


卷 53, 编号 6 (2018)
- 年: 2018
- 文章: 16
- URL: https://journals.rcsi.science/0015-4628/issue/view/9453
Article
Perturbation Growth Rate in Turbulent Couette Flow
摘要
The evolution of small perturbations in turbulent Couette flow is investigated numerically at the Reynolds numbers Reτ from 35 to 125. Steady-state turbulent flows calculated on the basis of solving the Navier-Stokes equations are used as the basic flow to study the process of development of the perturbations against their background. The values of the senior Lyapunov exponent λ1 which characterizes the maximum growth rate of small perturbations in the stochastic systems are determined. It is found that the exponent, being normalized by the near-wall scales, is equal to λ1 ~ 0.02. This is in agreement with the results of the previous investigations of turbulent flows in the circular pipe and plane channel. It is shown that the λ1, whose value is smaller by three times and which was obtained earlier in calculating the Lyapunov spectrum in Couette flow at Reτ = 35 in the so-called “minimum channel”, can be explained by insufficient dimension of the computational domain but not by smallness of the Reynolds number.



Stability Regimes of Flow in a Channel between Coaxial Cylinders
摘要
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.



Anisotropic Problem of Darcy Convection: Family of Steady Flows and Its Disintegration during the Destruction of Cosymmetry
摘要
Fluid convection in a porous rectangle is analyzed on the basis of the Darcy model with regard to anisotropy of the thermal characteristics and the permeability. Relations between the parameters for which the problem belongs to the class of cosymmetric systems are obtained. In this case explicit formulas for the critical numbers of the loss of stability of mechanical equilibrium are derived. Using a finite-difference method that retains the cosymmetry of the problem, families of steady convective regimes are calculated. The destruction of these families is demonstrated by means of the computational experiment in the case of violation of the cosymmetry conditions leading to appearance of a finite number of stationary regimes.



Effect of Fluid Viscosity on the Faraday Surface Waves
摘要
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.



Control of Laminar-Turbulent Transition on a Swept Wing Using Surface Microreliefs
摘要
The possibility of laminarization of flow around a swept wing using surface microreliefs in the form of riblets and separate plates (winglets) set at an angle to external streamlines is theoretically investigated. This relief is shown to reduce the crossflow velocity in the boundary layer due to a change in the direction of the near-wall streamlines. On the basis of very simple models based on an analysis of the relief period-averaged boundary layer flow an increase in the Reynolds number of laminar-turbulent transition obtained using the microrelief is estimated. The reasonable surface relief parameters for experimentally investigating the effectiveness of the proposed technique of controlling laminar-turbulent transition are determined.



Modelling of the Interaction of Unsteady High-Intensity Turbulence Flow with Heat- and Mass-Transfer in the Boundary Layer on the Surface
摘要
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.



Distinctive Features of Pressure Fluctuation Fields on the Surface of Steps
摘要
Parametric experimental investigations of the fields of turbulent wall pressure fluctuations on the surface of steps are performed. A considerable effect of the obstacle height on the spectral composition of the pressure fluctuations behind the leading edge is shown. The transformation of the spectral composition at different observation points is illustrated. The influence of the boundary layer thickness and the sweep angle is shown to be relatively weak. The spatial variation of the local correlation scales and the convective properties of the pressure fluctuation field on the step surface is illustrated.



Numerical Modeling of the Interaction between a Supersonic Boundary Layer and an Acoustic Wave
摘要
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.



A Model of Incompressible-Fluid Flow with a Free Surface in a Highly Porous Medium
摘要
A model is proposed for modeling a slow (inertialess) flow of a thin layer of viscous incompressible fluidwith a free surface inside a highly porousmediumabove an impermeable surface. The fluid flow is described by the Brinkman filtration law, on the bottom boundary the Navier slip condition is prescribed. The equation for the fluid layer thickness is obtained, which has the form of a special case of a nonlinear heat conduction equation. Solutions in the form of traveling waves and self-similar solutions describing the fluid spreading are investigated.



Application of Knudsen Thermal Force for Detection of CO2 in Low-Pressure Micro Gas Sensor
摘要
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.



Experimental Investigation on the Unsteadiness in Shock Wave/Boundary Layer Interaction
摘要
The unsteadiness of Shock Wave/Boundary Layer Interaction (SWBLI) generated by the two-dimensional compression ramps in hypersonic flow was investigated. The experiments were carried out in a Mach-6 low-noise wind tunnel under three different unit Reynolds number. The flow structures of 22◦ compression ramp were quantitatively analyzed by schlieren images. Through the measurements of the wall fluctuating pressure, the high-frequency unsteadiness with a characteristic frequency about 320 KHz is found in interaction regions by the Power Spectrum Density analysis and the influence area of this unsteadiness expands when the ramp angle increases. The results also show that the stronger adverse pressure gradient caused by the higher unit Reynolds number under the same ramp angle lead to the greater unsteadiness. Finally, the nonlinearity of the shock/boundary layer interaction is verified by bicoherence analysis.



Shock Focusing in the Interaction with a Local Area of Increased Gas Density
摘要
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.



Viscous Fluid Flows Induced by Translational-Oscillatory Motion of a Submerged Porous Sphere
摘要
Viscous fluid flows induced by a translational-oscillatory motion of a submerged porous sphere are determined. The fluidmotion inside and outside the sphere is considered in amoving noninertial reference frame fitted to the sphere. Exact analytical solutions of the Navier–Stokes equation outside the sphere and the nonstationary Brinkman equation inside the sphere are found. Examples of streamline patterns for some values of governing parameters are presented. It is shown that the results obtained earlier for particular cases of viscous flow around an impermeable solid sphere can be obtained from the solution presented.



Spectral Characteristics of Turbulent Velocity Fluctuations at Different Reynolds Numbers
摘要
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.



Energy Spectra and Fluxes in Dissipation Range of Turbulent and Laminar Flows
摘要
Two well-known turbulence models to describe the energy spectrum in the inertial and dissipative ranges simultaneously are by Pao (1965) and Pope (2000). In this paper, we compute energy spectrum E(k) and energy flux Π(k) using direct numerical simulations on grids up to 40963, and show consistency between the numerical results and predictions by the aforementioned models for turbulence flows. We also consider the laminar flow in which viscosity dominates over nonlinearity. For this case we suggest a modified model that predicts E(k) ~ k−1 exp(−k) and Π(k) ~ k exp(−k) in dissipation range of scales and verify it using numerical simulations. We emphasize the difference revealing local energy transfer for the turbulent flows and nonlocal one for the laminar flows at low Reynolds number.



Erratum
Erratum to: Optimal Axisymmetric Noses of Bodies in a Flow. Calculations and Experiments
摘要
Page 303, last line should read: (project No. 17-01-00126).


