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Toward a Microgravity Research Strategy (Appendix D)
Toward a Microgravity Research Strategy
D
Fluids, Interfaces, and Transport
STATUS
Fluid and interfacial transport processes are ubiquitous throughout
materials and biological systems. Gravitational effects influence these processes
through the sedimentation of particles, buoyancy-driven convection, and the
distortion of liquid and/or fluid interfaces by hydrostatic pressure. In many cases,
gravitational effects limit the operation of materials-processing systems, add
complexity, and potentially mask other physicochemical reactions that are under
way. Experiments in a microgravity environment provide conditions for unraveling
these complex transport processes in many systems. The new fundamental
understanding gained from these studies has led to improved system
performance on Earth, to new applications of the microgravity laboratory, and to
the better operation of fluids-handling systems in space.
REPORT MENU
These experiments and the supporting analyses of fluid and interfacial
NOTICE
processes have played a dominant role in past research programs, making this
MEMBERSHIP
area one of the best-developed and best-documented fields of microgravity
SUMMARY
research. Many review papers have appeared on aspects of this subject. The
CHAPTER 1
books by Feuerbacher et al.1 and Myshkis et al.2 are very good references to this
CHAPTER 2
material. An extended summary of many of the ideas presented here is available
CHAPTER 3
in the report of the discipline working group of NASA that focuses on this
CHAPTER 4
research area (Rosenberger3).
APPENDIX A
APPENDIX B
APPENDIX C
Microgravity research in fluids and interfaces is usually classified either by
APPENDIX D
the fundamental processes under study or by the area of application. Important
APPENDIX E
fundamental processes include (1) diffusive phenomena, (2) body force-driven
APPENDIX F
convection and surface-tension-driven convection in fluids, (3) capillary
phenomena, (4) nucleation and growth, (5) solidification and microstructure
formation, and (6) flow of multiphase fluids. Applications include transport
processes in materials-processing systems, the measurement of thermophysical
properties, and fluid and energy management in space.
Classification according to fundamental processes is used here to review
the accomplishments and future potential for microgravity research in fluid and
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Toward a Microgravity Research Strategy (Appendix D)
interfacial systems. Applications of understanding fundamental transport and
interfacial processes to particular systems are examined in the context of the
connections between this section and other portions of the report. The discussion
here focuses on the current level of understanding of physical phenomena and
on the role played by past microgravity research in achieving this understanding.
Potential applications of these fundamental results, both on Earth and in space,
are highlighted and key issues deserving future research are addressed.
Fundamental studies in transport processes can be divided into two
segments: research aimed at the analysis of the effects of coupled transport
processes where each process is well understood individually, and research
focused on understanding of fluxes of mass, heat, and species in complex
systems, where present theory gives a less than adequate description.
Many of the problems in the classical theory of fluid mechanics and
interface dynamics fall in the first category, in which mathematical descriptions in
terms of conservation laws and constitutive equations are known. Given the
availability of a database for the relevant transport coefficients (e.g., thermal
conductivities, diffusivities, and fluid viscosities) and the dependence of the
coefficients on temperature and concentration, these transport models can be
solved by numerical methods to give insight into coupled processes and to
design experimental systems. This approach is particularly appealing because of
the incredibly high cost of microgravity experiments and the increasing capability
of simulation methods to handle ever more complex problems.
There are two major difficulties to the numerical simulation approach.
First, the large uncertainty in a typical database for thermophysical properties
poses a major limitation. For most technologically interesting materials, these
properties are not known with any accuracy and, hence, this uncertainty is
transferred to the calculation of the transport processes through the simulation. In
fact, many microgravity experimental systems to measure these properties are
proposed. Second, the environment of microgravity offers a less symmetric and
less predictable laboratory than is typical on Earth. For example, the very
constant and unidirectional gravitational acceleration on Earth is replaced in
space by a jittering acceleration vector with direction and magnitude that depend
on time over a wide range of frequencies. The use of symmetry restrictions in the
calculations—a common mechanism used by analysts to cut the cost of the
simulations—is not justified, and, in many cases, totally predictive simulations
must be threedimensional and time-dependent. The difficulties of making these
calculations and the uncertainty of thermophysical data dictate continued reliance
on low-gravity experimentation.
MAJOR RESEARCH ACCOMPLISHMENTS
The major research challenges and accomplishments for each of the six
areas of interest listed above are described below.
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Diffusive Phenomena
The potential for convection-free experimentation in a microgravity
environment was a key motivation for many of the original experiments in space.
