Showing papers on "Fluid dynamics published in 1982"

An Album of Fluid Motion

Abstract: Keywords: visualisation de l'ecoulement ; tourbillon ; dynamique des : fluides ; aubes ; cylindre ; instabilite ; ecoulement : secondaire Note: moult photos Reference Record created on 2005-11-18, modified on 2016-08-08

Flames as gasdynamic discontinuities

Abstract: Early treatments of flames as gasdynamic discontinuities in a fluid flow are based on several hypotheses and/or on phenomenological assumptions. The simplest and earliest of such analyses, by Landau and by Darrieus prescribed the flame speed to be constant. Thus, in their analysis they ignored the structure of the flame, i.e. the details of chemical reactions, and transport processes. Employing this model to study the stability of a plane flame, they concluded that plane flames are unconditionally unstable. Yet plane flames are observed in the laboratory. To overcome this difficulty, others have attempted to improve on this model, generally through phenomenological assumptions to replace the assumption of constant velocity. In the present work we take flame structure into account and derive an equation for the propagation of the discontinuity surface for arbitrary flame shapes in general fluid flows. The structure of the flame is considered to consist of a boundary layer in which the chemical reactions occur, located inside another boundary layer in which transport processes dominate. We employ the method of matched asymptotic expansions to obtain an equation for the evolution of the shape and location of the flame front. Matching the boundary-layer solutions to the outer gasdynamic flow, we derive the appropriate jump conditions across the front. We also derive an equation for the vorticity produced in the flame, and briefly discuss the stability of a plane flame, obtaining corrections to the formula of Landau and Darrieus.

Turbulent Jets and Plumes

Abstract: Turbulent jets are fluid flows produced by a pressure drop through an orifice. Their mechanics, although studied for over fifty years, has recently received research attention that has resulted in a much improved understanding of the process by which they entrain surrounding fluid. Turbulent plumes are fluid motions whose primary source of kinetic energy and momentum flux is body forces derived from density inhomogeneities. Plumes have not been studied in the same detail as jets but nevertheless there have been some recent gains in the understanding of their mechanics. In this article we will review this progress, especially in relation to how jets and plumes interact with environmental factors, such as density stratification or uniform motion of the ambient fluid. As will become evident, many problems remain and, in some circumstances, we simply cannot describe precisely what does occur. In such cases we will try to provide current references and suggest approaches for future research.

Nonlocal stability analysis of the MHD Kelvin‐Helmholtz instability in a compressible plasma

Abstract: A general stability analysis is given of the Kevin-Helmholtz instability, for the case of sheared MHD flow of finite thickness in a compressible plasma which allows for the arbitrary orientation of the magnetic field, velocity flow, and wave vector in the plane perpendicular to the velocity gradient. The stability problem is reduced to the solution of a single second-order differential equation including a gravitational term to represent the coupling between the Kelvin-Helmholtz mode and the interchange mode. Compressibility and a magnetic field component parallel to the flow are found to be stabilizing effects, with destabilization of only the fast magnetosonic mode in the transverse case, and the presence of both Alfven and slow magnetosonic components in the parallel case. Analysis results are used in a discussion of the stability of sheared plasma flow at the magnetopause boundary and in the solar wind.

Low-Gravity Fluid Flows

Abstract: The behavior of fluids in micro-gravity conditions is examined, with particular regard to applications in the growth of single crystals. The effects of gravity on fluid behavior are reviewed, and the advent of Shuttle flights are noted to offer extended time for experimentation and processing in a null-gravity environment, with accelerations resulting solely from maneuvering rockets. Buoyancy driven flows are considered for the cases stable-, unstable-, and mixed-mode convection. Further discussion is presented on g-jitter, surface-tension gradient, thermoacoustic, and phase-change convection. All the flows are present in both gravity and null gravity conditions, although the effects of buoyancy and g-jitter convection usually overshadow the other effects while in a gravity field. Further work is recommended on critical-state and sedimentation processes in microgravity conditions.

A Review of Added Mass and Fluid Inertial Forces.

