Modares Mechanical Engineering
Year:2014 | Volume:14 | Issue:8|Papers Count:25

Plasma filtration in the renal glomeruli capillary network has been studied by physiologists for many years. Since they can access only to restricted information of plasma filtration, numerical modeling is employed to surmount this restriction. In this study, a three-dimensional CFD modeling of capillary network is presented for the analysis of blood distribution and local plasma filtration in the glomerular network. First, the homogeneous model is studied by solving the twodimensional equations of momentum and species transport. Next, specifications and parameters of capillary network are acquired by studying six previously proposed glomerular networks.Then, adequate network is proposed to investigate the effects of capillary networks on plasma filtration. Finally, three-dimensional momentum and species transport equations are solved for the network model. This model makes it possible to study the effects of various parameters locally on plasma filtration and this is the main advantage of the proposed model. In network model connection between lobules and their shapes just improve flow distribution and mass transfer.Otherwise the glomerular filtration rate in the flow rate of more than 150nl/min is evaluated more accurately in this model.

Single shaft gas turbine and the cycles based on it are sensitive to frequency drops and sudden change loads or large frequency dips might affect their stability. This phenomenon is related to reduction of air mass flow rate through the gas turbine during frequency dips, which might lead to interaction between the governor and temperature control loop. This interaction will prevent the gas turbine from being loaded further and might affect its stability. In this paper, the performance of the two well-known power generation cycles based on the gas turbine -combined cycle and steam injected gas turbine (STIG cycle)- are investigated during frequency dips and transient maneuvers. For this purpose, two similar units are developed based on these cycles and their performance are studied and compared in different scenarios. The simulation results show that the steam injected gas turbine has a better performance during frequency drops and it can handle larger step change loads. This superior performance of the steam injected gas turbine unit is almost twice as good as the similar combined cycle unit in some of the operating conditions.

Bipolar plates are the most important and expensive components used in fuel cells. Metallic bipolar plates are the best choice to replace graphite or machined thick metal plates due to their lightweight and low cost. Selection of suitable forming process is one of the main subjects in fuel cell technology. Nowadays, hydroforming process is commonly used for the production of metallic bipolar plates because of its capability in forming light weight and complex parts. Among the various patterns of bipolar plates, serpentine flow field pattern inevitably brings two main defects of rupture of material during forming process and uneven flow distribution in practical operations. In this research, forming of a slotted interdigitated serpentine pattern on SS304 stainless steel sheet by hydroforming process has been examined using finite element simulation and experimental approach. The effects of process parameters and die geometry on the thickness distribution and filling percent are also studied. It is concluded that by increasing the forming pressure, filling percent of the die increases and the thickness of critical region is more reduced due to the increasing of drawing ratio. Also, it was found that hydroforming process has high repeatability.

Parallel mechanisms are widely being used in industrial applications such as machine tool, metrology, earthquake simulator, fly simulator, medical equipment and etc. These mechanisms have some limitations like having erratic workspace, singular points in the workspace and complexity of control systems. These limitations should be studied for suitable usage of parallel mechanisms. In this article, a four degree of freedom parallel mechanism (three linear and one rotation degrees of freedoms) is proposed as machine tool and being studied and its workspace and singularity analysis are done by solving the kinematic relations and using Matlab software. So, at first the inverse and direct kinematic equations of mechanism were solved and then an algorithm is used to determine the workspace and singular points of proposed parallel mechanism. Finally, to investigate the results of workspace analysis the structure has been modeled in Solidworks software and the inverse kinematic relation and the obtained workspace have been validated using the simulation. At the last, to investigate the quality of robot performance and its dexterity in workspace, global condition index of mechanism using Jacobean matrix is calculated for different orientations of moving platform.

In this article, a mathematical analysis of a CCHP system following electrical demand management and thermal demand management in comparison to separate heat and power system for an office building in Tehran is presented. In the thermal demand management, two modes including with selling electricity to grid and without selling electricity to grid are investigated. In order to have a comprehensive evaluation of the performance of the CCHP system, four criteria including primary energy saving, CO2 emission reduction, operational cost reduction and rate of return are employed for a typical office building in Iran. Also the AHP method is used to specify the best operation mode of the CCHP system when all of the criteria are involved. Results show that from the energy and environmental viewpoints, the CCHP system following thermal demand management with selling electricity to the grid is the best operation mode. But the CCHP system following electrical demand management has the maximum rate of return. For the all of the operation modes, the CCHP system has lower operational cost than SHP system throughout the year. From an integrated view point, the CCHP system following thermal demand management with selling electricity to the grid is the most attractive option.

