12,322 research outputs found
Numerical Results for the System Noise Temperature of an Aperture Array Tile and Comparison with Measurements
The purpose of this report is to document the noise performance of a complex
beamforming array antenna system and to characterize the recently developed
noise measurement facility called THACO, which was developed at ASTRON. The
receiver system includes the array antenna of strongly coupled 144 TSA
elements, 144 Low Noise Amplifiers (LNAs) (Tmin =35-40K) and the data
recording/storing facilities of the initial test station that allow for
off-line digital beamforming. The primary goal of this study is to compare the
measured receiver noise temperatures with the simulated values for several
practical beamformers, and to predict the associated receiver noise coupling
contribution, antenna thermal noise and ground noise pick-up (due to the back
radiation).Comment: ASTRON repor
Variability and nature of the binary in the Red Rectangle Nebula
We present new observations of the central binary inside the Red Rectangle
nebula. The detection of zinc in the optical spectrum confirms that the
peculiar photospheric abundances are due to accretion of circumstellar gas.
Grey brightness variations with the orbital period are observed. They are
interpreted as being due to the variation of the scattering angle with orbital
phase. The small orbital separation of the system is not compatible with
previous normal evolution of the primary on the AGB. We point out the
similarity of the orbital history of this and other similar systems with those
of some close Barium stars and suggest that the nonzero eccentricity of the
orbit is the result of tidal interaction with the circumbinary disk.Comment: 4 pages, 3 figures, A&A Letters accepte
Transition Form Factors between Pseudoscalar and Vector Mesons in Light-Front Dynamics
We study the transition form factors between pseudoscalar and vector mesons
using a covariant fermion field theory model in dimensions. Performing
the light-front calculation in the frame in parallel with the
manifestly covariant calculation, we note that the suspected nonvanishing
zero-mode contribution to the light-front current does not exist in our
analysis of transition form factors. We also perform the light-front
calculation in a purely longitudinal frame and confirm that the form
factors obtained directly from the timelike region are identical to the ones
obtained by the analytic continuation from the spacelike region. Our results
for the decay process satisfy the constraints on the
heavy-to-heavy semileptonic decays imposed by the flavor independence in the
heavy quark limit.Comment: 20 pages, 14 figure
The Vector Meson Form Factor Analysis in Light-Front Dynamics
We study the form factors of vector mesons using a covariant fermion field
theory model in dimensions. Performing a light-front calculation in the
frame in parallel with a manifestly covariant calculation, we note the
existence of a nonvanishing zero-mode contribution to the light-front current
and find a way of avoiding the zero-mode in the form factor calculations.
Upon choosing the light-front gauge (\ep^+_{h=\pm}=0) with circular
polarization and with spin projection , only the
helicity zero to zero matrix element of the plus current receives zero-mode
contributions. Therefore, one can obtain the exact light-front solution of the
form factors using only the valence contribution if only the helicity
components, , and , are used. We also compare our
results obtained from the light-front gauge in the light-front helicity basis
(i.e. ) with those obtained from the non-LF gauge in the instant form
linear polarization basis (i.e. ) where the zero-mode contributions to
the form factors are unavoidable.Comment: 33 pages; typo in Eq.(15) is corrected; comment on Ref.[9] is
corrected; version to appear in Phys. Rev.
Elastic post-buckling behaviour of uniformly compressed plates
In this paper it is discussed how existing analytical and semi-analytical formulas for describing the elastic-post-buckling behavior of uniformly compressed square plates with initial imperfections, for loads up to three times the buckling load can be simplified and improved. For loads larger than about twice the buckling load the influence of changes in the buckling shape, assumed sinusoidal, cannot be neglected anymore. These changes can be taken into account by using the perturbation approach. The existing and improved formulas are compared to the results of finite element simulations
Parameter Study for First-Generation Sheeting Failure using a Theoretical and FE Model
First-generation sheeting is sheeting without longitudinal and transversal stiffeners. For the prediction of failure of this sheeting type, if loaded by j concentrated load and bending moment, several theoretical models and design codes exist. One of these theoretical models was developed recently and predicts failure by using a derivative of the web-crippling deformation due to the concentrated load as an imperfection for the compressed flange for which the behaviour is predicted by Marguerre's simultaneous differential plate equations. The quality of the model has been checked with a whole range of experiments, however, the experiments did not have such a variation of variables that the model could be checked systematically. In this paper, a FE model is used to predict failure for a systematic variation of sheeting variables and the failure loads are used to check the theoretical model. For varying web width, angle between web and flange, corner radius, yield strength, plate thickness, and span length, the theoretical model performs well, qualitatively and quantitatively, compared to the finite element model. For the compressed flange width and load bearing plate width, the theoretical model results show some divergence from the FE model results, although absolute differences remain acceptable
Ultimate Failure Behaviour of Second-generation Sheeting Subjected to Combined Bending Moment and Concentrated Load
Second-generation sheeting is widely used for cladding and roof construction. At interior supports, it is subjected to combined bending moment and concentrated load. Unfortunately, design rules for this loading are complicated and do not provide insight in the sheeting\u27s failure behaviour. This means there is a need for a new, insight providing design rule. For first-generation sheeting, a similar problem did exist. The Technische Universiteit Eindhoven (TUE) carried out three research projects [Bakk92a,Vaes95a,Hofm00a] that provided insight in the first-generation sheeting behaviour and resulted in a new, insight providing design rule. The TUE now uses the strategy of these three research projects for a new project on second-generation sheeting [Kasp01a], with the final aim of a new design rule for second-generation sheeting. In this new project, experiments on commonly used (in the Netherlands) second-generation sheeting were carried out. Second-generation sheeting behaviour was compared with first-generation sheeting behaviour. For sheeting with only stiffeners in flange, load falls occur before ultimate load. Stiffeners in the web only result in load falls after the ultimate load. For an experiment with only stiffeners in the web, a finite element simulation was made. The simulation predicts the sheeting behaviour fairly well and indicates how a stiffener affects the sheeting behaviour
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