نشریه مهندسی مکانیک مدرس
سال چهاردهم شماره 5 (امرداد 1393)

Ventilation is essential to provide a smoke-free path for safe passenger evacuation and effective rescue services in case of a tunnel fire. The critical ventilation velocity، VC، is defined as the minimum velocity which creates this safe passage by preventing smoke from spreading upstream in tunnels. This research discusses smoke flow control in tunnels with a focus on the important parameters affecting critical velocity. Critical velocity values are evaluated for different heat release rates and results show good compliance with model-scale experiments. The study is extended to investigate effects of fire source blockage ratio and lateral location، tunnel slope and ventilation air relative humidity on the behavior of critical velocity. Results show a drop in VC about equal to blockage ratio occurs in presence of fire source blockage. Investigation of critical velocity in sloped tunnels illustrates that for each %1 increment in negative slope، 2. 5% higher ventilation is required. Results also show that air relative humidity does not have significant effect. However، fire lateral location impacts critical ventilation velocity in such a way that about 10-20% higher airflow is necessary to suppress smoke in a near-wall fire in comparison with a middle-tunnel fire.

Forasmuch as، transition metal dichalcogenides (TMDs) are robust nanomaterials to sustain large strains without fracture، their application in new pliable electronic nanodevices is so appealing. Of these nanomaterials is tungsten disulfide which has specific electrical، optical and sensor properties; and due to possessing a non-planar structure، shows interesting responses under different plane strains. This investigation explores the mechanical properties of a monolayer tungsten disulfide (WS2) such as Young''s modulus، bulk modulus، shear modulus and Poisson''s ratio by applying the density functional theory (DFT) calculations based on the generalized gradient approximation (GGA). The results demonstrate that elastic properties of WS2 are less than those of graphene and its analogous inorganic hexagonal boron-nitride (h-BN) nanosheets. Unlikely، Poisson''s ratio is calculated higher than that of graphene and h-BN nanosheets. It is observed that، due to the special structure of WS2، the thickness of nanosheet (distance between S-S atoms)، bond length of W-S and the angle S-W-S change under different kinds of strains. Also، in the case of biaxial strain، the amount of variations in bond length، thickness and bending angle is higher than that in the cases of uniaxial and shear strain.

In this study hydrodynamics and noise behavior of a marine propeller is analyzed through numerical and experimental methods. In order to find out the conditions of initiation and development of cavitation، numerical analysis is carried out through finite volume method (FVM) for various rotational velocities. Moreover، hydrodynamics of the propeller is tested in the cavitation tunnel and the results are compared against numerical simulations. Second، the flow results obtained in the first step were used as the input to extract the sound pressure levels (SPLs) in the Ffowcs Williams–Hawkings (FW-H) formulation، to predict the far field noise. In addition، the behavior of the obtained SPL was studied and a good agreement was observed between our data and the previous works results. Similarly، experimental results collected from two hydrophones are compared with numerical simulations. In this case، cavitation is initiated and developed by either increasing the propeller’s rotational velocity in fixed pressure or dropping pressure while keeping the velocity constant. The signals registered at the two hydrophones are then filtered within one-third octave bands. The outcomes demonstrate a negligible deviation between numerical and experimental results for both noise and hydrodynamics tests.

Steel pipes with different cracks may be unsafe if the geometry of crack is larger than a specific size. The failure assessment diagrams are widely used to investigate the safety of such cracked pipes. These diagrams include three levels، in each of which the area under the failure assessment curve is the safe area، where as the outer are is unsafe. In this study، the API X65 gas transmission pipe with 48 inches outside diameter (commonly used in Iran''s energy transmission lines) is investigated under constant internal pressure of 7 MPa for the first time in Iran. In order to analyze the critical crack length، a pipe with fixed crack depth of 8 millimeter and crack lengths of 100، 130، 160، 190، 220 and 250 millimeter is considered. Similarly، for analyzing the critical crack depth، a pipe with fixed crack length of 150 millimeter and crack depths of 3، 5، 7، 8، 9 and 10 millimeter is assumed. Using ABAQUS modeling and standard BS7910 equations، the location of loading points is investigated in failure assessment diagram. The results indicated that، in the case of fixed crack depth (8 millimeter)، the critical crack length was 220 millimeter. For the case of fixed crack length (220 milimeter)، a critical crack depth of 9 millimeter was obtained.

