نشریه مهندسی مکانیک مدرس
سال دوازدهم شماره 1 (بهار 1391)

In this study, a single bubble behavior in dielectric viscous fluid under the uniform magnetic field has been simulated numerically by using a level set method in two-phase bubbly flow. The two-phase bubbly flow considered to be laminar and homogenous. Deformation of the bubble was considered due to buoyancy and magnetic forces induced from the external applied magnetic field. A computer code was developed to solve the problem with flow field, interface of two-phases, and the magnetic field. The Finite Volume method was applied using SIMPLE algorithm to differentiate the governing equations. Using this algorithm enables us to calculate the pressure parameter which was eliminated by previous researchers due to complexity of the two-phase flow. The Finite Difference method was used to solve the magnetic field equation. The results outlined in the present study well agree with the existing experimental data and numerical results. The results show that the magnetic field affects and controls the shape, size, velocity and location of the bubble.

In this paper, the influence of nanoclay Closite 30B on ballistic impact behavior of 2D woven E- Glass/Epoxy laminated composite has been investigated experimentally. The glass/epoxy/nanoclay laminate nanocomposites have 12 layers and 60% fiber volume fraction is manufactured by VRTM method. Fibers have a plain weave configuration with density of 200gr/m2, while The epoxy resin system is made of a diglycidyl ether of bisphenol A (DGEBA), Epon 828, as the epoxy prepolymer and a polyoxypropylene diamine with average molecular weight of 400 gr/mol, Jeffamine D-400, as the curing agent. The nanoclay Closite 30B is dispersed into the epoxy system in a 0%, 1%, 2%, 3%, 5% and 7% ratio in weight with respect to the matrix. Morphological studies using XRD revealed that nanostructures are mostly in intercalated form rather than exfoliated form. In additional to tensile test, ballistic impact test is carried out on the samples by flat-ended projectile with 14gr mass and 9.77mm diameter in 130m/s, 142m/s and 155m/s velocities. The results have shown that not only the mechanical properties, but also ballistic impact resistance can be improved with adding nanoclay.

In this paper, the normal flow permeability of an ordered fibrous structure, with square cross section is determined analytically. Porous structure is represented by a “unit cell” which is assumed to be repeated throughout the media. The selected unit cell is the space between four cylinders with square cross section. The analytical solution leads to a closed form equation for normal permeability of porous media including the geometrical parameters. Due to lack of experimental and analytical data for fluid flow through square cross section structures, a numerical study is also done and the obtained results are compared with the analytical solution. Moreover, a suitable scale analysis approach is employed to estimate the permeability of the fibrous structure. The presented method can predict the permeability of fibrous media, especially at high porosity. The present solution is successfully compared with the existing model in the literature over a wide range of porosity.

In the initial steps of grass-root projects, heat exchangers cost estimation for proper decision making regarding network optimization design through pinch technology is very important. It is necessary to forecast the cost of a heat exchanger through cost estimation equations. In this article, the cost estimation equations of heat exchanger and the heat exchangers network for shell and tube heat exchangers and plate and frame heat exchangers are obtained. These equations are the function of heat transfer area by available prices in Iran. The proposed relations for Shell and tube heat exchangers are in the maximum operating pressure region of shell and tube side between 1.2–41 bars and made of two types of material: carbon steel and stainless steel 316. Furthermore, plate and frame heat exchangers relations in two regions of the inlet cold flow lower than 30 cubic meters and between 30 to 50 cubic meters are obtained. These plate and frame heat exchangers are made of stainless steel 316. Moreover, here the error of the proposed equations is studied.

In this article, for the first time the numerical solution of Godunov method with HLLC Riemann solver is extended for a hyperbolic five equations two-fluid model. The flow field is considered for two-space dimensional case. So far, two main difficulties include non-monotonic behavior of mixture sound relation and inability of shock transition from interface was mentioned during working with this model. In this research these difficulties were overcome by selecting an appropriate mixture sound relation and appropriate discretization of non-conservative term. The mutual effect of shock wave impact with a droplet and two droplets with different diameters were simulated and studied. During the shock wave impact with 1.47 and 6 Mach with the droplet, a complicated shape of interface was formed with high pressure zone and low pressure zone of cavitations. The results obtained from the present attempt were compared with the experimental and related similar results of that obtained by the other numerical methods and models. The comparison of the results was good. It was also concluded that the numerical method used in the present work has enough accuracy with high capability in capturing two-phase flow interfacial instability and shock wave impact transmission from the droplet.

In this paper, the kinematic path planning of a special hyper-redundant manipulator with lockable joints is studied. In this manipulator the extra cables are replaced by a locking system to reduce weight of the structure and the number of actuators. In this research, the particle swarm optimization is used for path planning. In addition, the kinematic constraints such as joint limits are considered. In the first part of this paper, the minimum switch path planning is solved. This kind of path planning will decrease the vibration and energy consumption and increase the accuracy of manipulator. To validate the result, an innovative test is designed. According to the test results, the performance of the proposed method is shown. In the second part of the paper, the minimum time trajectory planning is studied based on the bang-bang theory. The inverse kinematic of manipulator is calculated such that the sum of legs length changes is decreased. Finally, the result of trajectory planning obtained from particle swarm optimization are compared to simulated annealing optimization results to confirm the performance and correctness of results.

