In the present study, vibrations in a piping system with a reciprocating compressor and gas flow have been investigated. On this basis, acoustical and mechanical analysis with the help of ANSYS software is performed. Also, the interaction between fluid and structure and vibrations caused by fluid pressure pulsation is considered. The natural acoustical and mechanical frequencies investigated and the system vibrations and fluid pressure pulsationin the system have been analyzed. Then, according to the API 618 standard, unallowable vibrations and pressure pulsation have been identified and the vibration reduction methods of this system have been investigated. In addition, the one-dimensional wave equation is solved by differential quadrature method (DQ). Systemtime response indicates that system pressure and vibration fluctuations are outside the standard range. Therefore, an acoustic filter is used to reduce the oscillations of the pressure and to reduce the vibrations, suitable supports are used. Ultimately, the range of pressure and vibration fluctuations will be within acceptable limits after making adjustments.

Abstract: In the past few decades, various studies have been done about watermelon modeling and its updating using vibration analysis method. In this research, two watermelons of Crimson Sweet and Charleston Gray are updated using experimental modal analysis method, finite element method and bees optimization method. In addition to modeling different parts of watermelons in layered mode, for the first time, Functionally Graded Material has been used in their modeling. After performing experimental modal analysis on number of watermelons and extracting their frequencies, the initial values of the properties in the red, white and green layers are also determined using experimental methods. For analysis, the finite element method has been used. The properties of the watermelons are considered once in layered mode and once again as Functionally Graded Material. The function used in the FGM mode is three-dimensional Basis Spline function. In layered mode, the modulus of elasticity and the density of each layer were considered as design variables. The design variables in FGM mode are also the modulus of elasticity and the density of the keypoints. After the initial model of the finite element design and experimental modal analysis was performed, the model is updated by information obtained from the experiment so that its natural frequencies approach the experimental values. To do this, the optimization process has been performed on the parameters of the finite element model of watermelons, using the bees optimization algorithm. The target function selected in this study is the sum of squares of the relative errors of the natural frequencies of watermelons. After the optimization process, the properties of the watermelons, which are the problem design parameters, are updated in layered and FGM model. The results show the accuracy of the proposed method in updating the properties of watermelons, for both layered and FGM mode.

 Ensure the health structures is an essential function. In this study, a non-destructive method to detect cracks in the pressure vessel is provided. The proposed method is based on changes in the dynamic characteristics of cracks. Using this approach, the location, depth, length and arc center specified in throughput. First, the design and construction of the pressure vessel has been done. Then thefinite element model is created. With this prototype, pre-tests and lie in the right spots to stimulate modal test and measurement, determination is dumped. Then on the pressure vessel, modal test is done by hammer excitation and the natural frequencies extracted. The finite element model has intrinsic error and the error due to lack of existing methods of modeling the structure. Then model updating of finite element model using optimization methods, is done. Then natural frequency of cracked pressure vessel obtained using partice swarm optimization(PSO).

 In this research, the failure of intermediate pressure blades of the steam turbine of Ramin power plant has been investigated. Cracks, according to amount of load, material structure and geometry of the blades, form in different parts.These cracks had spread principally under tensile stress and damage in blades and turbine parts. Therefore, it is necessary to pay more attention to the failure analysis of turbine blades. Here, the blade geometry has been made using a three-dimensional scaning camera. Next, the blade was put into the assembly of the intermediate pressure turbine stage and analysed by using ABAQUS finite element software. The dynamic analysis results showed that the resonance has not caused the failure of the blade. Crack modeling and fatigue growth of the crack have been performed by using ZENCRACK fracture mechanics software. To obtain higher accuracy and reduce the solution time, Sub-modeling method has been used. Because of the complex loading ,the growth of crack was studied in out of crack surface. CTOD and J-Integral methods were implemented for calculating the fracture mechanics quantities and Paris model was used to estimate the fatigue life in each stage of crack growth.

 In this research, vibration control of piping system was studied using active approach. A model of piping system was built in laboratory and was subjected to experimental modal analysis. Then, finite element model of the system was created in ANSYS and updated using an evolutionary algorithm; namely particle swarm optimization. In model updating process, the first four modes of the system were studied and their natural frequencies were modified. Damping ratios, in the form of Rayleigh's damping, of these modes were determined using experimental data. Then, using Gramian method, eighty location were tested for placing actuator and the best one was selected. The mathematical reduced model was obtained using Subspace method. Order of reduced model was determined using root mean square error factor. Finally, a control system was designed for reduced model. In design of the controller, natural frequencies of the system remained constant but damping ratios increased. Results show that amplitudes of vibration decreased in each mode while resonance did not appear in any other frequency.

