

مجله مكانيك سيالات كاربردي
Journal Of Applied Fluid Mechanics
ISSN 17353572
دوماهنامه داراي رتبه علمي  پژوهشي (علوم پايه) به زبان انگليسي
سال يازدهم، شماره 4، 2018




 Nozzle Displacement Effects on TwoPhase Ejector Performance: An Experimental Study K. Ameur * , Z. Aidoun Pages 817823 Abstract Full Text [PDF 388KB]   Experimental results of twophase ejector operation with refrigerant R134a as working fluid are presented in this paper. The tests carried out allowed evaluating the influence of the primary nozzle position in the mixing chamber and of operating conditions such as the thermodynamic state of the fluid at the inlet and outlet of the ejector. Various positions of the primary nozzle were tested and the operating conditions ranges were: primary inlet pressure 8.814.9 bars، subcooling 0.25 °C and ejector outlet pressure 3.74.7 bars. The tests have shown an optimal position of the primary nozzle (NXP=38.1 mm) in the ejector but this position was not very sensitive to operational conditions. The performance of the ejector dropped sharply when the nozzle was placed right at the inlet of the constantarea section in the mixing chamber. Pressures at the primary inlet and outlet had a limited impact on the entrainment ratio (<10%)، but it was found that the level of subcooling at the inlet of the primary flow had an important influence on the entrainment ratio with a variations of about 66%. Pressure monitoring inside the ejector showed a strong relation between NXP and pressure variations in the mixing section and the diffuser.
Keywords: Experiments; Twophase; Ejector; R134a; NXP
  
 Modeling and Simulation of an OxygenBlown Bubbling Fluidized Bed Gasifier using the Computational ParticleFluid Dynamics (CPFD) Approach A. Di Nardo * , G. Calchetti , S. Stendardo Pages 825834 Abstract Full Text [PDF 855KB]   Fluidized beds are conventional components of many industrial processes، such as coal gasification for energy generation and syngas production. Numerical simulations help to properly design and understand the complex multiphase flows occurring in these reactors. Two modeling approaches are usually adopted to simulate multiphase flows: the two fluids EulerianEulerian model and the continuous/discrete EulerianLagrangian model. Since fluidized beds account for an extremely large number of particles، tracking each of them could not assure to get results within a reasonable computational time. The Computational ParticleFluid Dynamics (CPFD) approach، which belongs to the EulerianLagrangian models class، groups together particles with similar key parameters (e.g. composition، size) into computational units (parcels). Parcel collisions are modeled by an isotropic solid stress function، depending on solid volume fraction. In this paper، the bubbling fluidized bed (BFB) upstream gasifier of the EU research infrastructure ZECOMIX (Zero Emissions of Carbon with Mixed technologies) has been simulated using a CPFD approach via Barracuda® software. The effect of different fluidizing agent injection strategies on bed bubbling and mixing، for nonreacting cases، has been studied. The numerical results for a reacting case have been compared to the available experimental data، gathered during the coal gasification campaign. The model has proved to be very useful in the choice of the more efficient injection configuration that assures a more effective contact of the gas with the solid bed and a good bubbling fluidization regime، together with a satisfactory prediction of the outlet gas composition. The numerical approach has turned out to be robust and timesaving and allowed to dramatically reduce the computational cost with respect the classical two fluids EulerianEulerian models.
Keywords: Fluidized bed gasifier; CPFD method; Multiphase flows
  
 2D CFD Computations of the TwoBladed DarrieusType Wind Turbine K. Rogowski * , M. O. L. Hansen , P. Lichota Pages 835845 Abstract Full Text [PDF 4102KB]   In spite of the attractiveness of CFD methods and advanced measurement methods، there is still no full analysis of aerodynamic blade loads for vertical axis Darrieustype wind turbines. Due to an inherently unsteady flow around the rotor blades، bladewakeblade interaction and the occurrence of dynamic stall، the aerodynamics of this type of wind turbine is very complex. A twobladed rotor have been investigated numerically for the tip speed ratio of 5.0. This paper compares results for aerodynamic blade loads obtained applying such turbulence models as: the standard kε; the RNG kε; the Realizable kε and the SST kω. As a result، quantitative instantaneous blade forces as well as instantaneous wake profiles behind the rotor have been obtained. Aerodynamic wake behind the rotor is also visualized by using streak lines. All CFD results are compared with experimental data taken from literature. Good agreement between the numerical results and the experiment is shown for the aerodynamic blade loads as well as for aerodynamic wake behind the rotor.
Keywords: Wind turbine; Aerodynamic blade loads; Aerodynamic wake; Streak line
  
 Numerical Investigations on Unsteady Flow past Two Identical Inline Square Cylinders Oscillating Transversely with Phase Difference M. G. Mithun , P. Kumar , S. Tiwari * Pages 847859 Abstract Full Text [PDF 1122KB]   Twodimensional numerical investigations have been carried out to study the temporal wake characteristics of laminar flow past two identical inline square cylinders performing transverse oscillations. Both the cylinders are forced to perform harmonic oscillations of same frequency and amplitude but with a phase difference. Computations are carried out using commercial software ANSYS Fluent 16.1 on a dynamically sliding mesh for fixed Reynolds number equal to 100. The oscillation frequency is varied from 0.4 to 1.6 times the frequency of vortex shedding behind a single stationary square cylinder. The amplitude of transverse oscillation is kept fixed equal to 0.4D (D = side of the cylinder). In addition، the effect of variation of intercylinder spacing has been investigated on wake interference which influences the modes of vortex shedding and resulting dynamic effects on the cylinders. Temporal signals as well as mean characteristics of lift and drag coefficients have been presented for different values of intercylinder spacing، phase difference between the two cylinders and frequency of oscillation.
Keywords: Transversely oscillating square cylinders; Phase difference; Wake characteristics; Wake interference; Modes of vortex shedding
  
