


Volume 54, Nº 4 (2019)
- Ano: 2019
- Artigos: 15
- URL: https://journals.rcsi.science/0015-4628/issue/view/9477
Article
Numerical Investigation of Three-dimensional Cavitating Flow around a Guide Vane of a Water Turbine
Resumo
The results of numerical simulation of cavitating flow around the experimental model of a guide vane of a water turbine are given. The possibilities of the detached eddy simulation (DES) method for calculating the flow dynamics around the hydrofoil in two regimes at 3° and 9° angles of attack are analyzed. The mean flow velocity inside the boundary layer is compared with the experimental data. The dimensions and shape of the three-dimensional vapor cavity are presented in various operating regimes. The cavity dynamics and induced pulsations are analyzed.



Periodic Convective Processes in a Magnetic Fluid in Vertical Channels
Resumo
The reasons for the experimentally observable periodic transitions between two possible magnetic fluid flow directions in vertical connected channels heated from below are analyzed. It is shown that the key role is played by thermal diffusion of colloid particles due to the horizontal temperature gradients in the cross-sections of the narrow channels. On one hand, convection favors fluid mixing and, on the other hand, it itself generates concentration inhomogeneities. The gravitational sedimentation of the particles is excluded from the list of possible reasons of these oscillations.



Three-Dimensional Flows in a Rotating Cylinder in the Presence of Turbulent Boundary Layers on End Disks
Resumo
An analytical model for calculating a viscous incompressible fluid flow in a rotating cylinder with a braking lid and the formation of turbulent boundary layers on the end surfaces is presented. The analysis is made with account for all nonlinear inertial terms in the equations of motion, within the framework of Loitsyanskii's integral relations. The approximate velocity profiles in the boundary layers are preassigned in accordance with the empirical 1/7 law. The main flow is subdivided into an inviscid quasisolid core and a lateral layer, where almost the entire upward part of a circulatory flow is concentrated. The unknown angular velocity of the core and its radial boundary are evaluated from the balance between the moments of the friction forces acting on the main rotating flow and the continuity condition for the circulatory flow.



Flow of a Thin Magnetic Liquid Layer in a Magnetic Field
Resumo
The problem of flow of a thin layer of an incompressible magnetic liquid with a given dependence of the magnetization on the magnetic field strength is analytically solved in the lubrication theory approximation in the case of inhomogeneous magnetic field. A partial differential equation that describes time variation in the layer shape is obtained. The general formulation of the problem is considered. The solution of the problem of the behavior of a thin magnetic liquid layer in the magnetic field of a vertical infinite cylindrical conductor carrying a given sufficiently heavy electric current is written.



Distinctive Features of Flow in the Hypersonic Boundary Layer in the Vicinity of the Plane of Symmetry of a Planar Wing Having a Bend of the Leading Edge
Resumo
The flow in the three-dimensional laminar boundary layer in the vicinity of the plane of symmetry of a semi-infinite planar wing having a bend of the leading edge is investigated in the case of strong interaction with the external hypersonic flow. The flow functions are asymptotically expanded in power series in the angular coordinate in the vicinity of the plane of symmetry. The corresponding boundary value problem for the principal terms of the expansion is formulated and solved. The possibility of the existence of several solutions in the vicinity of the plane of symmetry is shown. The sweep angle effect on the distinctive features of the flow is considered.



Circulatory Effects Developed during the Motion of Some Objects in the Marine Medium and the Atmosphere
Resumo
The expressions for the hydrodynamic load acting in circulatory flow on a dipole located in a stratified (two-layer) fluid current of finite depth are derived. The wave-making drag and the lift force are investigated as functions of the flow velocity and circulation. It is shown that taking the circulation into account can significantly change the hydrodynamic reaction on the dipole. The effect of sharp (reversible) change in the direction of action of the lift force over a relatively narrow velocity range of flow around the pipeline section modeled by the dipole is revealed. The possibility of manifestation of a similar effect during the motion of self-propelled underwater objects and flying vehicles is discussed.



