Plasmas and Fluids (1986) / Chapter Skim
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2. Fluid Physics
2. Fluid Physics
Pages 36-94
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From page 36... ...
Our need to understand the world of flow around us, encompassing the nature of transport across biological membranes to the appearance of solitary waves in planetary atmospheres, remains a constant stimulation and adventure. Fluid motion, which can exhibit the randomness of turbulent flow as well as much larger-scale coherent structures, provides one of the premier testing grounds for new developments in nonlinear dynamics.
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From page 37... ...
We have also attempted to gain a useful measure of the scope and level of effort that marks this field by a review of those agencies of the government that support fluid-physics research. However, such a review cannot be exhaustive in the sense that the definitions of fiuid-physics research tend to vary significantly with the nature and mission of the funding organization, nor are we able, in the time available, to explore private industry under whose sponsorship valuable contributions to the field have often been made.
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From page 38... ...
In conjunction with improved performance, time and costs have been reduced in the design of aircraft wings, internal combustion engines, nuclear fusion and fission devices, and surface and undersea naval vehicle components. In addition, computational fluid dynamics has increased our
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From page 39... ...
· Dimensional reasoning and recent theoretical understanding of jet noise, acoustic damping, and turbulent flows have led to a thousandfold reduction in the energy of acoustic emissions from aircraft leading to major reductions in perceived noise level near airports. · Important advances have been made in our understanding of the collective behavior of dilute particulate and aerosol suspensions.
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From page 40... ...
· As a result of the accelerating pace of physical understanding during the last decade, exciting improvements can be made in our ability to control turbulent flows and thus change their nature significantly, leading to novel drag and noise-reduction techniques; increased combustion efficiency; and control of separation, spreading, and mixing. Major advances in technology will be possible as a result of our ability to predict and control flows with turbulent zones.
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From page 41... ...
Applications range from improved fuel economy to fire safety. · We expect to see major advances in our understanding of multiphase flow systems, including macroscopic and microscopic interface phenomena, which are of interest in both industrial and geological processes, for example, the stability of the liquid-liquid interface leading to fingering in oil recovery, convective processes in the ocean, and the formation of layered structures in magma chambers.
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From page 42... ...
Employing Lagrangian mathematical techniques and instruments that move with the fluid, we anticipate new views of turbulent dispersion; of the interaction between waves, turbulence, and mean flow in boundary layers; and in ocean-atmosphere circulations. · The development of Monte Carlo computational techniques, which account for molecular motion in gas flows, will continue to be extended to higher-density flows, permitting meaningful modeling of highly nonequilibrium chemically reacting flow systems.
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From page 43... ...
INSTRUMENTATION TECHNIQUES · The past decade has spawned a remarkable growth in nonintrusive laser-based flow diagnostic techniques. Combined with equally spectacular developments in imaging, data storage, and manipulation techniques we have, during the decade, formed the beginning of what will become unprecedented advances in flow diagnostics cooperatively coupled to computational fluid dynamics.
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From page 44... ...
· We recommend a targeted research initiative to investigate and develop instrumentation for essentially simultaneous multipoint measurements of flow properties throughout large volumes. The instruments might be based on laser holographic methods, on multiprojection (tomographic)
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From page 45... ...
· Advances in numerical simulation and experimental techniques must not obscure the fundamental importance of analytical methods. These methods have been instrumental in advancing our understanding of complex flows and an aid in the development and verification of numerical methods for computing fluid flows.
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From page 46... ...
46 U' Ct o ~ EN 4 _` .0 ._ ._ Cal ._ V so Ct Cal V ._ 1 oo Cal so Ct V U
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From page 47... ...
Data from the National Bureau of Information indicate that approximately 900 Ph.D. theses are published each year in the general area of fluid dynamics.
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From page 48... ...
Branches of Fluid Physics COMBUSTION AND REACTING FLOWS Chemically reacting flows in general, and combustion in particular, are branches of fluid physics for which the underlying equations
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From page 49... ...
Fires are an ever-present threat to health and safety that necessitate continuing research on chemically reacting flows, e.g., to combat dangers associated with the rapidly changing mix of combustible materials in the modern urban environment. Importation of liquefied natural gas in large volumes has spurred research on the fire and explosion hazards of combustible clouds.
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From page 50... ...