Abstract: This report reviews the existing state of knowledge concerning the evaluation of the forces imposed on a body in a fluid due to acceleration of either the body or the fluid. It concentrates on those fluid inertial forces due to acceleration rather than on the drag/lift forces due to steady motion. The first part of the report presents a survey of the analytical background including the definition of added mass, the structure of the added mass matrix and other effects such as the influence of viscosity, fluid compressibility and the proximity of solid and free surface boundaries. Then the existing data base from experiments and potential flow calculations is reviewed. Approximate empirical methods for bodies of complex geometry are explored in a preliminary way. The possible dramatic effects of the proximity of the ocean bottom are further highlighted. The confused state of affairs regarding the possibly major effects of viscosity in certain regimes of frequency and Reynolds number is discussed. Finally a number of recommendations stemming from ocean engineering problems are put forward.

Geophysical fluid dynamics

Abstract: Preliminaries * Fundamentals * Inviscid Shallow-Water Theory * Friction and Viscous Flow * Homogeneous Models of the Wind-Driven Oceanic Circulation * Quasigeostrophic Motion of a Stratified Fluid on a Sphere * Instability Theory * Ageostrophic Motion

On fluid reserves and the production of superheated steam from fractured, vapor‐dominated geothermal reservoirs

Abstract: Vapor-dominated geothermal reservoirs produce saturated or superheated steam, and vertical pressure gradients are close to vapor static. These observations have been generally accepted as providing conclusive evidence that the liquid saturation must be rather small (<50%) in order that liquid may be nearly immobile. This conclusion ignores the crucial role of conductive heat transfer mechanisms in fractured reservoirs for vaporizing liquid flowing under two-phase conditions. We have developed a multiple interacting continuum method (MINC) for numerically simulating two-phase flow of a homogeneous fluid in a fractured porous medium. Application of this method to reservoir conditions representative of The Geysers, California, and results from an analytical approximation show that, for matrix permeability less than a critical value (≈2.5 to 5 microdarcies), the mass flux of water from the matrix to the fractures will be continuously vaporized by heat transported due to conduction. This gives rise to production of superheated steam even when the matrix has nearly full liquid saturation. Simple estimates also show that heat-driven steam/water counterflow can maintain a nearly vapor static vertical pressure profile in the presence of mobile liquid water in a reservoir with low vertical matrix permeability. The implication of these findings is that the fluid reserves of vapor-dominated geothermal reservoirs may be larger by a factor of about 2 than has generally been believed in the past.

Characterization of drained and undrained response of thermally loaded repository rocks

Abstract: The fluid pressure and mechanical response of a potential repository rock to heating is shown to be characterized by the isothermal parameters of the classic stress-strain theory for a porous medium, in combination with some nonisothermal parameters describing the fluid, solid, and pore volume expansivities. The isothermal coefficients are described in terms of easily interpretable parameters by noting that the fluid response can be formulated within the limits of drained and undrained behavior. The low permeability-high thermal conductivity environment generally considered to be ideal for nuclear waste storage would appear to favor an undrained response, at least within the isolated pores and cracks of a fractured rock medium. Several cases are presented that provide a qualitatively correct demonstration of the effects of heating in this environment. These include fluid pressure increases in excess of temperature-induced increases in in situ stress, elastic strain and the potential for inelastic crack propagation, and porosity-permeability augmentation. If the rocks are dry, or of a high permeability such that fluid flow takes place at constant fluid pressure, similar rock material alterations are possible. This follows from the fact that when the temperature is raised to some high value, say 80° or 90°C, and then decreased to its ambient value, the final volume of a polycrystalline substance will generally be greater than the initial one. Hence the effect of temperature is irreversible because of the differential thermal expansion of the composite mineral grains and the generation of new grain boundaries. The increase in porosity during such a heating episode is calculable and empirically related to increases in permeability.

Sand Stresses Around a Wellbore

Abstract: The authors have studied theoretically the stresses in a poorly consolidated sand around a cylindrical well, assuming axial symmetry. Applying theories of elasticity and plasticity on this three-dimensional (3D) model, analytical solutions for all three stress components have been worked out. The existence of a plastic zone around an uncased wellbore is confirmed, and the size of the zone is determined. When allowing an incompressible fluid to flow radially into the wellbore, a stability criterion describing the failure of the sand is found to exist. This criterion relates fluid flow forces to rock strength properties. Consideration also has been given to the stress distribution around a cased hole. It is shown that a decrease in the size of the plastic zone relative to an uncased hole occurs.