 Effects of Gas Crossover through the Membrane on Water Management in the Cathode and Anode Sides of PEM Fuel Cell K. Mohammadzadeh , H. Khaleghi * , H. R. Khadem Abolfazli , M. Seddiq Pages 861875 Abstract Full Text [PDF 666KB]   Water management in a proton exchange membrane fuel cell (PEMFC) is numerically modeled by considering the 2D، nonisothermal steady flow assumptions. Governing equations are solved in all cell layers including cathode and anode electrodes by finite volume method using a singleregion approach. The effect of gas crossover through the membrane is studied on cell performance. This consideration، not only improves the general accuracy of modeling but also makes it possible to model energy losses due to direct reaction of reactant gases. The effect of some key variables such as liquid water diffusivity، current density، membrane thickness، etc. on PEMFC conditions such as the amount of saturated liquid water، power density، cell temperature، crossover efficiency and so on are examined. It was observed that the amount of saturated liquid water on the anode side is considerably important. This observation addresses needs for further investigation of liquid water behavior in the anode electrode. The amount of liquid water saturation in both the cathode and anode electrodes is increased with increasing the current density. The results showed that at the current density of 0.2 A/cm2، crossover effect causes about 10% reduction in cell efficiency and by decreasing the current density this effect is enhanced.
Keywords: Numerical investigation; PEMFC; Water Management at the anode; Gas crossover through the membrane
  
 Large Scale Fluctuations in an Axisymmetric Sudden Pipe Expansion with Large Aspect Ratio A. Ringleb * , W. Schlüter , O. Sommer , G. Wozniak Pages 877883 Abstract Full Text [PDF 488KB]   The aim of the present work is the investigation of the turbulent flow field downstream of an axisymmetric sudden expansion with a large aspect ratio of D/d = 12،3. For the fundamentally understanding of the flow some numerical results are presented. They were achieved by using the RANS approach and SST turbulence model. The flow field is characterized by a jetlike flow near the nozzle exit and a large toroidal recirculation zone. The xcomponent of the velocity u was measured using onecomponent laser Doppler velocimetry. Axial and radial velocity distributions as well as some velocity spectra were measured. The spectra were calculated from the velocity signal using the SampleAndHold method together with the refinement technique. At the axial half length of the recirculation zone at the edge of the jet flow a narrow band peak was observed in spectra، suggesting the existence of largescale fluctuations or instability of the flow field. Further investigations reveal that this effect is locally limited and shows no sensitivity against changes of the inlet conditions، e.g. the Reynolds number and velocity profile.
Keywords: Axisymmetric sudden pipe expansion; Laser Doppler velocimetry; SampleandHold; Incompressible turbulent flow; Large scale fluctuations; Narrowband peak spectra
  
 CFD Investigation on the Steady Interaction between an Offset Jet and an Oblique Wall Jet N. Hnaien , S. Marzouk , L. Kolsi * , A. A. A. A. AlRashed , H. Ben Aissia , J. Jay Pages 885894 Abstract Full Text [PDF 1205KB]   In this paper a CFD investigation on the interaction between an offset jet and an oblique wall jet using twodimensional steady RANS equations is performed. This combination is denoted WOJ (Wall Offset jets). Several turbulence models such as the standard kω، SST kω، standard kε، RNG kε and realizable kε models are tested in the present study. A parametric study is performed to highlight the wall inclination effect on the WOJ flow maximum velocity decay as well as the shear layers spreading. Comparison between combined wall and offset jet (WOJ) and single offset jet (SOJ) flows is also established. Results show that increasing the wall inclination improves the combined wall and offset jets flow spreading. Furthermore، the outer shear layers spreading، is better than the inner shear layers one. Comparing to the combined wall and offset jet flow (WOJ)، a better spreading is found in the case of single offset jet flow (SOJ).
Keywords: Combined jets; Inclination; Maximum velocity; Shear layer; Steady RANS; Turbulent flow
  
 A BioThermal Convection in WaterBased Nanofluid Containing Gyrotactic Microorganisms: Effect of Vertical Throughflow S. Saini * , Y. D. Sharma Pages 895903 Abstract Full Text [PDF 253KB]   The effect of vertical throughflow on the onset of biothermal convection in a waterbased nanofluid containing gyrotactic microorganisms is investigated using more realistic boundary conditions. The Galerkin weighted residual method is used to obtain numerical solutions of the mathematical model. The effects of bioconvection Rayleigh number، gyrotaxis number، bioconvection Péclet number، Lewis number، Péclet number، particle density increment number، modified diffusitivity ratio، and nanoparticle Rayleigh number on thermal Rayleigh number are examined.The combined effect of Brownian motion and thermophoresis of nanoparticles، vertical throughflow، and gyrotactic microorganisms on the thermal Rayleigh number is found to be destabilizing and its value is decreased by first to third orders of magnitude as compared to regular fluids. Critical wave number is dependent on bioconvection parameters، nanofluid parameters as well as throughflow parameter. The results obtained using passive boundary conditions are compared with those of active boundary conditions. The present study may find applications in seawater convection at the ocean crust.
Keywords: Nanofluid; Vertical throughflow; Thermophoresis; Brownian motion; BioThermal convection; Gyrotactic microorganism
  