In studies of burning of individual fuel droplets, conditions have been measured and largely understood under which a bubble is generated and grows within the liquid, shattering the droplet and in the process producing more efficient combustion with less production of soot and oxides of nitrogen. Modern diagnostic experiments, computational methods, and analytical methods together have given greatly improved knowledge of structures and propagation mechanisms of premixed laminar flames, thereby offering ideas, for example, for achieving reliable combustion under highly fuel-lean conditions in spark-ignition engines to improve performance.
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From page 51... ...
NON-NEWTONIAN FLUIDS AND RHEOLOGY For over a hundred years, it has been customary in fluid mechanics to accept the Newtonian constitutive equation, i.e., the proportionality between stress and rate of deformation, as the standard fluid model that in conjunction with the laws of mechanics leads to the well-known Navier-Stokes equations of motion. Yet it has become increasingly apparent in recent years that there exist a great many fluids whose flow behavior differs in such a striking and fundamental way from that of their Newtonian counterparts that new constitutive equations need to be developed in order to properly model such systems theoretically.
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From page 52... ...
can significantly reduce the pressure drop in pipes under turbulent flow conditions even though, in a viscometer, these solutions behave as Newtonian substances having virtually the same viscosity as the solvent. Drag reduction is therefore not only of obvious practical importance it has already been employed in the Alaska pipeline as well as in many other instances but the explanation of its origin would go a long way toward helping us to understand the
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From page 53... ...
Such an example is the "reptation" theory recently developed by deGennes and by Doi and Edwards. If successful, this approach should provide us with an in-depth understanding of the striking flow properties of non-Newtonian fluids-which are often very unlike those observed with Newtonian substances and even help us to discover new nonNewtonian phenomena or construct fluids with preassigned flow characteristics.
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From page 54... ...
The tendency of the strong vorticity shed from heavily loaded wings to roll up quickly into rather concentrated vortex tubes leads to unusual trajectories, intense induced velocity fields, and frequently to asymmetric or nonsteady flow patterns and forces. The understanding and description of these complex flow fields is one of great importance in estimating the performance of high-speed aircraft, particularly high-performance fighters.
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From page 55... ...
We are beginning to understand the fundamental physics of these separated and unsteady flows and how, often in combination, they can be used to improve the efficiency and performance of technological devices. Both steady and unsteady mechanisms can be used to generate separated flows.
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From page 56... ...
Our understanding of the physics of these unsteady separated flows is just beginning and is especially important. MOLECULAR AND STATISTICAL PHENOMENA Molecular-scale phenomena are basic to all of fluid mechanics.
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From page 57... ...
Advances here would be an important step in describing highly nonequilibrium chemically reacting flows, which are important in applications such as high-energy chem ical lasers. We expect that there will be exciting developments in the biological application of statistical phenomena.
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From page 58... ...
The chemical processes involve such problem areas as modern filter technology, aerosols and sprays, sedimentation of particles and colloidal suspensions, fluid-fluid separation processes, electrokinetic and osmotic phenomena, hydrodynamic chromatography, surfactant technology, and molecular transport through biological and synthetic membrane es. The new biological applications derived from the need to obtain better understanding of a variety of cellular level biological phenomena, such as the vesicular transport of lipoproteins across arterial endothelium (thought to be related to arterial disease)
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From page 59... ...
FLUID PHYSICS 59 past. It is now realized that there are separated flow regions between particles when the separation is sufficiently small and that this flow separation will have an important effect on the heat- and mass-transfer characteristics of a bed of particles.
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From page 60... ...
Consequently, this branch of fluid mechanics figures prominently in oceanography, meteorology, and planetary sciences. It is similarly important in
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From page 61... ...
The theoretical advances have been made possible by developments in bifurcation theory, by systematic exploitation of multiple-scale and related singular perturbation techniques, by the advent of inexpensive large-scale computation, and by the introduction of increasingly sophisticated experimental techniques. Important new physical effects have been explored, in some cases leading to significant reassessments of phenomena previously misinterpreted.
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From page 62... ...
Turbulent heat and mass transfer in the magma are probably involved in crustal movement and temperatures. Turbulent heat transfer plays a
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From page 63... ...
Several analytical approaches related to techniques used in quantum field theory were applied to the problem; these have resulted more in added insight than useful computational techniques or major theoretical breakthroughs. The advent of large-scale computing has made it possible to solve the Navier-Stokes equations exactly for some simple turbulent flows.
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From page 64... ...
