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Abstract: A theoretical analysis of laminar-turbulent transition in supersonic gas flow is performed basing on the results of direct numerical simulation of flow around a flat plate within the framework of Navier—Stokes equations. The similarity law of the burst formation frequency is confirmed. The spectral analysis of the flow indicates that it is the resonant three-wave interaction that is realized in the turbulent boundary layer. The effect of the plate temperature on gas flow turbulization and the spectral composition of pressure fluctuations is studied. PubDate: 2022-04-01

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Abstract: The problem of hypersonic flow past an absolutely thin edge is considered. The solution is obtained using a model kinetic equation for polyatomic gases. The method of the problem solution, which makes it possible to distinguish the discontinuity of the molecule distribution in the velocity space, is described. The distribution of the normal stress over the plate surface and above it is calculated. The results obtained are in good agreement with the experimental data. It is shown that a disturbed flow region arises ahead of the thin edge of a plate in hypersonic flow PubDate: 2022-04-01

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Abstract: The solution to the problem on the behavior of an ice cover on the surface of an ideal incompressible fluid of finite depth under a local pressure domain in the presence of a shear current is constructed. The cases of the impulse action and the time-periodic axisymmetric load are considered. The Fourier transform is used within the framework of the linear wave theory. The deflection of ice cover as a function of the flow velocity gradient, the compression coefficient, and the ice cover thickness is investigated. PubDate: 2022-04-01

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Abstract: Three-dimensional (axisymmetric) flows of an ideal incompressible fluid are considered in multiply connected regions. In a coordinate system fitted to the velocity potential and the stream function the analogs of the powers of a complex variable are constructed for quasiconformal axisymmetric mappings; they are similar with those used for conformal mappings in the two-dimensional case. Quasianalytical polynomials of an arbitrary degree are considered and the functions conjugate to them are chosen. The solutions satisfying the Laplace equation in the cylindrical coordinate system are determined in the form of formal powers in the “operator” form. The solutions of problems of flow past bodies of finite dimensions, in semi-infinite domains, and in axysimmetric channels of complicated shape in the presence of an internal body are presented. PubDate: 2022-04-01

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Abstract: The laminar-turbulent boundary (edge) separates trajectories approaching a turbulent attractor from those approaching a laminar one, at least for a finite time. To investigate the flow dynamics on the edge we carried out direct numerical simulations of transitional pipe flow (here at Reynolds number Re ∈ [2200, 2800]) in a long computational domain. The studied solution has the form of a structure localized in space and traveling downstream. Its qualitative characteristics are similar to the turbulent puffs observed experimentally in the transitional Reynolds number regime. The dynamics within the saddle region of the phase space on the separatrix (hyper-surface in pipe flow) appears to be chaotic. Here, we report such localized solutions on the edge/separatrix for pipe flow and investigate their correlation to turbulent puffs using a minimal set of (artificial) restrictions to the states, i.e., the mirror symmetry, and investigate the resulting flow behavior in this subspace. In contrast to higher symmetry restricted solutions, here detected solutions on the separatrix turn out to be quite as complex as the full state solutions. Worth emphasizing that any solutions found in the subspace are also solutions of the full space and therefore represent physical (symmetric) flow states. PubDate: 2022-04-01

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Abstract: The parametric solution to the problem of flow of multimode viscoelastic fluids with the Giesekus and Phan-Thien–Tanner models in a circular pipe is given in the case of laminar isothermal steady-state flow regimes at moderate and high Weissenberg numbers. The distributions of the axial velocity and the normal stresses are presented. In addition, the distributions of the first normal stress coefficients for flow of the low-density polyethylene (DSM Stamylan LD 2008 XC43) and a 0.25% aqueous polyacrylamide solution are analyzed. The experimental and calculated data are in the adequate agreement for both (Giesekus and Phan-Thien–Tanner) rheological models. A particular case of the Giesekus viscoelastic fluid flow with the rheological model parameter in the interval (0.5, 1) is considered. PubDate: 2022-04-01

