The Experts below are selected from a list of 288 Experts worldwide ranked by ideXlab platform
Shigeki Inoue - One of the best experts on this subject based on the ideXlab platform.
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The test for suppressed dynamical friction in a Constant Density core of dwarf galaxies
Monthly Notices of the Royal Astronomical Society, 2009Co-Authors: Shigeki InoueAbstract:The dynamical friction problem is a long-standing dilemma about globular clusters (hereafter,GCs) belonging to dwarf galaxies. GCs are strongly affected by dynamical friction in dwarf galaxies, and are presumed to fall into the galactic center. But, GCs do exist in dwarf galaxies generally. A solution of the problem has been proposed. If dwarf galaxies have a core dark matter halo which has Constant Density distribution in its center, the effect of dynamical friction will be weakened considerably, and GCs should be able to survive beyond the age of the universe. Then, the solution argued that, in a cored dark halo, the suppression of dynamical friction is caused by a new equilibrium state constructed by the interaction between the halo and the GC, in which a part of the halo rotates along with the GC (co-rotating state). In this study, I tested whether the solution is reasonable and reconsidered why a Constant Density, core halo suppresses dynamical friction, by means of N-body simulations. As a result, I conclude that the true mechanism of suppressed dynamical friction is not the co-rotating state, although a core halo can actually suppress dynamical friction on GCs significantly.
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the test for suppressed dynamical friction in a Constant Density core of dwarf galaxies
Monthly Notices of the Royal Astronomical Society, 2009Co-Authors: Shigeki InoueAbstract:The dynamical friction problem is a long-standing dilemma about globular clusters (hereafter GCs) belonging to dwarf galaxies. GCs are strongly affected by dynamical friction in dwarf galaxies, and are presumed to fall into the galactic centre. But, GCs do exist in dwarf galaxies generally. A solution of the problem has been proposed. If dwarf galaxies have a core dark matter halo which has Constant Density distribution in its centre, the effect of dynamical friction will be weakened considerably, and GCs should be able to survive beyond the age of the Universe. Then, the solution argued that, in a cored dark halo, interaction between the halo and the GC constructs a new equilibrium state, in which a part of the halo rotates along with the GC (corotating state). The equilibrium state can suppress the dynamical friction in the core region. In this study, I tested whether the solution is reasonable and reconsidered why a Constant Density, core halo suppresses dynamical friction, by means of N-body simulations. As a result, I conclude that the true mechanism of suppressed dynamical friction is not the corotating state, although a core halo can actually suppress dynamical friction on GCs significantly.
Dimitrios Tsoulis - One of the best experts on this subject based on the ideXlab platform.
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A line integral approach for the computation of the potential harmonic coefficients of a Constant Density polyhedron
Journal of Geodesy, 2020Co-Authors: Olivier Jamet, Dimitrios TsoulisAbstract:A novel approach for the computation of the spherical harmonic coefficients of the gravity field of a Constant Density polyhedron is presented. The proposed method is based on the expression of these coefficients as the volume integral of solid harmonics. It is well known that the divergence theorem leads to an expression of these volume integrals as surface integrals. We show that these surface integrals can be expressed as the sum of line integrals along the edges of the polyhedron. In contrast to previous approaches, the values of the spherical harmonic coefficients at a given degree and order result directly from the computation of the line integrals. The performed numerical implementation revealed the stability of the proposed algorithm up to degree 360 for a prismatic test source.
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A line integral approach for the computation of the potential harmonic coefficients of a Constant Density polyhedron
Journal of Geodesy, 2020Co-Authors: Olivier Jamet, Dimitrios TsoulisAbstract:A novel approach for the computation of the spherical harmonic coefficients of the gravity field of a Constant Density polyhedron is presented. Based on the expression of the solid harmonics involved in the integrals in term of a gradient of the same function of higher degree, and on the ho-mogeneity and harmonicity of theses functions, the approach leads, after subsequent application of the divergence theorem in 3D and the Stokes theorem on the plane, to explicit line integrals defined along each polyhe-dral face, which are then summed for all faces of the polyhedral source. In contrast to previous approaches that involved recurrent relations for the integrals of the same functions, the proposed algorithm concludes to a numerical computation of line integrals linked directly to the coefficients. The performed numerical implementation revealed the stability of the proposed algorithm up to degree 360 for a prismatic test source.