In this study, the corrugated composite beam is actuated by shape memory alloy wire (SMAw). SMAw was placed on the surface of composite beam. Martensite to austenite transformation occurs by increasing of SMAw temperature. After transformation, SMAw length decrease and beam actuated. Beam displacement, force and current are measured and by A/D board transferred to computer. For evaluation of temperature in SMAw, the Heat transfer differential equation is used. Also Brinson’s model is used for modeling of SMA behavior. The results show that SMA behavior in Brinson’s model is good agreement whit experiments. But in lower temperatures than martensitic transformation state, the SMA stress is equal to zero in experiment unlike Brinson’s model. Also considering the SMA training and DSC test, for some temperatures in the experimental results, the start and end transformation temperatures are different to Brinson’s model. The results show as using SMAw in the corrugated composite, smart structures can be achieved that in corrugation direction is irritable, whereas in Perpendicular to the direction, corrugated composite bending strength is high that lead to using this structure in engineering application.

Numerical analysis and simulation of cavitating flows due to appearance and its application in the maritime industry, water turbomachinery, hydrofoils, underwater vehicles, etc. have specific importance. For this reason in this research, the effect of blowing on hydrodynamic behavior of cavitating flows over hydrofoils has been investigated. Jameson’s finite volume method and power-law preconditioning method with single-phase cavitation model (Barotropic model) have been used to the analyzing of cavitating flow. The stabilization of solution has been achieved with help of the second and fourth-order dissipation term. Explicit four step Runge-Kutta method has been used to achieve the steady state condition. As regards the cavitation often occurs at high Reynolds number, to facilitate the simulation the inviscid flow equations are considered. For apply the blowing from hydrofoil surface, a jet has been placed on hydrofoil’s upper surface. The parameters of jet location, blowing velocity ratio, blowing angle and width of jet are investigated and simulation has been performed for two different cavitation numbers. The numerical results show that the power-law precondition increases the convergence speed significantly. Blowing reduces the cavity length, lift and pressure drag coefficients compared to no blowing case. Also the increase of blowing velocity ratio, blowing angle and width of jet, decrease the cavity length, lift and pressure drag coefficients.

This paper presents an elastic parametric analysis for the purpose of investigating the limit angular speed, displacement and stresses in rotating disks made of functionally graded materials (FGMs) based on Tresca yield criterion. The material properties obey the power law in radial direction. The Poisson’s ratio due to slight variations in engineering materials is assumed constant. For different values of inhomogeneity constant, limit angular speed, displacement and stresses in radial direction are plotted and for the commencement of the plastic flow, different states are investigated. state1: onset of plastic flow at the inner radius, state2: onset of plastic flow at the outer radius, state3: onset of plastic flow as the simultaneously at both radii and state4: onset of plastic flow between the inner and outer radii. To the best of the researchers’ knowledge, so far, in the papers which have been dealing with the investigation of onset yield analysis, the density and yield stress has been assumed constant, however, in this paper by assuming varying density and yield stress in rotating disks made of functionally graded materials and comparing results obtained by fixing these parameters, it has been observed that taking the density as a constant value is wrong and varying it has significant effects on the stresses.

Laser Forming (LF) process is one of the thermal forming processes, which uses laser beam irradiation as a forming factor. In this process, temperature gradient along the sheet thickness produces the final bending angle. So far, various investigations are carried out on laser forming of low carbon steel sheets. However, LF process can be utilised in other metallic and non-metallic sheets. High surface reflectivity and thermal conductivity of aluminium sheets, compared to steel sheets, make them more difficult and more complicated to be laser formed than that of steel sheets. In this Article, using LF process simulation with the finite element software, effects of several process parameters such as laser power, scan speed, laser beam diameter and sheet thickness on final bending angle are investigated. Numerical results are validated with the same parameter assigned experimental results. This comparison shows a very good accordance between simulation and experimental results. Also, an equation is derived to predict the final bending angle correspond to the variations of mentioned parameters. This is derived by the use of Design of Experiment (DOE) and full factorial approach.

