نشریه مهندسی مکانیک مدرس
سال بیستم شماره 7 (تیر 1399)

In the current study, natural solar ventilation has been investigated aiming at reducing the consumption of fossil and thus, reducing greenhouse gas emissions in a hot and dry climate in which the behavior of various fluid variables (temperature, velocity, and flow rate) is considered in different conditions. Since solar radiation is not uniform throughout the day, passive solar ventilation is unstable. In this regard, the natural displacement flow in a solar ventilator with copper thermal absorber, double-glazed glass compartment to prevent thermal energy loss, as well as phase change materials for the storage of thermal energy has been investigated, experimentally. In the case of no phase change material, due to the creation of a suitable temperature difference, the panel has made the chimney effect possible for natural ventilation in some hours of the day, but in the early hours of the night, the temperature of the panel will be the same as the ambient temperature, and the chimney effect will not be available for proper ventilation. In a panel equipped with phase change materials, the system has acceptably been able to play an important role in reducing the temperature drop in the hours of the day with no solar radiation leading to a reliable air flow rate. In fact, the main purpose of using phase change materials in passive solar ventilation is the same effect, the use of excess energy in cases of energy shortages.

Employing nonlinear dynamic signature of the host structure for early damage detection and remaining useful life estimation purposes, is an emerging idea in the area of piezoelectric patches based structural health monitoring. Clamped support loosening is one of the defects that not only may cause disorder in system’s functioning, but also obstruct damage identification process through distorting the signals. In this study, support loosening induced contact acoustic nonlinearity (CAN) behavior was monitored by vibro-acoustic modulation (VAM) technique. Using miniaturized PZT patches with the capability to be installed on the host structure permanently for both pump and probe actuation as well as sensing the modulated signal, enabled online monitoring via VAM technique. An appropriate filter was designed to eliminate the unintentionally excited natural frequencies and to reveal the sidebands. In this study, the sensitivity of modulation strength to the pump excitation frequency was also investigated. According to the results, appearance of sidebands around the central probe frequency is an appropriate indicator for CAN identification. In order to study the mechanism of modulation phenomenon, a coupled field electromechanical finite element (FE) model was developed. Proper matching of the numerical and experimental results indicates sufficient accuracy of the developed FE model and its potential to predict the modulation behavior.

Thermal fatigue is one of the most important issues in different engineering fields. The importance of this phenomenon is its application in aerospace industries and considerable effects on the material properties. In this research, the effect of thermal fatigue on the machining quality of polymeric CNT-reinforced composites is studied. To follow this aim, initially the composite specimens with eight layers in symmetrical and unsymmetrical layups are fabricated and subjected to thermal cycling. Then, two different machining processes including conventional drilling and ultrasonic vibration assisted drilling are carried out and the thermal fatigue effects are experimentally studied. Additionally, the effects of various parameters including “addition of multi wall carbon nanotube”, “machining process” and “layup method” on machining quality of composites under thermal fatigue condition is investigated in order to obtain the least delamination. The results indicated that addition of multi-walled carbon nanotubes enhances the machining quality up to 13%. It was also revealed that the implement of ultrasonic assisted drilling could reduce the delamination damage up to 10%.

Today, the magnetic levitation system is widely used in various industries. This system is inherently unstable and nonlinear, which is presented by nonlinear equations. On the other hand, the existence of a time delay in these systems also causes system instability or even chaos, which creates additional problems in their control, thus requiring the design of robust and optimal control. In this paper, a robust adaptive intelligent controller based on the backstepping-sliding mode is proposed for the stability and proper tracking of the magnetic levitation system in the presence of time delay, uncertainty, and external disturbances. Due to changes in the equilibrium point, comparative control is used to update the system's momentary information and intelligent controller to estimate uncertainties and disturbances and non-linearity of the system. A robust controller is used to asymptomatic stabilize the Maglev system. The Lyapunov stability theory is used to analyze the stability of the magnetic levitation system with the proposed controller. In the end, in order to demonstrate the performance of the proposed controller, numerical simulations have been used in MATLAB software. The simulation results show that good tracking has been performed and the controller is very good against noise and disturbance.

