On transient development of ship waves on a running stratified ocean

Summary A theoretical study is made of the linearized problem of transient development of waves created by the free surface pressure disturbances on a running stratified ocean which is composed of two layers of fluids of different densities with the infinitely deep bottom layer. With the aid of the same method of the author (1969c), asymptotic solutions for the free and interfacial surface elevation functions are obtained explicitly. Solutions of the corresponding steady state problem of ship waves in a stratified ocean due to Crapper (1967) are recovered in the limit t without having to resort to the use of the radiation condition at infinity or an equivalent device. The solution of the problem, in general, consists of two different classes of waves. The waves of the first class are the usual surface waves for the homogeneous ocean with a little modification in amplitude; and the waves in the second class are the internal waves with large amplitude for small differences in density between the upper and lower layer of the ocean. Some discussions about the internal wave motions related to the infinitesimal density stratification and shallow upper layer are made.

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Sommario Si compie uno studio teorico sul problema linearizzato del transitorio creato da perturbazioni di pressione sulla superficie libera di un oceano stratificato formato da due strati di densità diversa con lo strato inferiore di profondità infinita. Per mezzo del metodo già sviluppato dall'autore (1969c) si ottengono soluzioni asintotiche esplicite per le funzioni che danno le quote della superficie libera e di quella di separazione. Le soluzioni del corrispondente problema a regime di onde prodotte da una nave in un oceano stratificato dovute a Crapper (1967) sono ritrovate al limite t senza dover ricorrere all'uso delle condizioni di radiazione all'infinito o ad un equivalente artificio. La soluzione del problema, in generale, consiste di due diverse classi di onde. Le onde della prima classe sono le solite onde di superficie di un oceano omogeneo con piccole modifiche nell'ampiezza; le onde della seconda classe sono onde interne con grande ampiezza per piccole differenze di densità fra lo strato superiore e inferiore dell'oceano. Si discute poi il moto ondoso interno relativo alla stratificazione infinitesimale di densità e allo strato superiore di piccole profondità.

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Summary presented to the Seventy-sixth annual meeting of the American Mathematical Society at San-Antonio, January, 1970.     

Numerical experiments on the transient motions of a flapping foil

The flow field generated by a foil during transient motions is investigated by means of numerical experiments. The numerical simulations have some advantages with respect to laboratory experiments. Indeed, having access to the velocity and pressure fields both in space and in time, it is possible to 'measure' quantities like vorticity, forces and torques which are quite difficult to obtain in laboratory. Moreover, data can be easily gained for different foil kinematics. The obtained results show that the time history of the propulsive force strongly depends on the details of the kinematics of the foil. Moreover, the numerical simulations have allowed to understand the main mechanisms employed by fish to propel themselves during fast starts and to identify the values of the parameters providing optimal propulsive performances.     

Nonlinear wave motions on liquid surfaces

A study is made of the formation of a shock wave (bore), produced by the movement of an initially weak discontinuity in the spatial derivatives of velocity and liquid depth in an area of stationary current in a channel of constant inclination. The formation of shock waves from compression waves was first studied by Riman [1]. Frictional resistance was considered in the Chezy form. The equations obtained therein for determination of the moment in time and spatial coordinates of the point at which the shock wave is formed, as well as the laws for propagation of shock waves are applicable to the problem of one-dimensional transient motion in a gas, the pressure of which is dependent on density. Instantaneous collapse of waves, as well as formation and movement of bores in rivers for an idealized flow model in a channel with horizontal bottom, neglecting friction, were described by Khristianovich, Mikhlin, and Devison [2], and Stoker [3]. Recently in the work of Sachdev and Bhatnagar [4], using numerical integration of the equation for bore intensity, the problem of shock wave propagation in a channel of constant inclination with consideration of fluid resistance in the Chezy form was studied. Gradual wave collapse and the bore formation mechanism were studied by Stoker [3] on the basis of the shallow-water theory. Neglecting friction on the horizontal channel bottom, he calculated the moment of time and coordinates of the point at which the shock wave is formed in the case where the initial disturbance is sinusoidal. The dependence of these values on wave amplitude for a channel of constant inclination was obtained by Jeffrey [5], who also neglected friction on the channel bottom and considered the initial disturbance to be sinusoidal. Lighthill and Whitham [6] discovered that for Froude numbers greater than two, the linear theory led to unlimited growth in the intensity of the flood wave. We note that the studies of flood-wave motion in the region of the first characteristic, performed in [3, 6], differ only in the forms of the resistance laws and dependences of the unknown functions on the variables. Physical peculiarities of various liquid wave motions were also examined by Lighthill in [7].Saratov. Translated from Izvestiya Akademii Nauk SSSR. Mekhanika Zhidkosti i Gaza, No. 2, pp. 62–66, March–April, 1972.     