Using of airbags is now increasing as a widely accepted safety measure designed to reduce morbidity associated with vehicle accidents. However, in many accidents the airbag, itself, causes the injury of passenger. One of the common injuries causes by air bag is the skin burning. Burns due to airbag deployment could be chemical or thermal. Chemical burns are not studied in this research. There are two mechanisms for thermal burns due to an airbag deployment, contact with the hot airbag itself, contact with the hot expelled gases from the airbag. Present research focus, is on numerical simulation of these two types of burns. A parametric study is done to investigate effect of airbag material, vent port diameter, number of vent ports and distance between driver and airbag. It was found that the most severe burns are due to PA66 and the less severe burns are due to PET. Increasing the vent port diameter and decreasing number of vent ports result burn in less skin depth. There is no clear relation between distance of driver with airbag and burns. Increasing the distance may cause less or more burn.

In this work, governing equations of the feed line, cut-off valve, and the starter system are analyzed mathematically and numerically. In the mathematical solution, the stability of the valve system is considered using the Laplace transform along with the linearization of the equations of the system. According to parameter design of the feed pipe–valve system, the system demonstrates the stable behavior in the effective parameter of the valve system on the basis of the Nyquist and Bode stability criterion. In the numerical solution, the steady state behavior of the cut-off valve is simulated during the cut-command. Then the rate of the pressure variation, mass flow rate through of the valve, gas pressure of the starter system, and the upstream pressure of the valve (water hammer) are considered based on the valve’’s poppet motion. The comparison of the simulation results with the experimental data depicts only 13 percent error in the mass flow rate through of the valve. In the last time of the closing valve, there is no variation in the mass flow rate in the valve due to the excessive loss factor of the valve when the valve approximately is closed. The results show that the closure of the cutoff valve shall be provided in accordance with allowable maximum pressure of the hydraulic shock on the established.

During the past few decades, growing global concern about environmental problems, caused by widespread use of fossil fuels, attracts more research attention toward adsorption systems technology. However, one of the main problems of these systems is the poor heat transfer rate in adsorbent bed due to its low thermal conductivity. In the present study, extended surfaces and metal piece additives are applied to the adsorbent bed in order to numerically investigate the effect of heat transfer enhancement on the adsorption system performance. Employing metal pieces increases effective thermal conductivity of the bed by at least 100%. Results indicate that decreasing fin space and fin height and adding metal pieces to the adsorbent bed reduce the cycle time which finally improves the system specific cooling power. However, it is worth mentioning that the effect of metal piece additives on the cycle time reduction and specific cooling power improvement decreases at smaller fin spaces. Moreover, results show that the increase of fin height improves the coefficient of performance while decreases the specific cooling power of the system. On the contrary, the reduction of fin space simultaneously increases the coefficient of performance and the specific cooling power of the adsorption system.

Nowadays, the baseboard heating systems have attracted the attention of many HVAC engineers because of its uniform temperature distribution and low feed water temperature. Despite this, the uniformity of indoor thermal conditions can be disturbed by some parameters such as exterior walls and air infiltration from window gaps. Therefore, the main goal of this study is to investigate the effects of air infiltration from window gaps on the performance of baseboard system and occupants’ thermal conditions. For this reason, a room has been considered under the terms of “ASHRAE-140 standard/Case 600” and climatic conditions of Tehran with winter outdoor design temperature of -10oC. Also, the heat power on the baseboard panel has been set as much as the average of occupants’ thermal dissatisfaction index stays within the allowable range (lower than 10%). The results show that the heating baseboard system can provide the appropriate thermal conditions for sitting occupants with average panel temperature of 43oC. In spite of this, the distribution of occupants’ dissatisfaction index near the floor is not uniform. The results indicate that the air infiltration can cause to increase the thermal dissatisfaction index up to 40% in the floor region.