Wheeled robots have various applications in industrial, laboratory, art, and filming environments. The choice of wheel and platform type in these robots depends on the motion and the degrees of freedom expected from the robot. With an appropriate choice of the wheel and platform, the degrees of freedom of 3 (known as holonomic robots) can be achieved in which the robot can move in both x and y directions and also rotate about the z axis in the general coordinate system. If the wheeled robot is designed to carry objects, it is necessary to consider a platform on top of the robot for this purpose. In this paper, a 3-DOF Stewart platform is used such that it provides rotation about x and y axes as well as motion in direction of z axis. The goal of this research is to develop a wheeled robot equipped with the 3-DOF Stewart platform to carry objects with ability of orientation control within the path. With integrating these two robots, the resultant robot will have 6 degrees of freedom, three of which are provided by the Stewart platform (α, β, Δz) and the other three are provided by the wheeled platform (Δx, Δy, γ). Therefore, the robot, with 6 degrees of freedom, can be controlled via the six parameters of Δx, Δy, Δz, α, β, γ.

Variable-area injectors are suitable for developing throttleable rocket engines because it is difficult to efficiently control thrust when fixed-area injectors are used. A pintle injector is a variable-area injector that can be used to control the mass flow rate of propellants. In practice, an injector plate containing several fixed-area injectors is replaced with a single pintle injector. In this research, a two-stage pintle injector is designed, manufactured, and tested for the effects of dimensionless numbers (Momentum ratio, Weber number, and discharge coefficient) on the injector’s performance, including the spray angle change, which is an important characteristic of the spray. The tests were done at ambient temperature and pressure conditions. The Weber number ranged from 19 to 1830, and the ratio of the fuel to the oxidizer momentum was varied from 0.2 to 13. Water is used instead of the oxidizer as a central propellant, and the air is used instead of the fuel as an external propellant. Shadowgraph and photography were used to measure the spray angle and study the desired parameters. Empirical relationships between functional parameters and dimensionless numbers were obtained that can be used in the design process.

Humans are always looking for ways to produce cheap and permanent electricity. One of these ways is to use wind turbines. The vertical axis wind turbines are less sensitive due to the problem of the setup and low efficiency compared to the horizontal axis turbines. One way to improve the performance of VAWTs is to change the angle of attack of the wind turbine blade. In this study, the computational fluid dynamics method is used to solve the finite volume flow equations. Different angles of attack range from -12 to +10 degrees and wind speeds of 10m/s and density of 1.225kg/m3 and constant dynamic viscosity of 1.825psi were used. The calculations showed that by increasing the angle of attack of the blade to +10 degrees  Cp and Torque decreased, by decreasing angle of attack of the blade to -4 degree, Cp and Torque increased, but by more decreasing AOA of -8 to -12 degrees Cp and torque decreased.

Design and safety of natural gas tanks Due to its high use in cars, it is of great importance. Therefore, in this paper, the empirical, numerical and optimization of these reservoirs is investigated. Experimental section designed and manufactured two metal and composite tanks that have been tested for internal pressure and their strength has been determined. Modeling of these tanks has been done in the numerical section with the help of Abaqus software 6.14. In addition to validating the results with experimental data, numerical simulation has been developed. Using the results of the development of numerical simulation and experimental design software, optimization of parameters and their relationship with pressure tolerance in these tanks have been investigated. The numerical and experimental results are in good agreement. Lightweight composite tanks are more resistant to internal pressures, which resulted in a 30% reduction in the weight of composite tanks and a 20% reduction in deformation under operating pressure.

Aluminum alloys, due to their high variety and favorable mechanical properties, are widely used in industries. Aluminum alloy 111H-5754 due to its properties such as high strength to weight ratio, ductility, toughness, and corrosion resistance, are applied in the manufacture of automotive body, offshore, and offshore oil equipment. The presence of 3% magnesium in the chemical structure of this alloy makes it susceptible to heat and therefore, it is not possible to perform most of the traditional machining processes on it. Water jet machining with abrasive particles (AWJM), because of the use of water and abrasive particles as cutting tools, can be a good method for machining these materials. In the present study, the effect of water jet and abrasive particle machining process parameters, including water jet pressure, traverse speed and loading coefficient on surface roughness, angle of striation, and burr formation in aluminum alloy 111H-5754 samples is discussed. The results showed that after traverse speed, water jet pressure and loading coefficient have the most effects on the surface quality characteristics, respectively. So, for a loading factor of 45% and a jet pressure of 300MPa, increasing the traverse speed from 200 to 300mm/min, the surface roughness value in the smooth area is about 50%, and the angle of striation of the lines in the rough area, increased by about 25%.

