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Vol11 , No 3
Investigation of the Numerical Methodology of a Model Wind Turbine Simulation
Pages : 527-544
Authors : N. Tabatabaei,  M. J. Cervantes,  C. Trivedi, 
The present work aims to investigate different methodologies for the numerical simulation of an upwind three-bladed wind turbine; which is supposed to be a base model to simulate icing in cold climate windmills. That is a model wind turbine for which wind tunnel tests have been completed at the Norwegian University of Science and Technology (NTNU). Using the assumption of axisymmetry, one-third of rotor has been modeled and periodic boundaries applied to include the effects of other blades. Then the full rotor was studied with transient simulation. To take in the effects of wind turbine wakes, the wind tunnel entrance and exit have been considered 4 and 5 diameters upstream and downstream of the rotor plane, respectively. Furthermore, the effects of tower and nacelle are included in a full-scale transient model of the wind tunnel. Structured hexa mesh has been created and the mesh is refined up to y+=1 in order to resolve the boundary layer. The simulations were performed using standard k-ε, Shear Stress Transport (SST) model and a sophisticated model Scale-Adaptive Simulation (SAS)-SST to investigate the capability of turbulence models at design and off-design conditions The performance parameters, i.e., the loads coefficients and the wake behind the rotor were selected to analyze the flow over the wind turbine. The study was conducted at both design and off-design speeds. The near wake profiles resulted from the transient simulation match well with the experiments at all the speed ranges. For the wake development modelling at high TSR, the present simulation needs to be improved, while at low and moderate TSR the results match with the experiments at far wake too. The agreement between the measurements and CFD is better for the power coefficient than for the thrust coefficient.
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Determination of the Zone with a Particularly High Risk of Endogenous Fires in the Goaves of a Longwall with Caving
Pages : 545-553
Authors : J. Brodny,  M. Tutak, 
During ventilation of longwalls, part of the air stream migrates into goaves with caving. In the case where these goaves contain coal susceptible to spontaneous combustion, such air flow through the goaves may lead to the formation of favourable conditions for coal oxidation and, subsequently, for its self-heating and spontaneous combustion. The ensuing endogenous fire may constitute a serious hazard to the mining crew and underground operations. The article presents the results of a numerical analysis of air stream flowing through the goaves of a longwall with caving ventilated with the U-type system from the exploitation field borders. The purpose of the analysis was to demarcate the zone with a particularly high risk of endogenous fires within the goaves. The hazardous values of air flow speed and oxygen concentration in the goaves, responsible for the commencement of low-temperature coal oxidation, were determined for specific mining and geological conditions
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Effect of Channel Boundary Conditions in Predicting Hydraulic Jump Characteristics using an ANFIS-Based Approach
Pages : 555-565
Authors : K. Roushangar,  R. Ghasempour,  R. Valizadeh, 
Hydraulic jump is a phenomenon which is used to dissipate the kinetic energy of the flow and prevent scour below overflow spillways, chutes and sluices. This paper applies adaptive neuro-fuzzy inference system (ANFIS) as a Meta model approach to estimate hydraulic jump characteristics in channels with different bed conditions (i.e. channels with different shapes and appurtenances). In hydraulic jump characteristics modeling, different input combinations were developed and tested using 1700 experimental data. The obtained results indicated that the applied method has high capability in modeling hydraulic jump characteristics. It was observed that the developed models for expanding channel with a block performed more successful than other channels. For rectangular channels, it was found that the basin with rough bed led to better predictions compared to the basin with a step. In the prediction of jump length, the superior performance was obtained for the model with input combinations of Froude number and the relative height of jump. From the sensitivity analysis, it was induced that, Fr1 (upstream Froude number) is the most significant parameter in modeling process. Also comparison between ANFIS and semi-empirical equations indicated the great performance of the ANFIS.