Rectangular plates-based resonant micro-sensors utilize the resonance frequency of electrically pre-deformed clamped micro-plates for sensing. Free vibration analysis of such systems in order to find their resonance frequency is the objective of present paper. For this aim, the modified couple stress theory (MCST) together with the Kirchhoff plate model is considered and the size-dependent equation of motion which accounts for the effect of axial residual stresses as well as the non-linear and distributed electrostatic force is derived using the Hamilton’s principle. The lowest frequency of the system as the resonance frequency of these micro-plates is extracted using a single mode Galerkin based reduced order model (ROM). It is found that the fundamental frequency of the system is decreased with an increase of applied voltage and becomes zero when the input voltage reaches the pull-in voltage of the system. The findings of present paper are compared and validated by available results in the literature and an excellent agreement between them is observed. Also it is found that using the MCST in pull-in analysis of clamped rectangular micro-plates can remove the existing gap between the results of classical theory (CT) and available empirical observations. Furthermore, it is observed that accounting for the size-effect on free vibration analysis of electrostatically pre-deformed micro-plates is more essential than flat ones.

The inertia of wave measurement buoys impresses transfer functions on the wave measurement data (i.e. heave, wave slopes with respect to the horizontal axes). This effect causes difference or error between the measured and actual wave data. Calculation or estimation of the buoy transfer functions and affecting the inverse of them, makes it is possible to achieve more accurate wave data. In this paper, an algorithm for estimation of the buoy transfer functions using in-situ wave data is introduced and the simulation results are presented. The effect of the buoy transfer functions on the spectral parameters is also investigated. This algorithm uses the intrinsic properties of the sea waves.

The autofrettaged thick-walled tube containing semi-elliptical crack is investigated. To study the variation of stress intensity factor on crack front, at first, two dimensional weight function is extracted. Stress intensity factor of all points on crack front can be calculate using proposed weight function, also, the complicated loading on crack faces including the loads due to axial gradient pressure in short cylinder and open-end tubes can be considered. Results show that, opposite of the cylinder subjected to uniform pressure, in pipes under gradient pressure, the maximum stress intensity factor are not necessarily on deepest point and surface points. The maximum stress intensity factor occurs on non-surface points in autofrettaged tubes. The results obtained from two dimensional weight function method have a good accuracy with the results obtained from finite element method. Prediction of fatigue crack configuration using two dimensional weight function can be more accurate than those obtained from one dimensional weight function.

In this paper, the immersed boundary-lattice Boltzmann method has been employed to simulate non-Newtonian flow around curve boundaries. The pressure base lattice Boltzmann equations have been used to solve the Eulerian domain to estimate proper pressure gradient in the Poiseuille flow. In addition Immersed boundary method (IBM) utilizes a discrete set of force density is also used to represent the effect of boundary on flow domain. In addition to simulate the real physical dominate problem and study the right effects of non-Newtonian fluid properties, scaling parameters have been introduced to notice the relationship between physical and lattice variables. At First, the capability of present method is examined for simulating the power-law fluid flow around a confined circular cylinder and the results show good agreement with previous study. In the following, the power-law fluid flow around elliptical cylinder in a channel is investigated for three aspect ratios h=1, 1.5, 2 and for 5£Re£40. Results show that, in comparison to aspect ratio, Reynolds number has greater effects on Drag reduction of elliptic cylinder. On the other hand power law index has the least effect on drag reduction.

The aim of this paper is to investigate a new control algorithm of gait rehabilitation robots that simultaneously provides more freedom for the patients and corrects their walking trajectory. The controller utilizes a gait-phase dependent reference trajectory and a gait-phase detection algorithm to determine the desired position and velocity of joints based on of their actual positions and velocities. Moreover, the controller uses two separate control blocks for the correction of the path and the cadence of walking of the patient. Since the reference trajectory is time independent, the patient can change the cadence of his/her walking. Furthermore, the separate control structure enables the controller to provide different levels of freedom and assistive force to be delivered to the patients. The control method has been implemented through ARMan, a gait rehabilitation robot and its effectiveness is evaluated on the walking trajectory of three healthy subjects and one stroke patient. The results of the experiments demonstrate that the proposed control method corrects the gait pattern of the subjects as good as impedance control method. In addition, this method provides more freedom for the patients to walk based on their desired cadence.

This study is investigated vibration analysis of a FG rectangular plate partially contacting with a bounded fluid. Wet dynamic transverse displacement of the plates is approximated by a set of admissible trial functions which is required to satisfy the clamped (CL) and simply supported moveable (SSM) and simply supported immoveable (SSI) geometric boundary conditions. The oscillatory behavior of fluid is obtained by solving the Laplace equation and satisfies the boundary conditions. The natural frequencies and mode shapes of the plate coupled with sloshing fluid modes are calculated by using the Rayleigh–Ritz method based on minimizing the Rayleigh quotient. The proposed method is validated with available data in the literature. In the numerical results, the effects of volume fraction coefficient, thickness ratios and aspect ratios of the FG plates and depth of the fluid, width of the tank, and boundary conditions on the wet natural frequencies are examined and discussed in detail.

