مجله مهندسی مکانیک
سال پنجاه و سوم شماره 2 (پیاپی 103، تابستان 1402)

In this research, by combining mathematical models based on the deformation mechanisms with finite element simulation of the deformation process, for the first time, the dislocation density content of commercially pure titanium was calculated and analyzed during its processing by the process of equal channel angular pressing (ECAP). For this purpose, first, the deformation behavior of commercially pure titanium during the ECAP process was simulated using ABAQUS software. In the next step, this information obtained from simulation was used as inputs to the mathematical model to estimate the dislocation density of the metal. A twodimensional simulation was carried out. In simulation, ECAP was performed at 250°C and in a die with a channel angle of Φ = 105 and a corner angle of Ψ = 20 degrees. In addition, ECAP was conducted up to 5 consecutive passes using two different processing routs A and C on the samples. Then, by using the mathematical model, the values of the dislocation density created in the samples were evaluated after different ECAP passes. The results indicate that by applying 5 ECAP passes using route C the mean dislocation density of titanium increases from the initial value (0.3× 1012 m-2) to 4.64 ×1015 m-2. Also, when ECAP is conducted by route A it leads to 10% higher density than route C. The distribution of dislocation density throughout the Ti bars has a good uniformity, except for a few millimeters from the beginning and end regions of the bars.

In this paper, a controller is designed to automatically control and keep the ship positioning, in a way that, it has ability to keep the ship at a coordination, automatically transfer the ship from one coordination to another one, and it’s also able to track a target without model uncertainties. To increase the accuracy and speed of the controller response, a backstep controller based on the terminal sliding mode is used, which guarantees the time-limited stability of the tracking error without depending on the initial conditions of the system. Also, in order to guarantee the controller's transient performance, a prescribed performance function is used in the controller design method. The designed controller, while guaranteeing the performance of the tracking error within a prescribed range, stabilizes the ship response in a constant time without dependence on the initial conditions of the system in a limited time so that the tracking error converges to a neighborhood of the target point. In order to validate the designed controller, the desired controller is tested in MATLAB environment on a ship model and the results show that the designed dynamic positioning controller is able to control the ship positioning well and all the theoretical findings are confirmed.

In this research, the uncertainty in the droplet size distribution has been studied as the most important characteristic of the pressureswirl atomizer spray. The size distribution of the spray droplets of a sample atomizer was measured using the shadowgraphy technique at different spray pressures and at different longitudinal, radial and peripheral positions of the spray cone. The minimum number of samples was obtained by optimizing the entropy of the non-parametric droplet size distribution. The log-normal, Rosin- Rammler and gamma parametric distribution functions were fitted to the experimental data and it was observed that the gamma distribution function is the most compatible with the experimental data. The effect of pressure on the gamma distribution function was investigated and it was observed that in the phase of changing the atomization regime, the two parameters of shape and scale change dramatically, but after that these parameters remain almost constant with the increase of pressure. Spray uniformity as another component of uncertainty was evaluated quantitatively with the two concepts of dispersion and heterogeneity. It was observed that in a certain axial distance, with the increase of spray pressure, the dispersion in different parts of the spray cone changes, but the total dispersion is almost constant.

Turbofan engines have attracted a lot of attention due to their high propulsion power. Compatibility of the simultaneous operation of the two main components of the turbofan engine, i.e., compressor and turbine, is one of the main concerns of designers and manufacturers. Because the form of characteristic curves of turbine and compressor are different and almost contradictory and will cause stagnation. In order to prevent turbofan surge, determining the matching of different components of this type of motors has become very important. The sensitivity of different parameters in the design of these types of engines is very high and achieving precalculated simulations that can have less calculation error will reduce the cost and time of the design stage. In this research, a lowerror simulation is designed to calculate the matching line of turbofan engines. In the stages of this research, first, the parameters affecting the matching line have been determined, and the method of calculating it has been extracted and simulated. Validation has been done for this simulation using the specifications of the RR-Trent 772-60, PW-PW4098 engines. According to the experiment performed, the thrust fuel consumption has been calculated with an error accuracy of less than 0.05 and the amount of thrust for different engines is less than 2000 (lbs). In another experiment, the correctness of determining the matching line for different components of the turbofan engine has been confirmed using the available resources. On the other hand, the accuracy of the overlap of the matching line of the simulation is 92% for the RR-Trent 772-60 engine and 94.2% for the PW-PW4098 engine.

