مجله مکانیک سازه ها و شاره ها
سال چهارم شماره 3 (پاییز 1393)

Most of structural mechanics problems can be solved efficiently by applying the optimization methods. In the structural analysis، a mathematical model is created from idealized structure، which consists of a set of equations with many variables and different matrices. Since the forms of these matrices are not unique، the optimal solution seeks for the most efficient format of the matrices. In this paper، an algorithm is developed for formation of optimal null bases in tetrahedron finite element models. In the proposed optimization model، the interface graph is defined to transfer the topological property of a tetrahedron element into the connectivity of a graph، and a generator is considered for each basis in order to satisfy the independency of statical bases vectors. Each sequence of generators is defined as a tour for an ant travel in ant colony optimization algorithm; therefore an efficient algorithm based on the ant colony system is presented. The optimal null bases result in highly sparse flexibility matrices and efficient force method. One significant feature of the optimal force method analysis is that the calculation is divided in to two independent part. The importance of this feature appears when solving a series of problems with fixed geometry but variable mechanical characteristics of members، such as redesign problems.

A novel control approach for an active suspension system of a vehicle is presented by using the concept of mechanical impedance. The dynamical behavior of the suspension system is controlled subject to road disturbances. The active suspension system is designed for a one–quarter model of vehicle with considering the effects of nonlinearity of the hydraulic actuator. The control design consists of two control loops; the outer loop is an impedance adaptive control while the inner loop is a proportional-integral force control. This research presents the stability analysis and verifies the control method by the stability analysis and simulation results. Adaptive control can overcome the parametric uncertainty. This control approach provides the passenger comfort when passing a bump and ensures both the passenger comfort and vehicle handling after passing the bump. The proposed approach is compared with the passive suspension system. Simulation results show the superiority of active suspension system over the passive suspension system in terms of the vehicle handling and passenger comfort.

Electro-thermo-mechanical transverse buckling of an embedded piezoelectric nanobeam (PNB) is investigated in this article based on Reddy beam theory (RBT). Surrounded elastic medium is simulated by the Pasternak foundation. The small scale effects are taken into account using strain gradient theory (SGT). In order to control the vibration characteristics، the PNB is subjected to an applied voltage in the thickness direction and a uniform temperature change. The governing equations are derived based on the energy method and Hamilton''s principle which are then solved by an analytical method to obtain the critical buckling load. The effects of temperature change، external electric voltage، the material length scale parameters and elastic medium on the buckling load ratio of the PNB are studied in detail. Moreover، a comparison between modified couple stress theory and strain gradient theory is carried out. The presented results indicate that increasing the external applied voltage increases the buckling load ratio of the piezoelectric nanobeam.

Progressive Collapse is a phenomenon in which a large part of a structure is destroyed by a small damage. In recent years، many codes and researches have investigated the behavior of structures against progressive collapse and proposed methods to evaluates the structural responses. One of the methods for estimation of structural response is alternative load path method using linear static، nonlinear static and nonlinear dynamic analysis. Nonlinear dynamic analysis is used to achieve more accurate responses. Since that، duration removed columns in the nonlinear dynamic analysis has a significant impact on the structural response، the purpose of this paper is to evaluate the effect of structural response due to duration removed columns. For this purpose، some ordinary buildings was selected and their reponses was evaluated against column elimination''s time and critical conditions was determined. Since that، in design codes، column elimination''s time، depends on the vertical period of the bays above the removed column، a proposed method is presented for determination this time.

Dam break time history have always been the intrest of many researchers. This phenomena can be modeled by Eulerian or Lagrangian approach which each of them have their benefits and disadvantages. In this paper isogeometric method is utilized for modeling flow in dam break analysis by Lagrangian approach. Mass and momentum conservation laws are governing equations of flow which are solved by pressure correction in Lagrangian approach. Least square method is used for discretization of space. Matrix size is notably smaller in isogeometric method in comparison with other methods (finite elements and meshless methods). Also stiffness matrix is symmetric and positive definitive. NURBS (Non - Uniform Rational B - Spline) functions are used as shape functions. Free surface profile and pressure values of isogeometric method in different times are compared with a meshless method. The results indicate the ability of the proposed method in solution of moving fluid with moving boundaries.

