bluuebunny/crossref_metadata_2025 huggingface dataset:
Calculation of Unsteady Flow in a Centrifugal Pump With Vaned Diffuser Using Staggered and Collocated Grid Methods
Authors: de Kleine D. , van Esch B. P. M. , Kuerten J. G. M. , Vreman A. W.
Published: July 2010

For the design of turbomachines like compressors, turbines, fans, and centrifugal pumps, more and more use is made of commercially available flow simulation software. To the authors best knowledge, in all cases discretization schemes based on a collocated grid are used. However, using a collocated grid method, the incompressible Navier-Stokes equations can suffer from odd-even decoupling. The adaptations necessary to suppress odd-even decoupling in incompressible flows result in calculations which are either less accurate or more time-consuming, especially for unsteady flows. In this paper we apply an alternative method based on a staggered-grid approach. The method is ideally suited for calculating unsteady incompressible flow on highly non-uniform block-structured grids. The unsteady flow in a radial pump with a vaned diffuser is calculated in 2D and compared with results obtained with a commercially available code. The time-dependent velocity and pressure fields are validated with experimental results available in literature.
Blade Interaction Forces in a Mixed-Flow Pump With Vaned Diffuser
Authors: Li Cheng , van Esch B. P. M.
Published: July 2010

Hydrodynamic forces in turbomachinery can cause problems ranging from excessive wear to vibrations. Of the many different causes of fluid-induced forces, their source attributed to blade interaction is perhaps best studied. Most studies, however, focus on high head pumps. This paper presents a study of blade interaction forces in a mixed-flow pump with vaned diffuser. It covers both experimental and numerical results. A mixed-flow pump is built into a closed-loop test rig. In order to measure the instantaneous forces and bending moments on the impeller, a co-rotating dynamometer is used, which is built in-between the impeller and the shaft of the pump. Extensive calibration proved necessary to determine the dynamic behavior of the shaft system. Measurements of forces at blade passing frequency show an unexpected dependency on flow rate. Another important observation is that blade excitation forces cause the impeller to whirl in a direction opposite to shaft rotation. Results of measurements are compared with unsteady CFD calculations using FLUENT®. Computed global characteristics show a good agreement with measurements. Also the magnitude of blade interaction forces shows a good qualitative agreement. Over a large range of flow rates, the dependency of magnitude on flow rate agrees well with the trend found in the measurements. Deviations are explained.
Modeling and Analysis of a Flow Divider Valve
Authors: Cheng Minter
Published: July 2010

Flow divider valves are often used in hydraulic systems to synchronize actuators. The basic structure of the flow divider valve is by incorporating a compensating spool to maintain equal pressure drops across metering orifices. Ideally, flow divider valve splits a single source flow into two parts under a specified ratio regardless of load conditions. In practical applications, any change in load pressure will cause force imbalance on the compensating spool, which will alter the flow rates through the metering orifices and affect the control accuracy consequently. In this study, the steady and dynamic performances of a flow divider valve are simulated numerically by solving the characteristic equations. The parameters studied in this research are centering spring constant, compensating spool mass, and metering orifice area. The simulation results show that flow force is the key factor to affect the flow division accuracy. Flow division error increases with increasing the load pressure differential, centering spring constant, and metering orifice area. Even though decreasing the spring force or the metering orifice area can reduce division error, the spring force still needs to be large enough to overcome the spool static friction and the orifice area cannot be too small to lose energy efficiency. Dynamic division error increases with increasing load pressure differential and metering orifice area but with decreasing spool mass. Increasing load pressure differential, spool mass, and metering orifice area will enhance the oscillatory tendency and increase the valve settling time. The centering spring constant has no obvious effect on the valve dynamic response.
Wake Structure of Heaving Elastic Airfoils and Their Dynamic Thrust
Authors: Fuchiwaki Masaki , Kurinami Tomoki , Tanaka Kazuhiro , Tabata Takahide
Published: July 2010

An unsteady flow in a low Reynolds number region attracts attention in recent years. The authors measured vortex flow in the wake of the rigid flat plate, rigid NACA0010, elastic flat plate and elastic NACA0010 with heaving motions and evaluated accelerating flows induced by the vortex flow patterns and vortex flows in the wake of the airfoils. Furthermore, we measured dynamic thrusts acting on these airfoils with heaving motions and clarified their relations with vortex flows. The clear thrust producing vortex street is formed in the wake of the elastic airfoils even at a comparatively low Strouhal number. The clear accelerating flow is formed in the wake of the elastic airfoils. The dynamic thrusts that act on the rigid NACA0010, elastic flat plate and elastic NACA0010 are strongly dependent on the Strouhal number based on the trailing edge amplitude regardless of heaving amplitudes, heaving frequencies, airfoil shapes and elastic deformations.
A Lagrangian Stochastic Model for Droplet Deposition Simulations in Connection With Wall Function Approaches
Authors: Aguinaga Sylvain , Simonin Olivier , Bore´e Jacques , Herbert Vincent
Published: July 2010