In this paper, the effect of the crack on the vibration behavior of a thick-walled cracked pipe conveying fluid is investigated. The presence of a crack on the pipe introduces considerable local flexibility at the crack location. This flexibility is modeled by the fracture mechanics approach. The accuracy of the model is validated through the experimental data reported in the literature. Then, by using the mentioned model, the vibration analysis of the cracked pipe conveying fluid has been accomplished. Moreover, in order to solve the equation governing the vibration of the cracked pipe conveying fluid, a new analytical technique based on the power series method is proposed. Then, by applying the boundary conditions and the compatibility conditions at the crack location, the frequency equation is obtained. The results are presented by appropriate curves showing the variation of the natural frequency of the cracked pipe conveying fluid in terms of the crack depth and the fluid flow velocity. Also, the results show that for a cracked pipe with a given depth and location for the crack, by increasing the fluid flow velocity, the natural frequencies of the pipe decrease. Also, as the fluid velocity approaches to a certain value, the fundamental natural frequency approaches zero and instability occurs.

In this paper, fatigue lives of tensile-shear multi-spot welded joints with four spot per specimen with different arrangements have been investigated. For this purpose, three types of joints with axial, transverse and almond-shaped have been selected. The prediction of fatigue life of spot welded joints has been carried out based on the available S-N curve of smooth specimens and the values of fatigue strength reduction factors. The experimental results and numerical predictions were compared and it was observed a good agreement between experimental and numerical ones. In addition, the Experimental Modal Analysis (EMA) was applied on the specimens before and after different fatigue loadings. Then percent of the identified natural frequencies decreasing was investigated and its correlation with crack initiation and total fatigue life of spot welded joints was studied.

Component of vapor-compression refrigeration cycle was modeled at steady state condition. Then, modeling and simulation of the whole cycle was performed to predict system parameters such as compressor work, cooling effect and coefficient of performance (COP) in various ambient conditions. The simulation results were compared with experimental results obtained from an experimental investigation on a split-type air conditioner. It was found that the experimental and simulation results are in good agreement and the model can predict the performance of the cycle successfully. Average difference between experimental and simulation results for prediction of COP was 4.5%. Simulation results show that for each 1℃ increase in ambient temperature, COP reduces 3.5%, and for 10% increase in ambient relative humidity, COP increases about 6.5%. Also, by increasing the air volumetric flow rate of condenser about 10%, COP increases about 5%. Effect of increasing the condenser area on its heat rejection rate was studied and it was found that increasing the condenser area, increases the heat rejection rate substantially only in a limited range and after that it does not change.

This paper presents the closed-form calibration procedure of a 5-Dof Mitsubishi robot. In this method only the joint angle information is required. But due to the limitation of the robot degrees of freedom it is not possible to attach the end-effector of the robot directly to the ground; however, we can use a bar with two ball end joints for this purpose. By doing this, the robot can move freely in space. The most limiting factor of the closed-loop calibration of robot is that we cannot measure the non-moving joints, and we have to use other joint to estimate the motion of these joints. A novel approach to estimate the non-moving degrees of freedom are presented in the paper that can be extended to other robots. Experimental results validate the proposed method and the deviation of the joint parameters compared with the nominal values of the robot parameters delivered in the catalogue is very limited and are in an acceptable ranges.

This paper deals with the effects of duty cycle on improvement of pressure distribution over a NLF0414 airfoil using the plasma actuators. Three Dielectric barrier discharges as the plasma actuators are flush mounted on the airfoil surface in different positions to improve pressure distribution at post-stall angles of attack. The experiments were performed in wind tunnel with pressure tabs measurements at Re_c=7.5×〖10〗^5. The main objective of these experiments is to find the most effective duty cycles for different excitation frequencies and different angles of attack. Results show that the plasma actuators in unsteady excitations are more effective in lower duty cycles on low excitation frequencies but the lower duty cycles lose their effectiveness on higher excitation frequencies.

There is a full connection between the electrochemical quantities of a fuel cell and the curves of the temperature and primary materials at the catalyst region. These quantities are strongly linked to the mass and heat transfer phenomena in the other regions. In the present paper, the lattice-Bolzmann method, as a microscale model with good computational capabilities in the problems such as the fuel cell, has been utilized to simulate the fluids flow and heat transfer in a two-dimensional cross section of a proton exchange membrane fuel cell including the channel, bipolar plate, gas diffusion layer and catalyst of the cathode and the electrochemical characteristics in the catalyst layer have been analyzed. By representing a method for estimation of the changes in the concentration along the channel, the serpentine arrangement has been modeled. The results reveal the essential role of the bipolar plate on the quantities at the catalyst layer.