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Microgravity Research Opportunities for the 1990s: Chapter 3
Microgravity Research Opportunities
for the 1990s
PART II—SCIENTIFIC ISSUES
3
Fluid Mechanics and Transport Phenomena
INTRODUCTION
The field of fluid mechanics and transport phenomena plays a dual role in
the microgravity research program. It stands as a distinct disciplinary area but
also appears as a theme running through other microgravity disciplines.
Presentations in other chapters of this report reflect the role of the fluids and
transport discipline as providing supportive science for other microgravity
disciplines.
Many fluid mechanics and transport phenomena (heat and mass transfer)
REPORT MENU
are influenced directly by gravity. Such phenomena are also vital elements in a
NOTICE
wide range of physical, chemical, and biological systems, many of which are
MEMBERSHIP
important in both terrestrial- and space-based technologies. The main
PREFACE
EXECUTIVE SUMMARY characteristics of a low-gravity environment in relation to fluid mechanics and
PART I transport phenomena are the diminished importance of buoyancy-driven flows
CHAPTER 1 and sedimentation, and the relative increase in importance of other forces, such
CHAPTER 2 as surface tension at fluid-fluid interfaces. These effects have important
PART II
consequences not only for the conduct of research in other microgravity
CHAPTER 3
disciplines but also in the development of many mission-enabling technologies.
CHAPTER 4
The fluid mechanics and transport phenomena area of the microgravity research
CHAPTER 5
program should have both fundamental and applied objectives:
CHAPTER 6
CHAPTER 7
PART III Fundamental problems. A key objective for fundamentally oriented
CHAPTER 8
research should be the identification and description of new phenomena that may
APPENDIX A
influence transport and other applications in microgravity due to the change in
APPENDIX B
parameter range from the ground-based norm.1 A second general objective for
fundamental research should be to focus on problems in which a low-gravity
environment can contribute to the fundamental understanding of observed
phenomena by providing a unique experimental window.
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Microgravity Research Opportunities for the 1990s: Chapter 3
Applications. Transport problems are relevant to the other disciplines
in the microgravity program and to mission-enabling technologies, including, for
example, understanding how altered transport phenomena in microgravity
influence the operation or adaptation of biological systems to space,
understanding how combustion processes change, or understanding how
materials processing is modified.2 Many mission-enabling technologies involve
transport phenomena, but predictive models for low-gravity performance and
operation are frequently inadequate. A knowledge base should be established for
the efficient design, development, and operation of such technologies.
STATUS
A few crude demonstrations, consisting primarily of the unusual effects of
surface tension on fluid behavior in microgravity, were performed aboard Skylab.3
These represented the first fluid experiments in a low-gravity environment. It
should be noted that despite the completion of more than 50 space shuttle flights
to date, relatively little time was allotted to microgravity research on fluid
dynamics and transport phenomena. Six fluid experiments by European scientists
were performed in 1983 aboard Spacelab-1. Five of those concerned the effects
of capillarity on fluids: isothermal meniscus between two plates of unequal radii;
free surface oscillations in drops; liquid-column stability with rotation, vibration,
and stretching; axisymmetric free surface behavior due to forced disturbances in
partly filled containers; and kinetics of the spreading of a tethered drop. The sixth
experiment was designed to study thermocapillary flows in a liquid bridge.
Some of these experiments were repeated on the D-1 mission in 1985,
along with seven other experiments-two dealing with thermocapillary flows in
rectangular containers; two with thermocapillary migration of bubbles and drops;
one with diffusocapillary convection in a rectangular container; and two with
separation of fluid phases (one in a liquid column and one with bubbles in a
reacting liquid).
The first U.S. fluid experiments were performed aboard Spacelab-3 in
1985. One was a geophysical fluid flow cell (i.e., a rotating hemispherical shell of
fluid with an electric field to simulate radial gravity forces) and another was
concerned with the dynamics of rotating and oscillating drops in acoustic fields.
The third, which was not primarily a fluid experiment, studied the growth from
solution of triglycine sulfate crystals by means of holograms and indicated the
reduction, during crystal growth, of buoyancy-driven convection. It was repeated
on the IML-1 mission in 1992.
The evaluation of Spacelab-1 fluid experiments noted a number of
problems, including power limitations, limitations on time available for
experiments, an insufficient theoretical basis, shortcomings in equipment
performance, and the poor quality of data transfer to the ground.4 Reviews of the
experimental results of the D-1 mission5 also indicated numerous problems in
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Microgravity Research Opportunities for the 1990s: Chapter 3
equipment performance, high g-jitter accelerations, and diagnostics. Thus, many
of the results were essentially qualitative and descriptive.