The deposition rate of droplets is strongly linked to their interaction with the boundary layer turbulence. In “industrial simulations”, droplets dispersion is usually modeled using Lagrangian stochastic simulations based on Reynold Average Navier Stokes (RANS) fluid calculations. Wall functions are also used to bound the number of mesh cells in the near wall region. But they also reduce the description of the boundary layer and lead to bad predictions of the droplets deposition rate. This study presents channel flow simulations using wall functions and run with the CFD code Fluent. In such configurations, the stochastic model of Fluent failed to represent the so-called “diffusion-impaction” regime of deposition. The “Concentration Wall Boundary Layer” model presented in this paper has been developed to predict deposition in simulations using industrial meshes with refinement such as y* > 20. This model calculates the deposition rate using only the intrinsic properties of the particles and the turbulent kinetic energy of the fluid expressed at the top of the boundary layer. The data provided by wall functions are then sufficient to calculate the deposition rate. This model is turned into a “Lagrangian stochastic wall boundary condition model” for the commercial CFD code Fluent. Various simulations have shown that this model improves remarkably the deposition predictions in channel flow. The dependence on the boundary cell size and the channel flow mean velocity has been tested. This model draws interesting perspectives to model deposition in complex configurations without requiring prohibiting mesh sizes.
Secondary Flow Velocity Field in Laminar Pulsating Flow Through Curved Pipes: PIV Measurements
Authors: Jarrahi Mojtaba , Castelain Cathy , Peerhossaini Hassan
Published: July 2010

Effects of different parameters on the secondary flow pattern have been studied experimentally by particle image velocimetry (PIV) for a developing laminar pulsating flow through a circular curved pipe. The curvature ratio is η = rc/r0 = 11 and the curvature angle is 90°. As different secondary flow patterns formed by oscillation cause different transverse mixings, the enhancement of transverse mixing is investigated here. A T-shaped structure installed downstream of the curved pipe allowed PIV measurements obviating light diffraction effects. From knowledge of the velocity components of the secondary flow, the variation in axial vorticity (ξ) and transverse strain (ε) were calculated. The experiments were carried out for the range of stationary Reynolds numbers 420≤Rest≤1000 (corresponding to Dean numbers 126.6≤Dn≤301.51), velocity component ratios 1≤(β = Umax,osc/Um,st)≤4 and frequency parameters 8.37<(α = r0(ω/v)0.5)<24.5. To guarantee being in the laminar regime, the higher values of β (β = 3 and 4) were studied just for Rest = 420. The effects of each parameter ((Rest, β and α) on transverse mixing are discussed by comparing the dimensionless vorticities (|ζP|/|ζS|) and dimensionless transverse strains (|εP|/|εS|) during a complete oscillation period.
Predictive Computational Fluid Dynamics Development and its Verification and Validation: An Overview
Authors: Lee H. B. , Bauer R. C.
Published: July 2010

A major focus for the development of computational fluid dynamics (CFD) technology is the attainment of an acceptable level of credibility of CFD analysis and simulations for the range of fluid flows of interest. Toward this end, a systematic program for CFD verification and validation (V&V) is being developed and implemented to enable Predictive-CFD (P-CFD) capability for use in the industrial design process. This paper provides an overview of a practical approach for CFD V&V that can support the initial use of CFD in the industrial design process and lays the foundation for providing a true predictive capability for CFD in the future. The approach emphasizes a bottom up view of CFD validation. In particular, validation assessments of fundamental, unit and separate-effects physics, flow configurations are performed to develop the large body of knowledge required to implement knowledge-based tools and procedures, e.g., best practices and design guidelines, for managing uncertainty and improving reliability of CFD analysis and simulations. In this approach, the flow field data obtained from validation experiments require a higher level of fidelity, resolution and documentation, including a complete and thorough description of the boundary and initial conditions driving the flows, the as-built geometry of the validation experiment and an appropriate uncertainty analysis of the experimental data. Datasets which meet these standards are termed validation-level datasets. It is expected that over time, the amount of validation-level data in the CFD V&V archives and the knowledge gained from CFD V&V assessments will be sufficient to assure the accuracy of the associated CFD predictions over a wide range of applications with a minimal amount of additional, confirmatory physical testing. In time, automated and standardized P-CFD methodology with associated best practices, design guidelines, and uncertainty quantification methods will provide a predictive capability in which sufficient confidence can be placed in CFD predictions that CFD analysis can replace large, semi-scale physical testing and allow for designs to be developed using CFD up front in the design process.
Research on Improving Efficiency of High Pressure Pump in Seawater Reverse Osmosis Desalination
Authors: Ye Xiao-yan , Yang Sun-sheng , Hu Jing-ning , Xiao Xia-ping , Zhou Guang-feng
Published: July 2010

