مجله مکانیک سازه ها و شاره ها
سال هشتم شماره 2 (تابستان 1397)

In this paper, a new type of spherical wrist is investigated. By using CAD/CAM software, simulation and analysis have been done. So, a new type of mechanism has been fabricated by involute axes in the form of conical gears that can create an unlimited revolution for the wrist without singularity. This wrist has the special ability due to its structure and mechanism to use robots for high precision applications and continuous movements without singularity. According to the drawings, the manufacturing of the model for molding and casting of pieces is carried out and then by machining, the dimensions of the parts are achieved to the standard of drawing. After the assembly, the mechanical work pieces are prepared. The direct and inverse kinematics of the involute angles are obtained for the directions of the wrist. Also, according to the kinematics of the robot, the position of the end-effector is calculated. At first, Matlab software is used to simulate the wrist model and also the experimental test is done for validation. At the end, the PD controller is designed to control the yaw, roll and pitch angles and the changes necessary to improve the wrist control have been applied in the controller and AVR-based microcontrollers. Finally, simulation results are verified through theoretical and experimental test.

In this paper a micro-accelerometer with a simple design, low fabrication cost, low volume and weight as well as sensitive to very small accelerations with acceleration measurement capability in two axes has been designed. In the proposed design, thermal-convection method has been used to measure the acceleration. This microsensor consists of a heater and two pairs detectors. By applying acceleration along the X or Y axis, a temperature change occurs in the detectors that is proportional to the acceleration applied to the system. Two stages of simulation and optimization of the micro-sensor were performed using finite element method and Taguchi method, respectively. Sensitivity is one of the most important output factors of the thermal accelerometers that geometric parameters and thermal conditions have huge effect on that. So effects of different levels of geometric and temperature variables have been studied and optimized for achieving the maximum sensitivity by analyzing the signal to noise ratio in Taguchi experimental design method. The sensitivity was improved 12.77 times compared to the average value of the simulations results, by adjusting the variables on the optimal levels. As a result, the sensitivity of 0.23 [˚c/g] for air-filled package and 0.70 [˚c/g] for CO2-filled package was obtained in this research.

The purpose of this paper is to present a new model based on the multiple regression method for estimating stress concentration factor of a quasi-triangular hole in composite lamina. One of the important applications of providing stress distribution around holes in terms of mechanical properties is the use of these relationships in the stress analysis of perforated viscoelastic plate using the effective modulus method or Boltzmann's superposition principle. First, using different values of the mechanical properties of the composites plates, and employing an analytical solution based on the complex variable method, the stress concentration factor of quasi-triangular hole is calculated for a number of these materials. Then, using multiple linear regression, An explicit expression for the stress concentration factor is given in terms of mechanical properties. The results show that the multiple regression model is able to predict the circumferential stress with a maximum error of less than 1%. An important point in this article is the reduction of the calculations by applying the aforementioned relations. In other papers in this field, hard and complex calculations were used to obtained the stress concentration factors, but with the help of the relation of this paper, it is to calculate the stress concentration factor with having the mechanical properties of the plate.

The purpose of this paper is to design a fractional order PID (FOPID) controller base on stochastic algorithms for attitude control of quadrotor. The model is based on usage of Fractional calculus in the PID Controllers which can provide novel and higher performance extension for FOPID controllers. Here, the main goal is to design an optimal PID and FOPID controller using stochastic algorithms. Therefor two stochastic algorithms are used for optimal tuning of FOPID parameters. Genetic Algorithm (GA) and Particle Swarm Optimization are compared in minimizing the performance criteria formula which leads to a better performance in controlling of the plant. A quadrotor is installed on an experimental test stand includes of accelerometers, gyroscopes and microcontroller with one and three degrees of freedom to test the performance of PID controller. Kalman filter is used to reduce the noises by combination of accelerometer and gyroscope output. The result of the experiments in the paper shows that the quadrotor can perform desired motions successfully with the controller in the response of step input and while it is subjected to external disturbances

The use of aluminum and magnesium alloys is limited due to their low formability at ambient temperature, in comparison to steels. The invention of new methods for increasing the formability of these alloys has always been investigated in the sheet metal forming industry. Studies have shown a dramatic increase in the formability of aluminum and magnesium alloys by increasing temperature. As a result, the necessity of application of warm forming processes requires the study and investigation of effective parameters on process. In this paper, numerical and experimental study of forming speed have been investigated in gradient warm deep drawing process of cylindrical parts for aluminum alloy 5083 sheet with 2mm thickness. To this end, some tests have been performed at three speeds of 60, 200 and 378mm/min at different temperatures with constant blank holder force. The effects of forming speed on punch force, thickness distribution and earing behavior, as well as limit drawing ratio and temperature gradient have been discussed and analyzed. Results show, by increasing temperature and reducing the punch speed, punch force has been decreased, drawing ratio has been increased and more uniform thickness distribution has been obtained. It has been also observed that by increasing temperature, the number and position of the ears have remained constant despite the reduction of the ears amplitude.