 Statistical Analysis of Wedge Effect on the Seakeeping of a Planing Hull in Irregular Waves at the Onset of the Planing Region P. Ghadimi * , S.M. Sajedi , P. Taghikhani Pages 905918 Abstract Full Text [PDF 640KB]   In the current paper، different experiments are conducted on a high speed planing craft in irregular waves، with and without a wedge. Performance and seakeeping aspects of these planing hulls in the form of trim، riseup، and resistance in regular waves and heave، pitch، bow، and center of gravity (CG) acceleration in irregular waves are extracted in time series. Irregular waves represent sea state 3 with 12cm height and peak period of 1.66. A model length of 2.63m and 1:5 scale is considered and all data for irregular waves are scaled، as well. The deadrise angle is constant and is taken to be 24 degrees. The targeted experimental tests are conducted for four longitudinal Froude numbers of 1.0، 1.18، 1.37، and 1.57، which are all in the planing regime. The results are analyzed for the mean height of wave، significant wave height، RMS، and spectrum. The comprehensive study of wedges' effects is also presented which indicates that a wedge can decrease the motions and accelerations، exceedingly. Ultimately، the obtained results are compared against those by Fridsma (1971) and Soletic (2010) and it is demonstrated that motions and accelerations are indeed reduced.
Keywords: Planing hull; Experimental seakeeping tests; Vertical accelerations; Statistical analysis; Irregular head sea; Wedge
  
 Performance Enhancement of Centrifugal Compressor with Addition of Splitter Blade Close to Pressure Surface A. Malik * , Q. Zheng , A. A. Zaidi , H. Fawzy Pages 919928 Abstract Full Text [PDF 1806KB]   This paper exemplified a way to increase pressure ratio and improve efficiency with addition of multi splitters in centrifugal impeller with a vaneless diffuser. DDA 404III back swept impeller of centrifugal compressor، studied through experiment is modified with the addition of splitters blades and a sample impeller is designed and analyzed with big splitter close to pressure surface and small splitter close to suction surface. Keeping all flow conditions and impeller definitions، same as experimentally validated impeller، in order to investigate effects of the location of the splitters between two main blades. It was observed that total pressure ratio is increased from 4.1 to 4.5 with 2 % increase in efficiency with big splitter close to pressure surface of main blade and small splitter close to suction surface of main blade. It was observed that relative Mach number reduces at inlet of modified impeller.
Keywords: Numerical simulation; Multi splitter blades; DDA 404III Impeller; Centrifugal Compressor
  
 MultiObjective Optimization of TwoStage Centrifugal Pump using NSGAII Algorithm M. Benturki * , R. Dizene , A. Ghenaiet Pages 929943 Abstract Full Text [PDF 1487KB]   Improving the efficiency and suction capability of a multistage centrifugal pump poses a major challenge for the designer of this type of equipment. This paper deals with the optimization of a two stage centrifugal pump using Nondominated Sorting Genetic Algorithm II (NSGAII)، coupled with threedimensional Reynoldsaveraged NavierStokes (3DRANS) flow solver. The first stage comprises a suction impeller with a diffuser while the second stage is formed by a second impeller connected to a volute. Both impellers are of different dimensions and are interconnected by a return channel. This arrangement increases the number of varying parameters and thus can add further constraints on the overall optimization process; as a result، a high computational complexity of NSGAII and a higher computational fluid dynamics (CFD) simulation cost is incurred. In order to save running time، optimization with CFD simulations are performed on each stage separately shall enable to obtain better parameterization flexibility; therefore، permitting to adopt only three objective functions in as well as limiting other geometrical constraints. The objectives of this study are to maximize the head and hydraulic efficiency at a time where the net positive suction head inception (NPSHi) is kept to minimum. The overall efficiency as well as the head of the optimized pump were increased by 9.8% and 15.7%، respectively، at best efficiency point (BEP) (rotational speed N=2600 rpm); the NPSHi of suction impeller was reduced by 13.6%. At N=1450 rpm (BEP)، an improvement of 14.9% in the head and 6.52% for the overall efficiency is observed. An important improvement in performance at different operating flow rates was obtained; this was in addition to other enhancements in the volumetric and hydraulic efficiencies. Unsteady CFD simulations were also performed to predict fluctuations in the pressure field، leakage flows and interactions between impellers and collectors. The obtained results were in agreement with experimental data. The head fluctuation of the optimized pump was also reduced by 22.5% in amplitude; this was favored by the presence of a tapered blade towards the trailing edge and the extended radial gap by 4.86% between the second impeller and cutwater، which was caused by the reduction of the impeller diameter.
Keywords: Multiobjective optimization; Twostage centrifugal pump; Unsteady; CFD; NSGAII; NPSHi
  