On the theory of transient wave propagation in a dispersive medium

Summary Choosing a simple parallel-plate waveguide, an exact expression is obtained for the transient response when a step-modulated carrier signal is applied. The analysis constitutes a modest extension of the early work of Sommerfeld and the more recent investigations of Rubinowicz and Knop. Numerical calculations are presented for a range of the parameters which have some relevance to propagation in the earth-ionosphere waveguide. The exact form of the transient envelope function is compared with an approximate version which is in the form of a Fresnel integral. It is shown that the approximate method gives a good qualitative estimate for the transient response characteristics. Thus, confidence is gained in applying it to other situations where an exact solution is not available.The research reported here was supported by the Advanced Research Projects Agency, Washington, D.C., under ARPA Order No. 183-62.Formerly the Central Radio Propagation Laboratory of the National Bureau of Standards.     

Localization of wave motions in a fluid-filled cylinder

The properties of harmonic surface waves in a fluid-filled cylinder made of a compliant material are studied. The wave motions are described by a complete system of dynamic equations of elasticity and the equation of motion of a perfect compressible fluid. An asymptotic analysis of the dispersion equation for large wave numbers and a qualitative analysis of the dispersion spectrum show that there are two surface waves in this waveguide system. The first normal wave forms a Stoneley wave on the inside surface with increase in the wave number. The second normal wave forms a Rayleigh wave on the outside surface. The phase velocities of all the other waves tend to the velocity of the shear wave in the cylinder material. The dispersion, kinematic, and energy characteristics of surface waves are analyzed. It is established how the wave localization processes differ in hard and compliant materials of the cylinder __________ Translated from Prikladnaya Mekhanika, Vol. 44, No. 4, pp. 72–86, April 2008.     

A second order stationary solution of the three dimensional wave diffraction potential

A theory on the second order wave diffraction by a three dimensional body fixed in a regular sea has been developed in the present paper. By regarding the sinusoidal disturb potential as a stationary solution of an initial value problem, and using Laplace transformation method and Tauberian theorem, the boundary value problems of stationary solution of the first and second order diffraction potential have been derived in this paper. Furthermore, the explicit solution of the second order stationary diffraction potential has been obtained with the method of the double Fourier transformation. It is found that the asymptotic behaviour of the second order stationary solution at far field is dependent on two wave systems, the first is “free wave”, travelling independently of the first order wave system, the other is “phase locked waves”, which accompany the first order waves. At the same time, the radiation conditions of the second order diffraction problems are derived. We also find that one can still pursue a steady state formulation with the inclusion of Rayleigh damping. Finally, as an example, the second order wave forces upon a fixed vertical circular cylinder have been calculated, and the numerical results agree well with the experimental data.     

The calculation of Green's functions in three dimensional hydrodynamic gravity wave problems

We have studied the problem of calculating Green's functions in three dimensional hydrodynamic gravity wave problems. A number of new expressions for these functions are presented for both finite and infinite depths. Various techniques for accelerating the convergence of some infinite series in these expressions are investigated and compared with the normal methods of evaluation. A significant improvement in the efficiency of the calculation is found using the results described in this paper.     