In stepped beams for finding natural frequency، the beam is divided into two segments in step location and using boundary conditions and continuity conditions between two segments eigenvalue problem has been computed. This procedure needs the numerical solution to determine frequency and cannot show the affects of various parameters on the frequency explicitly. In this paper، a polynomial function added to fundamental mode shape of hinged-hinged beam and obtained function considered as fundamental mode shape of stepped beam. then، the unknown coefficients of polynomial are determined by using the boundary conditions and continuity conditions in step location، and an analytical expression has been derived for fundamental frequency by using Rayleigh method which shows effects of different parameters in explicit form. the accuracy of derived formulation is confirmed by comparing with exact analytical solution of stepped hinged beam on elastic foundation for different values of step location and ratio. The derived analytical relation in this paper is a simple and accurate relation for determining the fundamental frequency of stepped hinged beam on elastic foundation.

In the present study، fluid flow around and heat transfer from a rotating circular cylinder confined in a channel are studied numerically using overset grid method. The governing equations consist of continuity، momentum and energy equations are solved for laminar flow. Numerical simulations are performed for Reynolds numbers varying from 100 to 500، non dimensional rotational velocities of 0، 1 and 2، blockage ratios of 0. 1، 0. 3 and 0. 5 and prandtl numbers of 0. 7 and 7. The results show that the transition from steady flow to unsteady flow is delayed as blockage ratio increases. Drag coefficient and heat transfer are reduced by rotation of the cylinder، In contrast as blockage ratio increases، Drag coefficient and heat transfer increases. In unsteady flow past a circular cylinder confined in a channel، the pattern of shed vortices is different from that observed in free stream flow past a circular cylinder and the vortices behind the cylinder moves criss-cross.

Tailor welded blanks are the blanks made of one material with two different thicknesses or two various materials which are joined together by welding process. Due to the wide application of these blanks، prediction of their formability limit is very important. In this investigation، first tensile test is carried out on the steel standard specimen with thicknesses of 0. 8 and 1. 2 millimeter، and tailor welded blank specimen and force-displacement diagram is determined for the standard specimen. Then، forming limit diagram of the tailor welded blank is achieved from performing the Erichsen''s test. Using the above diagram and ductile damage criterion، the standard tensile test، Erichsen''s test، and deep drawing processes are simulated in the ABAQUS/Explicit code and damage evolution، crack onset، and fracture in each processes are predicted. Finally، the numerical simulation results are compared and validated with the practical results.

In this paper، a novel technique is presented for determining of operating and surge point in compressor via sensor fault tolerant control. The analytical redundancy method and the dynamic neural network (DNN) based on robust identification scheme is presented to determine of compressor surge point accurately، even in the presence of uncertainty in the compressor and noise in the sensor. Generally، the main drawback of DNN method is the lack of systematic law for selecting of initial Hurwitz matrix. Therefore، in this paper the subspace identification method is proposed for selecting this matrix. The required data is obtained from compressor Moore-Greitzer simulated model. In the residual evaluation block، a specified algorithm is proposed for obtaining fault properties. Virtual sensor idea is utilized for fault tolerant control system. A number of simulation results are carried out to demonstrate and illustrate the advantages، capabilities، and performance of our proposed fault toleant control scheme.