 Influence of Stroke on Performance Characteristics of Synthetic Jet Fan K. Nishibe * , Y. Nomura , K. Noda , H. Ohue , K. Sato Pages 945956 Abstract Full Text [PDF 1322KB]   Synthetic jets، whose size and weight can be reduced easily، have become an attractive alternative to continuous jets. Many experimental and numerical studies have been conducted on synthetic jets to investigate their fundamental flow characteristics، including jet structure، for applied research such as boundary layer control and enhanced fluid mixing. However، few studies have focused on fluid transportation devices using synthetic jets as a driving source. Therefore، several issues concerning fluid transport characteristics still need to be resolved. In addition، although optimum operation of devices using synthetic jets is essential for their practical use، few studies have focused on this issue. The present study experimentally demonstrates the influence of the dimensionless stoke L on the performance characteristics of a synthetic jet fan under Reynolds number Re = 1800 and the same fan geometry; here، the stroke l is nondimensionalized by the primary slot width b. Furthermore، numerical simulations are conducted to complement the experiment. Velocity and pressure measurements are performed using a hotwire anemometer، differential pressure manometer، and pressure transducer. The influence of the dimensionless stroke L on the performance/efficiency curves، static pressure distribution on the duct surface، and unsteady flow characteristics are investigated. Moreover، the flow field inside the duct is observed through numerical simulation. The results show that the performance characteristics and pressure recovery process depend on the dimensionless stroke L، and an optimum range of dimensionless stroke L exists for operation.
Keywords: Synthetic jets; Jet pump; Fan performance curve; Pressure recovery
  
 Preliminary Experimental Investigation on a Low Profile MagnetoHydrodynamic Propulsive Blanket, Considering Plasma Generation M. A. Feizi Chekab , P. Ghadimi * , M. Sheikholeslami , A. Ghadimi Pages 957963 Abstract Full Text [PDF 325KB]   The use of magnetohydrodynamic (MHD) blanket propulsion system in ships، even with low efficiencies، has particular benefits that can make them an appropriate option for the marine designers. Accordingly، any attempt to increase the efficiency of these systems requires full recognition of their performance in different conditions. In the present study، as a continuation of previous numerical works by the current authors، a magnetohydrodynamic blanket propulsion system has been built and experimentally studied through examining the MHD forces produced in different voltages. Copper and gold have been used and compared as electrodes and the high advantage of gold has been demonstrated. The effect of electrolysis on the behavior of the blanket is analyzed. It has been demonstrated that although electrolysis restricts high currents in lower voltages (lower than ~140V) and the saturation of hydrogen decreases the MHD forces due to low electrical current (~140V up to ~160V)، the saturation of hydrogen around cathode at high voltages (more than ~160V)، makes a dielectric barrier which soon breaks down and make the production of plasma possible، which in turn highly increases the thrust force of the MHD blanket. Therefore، three regimes have been introduced and described for the MHD blanket; the electrolysis regime، the transition regime، and the hot plasma regime. Based on the obtained results، one may conclude that the present results have offered good evidence about the possibility of increasing the MHD blanket performance through plasma production in water.
Keywords: MHD propulsive blanket; Electrolysis; Plasma; Experimental investigation
  
 A Simple Method for the Estimation of the Axial Dispersion Coefficient in Gas Flow M. Wojcik * , M. Szukiewicz , W. Prochniak , P. Wiercioch Pages 965970 Abstract Full Text [PDF 224KB]   A simple method which is suitable for determining with reasonable precision the parameters of gas flow system has been proposed. An inverse boundaryvalue problem is considered. The model of gas flow with the Danckwert’s boundary conditions in a real measurement system has been analyzed and solved. The tracer technique was applied to determine axial dispersion coefficient of gas phase and Pèclet number. These parameters are commonly used to characterize the flow behavior of fluids. Axial dispersion coefficients were estimated by comparing model solution with recorded TCD signal (an inverse problem as a method for model parameter estimation) employing the Laplace transform technique. The GaverStehfest algorithm for the solution of the mathematical model has been applied. The proposed model of gas show a good agreement with the experimental data. The obtained results show that under operation conditions in the studied system the flow behaviour is neither plug flow nor perfect mixing. The described method is very fast in both experimental and computational part. Simple and errorless derivation of sophisticated model formulas has been possible by application of the Computer Algebra Systemtype program. The program also simplifies computations. Mathematical manipulations and computations were performed using program Maple®.
Keywords: Laplace transform; Numerical inversion of Laplace transform; Nonideal flow; Maple®
  
 On the Determination of the Yield Surface within the Flow of Yield Stress Fluids using Computational Fluid Dynamics N. Schaer * , J. Vazquez , M. Dufresne , G. Isenmann , J. Wertel Pages 971982 Abstract Full Text [PDF 466KB]   A part of nonNewtonian fluids are yield stress fluids. They require a minimum stress to flow. Below this minimum value، yield stress fluids remain solid. To date، 1D and 2D numerical models have been used predominantly to study free surface flows. However، some phenomena have threedimensional behaviour such as the appearance of the limit between the liquid regime and the solid regime. Here the aim is to use a Computational Fluid Dynamics (CFD) to reproduce the properties of the free surface flow of yield stress fluids in an open channel. Modelling the behaviour of the yield stress fluid is also expected. The numerical study is driven with the software OpenFOAM. Numerical outcomes are compared with experimental results from model experiment and theorical predictions based on the rheological constitutive law. The 3D model is validated by evaluating its capacity to reproduce reliably flow patterns. The depth، the local velocity and the stress are quantified for different numerical configurations (grid level، rheological parameters). Then numerical results are used to detect the presence of rigid and sheared zones within the flow.
Keywords: CFD; Yield stress fluids; Free surface flow; Yield surface; Regularized model
  