Modern intermetallic compound of gamma titanium aluminide (γ-TiAl) due to its low density, high elastic modulus, high resistance to oxidation, corrosion, and ignition has recently been considered in the aerospace and automotive industries. Traditional machining of this alloy is so difficult. In the current study, electrical discharge machining of γ-TiAl samples is investigated using different tool electrodes of graphite, copper, and aluminum. The results show that when using aluminum electrodes, tool wear rate is averagely 3.2 times more than copper and 5.8 times more than graphite tools. In addition, when using graphite electrodes, the average material removal rate is 4.2 times more than copper and 7.7 times more than aluminum. Machining by aluminum tool leads to formation of Al2O3 and TiO2 oxide compounds on the work surface but in machining by graphite electrode, TiC and Ti8C5 carbide phases are created on the work surface. In machining by graphite due to formation of hard carbide compounds in the recast layer, the microhardness is higher than the machined sample by the aluminum tool, where oxide compounds exist on the surface and the hardness of recast layer in the machined sample by copper electrode is less than the other two electrodes, because of existing phases such as copper oxide with less hardness. The highest electrochemical corrosion resistance belongs to the machined specimen using graphite tool and the lowest corrosion resistance is related to the machined sample by aluminum electrode. Reducing oxide and aluminum compounds and increasing carbide phases enhance the corrosion resistance of γ-TiAl machined samples.

Using proper dimensionless coefficients that are insensitive to various operating conditions is a crucial issue during the utilization of a yawmeter probe. These dimensionless coefficients produce the deviation angle of flow, stagnation and static pressures. In the current study, these coefficients are analyzed using SPM analytical and experimental methods. A comparison of experimental and analytical results shows that SPM analytical method predicts the flow deviation coefficients satisfactorily at the operational angle range of three-hole probe. This method also calculates the stagnation pressure coefficient precisely at the deviation angle range of ±10 degrees. The experimental results show that due to the assumption of constant speed on the probe, the analytical method cannot calculate the static pressure accurately. Experimental observations also demonstrate that velocity is increased and pressure is decreased over the probe. This is due to the suction region at the downstream of probe. Unlike analytical results, experimental observations depict that at zero degrees, the flow static pressure is equal to the average of pressure at the left and the right side of probe. Due to sensitivity of dimensionless coefficients of flow static pressure to variation of Reynolds number, various values are reported at different kinds of literature for these coefficients. These coefficients change with Reynolds number variations and their accuracies are decreased. In the current study, a new proper dimensionless coefficient is introduced which represents minimum sensitivity to Reynolds number.

Residual vibrations suppression of suspended payload transporting has numerous applications in the field of transporting. In previous studies, many control methods have been applied to reduce vibrations. Imprecise dynamic modeling, using sensor equipment, and high-cost designing of control systems decrease the performance of these methods. In the present study, an optimal trajectory of payload transport by dynamic programming algorithm is generated to reduce the residual swing. Dynamic programming algorithm is a computational technique by which breaking the problem down into sub-problems, an optimal trajectory recursively is executed with the sequence of sub-decision. In addition, input shaping method is applied to create the optimal trajectory. In this technique, the residual vibration is reduced by convolving an impulse sequence with a transport trajectory and consequently a desired trajectory creating. The simulation of optimal trajectories has been done in EDMS software. Regarding to the uncertainty of the dynamic modeling to which result error computational in input shaping technique, the dynamic programming algorithm is suggested for rapid transport of nonlinear systems. Experimental simulation section is carried out with connecting the pendulum to a robot to measure the vibration in ending of the transport and the time needed after swing stopping. Finally, the simulation results showed that the dynamic programming implementation leads to the reduction of the residual swing in the ending of the transport more than the prior method. Besides, the time needed for stop swing is 2 seconds lower than polynomial trajectory and 1 second lower than input shaping.

