Showing papers in "Fluid Dynamics in 2002"

Comparison of Calculated and Experimental Data on Supersonic Flow past a Circular Cylinder

Abstract: The supersonic perfect-gas flow past a circular cylinder is studied on the basis of a numerical analysis of the time-dependent two-dimensional Reynolds equations using a differential q–ω turbulence model with reference to the experimental conditions. The calculations are carried out at Reynolds and Mach numbers Re=2× 105 and M∞=1.1, 1.3, and 1.7 and the experimental investigations at Re=1.62×105–2×105 and Mach numbers on the interval 0.7 ≤ M∞ ≤ 1.7. The calculated and experimental data on the pressure coefficient distribution over the cylinder surface, the location of the separation point on the surface, and the pressure drag coefficient are compared.

Levitation of Magnets and Paramagnetic Bodies in Vessels Filled with Magnetic Fluid

Abstract: Formulas are obtained for the forces and moments acting on a spherical body made of a paramagnetic material in an uniform applied magnetic field and a magnet in a spherical vessel filled with magnetic fluid. An approximate formula is found for the force acting on bodies in ellipsoidal and cylindrical vessels or in a plane channel with a magnetic fluid in an uniform magnetic field. An analogy between the forces acting on a magnet and a paramagnetic body is demonstrated. The possibility of levitation of magnets and paramagnetic bodies in a vessel with a magnetic fluid is investigated.

Gas-Dynamic Methods of Temperature Stratification (a Review)

Abstract: The known methods of gas-dynamic temperature stratification are reviewed. Attention is concentrated on an analysis of the possibilities of the gas dynamic stratification method proposed by the author. The method is based on the difference between the equilibrium temperature of a thermally insulated wall in supersonic flow and the adiabatic stagnation temperature of the gas. Certain possible practical applications of the method to various types of energy-converting apparatus are considered. The basic trends of fundamental and applied research in the field of gas-dynamic temperature stratification are formulated.

Drop and Bubble Deformation in an Electric Field

Abstract: A steady problem of drop (bubble) shape in a uniform electric field is considered when the drop and the surrounding medium are immiscible. The electric-charge transport includes both the ohmic current across the interphase boundary and convective transport over the interface. If there is no convective transport, the drop (bubble) may be transformed into either an elongated or a flattened spheroid. Under these conditions, the sign of the deformation remains unchanged for arbitrary values of the problem parameters. Convective charge transport along the surface initiates additional motion in both the drop and the surrounding medium. However, with increase in the convective-transport intensity the deformed drops display different behavior. The compression of a flattened drop slows and, under certain conditions, compression is replaced by extension. However, an elongated spheroid cannot be transformed into a flattened spheroid. The calculations were performed under the assumption that the drop is convex. It was found that, for both an elongated and a flattened drop, the maximum ratio of the major and minor spheroid axes is 2:1. In experiments with oils, the possibility of both a decrease in the drop compression rate and deformation sign reversal was demonstrated.

Two-Dimensional Pressure Waves in a Fluid with Bubbles

Abstract: The specific features of wave evolution in a fluid with a finite bubble zone are studied. The two-dimensional effects are taken into account. The results for two-dimensional wave evolution in a uniform bubbly fluid are also presented.

Investigation of Natural Convection in a Cubic Cavity near 4°C

Abstract: The problem of the natural convection of water near the density inversion point is solved numerically for a cubic cavity with isothermal horizontal walls and thermally insulated vertical walls. For different Grashof numbers, six steady-state flows are obtained and the ranges of existence of these flows are found.