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Combined Methods in Preliminary Micro Scale Gas Turbine Diffuser Design – a Practical Approach
Pages : 567-575
Authors : M. Czarnecki,  J. Olsen, 
Micro scale gas turbines are low cost, simplified versions of full scale jet engines. A unique feature of their design are centrifugal compressor impellers that are selected from automotive low cost, high quality turbocharger components. The present article is dedicated to the practical design of a micro scale centrifugal compressor diffuser that suits a reduced scale, turbojet engine. The idea of using a simplified method comes from the requirement from fast geometry generation for a prototype design. The chosen approach is suitable when the time is crucial and available resources are limited. The chosen simplified analytical model is based on turbomachinery physics. The obtained results are verified by detailed data from successful designs such as KJ66, MW54 and TK50. For a prototype design, GT60 results where compared with a numeric simulation in the ANSYS CFX environment. The difference in isentropic efficiency, numerical prediction in comparison to compressor flow map was less than 3%.This is acceptable for preliminary calculations due to the difference in compressor stator design.
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Numerical Simulation of Turbulent Airflow and Micro-Particle Deposition in Upper Human Respiratory System
Pages : 577-584
Authors : A. R. Tahavvor,  P. Zarrinchang, 
The nasal cavity and sinuses are a component of the upper respiratory system and study the air passage into the upper component of human airway is consequential to amend or remedy deficiency in human respiration cycle. The nose performs many paramount physiological functions, including heating, humidifying and filtering inspired air, as well as sampling air to smell. Aforetime, numerical modeling of turbulent flow in authentic model of nasal cavity, sinus, pharynx and larynx has infrequently been employed. This research has tried to study details of turbulent airflow and particle deposition through all spaces in three-dimensional authentic model of human head which is obtained from computed tomography scan images of a 26-years old female head, neck and chest without any problem in her respiratory system that air can flow them. The particle size in this study was opted to be in the range of 5-30 µm. The particles are tracked through the continuum fluid discretely utilizing the Lagrangian approach.
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Numerical Investigation on Temperature and Flow Characteristics of a Tornado-Like Vortex by using Large-Eddy Simulation
Pages : 585-595
Authors : K. Li,  M. Xiang,  Y. Hu,  X. L. Huang,  H. Y. Qi, 
The present work is about numerical simulations of the tornado-like vortex flow generated by our group based on LES techniques which is executed on a three-dimensional computational grid and results have been compared with experimental ones. Different subgrid-scale stress model and finite volume method are adopted to solve the low Mach number compressible Navier-Stokes equations using the different computational domain and boundary conditions, which are mainly to assess the model feasibility. All the simulations were performed using ANSYS FLUENT14.5 in consistency with the real experimental model which avoided the performances of the different techniques and turbulence models introducing other variables. Numerical results suggest that the vacuum degree, temperature difference and the rotation strength decayed in the axial and radial direction regularly changed with inlet gauge pressure p0 from 100 to 400 kPa which are consistent with experimental results. The accurate numerical simulation of this specific flow, resulting in an improved prediction capability of the flow and thermal properties of tornado-like vortex, could allow a correct estimation of the vacuum and energy separating performance of this device in strong rotating jet operation. Furthermore, computational results illustrate that strong rotating jet turbulent flow under the conditions of the certain pressure, temperature, velocity profiles and distribution can formed tornados-like vortex evolution and maintaining mechanism due to a large gradient in radial temperature, pressure and velocity balanced by inertia force, centrifugal force and rotational kinetic energy dissipation.
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Ferrofluid Based Squeeze Film Lubrication between Rough Stepped Plates with Couple Stress Effect
Pages : 597-612
Authors : Y. D. Vashi,  R. M. Patel,  G. M. Deheri, 
This investigation purposes to study the magnetic fluid based squeeze film behavior on transversely rough stepped plates with the influence of couple stress. Using the well-known stochastic model of Christensen and Tonder the roughness effect has been evaluated. The magnetic fluid flow model of Neuringer - Roseinweig has been adopted to obtain the influence of magnetization. The governing Reynolds’ type equation is derived on the basis of stokes microcontinum theory for couple stress fluid. For the expression of pressure distribution, the stochastically averaged Reynolds’ type equation is solved. which results in calculation of load carrying capacity. The graphical outcomes also presented in tabular form suggest that although the bearing suffers on account of roughness, the magnetization and couple stress effect save the situation, as this combination does not allow the load carrying capacity to fall rapidly. However, in the case of negatively skewed roughness the magnetization goes a long way in dropping the adversarial influence of roughness by selecting an appropriate value of couple stress parameter when variance (-ve) is involved. It is found that the couple stress effect, alone may not be sufficient to counter the negative influence of transverse roughness and porosity. However, in almost all situations the ferrofluid lubrication adds significantly to the positive effect of couple stress to overcome the adversarial outcome of porosity and roughness. Further, the position of step plays a vital role for an all-round enhancement of the bearing performance.
