مجله مهندسی مکانیک امیرکبیر
سال پنجاه و یکم شماره 6 (بهمن و اسفند 1398)

In this paper, a two-dimensional numerical simulation is applied to study the Vortex-Induced Vibrations (VIV) of an elastically mounted rigid circular cylinder beneath a free surface of fluid. The effect of free surface in laminar flow (60 < Re < 130) with Fr=0.2 is investigated with considering two Gap-Ratios of 2.5, 1.5. The natural structural frequency of oscillator is assumed to match the vortex shedding frequency for a stationary cylinder at Re=100. Discretization of flow equations based on the Finite Volume method was implemented in CFD commercial software Ansys Fluent 14.0. Simulations of VIV and Free Surface of fluid flow have separately shown good agreement with previous results. This paper is the second part of an investigation about effects of Free Surface of fluid on VIV phenomena. The effects of Free Surface is investigated with using a comparison of vortex shedding modes and non-dimensional frequency diagrams for the two gap-ratios. With approaching cylinder to free surface, results shows changing type of vortex shedding modes, abatement in lock-in region, increasing Strouhal number and non-dimensional frequency ratio.

Today hydraulic operating system technology is wide spread into all sectors of manufacturing, agriculture, aerospace and marine industries where accuracy, precision and flexibility of applying force is very important. So due to the growing usefulness of hydraulic cylinders with precise and controlled operation in various industries, the need for control of piston speed is necessary. Therefore in this research, design, fabrication and evaluation of 5 different hydraulic cushions have been considered in order to optimize the stopping mechanism of pistons at the end of the course. In this article the comparison of 5 cushion spears including Circular, Conical, Sagittal, Double conical and Parabolic cushion have been studied with reviewing the motion behavior of piston and measuring the parameters such as displacement, speed, acceleration, flow rate and hydraulic pressure in an one way hydraulic cylinder. Results showed that the sagittal cushion with maximum pressure increasing of 1.98% for 200 kg load and 0.35% for 350 kg load had the lowest percentage of hydraulic pressure rise and circular cushion with maximum pressure increasing of 11.98% for 200 kg load and 3.92% for 350 kg load had highest percentage of hydraulic pressure rise.

In this study, the hydrodynamic behavior of fluid jet with/without application of electric field was studied and the effects of electric field intensity and direction on instability and breakup of jet were investigated experimentally. Results show that the characteristics of free falling jets depend on Reynolds number. Furthermore, jet mean diameter and its breakup length in this type of jets are directly related to Reynolds number. Electric field increases the jet mean diameter in the downstream region of electrodes and decreases the breakup length. Increase in electric field intensity shortens the breakup length and direct field is more effective on this area in comparison with reverse one. Applying a 6 kV electric potential difference, the upper jet breakup length can be reduced by 27% in comparison with neutral case. The standard deviation of produced droplets’ size for neutral case and reverse electric field, generated by 2kV, are equal to 1.3 and 1.1 respectively which indicates a more uniform droplets’ generation in the presence of reverse electric field. Considering the roundness of various generated droplets, it can be concluded that applying the electric field enhances the production of round droplets and reduces the number of irregular droplets.

The flow characteristics of water jets issuing from rectangular and elliptical injectors into quiescent air were experimentally investigated. Injectors were of the same cross-sectional area and a circular injector was also employed as the reference case. Digital images taken by a diffused backlight technique were processed to extract the main characteristics of the jet column at different jet velocities. The measurements were carried out for mass flow rates varying from 2 L/h to 120 L/h with small enough steps at low speeds to capture Rayleigh regime. Aside from the qualitative description of the jet flows, stability curve was plotted to make a comparison between different jets. The comparison revealed that the ellipse jet is the first one to reach the critical Weber number, while the circular jet remains laminar at higher velocities than the other two jets. Moreover, axis-switching phenomenon was carefully studied as the common characteristic of rectangular and elliptical jets. The wavelength and maximum amplitude of axis-switching were measured at different flow conditions and the results were compared. Though the axis-switching wavelength of both jets demonstrated a linear increment with Weber number, the rectangular jet was found to increase with a higher slope.

