Fluid Dynamics最新文献

The paper presents selections from a videofilm illustrating the rapid evolution of the fine flow structure during the merging of a water drop free-falling into a laboratory pool with tap water. In the initial stage of merging in the impact regime a packet of running periodic annular perturbations around the growing crown was visualized for the first time. The motion of the packet consisting of five to seven rings was traced according to the motion of a group of specks reflected from inclined sections of the fluid surface. The radial distances between the crests of perturbations in the packet (analogs of wavelengths) increase monotonically with the distance from the source. The time dependences of the lengths of individual components and the entire packet of annular perturbations, the velocities of its leading and trailing edges, and the frequency of disturbances at chosen points were determined. The dependences of the component dimensions on the frequency, as well as the cyclic frequency on the wave number of perturbations, were calculated. The conventional annular capillary waves of greater length are formed with a certain delay; they catch up the package of primary non-stationary disturbances and absorb them.

The specific features of flight of celestial bodies in the Earth’s atmosphere are studied, using mathematical modeling and numerical calculations, with regard to the motion of bodies about their center of mass. The ballistic parameters of strong spherical and elliptical bodies are compared in varying the parameters of their entry into the Earth’s atmosphere. It is found that, when flying in the atmosphere, the bodies with configuration close to the shape of an elongated ellipsoid tend to occupy a position at a certain angle to the flow. This affects the aerodynamic properties of the bodies. It was found that very different flight regimes are possible at small angles of entry of celestial bodies into the atmosphere when the body configurations differ from the “correct” (spherical) shape and the positions of the center of mass do not coincide with the center of the body shape.

The results of eddy-resolving ILES simulation of turbulent mixed convection in a rapidly rotating annular cavity with a central shaft under the conditions of the well-known experiment (Bohn, 2000) are given. The cavity is heated from the disk surfaces and from the periphery, while the cooling air flows through a narrow annular channel along the shaft. The use of a relatively fine computational grid (approximately 9 million hexahedral cells concentrating toward the walls) ensured acceptable resolution of thin quasi-laminar Ekman layers and small-scale eddies, that play an important role in the heat transfer processes near the disks. A particular attention is given to the formation of realistic flow conditions at the cavity inlet (including the presence of turbulent content), taking into account the characteristics of the cooling air supply duct of the experimental facility. The proposed modification of the boundary conditions led to a dramatic improvement in the quality of heat transfer calculations in the near-axial region of the disk as compared to calculations without turbulent content at the cavity inlet.

Pump-jet propulsion is gaining attention due to its multifunctional operation, strong hydraulic performance, and low noise levels, especially as traditional propulsion systems begin to reach their performance limits. This research combines computational fluid dynamics (CFD) and computational aeroacoustics (CAA) methods, validated through experiments conducted on a custom test equipment. The analysis explores how various factors such as the inlet flow rate, the impeller speed, and the number of blades affect non-cavitating noise in axial flow pumps. It is found that the lower flow rates, especially under off-design conditions, lead to increased overall sound pressure levels (OASPL). For instance, reducing the flow rate by 200 m³/h results in a 2.74 dB increase in OASPL, whereas a 100 m³/h increase in the flow rate causes only a 0.50 dB rise. Additionally, increase in the impeller speed has a more pronounced effect on overall sound pressure levels (OASPL) than decrease. There is also a nonlinear relationship between non-cavitating noise and the number of blades, indicating complex interactions. These results provide essential insights for predicting non-cavitating noise and optimizing acoustic design in the axial flow pumps.

The investigations performed in the Institute of Mechanics of Moscow State University on the evolution of velocity perturbations that propagate in a submerged incompressible jet with an extended laminar section are reviewed. The delay of laminar-turbulent transition is ensured by a developed unique jet-forming device. The first part of the study is devoted to the modal mechanism of perturbation growth in jet flow. Thin rings oscillating at various frequencies were placed in the jet at a small distance from the orifice to amplify the growing jet eigenmodes. A good agreement of the experimental and theoretically predicted wavelengths, the radial distributions of velocity fluctuations, and the growth rate of eigenmodes is demonstrated. The second part of the review considers a non-modal (algebraic) mechanism of perturbation growth. Special wave-like structures (deflectors) that excite a “roller-like” transverse motion were introduced in the jet. The specific features of turbulence transition caused by such steady-state perturbation are considered. Based on the experimental results, a non-modal growth mechanism for introduced perturbations, similar to the “lift-up” mechanism in near-wall flows, is identified. The development of disturbances qualitatively corresponds to the obtained theoretical optimal disturbances of the flow under consideration. The third part of the paper presents the results of computational study of the algebraic growth of perturbations in round submerged jets with the Michalke velocity profiles. A parametric analysis of the optimal growth of spatial disturbances was carried out.

The paper presents an overview of theoretical, computational, and experimental studies devoted to the modeling of the aerodynamics of different configurations of the high-speed Hyperloop train representing a pod (wagon) in motion within a tunnel, where a considerable rarefaction is ensured. A noticeable increase of the train velocity is expected under these conditions; it can be greater than 1200 km/h. The review gives a compact presentation of the information on the most characteristic conditions of the potential operation of Hyperloop, such as the gathered velocity, the pressure and temperature in the tunnel, and the ratio of the pod and tunnel cross-sectional areas. The main gasdynamic features of the high-speed Hyperloop motion in a tunnel with rarefied gas are listed; they have the governing effect on aerothermodynamic characteristics and the general efficiency of this transport system.

This integrated study quantifies the suction intensity (C_q) effects on shock-induced laminar separation bubbles (LSBs) and crossflow instability on a supersonic finite-span 65° swept wing at a Mach number equal to 2 and a unit Reynolds number 3.29 × 10⁷ m^–1. Results demonstrate that low-strength suction (C_q < 0.01) triggers the LSBs through recompression-shock-induced adverse pressure gradients. Conversely, high-strength suction (C_q > 0.01) suppresses the LSBs by establishing sustained favorable pressure gradients. At the threshold C_q = 0.01, flow control is balanced: uniform N-factor distribution is maintained while crossflow instability growth is suppressed; beyond this threshold (C_q > 0.01), however, suction amplifies crossflow instability. These results resolve the trade-off between separation suppression and instability mitigation, establishing C_q = 0.01 as the optimal design parameter for crossflow instability delay in supersonic swept wings.

New experimental results on studying lower Faraday wave modes on the surface of a liquid in a rectangular vessel are given. It is shown that the amplitude of oscillations of the standing wave nodes increases with decrease in the liquid depth and the wave profiles are characterized by local mobile disturbances associated with the manifestation of a second harmonic relative to the fundamental one. For the first time, the trajectories of tracer particles on the free liquid surface near a node were experimentally determined; the particles oscillate along curves with upward convexity. A comparison of experimental and theoretical data demonstrated the empirical validity of the Lagrangian approach.

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.