

مجله مكانيك سيالات كاربردي
Journal Of Applied Fluid Mechanics
ISSN 17353572
دوماهنامه داراي رتبه علمي  پژوهشي (علوم پايه) به زبان انگليسي
سال دهم، شماره 2، 2017




 Optimization of Swirling Flow Energy Parameters on the Velocity Profile after Local Obstacle by Firefly Algorithm Z. Glavcic , S. Bikić, , R. R. Bulatovic Pages 479489 Abstract Full Text [PDF 562KB]   The paper deals with swirling flow (SF) phenomenon which occurs in fluid flow after local obstacle. The phenomenon of SF has been considered in many studies، but here is some different approach to this problem. Based on the theorem of conservation and transfer of energy، a mathematical model of SF was developed in the form of differential equation which defines the velocity profile. During research، special accent was put on the following parameters: swirling parameter، swirling intensity، swirling flux and swirling coefficient. Firefly Algorithm (FA) optimization model، in conjunction with Simulink، was used to get velocity profile vs. time dependence and to verify the developed model. The diagrams of velocity profile were obtained for variation of the initial boundary values of given parameters and conclusions were derived upon mathematical model and simulation of the process.
Keywords: Circumferential component of velocity; Firefly algorithm; Swirling coefficient; Swirling flow; Swirling flux; Swirnling intensity; Swirling parameter
  
 Flow Characterizations and Drag Reduction on the Hydrophobic Surface with a Full Covering Gas Film P. Du , D. Song , F. Ren , Q. Xue , H. Hu Pages 491498 Abstract Full Text [PDF 361KB]   Characteristics of the flow on hydrophobic surfaces with a full covering gas film are investigated using the singlecomponent multiphase (SCMP) Lattice Boltzmann (LB) simulation. By adopting the ShanDoolen force model and incorporating equations of state (EOS) of real fluids، large density ratios are achieved. The height of the air film maintained is observed to increase with the improvement of the surface hydrophobicity. In the laminar flow، the state of the gasliquid interface is not influenced by the Reynolds number (Re). On the gas layer، the slip effect and wall shear stress reduction are sustained continuously regardless of their positions (on the space or on the microstructure). Even so، decreasing the solid area fraction is also significant to enhance the slip effect and decrease the wall shear stress on hydrophobic surfaces.
Keywords: Hydrophobic surface; Lattice Boltzmann method; Gas film; Slip; Drag reduction
  
 RBFDQ Solution of Natural Convection under the Effect of a Magnetic Field in a Tilted Cavity B. Pekmen Geridonmez Pages 499507 Abstract Full Text [PDF 636KB]   In this study، radial basis function based differential quadrature (RBFDQ) method is applied to the natural convection in an inclined unit square cavity under the effect of an applied magnetic field in different angles. The stream functionvorticity form of the dimensionless governing equations are concentrated on. The change in different Hartmann numbers، Rayleigh numbers and inclination angles of the cavity is investigated both in terms of streamlines، isotherms، vorticity contours and the average Nusselt number through the heated wall. The increase in Hartmann number causes heat transfer to be conductive due to the Lorentz force، and therefore the inclination angle of the cavity loses its effect. A remarkable effect of the inclination angle on heat transfer for 46 10 10 Ra is presented. The proposed method is a global method and provides to use small number of grid points as a result of DQ method.
Keywords: Radial basis functions; Differential quadrature method; Natural convection; Tilted cavity
  
 Thermal Marangoni Convection of Twophase Dusty Fluid Flow along a Vertical Wavy Surface S. Siddiqa , N. Begum , M. A. Hossain , R. S. R. Gorla Pages 509517 Abstract Full Text [PDF 812KB]   The paper considers the influence of thermal Marangoni convection on boundary layer flow of twophase dusty fluid along a vertical wavy surface. The dimensionless boundary layer equations for twophase problem are reduced to a convenient form by primitive variable transformations (PVF) and then integrated numerically by employing the implicit finite difference method along with the Thomas Algorithm. The effect of thermal Marangoni convection، dusty water and sinusoidal waveform are discussed in detail in terms of local heat transfer rate، skin friction coefficient، velocity and temperature distributions. This investigation reveals the fact that the waterparticle mixture reduces the rate of heat transfer، significantly.
Keywords: Marangoni convection; Boundary layer flow; Dusty fluid; Wavy surface
  
