75,663 research outputs found
Access in a Networked World: Scholars Portal in Context
published or submitted for publicatio
Method of making a rocket nozzle
A method is described for forming the interior of a nozzle having uneven walls so that a throat of smooth converging and diverging sides is provided for passing flow. A metallic insert material is placed within the flow passageway adjacent to the area where the sharper throat constriction is to be formed, so that the material will flow through the inlet into the throat space when liquefied
Radiant source tracker independent of nonconstant irradiance
A tracker of a radiant source is disclosed. The tracker is composed of a sectored matrix of detectors whose output voltages correspond to their respective levels of irradiance. The voltages are each sampled during a variable length period and the resulting samples are shaped into triangular samples by an integration process. They are supplied to a demodulator which provides an output corresponding to the difference of the time-voltage product of appropriate combinations of the samples. This output, after filtering, represents the angle of incidence on the detectors with respect to a central axis. The triangular samples are also supplied to a demodulator whose output is an AGC voltage which is a function of the sum of the time-voltage integrals of the triangular samples
Getting Comfortable with Change: A New Budget Model for Libraries in Transition
published or submitted for publicatio
Polarization properties and Earth-based radar measurements of Venus in the post-Magellan era
Studies of the polarization properties of reflected radar signals provide information about wavelength-scale surface and subsurface irregularities and can place constraints on the scattering models used to explain anomalously high backscatter cross sections, such as those measured for the surfaces of the icy Galilean satellites. The JPL aircraft-mounted synthetic aperture radars (SAR) provide cross-section and polarization information for terrestrial terrain types. Comparison of these measurements with results from the Magellan mission is helping to relate volcanic flow types on Venus to terrestrial equivalents. Unfortunately, the Magellan SAR transmits and receives a single linear polarization so that information concerning the polarization properties is dependent on past and future observations from the Earth, primarily with the 12.6-cm wavelength (the same as Magellan) radar system on the Arecibo telescope
Dynamical scaling in Ising and vector spin glasses
We have studied numerically the dynamics of spin glasses with Ising and XY
symmetry (gauge glass) in space dimensions 2, 3, and 4. The nonequilibrium
spin-glass susceptibility and the nonequilibrium energy per spin of samples of
large size L_b are measured as a function of anneal time t_w after a quench to
temperatures T. The two observables are compared to the equilibrium spin-glass
susceptibility and the equilibrium energy, respectively, measured as functions
of temperature T and system size L for a range of system sizes. For any time
and temperature a nonequilibrium time-dependent length scale L*(t_w,T) can be
defined by comparing equilibrium and nonequilibrium quantities. Our analysis
shows that for all systems studied, an "effective dynamical critical exponent"
parametrization L*(t_w,T) = A(T) t^(1/z(T)) fits the data well at each
temperature within the whole temperature range studied, which extends from well
above the critical temperature to near T = 0 for dimension 2, or to well below
the critical temperature for the other space dimensions studied. In addition,
the data suggest that the dynamical critical exponent z varies smoothly when
crossing the transition temperature.Comment: 14 pages, 13 figures, 9 table
Behavior and Breakdown of Higher-Order Fermi-Pasta-Ulam-Tsingou Recurrences
We investigate numerically the existence and stability of higher-order
recurrences (HoRs), including super-recurrences, super-super-recurrences, etc.,
in the alpha and beta Fermi-Pasta-Ulam-Tsingou (FPUT) lattices for initial
conditions in the fundamental normal mode. Our results represent a considerable
extension of the pioneering work of Tuck and Menzel on super-recurrences. For
fixed lattice sizes, we observe and study apparent singularities in the periods
of these HoRs, speculated to be caused by nonlinear resonances. Interestingly,
these singularities depend very sensitively on the initial energy and the
respective nonlinear parameters. Furthermore, we compare the mechanisms by
which the super-recurrences in the two model's breakdown as the initial energy
and respective nonlinear parameters are increased. The breakdown of
super-recurrences in the beta-FPUT lattice is associated with the destruction
of the so-called metastable state and hence is associated with relaxation
towards equilibrium. For the alpha-FPUT lattice, we find this is not the case
and show that the super-recurrences break down while the lattice is still
metastable. We close with comments on the generality of our results for
different lattice sizes
Development of 30 micrometers extrinsic silicon multiplexed infrared deterctor array
Two hybrid infrared (IR) detector arrays of antimony-doped silicon (Si:Sb) were produced and tested to evaluate their potential for use in low-background IR astronomy applications. The format of the arrays is 58 x 62 elements, with 76 micron-square pixels. A random-access, switched metal-oxide semiconductor field effect transistor (MOSFET) silicon multiplexer is used to read out the array elements. Reduced-background tests of signal, noise, and noise equivalent power were conducted over the temperature range 3.2 to 12 K. The arrays were found to have good sensitivity and good uniformity
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