16,168 research outputs found
Internal rotor friction instability
The analytical developments and experimental investigations performed in assessing the effect of internal friction on rotor systems dynamic performance are documented. Analytical component models for axial splines, Curvic splines, and interference fit joints commonly found in modern high speed turbomachinery were developed. Rotor systems operating above a bending critical speed were shown to exhibit unstable subsynchronous vibrations at the first natural frequency. The effect of speed, bearing stiffness, joint stiffness, external damping, torque, and coefficient of friction, was evaluated. Testing included material coefficient of friction evaluations, component joint quantity and form of damping determinations, and rotordynamic stability assessments. Under conditions similar to those in the SSME turbopumps, material interfaces experienced a coefficient of friction of approx. 0.2 for lubricated and 0.8 for unlubricated conditions. The damping observed in the component joints displayed nearly linear behavior with increasing amplitude. Thus, the measured damping, as a function of amplitude, is not represented by either linear or Coulomb friction damper models. Rotordynamic testing of an axial spline joint under 5000 in.-lb of static torque, demonstrated the presence of an extremely severe instability when the rotor was operated above its first flexible natural frequency. The presence of this instability was predicted by nonlinear rotordynamic time-transient analysis using the nonlinear component model developed under this program. Corresponding rotordynamic testing of a shaft with an interference fit joint demonstrated the presence of subsynchronous vibrations at the first natural frequency. While subsynchronous vibrations were observed, they were bounded and significantly lower in amplitude than the synchronous vibrations
Review of literature relating to the modeling of soil temperatures based on meteorological factors
Abstracts of 72 papers, journal articles, and other publications are presented. The applicabilities of each is assessed for use in improving winterkill parameters for a winter wheat model
Reference System Description
Several candidate antenna configurations are evaluated in terms of weight, efficiency, and structural rigidity. Particular emphasis is given to the waveguide slot array and its application to solar power satellites SPS. The electronic aspects of an SPS specific waveguide slot array are defined
Momentum transfer at the high-latitude magnetopause and boundary layers
How and where momentum is transferred from the solar wind to the magnetosphere and ionosphere is one of the key problems of space physics. Much of this transfer occurs through direct reconnection on the dayside, particularly when the IMF is southward. However, momentum transfer also occurs at higher latitudes via Alfvén waves on old open field lines, even for southward IMF. This momentum is transferred to the ionosphere via field-aligned currents (FACs), and the flow channel associated with these FACs produces a Hall current which causes magnetic variations at high latitude (the Svalgaard-Mansurov effect). We show examples where such momentum transfer is observed with multiple spacecraft and ground-based instruments
Scattering of first and second sound waves by quantum vorticity in superfluid Helium
We study the scattering of first and second sound waves by quantum vorticity
in superfluid Helium using two-fluid hydrodynamics. The vorticity of the
superfluid component and the sound interact because of the nonlinear character
of these equations. Explicit expressions for the scattered pressure and
temperature are worked out in a first Born approximation, and care is exercised
in delimiting the range of validity of the assumptions needed for this
approximation to hold. An incident second sound wave will partly convert into
first sound, and an incident first sound wave will partly convert into second
sound. General considerations show that most incident first sound converts into
second sound, but not the other way around. These considerations are validated
using a vortex dipole as an explicitely worked out example.Comment: 24 pages, Latex, to appear in Journal of Low Temperature Physic
Elastic forces that do no work and the dynamics of fast cracks
Elastic singularities such as crack tips, when in motion through a medium
that is itself vibrating, are subject to forces orthogonal to the direction of
motion and thus impossible to determine by energy considerations alone. This
fact is used to propose a universal scenario, in which three dimensionality is
essential, for the dynamic instability of fast cracks in thin brittle
materials.Comment: 8 pages Latex, 1 Postscript figur
Star-Like Micelles with Star-Like Interactions: A quantitative Evaluation of Structure Factor and Phase Diagram
PEP-PEO block copolymer micelles offer the possibility to investigate phase
behaviour and interactions of star polymers (ultra-soft colloids). A star-like
architecture is achieved by an extremely asymmetric block ratio (1:20).
Micellar functionality f can be smoothly varied by changing solvent composition
(interfacial tension). Structure factors obtained by SANS can be quantitatively
described in terms of an effective potential developed for star polymers. The
experimental phase diagram reproduces to a high level of accuracy the predicted
liquid/solid transition. Whereas for intermediate f a bcc phase is observed,
for high f the formation of a fcc phase is preempted by glass formation.Comment: 5 pages, 4 figures, PRL in pres
- …