Initial bias is a random parameter in micro-electro-mechanical rate gyroscopes that changes with each turn on and turn off. The bias can be estimated by averaging in static condition or by extended Kalman filter in other conditions. In addition, this parameter is affected by temperature or linear acceleration. Curve fitting on the bias variation of micro-electro-mechanical rate gyroscopes due to thermal effects is a usual method for thermal compensation of these sensors. However, these approximate curves cannot completely compensate the effect of the thermal bias in long-time applications. In this study, it is tried to improve the calculating accuracy by a combination of extended Kalman filter and the results of these curves and using advantages of both methods. Also bias estimation is improved using the switching algorithm in accelerated motions by avoiding improper data in the estimation process. Experimental tests show the effectiveness of this method especially in long-time accelerated motions.

Incremental forming method with lower cost and more flexibility can be a suitable alternative for traditional methods of the hole-flanging. In this study, the possibility of square hole-flanging of AL1050 aluminum sheet using incremental forming method has been investigated and the quality of the pyramid flange has been compared with conical flange. The final shape of the flange is defined so that wall angle increases with raising height. The process simulation was performed using Abaqus software and an experimental test was done to validate the simulation results. After performing the experimental tests, flange features such as the final size of the hole, flange height, and wall thickness were measured. The results showed that at the created flange around the circular hole, there is less spring back and more dimensional accuracy, however, it can be flanged a square hole by incremental approach with consideration of the height and hole size. The dimensional measurements showed that the final size of the hole will increase after the hole-flanging. By investigation of the various holes, it was found that in the larger initial hole, increasing the hole size after the flanging will be lower.

Electrochemical machining (ECM) has unique features and advantages which is a suitable method for machining when surface quality and residual stresses are of importance. Because of various parameters that influence this process, numerical and experimental studies play a key role in feasibility, practical utilization, and selection of optimal machining parameters in different materials and applications. On the other hand, with the high technology used in the casting of nickel-based single crystal superalloys, no grain boundaries are created in the material. Therefore, by improving the mechanical properties of this material, the traditional machining processes are not effective and economical. Also, they cause defects such as residual stresses, tool wear, and poor surface quality. The purpose of this research is to investigate numerically the electrochemical machining on this special superalloy. Comsol software is used for process modeling and numerical analysis. Firstly, the electrical current and voltage in the machining gap are determined, and finally, the workpiece displacement boundary is obtained. Then the numerical conditions of machining parameters are implemented for experimental investigation by electrochemical machining machine. About 8% error between the results of numerical simulation and experimental investigation shows the feasibility and capability of this modern machining for this particular superalloy.

Cooling and heating energy accounts for a significant portion of the total energy consumption in residential sector. Building envelope is exposed to sunlight and outside air and therefore have a significant role in determining the thermal loads of buildings. Meanwhile, roofs which are exposed to sunlight all the day long are important envelope components and have a significant share of buildings energy consumption. Therefore, applying appropriate roof solutions can significantly reduce building energy consumption for air-conditioning and improve indoor comfort conditions. This paper aims to investigate the effect of different roofing techniques on thermal performance of a single-storey residential building with two types of uninsulated and insulated configurations under different climatic conditions of Iran. For this purpose, different cool roof albedos 0.5, 0.7, and 0.9 and two types of green roofs, GR with actual local rainfall and wet GR, are considered. The thermal loads of the buildings are calculated using the DesignBuilder software and compared with a conventional cast concrete roof. The results show that by choosing an appropriate type of roof technique, the total air-conditioning energy requirement of the building can be reduced between 7-31%, depending on the building configuration and climatic condition.