 For many large structures such as bridges and for mechanical systems, experimental modal analysis will encounter some problems and limitations. On the other hand, in many cases, working conditions of the system differ from the experimental conditions and results will not meet the goals. In order to overcome these limitations, operational modal analysis methods will develop and apply. In this method, the vibration response of the system under environmental forces, will be measured and used for extraction of the modal parameters of the system. In this research, operational modal analysis has been performed using Continuous Wavelet Transform Method. The structure is a three-storey frame that connects to the ground with two clamped supports. To verify continuous wavelet transform method, first, the structure has been modeled with one-dimensional and two-dimensional elements in the Ansys software separately, and modal parameters are extracted. The same structure then has been imposed under impulse input and the displacement signal of some points of the structure is recorded. The resulting signals are imported into MATLAB software and continuous wavelet transform is applied on them. For this purpose, the Complex Morlet wavelet and Cauchy wavelet have been used. Thus, the modal parameters including natural frequencies, damping ratios and mode shapes are calculated. Comparison between these values with the results from the first finite element solution, shows high accuracy for the continuous wavelet transform method. For experimental researches, a three-storey three-dimensional frame with four clamped supports attached to the ground, has been imposed under impulse input and the displacement signal of some points of the structure is extracted. By applying continuous wavelet transform on these signals, the natural frequencies and damping ratios of the structure has been calculated. Finally, the .results of this method is compared with the experimental modal analysis

In this thesis, a dynamic vibration absorber is designed in order to control beam vibration in the first bending mode vicinity. The primary system is a cantilever beam, and the absorber is formed in a cantilever beam, too. The ANSYS software is used to design the absorber. The first step towards modeling primary beam by means finite element method, and performing numerical modal analysis. An experimental modal analysis is then performed for the primary beam. The boundary condition of primary beam finite element model is updated by comparing the results of numerical and experimental modal analysis and using optimization procedure. Next, the optimal parameters of absorber are determined by using the updated model of primary beam. After designing the absorber, the results of different finite element analysis such as harmonic analysis, transient analysis, and modal analysis are presented. The behavior of system is then studied before and after installing the absorber. The finite element results reveal that the absorber decreases the vibration amplitude of the primary beam significantly. Afterwards, an experimental analysis for experimental model is carried out. Finally, the finite element methods and empirical results are compared and validated. The experimental results are in a good agreement with the finite element ones.

Offering the best method for early crack detection in mechanical structures is the one of the most challenging subject in engineering researches. Present research has studied crack parameters detection of a post-buckled beam using reverse engineering without damaging the beam. For this purpose, a hinged-hinged cracked beam has been considered and crack has been modeled as a rotational massless spring. Governing equations including geometry relations, motion equation, continuity and boundary conditions have been extracted. System of governing equations is a system of nonlinear equations and because of post-buckled beam free vibration , the system has been solved in two stages. First, time-differential terms have been ignored and static equations have been extracted until post-buckling moment. These equations have been discretized by differential quadrature element method and solved by arc-length method. Dynamic equations have been discretized by DQEM too and solved as a eigenvalue problem. Post-buckled free vibration with considering structural damping has been studied too. After calculating natural frequencies, crack parameters assumed to be unknown and by using numerical frequencies and experimental frequencies which determined by other researchers and using a optimization algorithm, crack parameters have been predicted.

 One of the significant problems of gas-fired power stations currently in operation in iran, are turbine nozzles failures and especially their first stage that work in high temperatures. This part is due to the particular complexity of its manufacturing technology, many blades that are extremely needed in industry, every year with currency cost supply from major industries. In this research, the first stage nozzle of GE-F9 gas turbine have been investigated using the finite element method and checked in different working conditions. The vane geometry is modeled using the three-dimensional scanning camera. Static structural analysis without the creep effects, the creep analysis and crack analysis is carried out. Results have been obtained, with conditions that occurs in fact for the nozzle, be compared. Investigation shows the importance of nozzels cooling.

In this study one-dimensional continuous wavelet transform is used to detecting damage in plate. The parameter which used here as an input of wavelet transform is the first mode shape of defective plate. The plate is fixed-fixed with a flaw in certain depth and parallel with the lateral sides of plate, created on the surface of plate. At first by modeling a defected plate in finite element software the first mode shape is extracted and the process of finding damage using wavelet transform is described. Then the influence of damage’s dimensions and place on the results is studied, too. After that the problem is generalized to finding damage in the aluminum plate using experimental modal test and one-dimensional wavelet transform. Finally, in order to estimate the depth of the damage in addition to approximate location and size of defect, intensity factors compared in numerical and experimental results. From the results minimum number of vanishing moments required for this detection can be found by calculating Lipchitz exponent in this specific issue. Also, the defect’s dimensions and its position affecting on the intensity factor that this feature can be used to find the depth of the flaw.

The purpose of this research is to study the crack growth in the last row of the steam turbine in the plant Ramin of the finite element method. One of the common problems in the power industry is crack growth and fracture in the last row of turbine blades for many reasons, including the biphasic blades of work environment, blade of the centrifugal force, interaction fluid, unbalanced and vibration. Crack according to amount of load, material structure and geometry of the blades form in different parts. Therefore, it is necessary to pay more attention to the failure analyze of turbine blades for long life and reliability. The problem analysis using three-dimensional finite element modeling, evaluated in the state of appearing of triple modes of fracture mechanics. For modeling tunnel of crack, iso-parametric elements with 20 nodes to in the shape of single node in the front crack are used. In Ansys software environment, the tunnel of crack inserted in the more critical blade and then blades have exposed to combined loading and boundary conditions. The interaction integral method for stress intensity factors were calculated using a two degree of freedom model of the propagation of cracks in steam turbine sections with five centimeters long. At the end of each stage of crack growth of cracks was calculated by estimating the blade's life. In addition to it, because of this compound loading ,the growth of crack was studied in a state which is out of crack sheet