The Experts below are selected from a list of 18 Experts worldwide ranked by ideXlab platform
Lei Zhu - One of the best experts on this subject based on the ideXlab platform.
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Microstrip Leaky-Wave Antennas With Nonuniform Periodical Loading of Shorting Pins for Enhanced Frequency Sensitivity
IEEE Transactions on Antennas and Propagation, 2018Co-Authors: Danpeng Xie, Lei ZhuAbstract:This paper investigates on an effective approach to enhance frequency sensitivity of the classic wideband EH1-mode microstrip leaky-wave antenna (MLWA). Nonuniform Loadings of shorting pins are installed into the traditional EH1-mode microstrip line, which provide a unique Discontinuity affecting the mutual intersection of the passband and stopband, thus generating a newly created stopband within the continuous EH1-mode fast-wave region. After broadening the stopband, the slope of normalized phase constant is remarkably raised up and the original wide bandwidth of the operating region is effectively reduced, resulting in a dramatically enhanced frequency sensitivity. An equivalent-circuit model of the proposed EH1-mode MLWA with nonuniform Loading is built up at first, then its nonuniform Loading Discontinuity effect on dispersion transformation between passband and stopband is further explicated. Moreover, full-wave numerical extraction is also conducted to accurately demonstrate the modified phase constant under varied lateral pins’ Loading positions and strip widths. After thorough discussion, one set of optimized parameters are selected into antenna design. Compared with the conventional counterpart, the proposed EH1-mode MLWA with nonuniform Loading of shorting pins notably increases the frequency sensitivity from 0.038°/MHz to 0.135°/MHz.
Anthony N. Kounadis - One of the best experts on this subject based on the ideXlab platform.
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Extreme Instability Phenomena in Autonomous Weakly Damped Systems: Hopf Bifurcations, Double Pure Imaginary Eigenvalues, Load Discontinuity
Recent Advances in Mechanics, 2020Co-Authors: Anthony N. KounadisAbstract:The dynamic asymptotic instability of autonomous multi-parameter discrete systems under step compressive Loading either of constant direction (conservative load) or of varying direction (follower or nonconservative Loading) is thoroughly reconsidered using the efficient - and rather forgotten - Lienard-Chipart stability criterion. Attention is focused on the interaction of nonuniform mass and stiffness distribution with infinitesimal damping. Such parameters alone or combined with others may have a tremendous effect on the Jacobian eigenvalues and thereafter on the local asymptotic dynamic instability which – strangely enough –may occur before static (divergence) instability, even in the case of a positive definite damping matrix. It was also found that such systems when unloaded, although being statically stable, under certain conditions may become dynamically locally unstable to any small disturbance. Hopf bifurcations, double zero eigenvalues, double pure imaginary eigenvalues, Loading Discontinuity and other phenomena are properly established.
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Nonlinear dynamic buckling and stability of autonomous structural systems
International Journal of Mechanical Sciences, 1993Co-Authors: Anthony N. KounadisAbstract:Abstract A general approach for the nonlinear dynamic buckling and stability of dissipative or nondissipative structural systems governed by autonomous ordinary differential equations (ODEs) is presented. Geometrically imperfect systems with or without symmetric imperfections, as well as statically stable systems, which, in addition to a monotonically rising (stable) equilibrium path exhibit an unstable complementary path, are considered. The role of the dynamic buckling mechanism of the basin of attraction of a stable equilibrium point (on the prebuckling path), as well as the inset (stable) and outset (unstable) manifolds of a saddle (on a physical or complementary unstable path), are comprehensively explained. A static energy criterion for determining dynamic buckling loads for vanishing (but nonzero) damping and lower bound estimates of exact dynamic buckling loads are presented. Metastability, Loading Discontinuity and chaos-like phenomena are also revealed. The analysis is supplemented by two illustrative models of practical engineering importance.
Danpeng Xie - One of the best experts on this subject based on the ideXlab platform.
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Microstrip Leaky-Wave Antennas With Nonuniform Periodical Loading of Shorting Pins for Enhanced Frequency Sensitivity
IEEE Transactions on Antennas and Propagation, 2018Co-Authors: Danpeng Xie, Lei ZhuAbstract:This paper investigates on an effective approach to enhance frequency sensitivity of the classic wideband EH1-mode microstrip leaky-wave antenna (MLWA). Nonuniform Loadings of shorting pins are installed into the traditional EH1-mode microstrip line, which provide a unique Discontinuity affecting the mutual intersection of the passband and stopband, thus generating a newly created stopband within the continuous EH1-mode fast-wave region. After broadening the stopband, the slope of normalized phase constant is remarkably raised up and the original wide bandwidth of the operating region is effectively reduced, resulting in a dramatically enhanced frequency sensitivity. An equivalent-circuit model of the proposed EH1-mode MLWA with nonuniform Loading is built up at first, then its nonuniform Loading Discontinuity effect on dispersion transformation between passband and stopband is further explicated. Moreover, full-wave numerical extraction is also conducted to accurately demonstrate the modified phase constant under varied lateral pins’ Loading positions and strip widths. After thorough discussion, one set of optimized parameters are selected into antenna design. Compared with the conventional counterpart, the proposed EH1-mode MLWA with nonuniform Loading of shorting pins notably increases the frequency sensitivity from 0.038°/MHz to 0.135°/MHz.