The USML-1 mission, which was launched in June 1992, was the first
microgravity flight dedicated solely to U.S. scientists. Thirty-one investigations
were included in the payload, with two of the three major facilities for fluid
research. The first, the Surface Tension-Driven Convection Experiment (STDCE),
was dedicated to a single investigator; the second, the Drop Physics Module
(DPM), involved three investigators. A Space Acceleration Measurement System
(SAMS) was also included to enable the acceleration environment aboard the
shuttle to be described accurately.
The STDCE6-14 conducted in an interactive mode on the USML-1 mission
was the first thermocapillary flow experiment with state-of-the-art diagnostic
techniques to yield quantitative flow field and temperature data over a wide range
of conditions with two different heating modes and various initial interface
shapes, which are unique to the microgravity environment. The nature and extent
of large Marangoni number steady thermocapillary flows in a cylindrical chamber
were determined. The g-jitter due to thruster firings and other spacecraft activities
did not affect the experiments. No flow oscillations were observed even for
Marangoni numbers larger than 10, which indicates that the Marangoni number
alone is not sufficient to specify the onset of oscillations.
The STDCE was designed to determine quantitatively the nature and
extent of flows (velocity and temperature distributions) driven in a cylindrical
container by temperature gradients along the liquid free surface imposed by
different thermal signatures due both to various CO2 laser heat fluxes and to
various temperature differences between a heated center post and the cooled
wall.
The behavior of drops and shells subject to rotational torques by
acoustical fields was studied in the DPM. Another experiment involved
determining the surface properties of liquid drops in the presence of surfactants
and also the coalescence of droplets with surfactants. The third investigation in
the DPM measured the liquid-liquid interfacial tension of a compound drop.
A glovebox facility was also aboard that mission. Thirteen qualitative
experiments that were developed relatively inexpensively in a short period of time
were accommodated in this way. Six of these were related to fluid research: two
were on thermocapillary flows (with one generalizing the results of the STDCE),
three dealt with capillarity (with two related to the DPM experiments), and one
was concerned with the aggregation of fine particles in air. All of the detailed data
have not, as yet, been completely analyzed by the investigators, but the results
reported15 are interesting and encouraging.
RESEARCH AREAS
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Microgravity Research Opportunities for the 1990s: Chapter 3
Priority should be given to the study of phenomena that are unique to the
low-gravity environment and to those that are critical to space mission-enabling
technologies. Fundamental Research
Among the basic topics that may be studied with special advantage in the
low-gravity environment are the following:
Surface tension gradient-driven flows and capillary effects. The effects
of flow phenomena driven by surface tension forces are frequently masked by
competing gravitational forces in the terrestrial environment but become
significant or dominant in reduced gravity. This provides an opportunity for a
unique experimental window that can be used to study surface tension-driven
convection phenomena, as well as the microscale physics of the interfacial zone
(e.g., surface or interfacial rheology in systems with surfactants or the dynamics
of the contact line on a solid boundary), all of which remain poorly understood.
Fluid interfaces with surface tension variations are also common for many
spaceflight-enabling technologies (e.g., boiling, heat transfer, combustion,
welding, brazing, or soldering). Processes that rely on gravity-driven separations
in the terrestrial environment, such as the removal of gas bubbles from a liquid,
must be accomplished by other means in microgravity environments (e.g., by
thermocapillary-driven migration via nonuniform temperature fields). Frequently,
however, additional study will be necessary to enable practical implementation of
these processes.
Multiphase flows. Many important phenomena involve multiphase
flows or two-fluid systems (e.g., bubbles, drops). In ground-based studies, gravity
imposes a specific orientation on multiphase fluids, structures, and phase
organizations (e.g., gas-liquid, liquid-solid). Furthermore, the systems are
generally too complex for fundamental predictions and simulations. Thus, gravity
cannot be separated from the observed phenomena in ground-based studies. In
a reduced-gravity environment, the gravitational phase orientation is greatly
diminished, and it is expected that multiphase flows and associated transport
phenomena will become significantly altered. Low-gravity experiments might not
only contribute a foundation for space-enabling technologies, as discussed
below, but also contribute to an understanding of the essential physics of
terrestrial multiphase flows.
