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Fluid Physics INTRODUCTION AND OVERVIEW Study of the physics of fluid motions or, in the present context, fluid physics, is among the oldest branches of the physical sciences.* Despite this seniority, it continues to fascinate its practitioners with an eclectic collection of elegant problems. 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. We are all affected by wavelike fluid-mechanic teleconnections that transmit information about the Earth's tropical oceans over vast distances to alter patterns of global atmospheric circulation. Swimming creatures, governed by the laws of efficient underwater travel, provide insights into the evolutionary pathways stimulated by changing envi- ronments or vacant biological niches. This review of fluid physics is restricted to areas where fluid motions are of dominant importance and have only included developments in the understanding of the properties and statistical mechanics of liquids and gases that are directly related to fluids in motion. 36

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FLUID PHYSICS 37 In common with many other branches of physics, fluid physics also finds a driving force in the existence of important problems in engi- neering. The pacing element for advances in many applications such as the efficiency of flight, the effectiveness of heat engines, and the productivity of chemical-processing systems is our understanding of the fundamentals of fluid motion. There are striking examples in the machines of engineering as they exist today, compared with history, that measure the magnitude of advances in our understanding of fluid physics. As it is beyond the scope of this report to catalog all of these advances, only a few will be mentioned as examples. The modern transport plane, with swept wings and quiet engines, is a reflection of the progress in the last few decades of our understanding of high-speed flows. These configurations have been derived by a combination of originally empirical and more recently theoretical and conceptual constructs, made possible by advances in our understand- ing of the physics of flow. The gas turbine engine of today, although superficially similar to its historical counterpart, includes major im- provements made possible by extensive efforts in fluid physics. Our increased knowledge of combustion and heat transfer, which were bought with so much difficulty through research, have led to lower exhaust pollution and longer life of the critical engine components. Many of today's chemical engineering plants have a throughput and an efficiency increased severalfold over those of only a decade ago, brought about by careful analysis of fluid mixing and heat transfer. These examples illustrate that basic knowledge in fluid physics moves quickly from research in flow physics to application because of the intense competitiveness of today's technological society. In the following sections of this report we review significant recent developments as well as indicate where the next decade will provide compelling advances in our understanding. There is little doubt that these advances in understanding will in turn be matched almost immediately by innovations in technology. 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 valu- able contributions to the field have often been made. Nonetheless, these studies proved useful to the panel in its efforts to develop a series of findings with recommendations to both the funding and academic

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38 PLASMAS AND FLUIDS communities, which we hope will enable us successfully to support and extend this important field of physics. In the concluding section, we found it useful to subdivide fluid physics into branches distinguished by common phenomena. While these are certainly not unique, they do offer a convenience when one is attempting to obtain a feeling for the diverse activities in the field. There are also topical subject areas that are of current and future interest but that are not clearly highlighted by subdivision into phe- nomena-related branches. As a result, selected subject or discipline areas are also highlighted when they convey more clearly the main directions in research that rely on many phenomena. Finally, there are basic technical tools that are of fundamental importance to the ad- vancement of fluid-physics research. Their status and expected devel- opment are outlined. In summary, fluid physics remains intellectually stimulating because of the natural occurrence and importance of its problems. In addition, new levels of understanding of complex phenomena have further vitalized this field. Much of this understanding has been created by the development of powerful new tools that enable us to attack the nature of complex phenomena that hitherto have appeared to be intractable mysteries. Thus, the study of turbulence, complex high-speed flows, biological flows, and geological phenomena has been paced by new developments in powerful computational and instrumentation tech- niques. We look forward to the next decade as a time of excitement, adventure, and discovery. The associated implications for the mastery of many important practical problems so necessary to the well-being of our nation and the world serve as a further stimulus. SIGNIFICANT ACCOMPLISHMENTS AND OPPORTUNITIES IN FLUID PHYSICS Significant Recent Accomplishments The revolutionary development of computational fluid dynamics has been used to solve problems that have previously defied theoretical analysis and experimental simulation, such as convection and circula- tion within the Sun and planetary atmospheres and the nonequilibrium flow surrounding the Space Shuttle orbiter on re-entry. 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 compo- nents. In addition, computational fluid dynamics has increased our