Pulse Frequency Effect on the Flow Structure and Heat Transfer in an Impinging Gas-Saturated Turbulent Jet
Resumo
The turbulent flow structure and the heat transfer in an unsteady bubbly round impinging jet is numerically simulated for different pulse frequencies. The mathematical model is based on the Eulerian approach for describing the flow dynamics and heat transfer in the disperse phase (air bubbles). The problem is considered in the axisymmetric formulation; the system of unsteady Reynolds-averaged Navier—Stokes equations is solved. The turbulence of the carrier phase (liquid) is described invoking the model of the transport of the Reynolds stress components with account for the bubble effect on turbulence production. The pulse supply frequency effect on the flow structure and the heat transfer in an impinging gas-liquid jet is investigated. The pulsed nature of the jet causes both a considerable (almost twofold) increase in the liquid turbulence and heat transfer during the pulse interval and a considerable suppression of these parameters, when the jet is flow-off, as compared with the case of a steady impinging bubblyjet at the same time-average jet flow fluid rate.



Laminar Power-Law Fluid Flow in a T-Shaped Channel at Given Pressure Differences
Resumo
The steady laminar flow of an incompressible power-law fluid in a plane T-shaped channel is studied at given pressure differences between the inflow and outflow sections. A non-Newtonian fluid, whose rheological behavior is governed by the Ostwald—de Waele law is called the power-law fluid. The mathematical formulation of the problem includes the equations of motion and continuity. The no-slip condition is imposed on solid walls. The problem solution is obtained using a finite-difference method invoking the SIMPLE procedure. A parametric investigation of the kinematic and dynamic flow parameters is carried out for different values of the relevant parameters of the problem. The flow regime map is constructed for different pressure differences between the inflow and outflow boundaries and different nonlinearity exponents of the rheological model.



On Solution of the Riemann Problem Describing Injection of a Heated Salt Solution into an Aquifer
Resumo
The self-similar problem of breakdown of an arbitrary discontinuity is considered with reference to the processes of nonisothermal saline fluid flow through a porous medium. The possibility of salt precipitation in the form of a solid phase in the porous medium accompanied by reduction in its permeability is taken into account. The geometric method for solving the problem in plane is proposed under the assumption of incompressibility of the medium and neglecting thermal conductivity. It is shown that the solution should be constructed with the use of only the shock waves and zones with homogeneous parameter distributions whereas centered rarefaction waves cannot enter into the solution. The possible types of solutions of the problem are investigated. It is shown that three shocks propagate into the aquifer when the high-temperature saline fluid is injected, the temperature being discontinuous only on the inner shock.



Simulation of the Motion of a Mixture of Liquid and Solid Particles in Porous Media with Regard to Internal Suffosion
Resumo
The mathematical model of isothermal internal soil erosion is studied without regard for deformation of the porous medium. Soil particles are removed from the flow region when a certain velocity of filtration is reached. As the mathematical model of the problem, the mass conservation equations for water, moving solid particles and stationary porous skeleton are used along with an analog of Darcy's law for water and moving solid particles and a relation for the intensity of suffosion aquifer. The moving soil particles are considered to be an individual phase with its own filtration rate determined in solving the problem. This assumption makes it possible to construct the closed model. The algorithm of numerical solution of the initial boundary-value problem of groundwater filtration in which the internal soil erosion is taken into account is proposed. The numerical test calculations are carried out. The calculation results correlate well with the experimental data.



Experimental Investigation on a Single NACA Airfoil’s Nonlinear Aeroelasticity in Wake Induced Vibrations
Resumo
In order to meet the economic and environmental requirements, turbomachine blades and aircraft wings are becoming more light and flexible, and bearing more mechanical and aerodynamic loads. However aerodynamic excitation would bring more variables into the structural vibration, and becoming an aeroelasticity problem. Unlike mechanical resonance vibration, the structure would interact with the aerodynamic excitation, and the aerodynamic excitation frequency would lock into structural natural frequency even the frequency margin is more than 10%. This phenomenon extends the high amplitude response range and should be noticed in safety design in order to deal with the margin in specific resonance conditions. In this paper, the aerodynamic excitation induced forced response is investigated with experimental setup including upstream cylinder and a downstream single NACA airfoil in wind tunnel. The upstream cylinder generates the vortices imposed on the NACA airfoil, brings periodic excitation on the flexible blade. Flow velocity is measured with hot wire anemometer (HWA) at upstream and downstream of the blade synchronously. Numerical simulation is conducted based experimental condition and verified by the measurements. Proper Orthogonal Decomposition (POD) is applied to obtain the major flow structure at one typical flow condition. The structural properties of the airfoil including natural frequency and damping are evaluated through finite element analysis and hammer test. Based on the fluid and structure properties, coupled test and analysis can be conducted. The vibration characteristics of NACA airfoil at 1st and 2nd order modes are explored by altering the freestream velocity and cylinder diameter. The forced vibration of 1st order mode has the lock-in phenomenon, and the maximum amplitude point is not at the resonance point. But 2nd order mode shows typical resonance behavior.



