Investigation of wave propagation velocities in fluid mixtures

In order to determine the wave propagation velocities in fluid mixtures, the mixtures are approximated by block structures. These structures consist of identical cells containing eight blocks. The blocks may be filled with different fluids. In block structures, the passage to the limit is carried out under the conditions that the sizes of blocks tend to zero but the relative sizes of blocks remain constant. In the general case, the average wave field satisfies the equations of anisotropic fluids. Two special cases of mixtures of two fluids are considered. In the first case, both fluids are intermixed completely. In the second case, there are periodic inclusions of one fluid into the other. In both cases, the fluid mixtures are homogeneous and isotropic, and formulas for the velocities are obtained. These formulas determine the dependence of the velocities on the percent composition and the parameters of two mixed fluids. The velocity of propagation in the fluid mixture does not exceed the greatest velocity but may be less than the least velocity in mixed fluids. Bibliography: 6 titles. __________ Translated from Zapiski Nauchnykh Seminarov POMI, Vol. 332, 2006, pp. 149–162.     

Propagation of Seismic Waves in Block Fluid Media

The propagation of seismic waves in block two- and three-dimensional fluid media is investigated. For these media, effective models, which are anisotropic fluids, are established. Formulas for the velocities of wave propagation in these fluid media are derived and analyzed. Special investigation is conducted in the cases where blocks with different fluids alternate along the coordinate axes or where blocks filled with a fluid are surrounded by blocks with another fluid. In both cases, the dependence of the wave velocities in the entire medium on the differences of the densities and the wave velocities in fluid blocks is studied. Bibliography: 9 titles. Dedicated to P. V. Krauklis on the occasion of his seventieth birthday __________ Translated from Zapiski Nauchnykh Seminarov POMI, Vol. 308, 2004, pp. 124–146.     

Wave Attenuation in Effective Models Describing Porous and Fractured Media Saturated by a Fluid

Wave attenuation is introduced in the effective model of media that consists of alternating elastic and fluid layers. This attenuation is due to the friction on the boundaries between elastic and fluid layers and is described by additional terms in equations of the effective model. An investigation of these equations allows one to derive expressions of the attenuation coefficients for every body wave propagating along the layers. Bibliography: 9 titles.__________Translated from Zapiski Nauchnykh Seminarov POMI, Vol. 297, 2003, pp. 216–229.     

Propagation of perturbations in fluids excited by moving sources

A large series of A.A. Dorodnicyn’s works deals with rigorous mathematical formulations and development of efficient research techniques for mathematical models used in inhomogeneous fluid dynamics. Numerous problems he studied in these directions are closely related to stratified fluid dynamics, which were addressed in a series of works having been published in this journal by this paper’s authors and their coauthors since 1980. This paper describes the results of a series of works analyzing the propagation of small perturbations in various stratified and/or uniformly rotating inviscid fluids. It is assumed that each of the fluids either occupies an unbounded lower half-space with a free surface or is a semi-infinite two-component fluid layer. The perturbations are excited by a moving source specified as a periodic plane wave traveling along the interface of the fluids. Problems for five mathematical fluid models are formulated, their explicit analytical solutions are constructed, and their existence and uniqueness are discussed. The asymptotics of the solution as t → +∞ are studied, and the long-time wave patterns developing in five fluid models are compared.     

Estimating inequalities for velocities of wave propagation and for effective densities in fluid mixtures

Wave propagation in block fluid media is investigated on the basis of effective models, which are anisotropic fluids. For velocities of wave propagation and for effective densities, estimating inequalities are established. The propagation velocity in a fluid mixture cannot be greater than the greatest velocity in mixed fluids, but can be less than the least velocity in mixed fluids. Bibliography: 7 titles. __________ Translated from Zapiski Nauchnykh Seminarov POMI, Vol. 324, 2005, pp. 180–189.     

