St Petersburg Polytechnic University Journal-Physics and Mathematics最新文献

Flows around sets of airfoils with different shapes and thicknesses have been numerically investigated at relatively high Reynolds numbers (Re ≥ 10⁶) and low turbulence intensity (I ≤ 0.1%) using two-dimensional (2D) Reynolds-Averaged Navier-Stokes equations (RANS) closed by different turbulence models. The effects of a set of factors such as wind tunnel walls, the compressibility and the influence of the laminar-turbulent transition were investigated. The most likely reasons for the systematic disagreement between simulation and experimental data were established to be 3D effects ignored by 2D simulation, and imperfection of the existing turbulence models.

The influence of various factors on the stability of the performance parameters of compact CO₂ waveguide lasers (the LCD series) produced by Plazma JSC (Russia) has been considered for the first time. An angular misalignment of cavity mirrors was found to be the main reason for the relative instability of laser emission in the frequency range of 0–0.5 Hz. These lasers work under RF excitation and have a power output from 3 to 50 W in a non-selective oscillating mode. Lasers can work at a fixed emission wavelength or in the wavelength-change mode. Preprogrammed dynamic control of the laser's emission spectrum can be carried out in the automatic mode using the built-in servo driver from a personal computer. The CO₂ waveguide lasers of the LCD series can be used in laser lidar systems, particularly, for observation of small-sized objects and environmental monitoring.

The paper studies the applicability of the acoustoelasticity method to estimation of the strain-stress state under cyclic loading. It has been found that the uniform distributions of acoustic anisotropy along the working part of the sample and of the velocities of longitudinal and transverse ultrasonic waves become substantially non-uniform with an increase in the number of stress cycles. Moreover, the largest absolute values of acoustic anisotropy fell on the points with the largest plastic deformations, in particular, in the area of sample dispersion. The effect was being recorded from the early stages of sample loading until the fracture of the sample.

Electric and magnetic fields homogeneous in Euler's sense are a useful instrument for designing the systems of charge particle optics. The similarity principle for the charged particle trajectories in these fields was applied by Golikov for the first time to create spectrographic charge particle optical systems in a more systematic and intelligence way when using the fields being homogeneous in Euler's sense. This paper studies the Laplace potentials homogeneous in Euler's sense. The coefficients of the polynomials are functions of the two rest coordinates; they are presented not by the polynomial but ought to be the functions harmonic and homogeneous in Euler's sense. We have solved a finite chain of Poisson equations starting from the highest coefficients. By means of the proposed procedure we obtained new classes of potentials which provided a base for electric and magnetic spectrograph systems.

This paper studies the stress–strain state of an U-shaped expansion bellows under an internal-pressure induced load. The bellows is considered as a corrugated shell of revolution under axisymmetric load. The governing equations have been derived and the numerical calculations of the stress–strain state were carried out. A variant of the classical shell theory based upon Lagrangian mechanics was used. The finite-difference method was applied to solve the obtained system of ordinary differential equations. The ultimate internal pressure resulting in plastic deformations was determined. A simulation of the loss of equilibrium stability of the expansion bellows was performed. The ANSYS software was used for Finite-Element Method (FEM) in order to calculate the stress–strain state in the bellows.

The linear and nonlinear dielectric properties of composites based on KIO₃ and nanocrystalline Acetobacter Xylinum cellulose have been studied. This cellulose is structured so that it has a large amount of nano-channels with parallel arrangement and with diameters of 50–100 nm and the lengths being thousand-fold. The behavior of linear dielectric permittivity of bulk KIO₃ was found to have four anomalies at the temperatures corresponding to phase transitions. The temperature elevations of 20 and 24 K, respectively, for the IV → III and III → II structural transitions were revealed for KIO₃ in the pores of nanocrystalline cellulose relative to the corresponding transition temperatures in bulk KIO₃. Possible causes for the revealed temperature elevations were discussed.

A mock-up of а fiber-optic Doppler blood flow-velocity sensor has been made and described. The principle of its operation is based on the recording of the Doppler shift of scattered radiation of a monofrequent single-mode semiconductor laser. The radiation was inserted into a blood vessel model using a fiber-optic probe. The performance data of the mock-up in the blood vessel model was measured. The designed apparatus was shown to make possible the reliable measurement of the blood flow velocity in the blood vessels through inserting the fiber optic probes. The measurement accuracy depends on the accuracy of the used recording equipment. The performance data of the designed apparatus, that obtained using the blood vessel model, meets all modern requirements.

The behavior of acoustic anisotropy and the longitudinal wave velocity in the case of multiaxial stress-strain state of the plate under inelastic deformation has been studied experimentally. The plate had a stress concentrator in the form of the central hole. The results for several deformation levels, and the results of finite element analysis of active stresses were presented. The qualitative agreement between the calculated stress fields and the distribution fields of acoustic anisotropy was revealed. It was found that the absolute magnitude maximum of acoustic anisotropy fell on the areas with the biggest stresses near the concentrator. It was supposed that the non-uniform distribution of acoustic anisotropy in the material testified to a possible stress concentration at the corresponding points.

Axonometric projections together with the corresponding graphs for fullerenes are constructed in the range from 32 to 60. The growth of fullerenes is studied on the basis of a mechanism according to which a carbon dimer embeds in a hexagon of an initial fullerene. This leads to stretching and breaking the covalent bonds which are parallel to arising tensile forces. In this case, instead of a hexagon adjoining two pentagons, two adjacent pentagons adjoining two hexagons are obtained. As a result, there arises a new atomic configuration and there is mass increase of two carbon atoms. We considered the direct descendants of fullerene C32; namely, C2n, where n = 17–30.

A numerical study of the secondary flow in two geometrically different models of a common carotid artery has been carried out. One of the models (Model 1) is characterized by a statistically averaged curvature, and the second one (Model 2) is attributed to the maximal curvature of the artery. It was shown that the most intensive swirl occurred at the phase of flow rate decreasing, the maximum values of the swirl parameters were observed at the interface of the cervical and thoracic segments of the artery. This interface is the place where the Dean vortices are transformed into a single vortex forming a swirling flow. The swirl intensity averaged over the systole and characterized by the ratio of the maximal values of the axial and circumferential velocities was evaluated as 0.20 for Model 1 and 0.25 for Model 2. Generally, it was in accordance with the data of clinical measurements.