Larissa Ju. Fradkin

Bio: Larissa Ju. Fradkin is an academic researcher from London South Bank University. The author has contributed to research in topics: Diffraction & Ultrasonic testing. The author has an hindex of 10, co-authored 40 publications receiving 340 citations.

Low-frequency acoustic waves in nematic elastomers

Abstract: We study the spectral and polarization properties of acoustic waves propagating in nematic liquid-crystalline rubber materials. We apply the viscoelastic theory of nematic elastomers in the low-frequency (hydrodynamic) limit. Dynamic soft elasticity, exhibited by ideal nematic elastomers under certain geometries of shear at low frequencies, leads to anomalous anisotropy of energy transfer and attenuation of transverse waves. The results suggest an application of this class of materials as an acoustic polarizer medium.

The high-frequency asymptotic description of pulses radiated by a circular normal transducer into an elastic half-space

Abstract: A new method for simulating the propagation of pulses radiated by a circular normal ultrasonic transducer which is directly coupled to a homogeneous and isotropic elastic half-space is proposed Both nonuniform and uniform high-frequency asymptotics inside geometrical regions as well as boundary layers (penumbra, an axial region, and a vicinity of the critical rays) have been used to describe the transient field by means of harmonic synthesis The nonuniform asymptotic formulas involving elementary or well-known special functions elucidate the physics of the problem and give explicit dependence of the radiated waves upon the model parameters The formulas are applicable in the radiating near field which is the near-field with the evanescent wave zone excluded The code based on the uniform asymptotics has been tested in all regions against an exact numerical solution It is orders of magnitude faster, but in many realistic cases the accuracy does not suffer The limits of applicability of the model have been established

Propagation of acoustic waves in nematic elastomers

Abstract: We develop a theory of elastic waves in oriented monodomain nematic elastomers. The effect of soft elasticity, combined with the Leslie-Ericksen version of dissipation function, results in an unusual dispersion and anomalous anisotropy of shear acoustic waves. A characteristic time scale of nematic rotation determines the crossover frequency, below which waves of some polarizations have a very strong attenuation while others experience no dissipation at all. We study the anisotropy of low-frequency Poynting vectors and wave fronts, and discuss a "squeeze" effect of energy transfer nonparallel to the wave vector. Based on these theoretical results, an application, the acoustic polarizer, is proposed.

On Identification and Validation of Some Geothermal Models

Abstract: Various distributed and lumped parameter models of the Wairakei geothermal reservoir, New Zealand, are discussed within a unifying mathematical framework. The need for proper system identification is emphasized. The best lumped parameter model obtained by system identification techniques is presented and interpreted as a slow drainage model. Validation of different models is conducted by studying their forecasting powers with identified parameter values and by testing these identified values for compatibility with additional data.

Seismic Ray Theory

Abstract: Preface 1. Introduction 2. The elastodynamics and its simple solutions 3. Seismic rays and travel times 4. Dynamic ray tracing. Paraxial ray methods 5. Ray amplitudes 6. Ray synthetic seismograms Appendix. Fourier transform, Hilbert transform and analytical signals References Index.

Percolation, statistical topography, and transport in random media

Abstract: A review of classical percolation theory is presented, with an emphasis on novel applications to statistical topography, turbulent diffusion, and heterogeneous media. Statistical topography involves the geometrical properties of the isosets (contour lines or surfaces) of a random potential $\ensuremath{\psi}(\mathrm{x})$. For rapidly decaying correlations of $\ensuremath{\psi}$, the isopotentials fall into the same universality class as the perimeters of percolation clusters. The topography of long-range correlated potentials involves many length scales and is associated either with the correlated percolation problem or with Mandelbrot's fractional Brownian reliefs. In all cases, the concept of fractal dimension is particularly fruitful in characterizing the geometry of random fields. The physical applications of statistical topography include diffusion in random velocity fields, heat and particle transport in turbulent plasmas, quantum Hall effect, magnetoresistance in inhomogeneous conductors with the classical Hall effect, and many others where random isopotentials are relevant. A geometrical approach to studying transport in random media, which captures essential qualitative features of the described phenomena, is advocated.

Inverse methods for subsurface flow: A critical review of stochastic techniques

Abstract: The development of stochastic methods for groundwater flow representation has undergone enormous expansion in recent years. The calibration of groundwater models, the inverse problem, has lately received comparable attention especially and almost exclusively from the stochastic perspective. In this review we trace the evolution of the methods to date with a specific view toward identifying the most important issues involved in the usefulness of the approaches. The methods are critiqued regarding practical usefulness, and future directions for requisite study are discussed.