CFD numerical simulation of flow velocity characteristics of hydrocyclone

A CFD based numerical simulation of flow velocity of hydrocyclone was conducted with different structural and operational parameters to investigate its distribution characteristics and influencing mechanism. The results show there exist several unsymmetrical envelopes of equal vertical velocities in both upward inner flows and downward outer flows in the hydrocyclone, and the cone angle and apex diameter have remarkable influence on the vertical location of the cone bottom of the envelope of zero vertical velocity. It is also found that the tangential velocity isolines exist in the horizontal planes located in the effective separation region of hydrocyclone. The increase of feed pressure has almost no effect on the distribution characteristics of both vertical velocity and tangential velocity in hydrocyclone, but the magnitude and gradient of tangential velocity are increased obviously to make the motion velocity of high density particles to the wall increased and to make the cyclonic separation effect improved.

Direct numerical simulation of compressible turbulent flows

This paper reviews the authors＇ recent studies on compressible turbulence by using direct numerical simulation （DNS）,including DNS of isotropic（decaying） turbulence, turbulent mixing-layer,turbulent boundary-layer and shock/boundary-layer interaction.Turbulence statistics, compressibility effects,turbulent kinetic energy budget and coherent structures are studied based on the DNS data.The mechanism of sound source in turbulent flows is also analyzed. It shows that DNS is a powerful tool for the mechanistic study of compressible turbulence.

Unsteady Numerical Simulation of Flow around 2-D Circular Cylinder for High Reynolds Numbers

In this paper, 2-D computational analyses were conducted for unsteady high Reynolds number flows around a smooth circular cylinder in the supercritical and upper-transition flow regimes, i.e. 8.21×104〈Re〈1.54×106. The calculations were performed by means of solving the 2-D Unsteady Reynolds-Averaged Navier-Stokes （URANS） equations with a k-ε turbulence model. The calculated results, produced flow structure drag and lift coefficients, as well as Strouhal numbers. The findings were in good agreement with previous published data, which also supplied us with a good understanding of the flow across cylinders of different high Reynolds numbers. Meanwhile, an effective measure was presented to control the lift force on a cylinder, which points the way to decrease the vortex induced vibration of marine structure in future.

Research on numerical simulation of flow field in shallow-bath dense-medium separator

介绍了浅槽重介分选机的结构、工作原理和技术特点，并采用Fluent软件，对浅槽重介分选机内部的流场进行了分析和研究，得出了各主要位置的速度和速度变化规律。揭示了浅槽重介分选机在分选过程中的悬浮液体流场形态及运动规律，以为其结构设计和改进提供理论依据。

Numerical Simulations of the Flow Field around a Cylindrical Lightning Rod

As an important lightning protection device in substations,lightning rods are susceptible to vibration and potential structural damage under wind loads.In order to understand their vibration mechanism,it is necessary to conduct flow analysis.In this study,numerical simulations of the flow field around a 330 kV cylindrical lightning rod with different diameters were performed using the SST k-ωmodel.The flow patterns in different segments of the lightning rod at the same reference wind speed(wind speed at a height of 10 m)and the flow patterns in the same segment at different reference wind speeds were investigated.The variations of lift coefficient,drag coefficient,and vorticity distribution were obtained.The results showed that vortex shedding phenomena occurred in all segments of the lightning rod,and the strength of vortex shedding increased with decreasing diameter.The vorticity magnitude and the root mean square magnitudes of the lift coefficient and drag coefficient also increased accordingly.The time history curves of the lift coefficient and drag coefficient on the surface of the lightning rod exhibited sinusoidal patterns with a single dominant frequency.For the same segment,as the wind speed increased in a certain range,the root mean square values of the lift coefficient and drag coefficient decreased,while their dominant frequencies increased.Moreover,there was a proportional relationship between the dominant frequencies of the lift coefficient and drag coefficient.The findings of this study can provide valuable insights for the refined design of lightning rods with similar structures.