Myriad experiments, ranging from the solidification of metals and semiconductors
to electrophoretic separation of proteins, were performed with the hypothesis
that, in microgravity, the absence of bulk fluid motion driven by buoyancy forces
on Earth would lead to unique material properties and better separation
efficiency. These experiments were partially successful. Electrophoresis
experiments in space led to improvement in separation efficiency for model
systems. Experiments aimed at achieving diffusion-controlled crystal growth
showed the expected behavior for axial solute transport, but displayed the radial
nonuniformity that was shown computationally to be characteristic of weak
convection. These observations spawned both theoretical studies and Earth-
based experiments that have demonstrated the importance of controlling very
weak convective flows to obtain highly uniform solute concentrations in
microgravity solidification systems.
Removing the influence of convection also has uncovered the importance
of secondary mechanisms for diffusive transport. Such mechanisms as the Soret
effect for mass transport by a temperature gradient and the complementary
Dufour flux for heat transfer driven by a concentration gradient have been shown
to be important in many microgravity systems, where the effect would be masked
by convection on Earth. Theoretical understanding of these effects in anything
but very idealized model systems is severely hampered by the lack of
experimental data for transport coefficients.
Experimental measurement of diffusivities and other transport coefficients
is an exciting research application of the microgravity environment. Measurement
of diffusivities in space is particularly appealing, especially for liquids, because
the diffusivity is so low that fluid velocities of less than 1 m/s, hardly avoidable
on Earth, can lead to huge errors in the measurement. Recent measurements of
liquid diffusivities in space claim an accuracy of 2 percent.
Convection in Fluids
More complex transport processes arise when convection is driven by
multiple mechanisms. An example is double-diffusive convection, caused by
simultaneous temperature and concentration gradients. In many materials-
processing systems, these driving forces are affected differently by a low-gravity
environment. Other driving forces for convection are independent of gravity and
will become dominant in microgravity. Electrodynamic and magnetodynamic
forces both fall in this category and have been studied in relation to convection
during electrophoresis and to motion induced during magnetic levitation,
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respectively. Alternatively, magnetic fields have been applied to suppress
convection in electrically conducting fluids. In small-scale systems of only a few
centimeters on a side, the fields generated by conventional magnetic technology
are effective enough to alleviate the need for space experiments. In large
experimental systems, magnetic fields used in conjunction with a microgravity
experiment might be used to suppress residual convection that would occur
otherwise if only microgravity or magnetic fields were used. The possibility of
enhancing the microgravity environment through the use of magnetic fields
deserves more study.
Perhaps the most important and well-studied form of convection in
microgravity is caused by differences in surface tension that arise because of
temperature or concentration gradients along a liquid and/or fluid surface-so-
called thermocapillary or Marangoni convection. This convection is unavoidable
when a clean liquid interface exists in a nonisothermal system and, depending on
the geometry of the liquid, can be more intense than motion caused by buoyancy.
Moreover, thermocapillary flows show nonlinear transitions to time-periodic and,
finally, chaotic convection as the temperature or concentration gradient is
increased. Under these conditions, the bulk thermocapillary motion dominates
heat and solute transport, and there is little advantage to a space experiment.
Although much progress has been made in understanding thermocapillary
convection, it is impossible to eliminate it in many systems, and its presence
severely limits the possible configurations of low-gravity systems for performing
convectionless experiments.
Capillary Phenomena
Without the hydrostatic pressure of gravity, many fluids systems can exist
in space with only surface tension affecting a liquid surface. This class of capillary
phenomena includes liquid films, foams, drops, bubbles, and other configurations
of liquid and/or fluid surfaces that separate bulk phases. Statically levitated liquid
drops and suspended bubbles are excellent examples for which fundamentally
important experiments are possible only in space. Accordingly, microgravity
studies of drop dynamics and coalescence have flourished and have found
application as the basis for new measurement methods for liquid viscosity and
surface tension. Experimental observations of drop dynamics made early in the
NASA microgravity program and later results from Space Shuttle experiments
spawned many high-quality, theoretical studies of the fluid mechanics of drops
and bubbles and advanced this science considerably.
Because these levitation experiments are containerless (i.e., the droplet
does not contact a container wall), these techniques will be very important for
measurements at high temperature and of highly reactive materials once
experimental systems are available for precise measurement and environmental
control.
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Nucleation and Growth
The homogeneous nucleation and growth of a new phase, either liquid or
solid, from a parent phase, either gas or liquid, constitute a classical area of
physics and chemistry and are generic to many varying technologies, including
crystallization from liquid solutions, solidification from undercooled melts, and
aerosol formation from the vapor phase. Definitive experiments are extremely
difficult on Earth because of the complexities introduced by the need for confining
surfaces, which are sites for heterogeneous nucleation, and because of
gravitationally induced particle settling. The prolonged availability of a well-
controlled microgravity environment will make feasible the first experiments free
of these effects. To date, these experiments are limited to either minute samples,
which can be levitated effectively using acoustic or magnetic levitation devices, or
to very-short-duration experiments in NASA free-fall facilities.
