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H Moradpour - One of the best experts on this subject based on the ideXlab platform.

  • phantom wormhole solutions in a generic cosmological Constant Background
    Canadian Journal of Physics, 2015
    Co-Authors: Y Heydarzade, N Riazi, H Moradpour
    Abstract:

    There are a number of reasons to study wormholes with generic cosmological Constant Λ. Recent observations indicate that the present accelerating expansion of the universe demands Λ > 0. On the other hand, some extended theories of gravitation, such as supergravity and superstring theories, possess vacuum states with Λ < 0. Even within the framework of general relativity, a negative cosmological Constant permits black holes with horizons topologically different from the usual spherical ones. These solutions are convertible to wormhole solutions by adding some exotic matter. In this paper, the asymptotically flat wormhole solutions in a generic cosmological Constant Background are studied. By constructing a specific class of shape functions, a mass function, energy density, and pressure profile that support such a geometry are obtained. It is shown that for having such a geometry, the wormhole throat, r0, the cosmological Constant, Λ, and the equation of state parameter, ω, should satisfy two specific cond...

  • phantom wormhole solutions in a generic cosmological Constant Background
    Canadian Journal of Physics, 2015
    Co-Authors: Y Heydarzade, N Riazi, H Moradpour
    Abstract:

    There are a number of reasons to study wormholes with generic cosmological Constant Λ. Recent observations indicate that the present accelerating expansion of the universe demands Λ > 0. On the oth...

  • asymptotically flat wormhole solutions in a generic cosmological Constant Background
    2015
    Co-Authors: Y Heydarzade, N Riazi, H Moradpour
    Abstract:

    Research Institute for Astronomy and Astrophysics of Maragha (RIAAM), P.O. Box 55134-441, Maragha, Iran.There are a number of reasons to study wormholes with generic cosmological Constant Λ. Recentobservations indicate that present accelerating expansion of the universe demands Λ > 0. On theother hand, some extended theories of gravitation such as supergravity and superstring theoriesposses vacuum states with Λ < 0. Even within the framework of general relativity, a negative cos-mological Constant permits black holes with horizons topologically different from the usual sphericalones. These solutions are convertible to wormhole solutions by adding some exotic matter. In thispaper, the asymptotically flat wormhole solutions in a generic cosmological Constant Backgroundare studied. By constructing a specific class of shape functions, mass function, energy density andpressure profiles which support such a geometry are obtained. It is shown that for having such ageometry, the wormhole throat r

  • phantom wormhole solutions in a generic cosmological Constant Background
    arXiv: General Relativity and Quantum Cosmology, 2014
    Co-Authors: Y Heydarzade, N Riazi, H Moradpour
    Abstract:

    There are a number of reasons to study wormholes with generic cosmological Constant $\Lambda$. Recent observations indicate that present accelerating expansion of the universe demands $\Lambda>0$. On the other hand, some extended theories of gravitation such as supergravity and superstring theories posses vacuum states with $\Lambda<0$. Even within the framework of general relativity, a negative cosmological Constant permits black holes with horizons topologically different from the usual spherical ones. These solutions are convertible to wormhole solutions by adding some exotic matter. In this paper, the asymptotically flat wormhole solutions in a generic cosmological Constant Background are studied. By constructing a specific class of shape functions, mass function, energy density and pressure profiles which support such a geometry are obtained. It is shown that for having such a geometry, the wormhole throat $r_0$, the cosmological Constant $\Lambda$ and the equation of state parameter $\omega$ should satisfy two specific conditions. The possibility of setting different values for the parameters of the model helps us to find exact solutions for the metric functions, mass functions and energy-momentum profiles. At last, the volume integral quantifier, which provides useful information about the total amount of energy condition violating matter is discussed briefly.

Wei Xiang - One of the best experts on this subject based on the ideXlab platform.

  • steady euler flows with large vorticity and characteristic discontinuities in arbitrary infinitely long nozzles
    Advances in Mathematics, 2019
    Co-Authors: Guiqiang Chen, Feimin Huang, Tianyi Wang, Wei Xiang
    Abstract:

    Abstract We establish the existence and uniqueness of smooth solutions with large vorticity and weak solutions with vortex sheets/entropy waves for the steady Euler equations for both compressible and incompressible fluids in arbitrary infinitely long nozzles. We first develop a new approach to establish the existence of smooth solutions without assumptions on the sign of the second derivatives of the horizontal velocity, or the Bernoulli and entropy functions, at the inlet for the smooth case. Then the existence for the smooth case can be applied to construct approximate solutions to establish the existence of weak solutions with vortex sheets/entropy waves by nonlinear arguments. This is the first result on the global existence of solutions of the multidimensional steady compressible full Euler equations with free boundaries, which are not necessarily small perturbations of piecewise Constant Background solutions. The subsonic–sonic limit of the solutions is also shown. Finally, through the incompressible limit, we establish the existence and uniqueness of incompressible Euler flows in arbitrary infinitely long nozzles for both the smooth solutions with large vorticity and the weak solutions with vortex sheets. The methods and techniques developed here will be useful for solving other problems involving similar difficulties.

Miguel Onorato - One of the best experts on this subject based on the ideXlab platform.