Direct Numerical Simulation of the Pulsed Arc Discharge in Supersonic Compression Ramp Flow

Direct numerical simulation(DNS)of shock wave/turbulent boundary layer interaction(SWTBLI)with pulsed arc discharge is carried out in this paper.The subject in the study is a Ma=2.9 compression flow over a 24-degree ramp.The numerical approaches were validated by the experimental results in the same flow conditions.The heat source model was added to the Navier-Stokes equation to serve as the energy deposition of the pulsed arc discharge.Four streamwise locations are selected to apply energy deposition.The effect of the pulsed arc discharge on the ramp-induced flow separation has been studied in depth.The DNS results demonstrate the incentive locations play a dominant role in suppressing the separated flow.Results show that pulsed heating is characterized by a thermal blockage,which leads to streamwise deflection.The incentive locations upstream the interaction zone of the base flow have a better control effect.The separation bubble shape shows as"spikes",and the downstream flow of the heated region is accelerated due to the momentum exchange between the upper boundary layer and the bottom boundary layer.The high-speed upper fluid is transferred to the bottom,and thus enhances its ability to resist the flow separation.More stripe vortex structures are also generated at the edge of the flat-plate.Furthermore,the turbulent kinetic disturbance energy is increased in the flow filed.The disturbances that originate from the pulsed heating are capable of increasing the turbulent intensity and then diminishing the trend of flow separation.

NUMERICAL STUDY OF FLOW IN CONICAL DIFFUSER WITH VORTEX GENERATOR JETS

To develop vortex generator jet （VGJ） method for flow control, the turbulence flow in a 14° conical diffuser with and without vortex generator jets are simulated by solving Navier-Stokes equations with k-ε turbulence model. The diffuser performance, based on different velocity ratio （ratio of the jet speed to the mainstream velocity）, is investigated and compared with the experimental study. On the basis of the flow characteristics using computation fluid dynamics （CFD） method observed in the conical diffuser and the downstream development of the longitudinal vortices, attempt is made to correlate the pressure recovery coefficient with the behavior of vortices produced by vortex generator jets.

DIRECT NUMERICAL SIMULATIONS OF TURBULENT CHANNEL FLOWS WITH CONSIDERATION OF THE BUOYANCY EFFECT OF THE BUBBLE PHASE

Turbulent channel flows with consideration of the buoyancy effect of the bubble phase is investigated by means of the Direct Numerical Simulation （DNS）. This two-phase system is solved by a two-way coupling Lagrangian-Eulerian approach. The Reynolds number based on the friction velocity and the half-width of the channel is 194, and the gravitational acceleration varies from -0.5 to 0.5, ranging from the upflow to the downflow cases. This study aims to reveal the influence of buoyancy on the turbulence behavior and the bubble motion. Some typical statistical quantities, including the averaged velocities and velocity fluctuations for the fluid and bubble phases, as well as the flow structures of the turbulence fluctuations, are analyzed.

Numerical simulation of flow interference between four cylinders in in-line square arrangement

In the present paper,two-and three-dimensional numerical simulations of the flow interference between four cylinders in an in-line square arrangement at Re = 200 are performed.Assisted with the two-dimensional(2-D) numerical simulation,the mean and fluctuating forces,Strouhal number(St) and vortex shedding pattern in the wake for each cylinder were analyzed with the spacing ratio(L /D) ranging from 1.5 to 6.0.It was found that,four different vortex modes(viz.,flip-flopping,shielding anti-phase-synchronized,in-phasesynchronized and anti-phase-synchronized) gradually appear with the increase of the L/D ratio.The average drag coefficient of the upstream cylinders is larger than that of the downstream cylinders,while the downstream cylinders usually undergo serious fluctuating forces.When the L/D ratio ranges from 3.0 to 4.0,the dominant frequency of the drag coefficient is equal to the value of St of upstream cylinders.This indicates that a simultaneous resonance in the in-flow and cross-flow directions may occur for some single structures of a multi-body oscillating system.For the 3-D numerical simulation,the L/D and aspect ratios are kept constant as 5.0 and 10,respectively.It was found that some vortices are formed in the wake of the upstream cylinders.Besides,with the same spacing ratio,the calculated drag coefficient and lift coefficient fluctuation are slightly larger than the 2-D results,but with a phase difference.