The main aim of this paper is the study of the strength of AM-2 matting plate made of Al6061-T6 under impact loads due to aircraft landing. In this study, the damping effects of the aircraft tire have been modeled by means of Mooney-Rivlin hyperelastic criterion. Effective parameters such as the landing vertical velocity at 1.5, 2, 2.5, and 3 m/s; the landing horizontal velocity at 42, 45, 50, 55, and 60 m/s; the CBR value of subgrade at 5, 10, 15, 20 and 25; and the thickness of subgrade at 160, 200, 300, and 500 mm, have been studied. The results showed that the stresses in matting plate increases with increasing both horizontal and vertical components of landing velocity, but decreases by increasing the CBR value. On the other hand, considering the subgrade thickness greater than 300mm has no significant effect on stresses. In order to have safe situation, the CBR value be 15 and greater has been recommended. Comparison of the results with those obtained without modeling the damping effects showed that the stresses be 15% greater in conditions without modeling damping of the tire.

In this article, penetration of an armored piercing projectile into ultra-high strength steel plates (Hardox450 and Hardox500) will be investigated by finite element (FE) analysis and experimental measurement. The ballistic impact is simulated by Abaqus FE software and the FE model is verified the results of experimental test. The mechanical behavior of the selected material is defined by Johnson-Cook plasticity model and Mie-Grunisen equation for damage model. The experimental tests are carried out on 30mm thickness plate. A 14.5mm armored piercing projectile is fired to the plate. The finite element results revealed that the projectile penetrates partially in Hardox500 steel while it can perforates Hardox450 steel. The penetration depth of the projectile when collisioning by 911m/s speed and at perpendicular state are 13 and 30mm for Hardox500 and hardox450 steel respectively. The penetration depth will be increased by increase in the impact angle from 30o to 90o. In addition, the reflection state changes at a specific angle according to the type of material. The transition angle are 40o and 60o for Hardox450 and Hardox500 steel respectively. Finally, the ballistic limit is calculated and determined as 900m/s and 1200m/s for Hardox450 and Hardox500 steel respectively.

In this research, vibration behavior of a microgyroscope including beam structure is investigated. microgyroscope consists of a microcantilever beam and distributed proof mass that is actuated by electrostatic force. A developed model and formulation based on the distributed assumption for proof mass are presented to investigate the instability and vibration behavior of beam microgyroscope. Considering distributed assumption for proof mass, electrostatic force changes from concentrated force to distributed force and produces a moment that is effective on the mechanical behavior of the gyroscope.The equations of motion are reduced by Galerkin’s method and solved via numerical and analytical techniques. An important nondimensional parameter named as nondimensional length parameter is presented which it is the ratio of the proof mass length to beam length. Effects of the nondimensional length parameter on the static, dynamic, vibration behaviors and natural frequency of the microsyetem are investigated. Results show that by increasing the nondimensional parameter, the differences between the results of concentration and distribution hypothesizes for proof mass increase.

Aluminum alloys have been widely used in various industries, due to its special characteristics. On the other hand, due to its low formability, the forming of these alloys is difficult at room temperature. Therefore, in order to facilitate the forming, warm forming methods can be used, which will lead to the oxidation of the material. In Single Point Incremental Forming (SPIF), a sheet metal is formed by a progression of localized plastic deformation using a hemispherical-head tool. In this study, by applying ultrasonic vibration to the tool, Ultrasonic Vibration assisted SPIF (UVaSPIF) process was developed that improved the sheet formability at room temperature. In present paper, the effect of ultrasonic vibration and lubricant on behavior of the forming force has been investigated. Consequently, a hemispherical-head tool (D = 20 mm) with natural frequency of 20.4 kHz was designed and manufactured. The occurrence of longitudinal mode and resonance phenomenon has been confirmed by the results of modal analysis and experimental test. Then, Al 1050-O sheet was used as a work material. Experimental results obtained from straight groove test, indicate that in UVaSPIF, imposing of vibration without lubricant has the greatest impact on reducing the vertical component of the forming force whereas in SPIF, the lubricant has the greatest impact on reducing the vertical component of the forming force.

