The Experts below are selected from a list of 90081 Experts worldwide ranked by ideXlab platform

Lawrence Carin - One of the best experts on this subject based on the ideXlab platform.

  • multilevel fast multipole algorithm for scattering from conducting targets above or embedded in a lossy half space
    IEEE Transactions on Geoscience and Remote Sensing, 2000
    Co-Authors: Norbert Geng, Anders Sullivan, Lawrence Carin
    Abstract:

    An extension of the multilevel fast multipole algorithm (MLFMA), originally developed for targets in free space, is presented for the electromagnetic scattering from arbitrarily shaped three-dimensional (3-D), electrically large, perfectly conducting targets above or embedded within a lossy half space. We have developed and implemented electric-field, magnetic-field, and combined-field integral equations for this purpose. The nearby terms in the MLFMA framework are evaluated by using the rigorous half-space dyadic Green's function, computed via the method of complex images. Non-nearby (far) MLFMA interactions, handled efficiently within the multilevel clustering construct, employ an approximate dyadic Green's function. This is expressed in terms of a Direct-Radiation term plus a single real image (representing the asymptotic far-field Green's function), with the image amplitude characterized by the polarization-dependent Fresnel reflection coefficient. Examples are presented to validate the code through comparison with a rigorous method-of-moments (MoM) solution. Finally, results are presented for scattering from a model unexploded ordnance (UXO) embedded in soil and for a realistic 3-D vehicle over soil.

  • Multi-level fast-multipole algorithm for scattering from conducting targets above or embedded in a lossy half space
    2000
    Co-Authors: Norbert Geng, Anders Sullivan, Lawrence Carin, Senior Member
    Abstract:

    Abstract—An extension of the multilevel fast multipole algorithm (MLFMA), originally developed for targets in free space, is presented for the electromagnetic scattering from arbitrarily shaped three-dimensional (3-D), electrically large, perfectly conducting targets above or embedded within a lossy half space. We have developed and implemented electric-field, magnetic-field, and combined-field integral equations for this purpose. The nearby terms in the MLFMA framework are evaluated by using the rigorous half-space dyadic Green’s function, computed via the method of complex images. Non-nearby (far) MLFMA interactions, handled efficiently within the multilevel clustering construct, employ an approximate dyadic Green’s function. This is expressed in terms of a Direct-Radiation term plus a single real image (representing the asymptotic far-field Green’s function), with the image amplitude characterized by the polarization-dependent Fresnel reflection coefficient. Examples are presented to validate the code through comparison with a rigorous method-of-moments (MoM) solution. Finally, results are presented for scattering from a model unexploded ordnance (UXO) embedded in soil and for a realistic 3-D vehicle over soil. Index Terms—Fast algorithms, method of moments (MoM), numerical methods, scattering. I

Amin Mosallanezhad - One of the best experts on this subject based on the ideXlab platform.

  • Radiation driven outflows in agns revisiting feedback effects of scattered and reprocessed photons
    Monthly Notices of the Royal Astronomical Society, 2019
    Co-Authors: Amin Mosallanezhad, Feng Yuan, Jeremiah P Ostriker, Fatemeh Zahra Zeraatgari
    Abstract:

    We perform two-dimensional hydrodynamical simulations of slowly rotating accretion flows in the region of $ 0.01-7\, \mathrm{pc} $ around a supermassive black holes with $ M_\mathrm{BH} = 10^{8} M_{\odot} $. The accretion flow is irradiated by the photons from the central active galactic nucleus (AGN). In addition to the Direct Radiation from the AGN, we have also included the "re-Radiation", i.e., the locally produced Radiation by Thomson scattering, line and bremsstrahlung Radiation. Compare to our previous work, we have improved the calculation of Radiation force due to the Thomson scattering of X-ray photons from the central AGN. We find that this improvement can significantly increase the mass flux and velocity of outflow. We have compared the properties of outflow --- including mass outflow rate, velocity, and kinetic luminosity of outflow --- in our simulation with the observed properties of outflow in AGNs and found that they are in good consistency. This implies that the combination of line and re-Radiation forces is the possible origin of observed outflow in luminous AGNs.

Thomas Schildknecht - One of the best experts on this subject based on the ideXlab platform.

  • variation of the area to mass ratio of high area to mass ratio space debris objects
    Monthly Notices of the Royal Astronomical Society, 2012
    Co-Authors: C Fruh, Thomas Schildknecht
    Abstract:

    An unexpected space debris population with the unique property of a very high area-to-mass ratio (HAMR) was detected in 2004 by Schildknecht and colleagues. Ever since, attempts have been made to investigate the dynamical properties of these objects further. Their orbits are heavily perturbed by the effect of Direct Radiation pressure, and unknown attitude motion complicates orbit prediction. The area-to-mass ratio of the objects seems to be unstable over time. Only sparse optical data are available for these objects in drift orbits. This paper makes use of optical observations of five HAMR objects, observed over several years, and investigates the variation of their area-to-mass ratio and orbital parameters. A normalized orbit determination setup is established and validated with two low- and two high-area-to-mass-ratio-objects, to ensure that comparable orbits over longer time spans are determined even with sparse optical data.

