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Robertson, S. J.

Publications and source records attributed to Robertson, S. J..

At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

Effect of enclosure shape on natural convection velocities in microgravity

Natural convection velocities within two-dimensional enclosures of various shapes are estimated with reasonable accuracy, for Rayleigh numbers up to at least 5000, by considering the area to be equivalent to a circle of equal area. For the class of figures including the square, upright half-circle, and upright 2 x 1 aspect ratio rectangle, excellent agreement is obtained by considering the temperature difference across the maximum horizontal dimension to be equal to that across the equal area circle horizontal diameter. The agreement of two upright oblong shapes indicates probable agreement for any similar upright oblong shape of roughly the same aspect ratio. The horizontal oblong shapes have natural convection velocities approximately one-half that estimated based on the equal area circle, with the temperature difference across the maximum horizontal dimension taken to be equal to that across the equal area circle diameter.

Robertson, S. J.↗
NASA NTRS1985

Numerical analysis of flow non-uniformity in the hot gas manifold of the Space Shuttle main engine

Three-dimensional viscous flow in a conceptual hot gas manifold (HGM) for the Space Shuttle Main Engine High Pressure Fuel Turbopump (SSME HPFTP) was numerically analyzed. A finite difference scheme was used to solve the Navier-Stokes equations. The exact geometry of the SSME HGM was modeled using boundary fitted curvilinear coordinates and the General Interpolants Method (GIM) code. Slight compressibility of the subsonic flow was modeled using a linearized equation of state with artificial compressibility. A time relaxation method was used to obtain a steady state solution. The feasibility and potential usefulness of computational methods in assisting the design of SSME components which involves the flow of fluids within complex geometrical shapes is demonstrated.

Thoenes, J.↗
NASA NTRS1984

Manufacturing in space: Fluid dynamics numerical analysis

Thermocapillary convection in an air-jet Marangoni flow suppressing technique was numerically simulated for flow under various conditions of fluid properties, gravity, and temperature gradient. Results presented include computer generated plots of streamlines, velocities, and temperatures throughout the contained fluid flow, and plots of Marangoni flow velocities over the free surface.

Robertson, S. J.↗
NASA NTRS1982

Effect of enclosure shape on natural convection velocities

A numerical analysis was performed to compare natural convection velocities in two dimensional enclosures of various shape. The following shapes were investigated: circle, square, horizontal and upright 2 x 1 aspect ratio rectangles, horizontal and upright half circles, diamond. In all cases, the length scale in the various dimensionless parameters, such as Rayleigh number, is defined as the diameter of the equal area circle. Natural convection velocities were calculated for Rayleigh numbers of 1000 and 5000 with the temperature difference taken to be across (1) the maximum horizontal dimension, (2) the median horizontal line (line through centroid) and (3) the horizontal distance such that the temperature gradient is the same for shapes of equal area. For the class of shapes including the square, upright half circle and upright rectangle, the computed velocities were found to agree very closely with that of the equal area circle when the temperature difference is taken to be across the maximum horizontal dimension (condition (a)). The velocities for the horizontal rectangle and half circle were found to be approximately one half that of the equal area circle for the same condition. Better overall agreement among all shapes was obtained by setting the temperature difference across a distance such that the temperature gradients were equal for shapes of equal area.

Robertson, S. J.↗
NASA NTRS1982

Manufacturing in space: Fluid dynamics numerical analysis

Numerical computations were performed for natural convection in circular enclosures under various conditions of acceleration. It was found that subcritical acceleration vectors applied in the direction of the temperature gradient will lead to an eventual state of rest regardless of the initial state of motion. Supercritical acceleration vectors will lead to the same steady state condition of motion regardless of the initial state of motion. Convection velocities were computed for acceleration vectors at various angles of the initial temperature gradient. The results for Rayleigh numbers of 1000 or less were found to closely follow Weinbaum's first order theory. Higher Rayleigh number results were shown to depart significantly from the first order theory. Supercritical behavior was confirmed for Rayleigh numbers greater than the known supercritical value of 9216. Response times were determined to provide an indication of the time required to change states of motion for the various cases considered.

