1,671 research outputs found
High-frequency techniques for RCS prediction of plate geometries and a physical optics/equivalent currents model for the RCS of trihedral corner reflectors
Part 1 of this report continues the investigation, initiated in previous reports, of scattering from rectangular plates coated with lossy dielectrics. The hard polarization coefficients given in the last report are incorporated into a model, which includes second- and third-order diffractions, for the coated plate. Computed results from this model are examined and compared to measured data. A breakdown of the contribution of each of the higher-order terms to the total radar cross section (RCS) is given. The effectiveness of the uniform theory of diffraction (UTD) model in accounting for the coating effect is investigated by examining a Physical Optics (PO) model which incorporates the equivalent surface impedance approximation used in the UTD model. The PO, UTD, and experimental results are compared. Part 2 of this report presents a RCS model, based on PO and the Method of Equivalent Currents (MEC), for a trihedral corner reflector. PO is used to account for the reflected fields, while MEC is used for the diffracted fields. Single, double, and triple reflections and first-order diffractions are included in the model. A detailed derivation of the E(sub theta)-polarization, monostatic RCS is included. Computed results are compared with finite-difference time-domain (FDTD) results for validation. The PO/MEC model of this report compares very well with the FDTD model, and it is a much faster model in terms of computational speed
Stochastic models which separate fractal dimension and Hurst effect
Fractal behavior and long-range dependence have been observed in an
astonishing number of physical systems. Either phenomenon has been modeled by
self-similar random functions, thereby implying a linear relationship between
fractal dimension, a measure of roughness, and Hurst coefficient, a measure of
long-memory dependence. This letter introduces simple stochastic models which
allow for any combination of fractal dimension and Hurst exponent. We
synthesize images from these models, with arbitrary fractal properties and
power-law correlations, and propose a test for self-similarity.Comment: 8 pages, 2 figure
Three-electron anisotropic quantum dots in variable magnetic fields: exact results for excitation spectra, spin structures, and entanglement
Exact-diagonalization calculations for N=3 electrons in anisotropic quantum
dots, covering a broad range of confinement anisotropies and strength of
inter-electron repulsion, are presented for zero and low magnetic fields. The
excitation spectra are analyzed as a function of the strength of the magnetic
field and for increasing quantum-dot anisotropy. Analysis of the intrinsic
structure of the many-body wave functions through spin-resolved two-point
correlations reveals that the electrons tend to localize forming Wigner
molecules. For certain ranges of dot parameters (mainly at strong anisotropy),
the Wigner molecules acquire a linear geometry, and the associated wave
functions with a spin projection S_z=1/2 are similar to the representative
class of strongly entangled states referred to as W-states. For other ranges of
parameters (mainly at intermediate anisotropy), the Wigner molecules exhibit a
more complex structure consisting of two mirror isosceles triangles. This
latter structure can be viewed as an embryonic unit of a zig-zag Wigner crystal
in quantum wires. The degree of entanglement in three-electron quantum dots can
be quantified through the use of the von Neumann entropy.Comment: To appear in Physical Review B. REVTEX4. 13 pages with 16 color
figures. To download a copy with higher-quality figures, go to publication
#78 in http://www.prism.gatech.edu/~ph274cy
A physical optics/equivalent currents model for the RCS of trihedral corner reflectors
The scattering in the interior regions of both square and triangular trihedral corner reflectors is examined. The theoretical model presented combines geometrical and physical optics (GO and PO), used to account for reflection terms, with equivalent edge currents (EEC), used to account for first-order diffractions from the edges. First-order, second-order, and third-order reflection terms are included. Calculating the first-order reflection terms involves integrating over the entire surface of the illuminated plate. Calculating the second- and third-order reflection terms, however, is much more difficult because the illuminated area is an arbitrary polygon whose shape is dependent upon the incident angles. The method for determining the area of integration is detailed. Extensive comparisons between the high-frequency model, Finite-Difference Time-Domain (FDTD) and experimental data are used for validation of the radar cross section (RCS) of both square and triangular trihedral reflectors
Recommended from our members
Bone mineral density in patients with inherited bone marrow failure syndromes.