Due to developments in additive manufacturing (AM) techniques, design and producing cellular structures with complex topologies accompanied with appropriate mechanical properties and lightweight have become possible and the application of cellular porous materials has been increasing in various areas. In the current study, a novel cellular structure with adjustable radially graded relative density and properties inspired by bone tissue structure is designed and introduced. The cellular structure has five layers and is achieved by repeating a regular four-sided unit-cell in radial, peripheral, and axial directions by a specific pattern. Next, using analytical relations, the mechanical properties of the structure are derived. The obtained theoretical solution is validated by numerical modeling and experimental test of a polymeric specimen manufactured by SLA method. Comparison of the results shows good precision of the theoretical solution. Furthermore, the effect of design parameters including the height of representative volume element, the number of the sides of start shape, and radius of the struts on mechanical properties and their distribution is studied. Using genetic algorithms single-objective and multi-objective optimization is performed on elastic properties of the structure. The single-objective optimization results for structure with 70, 75, and 80% porosities led to 32.9, 35.92, and 35.68% improvement of elastic modulus to mass, respectively and 116.35, 96.48, and 73.62% increase of yield strength to mass at similar porosities compared to base models with same porosities. The results show proper ability of the structure in creating distribution of mechanical properties and porosity and its potential capability for use in bone replacement applications.

Combustion chamber has a crucial role in gas turbines and has a significant effect on the pollution and efficiency of them. Due to the complicated flow in combustion chambers because of high turbulence intensity, flow mixing, and flame behavior, prediction of the performance of such chambers is very complicated. There is a vital need for experimental investigations to study and understand the flame behavior in combustors. This experimental study was performed using a can type combustion chamber and LPG fuel at atmospheric conditions. First, stability curve, temperature distribution in the combustion chamber, and its exit plane in 6 flow conditions and then flow behavior were evaluated. The pollution at the outlet was obtained in different conditions and equivalence ratios. The results show that the flame tends to go downstream of the combustion chamber when the fuel mass flow rate increases (or in other words, by increasing the equivalence ratio) in constant air mass flow rate and finally exits from the chamber. By increasing the air mass flow rate in constant fuel mass flow rate, CO pollution is increased, and NOx pollution is decreased.

In the current study, the hot workability of W360 hot work tool steel was investigated by hot tension testing in the cast and wrought conditions in the temperature range of 900-1200°C at strain rate of 0.1s-1. The results showed that in both cast and wrought steels, ductility has increased with increasing temperature from 900 to 1000°C, due to dissolution of carbides and occurrence of dynamic recrystallization. The most recrystallization has occurred at 1050°C and the size of the grains has decreased. This reduction in wrought steel was more evident due to its smaller primary grain size. Wrought samples showed higher hot ductility and lower peak stress than cast samples. The ductility of cast steel depicted a significant decrease at 1200°C due to the presence of undissolved particles along grain boundaries and the stress concentration and thus formation of granular cracks surrounding them. It is while the breakdown of particles has prevented the stress concentration around them in the wrought steel. According to microscopic images of the samples after the hot tension test, in the wrought samples, the continuous alloyed carbide nets were broken during the rolling and occurrence of recrystallization and the carbides has become smaller and their distribution was more uniform. This issue reduces the stress concentration around the carbides in the wrought samples and thus leads to higher hot workability than the cast one. According to the results, the best hot deformation range of W360 steel was achieved in the temperature range of 1050 to 1150°C for both cast and wrought steels.

The present study is a numerical model for prediction of turbine flowmeter performance, using the equation of motion based on torque balance theory. In this model, numerical simulations were carried out for a 2-inch diameter G65 and PN/ANSI 150 gas turbine flowmeter which was made by Vemmtec Company, in steady state, using Multiple Reference Frame (MRF) model and Standard k-ε turbulence model using Fluent software. In order to model torque balance equation and calculate angular velocity of rotor, a UDF (User Defined Function) code was created and was added to the software. To evaluate the model's accuracy, simulation results were compared with experimental data which was obtained from manufacturer of the meter. The difference between the simulation results and experimental data was 0.16%, approximately, which indicates the validity of the proposed model in simulating of turbine gas flowmeter performance. The results obtained from the simulation depicted that the velocity distribution asymmetry was more than 0.4Qmax at the downstream of the meter, and because this phenomenon had no negative effect on flow measurement, the suitable length for the flow development for the downstream of meter was done using simulation at least 10 times the diameter of the pipe was proposed. Therefore, using the proposed model, the capital cost of design and optimization of turbine flowmeters can be reduced.