According to the energy consumption status and characteristics of high pressure pump in Seawater Reverse Osmosis desalination unit of 2500m3/d and 5000 m3/d capacity, reasonable types of high pressure pump were chosen. Research on improving the efficiency of the two types of high pressure pump was made. Without affecting the pump reliability, structural optimization was made centering a round improving the energy efficiency or making efforts to improve energy efficiency. CFD technology was adopted in the fluid end design of the reciprocating pump and high efficiency hydraulic model chosen of the multistage centrifugal pump. For multistage centrifugal pump, several hydraulic models with good hydraulic characteristics were chosen. Numerical method was used in the performance prediction and modeling. Test results show that the optimized reciprocating pump efficiency reaches 93.36% and the multistage centrifugal pump efficiency reaches 80%. This research provides theoretical guidance for reducing energy consumption of Seawater Reverse Osmosis (SWRO) and improving the efficiency of high pressure pumps.
CFD of Water Injection on Lift and Drag of a 2-D Hydrofoil With Cavitation
Authors: Izadi Mohammad J.
Published: July 2010

Undesirable effects such as a decrease in lift and an increase in drag can be the result of the occurrence of cavitation on hydrofoils. The goal of this research is to investigate the effect of water injection on the lift and drag coefficient of a 2-D hydrofoil. An unsteady uniform flow of water over a NACA hydrofoil (2D) is numerically simulated. For the numerical simulation of a cavitating flow, a bubble dynamics cavitation model is used to describe the generation and evaporation of the vapor phase. The RNG k-ε turbulence model is used as a turbulence model. To simulate the cloud cavitation, a modification to the turbulent viscosity which is necessary, is implemented. This simulation is done for various angles of attack and different injection velocities. Comparison between experimental data and the numerical simulation obtained here is done to validate the numerical results. The results presented here show that, as the mass flow of the water injection increases, the lift coefficient decreases for all angles of attack but the rate of this decrease decreases for higher angles of attack. As the mass flow rate increases, the drag coefficient increases more for small angles of attack and decrease for larger angles of attack.
A New Cavitation Model Based on Evaporation and Condensation Theory
Authors: Chen Gang , Liu Shuhong , Cao Guangjun , Wu Yulin , Fu Suhong , Zhang Weihua , Liu Deming
Published: July 2010

Cavitation is a phenomenon which occurs where the local pressure falls off under the vapor pressure. Over the past few years, numerical simulation models for cavitation have been developed significantly in order to investigate the mechanism of cavitation. In the paper, A local homogeneous cavitation model based on the theory of evaporation and condensation has been deduced, which is used to describe the phase change between water and vapor. The RNG k–ε turbulence model is used to simulate the turbulent flow and the finite volume method is employed to discrete the governing equations. The effects of surface tension of water, pressure fluctuations and non-condensable gases are included in the mass transfer cavitation model. Also in order to neglect the effects of the quantities such as the bubble number and bubble diameter, which is difficult to measure, the relations between the aerodynamic drag and surface tension forces is used to describe the bubble diameter. In order to evaluate the new cavitation model, the two phase cavitation flows around a NACA0015 hydrofoil at different attack angle and different cavitation number are simulated by the new cavitation model, and compared with references, which showed good agreement with the experiments.
CFD Validation of Gas Injection in Flowing Mercury Over Vertical Smooth and Grooved Wall
Authors: Abdou Ashraf , Wendel Mark , Felde David , Reimer Bernard
Published: July 2010

The Spallation Neutron Source (SNS) is an accelerator-based neutron source at Oak Ridge National Laboratory (ORNL).The nuclear spallation reaction occurs when a proton beam hits liquid mercury. This interaction causes thermal expansion of the liquid mercury which produces high pressure waves. When these pressure waves hit the target vessel wall, cavitation can occur and erode the wall. Research and development efforts at SNS include creation of a vertical protective gas layer between the flowing liquid mercury and target vessel wall to mitigate the cavitation damage erosion and extend the life time of the target. Since mercury is opaque, computational fluid dynamics (CFD) may be used as a diagnostic tool to visualize the behavior of the liquid mercury and guide the experimental efforts. In this study, CFD simulations of three dimensional, unsteady, turbulent, two-phase flow of helium gas injection in flowing liquid mercury over smooth, vertically grooved and horizontally grooved walls are carried out with the commercially available CFD code Fluent-12 from ANSYS. The Volume of Fluid (VOF) model is used to track the helium-mercury interface. V-shaped vertical and horizontal grooves with 0.5 mm pitch and about 0.7 mm depth were machined in the transparent wall of acrylic test sections. Flow visualization data of helium gas coverage through transparent test sections is obtained with a high-speed camera at the ORNL Target Test Facility (TTF). The helium gas mass flow rate is 8 mg/min and introduced through a 0.5 mm diameter port. The inlet mercury mass flow rate is 51 kg/s and the predicted local mercury velocity is 0.9 m/s. In this paper, the helium gas flow rate and the local mercury velocity are kept constant for the three cases. Time integration of predicted helium gas volume fraction over time is done to evaluate the gas coverage and calculate the average thickness of the helium gas layer. The predicted time-integrated gas coverage over vertically grooved and horizontally grooved test sections is better than over a smooth wall. The simulations show that the helium gas is trapped inside the grooves.
Numerical Study of Pressure Wave Transmission in Liquid Under Different Interface Conditions Using Particle Method
Authors: Sun Zhongguo , Xi Guang
Published: July 2010

