فهرست مطالب

Applied Fluid Mechanics - Volume:13 Issue:1, 2019
  • Volume:13 Issue:1, 2019
  • تاریخ انتشار: 1398/10/04
  • تعداد عناوین: 30
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  • R. Rabani, G. Heidarinejad*, J. Harting, E. Shirani Pages 1-13

    Nonequilibrium molecular dynamics simulations is applied to investigate the simultaneous effect of rarefaction and wall force field on the heat conduction characteristics of nano-confined rarefied argon gas. The interactive thermal wall model is used to specify the desired temperature on the walls while the Irving– Kirkwood expression is implemented for calculating the heat flux. It is observed that as the temperature differences between the walls increases by lowering the temperature of the cold wall, the number of adsorbed gas atoms on the cold wall increases notably due to the increment in the residence time of the gas atoms. Consequently, the interfacial thermal resistance between the gas and the cold wall reduces which results in a reduction of the temperature jump. Meanwhile, the increase in the temperature of the hot wall leads to a reduction of the residence time of gas atoms in the near-wall region which decreases the number of absorbed gas atoms on the hot wall. This results in an increase in interfacial thermal resistance which leads to a higher temperature jump. It is observed that the bulk, wall force field and interface regions form approximately 10%, 45% and 45% of the total thermal resistance, respectively. Furthermore, unlike the interfacial thermal resistance, the bulk and the wall force field thermal resistance are approximately independent of the implemented temperature difference.

    Keywords: Thermal resistance, Temperature jump, Gas atoms distribution, Molecular dynamics
  • M. J. Torres*, J. García, Y. Doce Pages 15-23

    The dispersion of solid particles in zones of turbulent recirculation flow is of interest in various technological applications. Many experimental studies have been developed in order to know the contribution of Stokes numbers and mean drift parameter on the entering and dispersion of particles in the recirculation zone however to our knowledge there are not numerical studies reported about it. In this work, we made a numerical study of the incompressible turbulent flow laden with solid particles in sudden expansion pipes with different expansion ratios and different Reynolds number upstream of the pipe, using LES and Germano dynamic model with JetCode program for the continuous phase (air). The solid particles movement (different diameters were considered) was solved by using a Lagrangian tracking algorithm coupled to JetCode taking into account only drag and gravity forces supposing one way coupling. Finally, we calculated Stokes numbers based on the different fluid time scales and the mean drift parameter for all the solved cases and studied their isolated effect on the solid particle dispersion in the recirculation zones by computing the concentration by means of the particle number within the recirculation zones. Our results coincided with the experimental findings reported by others authors: the particle concentration exhibits a maximum value as the Reynolds number upstream in the pipe is decreased, the pipe expansion ratio is increased and particle size is decreased. Regarding the results obtained numerically about the solid particle dispersion within turbulent recirculation zones in terms of Stokes numbers and the mean drift parameters, coincided adequately with the experimental results.

    Keywords: Turbulent flow, Sudden expansion pipe, Particle concentration, Multiphase flow
  • S. Jing wei*, W. Zhan xue, Z. Li, S. Xiao lin Pages 25-38

    Higher vector efficiency of fluidic thrust vectoring (FTV) technology results in less requirement on secondary flow mass, which helps to reduce the influence of secondary flow on the performance of an aero-engine. In the paper, a new concept of FTV, named as a hybrid shock vector control (SVC) nozzle, was proposed to promote the vector efficiency of a SVC nozzle. It adopts a rotatable valve with a secondary flow injection to enhance the jet penetration, so as to improve the vector performance. The flow characteristics of a hybrid SVC nozzle were investigated numerically by solving 2D RANS equations. The influence of secondary pressure ratio (SPR) and rotatable valve angle on vector performance were conducted. Then, the coupling performance of a hybrid SVC nozzle and an aero-engine was estimated, by using the approximate model of a hybrid SVC nozzle and the performance simulation model of an aero-engine. Results show that, a desirable vector efficiency of 2.96 º/ %-ω (the vector angle achieved by using secondary flow of 1% of primary flow) of a hybrid SVC nozzle was obtained. In the coupling progress, when a secondary flow of 5.3% of primary flow was extracted from fan exit to a hybrid SVC nozzle, a vector angle of 14.1°, and a vector efficiency of 2.91º/ %-ω were achieved. Meanwhile the thrust of the aero-engine thrust decreased by 5.6% and the specific fuel consumption (SFC) increased by 0.5%.

