Physics of Wave Phenomena最新文献

Further evidences of effectiveness of а two-dimensional approach to modeling of three-dimensional deep-water potential waves are given. The 2-D model is based on the same two surface conditions as 3-D, but instead of а 3-D Laplace equation (used routinely for calculation of surface vertical velocity) the surface projection of Laplace equation is suggested for use. This equation is not closed, since it contains both the vertical velocity and its vertical derivative. The closing scheme is based on consideration of vertical structure of a nonlinear component of the velocity potential. It was shown before that the surface vertical velocity and its derivative are linearly connected with a coefficient depending on some integral parameters of the problem. The applicability of the 2-D model for reproducing statistical properties of wave field was demonstrated before for relatively simple integral characteristics and spectra. The paper is devoted to comparison of more complicated statistical results generated by full 3-D model and current 2-D model. A good agreement between the high order moments for elevation and surface vertical velocity and some other characteristics proves the applicability of the model for reproducing of statistical structure of a multimode wave field with satisfactory accuracy. The main advantage of 2-D model is that it runs 30–80 times faster than a 3-D model with similar setting.

The electronic properties of magnetic semiconductors TlFeS₂ and TlFeSе₂ have been investigated experimentally by spectral ellipsometry and theoretically ab initio using the density functional theory (DFT). The imaginary and real parts of the dielectric function and the dispersion of the refractive indices, extinction coefficients, and absorption coefficients are found from ellipsometric measurements in the energy range of 0.7−6.5 eV. The direct band gap is estimated. The band structure, the origin of energy states, the optical functions, and the partial densities of states (PDOS) projected on atoms are determined from ab initio calculations. The calculation results are compared with the spectral ellipsometry data obtained in this study.

Variation in the parameters of the positron annihilation lifetime spectra in pure water presubjected to freeze–thaw processes is studied. It is shown that in freshly defrosted water (at 1–2°С), the ortho-positronium lifetime and formation probability tend to the corresponding values in ice (near its melting point), and after about 10 h they reach the values corresponding to ordinary water at the same temperature. It indicates that no complete (at molecular level) defragmentation of the ice lattice occurs immediately after ice melting, and rigid ice-like clusters, in which positronium formation is also possible remain in the liquid phase for several hours.

The dynamics of the reduction of hydrochloroauric acid (HAuCl₄) aqueous solutions and the formation and disintegration of gold nanoparticles under high-intensity irradiation by Ti׃Al₂O₃ laser pulses (τ = 130 fs, f = 1 kHz, N = 6 × 10⁴–3 × 10⁶ pulses, average radiation power of 0.8 W) has been investigated by optical absorption spectroscopy. This high-intensity irradiation is found to cause formation (both during irradiation and after it), along with single gold nanoparticles several ten nanometers in diameter, elongated (chain) submicron structures, consisting of individual nanoelements of different shape. Specific features of the mechanisms of formation and evolution of nanostructures, caused by the multiphoton ionization of a liquid, its heating, and modification of nanostructures as a result of intense impact of an ultrashort laser pulse, as well as liquid heating by radiation with a high average power, are discussed.

Laser-induced breakdown spectroscopy (LIBS) is first used to identify small impurity amounts in a three-dimensional ordered submicron dielectric structure exemplified by a synthetic opal matrix (SOM). A possibility of detecting metal-containing substances infiltrated in a SOM is demonstrated. The effect of the photonic bandgap and the excitation wavelength on the LIBS spectrum is shown.

High-intensity focused ultrasound (HIFU) is a therapy method to treat the tumors in prostate, liver, kidney, pancreas, bone, breast, and uterine fibroids. In the HIFU therapy process, the ultrasound generated in an ultrasonic transducer concentrates on a focal zone. At the zone, the temperature rises locally up to 56°C to provoke the necrosis of human tissue. Therefore, to control the therapy process, it is essential to perceive the main principle of heat generation in human tissue. We study FDTD-LBM (finite difference time domain—lattice Boltzmann method) as a method of predicting the temperature distribution in human tissue during HIFU therapy. The nonlinear Westervelt wave equation is employed for computing the pressure distribution in human tissue during ultrasound propagation, while the Pennes bio-heat transfer equation is used for calculating the temperature distribution in the tissue. Finite difference time domain (FDTD) is applied to solving the nonlinear Westervelt wave equation, and the lattice Boltzmann method can solve the Pennes bio-heat transfer equation. Simulation results have shown that the numerical simulation method proposed has improved the accuracy in analyzing the temperature field in human tissue.

The pressure field distribution of a ballistic shock wave (BSW) therapy device is a crucial factor for clarifying its treatment mechanism. We developed a lattice Boltzmann model (LBM) to describe the propagation of BSW. Based on the assumption that the propagation of BSW causes weak compressible flow, our simulaton was performed by coupling Tait equation of state. For a two-dimensional LBM, we used the density initial condition for initial turbulent region near the applicator. We first compared our simulation results with previous experimental measurements. Then we predicted the temporal and spatial distribution of pressure field. The pressure field of ballistic shock wave has a primary compressive region followed by a primary expansive region with the other disturbances. A secondary pressure pulse consists of a positive phase followed by a negative phase. Our results agree well with previous experimental data and provide additional data on the pressure field of BSW. Our model encourages further investigation to clear the biological mechanism of BSW therapy and to design more effective device.

Holographic processing of electrocardiograms is proposed on the basis of the windowed Fourier transform of the electrocardiosignal that converts it from the time representation to the frequency–time representation, to the modulus squared of which the two-dimensional frequency–time Fourier transform is applied. At the integrated transformation output the spectral density of the signal is localized in the form of focal spots containing information on the temporal distribution of spectral densities of separate waves in the electrocardiogram. Experimental results of holographic processing of electrocardiograms are presented, which are obtained for patients with a normal heart and with congestive heart failure.

For the first time, stimulated dynamic scattering of a light wave in a liquid nanodispersed medium is observed, which is caused by phase modulation of the dipole moment of the suspension nanoparticles subjected to acoustically induced vibrations in the electromagnetic field. Experiments have been carried out in an aqueous suspension of amorphous silicon dioxide SiO₂ nanospheres ≈100 and ≈350 nm in diameter under stimulated scattering excited by the second-harmonic radiation of the pulsed single-mode Nd³⁺:YAG laser. In addition to the first Stokes component of the stimulated Brillouin scattering with the frequency shift Ω typical of a solvent, a scattering line with the frequency shift ≈2Ω is observed in full agreement with the theory developed in the work.