Propagation of Seismic Waves in Block Elastic-Fluid Media. II

A two-dimensional medium consisting of alternating elastic and fluid blocks along the x and z axes is considered. For this block medium, an effective model described by a system of equations is constructed by the method of matrix averaging. An investigation of the equations of this model enables one to separate two body waves from the wave field, to construct their fronts, and to obtain expressions for their velocities along the axes. The effective model is considered in the cases where the block medium is converted to a layered elastic-fluid medium, where all the blocks are of the same size, and where an elastic or a fluid medium occupies the entire volume. Bibliography: 7 titles.__________Translated from Zapiski Nauchnykh Seminarov POMI, Vol. 297, 2003, pp. 254–271.     

Propagation of Seismic Waves in Block Elastic-Fluid Media. I

An approach of averaging block elastic-fluid media is proposed, and an effective model for a block medium in which every cell consists of three elastic blocks and one fluid block is constructed. An investigation of the model equations shows that in this model two longitudinal waves and one wave with a concave front set propagate. The limiting cases where the fluid block is narrowed down to a point or where the fluid block occupies the whole cell are considered in the paper. Bibliography: 10 titles.__________Translated from Zapiski Nauchnykh Seminarov POMI, Vol. 297, 2003, pp. 230–253.     

Propagation of Seismic Waves in Block Elastic-Fluid Media. III

The propagation of seismic waves in block two- and three-dimensional media is investigated. These media are composed of identical cells in which there are several fluid blocks and one elastic block. For these media, effective models, which are anisotropic fluids, are established. Formulas for the velocities of propagation in these fluids are derived and investigated. A special investigation is carried out in the cases where the elastic block occupies almost the entire cell or where the relative volume of the elastic block is very small. Bibliography: 9 titles. Dedicated to P. V. Krauklis on the occasion of his seventieth birthday __________ Translated from Zapiski Nauchnykh Seminarov POMI, Vol. 308, 2004, pp. 147–160.     

Nonlinear stability of viscous contact wave for the one‐dimensional compressible fluid models of Korteweg type

This paper is concerned with the large time behavior of solutions of the Cauchy problem to the one‐dimensional compressible fluid models of Korteweg type, which governs the motions of the compressible fluids with internal capillarity. When the corresponding Riemann problem for the Euler system admits a contact discontinuity wave, it is shown that the viscous contact wave corresponding to the contact discontinuity is asymptotically stable provided that the strength of contact discontinuity and the initial perturbation are suitably small. The analysis is based on the elementary L²‐energy method together with continuation argument. Copyright © 2013 John Wiley & Sons, Ltd.     

Long Time Behavior of the Cahn-Hilliard Equation with Irregular Potentials and Dynamic Boundary Conditions

The Cahn-Hilliard equation with irregular potentials and dynamic boundary conditions is considered.The existence of the global attractor is proved and the long time behavior of the trajectories,namely,the convergence to steady states,is studied.     

Homoclinic Bifurcations for Partial Differential Equations in Unbounded Domains

The connection between low-dimensional chaos in ordinary differential equations, and turbulence in fluids and other systems governed by partial differential equations, is one that is in many circumstances not clear. We discuss some examples of turbulent fluid flow, and consider ways in which they may be related to much simpler sets of ordinary differential equations, whose behavior can be reasonably well understood. (We are not advocating drastic Fourier truncation.) The generation of aperiodic solutions through the occurrence of homoclinic orbits is briefly analysed for ordinary differential equations, and the same kind of heuristic analysis is sketched for partial differential equations (in one space dimension). It is suggested that such an analysis can explain certain features of chaos, which have been observed in real fluids.     