Flow and heat transfer in underexpanded air jets issuing from the sonic nozzle of a plasma generator

Abstract: We present the results of an experimental investigation and numerical simulation of the gasdynamic structure of underexpanded dissociated-air jets and the heat transfer in these strongly nonequilibrium flows under the test conditions realized in the 100-kW electrodeless VGU-4 plasma generator of the Institute for Problems in Mechanics of the Russian Academy of Sciences (IPM RAS). The flow and heat transfer analysis is carried out on the basis of measurements of the static pressure in the plenum chamber, at the sonic nozzle exit, and on the low-pressure chamber wall, the stagnation pressure on the jet axis using a Pitot tube, and the heat transfer at the stagnation points of water-cooled models placed along the jet axis. The numerical simulation, based on complete Navier-Stokes equations, includes the calculation of (1) equilibrium air plasma flows in the discharge channel of the VGU-4 plasma generator; (2) underexpanded nonequilibrium dissociated-air jet outflow into the ambient space; and (3) axisymmetric jet flow past cylindrical models.

Evaporation and condensation fronts in porous media

Abstract: Flow of a fluid through a porous medium is considered with allowance for heat conduction. Both fronts at which the liquid is transformed into steam or a liquid-steam mixture and fronts with inverse transformations are studied. The evolutionarity conditions of these fronts are considered and a model of their structure is proposed.

Heat Transfer and Evaporation from a Plane Surface into a Half-Space upon a Sudden Increase in Body Temperature

Abstract: The one-dimensional time-dependent problem of evaporation from a plane body surface into a half-space filled by a gas (condensed phase vapor) upon a sudden increase in the body surface temperature is studied. The evaporation coefficient is the problem parameter and may take arbitrary values within the limits from zero to unity. The problem is formulated for the kinetic equation and solved by the finite-difference method. It is shown that a deviation of the evaporation coefficient from unity considerably modifies the gas phase flow pattern. However, the evaporation rate divided by the rate of evaporation into a vacuum at the given surface temperature is only weakly dependent on the evaporation coefficient.

Unsteady thermo-gravitational convection effects in a side-heated or cooled near-critical fluid

Abstract: Unsteady thermo-gravitational convection of a near-critical fluid in an enclosed cavity, whose side wall temperature increases or decreases while the other boundaries are thermally insulated, is considered. An effective numerical method based on a two-scale pressure representation is used for solving the complete Navier-Stokes equations with a Van-der-Waals equation of state. For the neighborhood of the critical point, a transformation of the similarity parameters, which allows the introduction of effective values of these parameters, is found. The characteristic times of rapid temperature equalization due to adiabatic compression (“piston effect”), heat conduction, and thermo-gravitational convection are compared. The reasons why, in an unsteady convective jet, the temperature of the near-critical fluid is higher than the fixed side-wall temperature are analyzed.

Turbulent Structure of a Gas-Liquid Impinging Jet

Abstract: The turbulent structure of a submerged axisymmetric impinging jet containing small gas bubbles is studied experimentally under conditions of periodic external excitation. On the basis of measuring the surface-friction pulsatory component in the jet impinging on an obstacle, the effect of the suppression of large-scale eddies at large gas volume fractions is registered. The conditions of resonant growth of coherent structures and the suppression of wide-band turbulence are determined for both the single-phase and the two-phase impinging jet. An analysis of the development of different pulsatory friction components in the impinging-jet gradient region is presented.

On the Viscous Drag of a Plate with Spherical Recesses

Abstract: The drag of flat plates with spherical recesses was measured using the direct balance method. The experiments were run in a low-turbulence wind tunnel with a cross-section measuring 1000 × 1000 mm and a length of 400 mm. Three surfaces with recesses 7.0, 3.9, and 1.3 mm in diameter and 0.5, 0.3, and 0.2 mm deep, respectively, were tested. It is shown that on the Reynolds and Mach number ranges Re= (3–9)· 106 and M ≤ 0.3 the spherical recesses add to the drag of a flat plate in turbulent flow. The recesses have almost no effect on the location of laminar-turbulent transition, which occurs at Ret ∼ 3· 106.