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Effect of Pitch Angle and Reynolds Number on Aerodynamic Characteristics of a Small Horizontal Axis Wind Rotor
Pages : 613-620
Authors : J. Y. Zhu,  P. Q. Liu, 
Wind tunnel experiments were conducted on a 2-blade horizontal axis wind rotor to investigate the effect of pitch angle and Reynolds number on aerodynamic characteristics. The experimental study was conducted in the start-up and operating stages and the results for the two stages are discussed, respectively. During start-up, with tip speed ratio less than 1, the power coefficient of the rotor increases with pitch angles, but remains almost constant with Reynolds number. In the operating stage, with tip speed ratio more than 1, the maximum power coefficient occurs at decreasing tip speed ratios as the pitch angle increases. In addition, the maximum power coefficient increases and the corresponding tip speed ratio decreases with Reynolds number. The aerodynamic characteristics of the rotor can be analyzed qualitatively based on reliable and full aerodynamic data of the airfoil, which contributes to selection of the airfoil and determination of the rated velocity.
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A Dramatic Enhancement of Heat Transfer in Dream Pipe with Viscoelastic Fluids
Pages : 621-635
Authors : P. Puvaneswari,  K. Shailendhra, 
A mathematical investigation on the combined effect of oscillation and conjugation on the enhancement of heat transfer in a heat pipe called Dream Pipe is carried out, when viscoelastic fluids (CPyCl/NaSal) are employed as the heat carriers. Closed-form solutions for the momentum and heat equations are presented. The physical and thermal properties of the polymer solution used are obtained by experiments. The effects of thermal conductivity and thickness of the wall, fluid thickness, Womersley number (α), Deborah number and Prandtl number on the enhancement of heat transfer are examined. Results obtained in the present analysis are in excellent agreement with those of the existing literature. The effective thermal diffusivity (κe) is maximized at optimum α where the fluid flow exhibits a resonant behavior. Several maxima occur in κe for several resonant frequencies and the dramatic increase in κe due to oscillation for the viscoelastic fluid is 5.63 x 109 times higher than that obtained by the molecular motion. This increase is much higher than that (1.84 x 104 times) obtained for the Newtonian fluid. κe is increased with increasing wall thermal conductivity and thickness in the viscous regime whereas in the elastic regime the effect of conjugation is saturated. In the viscous regime, a maximum increase of 50.63% in κe is obtained by optimizing the wall thickness. Also κe increases with increasing molar ratio of concentrations of counterion to surfactant. A maximum heat flux of 4.54 x 1010 W/m2 is achieved using a viscoelastic fluid with thermally conducting wall and this highest heat flux is 207 times higher than that (2.19 x 108 W/m2) obtained with the Newtonian fluid (liquid metal). Hence, viscoelastic fluids are preferable to liquid metals as working fluids in the Dream Pipe. The new insights gained by the present investigation are useful while designing viscoelastic Dream Pipes and micro channel heat exchangers.