The process of pressure wave transmission in liquid is simulated with the moving particle semi-implicit (MPS) method. The simulation is carried out in a tube full filled with an energy absorbing liquid. Here we studied the shapes and positions of pressure waves and investigated the behavior of the waves under different viscosities and densities of liquids. Some typical parameters of pressure wave, such as peak pressure value, wave length and transport speed are studied. Varying viscosity does not change the wave length and speed of the pressure wave evidently. The effect of interfaces which formed by viscosity difference or density difference is investigated. Reflection is found not always happened on such interfaces. Pressure wave transport to liquid-solid interface and free surface are also simulated. Pressure wave is vanished when closing to free surface. These results give useful qualitative suggestions on controlling the pressure wave in fluid engineering.
Research on the Design Process of Bellmouth Suction Passage of Vertical Pumping System
Authors: Cheng Li , Liu Chao
Published: July 2010

There are different types of suction box of vertical pumping system. In order to understand their relations and differences, the control volume method was used to simulate flow pattern of different suction boxes. The standard k-ε turbulent model and SIMPLEC algorithm were applied in solving the N-S equations. The steady calculation results indicated that the flow patterns of different suction boxes of vertical pumping system were similar to each other. The different kinds of suction boxes were evolved from the pump sump. They can be united if they have essentially the same controlled parameters. It is a useful idea for the design of suction box of vertical pumping system.
Numerical Analysis of Transport Phenomena in Resistance Spot Welding Process
Authors: Li Yong Bing , Lin Zhong Qin , Li Li , Chen Guan Long
Published: July 2010

Resistance Spot Welding (RSW) is a very complicated process involving electro-magnetic, thermal, fluid flow, mechanical and metallurgical variables. Since weld nugget area is close and unobservable with experimental means, numerical methods are mainly used to reveal the nugget formation mechanism. Traditional RSW models focus on the electro-thermal behaviors in the nugget, and do not have the ability to model mass transport caused by induced magnetic forces in the molten nugget. In this paper, a multi-physics model, which comprehensively considers the coupling of electric, magnetic, thermal and flow fields during RSW, temperature-dependent physical properties and phase transformation, is used to investigate the heat and mass transport laws in the weld nugget and to reveal the interaction of the heat and mass transports. Results show that the heat transport behaviors in the weld nugget, the profile of the nugget, and the thermal field evolution are significantly changed when the mass transport is considered. At the same time, a good agreement is also found between experimental and numerically calculated nugget sizes. As a result, when predicting crystal growth process, the effects of the mass transport should be considered in order to obtain a more accurate prediction results.
An Equation of Motion for Particles of Finite Reynolds Number and Finite Size
Authors: Loth Eric , Dorgan Andrew J.
Published: July 2010

In order to simulate the motion of bubbles, drops, and particles, it is often important to consider finite Reynolds number effects on drag, lift, torque and history force. Herein, an equation of motion is developed for spherical particles with a no-slip surface based on theoretical analysis, experimental data and surface-resolved simulations. The equation of motion is then extended to account for finite particle size. This extension is critical for particles which will have a size significantly larger than the grid cell size, particularly important for bubbles and low-density particles. The extension to finite particle size is accomplished through spatial-averaging (both volume-based and surface-based) of the continuous flow properties. This averaging is consistent with the Faxen limit for solid spheres at small Reynolds numbers and added mass and fluid stress forces at inviscid limits. The finite Rep corrections are shown to have good agreements with experiments and resolved-surface simulations. The finite size corrections are generally fourth-order accurate and an order of magnitude more accurate than point-force expressions (which neglect quadratic and higher spatial gradients) for particles with size on the order of the gradient length-scales. However, further work is needed for more quantitative assessment of the truncation terms and the overall model robustness and accuracy in complex flows.
Influence of Leading Edge and Spacing on the Near Wake of Cylinder Pairs
Authors: Tsikata Jonathan M. , Paul Samuel S. , Katopodis Chris , Tachie Mark F.
Published: July 2010

An experimental study of the effects of the shape of rectangular cylinder leading edge and the cylinder spacing on the wake dynamics in an open channel turbulent flow is reported. The shapes of the rectangular cylinder leading edges were square and round with each having a square trailing edge. In each experiment, measurements were conducted for a pair of cylinders that have identical leading edge shape. Three centre-to-centre spacing (b = 30 mm, 50 mm and 75 mm) were investigated for each cylinder leading edge shape. Particle image velocimetry system was used to conduct measurements around each cylinder pair. The results show that the mean velocity and the turbulence level increases with decreasing spacing. The turbulence levels downstream of a cylinder pair with a round leading edge are relatively higher than the corresponding cylinder pair with square leading edge.
Time-Average Drag Coefficient and Void Fraction in Gas-Liquid Two Phase Flow
Authors: Ghanbarzadeh S. , Hanafizadeh P. , Saidi M. H.
Published: July 2010