The management of consumption the reactive gas in PEMFC is classified into three types: open-end, recirculation and dead-end. In dead-end mode, reactant gasses due to accumulating of water and inert gas should be purged alternatively. In this paper a PEMFC stack with transparent end plates and a unique design for investigation of water management is designed, manufactured and fabricated. In this paper, for the first time, the discussion of water management in a dead-end anode and cathode PEMFC stack with details of form and remove of water has been investigated. The results have shown that at the current density lower than 200 mA/cm2 the produced water is in the form of separate droplets and there is no film flow and slug flow of water in the channel. Also, as expected, the accumulation of droplets and film flow in the lower half was more than the upper half and therefore the reduction of the number of channels to increase gas speed and effective water removal in this part was essential. The results have shown that for steady-state operation, the maximum time possible for closing the output valves is 5 seconds and the minimum time required to open it is 5 seconds.

An important types of density currents is called turbidity currents, where the density difference is due to the presence of suspended solid particles. In this paper, 3D numerical simulation of lock-exchange particle laden density current is presented in order to investigate the hydrodynamic of the current in density stratified environment. Simulations are carried out using Large Eddy Simulation method, three-dimensional Box filter, dynamic Smagorinsky method and Van driest damping function. The obtained results in stratified case are in good agreement with experimental data. Also, the presence of stratified environment significantly reduces the current velocity, but does not have any significant effect on the sedimentation pattern. In addition, the results show that increasing the slope leads to increase in sedimentation. It was also observed that increasing the particle diameter reduces the current momentum and increased sedimentation. For more accurate representation of the particle interaction, the particle settling velocity also varies with concentration. The analysis show that assuming the variable settling velocity in the early stages of the current leads to insignificant change in the front velocity, but when the flow advances more, the faster front velocity will be predicted.

Requirement of reducing undesirable noise from airplanes, wind turbines blades and fans resulted in the identification of noise sources of such equipment in many research works. Turbulent boundary layer trailing edge (T.E.) noise is one of the main sources of aerodynamic noise and extensive studies have been conducted on it during the past decades. Previous studies have shown that surface pressure spectra, the spanwise length scale of the SPFs and eddy convection velocity in the T.E. region are crucial quantities in determination of the far-field trailing edge noise. In the present study, for measuring these parameters and hence predicting the far-field trailing edge noise, a flat-plate model is designed and built. The flat plate is equipped with several streamwise and spanwise miniature condenser microphones, FG-23329-P07, acting as surface pressure transducers. All the experiments were carried out in a subsonic wind tunnel for three free-stream velocities: 10, 15 and 20 m/s. The spanwise length scale and eddy convection velocity are calculated by simultaneously measuring of unsteady surface pressure respectively in both streamwise and spanwise directions and using cross spectral density. The experimental results, including the surface pressure spectra, longitudinal and lateral coherence and eddy convection velocity provide many information about the flow field structure in the turbulent boundary layer. The results also show that the best collapses in the surface pressure spectra at low frequency and middle to high frequencies can be obtained by using outer and inner layer scales respectively. Furthermore, the longitudinal and lateral coherences can provide adequate information about the lifespan (or, inversely, the decay) of the eddies and their physical size. Finally, the trailing edge noise from flat plate has been predicted by using the analytical Amit-Roger model and results show the effectiveness of this model for prediction of far-field turbulent boundary layer trailing edge noise.

The motion and adhesion of cancer cells in a blood vessel during metastasis is a complex mechanism that occurs in the human body. In this analysis, a two-dimensional model of the movement of cancer cells has been developed that is solved in two different modes in a straight line in a blood vessel. These two modes are related to presence and absence of adhesion between cancer cell and blood vessel wall in the presence of adhesion between cancer cell and white blood cell. The analysis is performed using FEM and FSI equations. For FEM analysis, it is assumed that the properties of blood and cells are homogeneous and fluid is incompressible and Newtonian. Cancer cell is modeled as a rigid body and white blood cell is assumed as linear elastic. The model is time-dependent with 10-5 time step and a network of triangular elements is used. The analysis shows that the influence of adhesion between the cell and the vessel wall is more important from cell-cell adhesion. Through Consideration and scrutiny in the adhesion charts along with medical issues such as drug delivery to cancer patients has a considerable impact in the treatment or prevention of cancer metastasis.