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FLUID PHYSICS 39 understanding of combustion, chemically reacting, and multiphase flows. The pace of accomplishment in high-speed flows has been accel- erated by analytical methods, numerical simulation, and new experi- mental techniques. Inspired by these developments as well as by pressing social needs, significant advances have been made in the efficiency of commercial transport, manned re-entry from space, and the effectiveness of high-performance aircraft. Recent developments have led to exciting improvements in our understanding of turbulent flows. This has enhanced our ability to compute turbulent-flow characteristics and has provided new insight into how mechanical systems can display chaotic behavior. This understanding is being brought about by new measurement techniques combined with the availability of new powerful computational tools. Dimensional reasoning and recent theoretical understanding of jet noise, acoustic damping, and turbulent flows have led to a thousand- fold 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. New solution methods have been devised for treating large-amplitude drop- let deformation and the strong interaction between three or more particles with potential application to more dense systems. This progress has led to new insight into the behavior of clouds, fluid separation phenomena, geological magma chambers, climate dynam- ics, and complex theological fluids such as blood. The central unifying idea of modern geology is the fluid-convection interpretation of the motion of the Earth's upper mantle. Important implications have been demonstrated for planetary evolution, earth- quakes, volcanism, and mineral and petrochemical resources. Large-scale turbulent and coherent fluid-dynamic structures have been identified in the Earth's oceans and atmosphere and the at- mospheres of Jupiter and Venus. Their successful simulation using eddy-resolving computer models gives us a new view of laboratory turbulence and the general circulation, storms, and weather of the atmosphere and deep ocean. Simple wavelike connections have been discovered in terrestrial climate studies. Circulation changes like E1 Nino of the tropical Pacific are communicated great distances across the globe with massive effect on rainfall and winds. Single-photon and multiphoton excitation as well as scattering techniques have been developed to study the energy budgets of severe

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40 PLASMAS AND FF UlDS gas-dynamic environments such as flames, permitting us for the first time to see inside complicated chemically reacting flows. Noninvasive instrumentation techniques that detect blood-flow- initiated acoustic emissions from the human body, or neutrally buoyant probes that track, via satellite, the transient and mean circulation of the oceans, represent important achievements that have promoted under- standing of fluid-flow phenomena. Fluid-dynamic modeling has led to basic new knowledge of our cardiovascular, reproductive, and urinary systems as well as many of the internal organs of our bodies and the locomotion of biological organisms from a single-ciliate cell to the hummingbird and the tuna. Fluid-dynamic principles have been vital to the design of artificial organs, cardiovascular implants, prostheses, and the development of new clinical diagnostic methods. New constitutive models based on molecular physical structure have led to a better understanding of the striking flow properties of non-Newtonian fluids such as polymer solutions and drag-reducing agents. The synergistic interaction of chemical, fluid, and optical physics has created the new continuous high-power laser. The success of this example has led to the identification of the importance of fluid phenomena in the performance of electric discharge and other gas- media lasers as well. Significant Research Opportunities Rapid advancement will continue in our understanding of the characteristics and origins of turbulence, including investigations of the connection between the routes to chaos found for systems with a finite number of degrees of freedom and the continuous instability that is fluid dynamic turbulence. 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 signifi- cantly, leading to novel drag and noise-reduction techniques; increased combustion efficiency; and control of separation, spreading, and mix- ing. Major advances in technology will be possible as a result of our ability to predict and control flows with turbulent zones. Continued rapid growth in the development of advanced compu- tational fluid-dynamics procedures, together with the next generation of computer resources, will provide the opportunity to calculate and obtain a new level of physical understanding of complex three

