Modares Mechanical Engineering
Year:2016 | Volume:15 | Issue:12|Papers Count:16

In the present paper, nondeterministic CFD has been performed using polynomial chaos expansion and Gram-Schmidt orthogonalization method. The Gram-Schmidt method has been used in the literature for constructing orthogonal basis of polynomial chaos expansion in the projection method. In the present study, for the first time the Gram-Schmidt method is used in regression method. For the purpose of code verification, the output numerical basis of code for uniform and Gaussian probability distribution functions is compared to their corresponding analytical basis. The numerical method is further validated using a classical challenging test function. Comparison of numerical and analytical statistics shows that the developed numerical method is able to return reliable results for the statistical quantities of interest. Subsequently, the problem of stochastic heat transfer in a grooved channel was investigated. The inlet velocity, hot wall temperature and fluid thermal conductivity were considered uncertain with arbitrary probability distribution functions. The UQ analysis was performed by coupling the UQ code with a CFD code. The validity of numerical results was evaluated using a Monte-Carlo simulation with 2000 LHS samples. Comparison of polynomial chaos expansion and Monte-Carlo simulation results reveals an acceptable agreement. In addition, a sensitivity analysis was carried out using Sobol indices and sensitivity of results on each input uncertain parameter was studied.

Laser forming is a flexible forming process that needs no hard tooling or external forces. In this paper, laser forming of cylindrical surfaces with arbitrary radius of curvature is investigated analytically and experimentally. As the laser forming process is a die-less forming process, production of a desired shape from initial blank is very difficult with this process. Because in the laser forming process, there are some variable parameters such as laser power, laser beam diameter, laser scanning speed and dimensions of initial blank that directly affect the final shape of the produced part. Also, in addition to the above mentioned parameters, in the laser forming process of a cylindrical surface, a new parameter says the number of irradiating lines is added to variable parameters. Therefore complexity of laser forming of a cylindrical surface will be more than a simple laser bending. In this paper, an analytical method for laser forming of cylindrical surfaces with arbitrary radius of curvature is proposed. In the proposed method, with the aim of technical determining limitations of laser machine such as laser power, laser beam diameter and laser scanning speed, the number of irradiating lines and the distance between neighboring lines are proposed for production of cylindrical surfaces with arbitrary radius of curvature. Also, using experimental tests the performance and accuracy of the proposed method are investigated and verified. Analytical and experimental results show that with the proposed analytical method, cylindrical surfaces with any arbitrary radius of curvature can be produced with very good accuracy.

Early crack detection in structures prevents the occurrence of damage. Therefore, a challenge exists in the literature to provide efficient methods in the field of structural health monitoring. Many of the researches that have been done on the crack identification in structures are based on the models that ignore crack closure effects that make a significant error in the crack identification. Since it is more difficult to identify breathing crack than other damages, the purpose of this research is providing an efficient algorithm to identify breathing crack in beam type structures which are important elements in various types of structures. Because it's an important and significant issue to present a perfect model that is able to accurately explain behavior of damaged structure, in this paper a cracked beam has been modeled by ''breathing crack'' model which explains beam responses more accurately than other common models. In order to calculate natural frequencies of the beam accurately, in this research the fatigue crack model is used, which considers crack as breathing one with opening and closing behaviour. Then the problem of identifying crack parameters (location and depth of the crack) is defined as an optimization problem with the aim of minimizing the differences between natural frequencies calculated by the model and measured natural frequencies. In order to choose an appropriate algorithm to identify breathing crack, algorithms among various meta-heuristic algorithms are selected, which are able to identify the crack using only two natural frequencies. This ability is more valuable when it is desired not to damage structure during health monitoring procedure based on using frequency response of structure. Regarding the surveys conducted, the optimization problem is solved using cat swarm optimization (CSO) algorithm. Moreover, for validation the results obtained for different crack parameters have been compared with those of experimental tests. The results indicate that the proposed method has good accuracy.