Fortunately, most workers recognize that turbulent flows contain both order and disorder, and techniques are necessary that can encompass, and profit from, both. New statistical techniques are being applied to measurement, notably conditioned sampling to reduce the form of the coherent structures.
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From page 65... ...
We have little faith in breakthroughs in general, and particularly in a field as old as this one; we believe that progress is most likely to come in small increments by increased physical understanding, probably from interaction between careful machine calculations and modeling of various types compared to experiments. Much insight will probably come from investigation of the stability characteristics of turbulent flows and of models and from the inclusion of coherent structures in models.
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From page 66... ...
of these tectonic processes have led to an enormous unification of geology, with practical implications from fossil fuels to earthquakes. At smaller scales the flow and convection of fluids in the presence of crystallization is beginning to explain the structure of magma chambers, spreading centers, and some aspects of volcanism.
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From page 67... ...
Convection of course increases the overall transport rate of heat and mass and hence the growth rate; on the other hand, the morphology of the solid is usually affected adversely. INTERFACE PHENOMENA Flows with interfaces between two or more fluids occur frequently in nature and play an important role in a wide variety of physical phenomena.
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From page 68... ...
68 PLASMAS AND FLUIDS generally in time, is one of the primary goals of any theoretical or experimental investigation on the subject. The problem of determining this interface often requires novel experimental, analytical, or computational tools.
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From page 69... ...
FLUID PHYSICS 69 Spreading of Liquids on Solid Surfaces This topic is of considerable theoretical interest because it involves the motion of the contact line, i.e., the intersection between two immiscible fluids and a solid surface. Clearly, this motion, which is certainly observed experimentally, violates the universally accepted no-slip boundary condition, which must therefore be replaced by something more complicated.
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From page 70... ...
The application of these ideas accounts for sources of acoustic radiation beyond those recognized in the original acoustic analogy. RADIATION HYDRODYNAMICS One particular subfield of fluid dynamics that is both intellectually challenging and of great importance is radiation hydrodynamics, which also goes by the names radiation-coupled flows and radiation-induced flows.
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From page 71... ...
Thus the challenge has been, and continues to be, the development of accurate general-purpose numerical methods for solving the coupled set consisting of the fluid equations with radiation terms and the equation of transfer. To date, both Eulerian and Lagrangian formulations have been developed, in one- and two-dimensional generality.
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From page 72... ...
A final area of need relevant to this subfield of fluid dynamics concerns the basic data problem. Because of the high temperatures and pressures inherent in radiation hydrodynamics problems, very little relevant equation-of-state and opacity data can be obtained in the laboratory.
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From page 73... ...
Environmental problems include wind, water and sea erosion, the permeation of soil by seawater, and land reclamation through irrigation. Important biological porous media flows include the interstitial fluid flow in the articular cartilage, the drainage of intraocular fluid in the eye, blood perfusion in living tissue, and other applications that are mentioned in the section on Biofluid Dynamics.
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From page 74... ...
Classical fluid-mechanics techniques like stability theory are being applied, for example, instability theory to model the formation of dendrites in a supersaturated salt solution. The geometric aspects of these problems are often complex, but the range of valuable applications is large.
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From page 75... ...
The fluid mechanics of strongly vaporizing or condensing surfaces and the nonequilibrium statistical mechanics of the homogeneous formation of clusters of "multimers" have been studied in the last decade. Both phenomena have numerous potential applications in a variety of industrial processes and in understanding natural events.
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From page 76... ...
The condensation of water vapor into clouds requires condensation nuclei, and there is a complex of surface chemistry and particle kinetics that interacts with the fluid dynamics of the convecting cloud. Beyond the dramatic examples of these heat engines, like hurricanes, the dynamics of tropical cloud clusters is a central problem in atmospheric dynamics.
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From page 77... ...
Important recent advances include the development of efficient aerodynamic algorithms for simulating the flow about aircraft, detailed understanding of how viscous and inviscid flows interact, the aerodynamics of supercritical flows, and the utilization of shock-free flows once deemed physically unrealizable. There are manifold opportunities for major advances in high-speed flows that will extend the developments of the past decade.
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From page 79... ...
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From page 80... ...
It is clear that in some areas the Europeans are catching up with us in the application of fundamental advances in computational fluid dynamics (CFD) and turbulence.
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From page 81... ...
