No 5 (2023)

The forced nonlinear oscillations of a gas bubble in a liquid are studied when the frequency of oscillations of the external pressure of the liquid is equal to the natural frequency of the bubble oscillations (resonance). Using the averaging method, a simple formula is derived for the dependence of the amplitude of oscillations of a gas bubble on the amplitude of the external pressure and the thermophysical characteristics of the gas and liquid viscosity. Its good agreement with numerical calculations is shown up to the value of the amplitude of oscillations of the bubble radius comparable with its equilibrium value.

A research of the occurrence factors of narrow non-stationary movements formed in the Black Sea in the lower part of the constant piccoline and deeper is performed. Such currents spread along the continental slope against the predominant cyclonic direction of surface circulation (undercurrents) are observed from several days to several weeks and have a width of 8 to 12 km. The prerequisites for the generation of undercurrents in the Black Sea hydrophysical fields are studied on the example of its north-eastern region, where the undercurrents are fixed most often. The numerical experiment results are analyzed to calculate the hydrophysical fields based on the numerical z-model of MHI with a horizontal resolution of 1.6 km. The experiment is carried out for the 2016-2017 period with assimilation in the model of hydrological observations of temperature and salinity. ERA5 data are used to set atmospheric forcing. The undercurrents are  fixed twice in September 2016 and February 2017 according to the observations of the ARGO float ID6901833 in the area of the North Caucasus coast. A change in the direction of currents in the specified area (from cyclonic to anticyclonic and back) is reconstructed in the numerical experiment. Fields of basic hydrophysical parameters and some of their derivative characteristics are built and analyzed. The influence of mesoscale anticyclonic eddies, as well as gradients of the sea water density on the structure and variability of the velocity field in the north-eastern part of the sea is shown. According to the simulation results, narrow undercurrents into which Argo float fell  in the fall of 2016 and in the winter of 2017 are distributed within a few days in the anticyclonic direction along the continental slope on horizons from 50–100 to 500 m, and their formation occurred under the conditions of weakening cyclonic Main Black Sea current in the upper layer of the sea and the increase in the Kerch anticyclonic eddy.

Using the full Navier-Stokes equations, the linear stability of a plane Poiseuille flow in a channel with a corrugated bottom wall is considered. The wall is corrugated across the flow, and the main flow has one velocity component. The perturbations of the velocity and pressure fields are three-dimensional with two wave numbers. The generalized eigenvalue problem is solved numerically. It is found that the critical Reynolds number, above which time-increasing disturbances appear, depends in a complex way on the dimensionless amplitude and the corrugation period. The magnitude of the ratio of the amplitude and the corrugation period divides the area of the dimensionless amplitude of the corrugation into two, where the dependences of the critical Reynolds number on the parameters of the corrugation are qualitatively different.

New tests are presented for application packages and turbulence models for validation problems of high-intensity vortex flows near structured energy-efficient surfaces. The idea of testing is based on the discovery of anomalous intensification of separated flows and heat transfer in inclined grooves on plates and channel walls. In the grooves, due to the extraordinary pressure drops, confirmed by experiments, swirling flows are generated with high velocities of the return and secondary flow, comparable to the speed of the external flow. Also, high-gradient zones with friction and heat transfer are formed inside the grooves, many times (from 1.5–2 to 7–9 times) exceeding friction and heat transfer on a flat wall. As an example, the VP2/3 package, developed on the basis of original multi-block computing technologies and using multi-scale intersecting grids, is being tested. Comparison of numerical predictions with Zubin’s experiments confirmed high static pressure drops between the stagnation zone on the windward slope of the inclined groove and the negative pressure region at the site of generation of a tornado-like flow on the inlet spherical segment, and also demonstrated the acceptability of the RANS approach for predicting the characteristics of high-intensity swirling flows.

The stability of the boiling surface of water at rest in high-temperature rock, when the domain of ​​water is located above the domain of vapor is investigated. It is shown that the solution is not unique and there are two positions of the interface. As the parameters change, the solutions approach each other and the solutions seases to exist. The normal mode method was used to study the stability of the positions of the interface. A dispersion equation was obtained, which was studied numerically and analytically. It is shown that the transition to instability occurs with a decrease in pressure in the vapor region or an increase in pressure in the water region. Bifurcation diagrams are presented, illustrating the merging and nonexistence of solutions, and parts of the branches corresponding to stable and unstable flow regimes are highlighted.

The transverse impact of an impulsive water microjet on single cylindrical fibers is investigated. The stages of ejection, breakup, and collision of the microjet were recorded using high-speed photography methods. Significant slowing down of the microjet by the fiber and splitting into two parts were detected. The mechanisms of the observed phenomena and the influence of various factors are discussed.

The integral and temporal spectral characteristics of shock-heated air in the shock wave velocity range from 7.35 to 10.4 km/s at a pressure p0 = 0.25 Torr before the shock wave front were measured. The experiments were carried out on the DDST-M shock apparatus of the Moscow State University Institute of Mechanics. The radiation wavelength range  = 600-1100 nm, corresponding to the visible and near-infrared spectral regions, in which the main contribution to the radiation is provided by atomic nitrogen and oxygen lines, was investigated. The analysis of the obtained integral in time spectrograms of radiation is performed. The peculiarities of time oscillograms for the most typical atomic lines of the spectrum are highlighted. The measurement data are compared with experimental data of other authors.