linear wave equation
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Laser Physics ◽  
2021 ◽  
Vol 32 (1) ◽  
pp. 016002
Author(s):  
Punit Kumar ◽  
Nisha Singh Rathore

Abstract Relativistic and ponderomotive nonlinearities arising by the passage of a linearly polarized laser beam through a partially stripped magnetized quantum plasma are analyzed. The interaction formalism has been developed using the recently developed quantum hydrodynamic model. The effects associated with the Fermi pressure, quantum Bohm potential and electron spin have been incorporated. A nonparaxial, non-linear wave equation has been obtained by the use of source dependent expansion technique and spot size has been evaluated. The nonlinear relativistic self-focusing tends to focus the beam while the ponderomotive nonlinearity tends to defocus. The effect of magnetization and quantum effects on the spot size and the beam power have been studied.


2021 ◽  
Vol 81 (10) ◽  
Author(s):  
J. Mestra-Páez ◽  
J. M. Peña ◽  
A. Restuccia

AbstractWe show that in the Hořava–Lifshitz theory at the kinetic-conformal point, in the low energy regime, a wave zone for asymptotically flat fields can be consistently defined. In it, the physical degrees of freedom, the transverse traceless tensorial modes, satisfy a linear wave equation. The Newtonian contributions, among which there are terms which manifestly break the relativistic invariance, are non-trivial but do not obstruct the free propagation (radiation) of the physical degrees of freedom. For an appropriate value of the couplings of the theory, the wave equation becomes the relativistic one in agreement with the propagation of the gravitational radiation in the wave zone of General Relativity. Previously to the wave zone analysis, and in general grounds, we obtain the physical Hamiltonian of the Hořava–Lifshitz theory at the kinetic-conformal point in the constrained submanifold. We determine the canonical physical degrees of freedom in a particular coordinate system. They are well defined functions of the transverse-traceless modes of the metric and coincide with them in the wave zone and also at linearized level.


2021 ◽  
Vol 11 (4) ◽  
Author(s):  
Piero D’Ancona

AbstractWe study a defocusing semilinear wave equation, with a power nonlinearity $$|u|^{p-1}u$$ | u | p - 1 u , defined outside the unit ball of $$\mathbb {R}^{n}$$ R n , $$n\ge 3$$ n ≥ 3 , with Dirichlet boundary conditions. We prove that if $$p>n+3$$ p > n + 3 and the initial data are nonradial perturbations of large radial data, there exists a global smooth solution. The solution is unique among energy class solutions satisfying an energy inequality. The main tools used are the Penrose transform and a Strichartz estimate for the exterior linear wave equation perturbed with a large, time dependent potential.


CFD letters ◽  
2021 ◽  
Vol 13 (6) ◽  
pp. 1-14
Author(s):  
Serdar Hayytov ◽  
Wah Yen Tey ◽  
Hooi Siang Kang ◽  
Mohammed W. Muhieldeen ◽  
Omid Afshar

Among several numerical methods used to solve the hyperbolic model of the linear wave equation, single-step algorithms can be the more popular ones. However, these algorithms are time-consuming while incurring numerical inaccuracy. Thus, multistep methods can be a suitable option as it has a high order of accuracy. This study aims to investigate and compare the computational performance of these multistep schemes in solving hyperbolic model based on one-dimensional linear wave equation. The techniques studied in this paper comprise the two-step Lax-Wendroff method, MacCormack method, second-order upwind method, Rusanov-Burstein-Mirin method, Warming-Kutler-Lomax method, and fourth-order Runge-Kutta method. Finite difference method is applied in discretisation. Our simulation found that although higher-order multistep methods are more stable than single-step algorithm, they suffer numerical diffusion. The two-step Lax-Wendroff method outperforms other schemes, although it is relatively simple compared with the other three and four steps schemes. The second-order upwind method is attractive as well because it is executable even with a high Courant number.


Author(s):  
Shi-Zhuo Looi ◽  
Mihai Tohaneanu

Abstract We prove that solutions to the quintic semilinear wave equation with variable coefficients in ${{\mathbb {R}}}^{1+3}$ scatter to a solution to the corresponding linear wave equation. The coefficients are small and decay as $|x|\to \infty$ , but are allowed to be time dependent. The proof uses local energy decay estimates to establish the decay of the $L^{6}$ norm of the solution as $t\to \infty$ .


Author(s):  
Pascal Omnes ◽  
Stéphane Dellacherie ◽  
Jonathan Jung

Classical finite volume schemes for the Euler system  are not accurate at low Mach number and some fixes have to be used and were developed in a vast literature over the last two decades. The question we are interested in in this article is: What about if the porosity is no longer uniform? We first show that this problem may be understood on the linear wave equation taking into account porosity. We explain the influence of the cell geometry on the accuracy property at low Mach number. In the triangular case, the stationary space of the Godunov scheme approaches well enough the continuous space of constant pressure and divergence-free velocity, while this is not the case in the Cartesian case. On Cartesian meshes, a fix is proposed and accuracy at low Mach number is proved to be recovered. Based on the linear study, a numerical scheme and a low Mach fix for the non-linear system, with a non-conservative source term due to the porosity variations, is proposed and tested.


2021 ◽  
Vol 59 (5) ◽  
pp. 3737-3761
Author(s):  
Anna Kirpichnikova ◽  
Jussi Korpela ◽  
Matti J. Lassas ◽  
Lauri Oksanen

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