Showing papers in "Fluid Dynamics in 2018"

Problems of Evaporative Convection (Review)

Abstract: The theoretical foundations for mathematical modeling of the convective flows with evaporation are presented, and the topical research areas are given. The special attention is payed to models constructed within continuum mechanics, to comparison of the different formulations of corresponding problems including formulations of boundary conditions at the interfaces. Alternative analytical approaches and experimental studies are briefly discussed in the context of thermal convection accompanied by evaporation (or condensation) in the systems with thin liquid layers which are mostly sensitive to the phenomena of interphase exchange.

Energy Spectra and Fluxes in Dissipation Range of Turbulent and Laminar Flows

Abstract: 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.

Application of Model Kinetic Equations to Calculations of Super- and Hypersonic Molecular Gas Flows

Abstract: For the purpose of taking the internal degrees of freedom into account, threetemperature approximating model equations, which are a generalization of the R- and ES–BGKmodels, are proposed for a diatomic gas. The surface pressure, friction, and heat transfer coefficients are compared with the direct simulation Monte Carlo (DSMC) solution in the problem of flow past a cylinder in the super- and hypersonic flow regimes. The dependence of the surface coefficients on the rotational collision number is analyzed.

Three-Dimensional Problem of Radiative Gasdynamics of the Apollo-4 Command Module during Superorbital Atmospheric Entry

Abstract: The three-dimensional problem of radiative gasdynamics of the superorbital entry of the Apollo-4 command module into the dense terrestrial atmosphere at an angle of attack of 25° is numerically solved The flow conditions corresponding to the flight velocity V∞ = 105 km/s at an altitude H = 673 km are considered in detail The distributions of the densities of convective and radiative heat fluxes along the surface in a flow are obtained The spectral composition of the thermal radiation attaining the surface is studied The results of the calculations are successfully compared with the data of two-dimensional calculations

Mach Wave Effect on Laminar-Turbulent Transition in Supersonic Flow over a Flat Plate

Abstract: The effect of a Mach wave (N wave) on laminar-turbulent transition induced by the first instability mode (Tollmien–Schlichting wave) in the flat-plate boundary layer is investigated on the basis of the numerical solution of Navier–Stokes equations at the freestream Mach number of 2.5. In accordance with the experiment, the N wave is generated by a two-dimensional roughness at the computation domain boundary corresponding to the side wall of the test section of a wind tunnel. It is shown that the disturbance induced by the backward front of the N wave in the boundary layer has no effect on the beginning of transition but displaces downstream the nonlinear stage of the first mode development. The disturbance induced by the forward front of the N wave displaces the beginning of transition upstream.

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

Abstract: 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.

Mechanism of the Emergence of Rogue Waves As a Result of the Interaction between Internal Solitary Waves in a Stratified Basin

Abstract: The features of the interaction between internal solitary waves are investigated within the framework of the completely integrable Gardner equation with positive cubic nonlinearity. It is shown that the soliton polarity affects radically the result of the interaction between the solitons. The role of the pair interactions between solitons of different polarities proceeding when rogue waves emerge in the soliton fields in a stratified basin is demonstrated. The effect of such interactions on the higher-order moments of the wave field is studied.

Application of the Skeleton Model of a Highly Porous Cellular Material in Modeling Supersonic Flow past a Cylinder with a Forward Gas-Permeable Insert

Abstract: The supersonic (M∞ = 4.85) flow past a cylinder with a forward insertmade of a highlyporous cellular material is numerically modeled within the framework of the Reynolds-averaged Navier–Stokes equations. The air flow in the gas-permeable insert is described on the basis of a skeleton model of a highly-porous medium, whose determining parameters are the porosity coefficient (95%) and the pore dimensions (1 mm) of the actual cellular material. The aerodynamic drag coefficients of the model with different lengths of the porous forward insert are calculated on the unit Reynolds number range from 6.9 × 105 to 13.8 × 106 m−1. They are in agreement with the available experimental data, which indicates the adequacy of the proposed skeleton model in describing the actual properties of highly-porous materials.

Heat Exchange in a Cylindrical Channel with Stabilized Laminar Fluid Flow

Abstract: Based on the determination of the temperature perturbation front and additional boundary conditions, an approximate analytical solution is obtained for the stationary heat exchange problem when fluid flows in a cylindrical channel with a constant parabolic velocity profile (the Gretz–Nusselt problem), which allows us to investigate the temperature distribution in the fluid in a wide range of distances from the pipe inlet, including small and very small distances. Based on the data of numerical calculations of temperature change at a certain value of the spatial variable using the solution obtained by solving the inverse heat conduction problem, the Peclet number was found (in the case where it is unknown in the solution obtained), from which we can determine the velocity profile and the flow rate of the liquid. Graphs of the distribution of isotherms and the their velocities in space over time are plotted.