Two-phase flow simulations around a body were not studied before and considering these flows can play a significant role in long-term reliability and safety of industrial systems. In this paper, flow regimes, drag coefficient and void fraction around different cross-section prisms were considered. To achieve this aim, main equations of flow have been developed for investigation of drag coefficient in air-water two phase. Our numerical analyses were performed by a designed and written CFD package which is based on Eulerian-Eulerian approach. Geometries, which have been studied in this article, are: circle, rectangle and triangle, for different aspect ratio (length over width) and leading edge angle, respectively. Other parameters such as two phase Reynolds number, free stream void fraction and bubble size were considered, too. Drag coefficient is closely related to the turbulence and the bubble motion. Since these mechanisms vary over time, we used the time-average value of drag coefficient. Furthermore, some attempts were done to investigate flow field, void fraction distribution and especially wake region around the cylinders by an image processing scheme in an upward air-water bubbly flow. The results showed that drag coefficient strongly dependent upon Reynolds number. Numerical results were compared with available empirical correlations and experimental works of Yokosawa et al [1].
Numerical Prediction of Cavitation Free Zone Operation for Francis Turbine
Authors: Ahmed Anees , Kshirsagar J. T.
Published: July 2010

The Francis Hydro Turbine Performance is normally presented in the form of Hill Curves with Unit Head, Unit Flow and Unit Power etc. The Hill Curves give hydraulic performance at variety of flow, head and speed. The cavitation performance of the turbine is marked by curves additionally put on the Hill Curves. Though the turbine could be operated over the entire range specified on the Hill Curves, the site reports have indicated that the operation of turbine is restricted to what is called safe operating zone on Hill Curves. Beyond this region the machine is expected to operate under cavitation, the cavitation occurring at different location on blade, depending on which zone of Hill Curve Diagram the machine is operating. A model test data was available in the form of Hill Curves indicating safe zone of operation. Efforts were made to capture the safe operating zone for this Francis Turbine using numerical approach. A well validated commercial software was used to capture cavitation occurring. The paper describes the approach used and presents results in the form of cavitation, occurring or not occurring, by choosing two operating zones on either side of the safe operating area. The results were highly encouraging. The safe operating zone was clearly identified. The paper also presents results in the form of overall performance and volume fraction distribution etc in both the zones. The paper describes the approach used in numerical experiments and the results thereof.
Numerical Analysis of Blade Channel Vortex in Francis Turbine at Part Load of Middle-Low Head
Authors: Xiao Yexiang , Wang Zhengwei , Zhang Jin , Peng Guangjie , Liu Dingyou , Ma Xinhong , Xiao Ming
Published: July 2010

The unsteady flow behavior and pressure fluctuations in the runner of a Francis turbine were predicted numerically for the middle-low head at three small guide vane openings. The unsteady Reynolds-averaged Navier-Stokes equations with the k–ω based SST turbulence model were solved to model the unsteady flow within the entire flow passage. The eddy characteristics inside the runner passage were discussed for various operating conditions, the shape of the blade channel vortex illustrate by the instantaneous iso-surfaces of the vorticity was similar with the experimental observation. This study investigates the characteristics of the unsteady flow dominant frequencies at different monitored points of the runner by spectrum analysis. At small guide vane opening conditions, the pulse in the runner flow passage are due to the rotor-stator interference between the runner and the guide vanes, the blade channel vortex in the runner blade passage. And the dominant frequencies of blade channel vortex were a low frequency. The unsteady flow behavior of blade channel vortex in the runner was classified numerically at part load of middle-low head.
Pressure Fluctuations Reduction in Centrifugal Pumps: Influence of Impeller Geometry and Radial Gap
Authors: Solis Moise´s , Bakir Farid , Khelladi Sofiane
Published: July 2010

A numerical investigation was carried out in a volute type centrifugal pump to study the influence of splitter blades and radial gap and decrease the pressure fluctuations levels at the blade passage frequency for the same operating conditions. In this kind of turbomachinery, the strong interaction between the flow leaving the impeller and entering the volute casing generate pulsating pressures which propagate toward the inlet and the discharge of the pump. They are mainly associated to the blade passage frequency presenting high amplitudes near the tongue and at impeller discharge. Three configurations were considered in the numerical tests, the first hydraulic is the reference geometry to be optimized and the other two are the versions modified for decrease pressure fluctuations. The geometrical changes concern: adding splitter blades to the original impeller and increasing the radial gap between the splitter impeller and the volute tongue. The simulations for all configurations were performed at the same flow rate. It was necessary to decrease the rotational speed of the optimized configurations to reach the same operating point. The pressure signals were collected by virtual sensors placed at several locations of the pump mainly at the impeller-volute interface plus at inlet and at outlet ducts. Then they were treated and processed obtaining the frequency spectrum by means of the Fast Fourier Transform, so the numerical results were compared and discussed. A commercial CFD code has been used to predict the pulsating pressures and the Unsteady Reynolds Averaged Navier-Stokes (URANS) approach has been applied to solve the unsteady, incompressible and turbulent flow. The k-ω SST model was used to take into account the turbulence effects and the standard wall functions were applied. The numerical procedure employed for the unsteady simulations was determined after an investigation of the influence of boundary conditions on the pressure signals specially at the outlet duct, showing that the condition of outflow does not have influence on the numerical results.
Direct Numerical Simulation of Four Way-Coupled Gas-Solid Flow and Deposition in a Turbulent Channel Flow
Authors: Ahmadi Goodarz , Nasr Hojjat , McLaughlin John B.
Published: July 2010