    Keywords: Flow characteristics, Transverse injection, Vector performance, Coupling Performance, approximate model
  • M. Rostami Hosseinkhani, P. Omidvar *, S. Allahyaribeik, M. Torabi Azad Pages 39-54

    Marine oil spills can cause serious damage to the marine ecological environment. In the numerical modeling of oil plume rising and its advection, a better understanding of the oil plume transport may be effective on the sea pollution reduction and removing pollutants. In this paper, the effects of waves are investigated on the oil plume convection-diffusion pattern using smoothed particle hydrodynamics (SPH). Firstly, the rising patterns of an oil plume of different densities are simulated and the results are compared with the analytical solution. Then, the concentration distribution is shown for the oil plume rising problem. Afterwards, the suitability of the SPH method is examined by a cnoidal wave on shore effect. Finally, the plume of different conditions is located in waves and the advection of pollutant is studied with a fixed boom and different angles. It will be concluded that using a boom with a zero diversion angle would lead to minimum passing pollutant.

    Keywords: Smoothed particle hydrodynamics, Two-phase currents, Oil plume rising, Concentrationdistribution
  • M. Monfared*, M. A. Alidoostan Pages 55-65

    The main goal of this research is to study the drag reduction capabilities of blade-shape riblet surfaces in external flows. For this purpose, the ability of riblet surfaces for drag reduction of an underwater hydrodynamic model has been investigated. The surface geometry has been modified by applying shark skin inspired blade-shape riblets on the surface. These riblets have been modeled in various dimensions and applied on the exterior surface of an underwater hydrodynamic model, and their effects on the exerted drag force, at different flow velocities, have been studied numerically. For validating the numerical solution, the simulation results have been compared with the experimental data obtained by testing an underwater hydrodynamic model in a towing tank laboratory; and the validity of the numerical solution results has been confirmed. The results indicated that, riblet spacing has a significant effect on the reduction of drag force. Furthermore, by increasing the riblet spacing, the drag force is increased rather than decreased. Also, as the velocity increases, the performance of riblets in reducing the drag force is enhanced. In order to minimize the drag force applied on the underwater hydrodynamic model, by analyzing the numerical results, the most optimum riblet spacing has been obtained; at which the drag force is reduced by 7%. The achieved distance is a limit value; and at distances smaller or larger than this optimal distance, the effectiveness of the blade-shape riblet surface in reducing the drag force diminishes.

    Keywords: Underwater hydrodynamic model, Towing tank laboratory, Drag reduction, Riblet surface, Numerical simulation
  • D. Narayanan*, S. Anand, S. Anish Pages 67-77

    Several techniques are implemented to reduce the temperature rise in multistage compressors, which leads to the noticeable improvement in specific power output of a gas turbine. The objective of the present investigation intends to understand the effect of incidence angles on the aerodynamic performance of the compressor cascade under wet compression. Using large eddy simulations (LES) the effects of wet compression on compressor flow separation and wake formation are investigated. Experimental investigation was performed to validate the numerical results. The study reveals notable flow modifications in the separated flow region under the influence of wet compression and the total loss coefficient reduces significantly at the downstream side of the compressor for positive incidence angles. On the other hand, for negative incidence angles the wet compression enhances the total pressure losses inside the blade passage. Also, in the present investigation, particular emphasis has been given to understand the water film formation at negative and positive incidence angles.

    Keywords: Wet compression, Large eddy simulation, Linear compressor cascade, Incidence angle, Flow loss
  • S. Dong*, P. F. Geng, D. Dong, C. X. Li Pages 79-88

    A novel micromixer is presented in this study and the effect of DC or AC electric field on mixing efficiency is investigated numerically. Four types of AC waveforms are considered to explore the flow characteristic and mixing efficiency. The velocity field, concentration field and the mixing efficiency are analyzed in details. The results demonstrated that a pair of vortices with opposite rotating directions is generated when DC or AC voltages is applied to the electrode plates planted within the walls. The generated vortices greatly enhance the mixing of incoming fluids with different concentrations. The mixing efficiency firstly rises with time and then reaches a relative stable periodic state under different potential waveforms. As the voltage applied on the plates increases, the mixing efficiency is improved obviously. The mixing efficiency under full-wave AC signal is the highest, and it is up to 95.44% when the applied potential is 3 V and frequency is 5 Hz.

    Keywords: Mixing efficiency, Potential waveforms, Numerical simulation, Optimization, Electroosmoticflow, Microchannel
  • M. Namazizadeh, M. Talebian Gevari, M. Mojaddam*, M. Vajdi Pages 89-101

    Centrifugal pumps are among the most applicable machines in a wide variety of industrial systems for fluid pumping and transportation. Therefore their optimization has always been of great importance. Pump impellers play an important role in these machines as the energy transfer takes place in this part. In the present study, the impeller of a centrifugal pump is optimized by investigating the effect of adding splitter blades and modifying their geometry. A centrifugal pump is experimentally tested and numerically simulated and the characteristic curves are obtained. In the first stage, two different sets of splitter blades with different lengths are added to the impeller and the effect of splitter blade lengths on the results are explored. The case with the highest total head and overall efficiency is selected for the optimization process. The main blade and the splitter blade leading edge position and also the splitter blade distance between two successive blades are selected for the optimization process in the second stage. Efficiency and total head of the pump are considered as the optimization objectives. Using Design of Experiment (DoE) technique, the design space is created and response surface method is utilized to find the optimum geometry. The results show adding splitters can improve total head by about 10.6% and by modifying the geometry using DoE technique it could increase further by 4.4% with the negligible effect on the pump overall efficiency.