The accumulation of polymer waste in the environment is a threat to human life and other organisms on Earth. And it causes the necessity of recycling them. In this paper, a co-rotating twin-screw extruder was used to devulcanize the NR/SBR rubber compound. Then, the residence time was examined based on the different screw configurations and various angles of the kneading blocks. For this purpose, kneading elements were designed with different angles and set in the extruder. In the following, considering the extruder as a continuous reactor, equations and models for residence time and other parameters were defined. by studying solid and liquid (melt) models; It was found that the residence time is only affected by the spiral arrangement in the liquid part. This research aims to finding the optimal arrangement and angles to reduce the residence time, which improves quality and reduces energy consumption.

In this article, the effects of the vortex rope in the Francis turbine draft tube in one of Iran hydropower plant are studied experimentally and numerically. First, the experimental results of the unit at four operational loads of 22.52, 45.74, 68.73 and 85.60 MW was extracted by a pressure gauge which is located at a distance of 3.199 meters below the runner. According to the fast fourier transform, the load with a power of 45.74 MW had the most pressure fluctuations. Then numerical simulation of the whole turbine at different condition was performed using transient solution and SAS-SST turbulence model. At five injection condition namely 1.5% and 3% only water, 1% and 2% only air, and the combined injection of water and air (3% water and 2% air) order of vortex fluctuations were investigated. Also, an algorithm to optimize the parameter of efficiency multiplication in the pressure recovery coefficient over the peak-to-peak amplitude of fluctuations at different air and water injection was presented. The results showed that injection with 1.58% of water and 0.02% of air, cause the mentioned ratio to be maximum, which is the most optimal condition of operation.

The current research aims to simulate the collision of a car at several different speeds in ABAQUS and ANSYS software, to check the results and find the appropriate analytical relationship. For the results of the project to be accurate and reliable, five car models were analyzed in five phases. At first, for each phase of the project, the geometric model of the cars was modeled in an engineering design software, and their various components such as body, chassis, etc. were assembled on each other. After entering the models into ABAQUS or ANSYS software, the geometric defects of the model were repaired. Finally, things such as tension, deformation, energy, etc. were compared and an analytical relationship between energy and collision speed was proposed for the car body by MATLAB software. The results show that the most energy transfer is from the front part of the chassis to the cabin and the front points play a significant role in the accident.

Since forward kinematics in parallel robots has more difficulty and often does not lead to an analytical solution, in most studies, inverse kinematics has been preferred for parallel robot calibration. In this study, a technique was presented that does not need to completely solve direct kinematics. With the help of the forward kinematics, variation of the end effector with respect to the deviation of the kinematics parameters has been calculated. Then, by linearizing the equations, the possibility to use the linear least squares has been provided. The presented model like Zero-Reference model and Product of Exponential Formula, only uses one fixed reference frame and unlike D-H model does not have singularity problems. In the simulation, it was shown that the algorithm with a small number of selected poses and with high speed in a fraction of a second by a few iterations reaches an answer in the range of nanometers. Also, by using stereo vision, the presented theory was experimentally examined and shown that the proposed algorithm can reach the accuracy of the measuring instrument, similar to the simulation results.

Most available musculoskeletal models are made based on activities with a limited range of motion (ROM) like walking and running. Making sure about the squat simulation results, which include severe hip and knee flexion angles, is one of the most challenging problems in movement biomechanics. Choosing the appropriate musculoskeletal model as the first step of simulation has always been one of the main questions of researchers. This study has introduced the musculoskeletal models used in the most important previous studies conducted by domestic and foreign researchers using OpenSim. By summarizing the available models, a comprehensive guide has been provided which includes information about these models. F, a simulation step using the CAMS-Knee datasets was performed and the results were compared. The high error of knee joint contact force (RMS of the best model: 72% BW) emphasizes the necessity of providing more accurate models in this field. Developers of musculoskeletal models can benefit from the results of this research to improve the current models for simulating activities with severe degrees of lower limb joint flexion.

Spillways are one of the important hydraulic structures of a dam, which perform their duties during floods in order to maintain the health of the dam. In this research, the free ogee spillway of Sahand dam is analyzed with Flow-3D modeling software and the effective hydraulic parameters on the performance of this structure including the occurrence of cavitation and scouring of the stilling basin have studied and verified with lab model. The results show that cavitation occurs in three areas along the service spillway of Sahand Dam. The ogee crest part of the spillway, the part that slope changes in the middle of the chute and the flip bucket are the parts that have the problem of cavitation. For the ogee crest part, the tip curve should be modified and the cavitation-prone part should be made longer to create a vacuum and eliminate the possibility of cavitation. If this doesn't work, aeration should be done. In the middle of the chute, aeration should be used by creating a gap in the floor. Also, the end edge of the flip bucket should not be smooth and reshaped as a curved surface. The scouring depth for the flip bucket of the Sahand dam spillway is about 7.6 meters, and to deal with it, it is necessary to organize the downstream of the spillway using a excavated pool, protection and a stone riprap.