Source mechanism of volcanic tremor

Abstract: Low-frequency (<10 Hz) volcanic earthquakes originate at a wide range of depths and occur before, during, and after magmatic eruptions. The characteristics of these earthquakes suggest that they are not typical tectonic events. Physically analogous processes occur in hydraulic fracturing of rock formations, low-frequency icequakes in temperate glaciers, and autoresonance in hydroelectric power stations. We propose that unsteady fluid flow in volcanic conduits is the common source mechanism of low-frequency volcanic earthquakes (tremor). The fluid dynamic source mechanism explains low-frequency earthquakes of arbitrary duration, magnitude, and depth of origin, as unsteady flow is independent of physical properties of the fluid and conduit. Fluid transients occur in both low-viscosity gases and high-viscosity liquids. A fluid transient analysis can be formulated as generally as is warranted by knowledge of the composition and physical properties of the fluid, material properties, geometry and roughness of the conduit, and boundary conditions. To demonstrate the analytical potential of the fluid dynamic theory, we consider a single-phase fluid, a melt of Mount Hood andesite at 1250°C, in which significant pressure and velocity variations occur only in the longitudinal direction. Further simplification of the conservation of mass and momentum equations presents an eigenvalue problem that is solved to determine the natural frequencies and associated damping of flow and pressure oscillations. For a simple, constant pressure reservoir-conduit orifice fluid system, a change in orifice size can cause the source signal duration due to a single disturbance to change from several seconds to in excess of an hour. Fluid kinematic viscosity (0.412 m2/s) is both included and neglected in the analysis to illustrate its effect upon system damping. Tremor magnitude is related to the magnitude of the conduit wall motion occurring in response to oscillating fluid pressures. The entire volcanic fluid system does not generally experience a transient flow condition at a given time. Closed end and constant pressure fluid system boundaries reflect pressure waves and isolate components of the fluid system. Total fluid movement may not be accurately monitored by tremor data alone, since steady state and slowly changing flows are aseismic.

Schrödinger fluid dynamics of many‐electron systems in a time‐dependent density‐functional framework

Abstract: For an N‐electron system, a connection is explored between density‐functional theory and quantum fluid dynamics, through a dynamical extension of the former. First, we prove the Hohenberg–Kohn theorem for a time‐dependent harmonic perturbation under conditions which guarantee the existence of the corresponding steady (or quasiperiodic) states of the system. The corresponding one‐particle time‐dependent Schrodinger equation is then variationally derived starting from a fluid‐dynamical Lagrangian density. The subsequent fluid‐dynamical interpretation preserves the ’’particle’’ description of the system in the sense that the N‐electron fluid has N components each of which is an independent‐particle Schrodinger fluid characterized by a density function ρj and an irrotational velocity field uj, j = 1,⋅⋅⋅,N. However, the mean velocity u of the fluid is not irrotational, in general. The force densities and the stress tensor occurring in the Navier–Stokes equation are physically interpreted. The present work is another step towards the interpretation of physicochemical phenomena in three‐dimensional space.

An experimental study of small-amplitude drop oscillations in immiscible liquid systems

Abstract: Measurements of the characteristics of small-amplitude shape oscillations of drops immersed in a host liquid have been carried out by acoustical means. The resonance frequencies of the first few modes have been measured, as well as the damping constant for the fundamental mode, as functions of the drop radius and viscosities of the two liquids. A qualitative photographic study during steady oscillations has revealed a simple internal fluid-particle flow field with no circulation. The theory available at the present time has been found to provide results which are in general agreement with experimental findings for low-viscosity liquids.

A Theoretical Model for Fluid-Elastic Instability in Heat Exchanger Tube Bundles

Abstract: A simple theoretical model has been developed from first principles for the fluidelastic instability in heat exchanger tube bundles. A series of experiments were conducted to verify the basic assumption that only a single tube need be modeled in a flow channel which preserves the basic geometry of the array. The mechanism of instability is found to be one of flow redistribution due to tube motion and a phase lag resulting from fluid inertia. Quite good agreement is found with available experimental data for a parallel triangular array without the need for empirical fluid force coefficients. The model includes the effects of tube array pattern and pitch.