In this study, the design and optimization of a honeycomb energy absorber is performed using genetic algorithm. The main design goal is to absorb almost all the impact energy. Simultaneously, reduction of the shock force level is also considered as a main objective. In the first part, the crashworthiness behavior of honeycomb structure is parametrically studied. The results are utilized in the second part to optimize shock absorber design. In this part, aluminum honeycomb structure under dynamic loading is investigated using simulation in LS-dyna finite element code. Parametric studies are invoked to identify the influence of different model parameters on crashworthiness characteristics of honeycomb structure. Reducing the computational cost, a repeatable model of 'Y' cross section column is numerically simulated. The effects of changes in material properties including Young's modulus, yield stress, tangent modulus, geometrical properties such as cell size, foil thickness, as well as the effects of impact velocity on the deformation behavior of the structure were investigated. A number of 25 different geometries with same height and various cell sizes and thicknesses are studied and effects of thickness and cell size on the energy absorption properties are investigated. Results showed that crashworthiness parameters such as mean and peak stress depend mainly on cell size and thickness values, while the friction coefficient and young's modulus are of less importance. Any change in absorber’s geometry affects the mean collapse stress more severely than the peak stress. In the meantime, thickness change is more effective in comparison with cell size change.

In this study, honeycomb energy absorber is optimized using genetic algorithm. The design goal is to absorb whole impact energy within a limited shock load level. First the crashworthiness and parameter sensitivity of honeycomb structure are extracted as explicit functions that are utilized to find optimized shock absorber configuration. Energy absorber must depreciate the impact kinetic energy and mitigate its defects on the structure and aboard. So the energy absorption capacity while the shock load is kept limited is the main design objective. The volume and mass restrictions are also important objectives from an application point of view. Based on the simulation results available in the article Part I, the honeycomb response surfaces of crashworthiness parameters including the mean and peak crushing stresses are extracted. Utilizing the genetic algorithm based on response functions, the multi-objective optimized energy absorber is investigated. The main objective of the optimization problem is set to minimization of mass or volume while the maximum allowable shock and minimum energy absorption capacity are included as the problem constraints. The geometric specifications of honeycomb structure including cell-size, foil thickness, height and absorber face area are among the design variables with optimization outputs of energy absorption capacity, volume, mass, and shock level. Some optimization results are compared with those available in the literature and a typical problem is handled. Results show that mass and volume optimized geometries are almost similar and reduction of acceptable shock level causes the optimized geometry height to rise.

Accurate determination of the electro-elastic fields of quantum nanostructures within piezoelectric media is an important issue for realizing the electro-mechanical behavior of these nanostructures. In this paper, the governing partial differential equations corresponding to piezoelectric media containing quantum nanostructures are presented and subsequently, generalized analytical solutions based on Fourier series technique are developed for determination of the coupled electro-elastic fields in transversely isotropic piezoelectric barrier due to periodically distributed quantum nanostructures. The electro-elastic couplings of the piezoelectric barrier as well as the interactions between the quantum nanostructures are exhibited within the framework of the presented analytical solution. It is observed that no electric field and no electric potential will be induced anywhere in the medium for periodic distribution of quantum wires. The presented analytical solution is capable of treating different shapes and geometries of quantum wires/quantum dots. The electro-elastic fields of various shapes of sections of quantum wires and different geometries of quantum dots are studied and the effects of the geometry of periodically distributed quantum nanostructures are demonstrated. The results show that geometry of quantum nanostructures may considerably affect the induced electro-elastic fields and therefore, accurate determination of the geometry of quantum nanostructures as well as the induced electro-elastic fields would be essential for employment of these nanostructures in different fields of research and technology.