 Numerical Study on the Effect of an Annulus Injector on the Hydrodynamic Behavior of a Spray R. Esmaelnajad , M. T. ShervaniTabar * , M. Jafari , S. E. Razavi Pages 983994 Abstract Full Text [PDF 925KB]   In the internal combustion engines geometry of the injector orifice has significant effect on the improving of the fuel spray characteristics. In the present paper، effect of a conical annulus injector with three different aspect ratios and three different divergence angles of the annulus orifice on the hydrodynamic behavior of a fuel spray have been investigated numerically. The conical annulus injector aspect ratio is the ratio of the height of the annulus cone to the diameter of its circular base. The geometry of the annulus conical injectors inspires this idea that this type of injectors could inject possible large amount of liquid fuel into a combustion chamber symmetrically and homogeneously. The CFD software AVL Fire has been employed for numerical simulation of diesel fuel spray evolution. Numerical results show that the annulus conical injectors inject liquid fuel with an approximately homogenous distribution of droplets in the combustion chamber in comparison with the conventional injectors. In this kind of injector، fuel has been uniformly distributed in the cylinder. Numerical results also show that the annulus injectors significantly increase the cone angle of the liquid fuel spray and decrease its penetration length.
Keywords: Annulus conical injector; CFD; AVL Fire; Spray; Homogenous distribution of droplets
  
 Drag Experienced by a Composite Sphere in an Axisymmetric Creeping Flow of Micropolar Fluid V. Mishra , B. R. Gupta * Pages 9951004 Abstract Full Text [PDF 277KB]   This paper concerns an analytical study of a steady axisymmetric uniform flow of an incompressible micropolar fluid past a permeable sphere that contains a solid sphere. The mathematical expression for the flow fields are obtained in terms of stream function by using modified Bessel’s function and Gegenbauer function. Noslip condition، zero microrotation components، continuity of normal velocity which is equal to the filtration velocity on the surface of the sphere are used as boundary conditions. It is assumed that the fluid obeys Darcy law at the permeable surface. The internal and external drag force exerted by the fluid on the sphere، flow rate and the relevant quantities such as pressures، microrotation vectors have been calculated. It is observed that drag is greater for impermeable sphere as compared to permeable sphere. As permeability parameter increases the flow rate also increases rapidly. Various useful results are obtained and compared with the previous particular cases.
Keywords: Permeable sphere; Micropolar fluid; Drag force; Stream function; Darcy law
  
 Hydrodynamics Analysis on the HighSpeed Surface Vehicle with SuperCavitating Hydrofoils S. Zhao , M. Xiang* , H. Zhou , W. Zhang Pages 10051012 Abstract Full Text [PDF 932KB]   Expeditiously transferring personnel or cargo between seashores or vessels becomes an imperative requirement in ocean engineering. In this paper a novel highspeed surface vessel which has two symmetrical underwater torpedoshaped subwater bodies connected to the hull with two couples of supercavitating hydrofoils، which are located in series along the axis of the body، has been proposed. By using supercavitation technology in the subwater body and the hydrofoil، this vessel could achieve extreme high speed. Considering the sophisticated conﬁguration and the complex ﬂow ﬁeld around the vessel، this paper has investigated on the hydrodynamics of this vehicle through numerical simulation. The numerical method which couples the Schnerr and Sauer cavitation model into the mixture multiphase model has been validated by the case of twodimensional supercavitating hydrofoil. Then simulation has been carried out for this novel vehicle with different wetting depths. Based on analysing details of the ﬂow structure، the theredimensional effect for the supercavitating hydrofoil، as well as the interaction between the fore and the aft hydrofoils has been revealed. Then the hydrodynamics curves for both the fore and the aft hydrofoils are obtained، providing guidance for the design of the serial hydrofoils. Furthermore، hydrodynamic analysis has been made for the subwater body under the effect of hydrofoils. This work may give meaningful references for the design of highspeed surface vehicles.
Keywords: Numerical simulation; Hydrofoil; Supercavitation; Hydrodynamics
  
 Computational and Experimental Study on the WaterJet Pump Performance A. A. A. Sheha * , M. Nasr , M. A. Hosien , E. Wahba Pages 10131020 Abstract Full Text [PDF 591KB]   The effect of operational and geometrical parameters on the jet pump efficiency were determined experimentally and numerically. Numerical investigation was held firstly to determine the effect of diffuser angle، mixing chamber length، pump area ratio and driving nozzle position on the efficiency of jet pump. Commercial computational fluid dynamics (CFD) solver ANSYS FLUENT R 15.0 using SSTturbulence model was used. The numerical results showed that jet pump efficiency increases with decreasing both of diffuser angles and mixing chamber length up to a certain value and then pump efficiency decreases. Also، jet pump efficiency increases with increasing pump area ratio up to a certain value and then pump efficiency decreases. It was found that maximum numerical efficiency is 37.8 % for pump area ratio of 0.271. In addition، the numerical results showed that the optimum relative length of mixing chamber is 5.48 and the optimum value for diffuser angle at which the efficiency is a maximum value is 5º. Experimental tests were conducted to obtain the effects of various operational and geometrical parameters on the performance of the jet pumps. A test rig was constructed using the optimum design from the numerical results. The CFD’s results were found to agree well with actual values obtained from the experimental results.
Keywords: Jet pump; CFD; Pump efficiency; Geometrical parameters; Operational parameters
  