Direct Numerical Simulation of Incompressible Flows in a Zero-Pressure Gradient Turbulent Boundary Layer

Direct numerical simulation(DNS)of incompressible flows in a zeropressure gradient turbulent boundary layer(TBL)is conducted by a finite difference method in which a fourth order upwind scheme is applied to discretize the convective terms while a re-scaling approach is used to set inlet flow conditions.The Reynolds numbers based on free flow velocity and momentum thickness at the recycle section are respectively 687,1074,and 1430.The DNS has obtained favorable results indicating that the turbulence statistics is quite satisfactory as compared with the existing numerical and experimental results.The three dimensional turbulent structures at the momentum thickness Reynolds number of 1430 in several different instants are illustrated by the iso-surface of swirl strength square(the square of imaginary part of the complex eigenvalue of velocity gradient tensor)together with velocity vectors in three different cross sections.It is found that there are three kinds of vortical structures:quasi-symmetrical and asymmetrical hairpin vortices,and worm-like vortices.The DNS based on the numerical method can certainly reveal the main characteristics of the TBL flows at the given Reynolds numbers.

Direct Numerical Simulation of Vertical Rotating Turbulent Open-Channel Flow with Heat Transfer

Direct numerical simulation of vertical rotating open-channel flow with heat transfer has been carried out for the rotation number Nτfrom 0 to 0.1,the Prandtl number 1,and the Reynolds number 180 based on the friction velocity of non-rotating flow and the height of the channel.The ob jective of this study is to reveal the effect of rotation on the characteristics of turbulent flow and heat transfer,in particular near the free surface and the wall of the open-channel.Statistical quantities,e.g.,the mean velocity,temperature and their fluctuations,turbulent heat fluxes,and turbulence structures,are analyzed.The depth of surface-influenced layer decreases with the increase of the rotation rate.In the free surface-influenced layer,the turbulence and thermal statistics are suppressed due to the effect of rotation.In the wall-influenced region,two typical rotation regimes are identified.In the weak rotation regime with 0<Nτ<0.06 approximately,the turbulence and thermal statistics correlated with the spanwise velocity fluctuation are enhanced since the shear rate of spanwise mean flow induced by Coriolis force increases;however,the other statistics are suppressed.In the strong rotation regime with Nτ>0.06,the turbulence and thermal statistics are suppressed significantly because the Coriolis force effect plays a dominant role in the rotating flow.To elucidate the effect of rotation on turbulent flow and heat transfer,the budget terms in the transport equations of Reynolds stresses and turbulent heat fluxes are investigated.Remarkable change of the direction of streak structures based on the velocity and temperature fluctuations is discussed.

Three-Dimensional Numerical Investigation of the Separation Process in a Vortex Tube at Different Operating Conditions

Air separators provide safe, clean, and appropriate air flow to engines and are widely used in vehicles with large engines such as ships and submarines. In this operational study, the separation process in a Ranque-Hilsch vortex tube cleaning （cooling） system is investigated to analyze the impact of the operating gas type on the vortex tube performance; the operating gases used are air, nitrogen, oxygen, carbon dioxide and nitrogen dioxide. The computational fluid dynamic model used is equipped with a three-dimensional structure, and the steady-state condition is applied during computations. The standard k-c turbulence model is employed to resolve nonlinear flow equations, and various key parameters, such as hot and cold exhaust thermal drops, and power separation rates, are described numerically. The results show that nitrogen dioxide creates the greatest separation power out of all gases tested, and the numerical results are validated by good agreement with available experimental data. In addition, a comparison is made between the use of two different boundary conditions, the pressure-far-field and the pressure-outlet, when analyzing complex turbulent flows in the air separators. Results present a comprehensive and practical solution for use in future numerical studies.