 Prediction of Pressure Drop in Vertical Pneumatic Conveyors S. M. El Behery , A. A. El Haroun , M. R. Abuhegazy Pages 519527 Abstract Full Text [PDF 246KB]   This paper presents a steady state onedimensional twofluid model for gassolid twophase flow in a vertical riser. The model is solved using conservative variable approach for the gas phase، and fourth order RungeKutta method is used for the solid phase. The model predictions for pressure drop are compared with available experimental data and with EulerianLagrangian predictions، and a good agreement is obtained. The results indicate that the pressure drop increases as the solid mass flow rate، particle size، and particles density increase. In addition، the model predictions for minimum pressure drop velocity are compared with experimental data from literature and the mean percentage error. MPE for minimum pressure drop velocity is 9.89%. It is found that the minimum pressure drop velocity increases as the solid mass flow، particle size and particle density increase، and decreases as the system total pressure increases.
Keywords: Gassolid; Pneumatic conveying; Twofluid model; Pressure drop
  
 Effects of Blade Discharge Angle, Blade Number and Splitter Blade Length on Deep Well Pump Performance E. Korkmaz , M. Gö,lcü , C. Kurbanoğ,lu Pages 529540 Abstract Full Text [PDF 498KB]   Impellers with splitter blades are used for pumps and compressors in the design of turbomachines. Design parameters such as the number of blades، blade discharge angle and impeller discharge diameter impact affect pump performance and energy consumption. In this study، the effect of the number of blades (z=5، 6، and 7)، blade discharge angles (β2b=25، and β2b=35) and splitter blade lengths (40، 55، 70، and 85% of the main blade length) on Deep Well Pump (DWP) performance has been studied experimentally. In the experiments، pump casing، blade inlet angle، blade thickness، blade width and impeller inlet and discharge diameters have been kept fixed while other parameters such as the number of blades، blade discharge angles and splitter blade lengths have been allowed to vary. As a result of the experimental study، the highest efficiency of all the impellers for best efficiency point (b.e.p) has been obtained on the impeller with the number of blades z=6، blade discharge angle 2b=25 and 85% splitter blade addition compared to impellers without splitter blades.
Keywords: Deep well pump; Blade discharge angle; Pump performance; Impeller; Splitter blade
  
 Numerical Simulation of Highly Charged Droplets Dynamics with Primary Breakup in Electrospray at Sub Rayleigh Limit M. Rahmanpour , R. Ebrahimi Pages 541550 Abstract Full Text [PDF 335KB]   The size and the axial and radial velocity distributions of electrically controlled droplets generated from Taylor cone operating in the stable conejet regime are simulated by numerical modeling of electrosprays. A model is formulated as function of liquid flow rate، needletocounter electrode distance، applied voltage، and electrical conductivity and surface tension of the liquid in a DC electric field is presented with a 2D electrohydrodynamic model. The droplet size reduction can be explained by evaporation and/or Coulomb explosion. Results show that moving downstream، the average velocity of droplets decreases monotonically. This paper reports a numerical study of the effects of an externally applied electric field on the dynamics of drop formation from a vertical metal capillary. The fluid issuing out of the capillary is a viscous liquid، the surrounding ambient fluid is air، and the electric field is generated by establishing a potential difference between the capillary and a horizontal، electrode placed downstream of the capillary outlet. The Primary jet Breakup and droplet transport and evaporation of electrohydrodynamic sprays is investigated by modeling of droplet size and velocity distribution in spray cones and a series of drop migrations under the influence of an electric field were carried out and the results are in good agreement with other theoretical and experimental studies.
Keywords: Electro hydrodynamic; Electrospray; Primary breakup; Charged droplets; Rayleigh limit
  
 Mechanisms of Water Droplets Deposition on Turbine Blade Surfaces and Erosion Wear Effects G. Ilieva Pages 551567 Abstract Full Text [PDF 789KB]   Failure of turbine blades leads to various exploitation problems، efficiency decrease and economical losses، at all. A detailed research on aerodynamic features، in various exploitation conditions and regimes، and on reasons for failures، is a prerequisite to the obviated technical problems and increased reliability of turbine aggregates. Water droplets erosion is known as a very complex and crucial phenomena. It couples the effects of wet steam expansion، together with condensation (evaporation)، presence of second phase with the impact of water droplets over blade surfaces، erosion effects and fatigue mechanisms. The present research deals with a logical sequence for numerical simulations and research on erosion mechanisms in a low pressure stage of К10006 /1500 steam turbine، working at a Nuclear Power Plant. Attention is paid to the impact of droplets’ diameter on blade surfaces، their aerodynamic behavior and efficiency of energy conversion through turbine channels. Particular trajectories of water droplets، reasons for occurrence of erosion wear، over certain parts of the streamlined surfaces، are established and discussed. An approach to acquire incidence time to erosion appearance is implemented. Research methodology and obtained results are applicable to determine erosion effects on streamed complex surfaces، to replace expensive measurements campaigns، introduce approaches to decrease wetness in last stages of condensation turbines and prolong the reliability of blades operated in wet steam conditions
Keywords: Droplet diameter; Erosion impact; Turbine blade; Water droplet erosion; Water droplets trajectories; Water hammer pressure
  
