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No 4 (2024)
Articles
Absolute and global instability of plane submerged jets
Abstract
The family of velocity profiles of a submerged jet, which are absolutely unstable in the plane-parallel approximation, is considered. The profiles are specified by two parameters, the first of them is responsible for the location of the only inflection point in the velocity profile, and the second is responsible for the shear layer thickness. An algorithm for determining the length of the section of local absolute instability of the jet with a given input velocity profile, that is, the distance at which absolute instability gives way to convective instability, has been implemented. The dependence of this length on the parameters defining the input profile is obtained. A connection between the characteristics of local absolute instability calculated in the plane-parallel approximation and global instability of the jet evolving in space is analytically obtained. The input velocity profile that corresponds to sufficiently large length of the zone of local absolute instability, at which global instability of spatially developing jet occurs is demonstrated. Thus, the possibility of existence of global instability of plane submerged jets with special velocity distributions is demonstrated.
3-17
Self-induced displacement and rotation of a melting ice disk on the still water surface
Abstract
The results of an experimental investigation and physical modeling of self-induced displacement and rotation of an ice disk on the still water surface are presented. The dependence of the ice specimen rotation velocity on the water salinity and the depth of the experimental container is measured. It is shown that the reason for observable motions over the still water surface is the cellular convective flow generated by the ice melting process.
18-28
Interrelation between the local acceleration of laminar flow in a channel and the anomalous heat transfer enhancement in inclined two-row grooves
Abstract
The anomalous enhancement of laminar separation flow and heat transfer (AELSFHT) is studied in a channel with two rows of 26 densely packed grooves inclined at angles of ±45о in the case of uniform flow at the entry and the Reynolds number Re varying from 1000 to 5500. The local flow acceleration is validated, when the greatest flow velocity becomes of the order of 1.8 in dimensionless units and the wall layer becomes thinner above the spherical entry segments. In this case, the longitudinal velocity increases to a value of 1.4 at a distance y = 0.005 from the wall for Re = 2500. The interrelation between the local acceleration at the channel center and the AELSFHT is established, the minimum value of the negative acceleration amounting to –25 at Re = 5500 and the relative heat removal from the structured region of the channel reaching up to 5.2.
29-54
Distinctive features of propagation of a turbulent pulsed gas-droplet eddy cloud
Abstract
We present the results of the numerical modeling of the formation and motion of a blown solitary pulsed turbulent gas-droplet jet under the conditions approximately corresponding to human cough. The calculations are performed for the pulse duration t = 0.6 s and the greatest velocity of the gas phase of 20 m/s at the mass content of droplets ML1 = 1%. The drop phase in the exit section is monodisperse, while the initial dimension of particles in the calculations varied in the range D1 = 5‒30 μm. Two zones of elevated vorticity are formed within the cloud in the initial period of motion. They are situated in the mixing layer and in the region of deceleration of two-phase pulsed jet. The greatest levels of the longitudinal velocity and the kinetic energy of turbulence are attained in the interval of pulse blow-off. At the subsequent moments of time the turbulence velocity and level monotonically decrease. The vortex cloud produced by the solitary pulse exists for a fairly long time (t ≈ 4 s) and has a time to penetrate into the surrounding space at a distance greater than 3 m.
55-68
Convective flows of a colloidal suspension in a Hele-Shaw cell under vertical vibrations
Abstract
Numerical simulation of convection of a colloidal binary mixture in a Hele-Shaw cell under vertical vibrations of finite amplitude has been carried out. The boundary of convective instability in a modulated gravity field is found. Bifurcation diagrams are constructed and the concentration distributions of nanoparticles corresponding to various solutions are analyzed. Thermal vibration convective flows are studied in the case of positive thermal diffusion of nanoparticles and their gravitational stratification. It is shown that vibrations, depending on the frequency, can both increase the intensity of the convective flow and weaken it.
69-78
Two-layer equilibrium model of miscible inhomogeneous fluid flow
Abstract
Two-layer flow of a density-stratified fluid with mass transfer between the layers is considered. In the Boussinesq approximation, the equations of motion are reduced to a homogeneous quasilinear system of partial differential equations of mixed type. The flow parameters in the intermediate mixed layer are determined from the equilibrium conditions in a more general model of three-layer flow of a miscible fluid. In particular, the equilibrium conditions imply the constancy of the interlayer Richardson number in velocity-shift flows. A self-similar solution to the problem of breakdown of an arbitrary discontinuity (the lock-exchange problem) in the domain of hyperbolicity of the system under consideration is constructed. The transcritical flow regimes over a local obstacle are studied and the conditions under which the obstacle determines the upstream flow are determined. A comparison of steady-state and time-dependent solutions with the solutions obtained for the original three-layer models of miscible fluid flow is carried out.