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FLUID PHYSICS 41 dimensional, compressible viscous flows. It will then be possible to optimize more effectively the design of high-performance aircraft, improve the forecasting of severe storm formation, attempt to predict global seasonal and annual climate changes, and realistically simulate and model fundamental processes in planetary and astrophysical fluid dynamical behavior. Powerful laser-based optical instrumentation techniques will be developed for the rapid, multipoint measurement of flow-field proper- ties pressure, temperature, velocity, species concentration. In con- junction with rapidly developing numerical techniques, these data will be manipulated to provide new types of information as well as increase the usefulness oflargeexperimentalfacilities. In many technologically important fluid machines the flow is either separated or unsteady or both. With the help of modern instrumenta- tion and computerized data-analysis techniques, we are beginning to understand the physics of these types of flows and how, often in combination, they can be used to improve the efficiency of technolog- ical devices ranging from heart valves to aircraft. We expect these possibilities to present a major research challenge in the coming decade. The challenges of combustion and reacting flows are likely to yield new understanding resulting in important applications in the near future. Control of soot and other pollutants will result from understand- ing of their production mechanisms. Understanding of the interaction between chemical kinetics and fluid instabilities will result in an understanding of deflagration and the transition to detonation. Appli- cations 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. There will be an increasing interest in the behavior of more-dense particulate systems, from the multiparticle interaction of finite clouds of particles to, more generally, the flow through porous media and filters based on the hydrodynamic interaction with their microstruc- ture. Interdisciplinary cooperation in the study of basic cellular level biofluid dynamic processes in the presence of molecular forces will expedite explanations of such diverse phenomena as electrokinetic behavior in pores and membranes, the microstructure of osmosis, cell

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42 PLASMAS AND FLUIDS division, cellular transport function, gel hydration, and fluid motion in intracellular tissue matrix. All lead to a better understanding of basic cellular physiological function. Increased computational and data-handling capability will permit assimilation and understanding of the massive data sets required to describe complex natural flow phenomena as well as those in man- made devices. For example, using satellites and shipborne instru- ments, global-scale investigation is now possible of the oceans and climate dynamics. 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 circula- tions. 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. FINDINGS AND RECOMMENDATIONS Principal Findings SUPPORT STRUCTURE Support for basic research in fluid physics comes from a wide variety of sources. This is both a strength and a weakness, but the field suffers from the lack of an individual national identity. Despite the common technical threads that bind fluid physics, its basic research support is chaotic and limited. Considering its importance to techno- logical development and its potential for contribution to the under- standing of natural phenomena, fluid physics lacks sufficient visibility on a national scale and suffers from a lack of both amount and continuity of support from funding agencies, particularly for innovative new research directions. Many unique national experimental and computational facilities are not readily available to a large proportion of the research commu- nity. We do recognize and applaud the U.S. government's efforts to make time available for outside research in the National Aeronautics and Space Administration's (NASA) National Transonic Facility at Langley Research Center, in the 40 ft x 80 ft wind tunnel at Ames Research Center, as well as provide computer access through the Numerical Aerodynamic Simulation Program (NASP) also at Ames, and the National Center for Atmospheric Research (NCAR) Comput

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FLUID PHYSICS 43 ing Facility in Boulder. However, we believe that considerably more could be done, producing benefits for both the research community and the facilities that are involved. COMPUTATIONAL TECHNIQUES Numerous mathematical and experimental approaches are com- mon throughout fluid physics, such as the use of asymptotic methods and laboratory flow simulations. In the last decade a new theme has emerged: the importance of the computer with applications that range from the rapid organization of data and their subsequent analysis and display all the way to the direct numerical simulation of the major features of some turbulent flows. This expanding capability provides rich opportunities for technological development and increased under- standing of natural phenomena. It is now possible to use new scientific methods to tackle important but highly complicated phenomena, such as two-phase flow, which to date have been treated primarily from an empirical point of view. The mathematical techniques that have been developed and refined during the last 15 years have become increas- ingly important tools in advancing fundamental understanding of complex flows but also importantly in improving the methods for testing the results of numerical simulations as well. In the application of numerical simulation to technological problems, and most especially to aircraft design, the Europeans have been quick to acquire the latest high-speed computers and to implement the most advanced algorithms in the design of aircraft. INSTRUMENTATION TECHNIQUES The past decade has spawned a remarkable growth in nonintrusive laser-based flow diagnostic techniques. Combined with equally spec- tacular 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. EDUCATION The explosive growth of fluid physics into new areas involves increasingly interdisciplinary research. Acid rain prediction, gas lasers, blood flow, and the distribution of life in the sea are examples of strong interactions of fluid physics with chemistry, physics, and biology.

