ESTEGHLAL
Year:2007 | Volume:26 | Issue:1|Papers Count:27

Based on classical plate theory, standard and spectral finite element methods are extended for vibration and dynamic stability of axially moving thin plates subjected to in-plane forces. The formulation of the standard method earned through Hamilton's principle is independent of element type. But for solving numerical examples, an isoperimetric quadrilateral element is developed using Lagrange interpolation functions. The spectral method is, in fact, the solution of motion equation for an axially moving plate. Although this method has some limitations concerning boundary condition of plate and in-plane forces, it leads to an exact solution of free vibration and stability of plates traveling on parallel rollers. The method can be used as a benchmark of accuracy of other numerical methods.

A physical model of gabion overflow dams was studied to determine the velocity profile and Reynolds shear stress.Physical tests were done under two different conditions of dam crest, overflow dams with impermeable and with permeable crests.Instantaneous velocity components over dam crest were measured by an ADV (Acoustic Doppler Velocimeter) instrument. This instrument is capable of measuring instantaneous velocity components with frequencies up to 25 Hz. Average velocity components and bed shear stress were extracted from ADV measurements. The results of this research show the effect of crest permeability on velocity and Reynolds shear stress. The magnitude of Reynolds sheer stresses, horizontal velocity components, and absolute value of vertical velocity components under the permeable scenario are bigger than those of the impermeable scenario. Velocity distribution over the dam crest is different from the universal logarithmic profile.

In spite of the fact that the effect of earthquake on earth dams has been widely studied during the past decades, the complicated behavior of such earth structures against different seismological characteristics is still unknown. Such ambiguities necessitate more accurate studies using more comprehensive computation tools to achieve new results describing the behavior of such structures subjected to earthquake loading. In the present study, the simple soil model of elastic, perfectly plastic (based on the Mohr-Coulomb criterion), and Rayleigh damping criterion have been adopted for the soil. First, the numerical model employed was verified by dynamic analysis of real cases such as “Long Valley” and “Santa Felecia” earth dams. The computational results were then compared with real recorded data or with those reported by other researchers. In addition to evaluating seismic stability of earth dams, their seismic stability was verified using pseudo-static analyses. Therefore, the “Carsington” dam was analyzed to verify the results of pseudo-static analyses and to check the results of FLAC software in calculating the pseudo-static factor of safety. The values of calculated factors of safety in the present study are in good agreement with the published results in the literature. Furthermore, the failure behavior revealed in the analysis shows the ability of FLAC software in defining the failure surface. In the main part of the analyses, a parametric study was conducted for different selected conditions and specially the effect of dam height and the optimum size of crest width were investigated. The results are presented in relevant diagrams.

Marine structuers are one of the most important and susceptible facilities in Iran due to corrosion. The two methods of Cathodic Protection, namely, the cathodic protection with sacrificial anodes and cathodic protection using impressed current, are widely used for corrosion protection. According to the former, sacrificial anodes are installed at several points in the structure.Position of the anodes for achieving the required protection is a problem that engineers are very much interested in, and only empirical methods have so far been used to defermine these positions. Empirical rules, however, might cause either overprotection or underprotection. A major goal of this research is to develop a systematic way for analysis and automated design of Cathodic Protection systems that not only deliver almost uniformly protected structures but also minimize the costs. To this end, a Genetic Algorithm (GA) routine is used to determine the optimal position of anodes on the structure such that a uniformly protected design with minimum cost is achieved. The percentage of protection in each design has been taken as its fitness criterion. To figure out the situation of corrosion protection on the structure, the entire offshore structure with its complex system at anodes and surrounding electrolyte is modeled and analyzed by a finite element algorithm. Employing GA gradually modifies the generation of designs. The design which completely protects the structure and whose cost is minimum is introduced as the optimum design. To show the capability of the proposed method in achieving the optimum design, two examples are offshore presented.