Sand-Fluid Interface under Vibration

Abstract: The average dynamics of the interface between a pure fluid and a granular medium with fluid-filled pores in a closed vibrating cavity are investigated experimentally. Three types of vibration, namely, linear and circular translational in a horizontal plane and rotational about a vertical axis, are considered. In all cases, the excitation of a dynamic relief on the surface of the granular medium, preceded by fluidization of the sand, is observed. For more complicated vibration types, additional average effects are manifested, such as the generation of an average granular-medium motion relative to the cavity under circular vibration and the displacement of the fluidized granular medium toward the rotation axis under rotational vibration. In the cases considered, the regularities of the average dynamics of the fluidized granular medium are found. It is shown that the phenomena in a granular-medium-fluid system can be analyzed using the two-fluid theoretical model.

Effect of gas non-ideality on the performance of Laval nozzles with an abrupt constriction

Abstract: The flows in Laval nozzles with a ero-length region of abrupt constriction and in nozzles with smooth entrance regions are studied on the basis of the Reynolds equations supplemented by a differential turbulence model. It is established that the viscosity effect does not lead to flow separation in the vicinity of the minimum section of optimal nozzles with an abrupt constriction. In all the examples calculated, the thrust of these nozzles is greater than that of nozzles with smooth a constriction and an optimally contoured supersonic part, the flow rate through the nozzle being larger when viscosity is taken into account than in the ideal (inviscid) case.

Momentum and Energy Transfer in a Shock Wave

Abstract: The macroparameter profiles in a strong shock wave propagating in a single-component monatomic gas are investigated The interaction between the molecules is described by the variable-diameter sphere model Qualitative information concerning the shock wave parameters is obtained by direct statistical simulation (Monte-Carlo) method Using the data obtained, simple approximate gas-velocity dependences of the stress and the heat flow are found and the linear Newton and Fourier relations are generalized

Investigation of the Influence of Different Heterogeneous Recombination Mechanisms on the Heat Fluxes to a Catalytic Surface in Dissociated Carbon Dioxide

Abstract: A kinetic model of heterogeneous recombination in dissociated carbon dioxide on high-temperature heat-shield coatings is developed; the model takes into account the nonequilibrium adsorption-desorption reactions of oxygen atoms and their recombination in the Eley-Rideal and Langmuir-Hinshelwood reactions. On the basis of a comparison of the calculated heat fluxes in dissociated carbon dioxide with those measured in the VGU-3 plasma generator of the Institute for Problems in Mechanics of the Russian Academy of Sciences (IPM RAS) and the available literature data, the parameters of the catalysis model are chosen for the glassy coating of the Buran orbiter tile heat shield based on the SiO2–B2O3–SiB4 system. The effects of heterogeneous recombination proceeding in accordance with the Langmuir-Hinshelwood mechanism, as well as the processes involving carbon atoms and those involving physically adsorbed oxygen atoms, on the heat fluxes to the glassy coating are analyzed on the surface temperature range from 300 to 2000 K.

Profiling the Supersonic Part of a Plug Nozzle with a Nonuniform Transonic Flow

Abstract: The effect of transonic flow nonuniformity on the profiling of optimal plug nozzles is studied in the inviscid gas approximation. Sonic and supersonic regions providing maximum thrust for given nozzle dimensions and a given outer pressure are designed for given subsonic contours and calculated nonuniform transonic flows. As in the case of uniform flow on a cylindrical sonic surface, the initial regions of the designed contours satisfy the condition that in these regions the flow Mach number is unity or near-unity. In all the examples calculated, the optimal plug nozzles produce a greater thrust than the optimal axisymmetric and annular nozzles with a near-axial flow for the same lengths and the same gas flow rates through the nozzle. It is established that contouring without regard for transonic flow nonuniformity can result in considerable thrust losses. However, these losses are due only to a decrease in the flow rate, while the specific thrust may even increase slightly.

Laminar Base Flow Downstream of Slender Cones in Hypersonic Flow

Abstract: The base flow downstream of slender cones in a stream of perfect gas at Mach numbers 8 and 10 and Reynolds numbers 104 and 105 is numerically investigated. The calculated heat fluxes to the rear face of the body are compared with experimental data. It is shown that the friction and heat transfer coefficients increase without bound as the corner point is approached from both the lateral surface and the rear face, the sign of the latter coefficient being dependent on the body surface temperature factor.