Low-Reynolds-number flow between converging spheres

Abstract: Two spheres of different radii are approaching each other with equal and opposite velocities, the fluid flow around them being at low Reynolds number. The forces on the spheres can be calculated when they are very close by applying an asymptotic analysis — usually called lubrication theory — to the flow in the gap between the spheres. If the non-dimensional gap width is e, the force is calculated here correct to O(e In e) for all ratios of the two spheres' radii. The analysis can be combined with earlier numerical calculations to find all the constants in the asymptotic expansion correct to O(e).

Attenuation in partially saturated rocks

Abstract: Based on limited experimental data, the attenuation of acoustic waves in partially saturated rocks has been suspected to result from the effect of fluid flow. Global flow is accompanied by a viscous energy loss, which results in the attenuation of acoustic waves. In this theoretical study, we explore the effects of global flow and gas saturation level on acoustic wave attenuation. The attenuation predicted by theory is much smaller than that actually supported by experimental data for sandstones at low frequencies (≦1000 Hz). The technique involves the use of Biot's equations of two interacting continua that have been modified to accommodate confined gas and liquid by introducing effective and average values of fluid compressibility, fluid mass, and viscosity.

Lavage system with variable frequency, flow rate and pressure

Abstract: A mechanism, such as a motor driving a pump, creates a pulsating fluid flow. There is a control for varying the pulsation frequency independently of the fluid pressure or rate of flow. Another control varies the pressure or rate of fluid flow independently of the frequency of pulsation. Separate pulsatile lavage, irrigation, and aspiration functions are provided. In another aspect, the mechanism for producing the pulsating flow includes a fluid driving means, such as a reciprocating pump, and there is a means for varying the length of stroke of the driving means.

Magnetic levitation of liquid metals

Abstract: The process of levitation melting of metals is examined analytically and numerically for the case of axisymmetric toroidal high-frequency currents. The governing equations for the mean-velocity field and associated free-surface shape are derived under the assumption of low magnetic Reynolds number and the neglect of thermal effects. The form of the solution for high Reynolds number is discussed in general, and particularized to the case of high surface tension, in which limit a perturbation analysis about a spherical shape is presented. Finite-difference techniques are used to solve the Navier–Stokes equations in the sphere, and the surface perturbation is calculated. The asymptotic behaviour of the potential vorticity is illustrated by the numerical experiments.

Pulse decay permeability: analytical solution and experimental test

Abstract: Because of its usefulness in measuring very low permeability, the pulse decay technique has been discussed often in the literature. In this technique, a small pore pressure pulse is applied to one end of a jacketed sample, and the pressure vs. time behavior is observed as the pore fluid moves through the sample from one reservoir to another. Brace et al. gave an approximate solution to this problem with the assumption of a linear pressure gradient at all times. This simplification leads to a predicted exponential pressure vs. time decay. By means of numerical solutions, Lin and Yamada and Jones have shown that the Brace solution can lead to significant errors in calculating permeability. These numerical solutions, however, are inconvenient to use and require considerable computer programming time. The authors present an analytical solution based on realistic assumptions and boundary conditions.

A Dimensionless Parameter Approach to the Thermal Behavior of an Aquifer Thermal Energy Storage System

Abstract: To predict the performance of an aquifer thermal energy storage system, an understanding of the system's hydrothermal behavior is needed. One possibility is to run a detailed numerical simulation of the system. However, for a single-well system in which fluid flow is limited to steady radial flow, a characterization scheme based on a set of four dimensionless parameter groups allows production temperatures and energy recovery factors to be read from graphs. The assumption of radial fluid flow is valid when buoyancy flow can be neglected and a well is fully screened in a horizontal aquifer which is confined above and below by impermeable layers. Criteria for little buoyancy flow include a low permeability or vertically stratified aquifer, a small temperature difference between injected and ambient water, and short cycle length. The basic energy transport equations for the aquifer-confining layer system with steady radial fluid flow in the aquifer are nondimensionalized to derive the key parameter groups. Next a numerical model which calculates the heat transfer in the aquifer and confining layers for an injection-storage-production cycle is run for a range of values of these groups. The calculated production temperatures and energy recovery factors are then presented graphically as a function of the parameter groups. Comparisons between results of field experiments and recovery factors read from the graphs show good agreement.