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44 PLASMAS AND FLUIDS Fluid systems have motivated study of bifurcation theory, Lorenz attractors, and chaos, which are prominent in the study of physics and applied mathematics. This diversity of interests can be used to unify our understanding of fundamental fluid behavior and should be a more prominent part of university education. Education and university research in fluid physics is conducted primarily in engineering and applied mathematics departments in the United States. Last year only 1 percent of the Ph.D. theses in physics and astronomy in the United States were in fluid physics, and approximately 7 percent were in plasma physics, whereas approximately 30 percent of the engineering theses were on fluid-dynamics-related projects. This low emphasis on fluid physics in our physics curriculum has deprived physics research in this country of the opportunity to participate in many areas of technology that generate exciting new fundamental problems. Principal Recommendations RESEARCH SUPPORT We urge that a mechanism be established to provide a continuing survey of research support in fluid physics vis-a-vis the field's national and intellectual needs. While we are unable here to make a detailed suggestion about the form of this mechanism, it should provide information that will be useful in identifying basic research areas in this nationally important field that are neglected by omission or as a result of not being within the immediate sphere of influence of a support agency. Particularly, new research directions of great promise could be identified earlier. Areas that receive excessive overlapping support could also be identified. We recommend a targeted research initiative to investigate and develop instrumentation for essentially simultaneous multipoint mea- surements of flow properties throughout large volumes. The instru- ments might be based on laser holographic methods, on multiprojection (tomographic) techniques, or on a combination of these and other as yet unexplored methods. The measurements are important to many national programs in fluid physics. It should be recognized that the instruments will be expensive, and hence it is imperative that sufficient resources be made available to the research community for their development and eventual use. We recommend the provision of funds and organizational mecha- nisms to make unique national fluid-physics facilities available to the university and ~nongovernment communities for basic research. Direct

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FLUID PHYSICS 45 allocations of time and other resources will be necessary in order to maintain an appropriate balance between basic research and urgent development programs and to assure steady operational funding of these facilities. We strongly recommend the expansion of the role of the National Science Foundation (NSF) in supporting basic fluid-physics research, with a particular emphasis on the support available for basic fluid- physics research related to engineering science. There is funding for fluid-mechanics research embedded in the atmospheric and oceano- graphic sciences programs. However, only extremely limited funds are available for basic fluid-physics research in NSF's Engineering Direc- torate. No funds have been available from the Physics Directorate. EDUCATION In view of the pervasive importance of fluid physics in many areas of modern technology and the numerous unexplained phenomena associated with these technologies documented in this report, we strongly recommend that physics departments in this country consider the inclusion of a required undergraduate course in fluid physics. We similarly encourage engineering schools to consider a required upper- division undergraduate course in modern physics. This would be an important step in enhancing collaborative interdisciplinary relation- ships between the physical and engineering sciences. Fluid-flow instrumentation, especially optical techniques, are ex- pected to continue their recent exciting progress. Unfortunately this will cause the state of teaching laboratory equipment in our universities to be even more out of date. The need for dedicated, separate funding for modern laboratory equipment in fluid physics is at least as pressing as in other areas of science. 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. GOVERNMENT SUPPORT, MANPOWER, AND UNIVERSITY RESEARCH The major agencies that support external research in fluid mechanics and combustion are the Air Force Office of Scientific Research (AFOSR), Army Research Office (ARO), Department of Energy

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84 PLASMAS AND FLUIDS the bulk fluid, resulting in a complex of phenomena not present in the classical fluids described by the Navier-Stokes equations. These range from pure magnetohydrodynamic phenomena such as Alfven waves and forward propagation of viscous wakes to the more complex category involving thermal and ionization phenomena, such as the ionization or electrothermal instabilities in nonequilibrium plasmas. Studies of these complex phenomena have provided explanations for the behavior of the Earth's core, for events in the Sun's corona. The continued study of magnetohydrodynamics is essential to future progress in these areas, and in cosmology. Potential engineering applications of magnetohydrodynamics (MHD) include fusion power, electric circuit breakers, electric space propulsion devices, manipulation of molten metals, and MHD power generation. Fusion is addressed at length elsewhere in this report. After a very intensive effort over the last 20 years, funding for electric propulsion and MHD power generation is currently at a low level. Yet many important phenomena remain partially explored or perhaps undiscovered. It is important to continue fundamental work in this area, which, quite apart from its intellectual challenge, may have additional important applications in the future. GEOPHYSICAL FLUID DYNAMICS The fluid dynamics of the natural world encompasses a vast range of physical phenomena, from atmospheric and oceanic dynamics and climate change to geological processes in the Earth's mantle and core. The subject has evolved naturally to consider the atmospheres of the planets and fluid phenomena in astrophysics. What makes the geophysical fluid dynamics (GFD) of the atmo- sphere and oceans challenging is the ten decades of scale between the motions of planetary scale and the motions of smallest scale, where molecular diffusion is important. 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. A simulation of climatic change must in addition accurately account for the many years of weather, whatever its cumulative effect may be. A theory of the ocean circulation, on the other hand, must cope with its vastly slower response to a change in atmospheric winds or heating. It