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Abstract: The stability of advective flow in a rotating plane horizontal incompressible fluid layer with rigid boundaries is investigated. The linear temperature distribution is specified on the upper boundary of the layer and the lower boundary is heat-insulated. Advective flow developed due to horizontal convection is described analytically in the form of a new exact solution of the Navier–Stokes equations in the Boussinesq approximation. The stability of advective flow with respect to normal perturbations is investigated at a low Prandtl number over a wide range of the Taylor number within the framework of the linear theory. The most dangerous modes are determined and the neutral curves are constructed. The effect of rotation on the structure of finite-amplitude perturbations in the supercritical domain in the neighborhood of minima of neutral curves is studied within the framework of the nonlinear formulation of the problem. PubDate: 2022-04-01

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Abstract: Free internal waves in a two-dimensional vertically inhomogeneous stratified flow are considered in the Boussinesq approximation with account of the Earth’s rotation and turbulent viscosity and diffusion. The third-order implicite Adams method is used to solve numerically the boundary-value problem for the amplitudr of vertical velocity of internal waves. It is found that the eigenfunction and the wave frequency are complex. Taking turbulent viscosity and diffusion into account leads to the fact that the imaginary part of wave frequency is negative, i.e., wave is damped. The vertical wave momentum fluxes are nonzero and can be commensurable with or greater than the corresponding turbulent fluxes. PubDate: 2022-04-01

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Abstract: The problem of acid treatment in carbonate reservoirs at different scales is considered. The problem of two-phase multicomponent Darcy flow through a porous medium with chemical reactions is solved at the core scale. The regimes of acid injection into the core that lead to the wormholing phenomena are investigated. The present study makes it possible to determine the wormhole growth rate at various reaction kinetics parameters and injection rates and for various permeability distributions. The dependences obtained are used in the large-scale model of fracture acidizing in a reservoir (FAR) that also includes the model of two-phase multicomponent flow with chemical reactions through a porous medium and the model of acid transport through a fracture. Additional contribution of wormholes to the conductivity is simulated. Based on the model proposed, a series of calculations of fracture acidizing in a reservoir is carried out, the effect of taking into account the wormholes in the large-scale model proposed is investigated, and the influence of the constitutive parameters of the process considered such as the fracture length, the reservoir permeability, and the solution injection rate on the effectiveness of treatment estimated in terms of the production ability of well after treatment is studied. PubDate: 2022-04-01

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Abstract: Experimental investigations of liquid jet flows performed earlier in the presence of an artificial cavity having a negative cavitation number showed that under certain conditions cavitation self-oscillations with high pressure fluctuation intensity can occur in hydraulic systems. In this study, we consider the flow in a manifold consisting of a resistance (cavitator), an artificial gas cavity, and a convergent nozzle, through which the liquid and the gas flow out into the atmosphere. It is found that at moderate cavity ventilations, when the self-oscillations are absent, pressure pulses of impact nature are observable in the region of interaction between the jet and an obstacle (shield). Apparently, this effect is due to an intense development of Taylor structures at the boundary of the cavity with a negative cavitation number that forms in this flow. The dependence of the intensity of the periodic pulsed impact on the obstacle on the nozzle convergence is investigated for two frequency regimes of cavitation self-oscillations. The calculations performed within the framework of a one-dimensional model of the outflow of an incompressible fluid portion from the nozzle are in good agreement with the experimental data. PubDate: 2022-04-01