Development of the Strategy of Active Control of Instability Waves in Unexcited Turbulent Jets

Abstract: The possibility of controlling instability waves in the mixing layer of a subsonic unexcited jet is studied These waves can be noise sources in both free jets and jets as parts of configurations In the study the method of experimental diagnostics of the instability waves in the near field of a jet using an azimuthal multimicrophone array is realized The data on the near field fluctuations are used for testing the control strategy proposed by the authors The strategy consists in narrowband sliding filtration of the original signal and the formation of a narrowband controlling action on the basis of the linear principle of signal superposition The results of the study represent the next step toward the realization of an active control system suppressing natural instability waves in turbulent jets

Motion of an External Load over a Semi-Infinite Ice Sheet in the Subcritical Regime

Abstract: The three-dimensional problem of steady-state forced vibrations of fluid and semiinfinite ice sheet under the action of a local external load traveling along the rectilinear sheet edge at a constant velocity is considered. Two cases are analyzed. In the first case the fluid surface outside the ice sheet is free and in the second the fluid is confined by a rigid vertical wall and the ice sheet edge adjacent to the wall can be both clamped and free. The ice sheet is simulated by a thin elastic isotropic plate floating on the surface of fluid of finite depth. The load traveling velocity is assumed to be not higher than the minimum phase velocity of the flexural-gravity waves (subcritical regime). The solution to the linear problem is obtained by means of the integral Fourier transform and matching the expansions of the velocity potential in the vertical eigenfunctions. Examples of the numerical investigation of the ice sheet and fluid displacements are given.

Sustainment of Oscillations in Localized Turbulent Structures in Pipes

Abstract: The solution of the Navier–Stokes equations which reproduces some qualitative features of localized turbulent structures developed in circular pipes at transitional Reynolds numbers is numerically investigated. In the phase space this solution corresponds to the limiting state of the solution which evolves along the separatrix dividing the regions of attraction of the solutions corresponding to the laminar and turbulent flow regimes. Relative simplicity of the spatial and temporal behavior of the limiting solution on the separatrix makes it possible to investigate it in detail. In particular, the nonlinear mechanism of the onset of streamwise vortices responsible for sustainment of near-wall streaks whose instability ensures the presence of fluctuations is revealed.

Acoustic Waves of Various Geometry in Multi-Fraction Bubbly Liquids

Abstract: The propagation of acoustic waves of various geometry in mixtures of a liquid and a disperse phase consisting of small bubbles which differ from one another by both the radii and the thermophysical properties is investigated. A systemof differential equations of motion of the mixture is written and the dispersion relation is derived. The dispersion curves are constructed and damping of the pressure pulses is compared for the plane, cylindrical, and spherical waves in the bubbly liquids considered. The theory is compared with the experimental data.

Steady Flows in an Oscillating Spheroidal Cavity with Elastic Wall

Abstract: The steady flow arising in a spheroidal cavity with periodically-deformed elastic wall is studied experimentally It is found that average flows whose intensities and structures depend on the wall oscillation frequency and amplitude can develop in the fluid The average flow is generated in the Stokes boundary layer whose relative thickness is characterized by the dimensionless frequency of the vibrational action Flow in the form of a pair of toroidal vortices which occupy the entire cavity volume can be observed over the range of low dimensionless frequencies when the boundary layer thickness is comparable with the characteristic cavity dimension Increase in the dimensionless frequency (decrease in the relative thickness of the Stokes layers) leads to a displacement of the primary vortices towards the cavity boundary In this case secondary vortices with opposite swirling are formed in the central part of the cavity above the primary vortices The further increase in the dimensionless frequency leads to development of the secondary vortices and growth of the flow intensity The large-scale secondary vortices occupy almost the entire cavity volume over the range of high dimensionless frequencies The dependences of the regimes of average flows and their intensities on the control dimensionless parameters, the oscillation amplitude and frequency, are found on the basis of the results of the investigation

Effect of Fluid Viscosity on the Faraday Surface Waves

Abstract: 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.

Pore-Scale Investigation of Two-Phase Flows in Three-Dimensional Digital Models of Natural Sandstones

Abstract: The results of numerical simulation of the processes of two-phase flow through a porous medium in three-dimensional digital models of the porous space of three natural sandstone samples are given. The calculations are carried out using the lattice Boltzmann equations and the digital field gradient model over a wide range of the capillary numbers and the viscosity ratios of injected and displaced fluids. The conditions of flow through a porous medium with capillary fingering, viscous fingering and with stable displacement front are revealed.

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

Abstract: 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.