Water is needed to providing proper hydration of membrane and its ionic conductivity in polymer electrolyte membrane fuel cells، but excess water accumulation known as flooding phenomenon decreases the passing way of reactants in the GDL and reaction sites on catalyst layer and increases mass transport loss and leads to performance loss of polymer electrolyte membrane fuel cells. In the present work، the two-phase flow in the cathode channel of transparent polymer electrolyte membrane fuel cell with single serpentine flow field is visualized by direct optical imaging in unsteady and time averaged states. Then the water coverage length ratio and the average of water coverage length ratio are derived as a scale of water content of the cathode channel in the unsteady and time averaged states. In the unsteady and time averaged states، by increase the stoichiometry، decrease the relative humidity and inlet gases temperature in anode and cathode sides، the accumulated liquid water in the channels reduces. The effect of anode stoichiometry on the amount of water in the cathode channel in the unsteady and time averaged states is more than the cathode stoichiometry.

This paper investigates the multi-objective optimization design of planar cable-driven parallel robots by using the evolutionary optimization algorithm. Since in cable-driven parallel robots، the cables should remain in tension in all configurations، the extent of the controllable workspace is considered as one of the design indices. This objective function is of utmost importance to the design of cable-driven parallel robots، since it considers the unidirectional properties of the cables in the analysis. In addition، in order for the robot to have suitable dexterity and accuracy and to be able to manipulate any arbitrary task in all the required directions، various kinematic indices such as global condition number، translational and rotational kinematic sensitivity indices are used. Through analysis of the conflict of these objectives within the workspace of the robot، it is shown that use of multi-objective optimization is an effective method to reach to a suitable trade-off. Furthermore، by applying multi-objective optimization methods such as the non-sorting genetic algorithm and the adaptive weighted particle swarm optimization algorithm، the optimal pareto front for the design parameters for the cable robot is obtained such that to draw a compromise between the robot designs.

In practice، clearances in the joints are inevitable due to tolerances and defects arising from design and manufacturing. It is well-known that in the presence of clearance at one joint، the linkage gains an additional uncontrollable degree of freedom which is the source of error. Moreover، these joints undergo wear and backlash، resulting in poor accuracy and repeatability، and so cannot be used in precision mechanisms. The model of continuous contact is used for clearance model. Therefore، joint clearance was modeled as a small link with the length equals to the clearance. In this paper، a method is proposed to alleviate the undesirable effects of joints clearances. This improvement implemented using compliant joints. The compliances are introduced locally in the form of flexural hinges at the two oscillating revolute joints of a four-bar crank-rocker linkage. pseudo-rigid body model is used for modeling the flexures. An optimum design of the flexural hinge for minimum objective function is deduced and used to modify the conventional mechanism. The PSO method is used for optimization process. An example is included in which we show that introduction of compliance will modify the dynamics performances of the linkage.

In tube hydroforming process، due to friction condition، uniform wall thickness، as well as sharp corners may not be achieved. Use of ultrasonic vibration can improve the contact conditions at the tube-die interface. The current work studies the effect of applying ultrasonic vibration on wall thickness and corner radius of hydroformed tubes. By comparing the FEM models of tubes in two cases of with vibration and without vibration، it is possible to investigate the effects of vibration on process. In addition، the finite element analysis، as the modal and harmonic analysis، are used to design a set of ultrasonic tube hydroforming. A simple mechanism introduced in this paper is used to form the tube. The results indicate superimposing ultrasonic vibrations to the process will improve formability and spring back of the، and more uniform tube wall thickness will be achieved.