BackgroundPatients with inherited bone marrow failure syndromes (IBMFS) may have several risk factors for low bone mineral density (BMD). We aimed to evaluate the prevalence of low BMD in IBMFS and determine the associated risk factors.MethodsPatients with IBMFS with at least one dual-energy X-ray absorptiometry (DXA) scan were evaluated. Diagnosis of each IBMFS, Fanconi anemia (FA), dyskeratosis congenita, Diamond-Blackfan anemia, and Shwachman-Diamond syndrome was confirmed by syndrome-specific tests. Data were gathered on age, height, and clinical history. DXA scans were completed at the lumbar spine, femoral neck, and forearm. BMD was adjusted for height (HAZ) in children (age ≤20 years). Low BMD was defined as a BMD Z-score and HAZ ≤-2 in adults and children, respectively, in addition to patients currently on bisphosphonate therapy.ResultsNine of thirty-five adults (26%) and eleven of forty children (27%) had low BMD. Adults with FA had significantly lower BMD Z-scores than those with other diagnoses; however, HAZ did not vary significantly in children by diagnosis. Risk factors included hypogonadism, iron overload, and glucocorticoid use.ConclusionsAdults and children with IBMFS have high prevalence of low BMD. Prompt recognition of risk factors and management are essential to optimize bone health
Geotechnical Factors in Recent Earthquake-Induced Structural Failures in Greece
A review is made of geotechnical factors that played an important role in three recent earthquake-induced failures, two of which were deadly. The first two catastrophes concern two five-storey hotels that collapsed during the “Alkyonides earthquake” of 24 February 1981 (M=6.7) and the “Egion earthquake” of 15 June 1995 (M=6.2). The third failure is the collapse of a multi-storey factory caused by the “Athens earthquake” of 7 September 1999 (M=5.9). In the first two catastrophes, ground subsidence was estimated by two different methods and was found to be of the order of 0.13 to 0.46 m. These estimates are based on tentative assumptions that should be reviewed and possibly revised. Considerable differential settlements must have existed before the earthquake, as there were no basements that would have attenuated vertical loading and so even a moderate additional differential settlement could cause failure. In the third case, the structure was built near the edge of a steep slope of clayey soil. The co-seismic shear displacement caused the footings resting on the sliding mass to settle, thus causing severe distortions to the structure
RCS Analysis of Plate Geometries, parts 1 and 2
High-frequency techniques for Radar Cross Section (RCS) prediction of plate geometries and a physical optics/equivalent currents model for the RCS of trihedral corner reflectors are addressed. In part 1, a Uniform Theory of Diffraction (UTD) model for the principal-plane radar cross section (RCS) of a perfectly conducting, rectangular plate coated on one side with an electrically thin, lossy dielectric is presented. In part 2, the scattering in the interior regions of both square and triangular trihedral corner reflectors are examined
High-frequency techniques for RCS prediction of plate geometries and a physical optics/equivalent currents model for the RCS of trihedral corner reflectors, parts 1 and 2
Formulations for scattering from the coated plate and the coated dihedral corner reflector are included. A coated plate model based upon the Uniform Theory of Diffraction (UTD) for impedance wedges was presented in the last report. In order to resolve inaccuracies and discontinuities in the predicted patterns using the UTD-based model, an improved model that uses more accurate diffraction coefficients is presented. A Physical Optics (PO) model for the coated dihedral corner reflector is presented as an intermediary step in developing a high-frequency model for this structure. The PO model is based upon the reflection coefficients for a metal-backed lossy material. Preliminary PO results for the dihedral corner reflector suggest that, in addition to being much faster computationally, this model may be more accurate than existing moment method (MM) models. An improved Physical Optics (PO)/Equivalent Currents model for modeling the Radar Cross Section (RCS) of both square and triangular, perfectly conducting, trihedral corner reflectors is presented. The new model uses the PO approximation at each reflection for the first- and second-order reflection terms. For the third-order reflection terms, a Geometrical Optics (GO) approximation is used for the first reflection; and PO approximations are used for the remaining reflections. The previously reported model used GO for all reflections except the terminating reflection. Using PO for most of the reflections results in a computationally slower model because many integrations must be performed numerically, but the advantage is that the predicted RCS using the new model is much more accurate. Comparisons between the two PO models, Finite-Difference Time-Domain (FDTD) and experimental data are presented for validation of the new model
Strongly correlated wave functions for artificial atoms and molecules
A method for constructing semianalytical strongly correlated wave functions
for single and molecular quantum dots is presented. It employs a two-step
approach of symmetry breaking at the Hartree-Fock level and of subsequent
restoration of total spin and angular momentum symmetries via Projection
Techniques. Illustrative applications are presented for the case of a
two-electron helium-like single quantum dot and a hydrogen-like quantum dot
molecule.Comment: 9 pages. Revtex with 2 GIF and 1 EPS figures. Published version with
extensive clarifications. A version of the manuscript with high quality
figures incorporated in the text is available at
http://calcite.physics.gatech.edu/~costas/qdhelproj.html For related papers,
see http://www.prism.gatech.edu/~ph274c
Lithium and Lithium Depletion in Halo Stars on Extreme Orbits
We have determined Li abundances in 55 metal-poor (3.6 < [Fe/H] < -0.7) stars
with extreme orbital kinematics. We find the Li abundance in the Li-plateau
stars and examine its decrease in low-temperature, low-mass stars. The Li
observations are primarily from the Keck I telescope with HIRES (spectral
resolution of ~48,000 and median signal-to-noise per pixel of 140). Abundances
or upper limits were determined for Li for all the stars with typical errors of
0.06 dex. Our 14 stars on the Li plateau give A(Li) = log N(Li)/N(H) + 12.00 of
2.215 +-0.110, consistent with earlier results. We find a dependence of the Li
abundance on metallicity as measured by [Fe/H] and the Fe-peak elements [Cr/H]
and [Ni/H], with a slope of ~0.18. We also find dependences of A(Li) with the
alpha elements, Mg, Ca, and Ti. For the n-capture element, Ba, the relation
between A(Li) and [Ba/H] has a shallower slope of 0.13; over a range of 2.6 dex
in [Ba/H], the Li abundance spans only a factor of two. We examined the
possible trends of A(Li) with the characteristics of the orbits of our halo
stars, but find no relationship with kinematic or dynamic properties. The stars
cooler than the Li plateau are separated into three metallicity subsets. The
decrease in A(Li) sets in at hotter temperatures at high metallicities than at
low metallicities; this is in the opposite sense of the predictions for Li
depletion from standard and non-standard models.Comment: 29 pages including 3 tables and 12 figures Accepted by The
Astrophysical Journal, for the 1 November 2005 issue, v. 63
- …