  • variation of area to mass ratio of hamr space debris objects
    arXiv: Earth and Planetary Astrophysics, 2011
    Co-Authors: C Fruh, Thomas Schildknecht
    Abstract:

    An unexpected space debris population has been detected in 2004 Schildknecht et al. (2003, 2004) with the unique properties of a very high area-to-mass ratio (HAMR) Schildknecht et al. (2005a). Ever since it has been tried to investigate the dynamical properties of those objects further. The orbits of those objects are heavily perturbed by the effect of Direct Radiation pressure. Unknown attitude motion complicates orbit prediction. The area-to-mass ratio of the objects seems to be not stable over time. Only sparse optical data is available for those objects in drift orbits. The current work uses optical observations of five HAMR objects, observed over several years and investigates the variation of their area-to-mass ratio and orbital parameters. A normalized orbit determination setup has been established and validated with two low and two of the high ratio objects, to ensure, that comparable orbits over longer time spans are determined even with sparse optical data.

Fatemeh Zahra Zeraatgari - One of the best experts on this subject based on the ideXlab platform.

  • Radiation driven outflows in agns revisiting feedback effects of scattered and reprocessed photons
    Monthly Notices of the Royal Astronomical Society, 2019
    Co-Authors: Amin Mosallanezhad, Feng Yuan, Jeremiah P Ostriker, Fatemeh Zahra Zeraatgari
    Abstract:

    We perform two-dimensional hydrodynamical simulations of slowly rotating accretion flows in the region of $ 0.01-7\, \mathrm{pc} $ around a supermassive black holes with $ M_\mathrm{BH} = 10^{8} M_{\odot} $. The accretion flow is irradiated by the photons from the central active galactic nucleus (AGN). In addition to the Direct Radiation from the AGN, we have also included the "re-Radiation", i.e., the locally produced Radiation by Thomson scattering, line and bremsstrahlung Radiation. Compare to our previous work, we have improved the calculation of Radiation force due to the Thomson scattering of X-ray photons from the central AGN. We find that this improvement can significantly increase the mass flux and velocity of outflow. We have compared the properties of outflow --- including mass outflow rate, velocity, and kinetic luminosity of outflow --- in our simulation with the observed properties of outflow in AGNs and found that they are in good consistency. This implies that the combination of line and re-Radiation forces is the possible origin of observed outflow in luminous AGNs.

Norbert Geng - One of the best experts on this subject based on the ideXlab platform.

  • multilevel fast multipole algorithm for scattering from conducting targets above or embedded in a lossy half space
    IEEE Transactions on Geoscience and Remote Sensing, 2000
    Co-Authors: Norbert Geng, Anders Sullivan, Lawrence Carin
    Abstract:

    An extension of the multilevel fast multipole algorithm (MLFMA), originally developed for targets in free space, is presented for the electromagnetic scattering from arbitrarily shaped three-dimensional (3-D), electrically large, perfectly conducting targets above or embedded within a lossy half space. We have developed and implemented electric-field, magnetic-field, and combined-field integral equations for this purpose. The nearby terms in the MLFMA framework are evaluated by using the rigorous half-space dyadic Green's function, computed via the method of complex images. Non-nearby (far) MLFMA interactions, handled efficiently within the multilevel clustering construct, employ an approximate dyadic Green's function. This is expressed in terms of a Direct-Radiation term plus a single real image (representing the asymptotic far-field Green's function), with the image amplitude characterized by the polarization-dependent Fresnel reflection coefficient. Examples are presented to validate the code through comparison with a rigorous method-of-moments (MoM) solution. Finally, results are presented for scattering from a model unexploded ordnance (UXO) embedded in soil and for a realistic 3-D vehicle over soil.

  • Multi-level fast-multipole algorithm for scattering from conducting targets above or embedded in a lossy half space
    2000
    Co-Authors: Norbert Geng, Anders Sullivan, Lawrence Carin, Senior Member
    Abstract:

    Abstract—An extension of the multilevel fast multipole algorithm (MLFMA), originally developed for targets in free space, is presented for the electromagnetic scattering from arbitrarily shaped three-dimensional (3-D), electrically large, perfectly conducting targets above or embedded within a lossy half space. We have developed and implemented electric-field, magnetic-field, and combined-field integral equations for this purpose. The nearby terms in the MLFMA framework are evaluated by using the rigorous half-space dyadic Green’s function, computed via the method of complex images. Non-nearby (far) MLFMA interactions, handled efficiently within the multilevel clustering construct, employ an approximate dyadic Green’s function. This is expressed in terms of a Direct-Radiation term plus a single real image (representing the asymptotic far-field Green’s function), with the image amplitude characterized by the polarization-dependent Fresnel reflection coefficient. Examples are presented to validate the code through comparison with a rigorous method-of-moments (MoM) solution. Finally, results are presented for scattering from a model unexploded ordnance (UXO) embedded in soil and for a realistic 3-D vehicle over soil. Index Terms—Fast algorithms, method of moments (MoM), numerical methods, scattering. I