(this tutorial contains two large mpeg or quicktime format video clips)
Glossary: pH,
buoyancy-driven convection
Introduction
To most of us, the word "fluid" brings to mind images of water and other liquids. But to a scientist, the word fluid means much more. A fluid is any liquid or gaseous material that flows and, in gravity, assumes the shape of the container it is in. Gases fill the whole container; liquids on Earth fill only the lower part of the container equal to the volume of the liquid.
Scientists are interested in fluids for a variety of reasons. Fluids are an important part of life processes, from the blood in our veins and arteries to the oxygen in the air. The properties of fluids make plumbing, automobiles, and even fluorescent lighting possible. Fluid mechanics describes many processes that occur within the human body and also explains the flow of sap through plants. The preparation of materials often involves a fluid state that ultimately has a strong impact on the characteristics of the final product.
Scientists gain increased insight into the properties and behavior of fluids by studying their movement or flow, the processes that occur within fluids, and the transformation between the different states of a fluid (liquid and gas) and the solid state. Studying these phenomena in microgravity allows the scientists to examine processes and conditions impossible to study when influenced by Earth's gravity. The knowledge gained can be used to improve fluid handling, materials processing, and many other areas in which fluids play a role. This knowledge can be applied not only on Earth, but also in space.
Convection and Low-G
Gravity plays a major role in determining the motion of fluids on earth.
Small differences in density cause buoyancy-driven
convection. This type of convection occurs in many everyday situations.
Warm, less dense air rises, while cold, more dense air sinks. Many important
industrial and scientific processes involve steps in which components with
different densities are combined.
Performing experiments in reduced gravity enables scientists to study
the ways fluids behave in the absence of this type of buoyancy-driven convection.
Experiments on the Space Shuttle represent only one way to study fluid flows
in reduced gravity. NASA also supports reduced gravity research in special
aircraft. One such aircraft is the KC-135 (in an archive photos above, click
on images for a full-screen jpeg) operated at the Johnson Space Center in
Houston, Texas. This plane is used for both scientific research and astronaut
training. When flown in a special parabolic trajectory, the plane provides
approximately 20 seconds of reduced gravity. Under good conditions, the
plane can provide acceleration or gravity levels one hundred times smaller
than the 1g we feel on the ground. During the low-gravity portion of the
parabola (picture at right, above), the astronauts can float in order to
train for operations on the Space Shuttle and Space Station and scientists
can perform their experiments.
The two video clips (click on the links below) are from flow visualization
experiments. Researchers performed one in reduced gravity on the KC-135
and the other under the same conditions in 1g at the Space Sciences Laboratory
at NASA's Marshall Space Flight Center in Huntsville, Alabama. You will
see a mixing chamber loaded with green solution which is less dense than
the red fluid which will be injected into the chamber. The fluid colors
result from a special dye which shows the pH
of each solution. The red solution is diluted household vinegar. The green
solution is a water-based solution. Different colors show the various pHs
of solutions as they combine in the chamber and indicate the patterns of
fluid motion. The still photographs below are selected frames from the microgravity
movie (experiment performed on board KC-135) on the left, and the 1-G movie
(experiment performed at Marshall) on the right.
| Fluid Mixing in Low-Gravity (KC-135) | Fluid Mixing in 1-G (on Earth) |
| mpeg format, 1.8MBytes | mpeg format, 2 MBytes |
| quicktime movie, 3.3 MBytes | quicktime movie, 3.3 MBytes |
The systematic study of fluids under microgravity conditions holds the promise of refining existing theory or allowing the formulation of new theories to describe fluid dynamics and transport phenomena. Such research promises to improve the understanding of those aspects of fluid dynamics and transport phenomena whose fundamental behavior is limited or affected by the influence of gravity. Several research areas contain promising opportunities for significant advancements through low-gravity experiments. These research areas include: capillary phenomena, multiphase flows and heat transfer, diffusive transport, magneto/electrohydrodynamics, colloids, and solid-fluid interface dynamics.