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A Similarity Solution with Two-Equation Turbulence Model for Computation of Turbulent Film Condensation on a Vertical Surface
Pages : 637-645
Authors : M. Ziaei Rad,  A. Ahmadi Nadooshan,  S. Mahmoodi, 
In this paper, we presented a similarity solution for turbulent film condensation of stationary vapor on an isothermal vertical flat plate. In this method, some similarity transformations are employed and the set of governing partial differential equations (PDE) of conservation together with transport equations of turbulent kinetic energy and dissipation rate are transformed into a set of ordinary differential equations (ODE). Calculated data for the flow field, velocity profile, wall shear stress, condensate film thickness, turbulent kinetic energy, rate of dissipation, and heat transfer properties are discussed. The effect of Prandtl (Pr) number was also investigated in a wide range of variations. The obtained results showed that at high Prandtl numbers, the velocity profile becomes more uniform across the condensation film and therefore, the kinetic energy of turbulence is reduced. Furthermore, the effect of change in Pr is negligible at high Pr numbers and consequently, the flow parameters have no significant change in this range. The friction coefficient changes linearly through the condensation film and the slope of friction lines diminishes slightly by the Pr number. The rate of turbulent kinetic energy increases linearly from the wall up to about 20% of condensate film, then rises asymptotically and converges to a constant value near the liquid-vapor interface. Also, the rate of turbulent dissipation grows linearly up to 40% of condensate film thickness and then increases slightly while it oscillates.
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Flame Length Scaling of C2H4-Air Premixed Flames under Acoustic Forcing
Pages : 647-654
Authors : D. Hwang,  K. Ahn,  Y. Yoon, 
An experimental study has been carried out to investigate the effects of inlet velocity, equivalence ratio, and acoustic forcing on flame lengths and flame center lengths in a dump combustor. A premixed gas of ethylene and air was supplied to a combustor through an inlet section and an acoustic driver was used to generate acoustic forcing to simulate unstable combustion. By changing these parameters, combustion tests were performed and flame images were taken using an ICCD camera with a bandpass filter corresponding to a CH* chemiluminescence band. Flame lengths/flame center lengths were obtained from the flame images and were analyzed with respect to dimensional parameters. For a more general finding, the flame length and flame center length were normalized by the inlet width. The dimensional parameters were also replaced with non-dimensional parameters such as the Reynolds number, Strouhal number, Damköhler number, and normalized inlet velocity fluctuation, since dimensional parameters have a complex influence on these non-dimensional parameters. The normalized flame lengths and flame center lengths could be expressed well as a function of the non-dimensional parameters. It was found that an increase in the Reynolds number and a decrease in the Strouhal number, Damköhler number and normalized inlet velocity fluctuation caused the flame length/flame center length to become greater.
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On the Expedient Solution of the Boltzmann Equation by Modified Time Relaxed Monte Carlo (MTRMC) Method
Pages : 655-666
Authors : M. Eskandari,  S. S. Nourazar, 
In the present study, a modified time relaxed Monte Carlo (MTRMC) method is developed for numerical solution of the Boltzmann equation in rarefied regimes. Taylor series expansion is employed to obtain a generalized form of the Wild sum expansion and consequently the modified collision functions with fewer inter-molecular interactions are obtained. The proposed algorithm is applied on the lid-driven micro cavity flow with different lid velocities and the results for velocity and shear stress distributions are compared with those from the standard DSMC and TRMC methods. The comparisons show excellent agreement between the results of the MTRMC method with their counterparts from TRMC and DSMC methods. The present study illustrates appreciable improvement in the computational expense of the MTRMC method compared to those from standard TRMC and DSMC methods. The improvement is more pronounced compared to the standard DSMC method. It is observed that up to 56% reduction in CPU time is obtained in the studied cases.
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Electroosmotic Pumping Between Two Immiscible Electrical Conducting Fluids Controlled by Interfacial Phenomena
Pages : 667-678
Authors : A. Matías,  O. Bautista,  F. Méndez,  J. Escandón, 
In this study, the isothermal electroosmotic flow of two immiscible electrical conducting fluids within a uniform circular microcapillary was theoretically examined. It was assumed that an annular layer of liquid adjacent to the inside wall of the capillary exists, and this in turn surrounds the inner flow of a second liquid. The theoretical analysis was performed by using the linearized Poisson-Boltzmann equations, and the momentum equations for both fluids were analytically solved. The interface between the two fluids was considered uniform, hypothesis which is only valid for very small values of the capillary number, and shear and Maxwell stresses were considered as the boundary condition. In addition, a zeta potential difference and a charge density jump were assumed at the interface. In this manner, the electroosmotic pumping is governed by the previous interfacial effects, a situation that has not previously been considered in the specialized literature. The simplified equations were nondimensionalized, and analytical solutions were determined to describe the electric potential distribution and flow field in both the fluids. The solution shows the strong influence of several dimensionless parameters, such as μr, εr, , and , and , on the volumetric flow. The parameters represent the ratio of viscosity, the ratio of electric permittivity of both fluids, the dimensionless zeta potential of the microcapillary wall, the dimensionless charge density jump and charge density between both fluids, and the electrokinetic parameters, respectively.