The main goal of this study is to produce the copper-iron composite by powder metallurgy method and to investigate its microstructure and hardness after ECAP process. The copper with atomized iron with a grain size less than 63 microns were mixed with the ratio of 9:1, then pressed inside a cylindrical mold at the pressure of 450 MPa. The samples were compacted and sintered in a tunnel kiln with a temperature of 920 °C, then their surfaces were ground and subjected to a multi-pass ECAP process with back pressure. The performed evaluation indicate that the hardness for the sintered Copper-Iron metal matrix composite sample prior to the ECAP process was increased up to 190% after third pass. The average grain size of iron-copper composite in three pass decreased to 53% compared to the same composite on one pass. The theoretical density of the samples increased significantly after the ECAP process, so that the density value increased from 88% for the raw sintered sample to 98% for the samples in the three-pass ECAP condition.

In the topology and support optimization of continues structures, stability and buckling issues due to computational complexities are not usually considered, therefore in some cases, long and thin members are obtained in optimized configuration that can lead to the instability of structure. One of the major problems during the implementation of conventional buckling optimization processes that are based on changing the density or removing and replacing the material is the creation of pseudo buckling modes that lead to the divergence of the optimization problem, decreasing the rate of solving process or producing non-optimal shapes. In this article level set method with incorporating a fictitious interface energy is applied to find optimal configuration. The level set method is based on the movement of the structural boundaries and it has a high ability to control the complexity of the topology, so the idea of using this method has led to the elimination of pseudo buckling modes and the solving of this problem. Derivation of the required speed term in the level set method for buckling load factor is complicated and this derivation is another innovation of this research. Numerical examples are illustrated to prove the effectiveness of this method.

In this research, for complicated rectangular parts such as oil and gas tanks, and using hydroforming, forming of three geometries of simple, middle-step and the side-step, rectangular cups were investigated, and by examining the effect of geometrical parameters such as pressure, punch radius, and height, change in the initial shape of the sheet and the filling at different heights are compared. Moreover, by performing optimization and increasing the maximum pressure in middle-stepped and side-stepped cups to 30 and 40 MPa, the maximum filling can be increased by 8 and 12%, respectively. Also, by changing the height and drawing ratio in the stepped parts, the optimal shape of the initial sheet changes for conforming the curve of the formed sheet and the modified target contour. This change is variable for the coordinates of the node in the transverse wall, longitudinal wall, and corner radius of the optimal initial sheet.

In this paper, a novel method for model predictive control of switched multivariable systems under arbitrary switching signals is proposed. In this method, an accurate estimation algorithm based on Fuzzy-Wavelet Neural Networks is employed which is proposed for multivariable systems. Based on estimation of active switched dynamical mode, predicted sliding functions are calculated and according to corresponding candidate Lyapunov functions defined for various degrees of freedom, stabilizing MPC constraints are obtained. Existing methods in control of switched systems are based on worst-case switching configurations which result in conservatism of controller. In the proposed method, conservatism is eliminated based on incorporation of active switched dynamics which in turn leads to smaller control inputs and more precise tracking response.

The Chemiluminescence method is considered as one of the most widely spread optical methods which is used for identifying combustion systems. In this study, the CH* and OH* Chemiluminescence signals were measured with the use of PMT detector and other optical equipments for a bunsen burner. Also the effect of equivalence ratio and flow rate on these signals have been investigated. The results show that with increasing flow rate, CH* and OH* signals increase linearly. By changing the equivalence ratio, Chemiluminescence signals were maximized near stoichiometric condition. Therefore, these two types signals can be considered as an the heat release rate indicator and can be used to find the optimal working conditions for any desired burner. It was also shown that OH*/CH* is independent of mass flow rate and only affected by the equivalence ratio.Then, the calibration relationship between equivalene ratio and OH*/CH* signal has been extracted for a certain range of equivalence ratio, which is also true in other combustion condition. Finally, the application of this method has been evaluated to identify the dynamics of combustion systems. The results show that the premixed flames have a dominant peak frequency due to the Kelvin-Helmholtz vortices. In premixed flames, the peak frequency is decreased by increasing the equivalence ratio and the corresponding amplitude increases. Also, with increasing flow rate, the peak frequency is linearly increased and its amplitude decreases linearly. This work shows the potential of using the Chemiluminescence method as an effective tool in the field of experimental identification of combustion systems, as well as for controlling and manitoring the lean of fuel combustion chamber and identifying dynamic combustion phenomena.