Azimuthal wave motions on the surface of a rotating fluid cylinder

Wave motions in a fluid cylinder rotating about the axis are investigated within the framework of the linear theory. The cylinder is assumed to be fairly long. This makes it possible to restrict attention to the study of the plane oscillation pattern. The fluid is assumed to be ideal and incompressible. The models in which the fluid particles are confined by gravitational (body) or/and capillary forces (surface stress forces) are considered. A mode analysis is carried out and the dispersion relations are constructed. Traveling and steady-state waves on the surface of the fluid cylinder are investigated; qualitative effects ("wave inertia") are established. Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 3, pp. 128–133, May–June, 1998. The work was carried out with financial support from the Russian Foundation for Basic Research (project No. 96-01-00221). An erratum to this article is available at .     

Exterior reconstruction of a three dimensional scatterer

An explicit algorithm is constructed for the exterior Radon transform in three dimensions. An immediate consequence is the development of an exterior form for the inverse Born approximation. From this we discuss reconstruction from the time domain elastic wave scattering of a defect which is partly strongly scattering and partly weakly scattering.     

Nonlinear wave motions of a liquid in a vibrating axisymmetric cavity

The problem of the nonlinear wave deformation of the free surface of a liquid due to the translational motions of the containing vessel is examined. Bogolyubov's averaging method is used to investigate the characteristics of the wave motions of the liquid in the resonance zones in the case of a cylindrical vessel. Relations are obtained characterizing the variation of the amplitude of the circular wave with the frequencies of the external perturbations in the steady-state wave process; the conditions of occurrence and stability of such processes are analyzed.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 3, pp. 120–125, May–June, 1989.     

Unsteady wave motions of a fluid above an inclined floor

The propagation of unsteady waves above a flat inclined floor within the framework of a linear dispersion model was first studied in [1]. This paper shows how to solve the three-dimensional problem for the case = /4, where is the angle of inclination of the floor plane to the free surface. The two-dimensional problem was studied in [2–4]. In articles [2, 3] asymptotic solutions were found for the Cauchy-Poisson problem for certain values of . In [4], a method is proposed for solving the problem of the wave motion of a fluid due to the displacement of a section of the floor of the basin. However, the complicated structure of the expression obtained by reducing the problem to an inhomogeneous functional equation makes it impossible to study the solution. The aim of the present work is to obtain some exact solutions for the two- and three-dimensional problems of unsteady waves above an inclined floor, which are suitable for calculations and asymptotic estimates.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 6, pp. 65–70, November–December, 1984.     

Coulomb traction on a penny-shaped crack in a three dimensional piezoelectric body

The axisymmetric problem of a penny-shaped crack embedded in an infinite three-dimensional (3D) piezoelectric body is considered. A general formulation of Coulomb traction on the crack surfaces can be obtained based on thermodynamical considerations of electromechanical systems. Three-dimensional electroelastic solutions are derived by the classical complex potential theory when Coulomb traction is taken into account and the poling direction of piezoelectric body is perpendicular to the crack surfaces. Numerical results show that the magnitude of Coulomb tractions can be large, especially when a large electric field in connection with a small mechanical load is applied. Unlike the traditional traction-free crack model, Coulomb tractions induced by an applied electric field influence the Mode I stress intensity factor for a penny-shaped crack in 3D piezoelectric body. Moreover, compared to the current model, the traditional traction-free crack model always overestimates the effect of the applied electric load on the field intensity factors and energy release rates, which has consequences for 3D piezoelectric fracture mechanics.     

Steady and transient liquid sphere heating in a convective gas stream

 A finite-difference scheme has been developed to solve the equations governing the laminar forced convection heat transfer around and inside a spherical fluid droplet moving steadily in another immiscible fluid for both steady and transient thermal conditions. For large values of the external flow Reynolds number (Re), results not available in the literature have been obtained for circulating droplets at intermediate and high interior-to-exterior viscosity ratios (μ^*). Detailed results over a wide range of viscosity ratio (μ^*) and for 200≤Re≤1000 are presented for the temperature profiles outside and inside the sphere, Nusselt number, the time required to attain a uniform surface temperature and the time required to reach the steady-state temperature. Results show that convective heating is dependent on the external flow Reynolds number (Re) and the interior-to-exterior viscosity ratio (μ^*) where increasing Re or decreasing μ^* result in increasing heat transfer rate convected to the liquid sphere. Received on 1 March 1999