This paper is concerned with elastic local buckling of rectangular plates subjected to intermediate and end inplane loads. In this application, plates (or walls) may carry intermediate loads (whose direction is parallel to end loads) in addition to end loads. Since closed form solution for buckling analysis of plates with different end conditions and subjected to intermediate loads is complicated, numerical methods are more useful. Because of restrictions on the two finite strip methods (classical method and complex method), the spline finite strip method is used to solve the buckling. In spline finite strip method, the longitudinal B3 spline expressions combined with conventional transverse shape functions are used as displacement functions. This method is computationally more efficient than the finite element method, more flexible in boundary treatment, and more accurate in dealing with point forces and axial loads than the conventional finite strip method. Local buckling coefficients are presented for plates under intermediate and end inplane loads which are useful for design of steel walls or plates that support intermediate floors /loads.

The Present Serviceability Rating (PSR) is one of the major criteria in selecting road for rehabilitation. In this paper, statistically realistic models for are developed PSR and free speed correlation on uninterrupted flow facilities. Then, the previously developed relations between free speed and capacity are used to determine the effect of pavement condition on flow capacity. Two nearly identical cars were used and ten raters were selected to evaluate 23 test sections on two major separated roads and an expressway with similar geometric characteristics with a wide range of pavement conditions. Using SAS, it was shown that linear negative exponeatial models were the best regression models that fit the data according to –R2 and T-test.

Application of pile-raft foundations, which are known as “compound foundations”, is a suitable alternative in the case of heavy load structures. The interaction behavior of pile raft foundations makes these systems very complex to analyze.Different methods have been proposed to determine the bearing capacity of piled raft systems and distribution of loads between the components, i.e., pile group and mat. These methods are generally categorized into computer-based and conventional methods. In most of these methods, the bearing capacity of the mat, which is often a great portion of the total capacity, is neglected. Also, some model parameters used in these methods, as well as pile group or raft stiffness, cannot be determined by routine tests or calculations. In this study, a number of recent analytical methods of piled raft system are presented. A new method is then proposed which is based on settlement analysis of piled raft foundation and distribution of load between pile group and mat foundation, which regards the interaction of compound systems as an equivalent block foundation. In this approach, settlement is computed based on the concept of neutral plane according to which relative settlement of soil and pile group become the same. Two practical case studies are implemented for validation of the method. The comparison demonstrates favorable results for the proposed method.

This paper presents the preliminary results of an extensive parametric study on seismic response of two-dimensional semi-sine shaped hills to vertically propagating incident P- and SV-waves. Clear perspectives of the induced diffraction and amplification patterns are given by investigation of time-domain and frequency-domain responses. It is shown that site geometry, wave characteristics, and material parameters are the key parameters governing the hill’s response. Simple formula and some tables are proposed for estimating the characteristic site period and also the average amplification potential of semisine shaped hills, which could be easily applied in site effect microzonation studies of topographic areas.

Deterioration of concrete, which is mainly due to ignorance of environmental and service conditions, causes considerable costs for the construction industry. With this in mind, in this paper, results of investigation into the major causes of concrete deterioration in the Urumieh Lake are presented. For the purposes of this investigation, samples were obtained by mixing two types of cement (OPC types 1&2), micro silica, anti oxide, water proof and air entraining agent, with different w/c ratios and tested at the ages of 7, 14, and 28 days. In addition to compression strength, tensile strength of the samples was measured.Regarding the durability studies, abrasion resistance, electrical resistivity, chloride penetration, water absorption and freeze-thaw tests were carried out under both laboratory and real conditions in the Urumieh lake. Based on our findings recommendations are made about optimum w/c ratio, most suitable types of cement, optimum percentage of micro silica content, and additives.

Considering the vulnerability of double-layer grid space structures to progressive collapse phenomenon, it is necessary to pay special attention to this phenomenon in the design process. Alternate path method is one of the most appropriate and accepted methods for progressive collapse resistant design of structures. Alternate Path Method permits local failure to occur but provides alternate paths around the damaged area so that the structure is able to absorb the applied loads without overall collapse. Following the sudden initial local failure event, severe dynamic effects may arise which should be taken into account in determining the realistic collapse behavior of the structure. In this paper, a new methodology based on alternate path method is presented to apply dynamic effects of initial local failure. The method is called nonlinear dynamic alternate path method. Due to its capability to take account of dynamic nature of the failure, this method can be used to evaluate realistic collapse behavior of the structure and to investigate the vulnerability of the structure to progressive collapse phenomenon.