A. N. Kounadis - One of the best experts on this subject based on the ideXlab platform.
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Nonlinear Dynamic Buckling and Stability of Autonomous Dissipative Discrete Structural Systems
Nonlinear Stability of Structures, 1995Co-Authors: A. N. KounadisAbstract:The Nonlinear dynamic buckling and stability of nonlinearly elastic autonomous discrete systems under conservative step Loading (of infinite or finite duration), impact and impulsive Loading is examined in detail. Discrete structural systems which under the same Loading applied statically exhibit snap-through buckling are mainly considered. Emphasis is focused on the coupling effect of nonlinearities (geometric and/or material) and structural damping. A qualitative discussion of the dynamic buckling mechanism is comprehensively presented on the basis of energy and topological concepts in the light of recent progress of nonlinear dynamics and chaos. This leads to useful criteria for establishing exact, approximate and lower-upper bound dynamic buckling estimates without integrating the highly nonlinear equations of motion. It is shown that dynamic buckling can be defined as an escaped motion through a saddle of the (static) unstable postbuckling equilibrium path which leads either to an “unbounded” motion or to a point attractor associated with a remote stable equilibrium point. As byproducts of this analysis various phenomena such as restabilization (metastability), Loading Discontinuity, sensitivity to initial-conditions and to damping as well as chaoticlike Phenomena, are revealed. The theory is illustrated with analyses of single, two and three degrees of freedom models.
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Local (Classical) And Global Bifurcations In Non-linear, Non-gradient Autonomous Dissipative Structural Systems
Journal of Sound and Vibration, 1993Co-Authors: A. N. Kounadis, G.j. SimitsesAbstract:Abstract A general analysis for non-linear, non-gradient, multiple-parameter structural systems with or without damping described by autonomous ordinary differential equations is comprehensively presented. Attention is focused on bifurcational systems under follower Loading exhibiting a trivial stable fundamental path from which a post-divergence or post-oscillatory instability may occur. Conditions for establishing regions of existence of adjacent equilibria, critical and stability conditions as well as different types of bifurcations (with zero, double zero and pure imaginary eigenvalues), for a smooth variation of the control parameter, are thoroughly explored and discussed. Global (dynamic) bifurcations (associated with stable limit cycles), irrelevant to any characteristic properties of the Jacobian eigenvalues, are discovered in a certain small neighborhood of a compound branching. It seems that such global bifurcations with trajectories passing through the saddles of the trivial path are due to the interaction of the first and second buckling modes which occur in the aforementioned small region of adjacent equilibria. Loading Discontinuity phenomena for values of the control parameter defining the foregoing compound branching are detected. The effect of damping and other findings based on local (classical) analyses are compared with the results of this nonlinear analysis, and serious discrepancies are observed showing that a precise modelling must be generic; including both non-linearities and damping. The analysis is illustrated by using as a model Ziegler's classical model, the response of which is fully assessed.
G.j. Simitses - One of the best experts on this subject based on the ideXlab platform.
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Local (Classical) And Global Bifurcations In Non-linear, Non-gradient Autonomous Dissipative Structural Systems
Journal of Sound and Vibration, 1993Co-Authors: A. N. Kounadis, G.j. SimitsesAbstract:Abstract A general analysis for non-linear, non-gradient, multiple-parameter structural systems with or without damping described by autonomous ordinary differential equations is comprehensively presented. Attention is focused on bifurcational systems under follower Loading exhibiting a trivial stable fundamental path from which a post-divergence or post-oscillatory instability may occur. Conditions for establishing regions of existence of adjacent equilibria, critical and stability conditions as well as different types of bifurcations (with zero, double zero and pure imaginary eigenvalues), for a smooth variation of the control parameter, are thoroughly explored and discussed. Global (dynamic) bifurcations (associated with stable limit cycles), irrelevant to any characteristic properties of the Jacobian eigenvalues, are discovered in a certain small neighborhood of a compound branching. It seems that such global bifurcations with trajectories passing through the saddles of the trivial path are due to the interaction of the first and second buckling modes which occur in the aforementioned small region of adjacent equilibria. Loading Discontinuity phenomena for values of the control parameter defining the foregoing compound branching are detected. The effect of damping and other findings based on local (classical) analyses are compared with the results of this nonlinear analysis, and serious discrepancies are observed showing that a precise modelling must be generic; including both non-linearities and damping. The analysis is illustrated by using as a model Ziegler's classical model, the response of which is fully assessed.