Solidification and Microstructure Formation
Prediction of the microstructure of a solidifying interface is a key problem
in materials processing because of the crucial influence that this structure has on
the extrinsic properties of the solid. The dynamic behavior of the solidification
interfaces depends on heat and solute transport in the liquid phase as well as on
interfacial properties of the melt and/or solid system. In the last decade, the
problem of prediction of these microstructures has received intense theoretical
attention. The emphasis has been on the search for precise descriptions for the
morphogenesis of the microstructure patterns, such as solidification cells and
dendrites, that form as a function of changes in macroscopic operating
conditions—temperature gradient, growth rate, and alloy composition. The
formation of these small-scale microstructures during phase change is
ubiquitous, occurring not only in crystal growth from melts and solutions, but also
in electroplating and phase changes in liquid crystal systems. Significant
progress has been made on a number of problems, much of the research having
been sponsored by NASA. For example, several recent, intricate theories have
appeared to predict the dependence on growth rate and undercooling of the
shape of a single dendrite growing in an undercooled melt. Microgravity
experiments may be the only means of making distinctions between these
theories because convection in the melt has a strong effect on experiments, as
has been carefully documented. Similarly, the experimental growth of a rod-like,
eutectic material on Earth and in microgravity has demonstrated the influence of
convection on microstructure formation. On Earth, only thin-film solidification
experiments are relatively free of convection, but these experiments may be
altered significantly by the presence of the confining sidewalls.
Theories exist that are beginning to link the nonlinear transitions that
describe microstructure evolution during phase change with the flow in the
adjacent liquid. These theories have identified new types of interface dynamics,
which have not yet been confirmed experimentally.
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Flow of Multiphase Fluids
Multiphase fluids, such as bubbly gases and solid-laden slurries, form the
class of perhaps the most industrially important liquids. Not only are many
materials processed as multiphase liquids, but gas-liquid systems also appear in
heat transfer systems that employ nucleate boiling and in many gas separation
technologies. These fluids are not well understood in the sense that constitutive
descriptions do not exist that give accurate predictions of the average stress in
the multiphase liquid and of segregation of the components in nonhomogeneous
flows. The development of these descriptions is one of the outstanding problems
in fluid theology. Low-gravity experiments for isothermal gas-liquid flow will be
very important in separating the effect of gas buoyancy for viscous and inertial
interactions in the prediction of the rheological behavior of liquids containing
many gas bubbles. Studies of mass and heat transfer in two-phase flow will also
be important in understanding the effects of microgravity on the operation of
classical heat transfer and separation technologies in a space environment.
RESEARCH PROSPECTS AND OPPORTUNITIES
Further progress in the fundamental understanding of transport processes
and interfacial systems in microgravity will rely on the continued growth of
theoretical and computational research and on expansion of the experimental
database. The needs are not equal among these components; both the ground-
based and microgravity experimental programs are much less developed than
theory and should receive special attention. Experiments should aim at probing
new phenomena and at clarifying the link between theory and experiment. In
these experiments, emphasis should be placed on using well-characterized
materials; when the characterization is not complete, appropriate measurements
of the properties needed should be sponsored. Because many of the experiments
will rely on similar systems, for example, directional solidification and drop
levitation, integrated common equipment continues to be essential and should be
emphasized, but the equipment should be developed in close collaboration with
the principal investigators.
Platforms for these experiments should include short-duration drop tower
tests, sounding rocket flights, Shuttle bay experiments, and systems on the
Space Station. The microgravity needs for a specific experiment should be
considered carefully in order to leverage the most cost-effective environment for
the measurements.
Continued development of the ground-based research and analysis
program is essential to increasing the vitality of this program. This is particularly
true because of the important roles that theory and simulation play in unraveling
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complex transport processes in microgravity.
REFERENCES
1. Feuerbacher, N., H. Hamacher, and R.J. Naumann. 1986. Material
Science in Space. Springer-Verlag, Berlin.
2. Myshkis, A.D., V.G. Babskii, N.D. Kopachevski, L.A. Slobozhanin, and
A.D. Tyuptsov. 1976. Low-Gravity Fluid Mechanics. Springer-Verlag, New York.
3. Rosenberger, F. 1989. Report of the Fluids, Interfaces, and Transport
Discipline Working Group. Microgravity Science and Applications Division,
National Aeronautics and Space Administration.
Last update 7/13/00 at 3:51 pm
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