An approximate numerical method is presented for analysis and determination of modal characteristics in straight, pretwisted non-uniform helicopter blades. The analsysis considers the coupled flapwise bending (out of plane), chordwise bending (in plane), and torsion vibration of both rotating and non-rotating blades. The proposed method is based on the integral expansion of Green functions (structural influence functions) to develop the equations of motion for a clamped-free blade.Several examples are presented in various states such as flapwise bending, coupled bending-bending, coupled bending-torsion, and coupled bending-bending-torsion vibration analysis. The results obtained were compared with available numerical results in the literature. A modal testing and modal analysis were also carried out on a typical helicopter blade in static condition and the results were compared with the numerical ones. The results indicate that the proposed method is fast and robust and can be used for modeling of turbomachine blades, aircraft propellers and helicopter rotor blades.

Design of a natural dry cooling tower has been accomplished in two sections: the design of heat exchangers and the numerical solution of flow through the tower. Heat exchanger (Heller type) has been simulated thermodynamically and then coupled with a computer program, which calculated the turbulent natural convection flow through the tower. The computer program developed for this purpose can be used to obtain thermodynamic properties of the cooling tower such as mass flow rate of air, temperature of outlet water, distribution of temperature, distribution of velocity, and distribution of pressure through the tower. Numerical results have been compared with experimental data of Shahid Montazery Thermal Power Plant under different environmental conditions. Comparison between numerical results and experimental data showed good agreement.

In this paper, the equation of motion of an elastic 2 DoF wing model has been derived using Lagrange’s method. The aerodynamic loads on the wing were calculated via the Strip-Theory and the effect of compressibility was included. Wing deflections due to bending and twist motions were determined using the Assume-Mode method. The aeroelastic equations were solved numerically using the V-g method. The results obtained for different types of wings were in good agreement with experimental data.

In this article, turbulent flow heat transfer in the air gap between rotor and stator of a generator under nonhomogenous heat flux is studied experimentally. The rotor consists of four symmetrical triangular grooves. The stator surface is smooth and does not include any grooves. The relative heat flux between the rotor and the stator is 1 to 3. Temperature and heat flux are measured locally at three axial and two angular positions of inner and outer surface. The pressure drop of air flow through the air gap is also measured. In this work, the axial Reynolds number and rotational velocity of the rotor ranges are 4000<Rez<30000 and 300rpm<w<1500rpm, respectively. The results indicate that increasing the rotational velocity causes the rotor and stator heat transfer coefficient to increase considerablly and the respective value to the rotor is higher than that to the stator. In addition, the rotational velocity causes the air flow to be developed sooner.

The optimal path planning of cooperative manipulators is studied in the present research. Optimal Control Theory is employed to calculate the optimal path of each joint choosing an appropriate index of the system to be minimized and taking the kinematics equations as the constraints. The formulation has been derived using Pontryagin Minimum Principle and results in a Two Point Boundary Value Problem, (TPBVP). The problem is solved for a cooperative manipulator system consisting of two 3-DOF serial robots.

An intensive experimental investigation was conducted to study the effect of vertical tail, single and twin (with different cant angles) on the flow field and the corresponding aerodynamic forces and moments of a model of a fighter A/C.Aerodynamic forces under different flight conditions and different vertical tail settings were measured in a supersonic wind tunnel. Furthermore, effects of vertical tail on the model wake at subsonic speed were investigated. In addition to the force and pressure measurements, schlieren system was used to visualize the shock formation and movement at various locations on the model. The results show existence of a pair of symmetric vortices for the model equipped with a 22 degree vertical tail cant angle.The vortices burst symmetrically at moderate angle of attack. The drag coefficient increases with increasing cant angle at low to moderate alpha and decreases when alpha is further increased.

The blades of wind turbines are the most important parts in producing power output. In this study, a section of a 660 KW wind turbine blade will be installed in Iran in near future was tested in a wind tunnel. In addition to steady tests, various unsteady tests including the effects of reduced frequency mean angle of attack, and amplitudes were carried out. The preliminary results show strong effects of reduced frequency on the aerodynamic coefficients of the airfoil. Moreover, increasing the reduced frequency delays dynamic stall angle of attack but increases lift and drag coefficients compared to the static results. Further, the values of the aerodynamic coefficients in the upstroke motion (increasing angle of attack) are different form their corresponding values in the downstroke (decreasing angle of attack). These differences create a hysteresis loop where its width and shape are strong functions of reduced frequency, mean angle of attack, and amplitudes.