Swept-wing boundary layer receptivity to a steady free-stream vortex disturbance

Abstract: A single trailing vortex developed behind a micro-wing immersed in a free stream was used to study the vortex receptivity of a swept-wing boundary layer. As a result of the interaction, longitudinal-velocity disturbances develop in the boundary layer. On the swept wing, disturbance transformation occurs near the leading edge and is accompanied by the formation of a wave packet consisting of waves typical of cross-flow instability. Disturbances with other characteristics are also detected. These disturbances may be attributable to distributed boundary-layer receptivity to the free-stream vortex disturbance considered.

Weakly Nonlinear Analysis of Convection of a Two-Layer System with a Deformable Interface

Abstract: The onset of convection in a two-layer system heated from above and below is considered within the framework of the generalized Boussinesq approximations. In the longwave approximation an amplitude equation describing large-scale convective motions accompanied by a deformation of the interface is derived. Two types of steady-state regimes are revealed and their stability is studied.

Establishment of Surface Waves Generated by Hydrodynamic Singularities in Plane Flow

Abstract: A method of direct proof of the existence of steady-state regimes in classical problems of small waves on the surface of a plane flow of ideal heavy fluid of infinite depth accompanying the uniform motion of a hydrodynamic singularity inside the flow or the operation of a pulsating immersed source is proposed.

Investigation of the nitrogen shock wave structure on the basis of trajectory calculations of the molecular interaction

Abstract: The shock wave structure in a diatomic gas is investigated using the direct statistical simulation (Monte-Carlo) method. The energy exchange between translational and rotational degrees of freedom (TR-exchange) is calculated by solving the dynamic problem of the interaction between rigid-rotator molecules within the framework of classical mechanics. The density profiles calculated are compared with the experimental data and on this basis the nitrogen rotational relaxation time is estimated. The possibility of using simplified intermolecular interaction models, namely, the variable-diameter sphere model employed together with a phenomenological consideration of the TR-exchange, is studied. Gasdynamic parameter profiles in the shock wave are analyzed. Simple approximations of the velocity gradient and translational and rotational temperature profiles are obtained on the basis of a parametric calculation of the shock wave structure. This makes it possible approximately to describe the gasdynamic parameter profiles in terms of elementary functions.

Numerical Analysis of the Influence of Thermal Stresses on the Nonlinear Thermomolecular Pressure Difference Effect

Abstract: On the basis of a numerical analysis of the non-Navier-Stokes gas-dynamic equations for slow non-isothermal gas flows, the nonlinear thermomolecular pressure difference effect due to a large temperature gradient along the lateral surface of a capillary is investigated. It is shown that the magnitude of the effect is substantially different from the values calculated using the Navier-Stokes equations. For two models of molecular interaction (Maxwell molecules and hard spheres), the possibility of a quasi-one-dimensional interpretation of the effect for experimental estimation purposes is demonstrated. The solutions of the relaxation kinetic equation for flow in a plane capillary at small Knudsen numbers and the gas-dynamic equations for slow non-isothermal flows are compared and the range of their applicability is estimated.

Electromagnetic Radiation of an Oscillating Charged Viscous Drop of Finite Conductivity

Abstract: The dispersion equation for the capillary oscillations of a charged drop of viscous incompressible fluid of finite electrical conductivity with account for energy loss by electromagnetic-wave radiation is obtained. It is shown that the intensity of the energy loss associated with the electromagnetic radiation of the oscillating drop increases with its conductivity, charge and the surface mobility of the charge carriers. The intensity of the background radiation of a cumulus cloud is estimated.

Receptivity of the Blunt-Leading-Edge Plate Boundary Layer to Unsteady Vortex Perturbations

Abstract: The response of the boundary layer on a plate with a blunt leading edge to frozen-in vortex perturbations whose vorticity is normal to the plate surface is found. It is shown that these vortices generate an inhomogeneity of the streamwise velocity component in the boundary layer. This inhomogeneity is analogous to the streaky structure developing as the degree of free-stream turbulence increases. The dependence of the amplitude and shape of the boundary layer inhomogeneity on the distance from the leading edge, the streamwise and spanwise scales, and other parameters is found for periodic and local initial perturbations. It is shown that the receptivity of the boundary layer decreases with increase in the frequency and with decrease in the streamwise perturbation scale.