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Numerical Analysis of the Effects of Riblets on Drag Reduction of a Flat Plate
Pages : 679-688
Authors : A. Heidarian,  H. Ghassemi,  P. Liu, 
In recent years, marine transportation as well as land and air transportation has become in the centre of attention. There has been an attempt to reduce the drag exerted on vessels by various methods, such as rough surfaces and base bleed. Study of the high-speed seawater animals like sharks and dolphins has triggered the use of riblets on the hull of marine vessels. The use of riblet is easy among various methods of skin friction drag reduction. For this reason, there is an interest for construction and optimization of riblet as an efficient and economical way. In this research, the shark’s skin was modeled for the first time in ANSYS CFX using Computational Fluid Dynamics, through analyzing the effects of riblet on a submerged flat plate. Different types of riblet based on height and space between micro channels were studied, and then a comparison between a riblet-covered plate and a smooth plate in various velocities were performed. It was found that by selecting the most appropriate type of riblet, 11 % drag reduction can be attained. Then, by comparing different types of riblets, the best type for drag reduction was selected. Furthermore, the obtained results were compared with the available experimental data.
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Modeling and Optimization of the Magnetohydrodynamic Conduction Pump by Particle Swarm Method
Pages : 689-694
Authors : K. Bouali,  F. Z. Kadid,  R. Abdessemed,  A. Taleb Ahmed,  A. Toumi, 
The paper is dedicated to the study and the optimization of a magnetohydrodynamic conduction pump. Electromagnetic pumps have several advantages to mechanical pumps, they do not use a moving mechanical part, unlike traditional electric motors, and they transform electromagnetic energy directly into kinetic energy. A fluid is set in motion in a magnetic field by an electric field supplying an electric current to the terminals of electrodes immersed in the fluid. The optimization procedure based on the particle swarm method uses a fitness function as the minimum of the mass. The electromagnetic model is carried out by the finite volume method and the steady fluid flow by COMSOL software. The optimized results of the performance characteristics of the electromagnetic pump are obtained.
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An Optical Flow Measurement Technique based on Continuous Wavelet Transform
Pages : 695-707
Authors : A. B. Osman,  M. Ovinis,  I. Faye,  F. M. Hashim,  H. Osei, 
Flow measurement underwater oil leak is a challenging problem, due to the complex nature of flow dynamics. Oil jet flow associated with a multi-scale coherent structure in both space and time direction. Optical plume velocimetry (OPV) was developed by (Crone, McDuff, and Wilcock, 2008), and it was the most accurate technique that used for oil leak flow measurement. Despite its better estimation, the OPV measured the oil flow rate with high uncertainty of 21%. This is due to the multi-scale phenomena of oil flow, as well as the limited accuracy of direct cross correlation (DCC) typically used by OPV. This paper proposed a novel technique that considers the multi-scale property of turbulence in flow measurement. The proposed technique is based on continuous wavelet transform and estimates the flow using the following steps: Decomposition of turbulent flow signal by using continuous wavelet transform (CWT), correlation coefficient estimation in which Fast Fourier Transform (FFT) algorithm was used, interpolation and peak detection for the estimated correlation coefficients, and finally, the velocity field estimation. In order to validate the CWT-based technique, a turbulent buoyant jet, which has a similar flow-type of oil jet was experimentally simulated. Then, the CWT-based technique was applied to measure the jet flow, and the outcomes of the technique was compared to the experimental results. As a result, utilizing a smaller number of wavelet scales lead in better flow measurement as compared to the use of larger scales. CWT-based technique was accurately estimated the jet flow rate with standard error of 0.15 m/s, and outperformed the classical algorithms, including FFT, and DCC algorithms, which were measured with error of 3.65 m/s and 4.53 m/s respectively.