Diffusive transport processes. Under normal gravitational conditions,
multicomponent fluids experience various modes of thermosolutal convection
(depending on the relative orientations of temperature and concentration
gradients with each other and with the gravity vector). In addition, there are other
effects due to important convection phenomena that derive from the differences
in diffusivities for heat and mass. Such phenomena occur in many materials and
industrial processes (e.g., macrosegregation). With reduced-buoyancy
convection, there is a significant opportunity to obtain a better fundamental
understanding of other complex temperature- and concentration gradient-induced
interactions, such as Soret and Dufour effects, which are usually overshadowed
under terrestrial conditions.
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Microgravity Research Opportunities for the 1990s: Chapter 3
Colloidal phenomena. Colloidal suspensions are representative of a
larger class of systems in which surface forces play a critical role. In many
instances, however, understanding the consequences of these forces is
complicated by competing gravitational effects. Although gravitational effects can
be minimized in the terrestrial environment by density matching, this frequently
may change the chemical environment or require introduction of materials
different from those of direct interest. Thus, the microgravity environment
provides an opportunity for experimental studies of phenomena such as order-
disorder transitions in dispersions of hard spheres or electrohydrodynamic effects
such as drop deformation in electric fields, which can strengthen the fundamental
understanding of those physicochemical interactions and their macroscopic
manifestations in complex fluids.
The above topics are also important to biotechnology, combustion, and
materials science. Some of them are discussed in subsequent sections that deal
in detail with those areas. Beyond these specific discipline-based areas,
microgravity offers the possibility for experimental studies of fundamental fluid
physics phenomena that, in the terrestrial environment, cannot be done or can be
done only for limited parameter ranges. One example that has received
significant study already is the dynamics of individual bubbles or drops in
acoustic fields.
Technology Development
One of NASA's missions is to "provide technology for present and future
civil space missions and provide a base of research and technology capabilities
to serve all National space goals."16 Many of the technologies required for NASA
programs involve transport phenomena, including power systems; thermal
management systems; spacecraft fire hazard management; cryogenic engines;
fluid systems tankage; physical and chemical life-support systems; and user-
support subsystems such as refrigerators. More specifically, multiphase flow
phenomena, which are highly gravity dependent, are central to heat and mass
transfer in all systems, and gas-liquid contacting for air purification and energy
generation also have a major role in chemical processes such as catalysis and
ore beneficiation techniques. Fundamental studies of such phenomena will
contribute to better process design for the microgravity environment.
Safe and efficient heat transfer design for reduced-gravity applications is
currently not possible because fundamental knowledge of convection processes
at very low Reynolds number, and of boiling and condensation in a low-gravity
environment, is lacking.
NASA's long-term mission to explore the universe is also clearly
dependent on exploiting the behavior of materials and processes in low gravity.
For example, the mining, winning, and processing of extraterrestrial materials will
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Microgravity Research Opportunities for the 1990s: Chapter 3
be essential for extensive exploration of the solar system. Since it would be costly
to transport Earth-based materials to the Moon and other planets, in situ
materials must be considered. Current processing techniques will not suffice for
obtaining materials under the unusual conditions encountered on other,
nonterrestrial, bodies. Technologies to enable processes such as fluidized-bed
hydrogenation, electrowinning, and vapor-phase pyrolysis will eventually be
needed, and all involve transport phenomena that are affected by gravity. The
technologies for life- and operations-support systems, such as power generation
and storage, water purification, oxygen production, and fuel and fluid storage and
management, also involve gravity-dependent transport phenomena.
Thus, if reliable, cost-effective, and efficient technologies are to be
developed to enable NASA's mission to be successful, the fundamental
understanding of the effects of gravity on many transport phenomena is essential.
The committee believes that the following topics, in addition to being of intrinsic
scientific interest, are critical to provide the knowledge base required to design
effective and reliable space-based systems and facilities in which fluid processes
are involved:
Convective processes at low Reynolds number. Buoyancy affects the
transport of heat and mass when the Reynolds number becomes small enough in
the terrestrial flow of fluids with density variations. Investigation of low Reynolds
number flows with density variations, at reduced gravity levels, can reveal the
altered nature of the transport phenomena.
Transport processes with a phase transition. Aspects of condensation,
evaporation, and boiling are influenced by both gravity and interfacial forces. How
such phenomena differ in a low-gravity environment, with interfacial forces
dominant, requires further study.
