Rayleigh-Taylor instability of a compressible plasma in a horizontal magnetic field

A study of the Rayleigh instability of a compressible plasma, density stratified in horizontal planes and subjected to a horizontal magnetic field, is made. It is found that in the presence of the compressibility effects the magnetic field continues to affect the development of the Rayleigh instability even for disturbances that propagate across the magnetic field (taken perpendicular to gravity), contrary to the result for the case of an incompressible plasma. The special case of a plane interface separating two superposed uniform plasmas of different densities is treated as an example to illustrate the compressibility effects on the hydromagnetic Rayleigh instability. It is found that the compressibility effects have a stabilising nature.

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Zusammenfassung Es wird die Rayleigh-Instabilität eines kompressiblen Plasmas untersucht, mit Dichte-Schichtung in horizontalen Ebenen, in Gegenwart eines horizontalen Magnetfeldes. Es wird gezeigt, daß bei Kompressibilität das Magnetfeld auch die Instabilitätsentwicklung derjenigen Störungen beeinflußt, die sich quer zum Magnetfeld (welches senkrecht zur Gravitationsrichtung angenommen wurde) fortpflanzen; dies ist im kompressiblen Plasma nicht der Fall. Der Spezialfall einer ebenen Trennfläche zwischen zwei Plasmas mit verschiedenen gleichförmigen Dichten wird behandelt, um den Kompressibilitätseffekt auf die hydromagnetische Rayleigh-Instabilität zu zeigen; dieser erweist sich als stabilisierend.

     

Large-eddy-simulation of 3-dimensional Rayleigh-Taylor instability in incompressible fluids

The 3-dimensional incompressible Rayleigh-Taylor instability is numerically studied through the large-eddy-simulation ( LES) approach based on the passive scalar transport model. Both the instantaneous velocity and the passive scalar fields excited by sinusoidal perturbation and random perturbation are simulated. A full treatment of the whole evolution process of the instability is addressed. To verify the reliability of the LES code, the averaged turbulent energy as well as the flux of passive scalar are calculated at both the resolved scale and the subgrid scale. Our results show good agreement with the experimental and other numerical work. The LES method has proved to be an effective approach to the Rayleigh-Taylor instability.     

Nonlinear instability of superposed magnetic fluids in the presence of an oblique magnetic field

The nonlinear evolution of interfacial waves separating two magnetic fluids subjected to an oblique magnetic field is studied in two dimensions, with the use of the method of multiple scales. It is shown that the evolution of the envelope is governed by two partial differential equations. These equations can be combined to yield two alternate Schrödinger equations with cubic nonlinearity; one of them leads to the determination of the cutoff wave number separating stable from unstable deformations while the other Schrödinger equation is used to analyze the stability of the system. The stability of the system is discussed both theoretically and computationally, and the stability diagrams are obtained. It is found in the linear theory that the oblique magnetic field has a stabilizing influence if 0 ₁ + ₂ < /2, or 3/2 < ₁ + ₂ 2 and a destabilizing influence if /2 < ₁ + ₂ < 3/2, where 0 _j , (j=1, 2) and , is the angle between the field and the horizontal axis.In the nonlinear theory, the stability analysis reveals that there exist different regions of stability and instability. It is reported that the oblique magnetic field plays a dual role in the stability criterion and the angles ₁ and ₂ play a distinctive role in this analysis besides the effect of the variation of the magnetic permeabilities.     

Nonlinear Marangoni instability in dielectric superposed fluids

Summary The nonlinear Marangoni instability of two dielectric superposed fluids is investigated. The system is stressed by a normal electric field such that it allows for the presence of surface charges at the interface. The method of multiple scale perturbations is used in order to obtain uniformly valid expansions. Two nonlinear Schrödinger equations describing the perturbed system are obtained. One of these equations is used to describe analytically and numerically the necessary conditions for stability and instability near the marginal state, while the other equation is used to obtain the nonlinear electrohydrodynamic cutoff wavenumber separating stable and unstable disturbances for the system.     

Nonlinear instability for nonhomogeneous incompressible viscous fluids

We investigate the nonlinear instability of a smooth steady density profile solution to the three-dimensional nonhomogeneous incompressible Navier-Stokes equations in the presence of a uniform gravitational field, including a Rayleigh-Taylor steady-state solution with heavier density with increasing height (referred to the Rayleigh-Taylor instability). We first analyze the equations obtained from linearization around the steady density profile solution. Then we construct solutions to the linearized problem that grow in time in the Sobolev space H ^k , thus leading to a global instability result for the linearized problem. With the help of the constructed unstable solutions and an existence theorem of classical solutions to the original nonlinear equations, we can then demonstrate the instability of the nonlinear problem in some sense. Our analysis shows that the third component of the velocity already induces the instability, which is different from the previous known results.     