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Abstract: We consider the flow of a viscous compressible gas in a closed rectangular resonator induced by harmonic oscillations of its boundary on the first resonance frequency. The method of successive approximations is used to study the two-dimensional acoustic streaming in a resonator of arbitrary width. The existence of an acoustic streaming in the form of four Rayleigh vortices and four Schlichting vortices is revealed. The similarity between the acoustic streamings occurring in the cases of horizontal harmonic oscillations of an enclosure and oscillations of a resonator wall is shown, which indicates a weak influence of the means of standing wave generation on the acoustic streaming pattern. It is found that as the channel width decreases, the Schlichting vortex dimensions increase compared with those of Rayleigh vortices. When the channel width is less than six thicknesses of the acoustic boundary layer, the Rayleigh vortices disappear and only the Schlichting vortices remain. It is established that in the case of an oscillating enclosure the centers of the Rayleigh and Schlichting vortices lie in the same cross-section, while in the case of the resonator with an oscillating boundary the centers of the Schlichting vortices are displaced toward the vertical walls. PubDate: 2022-02-01

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Abstract: — The influence of the body shape on gas cooling (decrease in the stagnation temperature) in regions of reduced total enthalpy in the wake is studied. The problem is simulated by numerically solving the Navier–Stokes equations for a two-dimensional viscous perfect gas flow around thermally insulated bodies at the Reynolds number \({\text{Re}} = 1000\) and the Mach number \({{{\text{M}}}_{\infty }} = 0.4\) . Elliptic-cross-section cylinders and a pair of side-by-side circular cylinders located across the flow are considered. The accuracy of the results of simplified models for describing the phenomenon based on the velocity field in the developed wake is discussed. Some examples that demonstrate that neglecting the total enthalpy redistribution during the vortex formation process in such models can lead to considerable deviations from the data of numerical calculations in vortex cores are given. PubDate: 2022-02-01

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Abstract: — The nonlinear equations of multilayer shallow water in the Boussinesq approximation are used for simulation of unsteady hydrophysical processes in the shelf zone of a tidal sea. The vertical distributions of the temperature and the horizontal velocity component along a straight path are calculated on the basis of the data on variation in temperature at given horizontals obtained on one of the bottom stations. The results of numerical simulation are verified using the data of other checking stations located along the path considered. It is shown that the multilayer model reproduces qualitatively the structure of the wave train observed and the equations of two-layer flow describe dynamics of the central part of the thermocline even when the stratification differs significantly from the two-layer stratification. Comparison of the calculation results with the field data makes it possible to conclude that the hypothesis on the quasi-two-dimensional nature of shelf flow that underlies the used approach corresponds to reality. PubDate: 2022-02-01

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Abstract: Oscillations of plates with the doubly periodic structure of stiffeners in fluid are considered. The results of the experimental investigation of energy dissipation obtained from the damped oscillations of elastically suspended plates in a tank with fluid are given. The energy loss as a function of the dimensionless oscillation amplitude and the Reynolds number is investigated. Main attention is focused to determination of the energy dissipation per unit area of plates when the oscillation amplitudes are commensurable with the spatial period of stiffening ribs. PubDate: 2022-02-01

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Abstract: — The two-dimensional time-dependent problem of evolution of an initial perturbation in fluid with an ice cover floating on the fluid surface is solved in the linear formulation. In the undisturbed state the longitudinal fluid velocity varies linearly with depth. The ice cover is simulated by a thin elastic plate with taking into account the compressive forces. The integral representation of solution that describes the behavior of the ice cover is constructed. Asymptotics of this solution for large times which make it possible to describe the far fields of perturbations both in the neighborhood and far from the wave front are obtained. PubDate: 2022-02-01

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Abstract: — The problem of constructing axisymmetric nose parts having minimum aerodynamic drag in the range of large subsonic flight velocities under a given constraint on the aspect ratio is solved. The search for optimum shapes is based on the local linearization approaches, which were used in analyzing the results of simulations within the framework of Navier—Stokes equations and ensured the convergence with a limiting reduction in direct calculations of the numerical optimization process at large (more than 70) number of geometric parameters. The effect of additional restrictions imposed on the generator curvature on the drag is studied. The nose parts thus constructed and those having near-optimum characteristics are compared at subsonic and supersonic velocities; the latter forebodies are the Ryabouchinsky half-cavity and a truncated power-law body. The known feature of the bodies realizing zero or minimum wave drag at a given length, namely, the possibility of the formation of flat-faced nose as a region of boundary extremum, is confirmed. PubDate: 2022-02-01