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Numerical Investigation of Fluid Flow Mechanism in the Back Shroud Cavity of a Centrifugal Pump
Pages : 709-719
Authors : W. Dong,  W. L. Chu, 
A detailed analysis on the fluid flow distribution in the back shroud cavity is significant for accurately calculat-ing axial forces in the operation of centrifugal pumps. The numerical calculation results and the experimental results were basically consistent on the performance of the centrifugal pump and the fluid flow characteristics in the back shroud cavity. Distribution of velocity field was researched in the back shroud cavity. We plot the axial distribution curves of the dimensionless circumferential and radial components of velocity of the fluid inside the cavity with different angles and radii. We then analyze the fluid pressure distribution in the back shroud cavity and compare it with experimental results. Results show that the fluid flow in the back shroud cavity involves the core area and the fluid leakage. Results also show that the fluid in the core area behaves like a revolving rigid body. At the operating points of the same flow rate, the cross-sectional area of the volute directly affects the flow rate of the fluid in the back shroud cavity, significantly restricting the fluid flow in that component. How-ever, the flow pattern in the turbulent boundary layer is strongly affected by the leakage flow; hence, the distri-bution of velocity is not axially symmetric. When the flow rate increases from 0.8 Qsp to 1.2 Qsp, the radial dif-ferential pressure between the sealing ring and the volute decreases. Meanwhile, the disc friction loss of the impeller-to-wall inside the back shroud cavity tends to be more circumferentially or radially equal, whereas the radial leakage rate in the back shroud cavity tends to decrease. The fluid flow in the back shroud cavity compris-es the circumferential shear flow and radial differential pressure flow and is considered as a 2D viscous laminar flow.
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Experimental Investigation and Numerical Simulation of Morphological Changes in Natural Channel Bend
Pages : 721-731
Authors : S. K. Lee,  T. A. Dang, 
Sediment transport is an important process in maintaining balance of the form of river. The transport of bed load particles affects the processes of aggradation and degradation of the riverbed significantly. To predict the evolution process of the river morphology, the numerical model is considered as a useful tool. This study developed a two-dimensional (2D) depth-averaged model for the morphological change in the river bend. The flow module is represented by the shallow water equations, and the river morphological changes are represented by the sediment continuity equation. The sediment transport module treats bed load as mixtures of multiple grain-size sediments. A finite difference method was applied to solving the governing equations. The developed model was applied to predict bed-load transport rate on one set of the laboratory experiment. A field study was further applied to demonstrate the capability of the developed model in predicting morphological change in the curved river section in South Korea. The simulation results of the developed model were in good agreement with field data both laboratory experiment and natural channel bend.
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Effect of Boost Pressure on the In-Cylinder Tumble-Motion of GDI Engine under Steady-State Conditions using Stereoscopic-PIV
Pages : 733-742
Authors : Mohammed El Adawy,  M. R. Heikal,  A. Rashid A. Aziz,  S. Munir,  M. I. Siddiqui, 
This paper experimentally investigates the effect of boost pressure on the in-cylinder flow field under steady-state conditions using stereoscopic particle image velocimetry (Stereo-PIV) through increasing the pressure difference across the intake valves. The FEV steady-state flow bench was modified to provide an optical access into the cylinder region. The stereoscopic PIV measurements were carried out at various pressure differences viz., 300, 450, and 600 mmH2O across the intake valves of Gasoline Direct Injection (GDI) head for the mid cylinder vertical tumble-plane. Ensemble average velocity vectors were used to characterize the tumble flow structure and for the calculation of tumble ratio and average turbulent kinetic energy. Moreover, the Proper Orthogonal Decomposition (POD) technique was conducted on the PIV measured velocity vector maps to identify the most energetic structures generated at different valve lifts and pressure differences. The results of stereoscopic PIV measurements showed that the overall in-cylinder flow structures were mainly dependent on the valve lift irrespective of the applied pressure difference. However, the level of the turbulence kinetic energy increased as the boost pressure increased.
