Scientific and Technical Aerospace Reports Volume 39 April 6, 2001

colliding drops. The viscous resistivity tensor can be used together with a description of the van der Waals attractions between
the particles to specify the interparticle forces. We must then solve Newton’s laws of motion for the dynamic encounter of pairs
of coagulating drops. The fine particles under consideration have radii much smaller than the Kolmogorov length scale. Exploiting
this fact, we will conduct numerical simulations in which pairs of particles are subjected to a temporally fluctuating linear flow
field, whose statistics are chosen to reproduce results from previous direct numerical simulations for the Lagrangian correlation
functions of the strain and rotation rates in isotropic turbulence. Ensemble and time averages of these stochastic flow simulations
will provide the rate constant for turbulent coagulation. In addition to simulations corresponding to the ideal case of turbulent
coagulation of non-Brownian particles in microgravity, we will also consider mixed turbulent/sedimentation and turbulent/Brownian
simulations.
Author (revised)
Monte Carlo Method; Aerosols; Coagulation; Microgravity; Turbulence; Accumulations
20010024980 Cincinnati Univ., OH USA
Microscopic Flow Visualization in Demixing Fluids During Polymeric Membrane Formation in Low-G
Krantz, W. B., Cincinnati Univ., USA; Greenberg, A. R., Colorado Univ., USA; Todd, P., Space Hardware Optimization Technology,
Inc., USA; Proceedings of the Fifth Microgravity Fluid Physics and Transport Phenomena Conference; December 2000, pp.
1368-1377; In English; See also 20010024890; No Copyright; Avail: CASI; A02, Hardcopy; A10, Microfiche
Macrovoids (MVs) are large (10-50 micron) pores often found in polymeric membranes prepared via phase-inversion techniques.
They are generally considered undesirable since they can adversely affect the performance of polymeric membranes.
However, MVs can be useful in certain thin-film applications in which vapor transmission is necessary or for use as reservoirs
for enzymes or liquid membrane material. The goal of this research is to determine the mechanism of formation for MVs in order
to control their formation. The hypothesis to be tested in this research is that MV growth is determined by an interplay of solutocapillary
convection, viscous drag, and buoyancy forces. The results of preliminary ground-based experiments employing flow-visualization
videomicroscopy to study polymeric membrane formation via dry-casting (evaporative casting) are presented. These
experiments establish the viability of this real-time noninvasive technique for studying the MV formation process. Moreover, they
indicate that surface-active agents can significantly influence MV formation during the dry-casting process. This flow-visualization
videomicroscopy technique will be adapted to studying polymeric membrane formation via the wet-cast (precipitation bath)
process in both ground-based as well as KC-135 flight experiments in order to assess the role of both buoyancy as well as surfacetension
forces on MV formation.
Author (revised)
Flow Visualization; Membranes; Photomicrography; Voids; Diffusion; Convection
20010024981 Florida Univ., Chemical Engineering Dept., Gainesville, FL USA
Microgravity Driven Instabilities in Gas-Fluidized Beds
Ladd, Anthony J. C., Florida Univ., USA; Weitz, David A., Pennsylvania Univ., USA; Proceedings of the Fifth Microgravity Fluid
Physics and Transport Phenomena Conference; December 2000, pp. 1378-1379; In English; See also 20010024890; No Copyright;
Abstract Only; Available from CASI only as part of the entire parent document
A dense pack of solid particles can be fluidized by an upward flow of liquid, which counterbalances the gravitational force
on the particles. The resulting suspension is stable for sufficiently small fluid velocities, although the particle dynamics are complex
and still not fully understood. A packed particle bed can also be fluidized by a gas flow, but in this case a stable suspension
of moving particles cannot be produced in earth’s gravitational field. As the gas flow is increased, a stable ”expanded” bed can
be produced that has a lower solids concentration than the packed bed, but recent Diffusing Wave Spectroscopy (DWS) experiments
have shown that the particles are stationary. At slightly higher gas flows, necessary to set the particles in motion, the bed
is immediately unstable, with bubbles (large regions essentially void of particles) transporting a significant fraction of the gas
through the solid phase. A kinetic theory of gas-fluidized beds predicts that the bed is always unstable to particle inertia, which
is several orders of magnitude larger than fluid inertia in this case. In order to study the onset of this particle-induced instability,
the minimum fluidization velocity must be reduced by several orders of magnitude; we believe that this can be most successfully
accomplished in a reduced gravity environment. These microgravity experiments will offer the first opportunity to experimentally
study the effects of low-Reynolds number hydrodynamic interactions in a suspension where there is no liquid, and the particle
motion is predominantly ballistic. We expect that in a microgravity environment we can explore the transition between stable and
unstable flow regimes in gas-fluidized beds, and contrast the behavior of particle inertia (gas-fluidized) and fluid inertia (liquid
fluidized) driven instabilities. In this work we plan to investigate the effects of small amounts of inertia on the stability of fluidized
beds. We will compare and contrast the stability conditions and flow patterns arising from increasing fluid inertia (Re > 1,St about
Re) and increasing particle-inertia (Re is less than 1,St is greater than 1). In particular we will discover if the progression of instabi-
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