Numerical Simulations of Structural Deformation and Fluid Flow in Xiangshan Deposit

The Xiangshan deposit in Jiangxi province is one of the most important uranium deposits in China. The aim of this study is to achieve a better understanding of mineralization in the Xiangshan deposit through numerical simulation. In order to find the most favorable locations of mineralization and to help further mineral exploration, a coupling deforma- tion and fluid flow model has been established to describe the mineralization process. In this model, the simulation re- constructs the strata deformations under fields of compressive stress and thrust structure on the hanging wall of the Zou-Shi fault. Compared with practical information, the simulation results are consistent with the No. 51 exploration section of the western Xiangshan. In addition, on the basis of geological information provided by previous investigators, the model simulates the flow process of fluids under compressive stress fields. The result suggests that many tensional areas are formed, which can help the fluid flowing upward from deeper parts. The fluid is easy to concentrate on the breccia fractured zone between two volcanic layers, especially on the intersection parts with faults, resulting in the for- mation of favourable locations of mineralization. In addition, the model is significant in guiding the exploration of ura- nium deposits in the western Xiangshan and provides clues for further exploration of deposits.

Numerical Simulation of Unsteady-State Flowsin Bubble Column Reactors

Unsteady-state operation has been widely applied in chemical engineering, such as optimizing a process, increasing yield and saving energy, etc. But the knowledge of the flow characteristics in bubble column reactors(BCRs) under unsteady state control is far from enough. In order to study the flow structures in this operation, the volume of fluid (VOF) model and the standard k-ε model to simulate the evolution of gas-liquid flow in BCRs under the start-up state are combined. For both the symmetry and asymmetry flow, the layout of the gas-inlets, the gas-in velocity, the liquid viscosity and the aspect ratio of the BCR all have effects on the liquid velocity distribution. The simulation results could provide some information for the design and scale-up of the BCRs.

Numerical Study of Oil/Water Separation by Ceramic Membranes in the Presence of Turbulent Flow

Disposal of produced water during petroleum extraction causes serious environmental damage, hence the need to complete the water treatment before being disposed to environment within the criteria set established by environmental agencies in the countries. Ceramics membranes have been highlighted as a good device for separating oil/water. These act as a barrier to oil in the aqueous stream, because their essential properties for filtration, such as chemical inertness, biological stability and resistance to high temperatures. The limitation of the separation process is the decay of permeate flux during operation, due to concentration polarization and fouling. In this sense, this paper aims to evaluate numerically the feasibility of the process of separating oil/water by means of ceramic membranes in the presence of a turbulent flow induced by a tangential inlet. The results of the velocity, pressure and volumetric fraction distributions for the simulations different by varying the mass flow rate inlet and different geometric characteristics of the membrane are presented and analyzed.

Upwind Compact Method and Direct Numerical Simulation of Driven Flow in a Square Cavity*

A high-order accurate finite-difference scheme, the upwind compact method, is proposed. The 2-D unsteady incompressible Navier-Stokes equations are sOlved in primitive variables. The nonlinear convection terms in the governing equations are approximated by using upwind biased compact difference, and other spatial derivative terms are discretized by using the fourth-order compact difference. The upwind compact method is used to solve the driven flow in a square cavity. Solutions are obtained for Reynolds numbers as high as 10000. When Re≤5000, the results agree well with those in literature. When Re=7500 and Re=10000, there is no convergence to a steady laminar solution, and the flow becomes unsteady and periodic.