This study was concerned with the effects of particle-particle collisions and two-way coupling on the dispersed and carrier phase turbulence fluctuations in a channel flow. The time history of the instantaneous turbulent velocity vector was generated by the two-way coupled direct numerical simulation (DNS) of the Navier-Stokes equation via a pseudospectral method. The particle equation of motion included the drag and the shear induced lift forces. The effect of particles on the flow was included in the analysis via a feedback force that acted on the computational grid points. Several simulations for different particle relaxation times and particle mass loadings were performed, and the effects of the inter-particle collisions and two-way coupling on the particle deposition velocity, fluid and particle fluctuating velocities, particle normal mean velocity, and particle concentration profiles were determined. It was found that, when particle-particle collisions were included in the computation but two-way coupling effects were ignored, the particle normal fluctuating velocity increased in the wall region causing an increase in the particle deposition velocity. When the particle collisions were neglected but the particle-fluid two-way coupling effects were accounted for, the particle normal fluctuating velocity decreased near the wall causing a decrease in the particle deposition velocity. For the physical case that both inter-particle collisions and two-way coupling effects are present, a series of four-way coupling simulations was performed. It was found that the particle deposition velocity increased with mass loading. The results for the particle concentration profile indicated that the inclusion of either two-way coupling or inter-particle collisions into the computation reduced the accumulation of particles near the wall. Comparisons of the present simulation results with the available experimental data and earlier numerical results were also presented.
Demonstration and Validation of a 3D CFD Simulation Tool Predicting Pump Performance and Cavitation for Industrial Applications
Authors: Ding H. , Visser F. C. , Jiang Y. , Furmanczyk M.
Published: July 2010

Due to complexities in geometry and physics, Computational Fluid Dynamics (CFD) pump simulation has historically been very challenging and time consuming, especially for cases with cavitation. However, with the evolution and innovation of CFD technologies, pump cavitation simulation has improved significantly in recent years. In view of these developments, this paper will discuss a new generation CFD tool for pump cavitation simulation, using an axial flow water pump as a demonstration case. A novel CFD methodology and advanced cavitation model will be presented and discussed. Key components that are relevant to improvement of accuracy and CFD simulation speed will be discussed in detail. An axial flow water pump is chosen as the test case to demonstrate and validate the capability and accuracy of the code discussed. Simulation results include pump head, hydraulic efficiency and cavitation characteristic in terms of incipient Net Positive Suction Head (NPSHi) for the whole pump flow passages using both multiple reference frame (MRF) and transient approaches. Multiple operation conditions, from 70% to 120% of duty flow rate have been evaluated and will be projected against experimental data. Furthermore, simulated cavitation patterns will be compared with video images recorded during the experiments.
Numerical Analysis of Water Suction Effect on Lift and Drag Coefficient of a Hydrofoil With Cavitation
Authors: Izadi Mohammad J.
Published: July 2010

Cavitation is the formation of the vapor bubbles within a liquid where the flow dynamics, cause the local static pressure to drop below the vapor pressure. This phenomenon can cause undesirable effects on the hydrofoils such as a decrease in the lift and an increase in the drag. In the present study, the unsteady cavitating flow over a 3-D hydrofoil is numerically simulated. The purpose of this work is to investigate the effect of the upper surface suction in the cavitation area on the lift and drag coefficients of a hydrofoil. An unsteady uniform flow of water over a 3-D NACA hydrofoil is numerically simulated. The full cavitation model along with the RNG k-ε turbulence model is implemented. A modification to the turbulent viscosity, which is necessary to simulate the cloud cavitation, is implemented. The simulation is implemented for various angles of attack and various suction velocities. Comparison between some experimental data and the numerical simulation obtained here is done in order to validate the numerical results. The results obtained here show that, as the mass flow of the water suction increases, the drag coefficient is decreased for large angles of attack, but for small angles of attack it does not change as much. As the mass flow of the water suction increases, the lift coefficient is decreased for small angles of attack and for larger angles of attack the lift coefficient is increased.
Secondary Flows in Centrifugal Pump Impellers: PIV Measurements and CFD Computations
Authors: Westra R. W. , Broersma L. , van Andel K. , Kruyt N. P.
Published: July 2010

Two-dimensional Particle Image Velocimetry measurements and three-dimensional Computational Fluid Dynamics (CFD) analyses have been performed of the flow field inside the impeller of a low specific-speed centrifugal pump operating with a vaneless diffuser. Flow rates ranging from 80% to 120% of the design flow rate are considered in detail. It is observed from the velocity measurements that secondary flows occur. These flows result in the formation of regions of low velocity near the intersection of blade suction side and shroud. The extent of this jet-wake structure decreases with increasing flow rate. Velocity profiles have also been computed from Reynolds-averaged Navier-Stokes equations with the Spalart-Allmaras turbulence model, using a commercial CFD-code. For the considered flow rates the qualitative agreement between measured and computed velocity profiles is very good. Overall, the average relative difference between these velocity profiles is around 7%. Additional CFD computations have been performed to assess the influence of Reynolds number and shape of the inlet velocity profile on the computed velocity profiles. It is found that the influence of Reynolds number is mild. The shape of the inlet profile only has a weak effect at the shroud.
Motion of Liquid Drops Through Complex 3D Clothing
Authors: Nasr Hojjat , Ahmadi Goodarz , Tafreshi Homan , McLaughlin John B.
Published: July 2010