The fabrication of nano-composites is quite challenging because uniform dispersion of nano-sized reinforcements in metallic substrate is difficult to achieve using powder metallurgy or liquid processing methods. Friction stir processing (FSP) is a new solid-state process used to modify the refinement of microstructure, improvement of material’s mechanical properties and production of surface layer composites. In this investigation via friction stir processing, metal matrix composite surface (MMCs) was fabricated on surface of 5083 aluminum sheets by means of 5 mm and 80 nm TiO2 particles. The friction processed surface composite layer was analyzed through optical and scanning electron microscopical studies. Effects of reinforcing particle size and FSP pass number on the microstructure, micro hardness, on tensile and wear properties of the developed surfaces were investigated. Results show that the Nano composite layer created by TiO2 particles exhibits a microstructure with smaller grains and higher hardness, strength, and elongation compared to the composite TiO2 layer produced by particles. Increasing FSP pass numbers results in improved distribution of particles, finer grains, and higher hardness, strength, elongation, and wear resistance. The surface composite layer resulted in four passes with change in rotation direction with nano particle reinforcement exhibiting better properties in hardness, tensile behavior and wear resistance compared to the behavior of the base metal.

This paper presents implementation of position control for planar cable-driven parallel robots using Visual servoing. The main contribution of this paper contains three objectives. First, a method is used toward kinematic modeling of the robot using four-bar linkage kinematic concept, which could be used in online control approaches for real-time purposes due to decreasing of the unknown parameters and computation time. Second, in order to track the position of End-Effector, an online image processing procedure is developed and implemented. Finally, as the third contribution, two different controllers in classic and modern approaches are applied in order to validate the model with plant and obtain the most promising controller. As classic controller, pole placement approach is suggested and results demonstrate weaknesses in modeling the uncertainties although they represent acceptable performance. Due to the latter incapability, sliding mode controller is utilized and experimental tests represent effectiveness of this method. Result of the latter procedure is an inimitable operation on the desired task, however, it suffers from chattering effect. Moreover, results of these controllers confirm accommodation between the model and robot. The whole procedure imposed could be applied for any kind of cable-driven parallel robot.

Right selection of the type and number of driver nodes plays an important role in improving the controllability and performance of the dynamical networks. In this paper the controllability and performance of a network has been studied when a new approach for selecting the driver nodes, based on the three main node centrality criteria, has been proposed. For each criterion, the percentage of the least number of driver nodes to achieve the desired performance has been calculated for several network model structures. The results for pure random networks show that for the “betweenness centrality criterion’’ the number of driver nodes is minimal. Similar results hold for Small World networks subject to the fact that for dense, the number of driver nodes increases. It is shown that the “closeness centrality criterion’’ is the proper choice for the State Free networks, especially when the network is dense. Another important result is that in Small World networks, increasing the nearest neighborhood coefficient decreases the number of driver nodes for a desired performance. Similar results hold for Scale Free networks where increasing the heterogeneity coefficient improves the network pinning controllability.

A family of rotating disks used in Iranian turbine and compressor industry is investigated. Mechanical and thermal loads due to working condition leads to the crack initiation in the inner surface of the disk. The aim of this paper is the development of the two-dimensional weight function for the rotating disks containing semi-elliptical longitudinal cracks. The general form of the two-dimensional weight function is related to the proposed weight functions for embedded cracked domain in literature. In order to determine the unknown coefficient of the weight function, the reference stress intensity factors for cracked geometry subjected to reference loads are calculated. The analysis indicated that the results are independent of the number of terms in proposed weight function expansion. Extracting the weight function for disks from 90 to 420 mm thickness enables prediction of stress intensity factor for cracks in the structure subjected to arbitrary loading. The stress intensity factor for each point on the crack front subjected to one or two dimensional loads are calculated using the derived weight function. The results reveal that increasing the height to thickness ratio in rotating disks leads to the increase of the stress intensity factor for high depth ratio crack ones. Results show that the configuration of the disk sections affects the stress intensity factors of the same aspect ratio cracks in the structures. Comparison of the results obtained from the weight function method and those obtained with FEM are in good agreement.

