Fluid Dynamics最新文献

The methods of high-speed videorecording are applied for the first time to trace the evolution of the fine structure of the distribution of the free-falling coal slurry matter in a cuvette filled by tap water in different flow regimes. Multi-point illumination is used to reduce the unwanted light and the effect of complete internal reflection. In intrusive regime at small contact velocities, when the kinetic energy of the drop (KRD) is smaller than its potential surface energy (PSE), the drop of heavier suspension flowing smoothly into a receiving fluid forms a lenticular intrusion. The submerging intrusion transforms gradually into a vortex ring, which breaks down gradually into systems of new vortex rings, as in the classical experiments of J.J.Thomson and H.F. Newell. In the impact regime, where the ratio of the energy components is inverse, a confluent drop of the suspension deforms the fluid surface and breaks down into slender jetlets, whose traces form colored lineate structures and reticular formations on the fluid surface and within its thickness. The vortical head walls of the jetlets are slowly enlarged in motion and form colored ringlets after their stoppage. The suspension descents on the cavity bottom and penetrates into the fluid thickness, where it is gathered in an intermediate layer and distributed in a system of loops beneath the collapsing cavity. The pattern of the carbon microparticle distribution restructures itself rapidly with further flow evolution, as in the case of the confluence of an electrolyte drop.

The bubble collapse over a stainless steel surface under four distinct conditions: (i) surface inclination (α = 0°, 15°, and 30°), (ii) surface motion (U = 0, 50, and 75 m/s), (iii) surface roughness (K_s = 0, 0.015, and 0.030 mm), and (iv) multiple bubble interactions (bubble separation, s = 0, 1, and 2 mm) is investigated. The simulations were carried out using ANSYS Fluent software, applying the volume-of-fluid (VOF) approach to monitor bubble dynamics and estimate the pressure distribution. Two standoff distances d/R_max = 0.97 and 1.42, where d is the distance from the bubble center to the rigid surface and R_max is the maximum radius of the bubble taken as 3.5 mm, are used. The results show that surface inclination reduces the peak collapse pressures owing to oblique shockwave interactions, with the pressures decreasing by 38.24% at α = 30° as compared to a normal surface. Surface motion enhances collapse asymmetry, reducing the peak pressures by 34.57% at U = 75 m/s, particularly, at d/R_max = 0.97. An increase in in the surface roughness significantly lowers the localized pressure by 27.75% at a roughness height of 0.030 mm. In multiple bubble interactions, s = 0 generates up to 8.73 × 10⁶ Pa; however, this pressure decreases by over 60% at s = 2 mm, highlighting the influence of bubble spacing on the collapse intensity. These findings provide critical insights into bubble dynamics, erosion mechanisms, and material resilience and offer design guidelines for marine and industrial applications involving cavitation.

The HB-2 numerical simulation results are presented for the Mach numbers from 1.5 to 3 at angles of attack from –2° to +24°. The calculated results are compared with the experimental data [Vuković, D. and Damljanović, D., 2019] on the force and moment characteristics. The calculations were conducted using the method of splitting in physical processes realized on unstructured grids. The results calculated on hexagonal and prismatic grids are compared.

Various methods of the impact on the boundary layers that make it possible to control the Reynolds analogy factor are considered. The results of investigation of the effect of heat transfer enhancement, superposition of large-scale vortex structures, as well as the longitudinal pressure gradient are described. Dimples of various shapes and arrangements are considered as the way of heat transfer enhancement. Superposition of vortex structures on the wall in the cylinder wake is studied. An adverse pressure gradient is studied in compressible and incompressible nonequilibrium boundary layers, whereas a favorable pressure gradient is studied in a supersonic nozzle. The results obtained show that in the flows considered, the Reynolds analogy factor can be greater than the values for the zero-pressure-gradient boundary layer.

The article presents an overview of the studies carried out in the Institute of Mechanics of Moscow State University in 2021–2024. The results of experimental studies of the application of wave propulsors (WP) of the flapping wing type and direct-flow wave propulsors (DFWP) on a 1700 mm-long model of a small waterline area vessel (SWAV) are presented. The NACA0015 profile is used as a working element of both the flapping and direct-flow WPs. In the case of the direct-flow WP the profile is rigidly fixed against the hull with the wing chord inclination of 30°. The efficiency of using the WPs of the underwater sail type is also studied. Different WP types are tested on a smaller-scale DFWP model, 840 mm in length. Comprehensive studies of the efficiency of the direct-flow WPs and the underwater sail type WPs are carried out on a small model. An inclined plate is used as a working element of the direct-flow WP. The optimal parameters of the direct-flow WP (length, inclination, plate immersion) are experimentally determined. The effect of the hull immersion depth (draft) and the immersion depth of the underwater sail type WP on the vessel velocity counter waves is studied. The experiments show that the efficiency of flapping wings or underwater sail wave propulsors in the operating range of wave frequencies is slightly higher than that of DFWP. However, in stormy sailing conditions, DFWP has an advantage, since it shows the highest efficiency just in such conditions, while the other options considered are effective in the operating range of wavelengths that depends on the ship length and, generally speaking, does not coincide with the length of storm waves.