In this paper، elastic-plastic deformation of a rotating hollow FGM cylinder is analytically studied based on small strain theory and for plane-strain state. Variation of elasticity modulus، density and yield stress are assumed to obey power-law functions of radial coordinate. Material was assumed to obey Tresca yield criterion and its associated flow rule. To evaluate and validate the presented analysis، numerical results were compared with previously published results for homogeneous and also FGM cylinder with constant density and yield stress، as two special cases. Then the effect of density and yield stress variation، which was not considered in the previous researches، was investigated on the elastic-plastic deformation of the FGM rotating cylinder. The results show that when the variation of density and yield stress is ignored، considerable differences may arise not only in the magnitude of computed radial displacement and stress and strain components، but also in predicting the pattern of yield initiation and also plastic zone development.

A two dimensional numerical study is presented for steady state performance analysis of a catalytic radiant counter-diffusive burner. In these burners، the gaseous fuel enters from the rear of the burner and passes through the insulation and catalyst layers. The oxygen enters the catalyst layer from the burner surface and opposite to the fuel path. The reaction takes place over the catalyst layer. In this paper، the momentum، energy and species conservation equations in porous and non-porous media are solved using the finite element method in the COMSOL software. The simulations are based on proposed corrections on boundary conditions and combustion rate of methane equation. The simulation results compared with experimental measurements published in the literature for the same geometry and conditions which shows a considerable (10%) improvements. It is shown that diffusion of oxygen through the pad limits the catalytic combustion and controls the fuel conversion in the burner.

Multiple flaws are frequently occurred in actual components، such as pressure vessels and power plants. These flaws will in some circumstances lead to more severe effects than single flaw alone. Assessment of the interaction behaviour is based on an evaluation of the alignment and combination of these multiple flaws. In the current standards، multiple cracks are treated as an equivalent single crack if the distance between two cracks satisfies a prescribed criterion. First، this study introduces the current alignment and combination rules for through cracks. Following، to investigate the effects of the interaction of cracks، brittle fracture of a plate containig two adjacent cracks is simulated. The effect of cracks distances and crack lengths on stress intensity factors is evaluated. Also، crack growth analysis is simulated based on linear elastic fracture mechanics approach. The extended finite element method has been utilized to model the problem. This method enables the domain to be modeled by finite elements without explicitly meshing the crack surfaces، and hence crack propagation simulations can be carried out without remeshing. Based on the results، a new alignment and combination rule is proposed.

Precise ascertainment of the diffusion coefficient as the index of penetration of one species through another is essential for accurate physical modeling. The most precise and careful method for computation of these properties is Molecular Dynamics (MD). Unfortunately these methods have huge computational cost. The aim of this article is to introduce a suitable mesoscopic method and an efficient algorithm for calculating diffusion coefficient using this method. In this study we use Dissipative Particle Dynamics (DPD) for calculating diffusion coefficient of the water and lipid through the biological bilayer membranes. Eventually we validate our results with MD simulation and also experimental results. Then we can conclude about this method that besides being efficient from point of computational cost، the proposed algorithm is an accurate method for calculating diffusion coefficient. Therefore using this method، we are able to study the equilibrium behavior of bilayer membranes like calculating diffusion coefficient which with Molecular Dynamics simulations، it maybe takes more than hundreds of times.

In this paper، the natural convection of water-Al2O3 nanofluid in a square enclosure exposed to a magnetic field is numerically investigated. The enclosure is bounded by two isothermal vertical walls at different temperaturesof Th and Tc. The two horizontals walls of the enclosure are thermally insulated. A vertical plate (membrane separator) with a negligible thickness and a variable height is located in the middle of the chamber. Discretization of the governing equations are achived through a finit method and are solved using the SIMPLE algorithm. Based on the results of the numerical solution، the effects of the relevant parameters such as the dimensionless height of the membrane separator، Rayleigh number، the solid volume fraction and the Hartmann number on the flow field and the heat transfer rate are investigated. The results show that the heat transfer rate decreases with an increase of the dimensionless height of the membrane separator and an increase of the Hartmann number. The heat transfer rate، however، increases as the Rayleigh number increases. Depending on the Rayleigh number، the thermal performance of the enclosure is either improved or deteriorated as the solid volume fraction is increased.