79-93
Development of stationary disturbances in a spatially developing jet
Abstract
Nonmodal development of stationary three-dimensional disturbances in a circular jet is numerically investigated at the Reynolds number Re = 2850. The operating conditions of a laboratory experiment performed earlier in the Institute of Mechanics of Moscow State University are reproduced. A method for calculating optimal disturbances under the conditions of downstream developing main flow is developed. The disturbances associated with different azimuthal numbers are calculated. The shape, character of development, and growth degree of optimal disturbances are determined.
94-102
Bifurcations and stability of phase transition fronts in geothermal reservoirs
Abstract
The stability of a layer of water above a layer of vapor, separated by a boiling or condensation surface, in a geothermal reservoir is considered. In an unperturbed state in lowpermeability rocks, there is one interface, which can be either a water boiling surface or a vapor condensation surface. At relatively large permeability values, two new solutions can be formed, corresponding to other positions of the interface. The conditions for the existence and merging of stationary solutions depending on the parameters of the physical system are studied numerically. The stability of stationary positions of interfaces was studied using the normal mode method. It was found that the transition to instability precedes bifurcations of solutions and can occur both at finite and infinite wave numbers.
103-111
Experimental and theoretical investigation of the effect of dissolved surfactant on the dynamics of gas bubble floating-up
Abstract
We present the results of an experimental and numerical investigation of the floating-up of a solitary gas bubble in the presence and absence of dissolved surfactant at Reynolds numbers Re > 1. An original numerical technique is proposed, which makes it possible to investigate the dynamics of a solitary bubble with account for the effects occurring when a surfactant is introduced into the fluid. The effect of the surfactant concentration on the bubble dimensions, shape, and rise velocity are analyzed. Three stages of the bubble rise in the presence of a surfactant are established; they characterize its accumulation and distribution over the free surface of the bubble.
112-127
Round splashes of a viscous liquid
Abstract
The splashes of a highly viscous fluid (glycerol) resulting from its pulsed displacement from a gap between two rapidly approaching disks are studied. It is found that, outside the disks, the splash has the form of a thin film bounded by an annular rim. A physical model of the splash is formulated, and analytical solutions describing its trajectory are given. The calculation results are compared with experimental data. The effects of fluid viscosity, surface tension, and film breakdown are analyzed. It is shown that the key influence on the splash development scenarios is exerted by surface tension of the film connecting the rim to the disks.
128-141
Modelling of the process of volume annihilation in the gravitational self-field
Abstract
The process of volume annihilation of particles and antiparticles in the gravitational self-field is considered. The problem of homogeneous compression of such a system is solved both within the framework of the general relativity theory and in Newtonian mechanics. Two models of collapsing matter are considered, namely, hot and cold dust filled with radiation. The radiation pressure gradient is taken into account.
142-149
Acoustic resonance in an annular cavity with axial transit flow
Abstract
The results of a coordinated computational and experimental study of acoustic self-oscillations in an annular cavity surrounding a circular pipe with a local narrowing are given. In the experiment, pressure fluctuations were measured on the outer wall of the annular cavity for various volume flow rates; air entered the pipe at the atmospheric pressure. It was found that the flow regime with excitation of acoustic self-oscillations in the cavity is implemented in a certain range of flow rates. The oscillation frequency corresponds to the first natural frequency, and the root-mean-square values of pressure fluctuations reach a level of 2300 Pa. Numerical simulation based on the RANS approach, carried out for the geometry and conditions of experiment, reproduces the observed effect of acoustic excitation of the cavity and gives similar values of the fluctuation amplitude. The oscillation modes developed at various volumetric flow rates are analyzed based on the obtained calculated data.
150-162
Computational analysis of flight data on convective heating of the Martian descent vehicle within the framework of the perfect gas model
Abstract
The spatial problem of supersonic flow past the MSL descent space vehicle in the dense layers of the Martian atmosphere is solved using the perfect gas model. The system of Reynolds-averaged Navier-Stokes (RANS) equations is numerically integrated together with the Baldwin-Lomax algebraic turbulent mixing model. In addition to studying the flow field patterns in the vicinity of the descent vehicle for real trajectory conditions, the calculated data on convective heating of the surface on the windward and leeward sides are analyzed. Change in the heating conditions during laminar-turbulent transition near the surface is taken into account. A comparison with flight data is presented.
163-174