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The Impact of Air Fences Geometry on Air Flow around an ICE3 High Speed Train on a Double Line Railway Track with Exposure to Crosswinds
Pages : 743-754
Authors : M. Mohebbi,  M. A. Rezvani, 
Reduction in weight adjoined with the increase in the railway vehicle speed of travel added to the deteriorating effects of the crosswinds on the running behavior of high speed trains. During the past decade, many researchers have concentrated on examining the aerodynamic force and moment coefficients for the trains. Varieties of studies regarding the effects of crosswinds on the trains are accomplished. However, the need to restrain strong winds from disturbing trains running safety is not completed. This research is concerned with finding a proper solution for attenuating the worrying effects of the winds that hit the trains. Installation of air fences on the sides of the railway tracks is investigated. To serve the purpose, a variety of air fences with different heights, with and without the edges on top of the fences, at a variety of the edge lengths and angles are studied. The study covers double routed railway track while the air fences are installed on either side of the track. The train can be on the leeward or windward line. The problem solving is based on the Lattice-Boltzmann method. This research pioneers in using this method for the said purpose. It is found that by inserting the fences and increasing their heights for up to 1m, the drag forces decrease to 40 percent and the rolling moment coefficients decrease to 15 percent. The presence of the edge can also decrease the drag force for about 55 to 120 percent and decrease the rolling moment coefficients for about 30 to 115 percent in some cases. Variations in percentages of reduction are due to the different angles and the lengths of the edges.
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Flow Simulation of Various Riblet Shapes and a Uniform Model for Boundary Layer
Pages : 755-764
Authors : Y. Yu,  X. Ren,  X. Li,  C. Gu, 
Various riblet shapes are simulated through the computational fluid dynamics method for the elucidation of riblet effects on turbulent boundary layers and skin friction reduction. For the different shapes, seven typical riblet models are investigated by using renormalization group k-epsilon turbulence models. Simulation results are consistent with the existing theoretical data regarding flat plate and experimental results obtained from the riblet shapes. The riblet velocity profiles cannot satisfy the existing von Kármán’s constants in the logarithmic law boundary layer. The slope and intercept of the logarithmic law are strongly affected by geometric parameters and riblet shapes, and the effect of geometric parameters can be modeled. Meanwhile, the effects of riblet shapes can be modeled with a shape factor composed of a nondimensional cavity ratio and nondimensional top flatness. Therefore, a uniform model of the boundary layer can be obtained to illustrate the effects of various riblet shapes.
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Electrothermal Convection in Dielectric Maxwellian Nanofluid Layer
Pages : 765-777
Authors : V. Sharma,  A. Chowdhary,  U. Gupta, 
The influence of rheological behavior on the natural convection in a dielectric nanofluid with vertical AC electric field is investigated. The rheology of the nanofluid is described by Maxwell model for calculating the shear stresses from the velocity gradients. The employed model introduces the combined effects of movement of the molecules of the fluid striking the nanoparticles, thermophoresis and electrophoresis due to embedded nanoparticles. The exact solutions of the eigen model value problem for stress-free bounding surfaces are obtained analytically using one term Galerkin method to find the thermal Rayleigh number for onset of both non-oscillatory (stationary) and oscillatory motions. It is found that the oscillatory modes are possible for both bottom and top-heavy distributions of nanoparticles. It is observed that the value of critical Rayleigh number is decreased by a substantial amount with the increase in electric field intensity, whereas role of viscoelasticity (time relaxation parameter) is to hasten the occurence of oscillatory modes appreciably. The thermal Prandtl number is found to delay the occurence of oscillatory modes. These results are also shown graphically.