In this paper, properties of slab deformation in sizing press mill as one of the slab reduction processes in hot rolling mills have been evaluated using the elastoviscoplastic finite element method with explicit formulation. Effect of parameters such as initial slab width and thickness, reduction, feed pitch, and anvil speed on factors such as dog bone formation, head and tail fishtail profile, width necking at the leading end of slab, and slab edge quality have been studied. Furthermore, a comparison has been made between the two common width reduction methods, i.e. Vertical Rolling (Edging) and Sizing Press, in order to determine their differences and the efficiency of each process. The amount of width return (back spread), one of the most important factors related to width reduction efficiency and also slab formation after the first horizontal rolling pass, has been evaluated. Also, in order to validate the applied finite element method, the results obtained have been compared with experimental ones found in the literature. The results show that deformation in sizing press is more favourable and that its efficiency is better than that of the vertical rolling mill.

In this paper, an analytical method for noncircular shape extrusion is presented. Using this method, non-linear deformation field can be described with Hermit cubic spline which is prescribed by the boundary conditions of the die at its entry and exit. The Upper bound method has been used to obtain optimum coefficient of the tangential boundary conditions. The results show that the optimum tangential parameter and the extrusion force determined by this method have good agreement with those obtained from other established methods. Also physical modeling tests show that optimum non-linear die could reduce extrusion force and strain variation compared with those in a linear die.

Production of nickel-silvers by mechanical alloying was investigated. Effects of parameters such as milling duration, ball to powder weight ratio, and chemical composition on mechanical alloying process, and alloy’s color and microstructure were studied. The milled powders were characterized, using XRD and SEM. Results showed that nickel-silvers could be produced by mechanical alloying in a wide range of compositions. Alloyed powder with a bright silvery contrast and less than 15 nm grain size could be obtained by optimization of milling parameters. Zinc content of the powder mixture had a significant effect on the minimum alloying time. Ball to powder ratio up to 25 also reduced minimum alloying time but it had no significant effect above this value.

Polypropylene (PP) has poor adhesion to metals and other surfaces for its chemical structure. Hence, chemical modification of PP is necessary for metal surface coating application. In this research, grafting of maleic anhydride (MA) onto co (propylene-b-ethylene) in the presence of a dicumyl peroxide (DCP) was accomplished in a single screw extruder.Characteristics of the modified polymer were determined by Infra-red Spectroscopy (IR), Scanning Electron Microscopy (SEM), and adhesion test. Maximum grafting of MA was found to be 1.2832% for 1.5 pph of MA. Adhesion test showed that the samples containing 1 pph of MA (degree of grafting is 0.5816%) had better adhesion to steel surface (17.25 kgf).

Water-hammer is a transient condition which may occur in a network as a result of rapid or slow valve closures, pump failures, changes in turbine loading, etc. It creates high and low pressure waves which travel along the system and decay as a result of wall shear stress. Comparison of experimental and theoretical results revealed the failure of steady or quasi-steady models in correctly predicting the damping process of the pressure waves. In fact, the velocity profiles have greater gradients under unsteady conditions which results in higher shear stresses compared to the steady condition. In this paper, the transient condition in a network (valve-pipe-tank system) is investigated by implementing one of the unsteady friction models (Brunone model) into the method of characteristics (MOC). Results show that using the unsteady friction model damps the pressure waves more rapidly, the absence of which may result in disagreement between theoretical and experimental values. In addition, this work shows that pressure rise due to the water hammer phenomenon can not be correctly determined without effecting the unsteady friction factor. The valve closure law affects pressure rise prediction.

The aim of this research is to design a controlling loop that eliminates the irregularities in yarn tension during the winding process. In order to achieve this, we employed a relative feedback industrial control system. The yarn tension sensor measures the tension. Its output is analyzed in the automatic controlling unit. This unit adjusts the tension level according to feedback signals, thus adjusting the yarn tension to the desired value. The yarn package wound using this system will additionally experience the least yarn tension variations.