There is a growing body of evidence that strongly suggests that the small population of endothelial cells (inner cellular lining of blood vessels) involved in turnover (cell death and replacement)
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From page 82... ...
Prominent examples include the movement of interstitial water and solute diffusion in loaded articular cartilage, the filtration of blood plasma across the walls of the blood vessels, the drainage of interstitial fluid in the lymphatic circulation, the hydration of the cornea, and the drainage of aqueous humor from the anterior chamber of the eye. In all of these problems one is examining the consolidation of a biological porous matrix under pressure loading coupled with biochemical-physical phenomena.
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From page 83... ...
Thus, in order to understand ventilation and perfusion, fluid dynamics has to be coupled with nonlinear finite elasticity and biochemistry. There has been significant progress made in the past decade, but a detailed quantitative understanding awaits the future.
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From page 84... ...
Thus a theory or computer simulation of the weather must somehow incorporate the cumulative effect of all the smaller-scale fluid dynamics: internal waves, fronts, two- and three-dimensional turbulence, and convective clouds. Intense studies of these intermediate scales of motion are being pursued, for example, with much progress on severe storms, cloud modeling, and frontal dynamics being evident.
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From page 85... ...
A promising branch of study in this area is Lagrangian fluid dynamics, in which theory and measurements are carried out using the moving fluid particles as a reference. We are seeing rapid progress in the understanding of the oceanic general circulation, both the mechanical response to the stress exerted by the winds overhead and the thermodynamical response to heat flux and moisture flux between the air and sea.
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From page 86... ...
In horizontal pipes at low gas and liquid velocities a stratified configuration is attained whereby the liquid flows along the bottom of the pipe and the gas concurrently with it. Increases in the liquid velocity or a change of orientation to an upward inclination can give rise to a situation where the gas and liquid flow intermittently, thereby creating large pressure pulses, which in turn can cause vibrational damage.
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From page 87... ...
Here, in contrast to single-phase Hows where reliable correlations exist that do not require detailed knowledge of the turbulent flow field, in multiphase flows there are so many independent variables defining the system that dimensional analysis leads to too many dimensionless groups to be of use. Consequently, in multiphase flows one has to have a detailed model of the physics of flow in order to correlate test results in a meaningful way.
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From page 88... ...
These equations are then solved by asymptotic or analytical methods or by numerical means. This process includes the development of physically viable conceptual models based on a synthesis of available data, the generation of rationally derived governing equations and the corresponding and initial boundary conditions, and the development of solutions to quantify the physical process of interest.
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From page 89... ...
Numerical simulation must include assessments of accuracy and resolution so that physically viable solutions are discriminated from those that represent numerical artifacts. Analytically derived asymptotic solutions are not only useful as benchmarks to test numerical methods but also in providing the numerical time and length scales essential in resolving real physics.
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From page 90... ...
With the present and very near future advances in computer technology and numerical method development, we are now on the threshold of extending the Reynolds-averaged calculations to full aircraft at flight conditions. To pass over this threshold into the practical use of such calculations for aircraft design requires the solution of several topological problems in fitting a system of mesh points about a geometric shape as complex as an aircraft configuration, the development of convergence acceleration procedures to enhance the efficiency of numerical methods for solving the equations of compressible viscous flow, and the implementation of solution-adaptive grid systems.
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From page 91... ...
If this research continues at its present rate it is predicted that a fivefold increase in numerical efficiency will occur during the next 5 years and that a possible two-orders-of-magnitude speed increase is projected during the next 15 years for solving the equations of compressible viscous flow. EXPERIMENTAL METHODS Instrumentation Developments in fluid-dynamic instrumentation techniques over the past 10 years have involved combining extensive computer analysis with well-established techniques, such as conditional sampling of hot-wire probe outputs in the study of turbulence.
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From page 92... ...
Note that advances in recent years, say in the identification and study of large-scale structures in turbulent flows, have relied heavily on flow visualization methods including painfully reconstructed information in plane cuts through flows using point-by-point measurements from a few probes. Subsequently, the results are manipulated and displayed by a computer.
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From page 93... ...
The cryogenic wind tunnels for obtaining high Reynolds numbers now being brought into use are a recent example of this historical trend in facility development for aerodynamic purposes. The current efforts to develop an adaptive wall or "smart" transonic wind tunnels is an indication that significant new aerodynamic facilities will come on-line in the next decade.
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From page 94... ...
J Hanratty, University of Illinois (Multiphase Flows)
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