In this study, Lattice Boltzmann method (LBM) is used to investigate liquid water transport in a carbon paper gas diffusion layer (GDL) and gas channel (GC) of polymer electrolyte membrane fuel cells (PEMFC). The carbon paper GDL is reconstructed using the stochastic method. This work is done in a microscopic porous structure and behavior of liquid water clusters from catalyst layer / GDL interface to gas channel is studied. In this study, the dynamic behavior of liquid water during removal from gas diffusion layer (GDL) of a PEMFC electrode is simulated using LBM. The effects of GDL wettability on the removal process and liquid water distribution are investigated. In addition, liquid water dynamic behaviors and liquid water saturation within the GDL in two case of steady and transient are explored. This study focuses on the effects of surface wettability on the number of effective clusters, merging of clusters, and the required time for reaching the steady-state water distribution. The simulation results show that by increasing the contact angle of fibers in GDL, the required time to obtain a steady-state water distribution is reduced. Therefore, if the solid surface becomes more hydrophobic, water management will be improved in the gas diffusion layer.

One of the most important challenging subjects for scientists is the numerical simulation of the transport phenomena in heterogeneous media. The discontinuity in the properties causes computational errors leading to incorrect estimation of the exact values. Various methods were rendered to reduce the error. Most of these methods are based on the remeshing of the elements to align with discontinuities increasing the calculation cost significantly. One of the powerful methods introduced recently is extended finite element method (XFEM). XFEM is based on the modification of the interpolation functions to capture discontinuities throughout the domain. Using this method in fluid mechanic was merely addressed but recently the approach has been changed. So, in the present study, attempts were made to study the affecting factors on the deformation of Newtonian/Newtonian and non-Newtonian/Newtonian systems. The obtained results showed a good agreement with the experimental results by other scientists. Because of pressure field modification, studying droplet deformation becomes possible, even when rheological properties of component like viscosity ratio and interfacial tension are very different. The obtained results indicated that by fixing all rheological parameters, there will be an inverse relationship between the final droplet size and viscosity ratio of components.

simulation of blood flow in bypass grafts can help medical evaluation. Numerical simulation of blood flow in Configurations recommended by the surgeon Such as the configurations of Y is the aim of this study in order to predict hemodynamic parameters of this configuration in a patient with double stenosis 65 and 50 percent is examined at rest and during exercise. The computational domain was created from CT images from the human cardiac. In this study, blood is assumed homogeneous, non-Newtonian and pulsatile. For real modeling of flow and blood pressure, lumped model is used in outlet at rest and exercise states.The results indicate using this configuration is compensated the pressure drop and flow and time average wall shear stress has reduced in stenosis region and oscillatory shear index and relative residence time range has reduced in area pre and post-stenosis.Y bypass grafting investigation indicates time average wall shear is low at the bifurcation graft and There is possibility of creating restenosis in these areas, but These parameters are in the ideal range at the exercise state.

In present paper is to study numerical simulation by using Finite volume method for Newtonian and non-Newtonian fluid flow inside corrugated tube equipped with typical twisted tape (TT) and V-cut twisted tape (VTT) at constant heat flux. For validation of this simulation, this results compared with empirical correlations of researchers. In this analysis water was as Newtonian fluid and 0.2 wt % carboxymethyle cellulse (CMC) in water was as a non-Newtonian fluid, the range of Reynolds number for Newtonian and non-Newtonian fluid varied (5300 to 25700) and (2400 to 6800) respectively. In this analysis, the effects of using different turbulence models, variable heat flux and creating V-cut on Nusselt number and friction factor is investigated. The obtained results showed that standard κ -ω model of turbulence for Newtonian fluid is a proper model rather than the other models and it had good agreements between experimental data, and the average differences for Nusselt number in typical and V-cut twisted tapes were less than 15.2% and 14.4% respectively. On other hand, in identical condition for non-Newtonian fluid, using of standard κ - ε model of turbulence is a proper model and the average differences on Nusselt number for typical twisted tape were less than 18%.