 Development of an Acoustic Metamaterials for Aero Acoustic Noise Control S. Rezaei , M. Eskandari Ghadi , M. Rahimian Pages 569579 Abstract Full Text [PDF 304KB]   This paper aims at proposing a novel type tunable acoustic metamaterials with complete band gap composed of piezoelectric rods (Lithium Niobate) with square array as inclusion embedded polyimide aerogel background. The plane wave expansion method and the principles of BlochFloquet method used to get a band frequency and study the pass band for noise control. The results of this paper provide the required guidance for designing tunable wave filters or wave guide which might be useful in highprecision mechanical systems operated in certain frequency ranges، and switches made of piezoelectric; they also propose a novel type of tunable mechanical meta composite، where is independent of wave direction and has an equal sensitivity in all directions in which reacts omnidirectional and improves the aero acoustic noise control (e.g. bladeless fans) as well as general performance of vibrating structures (e.g. wind turbine).
Keywords: Phononic crystal; Acoustic band gaps; Passive control; Piezoelectricity; Low frequency regime shielding
  
 Micro Gas Turbine Combustor Performances in CO2/O2 Oxidizer Atmosphere A. Di Nardo , G. Calchetti Pages 581593 Abstract Full Text [PDF 883KB]   In fossil fuel energy power plants the oxycombustion technique، is one of the possible approaches to the problem of greenhouse gases emissions، through the CO2 capture and subsequent storage. It is realized using recirculated flue gas enriched with oxygen as oxidizer and it is suitable more than other techniques to retrofit existing plants. The commercial gas turbine combustors currently available are however designed and optimized for air combustion. In this work، through a series of CFD simulations، a typical commercial micro turbine burner has been tested in oxycombustion conditions، in order to verify the performances. Through this study it has been shown how these class of combustors cannot be used in an optimal way in terms of efficiency، pollutant emissions and oxygen consumption. Some possible solutions have been also proposed.
Keywords: Gas turbine burner; Oxycombustion; Reactive flow simulation; Kinetic mechanisms
  
 Effect of the Number of Injectors on the Mixing Process in a Rapidly Mixed Type Tubular Flame Burner Y. Chouari , W. Kriaa , H. Mhiri , P. Bournot Pages 595603 Abstract Full Text [PDF 626KB]   Threedimensional simulations are performed to study the nonreactive mixing process in a rapidly mixed type tubular flame burner (RTFB). The current work examines the effect of the number of injectors (N= 2، 4 and 6) on the mixing process by focusing on three criterions (Flow structure، local swirl intensity and mixing layer thickness). The Discrete Phase Model (DPM) is used to track the particle trajectories. Validation of the numerical results is carried out by comparing the predicted particle trajectories، central recirculation zone (CRZ) and tangential velocity results to the experimental data. It is concluded that the model offers a satisfactory prediction of the flow field in a RTFB. Numerical results show that، for the same geometrical swirl number (Sw) and the same Reynolds number (ReT)، the increasing of the number of injectors enhances the mixing process by generating a larger reverse flow and reducing the mixing layer thickness. It is also concluded that the local swirl intensity along of the RTFB can be correlated in terms of geometric swirl number and number of injectors.
Keywords: CFD; DPM; Number of injectors; Mixing layer thickness; Tubular flame burner
  