    Keywords: Centrifugal pump, Splitter blades, Optimization, Design of Experiment (DoE), Response surfacemethod
  • A. Bennia*, H. Fellouah, A. Khelil, L. Loukarfi, H. Naji Pages 103-117

    The mixing improvement by passive control is of wide practical interest. The lobed diffuser, which mixes the primary and secondary streams with high efficiency, has been widely used for heat and mass transfer in the field of fluid engineering. In addition, the jets through lobed generate streamwise vortices, which mix the ambient air and the jet fluid more effectively. The main objective of the present work is to develop new air diffusers for heating, ventilation, air conditioning (HVAC) systems using different jet geometries, in order to improve the users’ thermal comfort. Three free jets of air diffusers emitted from a tubular lobed, with six and five lobes, and from a swirl nozzle have been both studied experimentally and numerically. All diffusers have the same throat diameter. It turns out that the results obtained with the LES/WALE and LES/K-ET turbulence models are respectively in good agreement with the experimental results of the lobed and swirling jets. These results indicate that the best mixture is obtained using the six-lobed nozzle with respect to the five-lobed nozzle and the swirling nozzle. In addition, the importance of the jet type on the mixing capacity is highlighted.

    Keywords: Lobed jets, Swirling jet, Thermal homogenization, Experimental study, Numerical simulation, LES
  • B. Biçer, A. Yurtkuran* Pages 119-132

    Numerical modeling of internal nozzle flow can be regarded as an essential investigation in the field of gasoline direct injection system of combustion engines since it is directly connected with fuel spray atomization and subsequently efficiency of exhaust gas emission. Internal nozzle flow can be changed and formed according to several parameters such as; system pressure, chosen fuel type, the orientation of spray holes according to injector axis, conicity of spray holes and distribution of spray holes on valve-seat, etc. The changes in these parameters also affect the formation of cavitation inside of whole domain, spray angle and create wall-wetting on the spray hole surfaces. The present work investigates the parameter and design analysis in the valve-seat region of direct gasoline injection (GDI) injector using Computational Fluid Dynamics (CFD) and Design of Experiments (DOE). CFD is employed to study the behaviors of internal flow inside the valve-seat region according to several design parameters, whereas a mixed-level factorial design is used to test the significance of the effects on the response variables. In conclusion, the effects of the most significant factors on response parameters as amount of vapor formation, spray (Tau) angle, and pre-hole wall wetting are determined for further efficient design.

    Keywords: CFD, In-nozzle simulation, Cavitation, Gasoline direct injection, Design of experiments
  • L. Zhou*, Z. Wang, J. Shi Pages 133-145

    Inertial Particle Separator (IPS) is widely used as an important inlet protection device for turbo-shaft aero engine to protect the core engine in seriously polluted environment. In order to improve the separation efficiency of the IPS, an investigation was conducted to study the influence of critical geometrical and aerodynamic parameters on IPS performance, and Response Surface Method(RSM)was applied to explore the interaction between different parameters and obtain the response of the IPS performance on different parameters. Results show the separation of the sand particle in the IPS is achieved by the inertial accumulation of the sand particle, the trajectories of particles with small size are dominated by flow direction while paths of particles with larger size are dominated by the individual particle inertia and bounce characteristics from the IPS walls. The separation efficiency of the IPS is not only affected by the single geometrical parameters or aerodynamic parameter, but also apparently influenced by the interaction effects between different parameters. The most conspicuous influencing factor for the IPS separation efficiency on the AC-Coarse dust is Math and the interaction effects between Ro1 and Math. IPS separation efficiency on the AC-Coarse dust is improved by 3.8% by multi-factors optimization based on RSM, and the sand particle with size larger than 8 micron can be completely separated.

    Keywords: IPS, RSM, Separation efficiency, Geometrical parameter, Aerodynamic parameter
  • M. Prabhu, R. Ajith Kumar*, T. H. Gopikrishnan , P. J. Deshpande, U. Anandhakrishnan, A. S. Kiran, R. P. Govindu Pages 147-160