 Transition from Steady to Oscillatory Flow Natural Convection of LowPr Fluids in 3D Bridgman Configuration for Crystal Growth A. Atia , B. Ghernaout , S. Bouabdallah * Pages 10211031 Abstract Full Text [PDF 1034KB]   A numerical study of the transition from steady to oscillatory flow natural convection of low Prandtl number fluids inside the 3D Bridgman configuration has been carried out. The threedimensional NavierStokes and energy equations، with the Boussinesq approximation have been discretized by means of a finite volume procedure which employs a second order accurate central difference scheme to treat diffusive and convective fluxes. In natural convection، the buoyancy force is only driving the flow and its intensity can be move a harmful effect on the crystal growth، such as the striation. Naturally، the steady state flow is obtained for low Rayleigh number and shows a great dependence between the Rayleigh number، the flow structure and the heat transfer rate. A low increase in the Rayleigh number we guide to determine the critical point in which the 3D flow became oscillatory. This regime appears by a sinusoidal signal in the time and developed in each period of time.
Keywords: 3D Natural convection, Steadyoscillatory flow, LowPr fluid, Numerical study
  
 ReducedOrder Modeling of Unsteady Hypersonic Aerodynamics in MultiDimensional Parametric Space Z. Chen , Y. Zhao* , R. Huang Pages 10331045 Abstract Full Text [PDF 927KB]   A novel reduced order model (ROM) for unsteady hypersonic aerodynamics is developed، which is applicable for the variations of multiparameters. The key to the developed ROM lies in the CFDbased model reduction of the steady aerodynamic component، which stems from the quasisteady nature of aerodynamic forces in the hypersonic regime. Concretely، the proper orthogonal decomposition (POD) method، combined with Kriging interpolation، is used to construct the ROM for the steady aerodynamic component; meanwhile the unsteady part is directly obtained from Donov’s thirdorder piston theory. The new procedure is applied to a threedimensional low aspect ratio wing (Lockheed F104 Starfighter wing). It is shown that the developed ROM is able to accurately predict the unsteady hypersonic aerodynamic loads over a wide range of different flight conditions compared with the direct CFD computation.
Keywords: Reduced order model; Proper orthogonal decomposition; Kriging surrogate; Piston theory; Multidimensional parametric space
  
 Wave Motion due to a Ring Source in Two Superposed Fluids Covered by a Thin Elastic Plate N. Islam , R. Gayen * , B. N. Mandal Pages 10471057 Abstract Full Text [PDF 787KB]   The problem of wave generation by a horizontal ring of wave sources of the same timedependent strength present in any one layer of a twolayer fluid is investigated here. The upper fluid is of finite height above the interface and is covered by a floating thin infinite elastic plate (modeling a thin sheet of ice) while the lower fluid extends infinitely downwards. Assuming linear theory، the problem is formulated as an initial value problem and the Laplace transform in time is employed to solve it. For timeharmonic source strength، the asymptotic representations of the potential functions describing the motion in the two layers for large time and distance are derived. In these representations، the two different coefficients for each of the surface and interface wave modes have the same numerical values although it has not been possible to prove their equivalence analytically. This shows that the steadystate analysis of the potential functions produces outgoing progressive waves at the surface and at the interface. The forms of the surface and interface waves are depicted graphically for different values of the flexural rigidity of the elastic plate and the ring source being submerged in the lower or upper layer.
Keywords: Ring source potentials; Twolayer fluid; Thin elastic plate; Steadystate analysis
  
 On the Optimization of the Species Separation in an Inclined DarcyBrinkman Porous Cavity under the Effect of an External Magnetic Field R. Ahmed * , M. Hasnaoui , A. Abdelkhalek Pages 10591071 Abstract Full Text [PDF 727KB]   An investigation is conducted to study analytically and numerically the effect of a magnetic field on the species separation induced by the combined effects of convection and Soret phenomenon in an inclined porous cavity saturated by an electrically conductive binary mixture and provided with four impermeable walls. The long sides of the cavity are subject to uniform heat flux while its short ends are adiabatic. Uniform magnetic field is applied perpendicularly to the heated walls. The mixture satisfies the Boussinesq approximation and the porous medium، modeled according to DarcyBrinkman’s law، is assumed homogeneous and isotropic .The relevant parameters for the problem are the thermal Rayleigh number (RT = 1 to 106)، the Lewis number (Le = 10)، the inclination angle of the cavity (θ = 0º to 180)، the separation parameter (φ = 0.5)، the Darcy number (Da = 105 to 103)، the Hartmann number (Ha = 0 to 100) and the aspect ratio of the cavity (Ar = 12). The limiting cases (Darcy and pure fluid media) are recovered in this study. Optimum conditions leading to maximum separation of species are determined while varying the governing parameters in their respective ranges. Results show that the magnetic field can enhance the species separation in cases where the optimal coupling between thermosolutal diffusion and convection is not achieved in its absence. On the other hand، in cases where this optimal coupling is reached in the absence of the magnetic field، the application of the latter destroys the separation of species.
Keywords: DarcyBrinkman porous medium; Soret effect; Magnetic field; Separation of species; Analytical and numerical study
  