The most significant results of numerical study of the processes in heat exchangers with diffuser channels, obtained by the authors between 2020 and 2024, are reviewed. The plate and double-pipe heat exchangers with various gaseous and liquid coolants are considered. The performed studies showed that the amount of heat transferred from the “hot” to “cold” coolant increases as compared to heat exchangers with smooth channels of constant cross-section, due to the enhancement of heat transfer in heat exchangers with diffuser channels. The results obtained can serve as a basis for the development of new advanced heat exchangers.

Numerical simulations of turbulent separated air flow and heat transfer over a thermally insulated plate containing single V-shaped grooves with hemispherical ends and opening angles of 95°, 180°, and 275° on an isothermal rectangular insert were performed by solving the Reynolds-averaged Navier–Stokes and energy equations using multiblock computational techniques. The closure of the momentum equations employed the differential equations of the SST shear stress transport model. The results revealed a fundamental difference in the mechanisms that enhance separated flow and heat transfer in the grooves, associated with the formation of a U-shaped vortex at the bend of the V-groove and two tornado-like vortices at the upstream-facing ends of the Λ-shaped groove. A localized static pressure drop of about 0.3 between the external flow stagnation regions and low-pressure zones accelerated both the recirculating and secondary swirling flows. The total relative heat transfer from the rectangular section bounding the contour of the V-groove increased by nearly 30% compared to a flat plate, whereas for a transverse groove, the corresponding value reached 1.12.

The air-cushion craft that had appear in the thirties of the last century have been thus far among the best two-medium vehicles. However, the caterpillar tracked surface craft developed in recent years (snow mobiles) are in certain cases not inferior to them as concerns their running and operation performance. They are capable to roll over the water surface at a large velocity, their advantages being obvious in the case of motion over dry and shallow water. We note also that that the caterpillar tracks play the role of loth load-bearing surfaces and propulsors, which offers also certain advantages. In this paper, we discuss the hydrodynamic forces acting on caterpillar tracks in the case of the rolling over a free water surface and present certain experimental data.

The results of studies conducted at the EKUD experimental complex of the Institute of Mechanics of Moscow State University over the past ten years are analyzed. They include primarily the radiation characteristics of shock-heated gases measured in a wide range of wavelengths (λ = 115–1100 nm) at shock wave velocities up to 11.4 km/s and the gas pressure in front of the shock wave of 0.25 Torr, as well as the ignition characteristics of combustible mixtures based on hydrogen, propane, and propylene obtained at both low (T ≤ 1000 K) and high (T ≥ 1000 K) temperatures. Electron concentrations in the low-temperature plasma in the vicinity of a strong shock wave were measured.

Topical questions of constructing a model for measuring the rate constants of gas-phase chemical reactions are considered. These questions include aligning the experimental conditions and the initial data for a system of kinetic equations, constructing analytical models of kinetic curves, and estimating the rate constants for the reaction considered and the secondary processes. Using the example of one of the important chemical reactions, OH + O → O₂ + H, which is a part of the mechanism of combustion of a hydrogen-oxygen mixture, the problem of selecting the optimum method for processing the experimental kinetic curves is solved. The optimization criterion is obtaining a more complete volume of information on the rate constants of both primary and secondary processes, as well as the fulfillment of requirements for the convergence of the results of the iterative process for determining the rate constants for different experimental conditions for obtaining the kinetic curves and identical conditions that determine the measured quantity. The principle of “invariance of the measured quantity” is used to estimate the rate constants of secondary processes. This principle states that, under the experiment conditions that the measured quantity is constant, the method for determining the quantity should ensure a minimum range of variation in its values. In constructing the model and its optimization, for four processes in the base set (58 reactions), the changes in the rate constants were recorded at the temperature T = 298 K under the experimental conditions in which molecular hydrogen serves as the source of hydroxyl OH.