卷 53, 编号 6 (2018)

The stability of spiral flow occurring in the simultaneous presence of a pressure difference in the channel between two coaxial cylinders and rotation of one of the cylinders is studied. It is shown that depending on the azimuthal Reynolds number the modes with different azimuthal wavenumbers can be most unstable. The data of calculations are in good agreement with the available experimental data. It is shown that certain experimental results correspond to the earlier unknown regime of the zeroth azimuthal mode instability. The stability characteristics of spiral flow of a nanofluid based on water with zirconium oxide particles are studied. In all the cases considered the nanofluid is less stable than the baseline fluid. The degree of nanofluid flow destabilization increases with increase in the particle concentration and a decrease in their dimensions.

The comprehensive experimental analysis of the fluid viscosity effect on the standing gravity waves excited at parametric resonance is carried out. The viscous effects on the frequency range of excitement of the second wave mode, its resonance dependences, and the processes of damping and approaching the steady-state regime are quantitatively estimated by varying the viscosity over a wide range. It is found that the waves are regularized without breaking when the kinematic viscosity of the workingmedium becomes higher than a threshold value. A mechanism of viscous regularization of wave motion is suggested. In accordance with this mechanism, the effects observed experimentally relate to the presence of the shortwave cutoff domain in which viscous dissipation becomes the dominant factor and the shortwave perturbations responsible for breaking the standing wave are suppressed.

The dynamic and thermal characteristics of unsteady near-wall flows are investigated numerically on the basis of two-parameter turbulence models under conditions of high-turbulence free stream and impact of perturbing heat- and mass-transfer factors in the boundary layer. The effect of mass-transfer parameters considered on the permeable section on the development of dynamic and thermal processes in the steady-state turbulent boundary layer is studied and the boundary layer structure along the surface is investigated. The mutual action of time harmonic oscillations of the velocity of outer inviscid free stream and the heat-transfer parameters on wall on the development of time-dependent heat-transfer characteristics in turbulent flow is analyzed. The numerical results are compared with experimental and theoretical data.

The interaction between the supersonic boundary layer on an infinitely thin plate and acoustic waves is investigated on the basis of direct numerical simulation for Mach 2 incident flow. The parametric numerical investigations of the disturbances generated within the boundary layer by an acoustic wave arbitrarily oriented in space are for the first time performed. The calculations are carried out for different angles of incidence and sliding (in the latter case the wave vector is parallel to the plate surface) and frequencies. The main calculations are performed for the Reynolds numbers slightly greater than the critical values of the loss of stability. It is established that the velocity disturbance amplitude in the boundary layer is several times greater than that of outer acoustic wave. At small incidence and sliding angles the oscillation intensity increases with increase in the Reynolds number. At fairly large values of these angles the Reynolds-number-dependences of the disturbance amplitude contain maxima which are displaced toward the leading edge of the plate with increase in the angle. For a fixed point on the plate and a fixed frequency there exist critical sliding and incidence angles, at which the disturbances generated in the boundary layer are maximum. The excitation of oscillations in the boundary layer by a sound wave is more effective if the plate is irradiated from above. On the basis of the calculations performed at different frequencies it is shown that the location of a minimum in the dependence of the generated velocity disturbances coincides at a good accuracy with the position of the lower branch of the neutral stability curve.

Development of new techniques for detection of CO₂ gas is significant for decrease the dangers of CO₂. In this research, numerical simulations are performed to evaluate the performance of a new micro gas sensor (MIKRA) for the detection of CO₂ gas. This device works due to temperature difference inside a rectangular enclosure with heat and cold arms as the non-isothermal walls at low pressure condition. In this study, the pressure of CO₂ is varied from 62 to 1500 Pa correspond to Knudsen number from 0.1 to 4.5 to investigate all characteristics of the thermal-driven force inside the MEMS sensor. In order to simulate a rarefied gas inside the micro gas detector, Boltzmann equations are applied to obtain high precision results. To solve these equations, Direct Simulation Monte Carlo (DSMC) approach is used as a robust method for the non-equilibrium flow field. Our findings show that value of generated Knudsen force significantly different when the fraction of CO₂ in N₂–CO₂ mixtures is varied. This indicates that this micro gas sensor could precisely detect the concentration of CO₂ gas in a low-pressure environment. In addition, the obtained results demonstrate that the mechanism of force generation highly varies in the different pressure conditions.

The interaction between a shock wave and a round area of a gas of increased density is simulated numerically by solving Euler’s equations in the two-dimensional plane and axisymmetric formulations. These formulations of the problem describe the interaction of a shock with a heavy gas cylinder or bubble, respectively. The shock refraction and focusing processes are described considering two different regimes of interaction—external and internal. It is revealed that there are three successive pressure peaks reached on the axis (plane) of symmetry both outside and inside of the shocked inhomogeneity. The dependence of the peak pressures on the initial gas density in the inhomogeneity considered is determined for two different shock Mach numbers and it is found that in the plane and axisymmetric cases the highest pressures are reached in different regimes.

Different approaches to the theoretical description of the energy characteristics of turbulence are considered on a wide wavenumber range including both the inertial and the dissipation intervals. The description is based on the hypothesis on the locality of intermodal interactions and the cascade mechanism of energy transfer over the wavenumber spectrum. The concept of the Markovian nature of the transfer makes it possible to use the methods of renormalization group and to derive a renorm group equation supplementing the conventionally used dimensional considerations and the energy balance equation. Within the framework of a model that does not contain unphysical singularities of the type of the inverse spectral flux formulas for the spectral characteristics are obtained beyond the turbulence generation region at different Reynolds numbers determined by the turbulent energy pumping parameters on the small wavenumber range modeled by an external random force.