|
||||||||||||||||
|
||||||||||||||||
|
||||||||||||||||
|
| ||||||||||||||||||||||||||||||||
|
||||||||||||||||||||||||||||||||
Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 36
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
OCR for page 37
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
OCR for page 38
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
OCR for page 39
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
OCR for page 40
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
OCR for page 41
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
OCR for page 42
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
OCR for page 43
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.
OCR for page 44
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
OCR for page 45
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
OCR for page 46
46
U'
Ct
o ~
EN 4
_`
.0
-
._
._
Cal
-
._
V
so
Ct
Cal
V
._
1
-
oo
Cal
so
Ct
-
V
U.
.=
E_,
Cal
ED
Cd
Hi
o
_4
Ct
Hi
I:
.o ~
~ 3
C: ~
~ Hi
._
Ct
=O ~
~ O ~
. . ..
\0 ~ ~ O
rid
. . . . .
_ C) O ~ -
_ - r->
. . .. . .
- ~ - O ~ ~ O ~ ~ oo
_ _ _
of 0 V)
. . . . .
so ~ ~ 0 ~ rid
_ _ ~ _
~ O O ~
Cal
Ct
o~ ~
-- 0 - - ~) 3
~q
U.
._
._
C)
U~
C
oo ~ ~V~
· · · ts . ~s .
- ~ ~ ~ ~ ~
~_ _
oo
~O ~
-
_ ~:^
._ _ "C
E ~ ~ ~ ~= ¢ ¢
o pO: o ¢ ~ ~ ~ ~ O o Z ~ O
<~ZZZZZ ZOZ~
s~
o
_ ·~)
s~
<-o.
C~
Ct
._
4_
c:
:-
._
U.
U.
OCR for page 84
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
OCR for page 85
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
OCR for page 86
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.
OCR for page 87
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
OCR for page 88
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
OCR for page 89
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
OCR for page 90
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.
OCR for page 91
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
OCR for page 92
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
OCR for page 93
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
OCR for page 94
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) .
Representative terms from entire chapter:
fluid dynamics