 Laminar Instability of Parallel and Nonparallel Flows of Adiabatic Flat Plate Y. Mérida , M. Jelliti , T. Lili Pages 605614 Abstract Full Text [PDF 680KB]   In the present paper، spatial linear stability of adiabatic laminar flat plate boundary layer is computed numerically. This work is interested on the nonparallel compressible flow for a twodimensional (2D) and threedimensional (3D) disturbances. Stability diagrams، in the form of curves of constant spatial amplification rates are presented and constitute the original contribution of this paper. However، to assess the validity of the present computations، some important results of parallel flow stability are presented in the convenient and familiar form of contours of constant spatial amplification rates for both 2D and 3D waves in the Mach number range Me= 0.9 to 2.2. Comparisons results for the parallel flow agree with those obtained by Mack and Wazzan، Taghavi and Keltner، but differ from the nonparallel flow giving some important spatial stability results and showing the importance of the wave angle ψ and the Mach number، even the stability diagrams presented in this paper concerned the Reynolds number gives different results for the parallel and nonparallel flows.
Keywords: Laminar instability; Compressible boundary layer; Transonic flow; Supersonic flow; Stability diagrams
  
 Ferrofluid Appendages: Fluid Fins, a Numerical Investigation on the Feasibility of using Fluids as Shapeable Propulsive Appendages M. A. Feizi Chekab , P. Ghadimi Pages 615623 Abstract Full Text [PDF 919KB]   The present study focuses on the feasibility of using fluids، and in particular magnetic fluids، as “Fluid Structures” in designing external appendages for the submerged bodies and propulsive fins as a practical example. After reviewing the literature of the mathematical simulation of magnetic fluids and their applications، the concept of “Fluid Structures” and “Fluid Fins” are briefly introduced. The validation of the numerical solver against analytical solutions is presented and acceptable error of 1.21% up to 2.29% is estimated. Subsequently، the initial shaping of the ferrofluid as an external fluid fin، using three combinations of internal magnetic actuators، is presented which makes the way to the oscillating motion of the obtained fin، by producing a periodically changing magnetic field. It is demonstrated that the shape of the fluid fin is almost the replica of the magnetic field. On the other hand، it is illustrated that a fluid fin with a size under 0.005 m on a circular submerged body of 1cm diameter could produce 0.0158 N force which is a high thrust force relative to the size of the body and the fin. Based on the obtained results، one may conclude that، when a “Fluid Fin” is capable of producing this amount of thrust، other fluid appendages could be scientifically contemplated and practically designed.
Keywords: Fluid structures; Fluid appendages; Fluid fin; Ferrofluid; Numerical analysis
  
 Coupled Analysis of Unsteady Aerodynamics and Vehicle Motion of a Passenger Car in Crosswind Condition T. M. Huang , Z. Q. Gu , C. J. Feng Pages 625637 Abstract Full Text [PDF 950KB]   Highspeed passenger car requires a lighter weight for improving power performance and reducing fuel consumption; a car with higherspeed and lighter weight will lead to the passenger car more sensitive to the crosswind، which will affect the stability and drivability of the passenger car. This study employs the fullycoupled method to investigate a passenger car subjected “1cos” crosswind with consideration of the vehicle motion. Large eddy simulation (LES) and dynamic mesh is adopted to investigate the unsteady aerodynamic، and the vehicle is treated as a threefreedomsystem and driver’s control is considered to investigate the vehicle dynamic. The oneway simulation and quasisteady simulation are also conducted to compare with the fullycoupled simulation. The results of the three simulation methods show large difference. The peak value of the lateral displacement in fullycoupled simulation is the smallest between the three simulation approaches. While the change of aerodynamic loads and vehicle motion in fullycoupled simulation is more complicated than in oneway and quasisteady simulation. These results clearly indicate the significance of including of the unsteady aerodynamic loads in passenger car moving analysis.
Keywords: Unsteady aerodynamic; Largeeddy simulation; Vehicle motion; Fullycoupled simulation; Oneway simulation; Quasisteady simulation
  
 A New Formulation for Prediction of Permeability of Nano Porous Structures using Lattice Botzmann Method M. Mostafavi , A. H. Meghdadi Isfahani Pages 639649 Abstract Full Text [PDF 886KB]   The standard lattice Boltzmann equation، LBE، is inadequate for simulating gas flows in nano scale flows with Knudsen numbers higher than 0.1. In the present study، rarefied gas flow in nano porous structures is simulated using the modified Lattice Boltzmann Method، LBM، which is able to cover wide range of flow regimes. The present study، reports the effects of the Knudsen number and porosity on the flow rate and permeability in slip and transitional flow regimes. For the first time، the Knudsen’s minimum effect in micro/nano porous was observed. A new correlation between the permeability، the porosity and the Knudsen number is then proposed which is able to predict the permeability of inline and staggered nano porous structures in slip and transitional regimes.
Keywords: Lattice Boltzmann method; Nanoscale flow; Porous media; Transitional flow regime
  