In this paper، multi-objective Genetic Algorithm has been employed to find the communications satellite optimal transfer trajectory from geosynchronous transfer orbit (GTO) to geosynchronous orbit (GEO) as the destination operational zone. Because of satellite high specific impulse، Orbital maneuvers are considered as impulsive maneuvers in this paper and its validation is verified by comparison of results with continues one. Number of intermediate orbits has been found in order to physical constraints. In this systematic approach، both minimum-time and fuel-saving using constant acceleration simultaneously as the problem strategy to find the optimal transfer trajectory between two orbits. Set of optimal trajectories are plotted in Pareto Front and transfer trajectory can be selected from these points. Finally، the results are compared with the results obtained using STK software and good agreement has been revealed. The satellite access to ground station (GS) is analyzed and optimal position for GS is found by defining lack of access and mass consumption as objective functions. Finally، disturbance torques induced by liquid apogee motor (LAM) is investigated and thrusters'' fuel mass for this disturbance torque control is proposed.

In this study، the theoretical design of a vapor ejector used in an air-conditioning system is performed and the designed ejector is then optimized via computational fluid dynamics. Based on the numerical simulations، two geometrical parameters، throat diameter and nozzle position، are optimized. Then، the effects of the operating parameters on the performance of the optimized ejector are investigated numerically. The optimized ejector geometry is used as a variable-geometry ejector by using a spindle in the primary throat and the performance of the system in various spindle positions is studied. The results show the importance of using a analytical design to obtain the overall geometry of the ejector and numerical simulation in order to achieve the optimal ejector performance. The variable-geometry ejector designed based on the proposed method in this study with using solar energy، in conjunction with a cold storage system، might be able to provide the necessary refrigeration for all day long.

One of the most important parameters in prediction of sheet metal forming process is the influence of yield criteria on prediction of forming limit strains. In this paper، the effects of normal anisotropy coefficient on the Hill’s (1948) quadratic، Hosford’s (1979) higher-order، Hill’s (1979) non-quadratic anisotropic yield criteria، with attention to plane strain location is studied. Also the effects of these yield criteria and normal anisotropy coefficient، strain rate sensitivity factor، strain hardening exponent and yield criterion exponent on the forming limit diagram based on the M-K model have been discussed. The different effects of normal anisotropy coefficient on the limit strains of three above criteria have a good agreement with the effect of normal anisotropy coefficient on the yield different surfaces. The comparison has been made between yield criterion exponent and normal anisotropy coefficient effect on the Hill’s non-quadratic yield criterion. The results show that the proper selection of yield criterion have a significant influence on the forming limit diagram.

Electrohydrodynamic actuator is one of the newest devices in flow control techniques which can delay separation point and reduce the drag coefficient by inducing external momentum to the boundary layer of the flow. In this paper، a 2-D numerical approach was implemented to analyze the presence of electrohydrodynamic actuator on the incompressible، turbulent، steady flow over a NACA 4412 asymmetric airfoil. In this regards، the flow field and aerodynamic characteristics such as the drag and pressure coefficient were evaluated through the variety of attack angles، applied voltages، the location of emitting electrode، and the distance from the upper surface of the airfoil. The numerical results indicate that the drag coefficient with the presence of an electric field decreases with the enhancement of the supplied voltage but increases when the attack angle is augmented. In addition، the location of separation point significantly depends on the position of emitting electrode and the distance between the emitting electrode and the collecting electrode. On the other hand، according to the results، the Electrohydrodynamic effects cause the diminution of the wake region over the airfoil.

Point based 3D modeling has recently received greater attention، mainly due to its simplicity. One of the most fundamental operations for point set processing is to find the neighbors of each point in point clouds. This paper presents a new method called homogeneous neighborhood for determining neighbors in point clouds. This method of choosing neighbors، in addition to the distance takes into consideration the directional balance by improving the k nearest neighbors. The directional balance describes whether the neighbors are well spread around the point of concern. In this study effects of selecting neighbors on normal vector estimation are investigated. Normal vector is calculated using homogeneous neighborhood. For evaluation of the proposed method in determining neighbors، normal vector are calculated using the k nearest neighbors. The results show that the homogeneous neighborhood method is more accurate in normal vector estimation than the k nearest method. For evaluation of the homogeneous neighborhood method، it was employed in point cloud registration application. The results of registration by using the homogeneous neighborhood show that this method of neighbor selection yields reduced registration errors.