Investigation of flow patterns and behaviors has become a subject of great interest in recent decades because it is a basis in the recognition of particle movement and deposition in the human respiratory tract and helps in the prediction of the effects of inhaler drugs and pollutants, and respiratory diseases such as cancer. Therefore, in this study, the effects of two boundary conditions: (1) the same flow rate in all outlets and (2) the same static pressure equal to zero in all outlets has been numerically examined in an asymmetrical model of trachea-bronchial tract. Correspondingly, the effects of the inlet flow rate changes on the flow distribution, flow patterns and the reverse flow zones were studied in the range of breathing rates 12 to 48 lit/min for a 3D non-planar model of trachea-bronchial airways consists of 4 generations. The estimation of flow distribution obtained from the second boundary condition was more accurate when compared to the real distribution in the lungs. Using the first boundary condition, the flow distribution did not change when the inlet flow rate was increased. However, for the second boundary condition, little changes revealed. The flow pattern in the lower sections was more complex than the upper sections due to the bifurcations’ curvature that causes the Dean-flow particularly when this curvature is in non-planar to the previous bifurcation. When the second boundary condition was used, the number of generated reverse flow zones was more than the other conditions and increased with increasing the inlet flow rate.

In this paper, the effect of particle size on deposition in a compact heat exchanger was investigated numerically. The effect of flow velocity and particle mass on the deposition was also studied and discussed. Flow simulation was performed using Eulerian approach and particle motions were simulated using Lagrangian approach and discrete particle model (DPM) by ANSYS-FLUENT package. Turbulence was modeled with the k-ωSST model. Five fin channels of a compact heat exchanger were chosen as a 3D computational domain. The air flow was entered with velocity over a range from 1 m/s to 5 m/s and particles having various diameter sizes, were introduced to the computational domain from inlet boundary condition. The results showed that the pressure drop was increased with increase of particle size and particle mass. Besides, deposition ratio was grown with the increase of particle size. The results also demonstrated that most of the particles were settle down on the front of the channels and on the first and the second fins of channels. The effect of simultaneous injection of big and small particles was investigated and the results showed that small particles had more chance to deposit in the presence of the big particles.

In this study, the effects of frequency and dimensionless amplitude of the pulsating flow on the rate of heat transfer around elliptical cylinders with elliptical ratio of e=0.2 and 0.6 under different angles θ=0°, 30°, 60° and 90° and also for elliptical ratio of e=1 (circular cylinder) has been investigated. For this purpose, the flow around a cylinder with elliptical ratio of e=1 under pulsating and unpulsating flow been solved and then the results of elliptical cylinder under different angles of attack and elliptical ratio compared. So, pulsating flow over the cylinder, in the range of pulsating Strouhal number St=(0.1-2) and dimensionless amplitude of pulsating flow A=0.75 in Reynolds number of Re=100 has been studied. In all cases, elliptical cylinder have fixed temperature more than the temperature of the surrounding fluid. The pulsating flow is a factor that can be effective on the rate of heat transfer, according to results presented in this study, pulsating flow over elliptical cylinder, in some cases can increase the heat transfer rate. But in general, changes in the rate of heat transfer depend on the frequency and dimensionless amplitude of pulsating flow.

In this paper, the numerical analysis of the conjugate natural convection with surface radiation in a two-dimensional enclosure is carried out in order to search the optimum location of the boundary constant flux heat sources to minimize the temperature of the heat sources surface using the particle swarm algorithm. The air is considered as an incompressible fluid and a transparent media inside the enclosure with a steady and laminar flow regime. The surfaces of the enclosure are also considered to be opaque, diffuse and gray. The governing equations are solved using the stream function and vorticity formulation with the finite difference method. The maximum temperature and the location of heat sources are selected as the objective function and design variables, respectively. The results show that the minimum value of the maximum dimensionless temperature of the heat source decreases with the increase of emissivity or Rayleigh number. By increasing the Rayleigh number, the optimal location of heat sources shifts to the bottom for configurations with one or two heat sources. By increasing the emissivity in each Rayleigh number, the optimal value of the heat sources center location approaches to the center and in the configurations with two and three heat sources is close to each other.

