Analysis of the low gravity tolerance of Bridgman-Stockbarger crystal growth: I. Steady and impulse accelerations

Analysis of the low gravity tolerance of Bridgman-Stockbarger crystal growth II. Transient and periodic accelerations

This study is concerned with the effects of g-jitter, the residual acceleration aboard spacecraft, on selected classes of materials processes. In particular, the anticipated acceleration environment aboard Space Station Freedom (SSF) and its potential effects are analyzed, but the topic is covered with a sufficient level of generality as to apply to other processes and to other vehicles as well. Some of the key findings of this study include: The present acceleration specifications for SSF are inadequate to assure a quality level low-g environment. The local g vector orientation is an extremely sensitive parameter for certain key processes, but can not be controlled to within the desired tolerance. Therefore, less emphasis should be placed upon achieving a tight control of SSF attitude, but more emphasis should be focused on reducing the overall level of the g-jitter magnitude. Melt-based crystal growth may not be successfully processed in the relatively noisy environment of a large inhabited space structure. Growth from vapor or from solution appears more favorable. A smaller space structure and/or a free flyer can provide better alternatives in terms of g-jitter considerations. A high priority (including budgetary) should be given to coordinated efforts among researchers, SSF designers, and equipment contractors, to develop practical experiment-specific sensitivity requirements. Combined focused numerical simulations and experiments with well-resolved acceleration measurements should be vigorously pursued for developing reliable experiment-specific sensitivity data. Appendices provide an extensive cross-referenced bibliography, a discussion of the merits offered by g-jitter analysis techniques, as well as definitions of relevant nondimensional quantities and a brief description of available accelerometry hardware.

/pdf/an-examination-of-anticipated-g-jitter-on-space-station-and-3wyumdwzal.pdf

An examination of anticipated g-jitter on Space Station and its effects on materials processes

Transient thermal convection in an enclosure induced simultaneously by gravity and vibration

In this work, a computational study is presented for the investigation of gravity modulation (g-jitter) effects in thermally driven cavity flows at terrestrial and microgravity environments. The two-dimensional, time-dependent Navier-Stokes equations are numerically integrated by a time-split method using direct matrix solvers. Computations at terrestrial gravity are utilized to assess the effects of adiabatic side-wall boundary conditions as well as the full nonlinearity of the governing equations on the sinusoidally forced Benard problem studied by Gresho and Sani. The low-g calculations focus on the establishment of critical frequency ranges and consider the effects of modulation direction and randomness. The applicability of linear analysis in the excitable frequency range at low g is also discussed.

Computation of convective flow with gravity modulation in rectangular cavities

This paper discusses thermal convection in an enclosure induced by spacecraft vibrations (#-jitter). Under normal circumstances (no maneuvers, no intentional spinning of the spacecraft) the ^-jitter generates predominantly oscillatory velocity and temperature fields with zero time-mean values. The ^-jitter can also generate secondary flows with nonzero mean, but they are of much smaller order. Some implications of the g- jitter on materials processing in space are discussed. NE of the primary advantages foreseen for processing of materials in space is the reduction of natural convection associated with the Earth's gravity. However, there have been some indications (e.g., Apollo 14 experiments 1'2) that spacecraft vibrations might cause appreciable thermal con- vection. Such convection may be important in fluids ex- periments and also affect the quality of crystals grown in space. Therefore, to study the effectiveness of spacecraft vibrations in generating fluid flows, the present work theoretically investigates a case in which a fluid-filled con- tainer with differentially heated walls is subjected to spacecraft vibrations. Implications of the results for space processing are discussed. In an attempt to evaluate the effects of g-jitter on fluid motions Spradley el al. 3 made a numerical analysis for various configurations. They considered three g-jitter profiles, sinusoidal, absolute sinusoidal and saw tooth, and compared the flowfields with that for constant g level. They found that if the g-jitter is decomposed into a time mean part and an oscillatory part, the mean part is more important than the oscillatory part in determining the flowfield and heat- transfer rate. A somewhat related work has been done by Forbes4 where the effect of sinusoidal vibrations on natural convective heat transfer in a rectangular enclosure is studied experimentally as well as numerically under 1-g conditions. The results indicate that the vibrations have very little effect on the heat transfer when the flow is laminar. The present study is more comprehensive than the work by Spradley et al.,3 and is meant to give a better physical insight into the g-jitter in space. Some of the material presented herein is taken from the work by Prasad and Ostrach.5 Formulation of the Problem g-Jitter Consider an experimental setup which is in a spaceship orbiting the Earth (Fig. 1). The coordinates (X0,Y0,Z0) are fixed at the center of the Earth with origin 0 (fixed frame of reference), and the coordinates (X'f Y',Z') are attached to the spacecraft with origin 0' at its center of mass. The coor-

Thermal convection in an enclosure due to vibrations aboard spacecraft

The paper discusses thermal convection in an enclosure induced by spacecraft vibrations (g-jitter). Under normal circumstances (no maneuvers, no intentional spinning of the spacecraft) the g-jitter generates predominantly oscillatory velocity and temperature fields with zero time-mean values. The g-jitter can also generate secondary flows with non-zero mean, but they are of much smaller order. Some implications of the g-jitter on materials processing in space are discussed

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Thermal convection in an enclosure due to vibrations aboard spacecraft

Thermal convection in an enclosure due to vibrations aboard spacecraft