 Modelisation and Simulation of Heat and Mass Transfers during Solar Drying of Sewage Sludge with Introduction of Real Climatic Conditions N. Ben Hassine , X. Chesneau , A. H. Laatar Pages 651659 Abstract Full Text [PDF 160KB]   Sewage sludge presents a real problem with the urban and industrial expanding. So، the drying technique is indispensable in the sludge treatment process to minimize its volume and its revalorization. For cost and environmental reasons، the solar drying is becoming increasingly attractive for small and medium wastewater treatment plants. Therefore، the aim of this work is the modelisation of solar dryer of residual sludge. The model studied is a rectangular agricultural greenhouse. In the lower part، the sludge (assimilated to a porous medium)، acts as an absorber. It is subjected to a forced laminar flow. The transfers in the greenhouse and the porous medium are described respectively by the classical equations of forced convection and the DarcyBrinkmanForchheimer model. The implicit finite difference method is used to discretize the governing differential equation. The algebraic systems obtained are solved using the Gauss، Thomas and GaussSeidel algorithms. In order to complete the model and to determine the drying rate we associate a model of the sewage sludge drying kinetics. This work is realized with the meteorological data of the Tataouine region in the south of Tunisia. This data have undergone statistical treatment using the Liu and Jordan method. In order to show the advantages of solar drying، we especially studied the various transfer modes، the drying kinetics and the dryer performance.
Keywords: Heat and mass transfers; Forced convection; Solar drying; Sewage sludge; Dryer performance
  
 Onset of Darcy–Benard Penetrative Convection in Porous Media Y. H. Gangadharaiah Pages 661666 Abstract Full Text [PDF 166KB]   The onset of Darcy–Benard penetrative convection in a liquid saturated porous layer of high permeability of practical importance is investigated by employing the Brinkman–Forchheimer– Lapwood extended Darcy flow model with fluid viscosity different from effective viscosity. The lower boundary is taken to be rigid and isothermal and the upper surface is free and subject to the general thermal condition. The critical eigen values are obtained numerically، in general، using Galerkin method. The stability of the system is found to be dependent on the dimensionless internal heat source strength Ns ، permeability parameter e and the ratio of effective viscosity to fluid viscosity . It is observed that the increase in the value of permeability parameter is to delay while increase in the value of internal heat source strength is to hasten the onset of convection in a fluid saturated porous layer.
Keywords: Penetrative convection; Volumetric heat source; DarcyBenard convection
  
 A Strategy for Passive Control of Natural RollWaves in PowerLaw Fluids through Inlet Boundary Conditions C. Di Cristo , M. Iervolino , A. Vacca Pages 667680 Abstract Full Text [PDF 367KB]   The paper investigates the influence of the inlet boundary condition on the spatial evolution of natural rollwaves in a powerlaw fluid flowing in steep slope channels. The analysis is carried out numerically، by solving the von Kármán depthintegrated mass and momentum conservation equations، in the longwave approximation. A secondorder accurate scheme is adopted and a small random whitenoise is superposed to the discharge at the channel inlet to generate the natural rollwaves train. Both shearthinning and shearthickening powerlaw fluids are investigated، considering uniform، accelerated and decelerated hypercritical profiles as the unperturbed condition. Independently of the unperturbed profile and of the fluid rheology، numerical simulations clearly enlighten the presence of coalescence، coarsening and overtaking processes، as experimentally observed. All the considered statistical parameters indicate that the natural rollwaves spatial evolution is strongly affected by the unperturbed profile. Compared with the uniform condition، at the beginning of rollwaves development an accelerated profile reduces the growth of the rollwaves with a downstream shift of the nonlinear wave interaction. The opposite behavior is observed if the roll wave train develops over a decelerated profile. The comparison with the theoretical outcomes of the linearized near wavefront analysis allows the interpretation of this result in terms of stability of the base flow. It is shown that once the coarsening process starts to take place، the rollwaves spatial growth rate is independent of the unperturbed profile. Present results suggest that an appropriate selection of the flow depth at the channel inlet may contribute to control، either enhancing or inhibiting، the formation of a rollwaves train in powerlaw fluids.
Keywords: Natural rollwaves; Powerlaw fluid; Gradually varying flow; Shockcapturing method
  