Rayleigh-Taylor instability of compressible finitely conducting rotating fluid in the presence of a magnetic field

The character of the equilibrium of a non-viscous, compressible finitely conducting rotating fluid in the presence of a vertical magnetic field along the direction of gravitational field has been investigated. It is shown that the solution is characterised by a variational principle. Based on the existence of variational principle, an approximate solution has been derived for the case of a fluid having exponentially varying density in the vertical direction. Due to finite resistivity of the medium it is found that potentially stable or unstable configuration retains its character. Further the growth rate of disturbance has been obtained corresponding to short and long wavelengths and it is found that electrical resistivity suppresses the growth rate for large wavelengths but it increases the same for small wavelengths. It is further shown that magnetic field has a destabilizing influence for large wavelengths and a stabilizing influence for small wavelengths.     

Nonlinear dynamics and stability of two streaming magnetic fluids

This paper concerns the linear and nonlinear instability of Kelvin–Helmholtz flows in magnetic fluids under external driving. The fluids are subjected to an oblique magnetic field. With the use of the method of multiple scaling, a generalized derivation of the amplitude equation is obtained in marginally unstable regions of parameter space. A Melnikov function is formulated for such an instability and it is shown that there exist transverse homoclinic orbits leading to chaos.     

Rayleigh-Taylor instability and Rayleigh-type waves on a Maxwell-fluid

The maximum growth rate for Rayleigh-Taylor instability on a Maxwell-fluid has been found. The Rayleigh-Taylor instability mode exists for all wave-numbers and dimensionless time numbers. Further it was found that a propagating wave mode may exist at the same time. It was shown that this mode indeed was a Rayleigh-wave mode ask. A cut-off range may exist for the propagating wave-mode.

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Zusammenfassung Die maximale Wachstumsrate für Rayleigh-Taylor-Instabilitäten auf einer Maxwell-Flüssigkeit wurde berechnet. Die Rayleigh-Taylor-Instabilität existiert für alle Wellenzahlen und dimensionslosen (Memory)-Zeitzahlen. Gleichzeitig kann eine sich fortpflanzende Welle bestehen. Es wurde gezeigt, daß dies eine Rayleigh-Welle ist, wennk geht. Für die sich fortpflanzende Welle kann ein cut-off-Bereich existieren.

     

Thermal instability in compressible fluids in the presence of rotation and magnetic field

The thermal instability of compressible fluids pervaded by a uniform rotation and a uniform magnetic field, separately, is considered. For $\text{(}\text{Cp}\text{g}\text{)β < 1}$, with Cp, g, and β denoting the specific heat at constant pressure, the acceleration due to gravity, and the uniform temperature gradient, respectively, the system is shown to be stable. The magnetic field as well as rotation introduces oscillatory modes in thermal instability of compressible fluids, which are completely missing for $\text{(}\text{Cp}\text{g}\text{)β > 1}$ in the absence of rotation or magnetic field. The sufficient conditions which do not allow overstable modes are obtained.     

Nonlinear temporal dynamics in magnetic fluids between undulatory parallel walls

The effects of undulatory parallel walls and a normal magnetic field on the stability of weakly nonlinear waves at a horizontal interface of two magnetic inviscid fluids are investigated. We assumed that the walls have a weak sinusoidal undulation. The frequency of the main waves is similar to a problem having smooth boundaries. The breaker surface tension and the breaker magnetic field are obtained. The stability analysis concerns the interaction of two propagation wave numbers satisfying the resonance condition imposed by the periodicity of the sinusoidal walls. The first-resonance case occurs whenever the wall wave number is nearly equal to twice the propagation wave number while the second-resonance case occurs whenever the two kinds of wave numbers are nearly equal. When the wave number of the undulation is far from the propagation wave number, the sinusoidal walls have the same effect of the smooth walls on the stability criterion. The stability conditions and the transition curves in the two resonance cases are treated away from the critical state. The existence conditions and stability of Stokes waves near the critical state are discussed. Numerous illustrations and graphs amplify the work.     

Anisotropic transient wave motions in a conducting fluid under uniform magnetic and current fields

An initial value investigation into the development of two-dimensional anisotropic surface waves generated by a harmonically oscillating pressure distribution acting on the undisturbed free surface of an inviscid, incompressible homogeneous and electrically conducting fluid is made in this paper in considerable detail. The problem is solved by the use of generalized function treatment in conjunction with asymptotic methods. An asymptotic solution of the problem related to some physically realistic pressure distributions is presented. It is shown that an ultimate steady state is set up in the limit. Two limiting cases such as (i) very deep fluid and (ii) very shallow fluid, which are of particular interest have been examined with some emphasis. Finally, the effects of the imposed magnetic and current fields as well as the surface tension on the wave motions has been examined in some detail. Additionally, it is shown that the present method of solution provides an interesting example of the applicability of the generalized function method in problems of magnetohydrodynamics     

Nonlinear PDE with vector fields

A nonlinear PDE on a compact manifold is proposed where we use a given vector field. The nonlinear term involves the critical Sobolev exponent growth. To obtain the existence of solutions, conditions linking a critical point of the field and the scalar curvature are found. The second point is devoted to studying the viscosity limit of the solutions when the Laplacian term tends to zero.     