    This paper reveals the results of a study of vortex air core formation (Rankine vortex) when a rotated liquid (water) column in a cylindrical vessel is drained through two ports located at equal eccentricity (e) at the vessel base (diameter, 𝑑1and 𝑑2) simultaneously; 𝑑1is fixed whereas 𝑑2 is varied. Just before draining, a rotation (n rpm) is provided to the liquid column in controlled conditions. As draining progresses, when the liquid level reaches certain height called critical height (ℎ𝑐 ), initially a surface dip forms which further develops in to a vortex extending down till the drain port. Results show that critical height increases as the fluid rotation rate increases at the lowest eccentricity. But, at higher eccentricities, ℎ𝑐 , exhibits more or less an increasingdecreasing trend in most of the cases studied. Critical height is observed to be minimum for the largest value of 𝑑2 (equal to 𝑑1) irrespective of the values of the speed of fluid rotation, liquid initial height and port eccentricity. To particularly note, at the highest eccentricity, vortex formation is found to be completely suppressed for all values of port diameter (𝑑2) and initial fluid rotation (n) as indicated by the near-zero critical height values. The tangential velocity measurements using Particle Image Velocimetry are also reported. PIV results obtained for certain cases with induced fluid rotation (normal draining and faster draining) correlate well with the changes in the efflux (axial) velocity (deduced analytically) in these cases studied. The tangential velocity along radial direction obtained (PIV) also indicated the type of vortex formed in normal and faster draining cases. Video visualization of vortex formation carried out reveals that, vortex air core switching takes place between the drain ports maintaining an arched or curvilinear surface profile apart from demonstrating the nature of outlet flow discharge. All the vortex air core formation studies so far carried out were invariably with single drain port except the preliminary novel study by the same author group and the present study is a detailed extension of that novel study.

    Keywords: Vortexing, Twin drain ports, Liquid draining, Critical height, Time of emptying, PIV, Flowvisualization
  • J. R. Januário, C. B. Maia* Pages 161-168

    The hydraulic transport of solids has been adopted for many years. However, deposition of the solid particles in the pipe can cause equipment failure, pipeline erosion, and excessive pressure drop, resulting in financial and environmental problems. The critical velocity, also known as critical deposition velocity, is the limit of particle deposition when a moving bed of particles starts to form on the bottom of the pipe. The determination of the critical deposition velocity is an essential step in slurry pipeline`s design and operation. This paper assesses the influence of the velocity in the particle deposition of slurries containing coarse particles of apatite and hematite industrial concentrates and quartz. We used CFD to solve Navier-Stokes for the fluid phase and DEM to solve Newton`s equations governing the granular particles. CFD and the DEM modules adopted OpenFOAM and LIGGGHTS. Numerical results were compared with experimental data from the literature.

    Keywords: CFD-DEM, Slurry flow, Critical deposition velocity, Numerical simulation
  • M. Grioni*, S. A. Elaskar, A. E. Mirasso Pages 169-183

    Unsteady simulations of the flow around two cylinders arranged in tandem are carried out using Scale-Adaptive Simulation (SAS) turbulence model for high subcritical Reynolds number (Re = 2 × 105 ). Three-dimensional simulations are performed for different center-to-center distances between the cylinders (L/D varies 1.1 to 7, where D is cylinder diameter). The effects of the gaps between the cylinders are analyzed through the values of mean and fluctuating force coefficients, Strouhal number, pressure distribution, as well as through the wake flow structures behind both cylinders. The results are compared with published experimental data by different authors. The obtained results reveal good general agreement with the experimental data. Besides, to explore the effects of the interference, two tandem cylinders test are compared with a single cylinder case. The results show that this simple configuration (tandem) can strongly influence the flow pattern and forces on the cylinders. A critical nondimensional distance is obtained at L/D=3 at which two different flow patterns are identified, one pattern momentarily similar to the reattachment regime and another pattern similar to the co-shedding regime

    Keywords: SAS turbulence model, Circular cylinders in tandem arrangement, Wake interference, Vortexshedding
  • V. Kumar*, R. Kumar, H. Kumar Pages 185-197

    The novel controllable behaviour of magnetorheological (MR) fluid is the backbone of magnetorheological fluid-based finishing processes. MR fluid-based finishing processes facilitate better control over finishing forces as the stiffness of MR finishing fluid used in these processes can be controlled in accordance with the applied magnetic field and MR finishing fluid composition. Therefore, a detailed experimental investigation was carried out to find the effect of MR finishing fluid constituents on its yield stress through the Taguchi Design of Experiments. Rheological data obtained from a magneto-rheometer (MCR-102) was characterised by using Bingham plastic, Herschel–Bulkley and Casson’s fluid constitutive modelling. The coefficient of regression (R2 ) values of Herschel–Bulkley model were found to be best suited for all compositions of MR finishing fluid. Analysis of variance (ANOVA) has been used to find the contribution of selected parameters for improving the response characteristics. The optimized fluid has been then used for the finishing of biocompatible stainless steel AISI 316L, and the finishing results show that the average surface roughness value decreases down to 58 nm.