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Rollover Mechanism Methodology of LNG Tank with Gas-Liquid Stratification Based on Curvelet Finite Element Method and Large Eddy Simulation Technology
Pages : 779-786
Authors : B. Zhao,  S. Han,  L. Xu,  C. Shi,  D. Gao,  Y. D. Zhang, 
The rollover phenomenon of LNG tank has great threat to secure storage of LNG, therefore the Curvelet finite element method combing the large eddy simulation technology to analyze the rollover mechanism. Firstly, the basic principles of Curvelet transform are studied, the mathematical model and properties of Curvelet transform have been discussed. Secondly, theoretical models of rollover of LNG tank with gas-liquid stratification based on large eddy simulation are put forward, and then rollover Curvelet finite element of LNG tank with gas and liquid phase stratification is established. Finally, the rollover mechanism analysis of LNG tank with gas and liquid stratification is carried out based on different simulation method. The changing rules of mean velocity of interface between gas and liquid phase LNG gas phase and liquid phase LNG is obtained, and the changing rules mean velocities of gas and liquid phase LNG for different heat flux density are also obtained. Comparing results show that the Curvelet finite element method has higher computing precision than traditional finite element method.
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Simulating Fluid and Structure Interaction using Exponential Basis Functions
Pages : 787-799
Authors : H. Pary Abarghooei,  B. Boroomand, 
In this paper a meshless method using exponential basis functions is developed for fluid-structure interaction in liquid tanks undergoing non-linear sloshing. The formulation in the fluid part is based on the use of Navier-Stokes equations, presented in Lagrangian description as Laplacian of the pressure, for inviscid incompressible fluids. The use of exponential basis functions satisfying the Laplace equation leads to a strong form of volume preservation which has a direct effect on the accuracy of the pressure field. In a boundary node style, the bases are used to incrementally solve the fluid part in space and time. The elastic structure is discretized by the finite elements and analyzed by the Newmark method. The direct use of the pressure, as the potential of the acceleration, helps to find the loads acting on the structure in a straight-forward manner. The interaction equations are derived and used in the analysis of a tank with elastic walls. The overall formulation may be implemented simply. To demonstrate the efficiency of the solution, the obtained results are compared with those obtained from a finite elements solution using Lagrangian description. The results show that while the wave height and the oscillations of elastic walls of the two analyses are in good agreement with each other; the use of the proposed meshless analysis not only leads to accurate hydrodynamic pressure but also reduces the computational time to one-eighth of the time needed for the finite elements analysis.
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Lattice Boltzmann Numerical Investigation of Inner Cylindrical Pin-fins Configuration on Nanofluid Natural Convective Heat Transfer in Porous Enclosure
Pages : 801-816
Authors : Y. Jafari,  M. Taeibi-Rahni,  M. Haghshenas,  P. Ramian, 
Concerning the geometrical effect of inner cylindrical hot pins, the natural convective heat transfer of nanofluid in a homogenous porous medium in a squared enclosure is numerically studied, using lattice Boltzmann method (LBM). In order to investigate the arrangement of inner cylinders for better heat transfer performance, five different configurations (including one, three, and four pins) were compared, while the total heat transfer area of inner pins were held fixed. Squared cavity walls and inner cylinder’s surfaces were constantly held at cold and warm temperatures, respectively. In our simulation, Brinkman and Forchheimer-extended Darcy models were utilized for isothermal incompressible flow in porous media. The flow and temperature fields were simulated using coupled flow and temperature distribution functions. The effect of porous media was added as a source term in flow distribution functions. The results were validated using previous creditable data, showing relatively good agreements. After brief study of copper nano-particles volume fraction effects, five cases of interest were compared for different values of porosity and Rayleigh number by means of averaged Nusselt number of hot and cold walls; and also local Nusselt number of enclosure walls. Comparison of different cases shows the geometrical dependence of overall heat transfer performance via the average Nusselt number of hot pins strongly depending on their position. The four pin case with diamond arrangement shows the best performance in the light of enclosure walls’ average Nusselt number (heat transfer to cold walls). However, the case with three pins and downward triangular arrangement surprisingly gives promising heat transfer performance. In addition, the results show that natural convective heat transfer and flow field is intensified with increasing Rayleigh number, Darcy number, and porosity.
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