The stresses induced in the rubber by pressure loss and unbalanced velocities at exit is the main factor that affect the swelling of rubber cross-section after exiting the die. In this research, the effect of velocity distribution at the die exit on the rubber dimensions is experimentally and numerically studied with the aid of finite volume method. Three-dimensional simulation of non-Newtonian high-viscosity flow was performed to predict the distribution of velocity and pressure in the die channels. This can eliminates the experimental trial and error procedure for modifying the shape of channels to achieve a uniform velocity distribution and reduction of pressure loss. According to the importance of recognition of the soft and hard materials boundaries in multi-material cross-sections, the two-phase VOF method is employed. A comparison between primary and modified dies shows more precise dimensions of the modified die. The results show that in the narrow portions of the profile in the vicinity of wide regions, because of the impossibility of achieving a uniform velocity distribution, the produced cross-section is smaller than the design value. By the employed numerical method reduces the pressure loss in the modified die by 40% in comparison with that of the primary designed die.

A new approach using the genetic algorithms has been presented to estimate the uncertainties in numerical pressure calculation on a 3D wing. The amount of error in this method has been estimated in the form of power series as a function of the element size. The error tensor is expressed as the sum of squares and has been used as the fitness function in the genetic algorithm. The conventional method for error minimization has been differentiation which is replaced by the genetic algorithm in this paper. The error analysis along with a safety factor has been introduced as the uncertainties in numerical calculations. According to the results, refining the grids down to 25% of the initial size, reduced the error by an amount of 50%. The total uncertainty calculated in this paper was 0.03. This value determines a confidence level of 97.6%. The reliability of the results on three baselines higher than 97% approves the high accuracy of the present calculations. The highest and the lowest reliability in the present calculations was 99.16% and 97.6%, respectively.

In this work, injector regulating valve and pelton turbine impeller has been numerically and analytically designed and simulated. The impeller of the pelton turbine added on the shaft of a high pressure multistage pump which is used in sea water RO package to recover a part of input power from rejected flow return back after filter unit. Using ANSYS CFX, flow through the regulating valve for many outlet injector diameters has been numerically simulated to obtain head loss. For the point of operation, dimension of turbine impeller calculated using turbomachinery relations and some experimental data in order to synchronize as much as possible with the pump. The exact point of operation for the pump, turbine and injector obtained by intersecting performance curves of pump and turbine. In order to investigate the results, the full-scale pelton turbine and regulating valve manufactured with the material of duplex and installed on the pump. Performance test on the site shown about 26% decrease in input power. Because of the affinity relation for turbomachinery, the results can be validated for other point of operation due to change in pump speed.

Improvement of an airplane’s wing efficiency is one of the recent dominant steps toward the reduction of drag force and optimization of airflow and airborne structures interaction, as it has a functions similar to bird’s wings. In other words, these wings, in contrary with rigid ones which are currently being used, have the flexibility and consistency in various flight conditions.In this sudy, Numerical simulation of transonic flow around chordwise morphing airfoil have been accessed. Fluid-Structure interaction for analyzing flow field behavior in conjunction with deformable airfoil is used. In this regard, a two-dimensional finite element model is established and the arbitrary Lagrangian–Eulerian formulation (ALE), in flow field and structure configuration is applied to accommodate the deforming boundaries. The procedure incorporates one-equation Spalart-Allmaras turbulence model. The preferable Mach number for transonic regime is 0.7. Chordwise elastic deformability by uniformly varying extended load on both leading and trailing edge is considered for deformation purposes. The model is validated against conventional rigid airfoil for different angles of attack and the comparisons show considerable improvement in aerodynamic characteristics and prove the efficiency of elastic morphing airfoil. All the simulations are carried out by COMSOL Multiphysics software.

