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  • Other Sources  (65)
  • LUNAR AND PLANETARY EXPLORATION  (35)
  • Aerodynamics  (12)
  • SOLID-STATE PHYSICS
  • STRUCTURAL MECHANICS
  • 2015-2019  (4)
  • 2005-2009  (7)
  • 1985-1989  (54)
  • 1
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    The Critical Role of Computational Fluid Dynamics in Rotary-Wing Aerodynamics (1986)
    In:  Other Sources
    Details
    Publication Date: 2011-08-23
    Description: Helicopter rotor aerodynamics is basically the study of unsteady aerodynamic flows in a rotating and translating coordinate system. Current trends in this field are briefly reviewed by examining recent advances in lifting-surface theory, wake modeling, panel methods, and finite-difference models' Examples are used to illustrate selected current methods and some indications of promising future directions are highlighted.
    Keywords: Aerodynamics
    Type: Vertica (ISSN 0360-5450); Volume 11; Nos. 1, 2; 43-63
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  • 2
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    Morphological instabilities in directional solidification of a binary alloy - End effects (1989)
    In:  Other Sources
    Details
    Publication Date: 2011-08-19
    Description: This paper considers the unidirectional solidification of a binary alloy for which the liquid and solid phases are bounded by endwalls. In order to account for the transport of latent heat and solute, thermal and solutal boundary layers must be placed at the solidifying interface. Further, under the assumption of fixed temperature gradients, the presence of the endwalls leads to a velocity of solidification that decreases with time, and hence to an unsteady basic state having a planar interface. From a stability analysis of this state, a nonlinear long-wave evolution equation of Sivashinsky type is derived, with modified coefficients, that shows how the onset of cellular structure is delayed by the presence of endwalls.
    Keywords: SOLID-STATE PHYSICS
    Type: SIAM Journal on Applied Mathematics (ISSN 0036-1399); 49; 152-164
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  • 3
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    Directional solidification in an imperfect furnace (1987)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: The upward directional solidification of a binary alloy in which the rejected solute is more dense than the solvent is considered. The system is stably stratified both thermally and solutally. If heat is lost through the sidewalls, horizontal thermal gradients drive a steady double-diffusive convection. Solidification cells with small aspect ratios (width/length) A are considered, and asymptotic solutions to the steady double-diffusive system are obtained as A tends to 0 under the assumption heat losses are also small. When the thermal Grashof number is large, buoyancy boundary layers are at the sidewalls. The shape of the crystal-melt interface is determined by the coupled effects of local thermodynamic equilibrium, diffusion and convection.
    Keywords: SOLID-STATE PHYSICS
    Type: PCH/PhysicoChemical Hydrodynamics (ISSN 0191-9059); 9; 1-2,; 9-14
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  • 4
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    Ablation of Martian glaciers (1987)
    In:  CASI
    Details
    Publication Date: 2013-08-31
    Description: Glacier like landforms are observed in the fretted terrain of Mars in the latitude belts near + or - 42 deg. It was suggested that sublimation or accumulation-ablation rates could be estimated for these glaciers if their shapes were known. To this end, photoclinometric profiles were obtained of a number of these landforms. On the basis of analyses of these profiles, it was concluded that ice is chiefly ablating from these landforms that either are inactive rock-glaciers or have materials within them that are moving exceedingly slowly at this time. These conclusions are consistent with other geologic information. The analyses were performed using a two-dimensional model of an isothermal glacier.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Lunar and Planetary Inst., MECA Symposium on Mars: Evolution of its Climate and Atmosphere; p 78-80
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  • 5
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    Workshop on Structural Dynamics and Control Interaction of Flexible Structures (1987)
    In:  CASI
    Details
    Publication Date: 2013-08-31
    Description: The Hubble Space Telescope features the most exacting line of sight jitter requirement thus far imposed on a spacecraft pointing system. Consideration of the fine pointing requirements prompted an attempt to isolate the telescope from the low level vibration disturbances generated by the attitude control system reaction wheels. The primary goal was to provide isolation from axial component of wheel disturbance without compromising the control system bandwidth. A passive isolation system employing metal springs in parallel with viscous fluid dampers was designed, fabricated, and space qualified. Stiffness and damping characteristics are deterministic, controlled independently, and were demonstrated to remain constant over at least five orders of input disturbance magnitude. The damping remained purely viscous even at the data collection threshold of .16 x .000001 in input displacement, a level much lower than the anticipated Hubble Space Telescope disturbance amplitude. Vibration attenuation goals were obtained and ground test of the vehicle has demonstrated the isolators are transparent to the attitude control system.
