Engineering topics

Nadarajah, Arunan

Publications and source records attributed to Nadarajah, Arunan.

22 records · Page 2

NASA NTRS1993

Temperature dependence of diffusivities, preliminary definition phase

During the six months definition phase of the instrument development program, research personnel at the Center for Microgravity and Materials Research of the University of Alabama in Huntsville (UAH) were to furnish all of the necessary labor, services, materials, and facilities necessary to provide science requirement definition, initiate hardware development activities, requirements and timetable for integration and experimental accommodation of the GAS payload into the Shuttle cargo bay and an updated ground-based research flight program proposal consistent with the NRA selection letter. These activities were to be accomplished in parallel and consistent with the necessary research and development work toward the accomplishment of the overall objectives of the selected proposal.

Rosenberger, Franz↗

NASA NTRS1992

Effects of buoyancy-driven flow and thermal boundary conditions on physical vapor transport

A 2D numerical model was developed in order to ascertain if reduced gravity conditions are beneficial to physical vapor transport (PVT) and to determine its tolerance limits to residual accelerations. This was solved using the PHOENICS finite-volume code. Reduction of gravitational accelerations to less than 0.1 g0 was found to be sufficient to suppress buoyancy-driven convection to an extent that diffusion was the dominant transport mode, whence a greater uniformity in the growth rate could be obtained. It is shown that a uniform temperature gradient on the ampoule walls causes the vapor to be supersaturated throughout the ampoule, potentially resulting in undesirable nucleation at the walls. A 'hump' in the wall temperature profile can be used to avoid this. The prevailing transport conditions determine the size of the hump needed.

Nadarajah, Arunan↗

NASA NTRS1990

Process modelling for Space Station experiments

Examined here is the sensitivity of a variety of space experiments to residual accelerations. In all the cases discussed the sensitivity is related to the dynamic response of a fluid. In some cases the sensitivity can be defined by the magnitude of the response of the velocity field. This response may involve motion of the fluid associated with internal density gradients, or the motion of a free liquid surface. For fluids with internal density gradients, the type of acceleration to which the experiment is sensitive will depend on whether buoyancy driven convection must be small in comparison to other types of fluid motion, or fluid motion must be suppressed or eliminated. In the latter case, the experiments are sensitive to steady and low frequency accelerations. For experiments such as the directional solidification of melts with two or more components, determination of the velocity response alone is insufficient to assess the sensitivity. The effect of the velocity on the composition and temperature field must be considered, particularly in the vicinity of the melt-crystal interface. As far as the response to transient disturbances is concerned, the sensitivity is determined by both the magnitude and frequency of the acceleration and the characteristic momentum and solute diffusion times. The microgravity environment, a numerical analysis of low gravity tolerance of the Bridgman-Stockbarger technique, and modeling crystal growth by physical vapor transport in closed ampoules are discussed.

Alexander, J. Iwan D.↗

NASA NTRS1990

Modelling the solution growth of TGS crystals in low gravity

The experimental growth of triglycine sulfate (TGS) crystals from aqueous solution is modeled here in two dimensions using the PHOENICS finite volume code. Simulations are carried out for steady, impulsive, and periodic accelerations in order to determine tolerable acceleration levels. Scaling arguments are used to estimate the times required for thermal and solutal variations from the initial equilibrium state to be diffusively transported throughout the system, and to obtain order of magnitude information on the relative magnitudes of diffusive and convective transport. The computed concentration fields reflect the features of the concentration distributions found experimentally during experiments conducted aboard Spacelab 3 in 1985.

Nadarajah, Arunan↗