 Systematic Investigation of Thrust Production during Plunging Motion of the Airfoil H. Hamdani * , H. Zareen Pages 10731088 Abstract Full Text [PDF 827KB]   The effect of various conditions on the thrust generation of 2D airfoil in pure plunging motion has been investigated. These conditions include different airfoil shapes، different Reynolds numbers (Re) and reduced frequencies (K). The three different shapes used in this study are the NACA0014، the ellipse، and the flat plate airfoil، whereas، the three Re used in the study are 1000، 10000، and 25000 for the three values of K at 2.0، 1.0، and 0.5. For all these parametric studies، the thickness (t/c ratio) of all the airfoil has been kept as constant at 14% t/c ratio. During sinusoidal plunging motion، CL and CD varies in a sinusoidal manner however CL and CD lags with the airfoil motion and the time averaged lift coefficient over one complete cycle is zero whereas the time averaged drag coefficient is negative and nonzero i.e. thrust is produced. The reason behind the thrust generation is due to the formation of the Reverse Karman Vortex Street in the wake of the airfoil.NACA0014 airfoil produces more negative values of the drag coefficient as compared to the ellipse and flat plate which indicates that the shape effect is important for thrust generation which is due to the pressure changes that occur close to the leading edge of the airfoil and it is more pronounced for an airfoil with large Δy variation near the leading edge ، for instance NACA 0014. As the Re is increased، the time averaged drag coefficient becomes more negative and the thrust produced by the NACA0014 airfoil remains higher as compared to the other two airfoil which shows that the airfoil shape effect is dominant. As K reduces، time averaged drag coefficient (thrust) decreases and the airfoil shape effect becomes less prominent as K is decreased (or the unsteady effect decreases). It is seen that for all the cases، the CDv (drag due to viscous forces) is very small and major contribution of negative drag (thrust) comes from the pressure forces.
Keywords: Thrust; Reverse Vortex shedding; Airfoil, Vorticity; Plunge; Reduced frequency
  
 A Performance Analysis on Pressure Loss and Airflow Diffusion in a Chamber with Perforated VProfile Diffuser Designed for Air Handling Units (AHUs) M. S. Kamer , A. Erdogan , E. Tacgun , K. Sonmez , A. Kaya , I. G. Aksoy * , S. Canbazoglu Pages 10891100 Abstract Full Text [PDF 1444KB]   Outlet crosssectional area of fans used in air handling units is smaller than crosssectional area of chambers which are located next to the fan. In order to ensure efficiently operating of the air handling units، it is required that the air flows through a perforated diffuser to create a uniform air diffusion from fan outlet to following chamber with a minimum pressure loss and uniform velocity distribution. In this concept، numerical simulations and experiments were performed for the chamber with perforated Vprofile diffuser، which is often used in air handling units because of its simple geometry and easy manufacturing. Pressure losses were firstly obtained experimentally for different air velocities in the chamber. Then a performance analysis on the air flow diffusion and pressure losses inside chamber with perforated Vprofile diffuser for different geometric parameters such as entry length، apex angle، geometry and pattern of hole، plate thickness، porosity and surface roughness has been carried out numerically. It is seen that the experimental results validated with the numerical turbulence model results.
Keywords: Computational fluid dynamics (CFD); Pressure loss; Air handling units (AHUs); Perforated diffuser; Fan
  
 Numerical and Experimental Investigations on Aerodynamic Behavior of the Ahmed Body Model with Different Diffuser Angles P. Moghimi , R. Rafee * Pages 11011113 Abstract Full Text [PDF 1192KB]   Due to many restrictions applied by the necessity of fulfilling dimensional analysis in a numericalexperimental research and also the limits in experimental facilities a Low Reynolds Number simulation seems to be widespread. In this paper، effects of the diffuser angle on the aerodynamic behavior of the Ahmed body have been investigated for low Reynolds number flows. Numerical simulations were performed by solving the Reynolds Averaged NavierStokes (RANS) equations combined with different turbulence models. The Finite Volume Method (FVM) is used for simulations in Fluent 6.3.26 Software. The main objectives of the study are to improve the aerodynamic design of the body، analyzing the flow field to understand the nature of these improvements and reaching a suitable and reliable experimentalnumerical setup for such a flow. Finally، it was concluded that the SST kω turbulence model with transitional flow corrections is the best choice. From the flow simulation and obtained experimental data، it was concluded that that drag coefficient is a function of three main phenomena. Results showed that the drag coefficient has its minimum value at a specific diffuser angle (8◦) and further increases in the angle lead to higher drag coefficient. On the other hand، the lift coefficient constantly decreases by increasing the diffuser angle. In order to show the validity of the numerical results، experimental data were obtained by measuring the drag and lift coefficients of scaled standard Ahmed body and a model with the diffuser angle of 8 degrees in a wind tunnel. Results confirmed that improvement of drag and lift coefficients occurs when diffuser region is considered for the Ahmed body. In addition، the flow field around the body was studied in detail to show the effects of the diffuser geometry on the aerodynamic characteristics of the body.
Keywords: Lift Coefficient; Drag coefficient; Diffuser angle; Ahmed body; Low Reynolds Number flow
  