Friction stir processing (FSP) is utilized in various industries due to its unique properties. In this regard, a better insight into the material properties obtained from this method will facilitate a broader application. One of the most important properties that should be well renowned before any use is how a material behaves when it has cracks. Crack growth under shear loading is one of the most significant factors in component failure, and if the material behavior after the FSP method in the presence of cracks is known, it is possible to use it safely. In this research, the influence of the FSP on the mechanical properties and resistance of the material against crack growth under pure shear loading has been investigated. It was found that this process significantly increases the strength and ductility so that the yield strength, ultimate tensile strength, and elongation-to-failure improve by about 42%, 67%, and 125%, respectively, compared to the initial state. It was also understood that the processed sample demonstrates an average drop of 23% in strength against crack growth compared to the initial state. It seems that due to FSP application, the amount of brittle fractured surfaces has been substantially increased, causing a decrease in the critical fracture parameter values.

Cardiovascular diseases are the leading cause of mortality worldwide, and their treatment is crucial. Computational modeling is one of the new methods that can help specialists to provide more effective treatment and surgical solutions for patients. In this study, a multiphysics framework has been utilized for electromechanical modeling of the human heart to simulate the behavior of the heart in aortic valve stenosis in two hyperelastic and viscoelastic cases. By using COMSOL Multiphysics software, a completely new approach to determine ventricular pressure has been presented in this software, the implementation of which is more straightforward and does not require coding, and its accuracy has been verified. It was observed that in both hyperelastic and viscoelastic cases, ventricular pressure in aortic valve stenosis increases dramatically and stroke volume decreases. Moreover, the viscoelastic model showed relatively less deformation compared to the hyperelastic case and caused the damping of myocardial movements. The implementation of the presented model is simple and, in the future, it can be utilized to model different types of heart diseases.

Due to the complexity of the processes governing the flow in the gas turbine, it is necessary to perform experimental tests to ensure the performance, but creating the conditions governing the components in the operational conditions is very complex. most of the combustion chamber tests are performed in atmospheric conditions and then the results are generalized to the operational conditions. This work, along with the lack of test stands with real operational conditions, has been developed based on the fact that most of the processes governing the behavior of the chamber have not undergone significant changes with the change of operating pressure, and especially if the shape of the flame does not change, it is possible to study the behavior of the chamber even at low pressure. In the present research, this issue has been numerically investigated for a real combustion chamber and it has been observed that changes are characterized by increasing pressure. Also, NOx pollutant increases at higher pressures. The overall flow structures were not affected by the pressure increase, but the pressure loss rate increased. The current results show that in the desired combustion chamber, the atmospheric test can provide acceptable results and can be generalized to high pressure conditions.

In this research, the nonlinear vibrations of a cylindrical sandwich shell with a core of auxetic structure (negative Poisson’s ratio) are investigated. The intended shell consists of two layers with isotropic material and a core of auxetic material with a negative Poisson's coefficient which is the result of the geometric structure of the core. First, with the help of the constitutive equations of auxetic materials and isotropic materials and using the assumptions of the equivalent layer, the tensile, bending, and coupling stiffness of the structure is calculated. Then, according to the boundary conditions and for circular cylindrical, the displacement field is estimated. By replacing the displacement field in the equations of motion with partial derivatives, the equations are transformed into nonlinear equations with ordinary derivatives. In the following, Runge–Kutta numerical method is used to solve these equations. Then, the results are compared with the results in the research literature, and finally, the effect of the geometric characteristics of the auxetic core, such as the angle, length and thickness and geometric characteristics of the cylindrical shell, such as the radius and length of the cylinder, are examined.

In this study, Tube cyclic Extrusion Compression (TCEC) process was performed on copper pipes at ambient and 200°C (dynamic recrystallization activation threshold temperature) and the effect of process temperature on microstructure and mechanical properties were investigated. The results showed that by applying TCEC at ambient temperature, an equiaxed crystalline structure with an average grain size of 370 nm was obtained while by increasing the forming temperature to 200°C, the equiaxed grains with average size of 720 nm resulted. Applying the TCEC process at ambient temperature on a copper tube, increased its yield strength from 116 to 242 MPa and its average hardness from 69.2 to 101.4 Vickers while at the forming temperature of 200°C, the yield and ultimate strength were about 217 and 225 MPa, respectively, and the average hardness value was about 86.8 Vickers. Therefore, performing TCEC process at 200 °C on copper reduced the strength by 10% and increased the elongation to fracture by 21% compared to the cold forming state.