Oblique Interactions between Internal Solitary Waves

In this paper, we study the oblique interaction of weakly, nonlinear, long internal gravity waves in both shallow and deep fluids. The interaction is classified as weak when where Δ₁=|c_m/c_n?cosδ|, Δ₂=|c_n/c_m?cosδ|,c_m,n, are the linear, long wave speeds for waves with mode numbers m, n, δ is the angle between the respective propagation directions, and α measures the wave amplitude. In this case, each wave is governed by its own Kortweg-de Vries (KdV) equation for a shallow fluid, or intermediate long-wave (ILW) equation for a deep fluid, and the main effect of the interaction is an 0(α) phase shift. A strong interaction (I) occurs when Δ_1,2 are 0(α), and this case is governed by two coupled Kadomtsev-Petviashvili (KP) equations for a shallow fluid, or two coupled two-dimensional ILW equations for deep fluids. A strong interaction (II) occurs when Δ₁ is 0(α), and (or vice versa), and in this case, each wave is governed by its own KdV equation for a shallow fluid, or ILW equation for a deep fluid. The main effect of the interaction is that the phase shift associated with Δ₁ leads to a local distortion of the wave speed of the mode n. When the interacting waves belong to the same mode (i.e., m = n) the general results simplify and we show that for a weak interaction the phase shift for obliquely interacting waves is always negative (positive) for (1/2+cosδ)>0(<0), while the interaction term always has the same polarity as the interacting waves.     

Causality analysis of frequency‐dependent wave attenuation

The work is inspired by thermo‐ and photoacoustic imaging, where recent efforts are devoted to take into account attenuation and varying wave speed parameters. In this paper, we derive and analyze causal equations describing the propagation of attenuated pressure waves. We also review standard models, like frequency power laws, and the thermo‐viscous equation and show that they lack causality in the parameter range relevant for biological photoacoustic imaging. To discuss causality in mathematical rigor we use the results and concepts of linear system theory. We present some numerical experiments, which show the physically unmeaningful behavior of standard attenuation models, and the realistic behavior of the novel models. Copyright © 2010 John Wiley & Sons, Ltd.     

Wave motions in a stratified electrically conducting rotating fluid

The 3D dynamics equations for the stratified superconducting rotating fluid are studied. These equations are reduced to a scalar equation by representing the magnetic and density fields by a superposition of the unperturbed fields corresponding to the steady state of the fluid and the induced fields appearing due to the wave motion; the reduction also uses two auxiliary functions. The analysis of the scalar equation enables us to prove the solvability of the initial-boundary value problems of the wave theory for electrically conducting rotating fluids with nonhomogeneous density.     

On the development and generalizations of Cahn–Hilliard equations within a thermodynamic framework

We provide a thermodynamic basis for the development of models that are usually referred to as ??phase-field models?? for compressible, incompressible, and quasi-incompressible fluids. Using the theory of mixtures as a starting point, we develop a framework within which we can derive ??phase-field models?? both for mixtures of two constituents and for mixtures of arbitrarily many fluids. In order to obtain the constitutive equations, we appeal to the requirement that among all admissible constitutive relations that which is appropriate maximizes the rate of entropy production (see Rajagopal and Srinivasa in Proc R Soc Lond A 460:631?C651, 2004). The procedure has the advantage that the theory is based on prescribing the constitutive equations for only two scalars: the entropy and the entropy production. Unlike the assumption made in the case of the Navier?CStokes?CFourier fluids, we suppose that the entropy is not only a function of the internal energy and the density but also of gradients of the partial densities or the concentration gradients. The form for the rate of entropy production is the same as that for the Navier?CStokes?CFourier fluid. As observed earlier in Heida and Málek (Int J Eng Sci 48(11):1313?C1324, 2010), it turns out that the dependence of the rate of entropy production on the thermodynamical fluxes is crucial. The resulting equations are of the Cahn?CHilliard?CNavier?CStokes type and can be expressed both in terms of density gradients or concentration gradients. As particular cases, we will obtain the Cahn?CHilliard?CNavier?CStokes system as well as the Korteweg equation. Compared to earlier approaches, our methodology has the advantage that it directly takes into account the rate of entropy production and can take into consideration any constitutive assumption for the internal energy (or entropy).     