This study was concerned with the numerical simulation of a moving drop through a fabric due to a wettability gradient. The wettability gradient was introduced by varying the contact angle along the staggered fibers of a fabric. The unsteady laminar Navier-Stokes equation was solved using a fixed Eulerian unstructured grid. The Volume of Fluid Model (VOF) was used to account for tracking the gas/liquid interface. A water drop was placed on top of the fabric with an initial velocity, and the motion of the drop through the fabric was studied. Several computer simulations under different conditions such as the distance between fibers, contact angle distribution, and drop initial velocity were performed, and the results were compared with each other. In order to verify the accuracy of the computational model, the motion of a drop on a surface due to a wettabily gradient was simulated as a benchmark.
Transport and Deposition of Particles and Fibers in Human Tracheobronchial Tree
Authors: Tian L. , Ahmadi G. , Hopke P. K. , Cheng Y.-S.
Published: July 2010

Transport and deposition of ellipsoidal fibers in the human upper airways were analyzed using an asymmetric airway bifurcation model. The trachea and the first two generations (G0–G1) of the tracheobronchial tree were included in the study. The focus of the study was on prediction of transport and deposition of fibers and elongated particles. The laryngeal jet at the trachea entrance was modeled as an effective turbulence disturbance, and Reynolds stress transport turbulence model (RST) was used. For accurate modeling of the near wall airflow, the “two-layer zonal model” was used for boundary consideration, and the turbulence normal fluctuation close to wall is further corrected based on the “quadratic variation near wall model” (Tian and Ahmadi 2007). Lagrangian simulation of ellipsoidal fiber transport and deposition was developed where the coupled translational and rotational motions of the fibers were accounted for. The particle equations of motion included the hydrodynamic drag and torque, shear induced lift, gravitational sedimentation, and turbulence diffusion effects. The simulation results showed that the elongated fiber remained aligned with the main flow most of the time. On short duration occasions, the fibers rotated impulsively along their path. The fiber rotational motion was dependent on fiber geometry and the local flow shear. Fiber deposition pattern and deposition rate in the trachea and the first bifurcation were evaluated, and the results were compared with the experiments.
Numerical Study of Angle of Incidence Effect on Particle Deposition Over a Square Cylinder in a Channel Using Lattice-Boltzmann Method
Authors: Jafari S. , Salmanzadeh M. , Rahnama M. , Ahmadi G.
Published: July 2010

Particle dispersion and deposition over a square cylinder with different angles of incidence (α = 0°, 45°) in a channel flow was investigated numerically. A computational model using Lattice-Boltzmann Method (LBM) for flow simulation was developed. The interpolated bounce-back (IBB) boundary condition was applied to model the no-slip boundary conditions on curved boundary. The computational model was used to simulate the two-dimensional airflow field in the duct with obstructing square cylinder. A Lagrangian approach was used for simulation of spherical, solid particles suspended in the airflow in the duct. The Reynolds numbers were in the range of laminar. The formation of the von Karman vortex sheet behind the cylinder was predicted by the present LBM simulation. It was shown that the periodicity of the simulated vortex street agrees well with available experimental data. Transport and deposition of 1 μm to 100 μm particles in the duct were studied. The instantaneous flow field was used to evaluate the particle trajectories. The forces included in the particle equation of motion were drag, lift, gravity, and Brownian. The simulation results shows that the particle deposition efficiency decreases when the angle of incidence is increased from 0° to 45° and also more vigorous vortex shedding was observed at α = 45°.
Numerical Simulation of Cavitation Phenomena in a Centrifugal Pump
Authors: Jeanty Freddy , De Andrade Jesu´s , Asuaje Miguel , Kenyery Frank , Va´squez Auristela , Aguillo´n Orlando , Tremante Andre´s
Published: July 2010

Cavitation is a common phenomenon that appears during the operation of the hydraulic turbomachines reducing performance and life of Centrifugal pumps. The main goal of this work is primarily a CFD-simulation of the whole Centrifugal Pump-Turbine including the suction cone, impeller, diffuser blades and volute, in order to characterize and evaluate its performance under cavitation conditions. The CFD simulations results were compared with experimental data under cavitation and non-cavitation conditions. A good agreement has been obtained under non-cavitation conditions for global performance parameters. After the implementation of the Rayleigh Plesset cavitation model, the required Net Positive Suction Head (NPSHr) has been predicted from CFD simulations. Finally, a full cavitation test can be reproduced for a Hydraulic Turbomachine to avoid this dangerous phenomenon.
3-D Simulations of the Bubble Formation From a Submerged Orifice in Liquid Cross-Flow
Authors: Nabavi Majid , Siddiqui Kamran , Chishty Wajid A.
Published: July 2010