"Capillarity" describes the relative attraction of a fluid for a solid surface compared with its self-attraction. A typical example of capillary action is the rise of sap in plants. Research in capillary phenomena is a particularly fertile area for low-gravity experiments because of the increased importance of capillary forces as the effects of gravity are reduced. Such circumstances are always encountered in multiphase fluid systems where there is a liquid-liquid, liquid-vapor, or liquid-solid interface. Surface tension-driven flows also become increasingly important as the effects of gravity are reduced and can dramatically affect other phenomena such as the interactions and coalescence of drops and bubbles.
Capillary forces also play a significant role in multiphase flow and heat transfer, particularly under reduced-gravity conditions. It is important to be able to accurately predict the rate at which heat will be transported between two-phase mixtures and solid surfaces-for example, as a liquid and gas flow through a pipe. Of course, it is equally important to be able to predict the heat exchange between the two different fluid phases. Furthermore, when the rate of transferring heat to or from the multiphase fluid system reaches a sufficient level, the liquids or gases present may change phase. That is, the liquid may boil (heat entering the liquid), the liquid may freeze (heat leaving the liquid), or the gas may condense (heat leaving the gas). While the phase change processes of melting and solidification under reduced-gravity conditions have been studied extensively-due to their importance in materials processing-similar progress has not been made in understanding the process of boiling and condensation. Although these processes are broadly affected by gravity, improvements in the fundamental understanding of such effects have been hindered by the lack of experimental data.
Diffusion is a mechanism by which atoms and molecules move through solids, liquids, and gases. The constituent atoms and molecules spread through the medium (in this case, liquids and gases) due primarily to differences in concentration, though a difference in temperature can be an important secondary effect in microgravity. Much of the important research in this area involves studies where several types of diffusion occur simultaneously. The significant reduction in buoyancy-driven convection that occurs in a free-fall orbit may provide more accurate measurements and insights into these complicated transport processes.
The research areas of magnetohydrodynamics and electrohydrodynamics involve the study of the effects of magnetic and electric fields on mass transport (atoms, molecules, and particles) in fluids. Low velocity fluid flows, such as those found in poor electrical conductors in a magnetic field, are particularly interesting. The most promising low-gravity research in magneto/electrohydrodynamics deals with the study of effects normally obscured by buoyancy-driven convection. Under normal gravity conditions, buoyancy-driven convection can be caused by the fluid becoming heated due to its electrical resistance as it interacts with electric and magnetic fields. The heating of a material caused by the flow of electric current through it is known as Joule heating. Studies in space may improve techniques for manipulating multiphase systems such as those containing fluid globules and separation processes such as electrophoresis, which uses applied electric fields to separate biological materials.
Colloids are suspensions of finely divided solids or liquids in gaseous or liquid fluids. Colloidal dispersions of liquids in gases are commonly called aerosols. Smoke is an example of fine solid particles dispersed in gases. Gels are colloidal mixtures of liquids and solids where the solids have linked together to form a continuous network. Research interest in the colloids area includes the study of formation and growth phenomena during phase transitions-e.g., when liquids change to solids. Research in microgravity may allow measurement of large scale aggregation or clustering phenomena without the complication of the different sedimentation rates due to size and particle distortion caused by settling and fluid flows that occur under normal gravity.
A better understanding of solid-fluid interface dynamics, how the boundary
between a solid and a fluid acquires and maintains its shape, can contribute
to improved materials processing applications. The morphological (shape)
stability of an advancing solid-fluid interface is a key problem in such
materials processing activities as the growth of homogeneous single crystals.
Experiments in low-gravity, with significant reductions in buoyancy-driven
convection, could allow mass transport in the fluid phase by diffusion only.
Such conditions are particularly attractive for testing existing theories
for processes and for providing unique data to advance theories for chemical
systems where the interface interactions strongly depend on direction and
shape.
last updated March 25, 1997
Authors/Contacts for more information: Dr.
Jan Rogers, Barbara
Facemire, Dr. Don Frazier
Curator: Bryan Walls
NASA Official: John M. Horack