 Effect of Horizontal Magnetic Field and Horizontal Rotation on Thermosolutal Stability of a Dusty CoupleStress Fluid through a Porous Medium: a Brinkman Model K. Kumar , V. Singh , S. Sharma Pages 681692 Abstract Full Text [PDF 324KB]   The problem of the onset of double diffusive convection in a couplestress fluid saturated with a porous medium is studied under the effects of magnetic field، rotation and suspended dust particles. Linear stability analysis based on the method of perturbations of infinitesimal amplitude is performed using the normal mode technique for the case of freefree boundaries. The governing hydrodynamic and hydromagnetic equations of fluid flow are governed by the Brinkman model. The stability analysis examines the effects of various embedded parameters for the stationary mode both analytically and graphically. The principle of exchange of stabilities holds good in the absence of solute gradient parameter. Also، the sufficient conditions responsible for the existence or nonexistence of overstability are obtained.
Keywords: Couplestress fluid; Magnetic field; Rotation; Dust particles; Brinkman porous medium
  
 ThreeDimensional Numerical Simulation of Deformation of a Single Drop under Uniform Electric Field M. Akbari , S. Mortazavi Pages 693702 Abstract Full Text [PDF 491KB]   In this paper، deformation of a drop suspended in another immiscible fluid that is influenced by an external uniform electric field is investigated through fully 3D numerical simulations. The electric field is applied by imposing an electric potential difference in the ambient fluid. The Leaky dielectric model is used to obtain the electric field، charge distribution and eventually applied electric force at the interface. This force creates both oblate and prolate shapes، and also induces various Electrohydrodynamic flows inside and outside of the drop depending on the conductivity and permittivity ratio of the drop and the ambient fluid. A finite difference/fronttracking method is used. The results are presented for a wide range of nondimensional parameters for predicting the drop deformation quantitatively and qualitatively. Different flow patterns are induced inside and outside of the drop. The results show a good agreement with theoretical and experimental results in the literature. For the sake of consideration of the problem in more detail، four specific cases are investigated.
Keywords: Oblateprolate shape; Electric conductivitypermittivity; Fronttracking method
  
 Flow Mechanism for Stall Margin Improvement via Axial Slot Casing Treatment on a Transonic Axial Compressor H. Kuang , S. Wuli Chu , H. Zhang , S. Ma Pages 703712 Abstract Full Text [PDF 479KB]   Axial slot CTs were designed and applied on Rotor 67 to understand the physical mechanisms responsible for the improvement of the stall margin. Unsteady Reynoldsaveraged NavierStokes was applied in addition to steady Reynoldsaveraged NavierStokes to simulate the flow field of the rotor. The results show that aerodynamic performance and the rotor stability were improved. Stall margin improvement (SMI) improved by 26.85% after the CT covering 50% of the axial tip chord was applied، whereas peak efficiency (PE) decreased the least. The main reason for the rotor stall in the solid casing is the blockage caused by tip leakage flow. After axial slot CTs were applied، the tip leakage flow in the front part of the chord was obviously reduced، and the majority of the blockages in the tip region were removed. The absolute value of the axial momentum before 45% axial chord in CT_50 was reduced by 50%، whereas the maximum tangential momentum value of CT_50 was decreased by 70% relative to the solid casing. CT_50 configuration was located across the shock wave; thus، it can fully utilize the pressure gradient to bleed and remove the blockage region، and the across flow is considerably depressed.
Keywords: Casing treatment; Transonic axial compressor; Performance; Flow field; Stall mechanism
  
 Length Scale of Free Stream Turbulence and Its Impact on Bypass Transition in a Boundary Layer J. Grzelak , Z. Wierciń,ski Pages 713724 Abstract Full Text [PDF 573KB]   An experimental investigation was carried out to study the turbulent flow over a flat plate in a subsonic wind tunnel. The enhanced level of turbulence was generated by five wicker grids with square meshes، and different parameters (diameter of the grid rod d = 0.3 to 3 mm and the grid mesh size M = 1 to 30 mm). The velocity of the flow was measured by means of a 1D hotwire probe، suitable for measurements in a boundary layer. The main aim of the investigation was to explore the influence of the free stream turbulence length scale on the onset of laminarturbulent bypass transition in a boundary layer on a flat plate. For this purpose، several transition correlations were presented، including intensity and length scales of turbulence، both at the leading edge of a plate and at the onset of transition. The paper ends with an attempt to create a correlation، which takes into account a simultaneous impact of turbulence intensity and turbulence scale on the boundary layer transition. To assess the isotropy of turbulence، the skewness factor of the flow velocity distribution was determined. Also several longitudinal scales of turbulence were determined and compared (integral scale، dissipation scale، Taylor microscale and Kolmogorov scale) for different grids and different velocities of the mean flow U = 4، 6، 10، 15، 20 m/s.
Keywords: Turbulence scale; Turbulence intensity; Boundary layer; Transition; Grid; Isotropic turbulence
  