The results are presented from a 3-D simulation of the bubble formation from a submerged orifice in liquid cross-flow. VOF model is used for the simulations. The VOF equation is solved using an explicit time-marching scheme. A second order upwind differencing scheme is applied for the solution of momentum equation. The pressure-implicit with splitting of operators (PISO) scheme is used for the pressure-velocity-coupling scheme. Pressure is discretized with a PRESTO scheme. The computational domain has the dimensions of 100 mm length, 50 mm width and 16 mm height with an orifice of 0.25 mm radius, placed at the bottom of the channel and 10 cm from the water inlet. The water inlet velocity of 0.05 and 0.136 m/s and air inlet mass flow rate of 10−6 and 10−5 kg/s are considered. The simulation results are compared with the experimentally acquired images of the bubbles in the cross-flow stream using a high speed camera (3000 fps). A good agreement with respect to bubble shape and bubble terminal velocity is observed between the experimental and simulation results for both cases. The 3-D numerical model is compared with the 2-D model in order to highlight and emphasize the need for 3-D model to correctly simulate the dynamics of such flow configurations.
Simulation of Aerosol Deposition in 90° Square Cross Section Bends Using LES
Authors: Baghernezhad Nemat , Abouali Omid , Ahmadi Goodarz
Published: July 2010

The behavior of dilute turbulent particulate flow inside a 90° bend with a square cross section is investigated numerically. The continuous phase is simulated using the large eddy simulation technique. The particulate phase is simulated with a Lagrangian approach for various particle diameters. The curved bend has a squared cross section and the simulations are performed for a flow Reynolds number of 100,000 based on the hydraulic diameter of the bend duct. The details of the flow field predicted by LES are presented and compared with predictions of RSM model and the available experimental data. The results of LES are shown to be in a better agreement with the experimental data compared with the RSM model for certain conditions. Steady and unsteady particles tracking approaches are used and the collection fractions of the particles on the bend walls for various particles diameters are presented and discussed.
Examination of a Proposed Validation Data Set Using CFD Calculations
Authors: Johnson Richard W.
Published: July 2010

The United States Department of Energy is promoting the resurgence of nuclear power in the U. S. for both electrical power generation and production of process heat required for industrial processes such as the manufacture of hydrogen for use as a fuel in automobiles. The DOE project is called the next generation nuclear plant (NGNP) and is based on a Generation IV reactor concept called the very high temperature reactor (VHTR), which will use helium as the coolant at temperatures ranging from 450 °C to perhaps 1000 °C. While computational fluid dynamics (CFD) has not been used for past safety analysis for nuclear reactors in the U. S., it is being considered for safety analysis for existing and future reactors. It is fully recognized that CFD simulation codes will have to be validated for flow physics reasonably close to actual fluid dynamic conditions expected in normal and accident operational situations. To this end, experimental data have been obtained in a scaled model of a narrow slice of the lower plenum of a prismatic VHTR. The present article presents new results of CFD examinations of these data to explore potential issues with the geometry, the initial conditions, the flow dynamics and the data needed to fully specify the inlet and boundary conditions; results for several turbulence models are examined. Issues are addressed and recommendations about the data are made.
On the Dynamics of the Polygonal Satellite Vortices
Authors: Ait Abderrahamane Hamid , Siddiqui Kamran , Vatistas Georgios
Published: July 2010

This paper deals with the dynamics of polygonal shapes resulting from the symmetry breaking of hollow-vortex core in a shallow water layer produced by a rotating disk near the bottom within a stationary cylindrical container. These polygonal shapes are investigated through image analysis. It is found that a given polygon rotates at the frequency close to one–third of the corresponding disk frequency and the flow dynamics around the apexes of the polygon is characterized by a frequency which is close to one–third of the frequency of the given polygonal pattern. The results also suggest a possible resonance between the satellite vortices at the apexes of the patterns and the bulk flow.
Numerical Simulation of Confined Swirling Flows of Oldroyd Fluids
Authors: Majidi Sahand , Javadzadegan Ashkan
Published: July 2010

The effect of a fluid’s elasticity has been investigated on the vortex breakdown phenomenon in confined swirling flow. Assuming that the fluid obeys upper-convected Maxwell model as its constitutive equation, the finite volume method together with a collocated mesh was used to calculate the velocity profiles and streamline pattern inside a typical lid-driven swirling flow at different Reynolds and Weissenberg numbers. The flow was to be steady and axisymmetric. Based on the results obtained in this work, it can be concluded that fluid’s elasticity has a strong effect on the secondary flow completely reversing its direction of rotation depending on the Weissenberg number. Even in swirling flows with low ratio of elasticity to inertia, vortex breakdown is postponed to higher Reynolds numbers. Also, the effect of retardation ratio on the flow structure of viscoelastic fluid with the Weissenberg number being constant was surveyed. Based on our results, by decreasing the retardation ratio the flow becomes Newtonian like.