With the advancements of numerical upstream and central difference methods in modeling the subsonic and supersonic flows in different paths including the flow inside turbine blades, employing the numerical CUSP technique in the Jameson’s finite volume method can simultaneously benefit from the positive features of both mentioned methods. The novelty of this paper is first, improving Jameson’s finite volume method in modeling a 2D supersonic flow between the blades of a steam turbine using the CUSP method, and second, defining the most optimum control function mode using the Marquardt-Levenberg inverse method and by accounting for the mass conservation equation. By considering the importance of the shock regions in the blade’s surface suction side, the focus of the mentioned method is on this part which results in the significant improvement of the pressure ratio in Jameson’s finite volume method. The results of the first combined method (Jameson and CUSP) at the shock region of the blade’s suction surface desirably agree with the experimental data, and a decrease of numerical errors at this region is resulted. Furthermore, the results of the second combined method (Jameson, CUSP and inverse method) shows that in comparison with original Jameson’s method and the first combined method, by average, the conservation of mass condition is improved 15% at the shock region of the blade’s suction surface.

Conventional modal testing is known as a powerful tool for dynamic analysis of structures. However, for some engineering structures, conventional modal testing is difficult or even impossible to conduct due to the problems associated with the artificial excitation of structure. Operational Modal Analysis (OMA) is one solution to deal with these cases. In OMA the structure is excited by ambient forces and only the responses are measured and taken into account. Accelerometers are the traditional tools for measuring the responses of structure. It is well khonwn that the measured responses are contaminated by bias errors corresponding to the mass-loading effect of accelerometers. This causes the natural frequencies of structure are measured lower than the real values. In this paper a new method is proposed for eliminating the mass-loading effects of accelerometers from measured responses in OMA. A numerical model of a mass-spring-damper system is used for validation of the method. Also a steel plate is used for experimental validation of the proposed approach. The results are confirmed by those of conventional modal testing. Both numerical and experimental results show that the proposed method can effectively eliminate the mass-loading effects of accelerometers from measured responses in OMA. Also, the method has the ability to correct the measured natural frequencies from OMA accurately.

In this investigation, kinked crack path of friction stir Cu-Al7075-T6 alloy welded joints in four-point bending test conditions has been studied as well as the fatigue lifetime of the welded joints, numerically and experimentally. To do so, four-point bending and fatigue tests of welded specimens have been carried out and the experimental fatigue test data and the kinked crack angles in bending tests have been extracted. Maximum Tangential Stress (MTS) and (KII) min criteria have been used for estimating the kinked crack angles, and Paris law has been applied to predict fatigue crack propagation life of the welded specimens. Functionally graded materials concept has been employed for determining mechanical properties of different regions of welded joints. To do so, the mechanical properties of the weld region such as Young’s Modulus and Poisson’s ratio have been considered to be linear functions of the positions of the weld region points. It has been shown that, when the original notch is close to the material with the higher fracture toughness (Copper), the kinked crack angle becomes smaller. The results show good agreement between the experimental data and numerical estimations.

Wear phenomena in wheel-rail contact for railway vehicle is very important parameter. For this purpose, simulation and dynamic analyze of new and worn wheel profiles are done. Then wagon’s derailment factor along curved track is determined. Analytic solution is achieved by using Hertz contact theory and Kalker linear theory. Also, simulation and analysis is done in ADAMS/Rail software and for different wheel profiles and derailment factor is determining using derailment criteria. Results showing that derailment factor are low initially but vertical forces decreases and centrifugal force increases lateral forces and consequently derailment factor increases along wagon entering curved track and worn profiles have more tendencies exposed to derailment. However, permitted wear limit must be defined. In many cases, worn profiles havefewer tendencies to derailment. Using this method can determine wear limit of wheel profile to maintenance and re-profileoperation. It is revealed that the curve length does not affect on derailment factor. Also, damper coefficient does not affect on mean derailment factor But it is much more turbulence. Perturbation in curve beginning is considerable and at the end of the curve, it restores to initial value.