    Keywords: Magnetorheological finishing fluid, Magnetic field strength, Yield stress, Constitutive models, Surface roughness
  • X. F. Wu, X. Tian*, M. G. Tan*, H. L. Liu Pages 199-209

    A mixed flow pump with guide vanes was chosen as research model in this study, and eight parameters of the impeller were selected as optimization variables, including blade outlet inclination angle, blade wrap angle at hub, blade inlet angle and outlet angle at middle stream line, blade outlet width, front shroud inclination angle, hub inclination angle and vane number. Firstly, orthogonal experimental method and CFD numerical simulation method were used to produce samples, then the RBF neural network was adopted to establish the performance prediction model as the objective function, multi-island genetic algorithm was used for solving the objective function at last. Based on all the above, a method of multi-parameter optimization method on energy performance of mixed flow pump without changing the nominal diameter of impeller outlet was proposed and then verified by experiments. By this method, the pump head and efficiency at the design point of the model pump were increased by 11.5% and 4.32%, respectively. Meanwhile, the peak value of pressure pulsation coefficient at pump inlet, impeller outlet, guide vane outlet and pump outlet all decreased obviously, by a maximum decrease of 62.9%. Compared to the original model, the static pressure in the optimization model increased by 30kPa and the gradient of static pressure distribution after optimization becomes larger and more uniform. The turbulent energy intensity at the impeller outlet was reduced by 0.2m2 /s2 . The pressures at the 60% blade position and 80% blade position both increased by nearly 65kPa and the pressure decreased by 50kPa at the blade pressure side.

    Keywords: Energy characteristics, Mixed flow pump, Multi-island GA, Optimization design
  • S. Zirak*, M. Seifi, A. Ramesh Pages 211-219

    Flow behavior through a gas turbine double-container fuel valve is numerically studied. Normally the gas fuel supply pressure of the gas turbine sites is over 20+ barg while the combustion chamber pressure is around 12 barg in base load operation and slightly more than atmospheric during start-up. Therefore, the flow control through this high range of pressure ratios is a very difficult and costly task with a single-container control valve. The double-container valve is an innovative design which consists of two parts, SRV (Stop Ratio Valve) followed by GCV (Gas Control Valve), in a compact unit. SRV maintains a significantly low pressure upstream of the GCV during gas turbine firing to establish flame and control fuel flow during acceleration. It opens the GCV to a position where it is much easier to control the flow through the valve. The same situation exists in base load operation when the turbine load is changing. The obtained results prove the special design of the valve to maintain linear characteristics of flow with stroke position in GCV. The results of the mass flow are given for various GCV stroke openings at various valve pressure ratios. Also, the range of pressure ratios for a proper operation of GCV is determined. SRV regulates the middle pressure between the two parts based on rotor speed. Therefore, a sensitive combination of globes position takes place during gas turbine operation.

    Keywords: Double-container, Control valve, Gas turbine, Gas fuel, Numerical analysis, Operatingconditions
  • N. Ahmad, J. Bin*, Z. Qun, S. Abdu Ahmad, H. Fawzy Pages 221-232

    The stability range of the gas turbine engine compressors is being challenged in the modern days due to the intention of increasing per stage maximum loading. Casing treatment has been widely adopted as a realistic passive flow control means to improve the stall margin with a slight decrease of efficiency at the same time by various grooves of which shape (location, angles and so on) has a significant influence on controlling effect. However, the influence of some details in grooves is ignored in most of the case that may impact the specific flow field in the groove. A research on the impact of chamfer and fillet corners on the performance of casing treatment was proceeded by numerical simulation in this paper. The performance of different models of grooves on NASA Rotor 37 was investigated by discretizing 3D RANS based on finite volume method. Firstly, steady simulations were performed on NASA rotor 37 for validation. The CFD results and experimental data for adiabatic efficiency and pressure ratio were in good agreement. According to convergence criteria, the initiation of the stall was predicted. Few numbers of circumferential grooves casing treatment (CGCT) models have been proposed and tested numerically. Rectangular CGCT shape and smooth wall casing performances were analyzed. Moreover, the highest adiabatic efficiencies and stall margins of smooth wall casing, rectangular grooves and rectangular grooves with chamfer and fillet corners shapes models were compared to evaluate the influence of the shape of grooves on the stability and performance on axial flow compressor. The rectangular circumferential casing grooves and rectangular grooves with chamfer and fillet corners enhanced significantly stall margin and an operational range of the transonic axial flow compressor but adiabatic efficiency was slightly decreased.