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Recursive algorithms for the computation of the potential harmonic coefficients of a Constant Density polyhedron
Journal of Geodesy, 2009Co-Authors: Dimitrios Tsoulis, Jérôme Verdun, Olivier Jamet, Nicolas GonindardAbstract:The gravitational potential of a Constant Density general polyhedron can be expressed both in terms of a closed analytical expression and as a series expansion involving the corresponding spherical harmonic coefficients. The latter can be obtained from two independent algorithms, which differ not only in their algorithmic architecture but in their efficiency and overall performance, especially when computing the coefficients of higher degree and order. In the present paper a comparative study of all these three approaches is carried out focusing on the numerical implementation of the recursive relations appearing in the two algorithms for the computation of the polyhedral potential harmonic coefficients. The performed numerical investigations show that the linear algorithm proposed by Jamet and Thomas (Proceedings of the second international GOCE user workshop, 'GOCE, The Geoid and Oceanography', ESA-ESRIN, Frascati, Italy, 8-10 March 2004, ESA SP-569, 2004), but so far not implemented, achieves a reasonable accuracy at a computational expense that opens to practical applications, for instance in the field of satellite gravimetry/gradiometry interpretation. The convergence behavior of the linear recursion algorithm is studied thoroughly and a computational procedure is proposed that enables the stable computation of potential harmonic coefficients up to degree 60 when referring to an arbitrarily shaped polyhedral body.
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Assessment of a numerical method for computing the spherical harmonic coefficients of the gravitational potential of a Constant Density polyhedron
2008Co-Authors: Olivier Jamet, Dimitrios Tsoulis, Jérôme Verdun, Nicolas GonindardAbstract:This study focuses on the assessment of a linear algorithm for computing the spherical harmonic coefficients of the gravitational potential of a Constant Density polyhedron. The ability to compute such an expansion would favor several applications, in particular in the field of the interpretation and assessment of GOCE gravitational models. The studied algorithm is the only known method that would achieve this computation at a computational cost depending linearly on the number of computed coefficients. We show that although this methods suffers from severe divergence issues, it could be applied to retrieve band-limited estimates of the potential generated by a Constant Density polyhedron.
Olivier Jamet - One of the best experts on this subject based on the ideXlab platform.
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A line integral approach for the computation of the potential harmonic coefficients of a Constant Density polyhedron
Journal of Geodesy, 2020Co-Authors: Olivier Jamet, Dimitrios TsoulisAbstract:A novel approach for the computation of the spherical harmonic coefficients of the gravity field of a Constant Density polyhedron is presented. The proposed method is based on the expression of these coefficients as the volume integral of solid harmonics. It is well known that the divergence theorem leads to an expression of these volume integrals as surface integrals. We show that these surface integrals can be expressed as the sum of line integrals along the edges of the polyhedron. In contrast to previous approaches, the values of the spherical harmonic coefficients at a given degree and order result directly from the computation of the line integrals. The performed numerical implementation revealed the stability of the proposed algorithm up to degree 360 for a prismatic test source.
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A line integral approach for the computation of the potential harmonic coefficients of a Constant Density polyhedron
Journal of Geodesy, 2020Co-Authors: Olivier Jamet, Dimitrios TsoulisAbstract:A novel approach for the computation of the spherical harmonic coefficients of the gravity field of a Constant Density polyhedron is presented. Based on the expression of the solid harmonics involved in the integrals in term of a gradient of the same function of higher degree, and on the ho-mogeneity and harmonicity of theses functions, the approach leads, after subsequent application of the divergence theorem in 3D and the Stokes theorem on the plane, to explicit line integrals defined along each polyhe-dral face, which are then summed for all faces of the polyhedral source. In contrast to previous approaches that involved recurrent relations for the integrals of the same functions, the proposed algorithm concludes to a numerical computation of line integrals linked directly to the coefficients. The performed numerical implementation revealed the stability of the proposed algorithm up to degree 360 for a prismatic test source.