Transport and instability for perfect fluids

Incompressible perfect fluids are described by the Euler equations. We provide a new simple proof for well-posedness for velocities in and linear and nonlinear instability results using transport techniques. The results have an important consequence: the topology of is too fine for interesting questions about large time behavior. Received: 14 September 2001 / Published online: 4 April 2002     

Sloshing characteristics in half-full horizontal elliptical tanks with vertical baffles

The effects of surface-piercing or bottom-mounted vertical baffles on two-dimensional liquid sloshing characteristics in a half-full non-deformable horizontal cylindrical container of elliptical cross section is investigated. The problem solution is achieved by employing the linear potential theory in conjunction with the successive conformal transformation technique, leading to matrix eigenvalue problems on simple (rectangular and semi-infinite strip) regions. Plots of the resonant eigen-frequencies as a function of the baffle extension at selected container aspect ratios are presented and discussed for the three lowest antisymmetric and symmetric transverse oscillation modes. Also, the effects of vertical baffles on the hydrodynamic pressure mode shapes and sloshing flows are examined through appropriate 2D images. The surface-piercing vertical baffle is demonstrated to be an effective tool in reducing the antisymmetric sloshing frequencies, especially for lower aspect ratio tanks and higher modes. On the other hand, the bottom-mounted baffle is shown to have a great influence on the higher antisymmetric slosh modes only when its tip approaches the liquid free surface. Limiting cases are considered and good agreements with available analytic and numerical solutions as well as experimental data are obtained.     

Nonlinear instability in advection-diffusion numerical models

The phenomenon of nonlinear instability, which affects all propagation equations, is simply presented and explained; its origin lies in the numerical dispersion of fundamental solutions by the discrete schemes. Methods to avoid it or to minimize its effects on solutions are proposed and discussed.     

Study of the effect of viscosity and homogeneous horizontal magnetic field on Rayleigh‐Taylor instability

Effect of the viscosity on Rayleigh‐Taylor instability for two contiguous semi‐infinite fluids, in presence of a homogeneous horizontal magnetic field permeating both fluids is investigated. These fluids are incompressible, are arranged in horizontal strata and infinitely conducting. Only the linear terms in the magnetohydrodynamic (MHD) equations are considered. The gravitational acceleration was constant. The dispersion relation that defines the growth rate σ for the system has been defined as a function of the physical parameters of the system and was solved numerically.     

The global instability of uniform flows in non-one-dimensional regions

The conditions for the instability of flows or states, which are independent of time and coordinates, in extended non-one-dimensional regions are considered in a linear approximation. An extension of the idea of global instability, previously introduced for the one-dimensional case, is given. A method is proposed for weakly unstable flows, which enables one to investigate under what conditions perturbations, which grow without limit with time, and which do not depend on the specific form of boundary conditions (provided they are not degenerate), exist. The case of a two-dimensional rectangular region is considered in detail.     

Nonlinear analysis of capillary instability with heat and mass transfer

The nonlinear capillary instability of the cylindrical interface between the vapor and liquid phases of a fluid is studied when there is heat and mass transfer across the interface, using viscous potential flow theory. The fluids are considered to be viscous and incompressible with different kinematic viscosities. Both asymmetric and axisymmetric disturbances are considered. The analysis is based on the method of multiple scale perturbation and the nonlinear stability is governed by first-order nonlinear partial differential equation. The stability conditions are obtained and discussed theoretically as well as numerically. Regions of stability and instability have been shown graphically indicating the effect of various parameters. It has been observed that the heat and mass transfer has stabilizing effect on the stability of the system in the nonlinear analysis for both axisymmetric as well as asymmetric disturbances.     

Allowed regions in the problem concerning dynamics of a charged particle in a superposition of dipole and uniform magnetic fields

The motion of charged particles in the Earth’s magnetic field has been of interest to mathematicians and physicists in connection with the study of the polar aurora and cosmic rays. In 1907, Norwegian mathematician Stromer gave the mathematical formulation of this problem. It became the problem of great importance with the discovery of the Van Allen radiation. As is known, the Earth’s magnetic field can be considered approximately as a superposition of dipole and uniform magnetic fields, and the dipole’s magnetic moment is either parallel or antiparallel to the induction of the uniform field. Thus, the problem concerning the dynamics of the charged particle in the magnetic field of the Earth is reduced to that of charged particle dynamics in the composed field. The paper is devoted to the construction and investigation of the allowed regions in a superposition of dipole and uniform magnetic fields for positive values of Stormer’s constant γ and the same orientation of magnetic moment and uniform field.     

Stability and instability in adiabatic shearing motions of incompressible fluids

We investigate the effect of temperature dependence of the viscosity on the stability of the adiabatic shearing flows of an incompressible Newtonian viscous fluid between two parallel plates. When the viscosity strongly decreases with temperature, the shearing flow caused by a steady motion of the upper plate (steady shearing) becomes unstable, while the shearing flow caused by a time-dependent body force is found to be stable.