Single-Fluid Model of a Mixture for Laminar Flows of Highly Concentrated Suspensions

Abstract: A model of laminar flow of a highly concentrated suspension is proposed. The model includes the equation of motion for the mixture as a whole and the transport equation for the particle concentration, taking into account a phase slip velocity. The suspension is treated as a Newtonian fluid with an effective viscosity depending on the local particle concentration. The pressure of the solid phase induced by particle-particle interactions and the hydrodynamic drag force with account of the hindering effect are described using empirical formulas. The partial-slip boundary condition for the mixture velocity on the wall models the formation of a slip layer near the wall. The model is validated against experimental data for rotational Couette flow, a plane-channel flow with neutrally buoyant particles, and a fully developed flow with heavy particles in a horizontal pipe. Based on the comparison with the experimental data, it is shown that the model predicts well the dependence of the pressure difference on the mixture velocity and satisfactorily describes the dependence of the delivered particle concentration on the flow velocity.

Investigation of Submerged Jets with an Extended Initial Laminar Region

Abstract: The method of producing laminar submerged jets using a device, whose length is comparable with the jet diameter, is described. A submerged air jet, 0.12 m in diameter, produced by means of this technique is experimentally investigated in the Reynolds number range from 2000 to 13 000. Hot-wire anemometer measurements of the flow parameters and laser visualization of the flow are performed. It is shown that the device developed makes it possible to produce submerged jets with the laminar regions as long as 5.5 jet diameters. The initial regions of such jets can be used to study the development of disturbances in submerged jets, as well as used in medicine and engineering in organizing various gasdynamic curtains which produce zones with given properties with respect to purity and composition inside another gas media.

Status of the Navier–Stokes Equations in Gas Dynamics. A Review

Abstract: The existing ideas on the status of the Navier–Stokes equations are changed in taking into account the following facts: generally speaking, the terms of these equations neglected in the boundary layer equations are of the order of certain Burnett terms in the conservation equations; the Navier–Stokes equations cannot be used to describe slow nonisothermal gas flows since in this case it is necessary to take the Burnett temperature stresses into account; and in the transport relations the Burnett terms determine certain effects (for example, the mechanocaloric effect).

Dipole Electromagnetic Radiation by a Charged Drop Oscillating in a Uniform Electrostatic Field

Abstract: The intensity of electromagnetic radiation generated by a charged drop oscillating in a uniform electrostatic field is studied within the framework of analytical calculations retaining the terms of the second order of smallness with respect to the ratio of the droplet oscillation amplitude to the droplet radius. It is found that the charge induced in the drop surface oscillations generates a dipole radiation detected in the first-order calculations and a self-charge detected with allowance for the second-order terms only. It is shown that the order of the magnitude of the total intensity of radiation generated by a cloud can be determined from small-droplet radiation. Among two radiation sources, namely, the radiation generated by small droplets oscillating at low modes and the radiation generated by hydrometeors oscillating at high modes, the first plays a dominant role.

Experimental and Numerical Investigation of Heat Exchange between Underexpanded High-Enthalpy Air Jets and Cylindrical Models

Abstract: Experiments on heat transfer in supersonic underexpanded high-enthalpy air jets are conducted on the VGU-4 induction plasmatron at the pressure in the compression chamber of 8.5 hPa. At the air flow rate of 3.6 g/s and the high-frequency generator powers of 45 kW(regime 1) and 64 kW (regime 2) the heat fluxes to the copper surface at the stagnation point of watercooled cylindrical models along the axes of dissociated air jets are measured. The models, 30 mm in diameter, could have a flat face or a hemispherical nose. In the same regimes, the stagnation pressures are measured using the Pitot tube in the shape of a cylinder, 30 mm in diameter, having either a flat face or a hemispherical bluntness with a receiving hole, 14 mm in diameter. For the experimental conditions calculations of flows in the plasmatron discharge channel and supersonic underexpanded jets issuing from the discharge channel are performed within the framework of the Navier–Stokes and Maxwell equations. The heat fluxes to the experimental models are computed and compared with the experimental data.

Numerical and Experimental Study of Bubble Dynamics in Contact with a Solid Surface

Abstract: A complex numerical and experimental method is proposed for studying 3D dynamics of a bubble contacting with a surface in the presence of an acoustic field The numerical approach is based on the boundary element method for potential flows, which is most efficient for solving the problems in a 3D formulation The use of heterogeneous computer architectures consisting of central graphic processors and becoming more and more popular makes it possible to increase the scale of the problem and sufficiently reduce the calculation time The mesh destabilization problems are solved using a spherical filter To describe the contact line dynamics, a semi-empirical law of motion is used The experimental method is based on high-speed recording and optical microscopy An air bubble contacts with the inner surface of an experimental cell made from acrylic glass and filled with distilled water The acoustic field in the cell generated by a disk-shaped acoustic radiator is measured using a hydrophone The behavior of the bubble contacting with a hydrophillic surface is considered for the cases of a fixed or moving contact line The shape and volume oscillations of the bubble are investigated The results of numerical simulations agree qualitatively with the experimental data