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FLUID PHYSICS 85 must account also for the differing behavior of salinity and tempera- ture, both of which influence the fluid buoyancy. At high latitude the dynamics is made complex by sea ice. The oceans act as a flywheel in the climate system with a time scale as great as a thousand years. The worldwide disruption of weather by the 1982-1983 El Nino event in the tropical Pacific Ocean shows us the powerfully interactive nature of the oceans and atmosphere. Wave-propagation theories have successfully described several of the links in the sequence of tropical and global change. Beyond these short-term events we are soon to experience the global effect of increasing carbon dioxide in the atmosphere. The prediction of climate change over the next half-century relies on complex fluid- dynamical modeling of the general circulation and its heat and moisture balances. These important problems involve, in addition to classical fluid dynamics, interactions with chemistry (for example, of aerosols in the atmosphere and carbon in the oceans), radiative effects, multiphase and multicomponent fluids (as in convective clouds and in sea ice), and biology. (The biosphere interacts with the fluid atmosphere and oceans in many ways.) Such interactions are crucially important in the possible aftermath of nuclear war, in which the particulate load of the atmosphere may be great and the Sun obscured for months or years. A promising branch of study in this area is Lagrangian fluid dynam- ics, 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. The complexity of the system would defy any brute-force solution by computer simulation, but there is much optimism that new techniques will lead to a solution: first, radically new measurements of the atmosphere and oceans are now possible using microelectronics, remote sensing (especially from orbiting satellites), and computer analysis, and, second, simple theo- retical models of the circulation are emerging that help to reduce the apparent complexity of the system. These theories of the circulations, wave propagation, turbulent cascades, and the induction of mean circulation by eddy motions are laying the groundwork for the coupled model of the ocean-atmosphere system. The close interaction of theory, observation, and computer and laboratory experiment are characteristic of the work. The study of the atmosphere of other planets has a close connection with GFD: while many new physical and chemical ejects are present

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86 PLASMAS AND Ff UlDS on the planets, some remarkable tentative similarities have been found with terrestrial flows. Beyond their own intrinsic interest, the value of studying the other planets is to better understand our own. Intense, isolated vortices, for example, have been observed in Jupiter's circu- lation; models of them have aided in understanding terrestrial flow, from small severe storms to the intense eddies cast off from the Gulf Stream. Terrestrial general circulation models have been able to simulate some of the banded flows of the outer planets, simply by altering appropriately their planetary rotation and density stratifica- tion. MULTIPHASE FLOWS The analysis of flows in which more than one phase is involved (multiphase flows) offers problems of far more complexity than are encountered with single-phase flows. The reason for this is that the different phases, in general, are not uniformly mixed, and a detailed understanding of how these phases are distributed in a flow field is needed. The importance of these flows can be realized by considering a few examples: The transport of crude oil in a pipe usually involves the flow of both liquid and gaseous hydrocarbons. 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. Thus, it is usually desirable to design so as to avoid slugging, but, unfortunately, currently available scaling laws are unable to predict either the conditions under which slugs will appear or their properties. Another example is found in nuclear reactors, which typically employ water to remove the heat generated by the nuclear decay. A two-phase flow of vapor and liquid occurs in the cooling passages because of the boiling of the liquid. The flow character can vary from a bubbly flow, which consists of a mixture of vapor bubbles and liquid, to an annular flow, whereby a mixture of vapor and liquid droplets flow concurrently with a liquid film on the wall. It is critically important to design these cooling systems so that the wall film does not dry out, because under these circumstances the cooling is insufficient and a runaway reaction can occur.