 Numerical Investigation of Detonation Combustion Wave in Pulse Detonation Combustor with Ejector P. Debnath , K. M. Pandey Pages 725733 Abstract Full Text [PDF 442KB]   Detonation combustion based engines are more efficient compared to conventional deflagration based engines. Pulse detonation engine is the new concept in propulsion technology for future propulsion system. In this contrast، an ejector was used to modify the detonation wave propagation structure in pulse detonation engine combustor. In this paper kε turbulence model was used for detonation wave shock pattern simulation in PDE with ejectors at Ansys 14 Fluent platform. The unsteady Euler equation was used to simulate the physics of detonation wave initiation in detonation tube. The computational simulations predicted the detonation wave flow field structure، combustion wave interactions and maximum thrust augmentation in supersonic condition with ejectors at time step of 0.034s. The ejector enhances the detonation wave velocity which reaches up to 2226 m/s in detonation tube at same time step، which is near about CJ velocity. Further the time averaged detonation wave pressure، temperature، wave velocity and vortex characteristics interaction are obtained with short duration of 0.023s and fully developed detonation wave structures are in good agreement with experimental shadowgraph، which are cited from previous experimental research work.
Keywords: Detonation; Ejector; Computational fluid dynamics; Pulse detonation engine; Thrust
  
 Experimental Study of Natural Convective Flow over a Hot Horizontal Rhombus Cylinder Immersed in Water via PIV Technique M. Karbasi pour , M. Nili , Ahmadabadi , G. Taherian , A. Minaean Pages 735747 Abstract Full Text [PDF 511KB]   Natural convective flow over a horizontal cylinder is a phenomenon used in many industries such as heat transfer from an electrical wire، heat exchanger، pipe heat transfer، etc. In this research، fluid dynamics of natural convective flow over a horizontal rhombus cylinder، with uniform heat flux، is investigated by using twodimensional Particle Image Velocimetry (PIV) Technique. Experiments are carried out in a cubical tank full of water having an interface with air and the cylinder is placed horizontally inside the tank. The heater is turned on for 40s and the effects of heater's power and the height of water above the cylinder are surveyed. The experiments are carried out in three different heights of water and two different heater’s powers in which Rayleigh number changes from 1.33×107 to 1.76×107. The emitted heat flux causes the buoyancy force to be made and the main branch of flow to be formed. Then، moving up the main branch flow through the stationary water generates two equal antidirection vortexes. These vortexes are developed when they reach the free surface. The results indicate that the flow pattern changes for different values of water height and heater’s power.
Keywords: Natural convective; Rhombus cylinder; Heater; Velocity fields; PIV technique
  
 Flow Simulations with UltraLow Reynolds Numbers over Rigid and Flexible Airfoils Subject to Heaving and Flapping Motions D. Antonelli , C. Sacco , J. Tamagno Pages 749762 Abstract Full Text [PDF 1058KB]   Numerical simulations of flow patterns at ultralow Reynolds numbers over rigid and flexible airfoils are presented، and the influence of flexibility on main aerodynamic properties are discussed. Typical unsteady flights like heaving and flapping are، in terms of Reynolds and Strouhal numbers، reduced frequencies and FSI (Fluid Structure Interaction) factor، are valuated. It has been found that for some flexibility levels، the aerodynamic forces and propulsive efficiency are enhanced if compared with a rigid airfoil. The mathematical technical approach used to solve the laminarincompressible flow equations coupled with structural algorithms، is described.
Keywords: Aerodynamic wing sections; CFD; Fluid structure interaction; Unsteady flows; Low Reynolds; Flexible airfoil; Partitioned method; Finite element method
  
تاريخ انتشار: 26/1/96 تلفن: 33915271 (031)
تاريخ درج در سايت: 26/1/96
شمار بازديدکنندگان اين شماره: 70