    Keywords: Numerical simulation, Axial compressor, Circumferential grooves, Adiabatic efficiency, Stallmargin
  • K. Xie*, Y. J. Cui, X. Q. Qiu, J. X. Wang Pages 233-243

    It is a difficult scientific problem of applied fluid mechanics that the flame is too long and does not match the furnace chamber in a small restricted heating space. This paper aims to investigate the effect of the air quantity distribution ratio on the flame height of a flue gas self-circulation burner. In order to obtain a better combustion emission effect and a shorter flame height, a burner head structure with a small flue gas selfcirculation was designed. Numerical simulation was employed to investigate the effect of the different distributions of central air, swirling air and secondary air on flame height. The periodic boundary condition model was adopted and the numerical model was compared and validated by experiment. Correlation analysis was used to determine the influence of the air inlet ratio of each part on the flame height and recirculating flue gas ratio (RFGR). The results show that the influence of different air quantity distributions on flame length is very significant. A reasonable central air ratio is a necessary condition for the good combustion of this flue gas self-circulation burner. Secondary air can effectively increase the RFGR, and flame height was significantly shorter with the increase of RFGR, but when it increased to more than 12%, the flame length was basically no longer shortened. On the premise of stable combustion, when the ratio of central air, swirling air and secondary air are respectively 25%, 35% and 40%, the shortest flame length is achieved. This work reveals an influence mechanism of the flame height of a small burner with a flue gas circulation structure. These results can provide theoretical support and an engineering design basis for the short flame problem in a small restricted space.

    Keywords: Air quantity distribution, Central air, Swirling air, Secondary air, RFGR, Flame height
  • M. Moghanloo, M. Vaghefi*, M. Ghodsian Pages 245-260

    In this paper, the effect of increase collar thickness has been investigated by considering other parameters affecting the flow pattern around the oblong bridge pier located at 90˚ from the 180˚ bend constant. Data acquisition was performed using the Vectrino 3D velocimeter. The leveling of the collar in both tests is constant and 0.4 times the pier diameter above the initial bed surface. The flow conditions were close to the motion threshold and bed balanced. The results showed that with an increase in the thickness of the collar by a factor of 4, the barrier surface is located in the flow path and the flow is deviated to the bed by colliding with the edge of the collar. This flow deviation that was due to the increase the collar thickness, led to 30% increase in the maximum depth of the scour hole upstream of the pier. Also, with the increase in the thickness of the collar, the tendency of the streamlines upstream of the pier to the inner bank increased, resulting in a 20% increase in the maximum sedimentation near the inner bank. The maximum turbulent kinetic energy at 80 degrees of the bend, corresponds with a protected pier with 3 mm thickness, equal to 620 cm2 /s2 .gr, which is reduced by 60% with increasing collar thickness.

    Keywords: 180˚ sharp bend, Bridge pier, Collar, Flow pattern, Collar thickness, Kinetic energy
  • M. Mohammadi Amin*, A. Kiani Pages 261-273

    The stability of aerospace vehicles is one of the concerning subjects for aerospace engineers and researchers and there are different solutions for the stabilization purpose including rigid and flexible stabilizers use. Design and analysis of such systems generally need multi-disciplinary analysis tools and also efficient design strategies. The first goal of this paper is to develop a computational framework for the simulation of bodyfluid-structure interactions (BFSI) due to the oscillations of a flexible stabilizer connected to the end of body. For analyzing fluid-structure interactions, an iterative partitioned coupling algorithm is utilized. With combining a dynamic simulation tool for body motions, the ultimate multidisciplinary framework is arranged. As the second goal of the work, for the sensitivity analysis and also constructing a cost-efficient basis for parametric study, the design of experiments (DOE) methodology is implemented. The proficiency and efficiency of computations is evaluated with the results obtained in a variety stabilizing conditions and various strip characteristics such as length, width, and bending stiffness. The results of different simulations shows that the proposed framework is capable to capture the multi-physic nature of the problem with reasonable cost, especially useful for frequent analyses needed during product design and development loops.

    Keywords: Fluid-structure interaction, Partitioned coupling algorithm, Strip stabilizer, DOE, Reentrycapsule
  • Y. Zhong, X. Dong, Z. Yin, H. Fang* Pages 275-286

    Inkjet technology is an essential tool for precise and quick delivery of liquids in micro-droplets. A key topic of the technology is to deliver the droplets efficiently by designing the nozzle that is related to the droplet speed and the droplet volume in a stable inkjet process. The ejected droplets are usually too small to determine their physical states through onsite measurement. Complex physical phenomena, such as the coupling effects of surface tension, viscous force and inertial force, make it difficult to optimize the nozzle design by experiments alone. In the paper, we adopt computational fluid dynamics to investigate the inkjet process with the orthogonal test method to arrange the studied cases. The computational results firstly have been verified through measuring a simulated case that could be observed in the experiment. Different nozzle structures then have been examined by numerical simulation. It is found that the Laval-shaped nozzle can improve the droplet speed significantly to deliver the droplets fast, and that the curvilinear-triangle-shaped nozzle can minimize the droplet volume to improve the printing accuracy. It is further revealed that a large ink viscosity and surface tension, as well as a low ink density can improve the process stability. Additionally, a parameter combined by the droplet speed, the droplet volume and the stability level is proposed to evaluate the comprehensive performance of the inkjet nozzle.