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FLUID PHYSICS 87 As a final example, we mention the transportation of solids, such as coal, in a slurry in a pipeline. Here one of the chief engineering considerations is to avoid settling of the particles, which is accom- plished by selecting a proper size range for the particles and a judicious pipeline design. In a long straight pipeline the particles settle because of gravitational forces; this is opposed by turbulence and other hydrodynamic effects, in a manner that is not yet understood. Further basic research on these hydrodynamic effects is needed to provide a solid theoretical basis for the design of slurry pipelines. In fact, it is not an exaggeration to claim that almost every aspect of a manufacturing facility in the chemical process industry is confronted with multiphase problems. This can involve the contacting of gases and liquids or of solids and liquids, the design of condensers and boilers, the evolution of gas in a chemical reaction, the design of pressure relief valves, or the separation of phases. Quite often the failure of a process design (usually at great cost) can be traced to a poor understanding of the consequences of a scaleup of some part of the system involving a multiphase flow. The recognition of this problem has led large compa- nies to identify critical parts of the flow system and to do full-scale tests to ensure a safe process design. The problem of scaling multiphase problems can be illustrated by considering the prediction of pressure drop in a long straight pipe. 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. Current design methods, given in engineering handbooks, usually involve the modification of single-phase relations by using fluid prop- erties that are some combination of the properties of the different phases, an approach the inadequacy of which has been recognized for 25 years. It is quite clear that predictive methods for pressure drop must be tailored to the flow configuration that is expected to exist. Research in this area has three main aspects: (1) basic studies of multiphase phenomena, (2) the prediction of how the phases distribute for different flows, and (3) the development of design equations as well as computer codes for predicting the distribution of phases in complex flow situations. Basic studies would involve such issues as the mech- anism by which particles are entrained and moved by turbulence in a

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88 PLASMAS AND FL UlDS flowing gas stream or the mechanism by which waves are generated by air flowing over a liquid film. The second aspect of this research involves the use of diagnostic tools to determine how the phases distribute in a particular flow and the use of basic studies to provide an explanation of the observed distribution. The final aspect of research on multiphase flows is the development of design equations and computer codes to predict phase distribution. To date, it has become customary to consider separate differential equations for each phase and to use these as the basis for the computations. Unfortunately, it is quite likely that this aspect of research on multiphase flows is danger- ously ahead of our basic knowledge. Technical Disciplines MODELING AND ANALYTICAL METHODS Phenomena found in the natural world and in the industrial environ- ment are identified and described in physical terms. This physical description must then be expressed mathematically in nondimensional form. This delineates the dominant physical mechanisms. These equa- tions 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 corre- sponding and initial boundary conditions, and the development of solutions to quantify the physical process of interest. All Newtonian fluid-physics processes are described ultimately by a suitably generalized set of Navier-Stokes equations in which chemical effects and radiative transfer may be included. Suitable analogs are developed for non-Newtonian fluids. Unless the solution process is to be based on numerical simulation of the complete general equations, rational approximation schemes are needed to reduce the full mathe- matical system to a simpler form compatible with the physical model. Significant parameter groups are identified and then employed to develop asymptotic representations of the complete equations. Meth- ods of this genre have permitted an enormous improvement in the understanding of classical ad hoc approximations (for example, bound- ary-layer theory and potential flow theory) and facilitated the develop- ment of techniques for describing very complex flows including, for example, multiple-deck descriptions of trailing-edge flows, shock/ boundary-layer interactions, and delta-wing aerodynamics. In fact, major processes in every branch described in this chapter, with the

