Scientific and Technical Aerospace Reports Volume 39 April 6, 2001

particle size was studied. In a density-matched suspension, the inertia of the particle and that of the fluid are indistinguishable;
hence, the results for the scaling of the stresses may not be applicable to situations in which the densities differ. Additional questions
surround the normal stress, which was measured by using a flexible membrane for the inner stationary cylinder that would
deflect in response to the applied pressure. The system was insensitive to small pressure changes, and the membrane deflection
may have altered the gap size and the mean solid concentration. The current work at Caltech focuses on experiments and simulations
for conditions similar to those originally examined by Bagnold. The low-gravity environment offers the opportunity to reexamine
Bagnold’s experiments for particles of varying densities. These experiments should provide an opportunity to separate the
effects of the inertia of both the fluid and solid phases on the shear and normal stresses. We plan to conduct experiments using
different particle sizes and density, varying particle concentrations and shear rates that would extend the Bagnold number well
into the grain inertia regime. In addition, the work involves a computational component that uses a combination of smooth-particle
hydrodynamics and the discrete element method to compute liquid-solid flows. The experimental apparatus will consist of a coaxial
shear cell with a rotating outer cylinder. The inner cylinder will be mounted on a friction-free air bearing and constrained from
rotation by a load cell to measure the shear force. For the flight instrument, pressure sensors will be inserted in the wall of the inner
cylinder to simultaneously measure the normal force. The granular material and the fluid will be contained in the annular region
of the experiment. We have a prototype shear cell already constructed that we are using as a basis for the design of the flight instrument.
In addition to the experimental measurements, the project also involves a computational phase that uses a combination of
the smoothed particle hydrodynamics technique (SPH) and the discrete element method (DEM) to model flows containing a viscous
fluid and solid macroscopic particles. The two-dimensional numerical simulations are validated by comparing with experimental
measurements for wake size, drag coefficient and heat transfer for flow past a circular cylinder at Reynolds numbers up
to approximately 100. The comparisons demonstrate that the technique can be successfully used for incompressible flows at
Reynolds numbers up to approximately 100. The central focus of the work, however, is in computing flows of liquid-solid mixtures.
Hence, the simulations were run for neutrally buoyant particles contained between two shearing plates for different solid
fractions, fluid viscosities and shear rates. The simulations involved up to 18 solid particles sheared between two plates separated
by a distance of approximately 8 particle diameters. The no-slip boundary condition was satisfied on the surface of the particles
and the bounding walls. The tangential force resulting from the presence of particles shows an increasing dependence on the shear
rate, from a linear regime for macro-viscous flows to a square dependence for grain inertia flows. The normal force showed considerable
variation with time, which is not fully understood and is part ongoing numerical simulations. In addition, the simulations
indicated stability problems at higher shear rates; the instability has also been observed in other SPH simulations. Hence, the focus
of the current work involves the effect of the smoothing function and other features of the simulation technique on the flow parameters.
Author (revised)
Granular Materials; Hydrodynamics; Interstitials; Viscosity
20010024978 Rennes Univ., France
Sheet Flows, Avalanches, and Dune Migration on Earth and Mars
Jenkins, J., Cornell Univ., USA; Bideau, D., Rennes Univ., France; Hanes, D., Florida Univ., USA; Proceedings of the Fifth Microgravity
Fluid Physics and Transport Phenomena Conference; December 2000, pp. 1355-1364; In English; See also 20010024890;
No Copyright; Avail: CASI; A02, Hardcopy; A10, Microfiche
We have begun a study of sheet flows and avalanches of granular materials in terrestrial and Martian environments. Sheet
flows are relatively thin, highly concentrated regions of grains flowing near the ground under the influence of a strong turbulent
wind. In them, grains are suspended by inter-particle friction, inter-particle collisions, and/or the velocity fluctuations of the turbulent
gas. Avalanches are flows of dry, cohesionless granular materials that are driven by gravity down inclines against the frictional
and collisional resistance of the grains. The study will employ and extend existing theories involving particle-particle and gas-particle
interactions to apply to the evolution of a typical terrestrial sand dune during a sandstorm. Experiments will explore the generation
of airborne particles in collisions, the flow of air and particles over particle beds, and the dependence of the angle of repose
on particle packing and friction. We will also investigate the influence of particle size, reduced gravity, gas density, and gas viscosity
in order to extend our results to the Martian environment. The focus will be on modeling mechanisms for the transport of granular
materials by the wind and gravity. The anticipated result of this activity will be continuum equations for the balance of mass,
momentum, and energy for both the particle phase and the gas phase that may be employed in numerical codes to study unsteady,
inhomogeneous flows. In the study of suspension and transport by the wind, an effort will be made to understand the transition
between successive regimes of flow that are observed as the strength of the wind increases. In particular, the means by which
momentum and energy are exchanged between particles and between particles and the gas will be described and quantified. Also,
the interface between the bed and the flow will be studied in experiments and computer simulations in order to understand better
the balance of momentum and energy there. This will lead to improved boundary conditions for the continuum equations. In the
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