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Abstract: — An experimental investigation of the effect of small angles of attack on laminar-turbulent transition in the supersonic boundary layer on a swept wing with the sweep angle of the leading edge 72° is performed for the Mach numbers 2 and 2.5 (subsonic leading edge) and 4 (supersonic leading edge). The measurements were carried out in the T-325 wind tunnel of the Institute of Theoretical and Applied Mechanics of the Siberian Branch of Russian Academy of Sciences using a constant temperature hot-wire anemometer. The growth curves and the amplitude-frequency fluctuation spectra are obtained and the locations of laminar-turbulent transition in the supersonic boundary layer on a swept model wing are determined for several values of the angle of attack. It is shown that variations in the angle of attack have a strong effect on the transition Reynolds number at the freestream Mach numbers М = 2 and 2.5. At Mach 4 a small variation in the angle of attack does not lead to a considerable increase in the transition Reynolds number. A decrease in the transition Reynolds number with an increase in the Mach number fixed for the model wing with the sweep angle χ = 72° is in good agreement with the data for the model wing with χ = 45°. PubDate: 2022-02-01

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Abstract: — Flow structures around a wavy square cylinder with a perturbation wavelength of 5.6D are investigated using large eddy simulation at the Reynolds number of 23 500. The detailed force characteristics and wake flow structures of the wavy square cylinder are captured and compared with a square cylinder. Under the effect of wavy leading edge up to 27% and 98% reduction in the mean drag and the lift fluctuations are achieved, respectively. The 3D mean flow field implies a high shear flow between node and saddle positions, which is associated with the generation of additional streamwise and vertical vortex pairs. These vortex structures are responsible for the three-dimensionality of the wake flow behind the wavy square cylinder. The instantaneous flow patterns suggest that the staggered pattern of von Kármán vortex shedding is suppressed and replaced by symmetric vortex shedding into the near wake region of the wavy square cylinder. This symmetric vortex structure in the near wake plays a role in preventing the upper and lower shear layers from interacting with each other. The time–frequency analysis exhibits a reduction in the dominating vortex shedding frequency for the case of wavy square cylinder, indicating a relatively steady wake flow. This can be attributed to the vortex dislocation behind the wavy square cylinder. In the range of high frequencies, small streaks associated with small-scale fluctuations are enhanced by the wavy leading edge. The present study suggests that the perturbation wavelength obtained from the wavelength of Mode A instability at laminar flow regime can be served as a basis for flow control in turbulent flow regime. PubDate: 2022-02-01

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Abstract: The aim of the study is to interpret physically the effect of restructuring of the near wake produced by two cylinders under the action of a glow discharge. For this purpose, a version of the simple model of near wake is constructed on the basis of perturbation theory. Within the framework of this model, the complex wake is represented by two (according to the number of generating cylinders) von Karman partial streets which interact between each other in the zone of wake formation in the neighborhood of cylinders. Each of the streets can be simulated by an oscillator of the Van der Pole–Stuart–Landau type. On the basis of comparison of the experimental data for flow velocity oscillations and the model predictions, the wake regimes observed in gas and plasma flows are identified with global model modes. It is stated that the restructuring of the wake from a global hydrodynamic mode into intermittent modes was observed and this bifurcation is caused by the strengthening of interaction between the zones of formation of the von Karman partial streets resulted from the discharge switching-on. PubDate: 2022-02-01

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Abstract: The results of an experimental investigation of subsonic turbulent flow past a lengthy cavity in a flat plate or the wall of a plane-parallel channel are presented. The cavity represents a cylindrical trench of finite span with rounded spherical ends. The pressure coefficient distributions are obtained and the streamline patterns on the cavity surface are visualized for different angles of inclination of its longitudinal axis to the oncoming flow. The effect of these parameters on the intensity of separated swirl flow in the cavity is discussed. PubDate: 2022-02-01