Robertson, S. J.↗
NASA NTRS1982

Effect of Rayleigh accelerations applied to an initially moving fluid

The General Interpolant Method computer code was used to analyze two-dimensional unsteady thermal convection in circular cylinders under variable low-g conditions associated with space flight. When an acceleration vector was applied parallel to the thermal gradient, in the case of a fluid at rest, no convection resulted for the stable direction, and an instability led to Rayleigh convection for the opposite direction. However, when the acceleration had a component orthogonal to the gradient, convection resulted at any Rayleigh number. The effect on convection of both types of acceleration, applied concurrently or sequentially, was investigated, including the case when the resultant vector varied in direction with time. An analysis of experimental results shows that for space flight conditions, the Rayleigh accelerations induce significant, but not dominating, changes in the established convection even when the Rayleigh number is less than critical.

Dressler, R. F.↗
NASA NTRS1981

Manufacturing in space: Fluid dynamics numerical analysis

Natural convection in a spherical container with cooling at the center was numerically simulated using the Lockheed-developed General Interpolants Method (GIM) numerical fluid dynamic computer program. The numerical analysis was simplified by assuming axisymmetric flow in the spherical container, with the symmetry axis being a sphere diagonal parallel to the gravity vector. This axisymmetric spherical geometry was intended as an idealization of the proposed Lal/Kroes growing experiments to be performed on board Spacelab. Results were obtained for a range of Rayleigh numbers from 25 to 10,000. For a temperature difference of 10 C from the cooling sting at the center to the container surface, and a gravitional loading of 0.000001 g a computed maximum fluid velocity of about 2.4 x 0.00001 cm/sec was reached after about 250 sec. The computed velocities were found to be approximately proportional to the Rayleigh number over the range of Rayleigh numbers investigated.

Robertson, S. J.↗
NASA NTRS1981

Numerical simulation of natural convection in a spherical container due to cooling at the center (idealization of the Lal/Kroes experiment)

Natural convection in a spherical container with cooling at the center was numerically simulated using a numerical fluid dynamics computer program. The numerical analysis was simplified by assuming axisymmetric flow in the spherical container, with the symmetry axis being a sphere diagonal parallel to the gravity vector. This axisymmetric spherical geometry was intended as an idealization of the proposed Lal/Kroes crystal growing experiment to be performed on Spacelab. Results were obtained for a range of Rayleigh numbers from 25 to 10,000. The computed velocities were found to be approximately proportional to the Rayleigh number over the range of Rayleigh numbers investigated.

Robertson, S. J.↗
NASA NTRS1980

Numerical analysis of natural convection in two-dimensional square and circular containers in low gravity

A numerical study of natural convection in circular cylinder and square enclosures shows that the analytic low Rayleigh number theory of previous investigators is valid for Rayleigh numbers up to 1000. For a Rayleigh number of 5000, steady state values of maximum fluid velocity differ by 20 percent. This deviation between analytic theory and numerical results increases for higher Rayleigh numbers. In addition, the low Rayleigh number theory is shown to be valid for higher Rayleigh numbers for a portion of the transient phase before significant deviation becomes apparent. It is also shown that square shaped experimental configurations may be analytically approximated with good accuracy by circular cylinders of equal cross sectional area for the prediction of convection velocities and flow patterns at low Rayleigh number.

Robertson, S. J.↗
NASA NTRS1980

The direct observation of unidirectional solidification as a function of gravity level

A saturated solution of NH4 Cl-H2O was solidified unidirectionally on earth at 1 g and in a suborbital rocket at 0.001 g. In the 1 g experiment, extensive fluid flow was apparent, creating plumes which carry numerous crystallites vertically above the interface in a manner associated with freckling in castings. During growth the interface grew into the liquid at the same rate as the individual dendrites. When solidified in low g, the interface grew at a slower rate than the individual dendrites. No plumes of liquid carrying crystals were seen, but several crystals did appear and grow ahead of the interface. Growth rates, dendrite morphologies and preferred crystallographic orientations are compared for the two experiment conditions. Results indicate that solidification in low gravity produces more uniform dendrite arm spacings, and tends to be controlled by diffusion and crystallographic orientation.

Johnston, M. H.↗
NASA NTRS1979

Thermal analyses of a materials processing furnace being developed for use with heat pipes

A special materials processing furnace is being developed for the forthcoming Spacelab missions to study the solidification under closely controlled conditions of various sample materials in the absence of gravity. The samples are to be rod shaped and subjected to both heating and cooling simultaneously. The thermal model is based on a developed Thermal Analyzer computer program. The model was developed to be very general to enable the simulation of variations in the furnace design and, hence, serve as an aid in finalizing the design. The thermal model is described and a user's guide given. Some preliminary results obtained in testing the model are also given.