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Recursive algorithms for the computation of the potential harmonic coefficients of a Constant Density polyhedron
Journal of Geodesy, 2009Co-Authors: Dimitrios Tsoulis, Jérôme Verdun, Olivier Jamet, Nicolas GonindardAbstract:The gravitational potential of a Constant Density general polyhedron can be expressed both in terms of a closed analytical expression and as a series expansion involving the corresponding spherical harmonic coefficients. The latter can be obtained from two independent algorithms, which differ not only in their algorithmic architecture but in their efficiency and overall performance, especially when computing the coefficients of higher degree and order. In the present paper a comparative study of all these three approaches is carried out focusing on the numerical implementation of the recursive relations appearing in the two algorithms for the computation of the polyhedral potential harmonic coefficients. The performed numerical investigations show that the linear algorithm proposed by Jamet and Thomas (Proceedings of the second international GOCE user workshop, 'GOCE, The Geoid and Oceanography', ESA-ESRIN, Frascati, Italy, 8-10 March 2004, ESA SP-569, 2004), but so far not implemented, achieves a reasonable accuracy at a computational expense that opens to practical applications, for instance in the field of satellite gravimetry/gradiometry interpretation. The convergence behavior of the linear recursion algorithm is studied thoroughly and a computational procedure is proposed that enables the stable computation of potential harmonic coefficients up to degree 60 when referring to an arbitrarily shaped polyhedral body.
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Assessment of a numerical method for computing the spherical harmonic coefficients of the gravitational potential of a Constant Density polyhedron
2008Co-Authors: Olivier Jamet, Dimitrios Tsoulis, Jérôme Verdun, Nicolas GonindardAbstract:This study focuses on the assessment of a linear algorithm for computing the spherical harmonic coefficients of the gravitational potential of a Constant Density polyhedron. The ability to compute such an expansion would favor several applications, in particular in the field of the interpretation and assessment of GOCE gravitational models. The studied algorithm is the only known method that would achieve this computation at a computational cost depending linearly on the number of computed coefficients. We show that although this methods suffers from severe divergence issues, it could be applied to retrieve band-limited estimates of the potential generated by a Constant Density polyhedron.
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A Linear Algorithm for Computing the Spherical Harmonic Coefficients of the Gravitational Potential from a Constant Density Polyhedron
2004Co-Authors: Olivier Jamet, Emilie ThomasAbstract:A new method is proposed to derive the spherical harmonic coefficients of the contribution to the gravitational potential of a Constant Density polyhedron of arbitrary shape. This method relies on a set of recurrent relationships between the involved integrals, and achieves a linear complexity in function of the number of edges of the polyhedron and the number of coefficients to be computed. In this paper we present the mathematical basis of the algorithm and discuss its possible applications.
Andrea W Richa - One of the best experts on this subject based on the ideXlab platform.
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MANET Routing with Provably Low Complexity Through Constant Density Clustering and Route Request Broadcast
Wireless Personal Communications, 2007Co-Authors: Hyo-sik Yang, Andrea W Richa, Luke Ritchie, Martin ReissleinAbstract:As mobile ad hoc networks (MANETs) are emerging as important components in critical and large-scale applications, it is crucial to develop MANET routing mechanisms with provably low complexity. In this paper, we give a tutorial overview of the efficient use of elementary node clustering and route request broadcast mechanisms for low-complexity MANET routing. We explain these mechanisms with illustrative examples and discuss their theoretical performance characteristics. We demonstrate that node clustering with Constant Density and route request broadcasting with a doubling radius technique over the network of cluster leaders can be employed for MANET routing with theoretically proven low complexity. Moreover, we contrast these efficient elementary clustering and route request broadcast mechanisms with clustering and route information accumulation mechanisms in the widely studied AODV and DSR routing protocols and discuss the implications of these various mechanisms for scalable MANET routing.