Experimental and Numerical Investigations of the Characteristics of Pulsed Thermal Actuators

Abstract: The purpose of the study is to numerically, experimentally, and analytically investigate the characteristics of plasma pulsed thermal actuators (PT actuators) and to assess their possibilities in controlling flow around airfoils, wings, and configurations at large subsonic freestream velocities. For the PT actuators of the types considered the mathematical models adequately describing their effect on flow past bodies are developed. The characteristics of a prototype PT actuator are experimentally investigated on a specially developed rig. A new type of the PT actuator equippedwith a channel (PTC actuator) is proposed; it is designed to operate at a high pulse repetition frequency and at large flow velocities. Numerical investigations show that the PTC actuators are free of the essential and fundamental shortcoming of the PT actuators which consists in the working zone superheating at high pulse repetition frequencies.

Thermodynamic Conditions of Formation of CO 2 Hydrate in Carbon Dioxide Injection into a Methane Hydrate Reservoir

Abstract: The mechanism of replacement of methane by carbon dioxide in the hydrate in the process of CO2 injection into a reservoir with formation of fronts of methane hydrate dissociation and carbon dioxide hydrate generation is investigated. It is found that such a replacement regime can be implemented in both low- and high-permeability reservoirs. It is shown that in the highintensity injection regime the heat flux from the well does not affect propagation of the fronts of methane hydrate dissociation and carbon dioxide hydrate generation. In this case the replacement regime is maintained by only the heat released at formation of carbon dioxide hydrate. An increase in the injection pressure may lead to suppression of methane hydrate dissociation and termination of the replacement reaction. The critical diagrams of existence of the regime of conversion of methane hydrate to carbon dioxide hydrate are constructed.

Vortex Shedding: A Review on Flat Plate

Abstract: Flat plates, both single and in tandem or side by side arrangement, are widely used in many engineering applications. Despite vast investigations of the flow structures and wakes downstream of these bluff bodies, this unsteady phenomenon yet remains a fundamental issue in many industrial applications. This paper reviews the state of the art concerning the flow over flat plates in different arrangements focusing on plates normal to the flow. Turbulent wake regions are discussed for the flat plates in side by side or tandem arrangement. Numerical studies are reviewed with emphasis on the realized turbulent models. The effect of the chosen turbulence model on the prediction of the wake region is discussed.

Asymptotic Analysis of Viscous Fluctuations in Turbulent Boundary Layers

Abstract: A two-dimensional boundary layer of an incompressible viscous fluid is investigated in the presence of velocity and pressure fluctuations. The characteristic Reynolds number is high and, as a consequence, the unsteady (turbulent) boundary layer is thin. An asymptotic approach is used to analyze the complete unsteady Navier–Stokes equations, which makes it possible to separate out the characteristic viscous and inviscid flow zones in the boundary layer and to solve the corresponding problems. The analytical expressions for the viscous fluctuations governed by the Hamel equation with a large value of the parameter are derived.

On the Reasonability of Taking the Volume Viscosity Coefficient into Account in Gas Dynamic Problems

Abstract: The flow models are considered in the first (Navier–Stokes) approximation. The effect of the volume viscosity coefficient on the solutions obtained is investigated with reference to the problem of the plane shock wave profile. It is shown that taking the volume viscosity coefficient into account makes it possible to improve the solution for the flows with low dynamic nonequilibrium for which the first-approximation models are justified. It is preferable to use two-temperature flow models under the conditions of high dynamic nonequilibrium.

Generation of Three-Dimensional Disturbances in a Boundary Layer on Strong Interaction with a Hypersonic Flow

Abstract: Laminar boundary layer flow over an infinite-span, finite-length flat plate is investigated in the regime of strong interaction with a hypersonic gas flow. Under the assumption that an additional condition dependent on the transverse coordinate can be imposed on the trailing edge of the plate the flow functions are expanded in power series in the vicinity of the leading edge. It is shown that these expansions include an indefinite function dependent on the transverse coordinate. The corresponding boundary value problems are formulated and solved and the eigenvalues are determined. It is established that in this case the two-dimensional boundary layer can rearrange itself into a three-dimensional boundary layer.

Static Hysteresis of the Aerodynamic Characteristics of a Model Aircraft in the Landing Regime

Abstract: Flow past a simplified model of the civil aircraft is numerically investigated in the landing regime. The numerical modeling is performed within the framework of the Reynolds equations. The aerodynamic characteristics of the model are studied. Regimes with the static hysteresis are found.