In polymer electrolyte membrane fuel cells (PEMFCs) more than half of the chemical energy of hydrogen is converted into heat during electricity generation. This heat، If not exhausted properly، impairs the performance and durability of cell. In this paper، several different parallel channel designs are proposed for cooling flow field in PEMFC، and heat transfer in cooling plates is simulated through computational fluid dynamics. The performance of suggested designs are assessed in terms of maximum surface temperature، average surface temperature، temperature uniformity and the pressure drop characteristics. The results indicate that the model with wavy channels، has the least temperature uniformity index، the least maximum surface temperature and the least average surface temperature; therefore in terms of cooling performance، is the best model among the models investigatedin this article; while this proposed model incurs more pressure drop. The model with straight channels with trapezoidal cross section area، has weaker temperature characteristics than the model with wavy channels; while this model incurs less pressure drop. These two models are more suitable for cooling than available models; but for choosing one model between them، a compromise must be made between temperature characteristics and minimum pressure drop.

In this paper، metal foams as flow field are used in structures to effectively distribute the gaseous reactants for the electrochemical reactions، reduce weight، save the cost of machining of flow-field channels and enhance performance in the PEM fuel cells. A 3D model is considered and a set of equations consist of continuity، momentum، species and charge together with electrochemical kinetics in the form of single domain is developed and solved numerically. The comparison is made between the PEM unit cell with metal foam and parallel channel as flow-field gas distributor. The results show that after using metal foam flow field designs، the reactant gases transfer and current density increases and gases and current density distribution improves. The performance of the cell with metal foam are similar to cell with paralle channels for low and intermediate current densities; however by increasing current density، the cell with metal foam has better performances، due to omit of ribs effects. Furthermore، metal foam high permeability and foam structure help distributing the reactant gases evenly while keeping pressure loss and therefore parasitic power loss. low.

According to the ever-increasing need for low-cost and unlimited energy sources، renewable energies have been taken into consideration. Solar energy is one of the main sources of renewable energies. In this article، with the use of solar energy، conventional and sloped solar chimney power plant performance in different climates of Iran has been examined. To do this، the appropriate mathematical models for the radiation of the Sun، solar collector and chimney are used. For a better analysis، three types of horizontal collectors and 30 ° and 60 ° sloped ones are considered and their received radiation and electric power outputs are compared together. The results show that the horizontal collectors produce more power in the summer، while in the winter، power output increases with an increase in collector slope. Also، 300 sloped collectors get the most Sun''s radiation، while 60° sloped ones produce maximum electrical power. Finely، among the considered cities، Yazd has the best performance.

The aim of this paper is to investigate cogeneration of electricity and hot water. In the presented design، thermoelectric module has been devised in the path of heat transfer from solar heat absorber surface to a water reserve. Besides، Fresnel lens is employed to concentrate sunlight as well as to reach justified temperature by the means of thermoelectric modules. The Fresnel lens belongs to a class of point focus lenses، which its dimension is 30*30 cm2. In order to optimal use of the thermoelectric capacity، it is proposed to devise an array of Fresnel lenses which can transfer heat to the thermoelectric module via mineral oil. The results show that approximately 186W thermal power and 6W electric power can be obtained by use of a thermoelectric module (type TEP1-12656-0. 6) and 6 Fresnel lenses. Results indicate that the maximum thermoelectric power generation is 1. 08W under radiation intensity of 705. 98 W/m2. Moreover، thermal efficiency of system is about 51. 85%.

Entropy generation of nanofluids during natural convection in rectangular porous enclosures is analyzed in this paper. The objective is to find optimum circumstances from the standpoints of the First Law and the Second Law of thermodynamics. For this purpose، the mass، momentum، and energy conservation equations are solved numerically. Thereafter، the generation of entropy is calculated and discussed. Computations are undertaken for Cu، Al2O3، and TiO2 nanoparticles in a base fluid of water and the corresponding results are compared. Moreover، the influences of volume fraction of the nanoparticles، Rayleigh number، enclosure aspect ratio، radiation exchange، and non-Darcy effects on heat transfer and entropy generation in the porous enclosure are analyzed. Inspection of the presented results demonstrates that radiation exchange and non-Darcy effects possess prominent consequences on heat transfer and entropy generation inside the enclosure. Among the current nanofluids، the highest heat transfer and entropy generation appears in the Cu-water nanofluid.