 MicroVibration Analysis and Optimization of Aerostatic Bearing with Pocketed OrificeType Restrictor Y. F. Li , Y. H. Yin* , H. Yang , X. E. Liu , J. Mo , H. L. Cui Pages 11151124 Abstract Full Text [PDF 489KB]   For an aerostatic circular thrust bearing with a single pocketed orificetype restrictor، the flow field in the bearing clearance is analyzed numerically، and the formation mechanism of the bearing microvibration is investigated. Through flow field analysis، the flow structures in the bearing clearance are discussed and classified. The formed vortex flow in flow field is analyzed، and the influence of the vortex flow on bearing dynamic stability related to microvibration is discussed. For each flow structure، the vortex flow always exists and induces the bearing microvibration. The Reynolds number is used to represent the degree of bearing microvibration and the rationality is verified. Based on the flow analysis results، the maximum Reynolds number in the bearing clearance flow field is taken as the optimization objective to reduce the microvibration amplitude، the approximate model for design optimization is established by using the radial basis functions method and the optimization methodology is illustrated. Several cases of optimization are carried out with different given bearing loads. Through optimization، the maximum Reynolds number is reduced greatly، which means the enhancement of the bearing dynamic stability. The optimization results show that in order to suppress the microvibration، the air supply pressure should be kept as small as possible، the small air pocket diameter and orifice diameter are also needed.
Keywords: Gas lubrication; Aerostatic bearing; CFD simulation; Microvibration; Dynamic stability; Design optimization
  
 Comparative Flow Field Analysis of Boundary Layer Diverter Intake and Diverterless Supersonic Intake Configuration I. Arif * , S. Salamat , M. Ahmed , F. Qureshi , S. Shah Pages 11251131 Abstract Full Text [PDF 511KB]   In this paper comparative flow field analysis of two intake configuration i.e. Boundary Layer Diverter Intake and Diverterless Supersonic Intake is carried out based on dimensionless parameters under various flow conditions. Numerical analysis of aircraft intake is a complex phenomenon which involves both external and internal flow analysis. In this research، both external and internal flow characteristics of intake duct are analyzed in detail. A comprehensive mesh scheme is devised and implemented to accurately capture the flow behavior in external surrounding of intake duct and flow passing through the intake duct. The analysis is carried out at different flow conditions to analyze the flow behavior in subsonic and supersonic regimes. Engine design mass flow rate is used for accurate intake analysis and results are validated with available literature. Boundary layer diversion and pressure recovery are examined for each intake configuration and comparative analysis based on pressure recovery is carried out subsequently. The analysis reveals that at subsonic and transonic regimes، Boundary Layer Diverter intake is much more effective than Diverter less Supersonic Intake، however، in supersonic regime Diverter less Supersonic Intake is found be to more effective. The research can further help in modifying/ improving the design of an existing intake configuration for enhanced intake efficiency.
Keywords: Aerodynamics; Boundary layer Diverter intake; Diverterless supersonic intake; Pressure recovery
  
 Numerical Investigations into the Origin of Tip Unsteadiness in a Transonic Compressor G. An , Y. Wu * , J. Lang , Z. Chen , B. Wang Pages 11331141 Abstract Full Text [PDF 2004KB]   Threedimensional numerical simulations are conducted to investigate the origin of flow unsteadiness and its associated unsteady flow phenomena in a transonic compressor rotor. The predicted results are compared with the available experimental data and a good agreement is achieved. The numerical monitoring results and further analyses of the flow field indicate that flow unsteadiness is detected in the passage with the operating condition approaching the stability limit، and the highest oscillating region is at the leading edge of the blade pressure surface; the tip leakage vortex breakdown is not a decisive factor for the flow unsteadiness، and the shock oscillation is a unsteady flow phenomenon resulted from the vibration of the recirculation region; a Utype vortex emerges in the tip leakage vortex breakdown region، and its periodic impingement on the pressure surface of the adjacent blade is treated as a trigger that leads to the flow unsteadiness.
Keywords: Transonic compressor rotor; Flow unsteadiness; Vortex breakdown; Shock wave oscillation
  
 Effect of Fin Geometry on FlowInduced Vibration Response of a Finned Tube in a Tube Bundle H. Arshad * , S. Khushnood , L. Ahmad Nizam , M. Ameer Ahsan , O. Ghufran Bhatti Pages 11431152 Abstract Full Text [PDF 553KB]   An experimental study is carried out on a parallel triangular finned tube array with P/Deff ratio 1.62 to examine the effect of fin geometry on flowinduced vibration response. Fins on a tube increase the heat transfer rate but these also affect the fluid dynamics around the tube. The flow pattern across the finned tubes is complex as compared to bare tube arrays. There are numerous parameters that affect the finned tube vibration subjected to air crossflow in a tube array. In the current study، some of these parameters i.e. fin thickness and fin density are focused and their effects on flowinduced vibration response are analyzed in different rows of fin tube array. The current experimentation is performed in a subsonic wind tunnel using a single flexible Aluminum finned tube in a rigid array. Seven tubes with similar specifications but distinct fin thickness and fin density are used for the testing purpose. Their amplitude response suggests that the flowinduced vibration behavior is greatly affected by changing the finned tube parameters. It has also been observed during spectral analysis that the Strouhal number is independent of fin geometry since it remained constant in different rows of the array for finned tubes under study. It suggests that the vortex shedding has also contributed towards the finned tube vibration predominantly in the first، second and the fourth row of tube array.
Keywords: Finned tube bundle; Flexible tube; Mass ratio; Fin geometry; Parallel triangular
  
تاريخ انتشار: 20/4/97 تلفن: 33915271 (031)
تاريخ درج در سايت: 20/4/97
شمار بازديدکنندگان اين شماره: 147