In this research, water droplet impact process on a solid surface is simulated using a sharp approach for interface modeling. This approach is based on the solving momentum and continuity equations and imposing appropriate jump conditions at the interface. The level set method is used for interface tracking and the ghost fluid method is used to impose jump conditions at the interface accurately. In this way, smearing of quantities across interface is prevented and discontinuities are preserved at interface. The accuracy of numerical procedure is approved via comparison of simulation results with experimental and numerical data. Simulation results show that the used numerical method in comparison with the VOF method, represents more accurate prediction of droplet behavior during impact process. The effect of contact angle between water droplet and surface on the impact process is investigated. For contact angles less than 90°, water droplet spreads on the surface after impact. But, for contact angles greater than 90°, droplet starts to recoil after spreading. In this case, it is possible that droplet rebound from surface after recoiling. Maximum spreading radius of droplet decreases by an increase in contact angle.

The purpose of this research is to investigate the performance of hybrid Darrieus-Savonius wind turbines to achieve a model with high starting torque and suitable performance range. Straight-Bladed Darrieus wind turbines have high-amplitude fluctuations in torque and, at some angles, this torque is not enough to start the turbine motion. The hybrid turbine is compared with two equivalent models of straight-bladed Darrieus wind turbines. The first model has equal available power and the second model has equal height with the hybrid turbine. 3D simulation is performed using CFD and solving Reynolds averaged Navier-Stokes equations with finite volume method, using turbulence model and rotating mesh for rotation of the turbine. According to the results, the hybrid turbine in the starting mode has 22.24% and 17.5% lower standard deviation and 69.8% and 56.9% higher average torque, respectively, compared to the first and second equivalent turbines. In operational mode, the hybrid turbine at the rotational speed of 30 RPM has 16.1% and 27.3% lower standard deviation and 19.1% and 1.03% higher average torque, respectively. Therefore, the hybrid turbine at the start of the motion, as well as at low rotational speeds, has higher average torque and less fluctuations compared to equivalent Darrieus turbines.

In this study, numerical simulation of a pulse-jet filter cleaning system is conducted and its performance is investigated under pre-defined conditions. In the first step, 3D simulation of the system from high pressure tank to filter inlet is performed and the output is used as the input of second step. In second step, simulation is performed for filters with inlet mass flow rates calculated from the previous step. To validate the results, the results are compared to experimental data showing acceptable agreement. The results show that regardless of valve type, cleaning pulse generates after 0.5s and suddenly decreases afterward. No shock or choking is observed in the system. Another interesting result is induced flow generated after the nozzles, which increases filter inlet mass flow rate. In addition, axial distribution of filter outlet flow rate is not uniform, degrading from inlet to outlet. Finally, a complete parametric study is performed to investigate the effect of the tank pressure on the pressure difference in the filter which is an important index in cleaning performance analysis.

Turbocharger systems, can increase volumetric efficiency and decrease fuel consumption and emissions of an engine due to compressing the entering air to the engine. Flow characteristic inside turbine is sophisticated and several phenomena like flow separation and high turbulent flow can occur inside turbine. Determining exact performance behavior of turbine can alter the matching process of the turbocharger with the engine. The main goal of this research is 3D and steady simulation of the flow inside turbocharger’s turbine and analysis of the performance behavior of turbine under different working conditions. To this end, 3D flow inside turbine including volute, rotor, diffuser and wastegate passage is investigated steadily. Validating simulation results by experimental results shows that there exist 3 to 9 percent of error between these. In order to control the rotational speed of turbine, turbocharger is equipped with wastegate. By measuring the exact amount of wastegate opening in different working conditions of turbine on a test cell, flow simulation inside turbine is accomplished in different wastegate openings and the effect of wastegate opening on the turbine performance and isentropic efficiency is investigated. The results shows that opening of wastegate can reduce isentropic efficiency and power produced by the turbine significantly.