    Keywords: Inkjet technology, Physical properties, Droplet delivery, Nozzle design
  • S. Kaliappan*, S. Mohanamurugan, P. K. Nagarajan Pages 287-298

    This paper is aimed at a comparative investigation on two different velocity profiles for piston movement namely Sinusoidal and Trapezoidal Profiles for an IC Engine. In conventional IC Engine, velocity profile of piston motion is Sinusoidal. It has many disadvantages such as high mean velocity that leads to high inertial force, frictional losses, wear and high rate of heat leakages. Nearly 20% of the total power produced by the engine is dissipated into heat because of friction. Of this 20%, about 75% is due to friction of piston rings on the cylinder walls. This is an irreversible loss and can be seen as a consequence of high mean piston velocity associated with the existing Sinusoidal Piston Velocity Profile. In addition, varying velocity profile can cause rapid acceleration and finally jerks which lead to considerable mechanical vibration and noise. As a result the mechanical strength of engine material will be high to withstand the inertial force, friction and wear. To overcome these difficulties, an extensive attempt is made to improve the piston movement by restructuring the piston velocity profile with reduced mean velocity which is constant for most of the crank angle. A comprehensive experimental examination is conducted for the Sinusoidal velocity profile, which are utilized in arriving at an optimal CFD procedure through validation study. A proposed connecting rod configuration with internal gear and pinion arrangement is proposed to achieve different Trapezoidal Profiles. The optimum CFD procedure found from validation study is used to analyze and understand the engine with modified Trapezoidal Velocity Profiles. There is almost 20% reduction of mean piston velocity that considerably improves hydro-thermo dynamic and mechanical characteristics of the existing engine.

    Keywords: CFD, Sinusoidal, Trapezoidal, Hydro-thermo dynamic, Mechanical characteristics
  • Ö. Arslan* Pages 299-315

    The main objective of this study is to evaluate the emissions of volatile organic compounds (VOCs) from coal-fired power stations. A quantitative understanding of the chemistry controlling the formation and destruction of these intermediate species is a prerequisite for the realistic modelling of pollutant formation in flames. Therefore, well investigated skeletal reaction mechanism has been built and introduced into perfectly stirred reactor model in order to accomplish prediction of some of these hazardous important intermediate species. This can be great help in considering lack of experimental VOCs data from fossil fuel fired power stations. Predicted results have been validated against ICSTM Furnace data where possible. The performance of the model against the large laboratory scale experimental data has resulted in considerable confidence in the use of this model for a full-scale boiler configuration. But, more confidence will be forthcoming from an increase in the amount of validation data, which is unluckily lacking at the moment. Furthermore, the model can be used in order to provide deeper insight into the formation processes of VOC species emitted from coalfired power stations. However, this can be accomplished far better by including more elementary reactions into the skeletal reaction mechanism.

    Keywords: VOCs, PAHs, Power stations, PSR model, Toxic, Carcinogens
  • H. Koochi, N. Maleki Jirsaraei*, S. Azizi Pages 317-325

    In this paper, we have numerically examined models that are capable of describing free falling regimes of a rigid sphere in a thixotropic fluid as Laponite. By simultaneously solving the dynamical equations governing both sphere and fluid systems, different regimes referred to in the experimental methods, are obtained. Three common behavioral regimes are: i) quickly stopping object, ii) fall with decreasing velocity, and iii) falling with a constant velocity in a steady-state mode. The initial state of the fluids (which is a function of the aging time), the characteristic relaxation time of microstructure's building up under shear, the characteristic thickness of the yielded zone around the sphere, and critical yield stress value besides the diameter and density of the sphere are effective parameters that change the regime.

    Keywords: Free falling, Thixotropy, Aging time, Yield stress, Drag force, 4th Runge-kutta method
  • S. Touzani, A. Cheddadi*, M. T. Ouazzani Pages 327-334

    Natural convection is numerically investigated in a finned horizontal cylindrical annulus filled with air where two isothermal blocks are attached to the inner cylinder in a median position. A bi-vortex flow is observed and the influence of each vortex on heat transfer is analysed. Heat flux and transfer rate calculated per zone (superior, central and inferior) show that heat transfer is more important for the annulus superior zone. Compared to without blocks configuration, blocks presence contributes in the overall heat transfer improvement. This improvement is observed at the inferior zone (inferior vortex effect) and the central one. For the superior zone, despite its importance, heat transfer is deteriorated. This study also shows that the addition of inverted trapezoidal blocks to the medium increases heat transfer, relatively to the without blocks case, by 10.54% to 29.62% depending on Rayleigh number.