DOMAIN DECOMPOSITION HYBRID METHOD FOR NUMERICAL SIMULATION OF BLUFF BODY FLOWS——THEORETICAL MODEL AND APPLICATION

The discrete vortex method is not capable of precisely predicting the bluff body flow separation and the fine structure of flow field in the vicinity of the body surface. In order to make a theoretical improvement over the method and to reduce the difficulty in finite-difference solution of N-S equations at high Reynolds number, in the present paper, we suggest a new numerical simulation model and a theoretical method for domain decomposition hybrid combination of finite-difference method and vortex method. Specifically, the full flow field is decomposed into two domains. In the region of O(R) near the body surface (R is the characteristic dimension of body), we use the finite-difference method to solve the N-S equations and in the exterior domain, we take the Lagrange-Euler vortex method. The connection and coupling conditions for flow in the two domains are established. The specific numerical scheme of this theoretical model is given. As a preliminary application, some numerical simulations for flows at Re=100 and Re=1000 about a circular cylinder are made, arid compared with the finite-difference solution of N-S equations for full flow field and experimental results, and the stability of the solution against the change of the interface between the two domains is examined. The results show that the method of the present paper has the advantage of finite-dlfference solution for N-S equations in precisely predicting the fine structure of flow field, as well as the advantage of vortex method in efficiently computing the global characteristics of the separated flow. It saves computer time and reduces the amount of computation, as compared with pure N-S equation solution. The present method can be used for numerical simulation of bluff body flow at high Reynolds number and would exhibit even greater merit in that case.

NUMERICAL STUDY OF TOPOLOGICAL STRUCTURE OF 3-D TRANSONIC VISCOUS FLOW FIELD (TVFF) INSIDE TURBINE CASCADE

The major purpose of this paper is to numerically study the complex structure of vortex system occurring within transonic turbine cascade. The transonic viscous flow in turbine cascade is simulated by solving full 3D Reynolds average N S equations, and then detailed analyses of vortex system structure are presented. Under guidance of topology, the wall limiting streamlines are employed to reveal the flow structure near the wall, and an analysis of space streamlines and cross section streamlines is given for the investigation of flow structure in the flow field. Through the analysis, the formation and evolution of the vortex system and the whole process of separation occurring within this turbine cascade are revealed

Numerical Simulation and Experimental Investigation of 3 D Separated Flow Field around a Blunt Body

To study the effects of factors such as electromagnetic field and electron temperature on the ion extraction characteristics (extraction time, collision loss ratio) in atomic vapor laser isotope separation (AVLIS), 2 D electron equilibrium model was used to analyze and simulate the ion extraction process. The collisions between particles mainly considered charge exchanges between isotopic ions and atoms, which were treated by using cross section. The results show that the electric field and electron temperature have significant effects on the ion extraction characteristics: the stronger the electric field is, the higher the electron temperature is; the shorter the extraction time is, the less the collision loss ratio is, and moderate constant magnetic field has few effects on them. Key words atomic vapor laser isotope separation (AVLIS); ion extraction; charge exchange cross section; collision loss ratio

Grid Convergence Property of Three-Dimensional Measurement-Integrated Simulation for Unsteady Flow behind a Square Cylinder with Karman Vortex Street

The purpose of this study was to clarify grid convergence property of three-dimensional measurement-integrated (3D-MI) simulation for a flow behind a square cylinder with Karman vortex street. Measurement-integrated (MI) simulation is a kind of the observer in the dynamical system theory by using CFD scheme as a mathematical model of the system. In a former study, two-dimensional MI (2D-MI) simulation with a coarse grid system showed a fairly good result in comparison with a 2D ordinary (2D-O) simulation, but the results were degraded with grid refinement. In this study, 3D-MI simulation and three-dimensional ordinary (3D-O) simulation were performed with three grid systems of different grid resolutions, and their grid convergence properties were compared. As a result, all 3D-MI simulations reproduced the vortex shedding frequency identical to that of the experiment, and the flow fields obtained were very close, within 5% difference between the results, while the results of the 3D-O simulations showed variation of the solution under convergence. It is shown that the grid convergence property of 3D-MI simulation is monotonic and better than that of 3D-O simulation, whereas those of 2D-O and 2D-MI simulations for streamwise velocity fluctuation are divergent. The solution of 3D-MI simulation with a relatively coarse grid system properly reproduces the basic three-dimensional structure of the wake flow as well as the drag and lift coefficients.