    Keywords: STRUCTURAL MECHANICS
    Type: Structural Dynamics and Control Interaction of Flexible Structures; p 669-690
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  • 6
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    Orbital Resonances and Planetary Accretion in the Solar Nebula (1985)
    In:  CASI
    Details
    Publication Date: 2014-10-08
    Description: Planetesimal orbital evolution in a resisting medium near an accreting protoplanet was studied to explore mechanisms for capture into Trojan and satellite orbits. Various mechanisms for capture into libration were proposed, e.g., increase in Jupiter/Sun mass ratio, change in Jovian orbital radius, and collisions of asteroids with interplanetary dust. Studies include effects of solar nebula gas drag on orbital evolution. In general, the gas deviates from Keplerian motion, causing secular decay of planetesimal orbits, as well as damping eccentricity. The motion of bodies near Jupiter under the effect of a resisting medium was numerically explored. The equations of motion were integrated using the formalism of the planar restricted three body problem, modified to include effects by gas drag and a growing Jupiter.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: NASA, Washington Repts. of Planetary Geol. and Geophys. Program, 1984; p 134-136
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  • 7
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    Models of Angular Momentum Input to a Circumterrestrial Swarm from Encounters with Heliocentric Planetesimals (1985)
    In:  CASI
    Details
    Publication Date: 2014-10-08
    Description: Models of lunar origin in which the Moon accretes in orbit about the Earth from material approaching the Earth from heliocentric orbits must overcome a fundamental problem: the approach orbits of such material would be, in the simplest approximation, equally likely to be prograde or retrograde about the Earth, with the result that accretion of such material adds mass but not angular momentum to circumterrestrial satellites. Satellite orbits would then decay due to the resulting drag, ultimately impacting onto the Earth. One possibility for adding both material and angular momentum to Earth orbit is investigated: imbalance in the delivered angular momentum between pro and retrograde Earth passing orbits which arises from the three body dynamics of planetesimals approaching the Earth from heliocentric space. In order to study angular momentum delivery to circumterrestrial satellites, the near Earth velocities were numerically computed as a function of distance from the Earth for a large array of orbits systematically spanning heliocentric phase space.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: NASA, Washington Repts. of Planetary Geol. and Geophys. Program, 1984; p 129-131
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  • 8
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    Unsupervised Classification of Global Radar Units on Venus (1985)
    In:  CASI
    Details
    Publication Date: 2014-10-09
    Description: Characterization of the Venusian surface in terms of its radar properties was accomplished by application of an unsupervised, linear discriminant algorithm to two Pioneer-Venus (PV) Orbiter radar data sets: the RMS-slope (surface roughness) and reflectivity. Both databases were spatially filtered to the same effective resolution of 100 km prior to classification. A recent supervised classification study using these data was based on presupposed morphologic significance of selected data ranges. The knowledge of both Venusian geology and the geologic significance of the radar data is so limited that the data warrant a more unsupervised approach; for this study a linear discriminant classifier was chosen. This approach is purely statistical, thereby removing any observer bias. Statistical significance of the resulting clusters was evaluated by an ancillary program in which an F test utilizing the Mahalanobis' distance.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: NASA, Washington Repts. of Planetary Geol. and Geophys. Program, 1984; p 450-452
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  • 9
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    Asteroid Collisional Evolution Studies (1985)
    In:  CASI
    Details
    Publication Date: 2014-09-30
    Description: Understanding asteroid collisional evolution is important for characterizing the physical state of asteroids today and for learning about the processes that acted in this region of the solar system early in its history. The collisional outcome algorithm in the numerical simulation of asteroid evolution was revised to reflect pressure-strengthening. Asteroid collisions are now treated as a distribution of oblique impacts rather than as only head-on collisions. The initial and evolved size distribution of a plausible asteroid population is compared with the observed size distribution. Asteroid accretion times and reconstruction of the primordial solar nebula suggest that there was significantly more mass in this part of the solar system when the asteroids were accreting.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: NASA, Washington Repts. of Planetary Geol. and Geophys. Program, 1984; p 77-79
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  • 10
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    Small Martian valcanoes (1987)
    In:  CASI
    Details
    Publication Date: 2013-08-29
    Description: Various types of volcanoes were identified on Mars, mainly on the basis of qualitative morphologic criteria such as relief, circularity, summit craters, and alignment with structural trends. A survey of Viking Orbiter images is being conducted to identify possible candidates for Martian volcanoes. The topographic profiles of the selected Martian volcanoes that are currently being acquired will be compared with tabulated data for terrestrial and lunar volcanoes. Variation in volcanic style with age will also be examined.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Lunar and Planetary Inst., MEVTV Workshop on Nature and Composition of Surface Units on Mars; p 49-50
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