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FLUID PHYSICS 89 notable exception of turbulence phenomena, can be attributed to the use of contemporary rational approximation methods. Once a reduced equation system has been specified, a method for their solution must be found. One can employ exact analytical meth- ods, asymptotic analytical methods, computational techniques, and even formal mathematical methods to find solutions as well as solution bounds, properties, and uniqueness. For most fluid-physics processes useful exact analytical solutions are seldom found. Asymptotic solu- tions provide a quantitative description of the physical phenomena for limited ranges of parameter values. In highly nonlinear problems (for example, chemically active systems) novel perturbation techniques are needed to find concise, uniformly valid expansion-based solutions. This is particulary important in systems with disparate time and length scales. Numerical simulation must include assessments of accuracy and resolution so that physically viable solutions are discriminated from those that represent numerical artifacts. Analytically derived asymp- totic 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. The need to benchmark numerical simulations is especially important when the numerical model employs the full equations describing the processes involved. In this case, the ensemble of physical processes occurring concurrently is large and the resolution of disparate length and time-scale processes is essential. Only in the area of turbulence is the mathematical modeling hindered by the lack of definitive conceptual models. Averaged equations derived from the Navier-Stokes equations always have undefined terms that are only described in terms of ad hoc closure approxima- tions. So far, mathematical methods have not yielded rationally derived rules for closure, and a more focused effort toward providing better answers to this question may prove fruitful. COMPUTATIONAL FLUID DYNAMICS In recent years rapid progress has been made in computational fluid dynamics. The moving force for this development was largely provided by the availability of reliable and powerful computer resources. This in turn has stimulated both theoretical and experimental research toward the understanding of fundamental processes in fluid dynamics. As a result, we currently have the capability to calculate many complex unsteady two-dimensional and steady three-dimensional flows in- cluding the effects of compressibility and viscosity that were impos

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90 PLASMAS AND FL UlDS sible or impractical only a few years ago. There are, however, many limitations that must still be overcome. Computational aerodynamics has progressed during the last two decades from linear theory for slender-body-flow calculations to nonlinear inviscid theory for flows about aircraftlike configurations. During the last decade there has been much activity in calculating three-dimensional compressible viscous flows past relatively simple aerodynamic shapes using the Reynolds-averaged Navier-Stokes equa- tions with turbulence modeling. These calculations, representing the present stage of development, require only the space-time resolution of the gross turbulence effects and leave the representation of the remaining, although highly significant, turbulence effects to realistic modeling. The computer storage and speed requirements of this stage are much less than those of the next and final stage, which represents by mesh and time-step resolution all sizes of the significant energy- bearing turbulent eddies. 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 proce- dures to enhance the efficiency of numerical methods for solving the equations of compressible viscous flow, and the implementation of solution-adaptive grid systems. Much progress is also required before the currently available turbu- lence models will be able to account for the effects of strongly interacting flow fields with moderate or large amounts of separation. There is no assurance that such capability will be forthcoming in the near or even distant future. However, present models can predict to engineering accuracy turbulence boundary-layer interactions with few or no regions of separation. Development of the procedures required to extend viscous flow calculations to complex three-dimensional flows, soon to be possible with forthcoming computers, without waiting for further improvements in turbulence modeling is still a logical next step. These calculations will also be of engineering accuracy for flows near design conditions and can be used to predict incipient separation, shock and vortex boundary-layer interactions, buffet, reduced lift, and interference phenomena. Improvements in turbulence modeling will further extend the range of application, eventually to tactical aircraft in maneuver.

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FLUID PHYSICS 91 Within the next 2 years computer resources will become available that can process data at a rate of the order 5 x 108 floating-point operations per second two orders of magnitude faster than current machines with core memories exceeding 16 x 106 words. This capability will enable Reynolds-averaged Navier-Stokes calculations to be made about body shapes as complex as modern aircraft at the same cost and time as present calculations for fairly simple geometric shapes. Further improvements in reducing computer cost and time are required to make such calculations practical for aerodynamic design. However, complementary to advances in computer technology, new numerical methods are being developed to increase numerical effi- ciency. 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. There has also been an explosive application of laser techniques. For example, we have Raman scattering for rotational temperature, Doppler velocimeters, and excitation techniques (LIF, or laser-induced fluorescence) along with particle and droplet sizing instruments based on laser scattering. These techniques follow the historical trend in gas dynamics and combustion research of striving for ways to measure the energy budgets in fluid flows. The distribution of energy among the classical and quantum-mechanical states of a gas or fluid is fundamental to many areas of fluid-physics research. The newly developed techniques permit one to investigate flows in ways that have previously not been possible. The drawback of most of them is that they are relatively complicated and time-consuming to use. However, because of a convergence of developments in several fields (medical imaging, large-array processors) it is now possible to antici- pate a revolution in fluid-dynamic instrumentation. The ideal fluid-dynamic instrument is capable of approximately point resolution, is noninvasive, and can obtain data from a relatively large

