16,469 research outputs found
An alternative method of analysis for base accelerated dynamic response in NASTRAN
An alternative method of analysis to determine the dynamic response of structures subjected to base accelerations is presented. The method is exact as opposed to the approximate technique of using unusually large masses and loads to enforce desired base accelerations. This paper presents the relevant equations to motion, ALTERs for direct and modal frequency-, random- and transient-response rigid formats, and illustrative examples
Acoustical modes of arbitrary volumes using NASTRAN transient heat transfer RF9
An equivalence between temperature and pressure, heat conduction and stiffness and heat capacity and mass is defined, enabling acoustical modal analysis of arbitrary three dimensional volumes. The transient heat transfer analysis rigid format in NASTRAN, RF9, has been altered providing the acoustical analysis capability. Examples and ALTERs are included
Probing the geometry and motion of AGN coronae through accretion disc emissivity profiles
To gain a better understanding of the inner disc region that comprises active
galactic nuclei it is necessary to understand the pattern in which the disc is
illuminated (the emissivity profile) by X-rays emitted from the continuum
source above the black hole (corona). The differences in the emissivity
profiles produced by various corona geometries are explored via general
relativistic ray tracing simulations. Through the analysis of various
parameters of the geometries simulated it is found that emissivity profiles
produced by point source and extended geometries such as cylindrical slabs and
spheroidal coronae placed on the accretion disc are distinguishable. Profiles
produced by point source and conical geometries are not significantly
different, requiring an analysis of reflection fraction to differentiate the
two geometries. Beamed point and beamed conical sources are also simulated in
an effort to model jet-like coronae, though the differences here are most
evident in the reflection fraction. For a point source we determine an
approximation for the measured reflection fraction with the source height and
velocity. Simulating spectra from the emissivity profiles produced by the
various geometries produce distinguishable differences. Overall spectral
differences between the geometries do not exceed 15 per cent in the most
extreme cases. It is found that emissivity profiles can be useful in
distinguishing point source and extended geometries given high quality spectral
data of extreme, bright sources over long exposure times. In combination with
reflection fraction, timing, and spectral analysis we may use emissivity
profiles to discern the geometry of the X-ray source.Comment: 15 pages, 12 figures. Accepted for publication in MNRA
NASTRAN forced vibration analysis of rotating cyclic structures
Theoretical aspects of a new capability developed and implemented in NASTRAN level 17.7 to analyze forced vibration of a cyclic structure rotating about its axis of symmetry are presented. Fans, propellers, and bladed shrouded discs of turbomachines are some examples of such structures. The capability includes the effects of Coriolis and centripetal accelerations on the rotating structure which can be loaded with: (1) directly applied loads moving with the structure and (2) inertial loas due to the translational acceleration of the axis of rotation (''base' acceleration). Steady-state sinusoidal or general periodic loads are specified to represent: (1) the physical loads on various segments of the complete structure, or (2) the circumferential harmonic components of the loads in (1). The cyclic symmetry feature of the rotating structure is used in deriving and solving the equations of forced motion. Consequently, only one of the cyclic sectors is modelled and analyzed using finite elements, yielding substantial savings in the analysis cost. Results, however, are obtained for the entire structure. A tuned twelve bladed disc example is used to demonstrate the various features of the capability
Proxying the socio-economic background through real estate values. An application on performances of university students
This study shows how the socio-economic background of students in tertiary education can influence their performances and, in particular, the obtained graduation mark. Relying on administrative records on graduated students of the University of Modena and Reggio Emilia (UNIMORE) and aggregated statistics from the Immobiliare.it website, we explore the role of socio-economic background on students’ performances through two different proxies. One refers to the group of the Italian indicator of the household equivalised economic situation (or ISEE) which the student belongs, while the other consists of the average real estate price featuring the postcode where the student resides. Econometric results show a positive influence of both proxies of the socio-economic background on the graduation mark. Specifically, we observe that belonging to highest ISEE groups has on the graduation mark a similar effect with respect to the average real estate price of the student’s postcode of residence. This evidence confirms that the latter may be an effective alternative dimension to proxy the individuals’ socio-economic background when income/wealth variables are not available, interval-censored, or also present relevant issues of reliability
Slow dynamics of a confined supercooled binary mixture II: Q space analysis
We report the analysis in the wavevector space of the density correlator of a
Lennard Jones binary mixture confined in a disordered matrix of soft spheres
upon supercooling. In spite of the strong confining medium the behavior of the
mixture is consistent with the Mode Coupling Theory predictions for bulk
supercooled liquids. The relaxation times extracted from the fit of the density
correlator to the stretched exponential function follow a unique power law
behavior as a function of wavevector and temperature. The von Schweidler
scaling properties are valid for an extended wavevector range around the peak
of the structure factor. The parameters extracted in the present work are
compared with the bulk values obtained in literature.Comment: 8 pages with 8 figures. RevTeX. Accepted for publication in Phys.
Rev.
Revealing structure and evolution within the corona of the Seyfert galaxy I Zw 1
X-ray spectral timing analysis is presented of XMM-Newton observations of the
narrow line Seyfert 1 galaxy I Zwicky 1 (I Zw 1) taken in 2015 January. After
exploring the effect of background flaring on timing analyses, X-ray time lags
between the reflection-dominated 0.3-1.0keV energy and continuum-dominated
1.0-4.0keV band are measured, indicative of reverberation off the inner
accretion disc. The reverberation lag time is seen to vary as a step function
in frequency; across lower frequency components of the variability, 3e-4 to
1.2e-3Hz a lag of 160s is measured, but the lag shortens to (59 +/- 4)s above
1.2e-3Hz. The lag-energy spectrum reveals differing profiles between these
ranges with a change in the dip showing the earliest arriving photons. The low
frequency signal indicates reverberation of X-rays emitted from a corona
extended at low height over the disc while at high frequencies, variability is
generated in a collimated core of the corona through which luminosity
fluctuations propagate upwards. Principal component analysis of the variability
supports this interpretation, showing uncorrelated variation in the spectral
slope of two power law continuum components. The distinct evolution of the two
components of the corona is seen as a flare passes inwards from the extended to
the collimated portion. An increase in variability in the extended corona was
found preceding the initial increase in X-ray flux. Variability from the
extended corona was seen to die away as the flare passed into the collimated
core leading to a second sharper increase in the X-ray count rate.Comment: 18 pages, 11 figures. Accepted for publication in MNRA
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