Mcanally, J. V.↗
NASA NTRS1979

Thermal and fluid flow analyses of sounding rocket experiment 74-21/2R and 74-21/3R

The thermal convective velocities during the 4-21/2R and 74-21/3R flight tests were calculated. The 74-21/2R and 74-21/3R ground tests were analyzed using a convection formula. Thermal convection velocities of 0.1 to 0.5 cm/sec were calculated for the 74-21/2R and 74-21/3R ground tests which were two to four orders of magnitude greater than for the flight tests. For the 74-21/2R flight test, solutal effects do not produce any convective motion. For the 74-21/3R flight test closed form convection equations estimated the solutal convective velocities on the order of 1/0.003 cm/sec.

Robertson, S. J.↗
NASA NTRS1977

Bhatnagar-Gross-Krook model solution of back-scattering of outgas flow from spherical spacecraft

The BGK kinetic model for a binary mixture of gases was used to predict the backscattering return flux of molecules outgassing from a spherical spacecraft. For simplicity, the outgassing molecules were considered to be a single species, with the ambient atmospheric molecules being the second species in the binary mixture. Two basic collision mechanisms were considered: 1) collisions involving only outgas molecules (self-scattering); and 2) collisions between outgas molecules and molecules in the ambient atmosphere (ambient-scattering). Results were obtained in the form of simple equations relating outgas return flux to parameters based on outgas characteristics, spacecraft size, and the characteristics of the ambient flow.

Robertson, S. J.↗
NASA NTRS1976

Space shuttle contamination due to backflow from control motor exhaust

Spacecraft contamination of the space shuttle orbiter and accompanying Spacelab payloads is studied. The scattering of molecules from the vernier engines and flash evaporator nozzle after impingement on the orbiter wing surfaces, and the backflow of molecules out of the flash evaporator nozzle plume flow field due to intermolecular collisions in the plume are the problems discussed. A method was formulated for dealing with these problems, and detailed results are given.

Robertson, S. J.↗
NASA NTRS1976

Spacecraft self-contamination due to back-scattering of outgas products

The back-scattering of outgas contamination near an orbiting spacecraft due to intermolecular collisions was analyzed. Analytical tools were developed for making reasonably accurate quantitative estimates of the outgas contamination return flux, given a knowledge of the pertinent spacecraft and orbit conditions. Two basic collision mechanisms were considered: (1) collisions involving only outgas molecules (self-scattering) and (2) collisions between outgas molecules and molecules in the ambient atmosphere (ambient-scattering). For simplicity, the geometry was idealized to a uniformly outgassing sphere and to a disk oriented normal to the freestream. The method of solution involved an integration of an approximation of the Boltzmann kinetic equation known as the BGK (or Krook) model equation. Results were obtained in the form of simple equations relating outgas return flux to spacecraft and orbit parameters. Results were compared with previous analyses based on more simplistic models of the collision processes.

Robertson, S. J.↗
NASA NTRS1973

Research study on materials processing in space experiment M512

A study program was conducted to clarify the role of gravity in the fluid mechanics of certain molten metal processes of potential significance to manufacturing in space. In particular, analyses were conducted of the M551 Metals Melting Experiment and the M553 Sphere Forming Experiment to be conducted in the M512 Facility onboard Skylab. The M551 experiment consisted of a study of electron beam welding of various metals, and the M553 experiment studied the formation of molten metal spheres by free-floating in a near zero-gravity environment. The analyses of these experiments and a comparison with ground-based and KC135 experimental results are presented.

Brashears, M. R.↗
NASA NTRS1973

Research study on materials processing in space experiment M512

A study program was conducted to clarify the role of gravity in the fluid mechanics of certain molten metal processes of potential significance to manufacturing in space. In particular, analyses were conducted of the M551 Metals Melting Experiment and the M553 Sphere Forming Experiment to be conducted in the M512 Facility onboard Skylab. The M551 experiment consisted of a study of electron beam welding of various metals, and the M553 experiment studied the formation of molten metal spheres by free-floating in a near zero-gravity environment. The analyses of these experiments and a comparison with ground-based and KC135 experimental results are presented.

Brashears, M. R.↗