Maintaining and restoring robot balance in the presence of external disturbances is an important issue for a quadruped robot. This is due the fact that these robots move over uneven terrains which may be the sources of the disturbances. In this article, the balance recovery problem of a quadruped robot after an external disturbance will be investigated. To this end, in the first step, the equations of motion of a whole-body model of a robot and also a constraint elimination method will be proposed. In order to recover robot balance, the desired accelerations will be computed based on the concepts of a PD controller and by using the desired velocities and the positions of the main body. However, these accelerations may lead to slipping the stance feet or losing robot stability. Therefore, an optimization problem will be defined to calculate the admissible accelerations and the contact forces simultaneously. The optimal regulation of the contact forces will be done to distribute the contact forces among all stance legs to avoid feet slippage. Since the stability and the slippage avoidance conditions are formulated as linear constraints, the optimization can be solved as a linear constrained least squares problem. To evaluate the effectiveness of the proposed algorithm, it will be examined on a quadruped robot in the simulation in two different case studies: in standing situation and walking gait. Finally, obtained results will be discussed.

"DEFORM" three-dimensional finite element software is used to describe the behavior of plastic deformation of Ti-6Al-4V workpiece during blade preform extrusion process. Under different conditions of extrusion, numerical analysis of the process force parameter during extrusion process is presented. The relative effects of billet temperature, friction coefficient and die temperature on process force were investigated. To determine the process friction coefficient, the ring compression test of Ti-6Al-4V alloy with glass lubrication was performed. Also experimental tests were successfully done in order to manufacture blade preform. It was observed that billet temperature has much effect on force of Ti-6Al-4V alloy blade preform extrusion process. Die temperature has effect on the process force but its effect is not as much as the effect of the billet temperature. By increasing of the die temperature, the process force decreases. Experimental tests showed that the billet transfer process from the furnace to die has important effect on done or not done of the extrusion process because the billet transfer process from the furnace to die is cause of alters the billet initial temperature just before extrusion process. By reducing of the placing and transfer time of billet from the furnace to die, due to the vicinity of the billet and air, billet temperature have less reduction and therefore it becomes easier to shape. Also by increasing the friction coefficient, the force required for extrusion of Ti-6Al-4V alloy blades preform increased.

This research investigates notch effect on fatigue life of HSLA100 steel which is widely applicable in the marine industry. Experimental tensile tests and rotating bending fatigue tests were performed on both smooth and notched cylindrical specimens and the corresponding mechanical properties and S-N curves were obtained. To better investigate the notch and also size effect on fatigue life of the specimens, two different notch geometries and specimen dimensions were used. To calculate the fatigue strength factor, stress distribution under bending load is simulated for smooth and notched specimens. Then, the stress distribution under bending load is converted to stress distribution under rotating bending load using an in-house developed code. Finally, using an in-house developed code, the fatigue strength factor of the specimens is calculated by weakest link theory. In order to better investigate the weakest-link theory, in calculating the fatigue strength factor, this factor is calculated using the classical methods and compared with experimental results. Finally, comparison of theoretical with experimental results shows that the weakest-link theory gives better predictions than other classical methods and the results are closer to experimental ones. Also, Weakest-link theory uses the finite element results to predict notch effect. This facilitates the use of this theory in fatigue design of complicated specimens.

The modeling and optimization of rectangular finned multi stream plate-fin heat exchangers are presented in this paper. The proposed method for thermal modeling of this type of heat exchangers is based on uniform heat distribution along the plates. So, the heat streams are distributed along the multi stream heat exchanger based on two principles: equal quantity of stream channel distribution and uniform heat distribution in each of the channels. The geometric, thermal and hydraulic modeling and design of the multi stream heat exchanger is carried out based on rectangular fin specifications. The total annual cost (TAC), the summation of capital investment and operating and maintenance costs are regarded as objective function to be minimized. The main variables are heat exchanger core dimension such as length, width, height and the fin geometric parameters such as fin pitch and height. The genetic algorithm is utilized as optimization tool to minimize the total annual cost of the multi stream plate-fin heat exchanger. The proposed method is applied to a case study. The results of the current method are compared with the literatures.