Journal of Applied Mechanics and Technical Physics最新文献

The dynamics of an acoustic signal passing through a bubble curtain has been studied by mathematical modeling. The case is considered where the gas (methane) in the bubbles is under hydrate formation conditions. The effects of the hydrate formation process, the initial bubble radius, and the volume gas fraction on the attenuation coefficient and phase velocity of the wave propagating in a deep-water bubble curtain are investigated. The reflection and transmission coefficients of acoustic waves at the boundaries separating single- and two-phase regions are obtained. It is found that the hydrate formation process increases the wave attenuation coefficient by more than two orders of magnitude at low frequencies (below 1 kHz).

Solutions having the form of a diffusion wave propagating in a medium at rest with a finite velocity to a nonlinear parabolic reaction–diffusion system are constructed and studied. A problem of initiating a diffusion wave by boundary conditions specified on a sphere (circular cylindrical surface) is considered in the cases of spherical and cylindrical symmetry for the first time. An existence and uniqueness theorem for a solution in the class of analytical functions is proven. An exact solution is constructed in the form of explicit formulas. A step-by-step iterative algorithm based on the collocation method and expansion in radial basis functions is proposed. Numerical calculations are performed, and their results are verified using the exact solution.

The laminar–turbulent transition in transonic flows is studied with the use of hot-wire thin-film surface sensors. For determining fluctuations recorded by the film sensor in the boundary layer, coherence spectra are constructed between the data obtained by the film sensor and by the hot-wire probe, which was used to perform measurements across the entire boundary layer. Based on the analysis of these spectra, it is demonstrated that the most accurate and complete data are obtained by the thin-film sensor in the intermittency region.

A countercurrent flow of a liquid film and a turbulent gas in a narrow vertical channel is considered. In the case of small Reynolds numbers and under certain conditions of the flow parameters, the problem is reduced to studying one evolutionary integrodifferential equation for the deviation of film thickness from an unperturbed level. Evolution of periodic disturbances is numerically studied. Several solutions to the model equation are presented.

A mathematical model is proposed that describes the formation of internal hydraulic jumps and the mixing of codirectional flows of an ideal stratified fluid in the Boussinesq approximation. The model is based on a three-layer representation of the flow taking into account the entrainment of fluid from the outer layers into the intermediate vortex layer and is represented as a system of inhomogeneous conservation laws. The rate of entrainment is given by the equilibrium condition within the framework of a more general model for the evolution of the mixing layer. The velocity of propagation of disturbances and the concepts of subcritical and supercritical flows are formulated. It is shown that the model is suitable for describing flow mixing and splitting in deep-sea currents. Solutions were constructed that correspond to the flow around an obstacle with the formation of an internal hydraulic jump and a region of intense mixing. The results of numerical simulation were shown to be in good agreement with experimental data.

A method of determining the dynamic yield strength of heterogeneous materials is proposed, based on comparisons of the processes of cavity formation in the reference and examined materials. A series of computations is performed for penetration of an elongated rod into homogeneous and heterogeneous massive targets. A nonmonotonic dependence of the penetration depth on the motion velocity is derived. The yield strength limit of a heterogeneous medium is obtained from the analysis of computation results in the coordinates of the specific kinetic energy of the rod and the work of stresses on plastic strains.

Results of studying gas flows in a setup used for cold gas-dynamic spraying are reported. The setup consists of a cylindrical input channel with a central rod connected at a right angle with a rectangular slotted channel. Injection of a high-pressure gas through the input channel leads to gas acceleration. It is shown that there are velocity and pressure disturbances in a supersonic overexpanded jet exhausting from the channel, which arise due to the presence of compression and expansion waves, as well as streamwise vortices. The contribution of streamwise vortices to pressure and velocity disturbances is approximately 30%. Streamwise vortices are formed at the entrance of the slotted channel owing to the curvature of streamlines.

In this study, the effect of patient’s physical activity in terms of the heart rate on the growth of the thoracic aortic aneurysm (TAA) is studied. Using medical images of the patient, a patient-specific geometry model is constructed. Then the hemodynamic parameters of the blood flow are numerically analyzed for different heart rate conditions. The simulation results show that the maximum wall shear stress, the maximum velocity, and the maximum pressure during a cardiac cycle increase by 19.1, 12.7, and 50%, respectively, as the heart rate increases from 60 to 174 beats per minute. Results also indicate that an increase in the heart rate leads to reduction of the time-averaged wall shear stress and simultaneously to an increase in the wall shear stress oscillations. According to the literature, these hemodynamic conditions are undesirable and can increase the likelihood of aneurysm development and aortic rupture.

This paper describes the study of a method for controlling the physical and chemical properties of an oil reservoir in order to increase its oil recovery by injecting a GBA-F acid composition into the reservoir. An experimental testbed is developed to physically simulate the effect of the acid composition on the oil reservoir. Experiments are carried out using the flat model with a single injection of the reagent into the central wellbore. Experimental results confirm the effectiveness of the acid composition.

A method is proposed for three-dimensional quantification of the shagginess of a pathologically altered aorta with account for a three-dimensional lumen morphology. An algorithm for assessing local shagginess is developed and implemented, and an integral criterion for shagginess in the anatomical region is proposed. The algorithm is tested on data from two patients with a shaggy and a smooth lumen. The results obtained in this study are in good agreement with visual assessment performed by expert surgeons and are also confirmed by clinical outcomes during surgery.