  • phase suppressed hydrodynamics of solitons on Constant Background plane wave
    Physical Review Fluids, 2020
    Co-Authors: Amin Chabchoub, Takuji Waseda, Marco Klein, Stefano Trillo, Miguel Onorato
    Abstract:

    The significance of local phase shifts in the experimental realization of soliton and breather waves in finite water depth as well as in deep water is explored. When suppressing the corresponding phase shift, the coherence of the wave envelope disintegrates by forming distinct soliton patterns. All experimental results are in excellent agreement with the predictions based on the nonlinear Schr\"odinger equation framework, suggesting the universality of observed dip and extreme wave pattern dynamics.

  • phase suppressed hydrodynamics of solitons on Constant Background plane wave
    arXiv: Pattern Formation and Solitons, 2020
    Co-Authors: Amin Chabchoub, Takuji Waseda, Marco Klein, Stefano Trillo, Miguel Onorato
    Abstract:

    Soliton and breather solutions of the nonlinear Schrodinger equation (NLSE) are known to model localized structures in nonlinear dispersive media such as on the water surface. One of the conditions for an accurate propagation of such exact solutions is the proper generation of the exact initial phase-shift profile in the carrier wave, as defined by the NLSE envelope at a specific time or location. Here, we show experimentally the significance of such initial exact phase excitation during the hydrodynamic propagation of localized envelope solitons and breathers, which modulate a plane wave of Constant amplitude (finite Background). Using the example of stationary black solitons in intermediate water depth and pulsating Peregrine breathers in deep-water, we show how these localized envelopes disintegrate while they evolve over a long propagation distance when the initial phase shift is zero. By setting the envelope phases to zero, the dark solitons will disintegrate into two gray-type solitons and dispersive elements. In the case of the doubly-localized Peregrine breather the maximal amplification is considerably retarded; however locally, the shape of the maximal focused wave measured together with the respective signature phase-shift are almost identical to the exact analytical Peregrine characterization at its maximal compression location. The experiments, conducted in two large-scaled shallow-water as well as deep-water wave facilities, are in very good agreement with NLSE simulations for all cases.

Nail Akhmediev - One of the best experts on this subject based on the ideXlab platform.

  • Breather solutions of a fourth-order nonlinear Schrödinger equation in the degenerate, soliton, and rogue wave limits.
    Physical Review E, 2017
    Co-Authors: Amdad Chowdury, Wieslaw Krolikowski, Nail Akhmediev
    Abstract:

    : We present one- and two-breather solutions of the fourth-order nonlinear Schrodinger equation. With several parameters to play with, the solution may take a variety of forms. We consider most of these cases including the general form and limiting cases when the modulation frequencies are 0 or coincide. The zero-frequency limit produces a combination of breather-soliton structures on a Constant Background. The case of equal modulation frequencies produces a degenerate solution that requires a special technique for deriving. A zero-frequency limit of this degenerate solution produces a rational second-order rogue wave solution with a stretching factor involved. Taking, in addition, the zero limit of the stretching factor transforms the second-order rogue waves into a soliton. Adding a differential shift in the degenerate solution results in structural changes in the wave profile. Moreover, the zero-frequency limit of the degenerate solution with differential shift results in a rogue wave triplet. The zero limit of the stretching factor in this solution, in turn, transforms the triplet into a singlet plus a low-amplitude soliton on the Background. A large value of the differential shift parameter converts the triplet into a pure singlet.

  • modulation instability fermi pasta ulam recurrence rogue waves nonlinear phase shift and exact solutions of the ablowitz ladik equation
    Physical Review E, 2011
    Co-Authors: Nail Akhmediev, Adrian Ankiewicz
    Abstract:

    We study modulation instability (MI) of the discrete Constant-Background wave of the Ablowitz-Ladik (A-L) equation. We derive exact solutions of the A-L equation which are nonlinear continuations of MI at longer times. These periodic solutions comprise a family of two-parameter solutions with an arbitrary Background field and a frequency of initial perturbation. The solutions are recurrent, since they return the field state to the original Constant Background solution after the process of nonlinear evolution has passed. These solutions can be considered as a complete resolution of the Fermi-Pasta-Ulam paradox for the A-L system. One remarkable consequence of the recurrent evolution is the nonlinear phase shift gained by the Constant Background wave after the process. A particular case of this family is the rational solution of the first-order or fundamental rogue wave.

Niclas Wyllard - One of the best experts on this subject based on the ideXlab platform.

  • derivative corrections to d brane actions with Constant Background fields
    Nuclear Physics, 2001
    Co-Authors: Niclas Wyllard
    Abstract:

    Abstract We study derivative corrections to the effective action for a single D-brane in type II superstring theory coupled to Constant supergravity Background fields. In particular, within this setting we determine the complete expression for the (disk-level) four-derivative corrections to the Born–Infeld part of the action. We also determine 2n -form 2n -derivative corrections to the Wess–Zumino term. Both types of corrections involve all orders of the gauge field strength,  F . The results are obtained via string σ -model loop calculations using the boundary state operator language. The corrections can be succinctly written in terms of the Riemann tensor for a non-symmetric metric.

  • derivative corrections to d brane actions with Constant Background fields
    arXiv: High Energy Physics - Theory, 2000
    Co-Authors: Niclas Wyllard
    Abstract:

    We study derivative corrections to the effective action for a single D-brane in type II superstring theory coupled to Constant Background fields. In particular, within this setting we determine the complete expression for the (disk level) four-derivative corrections to the Born-Infeld part of the action. We also determine 2n-form 2n-derivative corrections to the Wess-Zumino term. Both types of corrections involve all orders of the gauge field strength, F. The results are obtained via string sigma-model loop calculations using the boundary state operator language. The corrections can be succinctly written in terms of the Riemann tensor for a non-symmetric metric.