Investigation of Different Condensation Regimes in Isobaric Turbulent Air-Steam Jets

Abstract: Condensation in axisymmetric turbulent air-steam jets is studied theoretically and experimentally under bench experiment conditions in which a hot mist jet is injected from a nozzle into air. On the basis of the physico-mathematical model developed, four problems are considered: homogeneous condensation in the jet at a fairly low ambient air temperature, heterogeneous condensation on particles introduced into the jet at the nozzle outlet, heterogeneous condensation on particles ejected into the jet from the surrounding space, and condensation on ions entering the jet from a corona point on the flow axis. The local characteristics of the dispersed phase (mean particle size, standard deviation of the particle size, particle number and volume concentrations) and its integral characteristics (coefficient of vapor conversion into condensed phase and the optical thickness of the jet in different sections) are determined. The calculation results are compared with experimental data. As an application of the model developed, the characteristics of heterogeneous condensation in the jets of certain modern aircraft engines (IL-96-300, Tu-204, MiG-29, Boeing-707) are found on the assumption that the condensation occurs on particles entering the jet at the nozzle outlet and the particle growth rate in all stages (including the initial stage of particle irrigation) coincides with the growth rate of liquid drops.

Simple Regimes of Fluid Motion in the Neighborhood of the Intersection of Bifurcations Initiating Nonisothermal Taylor Vortices and Azimuthal Waves

Abstract: Steady-state, self-oscillating, and two-frequency quasi-periodic regimes of fluid motion between heated rotating cylinders are investigated in a small neighborhood of the point of intersection of the neutral curves of monotonic axisymmetric and oscillatory three-dimensional instability of the nonisothermal Couette flow. Methods developed in studies [2–4], which make it possible to reduce the problem to the investigation of an autonomous dynamical fourth-order system whose coefficients can be found numerically by integrating a series of linear boundary-value problems for systems of ordinary differential equations, are used.

Effect of the Nozzle Edge Shape on the Acoustic Sensitivity of a Jet

Abstract: The effect of the nozzle edge shape on the acoustic sensitivity of jets, that is, on the dependence of the jet parameters on the amplitude and frequency of the acoustic oscillations produced by an external source, is experimentally studied. The investigation was performed for nozzle edge configurations, the variation of which did not result in a change in the jet characteristics without external acoustic excitation. This means that the change in the edge shape alone had no influence on the flow pattern at the nozzle exit or the boundary layer flow regime on the nozzle walls. Measurements of the dependence of the mean velocity and the velocity fluctuation intensity on the jet axis on the distance from the nozzle exit showed that a change in the nozzle edge shape can lead to a change in the acoustic sensitivity of the jet when the jet is exposed to external acoustic action.

Investigation of the flow in an annular cavity in a cylindrical body in a supersonic stream

Abstract: The results of an experimental investigation of the flow over an annular cavity in a cylindrical body are presented; the cavity-to-body diameter ratio was 0.7 and the incident flow Mach number was 2.84. Using the data on the pressure distribution and optical measurements of the flow pattern, the structure of the flow inside the cavity was studied on the relative cavity length range from 0.5 to 14 including the regimes with both open and closed separation zones.

Existence and stability of the inner boundaries of the domain of multiple hysteresis of static aerodynamic forces and moments

Abstract: The results of experimental investigations of the multiple static hysteresis of the aerodynamic characteristics of a rectangular high-aspect-ratio wing are presented. Schematic wing-flow structure patterns, time dependences of the coefficients cy(t), mz(t), and mx(t) and their frequency spectra obtained for a fixed model are given for different boundaries of the hysteresis domain. The time dependences of aerodynamic forces and moments are analyzed at angles of incidence at which sharp changes are observed. It is shown that the static hysteresis can be described by a mathematical model used in catastrophe theory.