A two-dimensional (2D) eXtended Finite Element Method (XFEM) simulation is presented for propagation of hydraulic fractures from wellbore that the minimum principal in-situ stress is in horizontal plane. One primary role of hydraulic fracturing is to provide a high conductivity pathway along which reservoir fluids can flow to the well. In this study، enriched element has been applied with a maximum principal stress damage criterion، for initiation and propagation of crack in Abaqus 6. 12. The properties and input data for rock models were extracted experimentally from Ahwaz reservior- Bangestan wellbore specimens. The specifications of crack were studied by analyzing the rock model without any crack or flaw and under different condition، such as in-situ stresses، pore pressure and elastic modulus. The results show that the critical position of crack initiation is perpendicular to minimum principal in-situ stress and stress condition of borehole and by increasing the pore pressure to the rock models، the pressure of injection fluid decreases. The results show that the pressure of injection fluid decreases at initiation step to constant amplitude after crack propagation. These results are in close agreement with the theoretical data، so that our simple procedure is efficiency and flexible.

In this paper، the thermoelastic behavior of cylindrical sandwich shell with functionally graded (FGM) core under thermal shock is presented. Thermo mechanical properties of FGM layer are assumed to be independent of temperature and also، very continuously and smoothly functions in the radial direction as a nonlinear power function. The analytical solutions of governing partial differential equations for each layer of cylinder are solved by using Laplace transform and power series method. Mechanical boundary conditions and continuity equations for interfaces are used to obtain unknown parameters that get in recurrence equations for each layer of cylinder. The results in Laplace domain transferred to time domain by employing the fast inverse Laplace transform method (FLIT). The effects of FGM’s power on the dynamic characteristics of the FG thick sandwich cylindrical shell are studied in various points across the thickness of cylinder. The analytical presented method provides an appropriate field for analysis of transient radial and hoop stresses in a cylinder on various thermo mechanical load. Accuracy of gained equations is evaluated by similar articles. The results have a good agreement with published data in pervious researches.

The accurate، correct، and quick calculation of vehicle longitudinal velocity during braking plays a vital role in the precise operation of Anti-lock Brake System (ABS). Therefore، different researches have been conducted in the field of vehicle longitudinal velocity estimation. But، most these researches have been faced with a problem so called using braking torque as a known input to an estimator. These researches have addressed the issue while measuring the braking torque is not easy and needs expensive and additional sensors which causes the increase of costs and also requires more attention to maintenance and repair problems. In this paper، two approaches، Unknown Input Iterated Extended Kalman Filter (UIIEKF) and Modified Nonlinear Adaptive Filter (MANF) are proposed in order to estimate vehicle longitudinal velocity so that they do not need a braking torque and both methods have acceptable accuracy. The main difference between these two approaches is that the UIIEKF requires the dynamic model of vehicle motion during the braking process to estimate the longitudinal velocity while the MANF is model-free. Different aspects of both methods are analyzed by experimental tests on the vehicle and finally advantages and disadvantages of the both methods are compared.

In this paper the experimental and numerical study on flexural behavior of new type sandwich structures with glass-epoxy skins and a combinatorial core consist of foam and corrugated composite with trapezoidal geometry was investigated. After sample production by vacuum assisted resin transform molding technique; samples were tested according to standard and then the related load-deflection curves were obtained. Finite element analysis was applied for determining maximum deflection of samples by ABAQUS software. In experimental three series of samples with combinatorial core and three different pitch for trapezoidal corrugated composite in a foam core and one series of sample with simple foam core; have been product. In order to experimental accuracy of problem; three same samples have been product in each case and the average data have been used for results. It has been shown that the flexural stiffness was increased with decreasing in pitch; and the flexural stiffness to mass ratio was increased at first but then was decreased with decreasing in pitch. Finally the experimental and numerical results were compared and a reasonable agreement between them has been observed.