed in a zone of recir­ culating flow leading to a stable, compact flame. In this instance, discrete­ frequency noise and vibration may be undesirable side effects. Vibration due to excess swirl also represents a problem in the draft tubes of water turbines, and the features of vortex flows can also adversely affect the performance of axial-radial outlet casings of axial turbines and com­ pressors. The basic characteristics of fluidic vortex valves, which range in size from miniature fluidic devices to flood-dam control valves, are a consequence of the dissipative nature of vortex flows. Examples of other devices where the vortex character of the flow plays a central role include dust cyclones, hydrocyclones, and the Ranque-Hilsch refrigeration tube. As a preliminary, we review some basic aspects of confined vortex flows-in particular, the concepts of supercritical and subcritical flow and vortex breakdown.

Recent Developments in Rapid-Distortion Theory

Abstract: Utilisation de la theorie de distorsion rapide pour le calcul de la reponse de la turbulence a des deformations appliquees

Computation of flows with shocks

Abstract: Revue des techniques utilisees pour calcul des ecoulements uni- et bi-dimensionnels avec chocs

Rarefaction waves in liquid and gas-liquid media

Abstract: The thermodynamics of a substance in the critical state has been exten­ sively studied, and a strong dependence of the thermodynamic parameters on the temperature and pressure near the critical point has been revealed. However, processes such as heat exchange and finite-amplitude waves (shock waves) have not been as thoroughly investigated in the region of the critical point. In fact, the question of the dynamics of finite-amplitude perturbations of pressure, density, and temperature in the critical region has not been considered at all. One should mention just one study devoted to shock waves near the critical point, namely that by Zel'dovich (1946), who analyzed the entropy condition for shock-wave stability and theo­ retically the possible existence of rarefaction shock waves. The effect of the sign of (iJ2p/iJV2)s on the structure of compression and rarefaction waves was discussed by Thompson & Lambrakis (1973).