Scientific and Technical Aerospace Reports Volume 39 April 6, 2001

equations of motion in geophysical fluid flows have lead to a dependency on asymptotic mathematical methods and computational
methods for theoretical understanding. Much of the data regarding geophysical flows has come from direct planetary observations.
These observations are made with little or no control over the many complicated factors influencing the flow. Although no
experimental system is perfect, an experiment never misses a nonlinear interaction and may be accessible to a wide range of parameters.
At the very least, a well-controlled experiment can provide important information to help validate numerical models. In our
proposed experimental system, a central-force is applied to, incompressible, compressible, and two-phase fluids in spherical
geometry. The central force is created using an AC electric field in a spherical capacitor filled with dielectric fluid between the
spheres. A fluid may be made compressible or two-phase by controlling a sample fluid’s density and ambient temperature. In other
words, our system is filled to the critical density and is placed near its liquid-gas critical point by controlling the ambient temperature.
Because of the temperature dependence of the dielectric constant and the temperature dependence of the fluid density, a buoyancy
force drives a flow when the inner sphere is heated to a higher temperature than the outer sphere. Our previous work was
limited in geophysical similarity because our system was not rotated and it is also subjected to the usual gravitation buoyancy force
from the earth that breaks the spherical symmetry. Our proposed new system will be rotated in micro-gravity. In this talk I will
survey our previous results and discuss our proposed experimental system. Real time holographic interferometry and shadowgraph
visualization are used to study convection in the fluid between two concentric spheres when two distinct buoyancy forces
are applied to the fluid. The heated inner sphere has a constant temperature that is greater than the outer sphere’s constant temperature
by (Delta)T and an AC voltage difference, (Delta)V, is applied between the inner and outer spheres. The resulting electric
field gradient in this spherical capacitor produces a central polarization force. The temperature dependence of the dielectric
constant results in a radial buoyancy force that is especially large near the inner sphere. The normal buoyancy is always present
and, within the parameter range explored in our experiment, always results in a large-scale axi-symmetric cell with the axis along
the vertical. We have found that this flow becomes unstable to toroidal or spiral rolls that travel vertically upward when (Delta)T
and (Delta)V are sufficiently high. These rolls start near the center sphere’s equator and travel vertically upward. The onset of
this instability depends on both the temperature difference (Delta)T and the voltage difference (Delta)V and these two quantities
appear to be related, within the parameter range accessible to our experimental system, by a power law. Measurements of the heat
transfer show that these traveling rolls increase the heat transfer at onset. Far above onset the heat transfer may actually decrease
with increasing (Delta)T. The traveling roll’s frequency increases with increasing (Delta)T near onset and with increasing
(Delta)V far above onset. These results have been interpreted in terms of the flow structure that includes a thermal boundary layerlike
behavior with a width that increases from the bottom pole to an unstable ”latitude” near the equator where the rolls appear.
by varying the ambient temperature of the capacitor filled with near-critical fluid, we have observed the effects that an applied
voltage has on the fluid during a parabolic flight. Although we had hoped to observe and verify that a spherical density gradient
is produced in the fluid, the actual conditions in flight did not allow this to happen. We did, however, see clear evidence of the
existence of the influence of this force on the fluid. Below the critical temperature the liquid was clearly attracted by the polarization
force while the gas was repelled. It shows the this repelling effect on a bubble that is pushed to the outer sphere. As can be
seen the electric field attracts the higher density liquid toward the center electrode where the electric flux is higher. In the 0(suub
g) to 2(sub g) part of a parabolic flight and above the critical temperature, a supercritical fluid forms a vertical density gradient
because of its very high compressibility. The resulting horizontal near-critical fluid layer is clearly modified by the electric field,
i.e., the higher density fluid is attracted toward the inner sphere. It shows that the initially horizontal layer is deformed by attracting
higher density fluid toward the inner sphere while the lower density fluid moves away producing the curved layer shown. These
experiments clearly suggest that geophysical analogies using near-critical fluids in micro-gravity are possible when this system
in heated and rotated.
Author (revised)
Fluid Flow; Spheres; Rotation; Real Time Operation; Temperature Dependence; Convection; Coriolis Effect
20010025020 Massachusetts Inst. of Tech., Cambridge, MA USA
The Evolution of Tensile Stresses in Uniaxial Elongational Flows of Dilute Polymer Solutions Subjected to a Known Preshear
History
Mckinley, Gareth H., Massachusetts Inst. of Tech., USA; Anna, Shelley L., Harvard Univ., USA; Proceedings of the Fifth Microgravity
Fluid Physics and Transport Phenomena Conference; December 2000, pp. 1869-1887; In English; See also 20010024890;
No Copyright; Avail: CASI; A03, Hardcopy; A10, Microfiche
Complicated deformation histories make the extensional behavior observed in many strong flows of polymeric liquids
extremely difficult to characterize precisely. The generation of a homogeneous uniaxial deformation flow in a microgravity environment
is the goal of the Extensional Rheology Experiment (ERE). In advance of obtaining flight data from this experiment, we
attempt to gain insight into the effect of complex deformation histories by imposing a known and controlled preshear history on
a liquid bridge prior to imposing a nearly ideal uniaxial elongational flow in a Filament Stretching device. The instrument can
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