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92 PLASMAS AND FLUIDS volume of flow simultaneously and display it in arbitrary planes cut through the volume. Using the computer technology developed for axial tomography in medical imaging combined with single- and multiphoton excitation or scattering techniques in multiangular projec- tion geometry, it is possible to project that with significant support over the next few years many of the characteristics of the ideal fluid- dynamic instrument can be achieved. 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. Subse- quently, the results are manipulated and displayed by a computer. Rapid collection of volume data is an essential part of improving wind-tunnel testing efficiencies since only then can full advantage be obtained from introducing on-line computational techniques into ex- perimental studies. There has been an explosion of instrumentation effort in the atmo- spheric sciences. Ground-based remote sensing now allows us to measure the turbulence structure of the atmosphere, showing internal waves, turbulence, clouds, severe storms, and jet streams in detail. The impact on theoretical studies has been great, with a new picture of mesoscale structure emerging. New developments in technology have also led to the development of new oceanographic sensors that drift with the sea motion and are interrogated by satellites. These are expected to provide a wealth of flow information in the next decade: One area where instrumentation is particularly important and dif- ficult is combustion research. In combustion we seem now to be in a period of active development of experimental techniques. Certain quantities that could not be measured 10 years ago currently can be measured routinely (e.g., rotational temperatures). There are a number of key quantities that cannot be measured today but are likely to be measurable routinely in 10 years (e.g., certain joint probability-density functions). A large fraction of the progress being made concerns optical techniques. Optical methods developed during the past 10 years include Raman spectroscopy (of various types) for measuring temperatures and con- centrations of various chemical species, Rayleigh scattering for mea- suring densities, laser-Doppler velocimetry for measuring velocities, resonance fluorescence for measuring radical concentrations, and laser holography for measuring temperature fields. Since combustion envi

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FLUID PHYSICS 93 ronments are rather hostile, the remote nature of optical devices can possess importance beyond their obvious nonobtrusive benefits. Ca- pabilities in time and space resolution by optical methods have progressed to a point at which many quantities of interest in turbulent reacting flows can be measured on a space-time resolved basis. Much important knowledge has been obtained in recent years by application of optical techniques to studies of reacting flows in well-equipped laboratories. For example, the nature of the quench layers at the walls for applications in piston engines has been clarified to a large extent by these methods; and contrary to earlier belief, it was established that the wall-quench layer is not a source of unburned hydrocarbons. Progress of this type could not have been made without the new optical methods. There is still important information that is not fully accessible. For example, the joint probability-density functions for the concentration and the magnitude of the gradient of the concentration (effectively the so-called scalar dissipation rate) in turbulence diffusion flames plays a central role in theories of heat-release rates and of extinction, but no experimental information is yet available. This is just one example of measurement at the frontier of optical techniques in combustion. The results needed are quite likely to be obtained over the next 10 years. There are good prospects for continued improvement in capabilities of the optical methods. Moreover, there are theories in need of testing (and of input parameters) that can benefit from these improvements. Therefore, we can visualize experimental techniques (especially opti- cal techniques) in combustion to be an active area during the next 10 years. Flow Facilities Facilities associated with direct application of fluid mechanics, such as wind tunnels for airplanes, have always been available. The cryo- genic 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. It is difficult, however, to build significant facilities simply to investigate questions of fluid physics. It seems to us that there may be a need for facilities that are designed for and dedicated to the study of specific areas of the physics of fluid motion. We suggest that the

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94 PLASMAS AND FL UlDS fluid-physics community be alert to possible needs in this area and, if appropriate, develop open discussion at the national meetings on this subject. It would be a benefit to fluid physics if unique national experimental facilities could be used on a regular basis by university and other researchers. We are of the opinion that such a program would inject new ideas into the organization operating the facility, as well as permit state-of-the-art experiments by a widened pool of talented researchers. Typically, large national facilities tends to be equipment rich compared with university laboratories. Providing access and attractive arrange- ments for conducting experiments by visiting investigators may be an efficient way to increase the productivity of these facilities. ACKNOWLEDGMENTS The panel members thank the following people for their contribu- tions of sections to this chapter: T. J. Hanratty, University of Illinois (Multiphase Flows); D. R. Kassoy, University of Colorado (Modeling and Analytical Methods); S. Leibovich, Cornell University (Stability); G. C. Pomraning, University of California, Los Angeles (Radiation Hydrodynamics) .