Rarefied Gas Flow Reflection from a Wall with an Orifice and Gas Outflow into a Vacuum
Resumo
The unsteady two-dimensional problem of the reflection of a uniform supersonic rarefied-gas flow incident normal on a wall with an orifice and the gas outflow through the orifice (slot or channel) is solved on the basis of the kinetic S-model. The slot influence on the reflection nature, the velocity of the shock wave reflected from the wall, and the formation of a gas jet flowing out into a vacuum is studied as a function of the incident flow velocity, the gas rarefaction degree, and the conditions imposed on the wall. The effect of the wall thickness, that is, the length of the channel, through which the gas flows out, is also investigated. The gas flow rate through the orifice is calculated as the main integral characteristic. The attainment of steady reflection regime in the outflow process is traced. The kinetic equation is solved numerically using the conservative difference method of the second order with respect to all variables.



Flow in a Narrow Channel with Chemical Reactions on the Wall
Resumo
The solution of the problem of gas flow in a narrow plane or cylindrical channel with the internal rod on the axis and chemical reactions on the walls is obtained. The problem reduces to solution of an ordinary differential equation for the transverse flow parameter distribution. In a certain degree, this example simulates flows in chemical reactors used sometimes for investigation of heterogeneous chemical reactions.



Approximation Formulas for the Radiative Heat Flux at High Velocities
Resumo
The aim of the study is to obtain the analytical expression for the radiative heat-transfer coefficient at the stagnation point of a blunt body as a function of its velocity and size and the atmosphere density over the parameter range characteristic of large meteoroid entering into the Earth's atmosphere. Analytical approximations, available in the literature, of the numerical calculations of the radiative heat flux to the stagnation point of a body with the nondestructible surface obtained over restricted altitude, velocity, and nose radius range are given. These approximation formulas are tested over a wider flow parameter range by comparing with calculations carried out by other authors. A new approximation relation for the radiative heat flux at the stagnation point is suggested on the basis of an analysis of these comparisons and performed correlations for the body velocity and radius and the atmosphere density.



Influence of the Excitation Frequency and Orifice Geometry on the Fluid Flow and Heat Transfer Characteristics of Synthetic Jet Actuators
Resumo
Synthetic jet is a novel fluctuating time dependent flow technique that transfers linear momentum to the surroundings by ingesting and expelling fluid from a cavity containing an oscillating diaphragm and is conceivably useful for electronic cooling. In this paper numerical analysis is performed to address the effects of the variation in geometric parameters of orifice and excitation frequency on the synthetic jet fluidic. The moving piezoelectric diaphragm is modeled with constant voltage amplitude boundary condition. Computations are carried out by using COMSOL 5.3a Multiphysics software. The present study focuses on the synthetic jets which are formed from a single cylindrical cavity but with different orifices, such as single-hole, three-holes, single-rectangular slots and three-rectangular slots. The exit areas for single-hole and single rectangular-slot has been chosen as 7.0 mm2, while exit areas for multiple holes and slots has been chosen as 21.0 mm2. The velocity of the synthetic jet reaches a maximum value when the diaphragm is excited at an optimum frequency. In case of single-hole synthetic jet, the optimum frequency is nearly the same as that of single rectangular- slot type orifice; however the optimum frequency for the multi-orifices is lower than that of singleorifice synthetic jet. The quality of the simulation results is verified by grid, time and domain independence studies, and validated with the existing experimental data. The simulation results obtained in this study are remarkable as they provide primary design guidepost for the excitation frequency and orifice shape. The present investigation also indicates the maximum value of average heat transfer coefficient is 86.5 W/m2 K with single rectangular-slot orifice, which is 21% higher as compared with a single-hole orifice. For single rectangular-slot orifice when distance ratio of orifice-to-heater is (Z/b) = 80 the value of heat transfer coefficient is 112.5 W/m2 K which is about 20.3% higher as compared to that of single-hole at (Z/d) = 14 thereby leading to better performance.