ANALYSIS OF THE STICK-SLIP PROBLEM FOR NON-NEWTONIAN FLOWS

In this paper we consider the stick-slip problem for non-Newtonian flows: the fluid emerges from a 2-dimensional bounded strip {(x, y); ?N < x < 0, |y| < 1} into the space {0 < x < N} and its free boundary is linearized to {x > 0; |y| = 1}. The fluids we consider are a class of Oldroyd models, excluding some special models such as the upper convected Maxwell model. We are not studying the full non-Newtonian flow, but rather the simpler problem where, in some terms of the constitutive equations of the model, the underlying velocity is assumed given, and Newtonian-like.     

Higher-order asymptotic solutions in fluids of various configurations

Applying perturbation methods, symbolic computation, and generalizing the solution method, higher-order asymptotic solutions are constructed in Lagrangian variables for several models describing 2D standing wave motions in fluids of various configurations. Three main parameters of the fluid configuration, depth, capillarity, and stratification layer, are considered. The frequency-amplitude dependences are obtained and compared with those known in the literature in Eulerian and Lagrangian variables. The comparison shows that the analytical frequency-amplitude dependences are in complete agreement with previous results known in the literature and with the results obtained for other models. A generalization allows us to investigate critical phenomena for standing waves in fluids of various configurations. Namely, special attention is focused on critical values of one parameter, the fluid depth. The frequency-amplitude dependences are analyzed from the point of view of critical values: critical points and critical curves are determined for several models describing standing waves in fluids of various configurations.     

Dynamics of a rotating layer of an ideal electrically conducting incompressible fluid

A system of nonlinear partial differential equations is considered that models perturbations in a layer of an ideal electrically conducting rotating fluid bounded by spatially and temporally varying surfaces with allowance for inertial forces. The system is reduced to a scalar equation. The solvability of initial boundary value problems arising in the theory of waves in conducting rotating fluids can be established by analyzing this equation. Solutions to the scalar equation are constructed that describe small-amplitude wave propagation in an infinite horizontal layer and a long narrow channel.     

Analysis of a fast method for solving the high frequency Helmholtz equation in one dimension

We propose and analyze a fast method for computing the solution of the high frequency Helmholtz equation in a bounded one-dimensional domain with a variable wave speed function. The method is based on wave splitting. The Helmholtz equation is split into one-way wave equations with source functions which are solved iteratively for a given tolerance. The source functions depend on the wave speed function and on the solutions of the one-way wave equations from the previous iteration. The solution of the Helmholtz equation is then approximated by the sum of the one-way solutions at every iteration. To improve the computational cost, the source functions are thresholded and in the domain where they are equal to zero, the one-way wave equations are solved with geometrical optics with a computational cost independent of the frequency. Elsewhere, the equations are fully resolved with a Runge–Kutta method. We have been able to show rigorously in one dimension that the algorithm is convergent and that for fixed accuracy, the computational cost is asymptotically just O(w^{1/ p})\mathcal {O}(\omega^{1/ p}) for a pth order Runge–Kutta method, where ω is the frequency. Numerical experiments indicate that the growth rate of the computational cost is much slower than a direct method and can be close to the asymptotic rate.     

Three‐dimensional internal gravity‐capillary waves in finite depth

We consider three‐dimensional inviscid‐irrotational flow in a two‐layer fluid under the effects of gravity and surface tension, where the upper fluid is bounded above by a rigid lid and the lower fluid is bounded below by a flat bottom. We use a spatial dynamics approach and formulate the steady Euler equations as an infinite‐dimensional Hamiltonian system, where an unbounded spatial direction x is considered as a time‐like coordinate. In addition, we consider wave motions that are periodic in another direction z. By analyzing the dispersion relation, we detect several bifurcation scenarios, two of which we study further: a type of 00(is)(iκ₀) resonance and a Hamiltonian Hopf bifurcation. The bifurcations are investigated by performing a center‐manifold reduction, which yields a finite‐dimensional Hamiltonian system. For this finite‐dimensional system, we establish the existence of periodic and homoclinic orbits, which correspond to, respectively, doubly periodic travelling waves and oblique travelling waves with a dark or bright solitary wave profile in the x direction. The former are obtained using a variational Lyapunov‐Schmidt reduction and the latter by first applying a normal form transformation and then studying the resulting canonical system of equations.