Winglets are one of the tools used for reducing the effects of the tip vortex. In this research, the effects of installing split blended winglet on Micro-Aerial Vehicle half model in order to the reduction of wing tip vortex were investigated experimentally. The experiments were conducted in an open circuit wind tunnel at velocity 16 m/s and angle of attack 10ᵒ. The Reynolds number of the model was about 3.8×104 based on Mean-Aerodynamic chord. In order to measure the mean velocity and turbulence intensity, the Hot-Wire Anemometry (HWA) has been used. Experimental results in the absence of installing winglet indicated the existence of a vortex structure behind the tip of the wing which is due to the pressure difference created on the upper and lower surfaces of the wing. Turbulence intensity has it highest value in vortex core which is also observed to decrease gradually with downstream distance. The results show that in the presence of winglet, there are two clear vortex structures which one at the winglet/wing junction and the other at the tip of the winglet.

In this study, a new solar water desalination system is designed and fabricated. In this system, solar energy intensity has been concentrated by means of parabolic trough collector, which reflects the sunrays to its focal line, where the still is located. Performance of the desalination system was studied in real conditions. The effect of adding steel sponge to the water inside the still and the use of a canopy on the condenser fins has been studied. Results show that the performance of the desalination system increase by 17.7% and 6.3% by using steel sponge and canopy, respectively. In order to investigate the effect of various factors on the performance of the present desalination system, a thermal model is developed based on the conservation of energy. The governing equations are implemented and solved numerically in MATLAB software. Then results of this modeling, including water temperature, adsorbent plate temperature and rate of fresh water production were compared with empirical data and a good agreement between them was observed. Average relative error between experimental and thermal model results for all variables was less than 6 percent. Finally, the model is used to study the behavior of the machine in different seasons.

In the present study, three different fluids with different volumes of nano materials(Al2O3) in the Reynolds range of 3 to 48 thousand were investigated for the purpose of evaluating the effect of using nanofluid in heat exchangers. The converter in the submarine engine come to use of as a motor oil cooler. The results show that the addition of nanoparticles to the base fluid in the lower reynolds results in a higher performance factor in the exchanger. By increasing the volumetric percentage of nanoparticles, the converter's performance factor decreases. Optimum mode and maximum heat exchanger performance, in all volumetric percentages, occurs in Reynolds 20,000 for water and 12,000 for ethylene glycol. In other words, the choice of base fluid with a higher performance factor depends on the Reynolds number range. Also, this study examined sea water as a cooling fluid available to the marine diesel engine.

The Guidance commands are mostly generated by determining the line of sight (LOS) angle. Among the existed guidance laws, proportional navigation (PN) is the most widely used algorithm. In this way, the differentiation of LOS angle is needed. Then the LOS rate are usually found by numerical differentiation of the LOS angle. The LOS rate can be directly measured by an embedded internal seeker which it would be a differential free technique. Nowadays, in practical, guidance algorithms equipped with a seeker have been interested. Such a guidance loop includes some subsystems like vehicle dynamics, the type of flight path and also navigation sensors which each subsystems may have some parameters like aerodynamic coefficients. Therefore the guidance system performance would be affected by changing in these parameters. In this paper, in a typical vehicle equipped with a seeker, the guidance system efficiency are numerically investigated in terms of the aerodynamic coefficients change.

Today, dust, biological and technical implications of them has become a problem in which researchers and industrialists have been involved. In the present study, expressed an overview of the status of dust and environmental problems in the country and described how to model the motion of dust in the range of Qom University using computational fluid dynamics tools. Then it is said how are atmospheric conditions effect and the deposition of these particles on the part of the University of Qom numerically. The results showed that the building architecture has a significant effect on the distribution of dust concentration around the building. Then, the effects of wind direction are also studied. The trapped dust particles will be minimized for a specific angle between the wind and building directions. The effects of temperature gradient (daily and seasonal) on dust concentration are also studied and then the effects of various temperatures on the dust emission studied. It has been observed that considering the temperature variation in computational model leads to reduction of concentration of dust particles up to %90. Finally, accumulation of settled particles on the ground level was considered. The results proved the considerable influence of wind direction on accumulation of settled particles in the central yard of the building.