    Keywords: Natural convection, Horizontal annulus, Isothermal blocks, Median position, Heat transfer rate
  • S. Babu*, S. Anish Pages 335-347

    Secondary air bled from the compressor which bypasses the combustion chamber is used to seal the turbine components from incoming hot gas. Interaction of this secondary air or purge flow with the mainstream can alter the flow characteristics of turbine blade passage. An in depth analysis of secondary loss generation by purge flow in the presence of upstream disturbances has huge relevance. The objective of present study is to understand the aerodynamic and thermal effects caused by the purge coolant flow in the presence of an upstream wake. A linear turbine cascade is selected for the computational study and a stationary cylindrical rod which resembles the trailing edge of nozzle guide vane is kept 20 mm before the leading edge to generate the upstream wake (or disturbance). Purge flow disturbances includes strong formation of Kelvin-Helmholtz vortices at trailing edge and additional roll-up vortices at leading edge. Detailed analysis is carried out by varying the velocity ratios as well as the ejection flow angle. Higher velocity ratio and perpendicular coolant ejection reduces the mainstream axial momentum which enhances the passage cross flow. Even though the mass averaged total pressure loss is linearly dependent on the velocity ratio, a reduction in the ejection angle brings down the loss coefficient at the blade exit. A lower ejection angle will improve the film cooling effectiveness also. The presence of purge flow causes an increase in the overturning and underturning.

    Keywords: Secondary vortex, Q Criterion, Film cooling effectiveness, Ejection angle, Exit yaw angle
  • H. A. Abdul Wahhab*, M. A. Mashkour, S. A. Madodi Pages 349-355

    The idea of using water-in-Diesel (W/D) emulsion in recent studies as fuel for diesel engines is to reduce the emissions. The introduction of water into a diesel engine using W/D emulsion has a number of potential benefits and can be used as an alternative fuel. One of important factors to use this fuel was the distribution of water droplets in emulsion and emulsifier stability. In the present work, the effect of emulsifier dosage (water in diesel ratio) and heating of W/D emulsion on the stability period with using optical technique was investigated. Five samples of W/D emulsion at different emulsifier dosages (5%, 10%, 15%, 20%, and 25%) water content were studied, whereas the heating of emulsions was carried out for 40oC, 60oC, and 80oC. The results obtained from the current work manifested that an increase in water dosage to W/D emulsion had bad effects on the stability period, also, the increase in heating temperature for W/D emulsion revealed a negative effect on the emulsion stability.

    Keywords: Water-diesel emulsion fuel, Optical technique, Stability of mixtures, Surfactant
  • R. Gayen*, S. Gupta Pages 357-369

    In this paper we analyze the interaction of water waves with a permeable barrier which is slightly perturbed from its vertical position within the framework of linearised water wave theory. The barrier is placed in water of finite depth. Two different kinds of barriers are examined, namely, (I) a partially immersed barrier and (II) a submerged bottom standing barrier. The governing boundary value problem involving the velocity potential function is split into two boundary value problems involving the zeroth order as well as the first order velocity potential functions by using a simplified perturbation technique. The zeroth order reflection and transmission coefficients which are due to a vertical permeable barrier are evaluated by solving a Fredholm integral equation of second kind numerically by using a one term Galerkin approximation. Green’s theorem is applied to evaluate the first order reflection and transmission coefficients. The first order transmission coefficient vanishes irrespective of the shape of the barrier. The numerical values for the first order reflection coefficient are determined by choosing some appropriate shape functions. The numerical results for the zeroth order reflection coefficient which stand for the case of a vertical barrier are validated against the known results for both the permeable and impermeable barriers. The first order reflection curves are also compared by making the porosity constant to be zero with those available in the literature for an impermeable nearly vertical barrier.

    Keywords: Water waves, Permeable barrier, Perturbation technique, Galerkin approximation
  • V. Venkateswarlu, D. Karmakar* Pages 371-385

    The significance of multiple porous structures with finite spacing upon elevated seabed in the presence and absence of the leeward wall is examined under oblique wave impinging. Fluid propagation is assumed over the impermeable elevated bottom, and the fluid realm is separated into open water and porous structure regions. Continuity of the dynamic pressure and mass fluxes at the interfaces of the porous structure and the open water regions are adopted. The resistance and reactance due to the presence of the porous structure are taken into account using the porous structure dispersion relation. The numerical model is developed based on the eigenfunction expansion method along with matched velocity potentials at the interfaces of open water and the porous block regions. The wave reflection and transmission characteristics, energy damping and wave force impact on the leeward wall is analysed. The significance of the porosity, structural width, angle of incidence, width between the two structures and water chamber length is studied considering multiple porous blocks with finite spacing under oblique wave impinging in the presence and absence of leeward wall. The numerical results obtained in the present study agrees well with the theoretical and experimental results available in the literature. The present study illustrates that, with the increase in the number of porous blocks and gap between the porous blocks, the resonating trend is observed in the wave transformation and the influence of the elevated step height is revealed for the wave trapping.

    Keywords: Multiple porous structures, Energy damping, Impermeable elevated bottom, Eigenfunctionexpansion method, Wave transformation