Complex materials. The structure and transport of complex materials
are often influenced by gravitational effects; for example, multiphase flows in
porous media, flows of powders and granular materials, the motion and stability
of foams, and the sedimentation of colloidal dispersions. Research is necessary
to understand the behavior of such materials in a microgravity environment. This
research may also lead to a better understanding of the behavior of complex
materials at 1 g.
Materials processing. Buoyancy, sedimentation, and interfacial
phenomena influence such important processing methods as fluidized-bed
hydrogenation, electrolysis and electrowinning, and vapor-phase pyrolysis. Their
features in microgravity should be investigated.
Physical processes in life- and operating-support systems. Some of
the effects indicated above apply to such processes as power generation and
storage, water purification, oxygen production, and fuel and fluid storage and
management.
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Microgravity Research Opportunities for the 1990s: Chapter 3
SUMMARY
The modifications in fluid flows and in heat and mass transfer processes
that occur in the transition between the terrestrial environment and microgravity
underlie modifications that occur in many biological processes, materials
behavior, combustion, and a number of mission-enabling technologies. These
changes result from the diminished significance of buoyancy for gravity-driven
motions and the relatively greater importance of other forces. The microgravity
environment also provides unique conditions for experimental studies that can
contribute new understanding of fluid flow and transport processes that are
significant primarily to Earth-based applications.
For these reasons, a research program on fluid physics, aimed at
primarily fundamental studies of fluid mechanics and transport phenomena that
are partly or completely masked at 1 g, has been under way for the past several
decades, and the committee recommends that this program be continued.
Among the topics that this program should cover are surface tension-driven flows
and capillary effects; multiphase heat transfer and fluid flows; and the behavior of
complex fluids such as colloids, granular media, and foams, whose dynamical
behavior can be strongly influenced by gravity. In addition, some emphasis
should be placed on microgravity transport processes that will play a role in the
design and operation of future space facilities, such as low Reynolds number
convection, boiling, evaporation or condensation, or methodologies that may be
required for materials processing, such as fluidized-bed hydrogenation,
electrowinning, or vapor-phase pyrolysis.
It is further recommended that steps be taken to enhance interaction
between the research community and the engineering community that will design,
build, and operate future space facilities to help prioritize studies related to
"enabling technologies." Similarly, steps should be taken to foster interdisciplinary
research involving fluid dynamics and transport processes in other areas such as
the life sciences, materials processing, and combustion.
REFERENCES
1. Ostrach, S. 1982. Low-gravity fluid flows. Ann. Rev. Fluid Mech.,
14:313-345.
2. Ostrach, S. 1988. Industrial processes influenced by gravity. NASA
Contractor Report 182140.
3. Ostrach, S. 1974. Skylab science demonstrations. Proceedings of the
ESRO Symposium on Processing and Manufacturing in Space.
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Microgravity Research Opportunities for the 1990s: Chapter 3
4. European Space Agency. 1985. Assessment of the Results of Fluid
Science and Materials Science Experiments Conducted During the Spacelab-1
Mission.
5. Natur Wissenschaften. 1986. D-l Erste Ergebnisse der deutschen
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Driven Convection Experiment: Preliminary analysis of the data. In International
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Tension Driven Convection Experiment. AIAA Paper 93-4312. Presented at AIAA
Space Programs and Technologies Conference, Huntsville, Ala.
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convection experiment. 1993 ASME Winter Annual Meeting, New Orleans, La.
10. Kamotani, Y., S. Ostrach, and A. Pline. 1994a. Some results from the
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AIAA Aerospace Sciences Meeting, Reno, Nev.
11. Kamotani, Y., S. Ostrach, and A. Pline. 1994b. Some velocity field
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data taken in Surface Tension Driven Convection Experiment in microgravity.
Physics of Fluids, 6:11.
13. Kamotani, Y., S. Ostrach, and A. Pline. 1994d. Summary of results
from the Surface Tension Driven Convection Experiment in microgravity. 45th
International Astronautical Congress, Jerusalem, Israel, October 9-14.
14. Kamotani, Y., S. Ostrach, and A. Pline. 1995. A thermocapillary flow
experiment in microgravity. Journal of Heat Transfer, in press.
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USML-1 and USMP-1 with the Microgravity Measurement Group. NASA
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16. Space Systems and Technology Advisory Committee. 1991.
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Microgravity Research Opportunities for the 1990s: Chapter 3
Integrated Technology Plan for the Civil Space Program. Preliminary draft.
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