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Constant Density spanners for wireless ad hoc networks
ACM Symposium on Parallel Algorithms and Architectures, 2005Co-Authors: Kishore Kothapalli, Christian Scheideler, Melih Onus, Andrea W RichaAbstract:An important problem for wireless ad hoc networks has been to design overlay networks that allow time- and energy-efficient routing. Many local-control strategies for maintaining such overlay networks have already been suggested, but most of them are based on an oversimplified wireless communication model.In this paper, we suggest a model that is much more general than previous models. It allows the path loss of transmissions to significantly deviate from the idealistic unit disk model and does not even require the path loss to form a metric. Also, our model is apparently the first proposed for algorithm design that does not only model transmission and interference issues but also aims at providing a realistic model for physical carrier sensing. Physical carrier sensing is needed so that our protocols do not require any prior information (not even an estimate on the number of nodes) about the wireless network to run efficiently.Based on this model, we propose a local-control protocol for establishing a Constant Density spanner among a set of mobile stations (or nodes) that are distributed in an arbitrary way in a 2-dimensional Euclidean space. More precisely, we establish a backbone structure by efficiently electing cluster leaders and gateway nodes so that there is only a Constant number of cluster leaders and gateway nodes within the transmission range of any node and the backbone structure satisfies the properties of a topological spanner.Our protocol has the advantage that it is locally self-stabilizing, i.e., it can recover from any initial configuration, even if adversarial nodes participate in it, as long as the honest nodes sufficiently far away from adversarial nodes can in principle form a single connected component. Furthermore, we only need Constant size messages and a Constant amount of storage at the nodes, irrespective of the distribution of the nodes. Hence, our protocols would even work in extreme situations such as very simple wireless devices (like sensors) in a hostile environment.
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SPAA - Constant Density spanners for wireless ad-hoc networks
Proceedings of the 17th annual ACM symposium on Parallelism in algorithms and architectures - SPAA'05, 2005Co-Authors: Kishore Kothapalli, Christian Scheideler, Melih Onus, Andrea W RichaAbstract:An important problem for wireless ad hoc networks has been todesign overlay networks that allow time- and energy-efficientrouting. Many local-control strategies for maintaining such overlaynetworks have already been suggested, but most of them are based onan oversimplified wireless communication model. In this paper, we suggest a model that is much more general thanprevious models. It allows the path loss of transmissions tosignificantly deviate from the idealistic unit disk model and doesnot even require the path loss to form a metric. Also, our model isapparently the first proposed for algorithm design that does notonly model transmission and interference issues but also aims atproviding a realistic model for physical carrier sensing. Physicalcarrier sensing is needed so that our protocols do not requireany prior information (not even an estimate onthe number of nodes) about the wireless network to runefficiently. Based on this model, we propose a local-control protocol forestablishing a Constant Density spanner among a set of mobilestations (or nodes) that are distributed in anarbitrary way in a 2-dimensional Euclidean space. More precisely,we establish a backbone structure by efficiently electing clusterleaders and gateway nodes so that there is only a Constant numberof cluster leaders and gateway nodes within the transmission rangeof any node and the backbone structure satisfies the properties ofa topological spanner. Our protocol has the advantage that it is locallyself-stabilizing, i.e., it can recover from anyinitial configuration, even if adversarial nodes participate in it,as long as the honest nodes sufficiently far away from adversarialnodes can in principle form a single connected component.Furthermore, we only need Constant size messages and a Constantamount of storage at the nodes, irrespective of the distribution ofthe nodes. Hence, our protocols would even work in extremesituations such as very simple wireless devices (like sensors) in ahostile environment.
Robert A. Werner - One of the best experts on this subject based on the ideXlab platform.
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Spherical harmonic coefficients for the potential of a Constant-Density polyhedron
Computers & Geosciences, 1997Co-Authors: Robert A. WernerAbstract:Abstract Recurrent relationships for the definitions of fully normalized spherical harmonic coefficients C n , m and S n , m are derived and integrated analytically to yield the gravitational potential of a Constant-Density polyhedron. The algorithm is expressed in a C language computer program.