Sharif Digital Repository

The use of air bearings was began the last decade. Oil-free air bearing technology is implemented in rotating machines such as turbines, Micro gas turbines and turbocharged, this technology has improved the efficiency of rotating systems. In rotating machines, at high speeds, passing through the critical speed happens. As a result, the natural frequency of the system is crossing the critical speed, which causes system instability and lack of system amplitude control. Instability causes damage to the system. The aim of this thesis is to investigate the nonstationary vibrations of a flexible rotor with air bearings in the passing critical speeds. Behavior of air bearings is also considered to... 

A simple control system contains at least an actuator, a controlled link, and a foundation. These structures are not ideally rigid, this flexibility can appear willingly or unwillingly in any parts of the system or the connections between them. As a consequence of this flexibility, the movement of the controlled part will fluctuate unintentionally. In this thesis, a control system with rigid controller and flexible torsional base is investigated and controllers are designed and compared to reduce these unwanted vibrations. Finally, by implementing the controllers on the constructed system, the experimental results are obtained and compared with the analytical results.To fabricate the... 

In this project, a system designed for balancing a flexible rotor, without any need for shutting down the rotor. Then, functionality of this system, studied with experimental assessments. The method used in this plan, is classified into active online balancing methods of rotors which can change the mass distribution of the rotor, to improve the balancing condition, during operation. In order to reach this goal, a balancer plane is designed that by angular displacement of two nonconcentric masses, each driven with an electrical actuator, sets a desired unbalancy vector. Then, a rotor designed, which after mounting balancer plane, could be classified into the flexible rotors according to... 

Fatigue crack growth can have harmful effects on the reliability rotary machine. While an early warning to crack, can be used to extend the life of these expensive machines and increase reliability.Structural Vibration Condition Monitoring as a primary fault detection method, achieved to a lot amazing result and for this reason studies vibration behavior of cracked rotational shaft is a big issue for engineers of dynamic machine.In this research, first check the fault detection methods based on structural vibration condition monitoring, mode shape methods were extracted. Then by modeling rotational shaft in computer and creation of crack on it, fault detection quality of these methods have... 

The main purpose of this dissertation is to investigate and improve the efficiency of Sparse Reconstruction method for structural health monitoring application in order to solve the challenges including; finding multiple simultaneous damages, damage detection and location in anisotropic structures and real engineering complex structures. The main approach of this research is based on presenting theoretical concepts and developing practical methods based on the presented concepts, as well as evaluating and implementing the developed methods using numerical simulations and experimental tests. In this work, the finite element method is used for numerical simulation. In the experimental test,... 

In this thesis, the nonlinear vibrations of the circular plate is examined using multiple scale methods and experimental tests. At first step, the nonlinear equations governing the problem are written using von Karman's assumption. In the next step, to solve nonlinear equations and calculate natural frequency, the problem is solved using multiple scale method and frequency charts are extracted as a function of the amplitude of vibration. . The design of the setup has been done in such a way that it is possible to simulate the boundary conditions. In the first phase, experimental tests was performed on a 0.3 mm thick aluminum sheet, which did not produce the desired results due to the... 

This research investigates the impact of boundary conditions on the performance of a Coriolis flow meter equipped with a pair of straight tubes. Boundary condition here refers to the support plates that connect the two tubes. Initially, the tube containing the fluid was considered as a pair of straight tubes with adjustable boundary conditions. The equations of motion for the fluid inside the tube were written under constant pressure and temperature, and the Galerkin method was used to eliminate the spatial term of the differential equations. The problem was analytically solved to the extent possible, examining the effect of boundary conditions on modal properties, the phase shift readings,... 

In the present work, the dynamic charactristics of an oil lubricated journal bearing are identified by experimental modal test. First other similar articles were studied. Then according to requrements and specifications of the bearing, a test bed is designed and constracted by existing technologies to perform tests and to collect required data. The test bed consists of several parts, including rotary shaft, stator, oil supply pack, dynamic force shaker and various sensors, which are constructed and assembled with very precise tolerances. In order to identify the dynamic coefficients of the bearing, the test process was performed using dynamic and static forces and collecting required data.... 

The present study aims to achieve two major objectives in the field of Rotordynamics. The first one is to investigate the frequency response function by considering rubbing between fixed and rotating components of the system and also the presence of nonlinear support such as tiling-pad bearings using analytical approach and the second one is to study the stability of the system using both analytical and numerical methods. In first step, by using assumptions and laws of physics, a model for dealing with the rubbing phenomenon and the friction force between fixed and rotating components of the system is presented. In the next step, the rubbing force is modeled. Then using the Jeffcott rotor... 

This thesis aims to provide a method to obtain the radial dynamic properties of rolling element bearing, including stiffness and damping by experimental test methods. Methods such as modal test, vibration test, and static tests were known as experimental methods to obtain these properties, and in this research, all three of these methods were used to obtain these properties. After studying the theoretical relations governing the stiffness coefficients of the bearings, a test bed platform was designed and built to conduct the tests After the construction of this platform, the test design stage was reached done and different parameters changed during these tests several tests were it is noted... 

This thesis aims to provide a method for obtaining the dynamic properties, including stiffness and damping, of rolling bearings in radial and axial directions using an experimental approach. Experimental and laboratory methods such as modal testing, vibration testing, and static tests are recognized as practical approaches to obtain these properties. In this research, all three of these methods were employed to determine these properties. After studying the theoretical relationships governing bearing stiffness coefficients, a test setup was designed and constructed for conducting experiments. Subsequently, various tests were conducted, and data was collected, including shaft displacement... 

Three dimensional analyses have been carried out for predicting the behavior of the jointed rock slope of the abutments of the bridge which is proposed to be constructed across the river Karun4 in Iran using 3DEC. The rock overall slope angle is approximately 60 to 70 degrees, composed of highly jointed rock mass and the joint spacing and orientation are varying at different locations. Since 3DEC is a three-dimensional numerical code, utilizes a Lagrangian calculation scheme to model large movements and deformations of a blocky system, allows for modeling of large movements and rotations, and including complete detachment of rigid or deformable discrete blocks has been utilized for the... 

In this study, high frequency vibration analysis of two connected plates by a beam interface is carried out using Energy Finite Element Analysis (EFEA). In structural vibration analysis at low frequencies, classical methods such as Finite Element Method or Boundary Element Method present accurate results. But, using these methods at high frequencies requires expense of high calculation cost and time. Therefore, using of statistical methods that are based on average estimates of vibrations, at high frequencies is most popular. EFEA is a method that gives average response of structure at a certain point and frequency band. This method is developed on the basis of finite element analysis of... 

In the present study, the nonlinear vibrations of the rotor on the nonlinear bearing have been studied by the multiple scale method. To achieve this goal, Jeffcott models, two degrees of freedom and rotating Euler-Bernoulli beam have been used as rotors and insulators and nonlinear tilting-bearing bearings have been used as supports.To better understand the importance of the supports in the analysis of rotating systems, the rotor joints with isolates with high static stiffness and low dynamic stiffness have been considered and the dynamic behavior of the system has been studied using the multiple scale method. The results of this study showed that this type of nonlinear isolator has a good... 

This dissertaion deals with the vibration and aeroelasticity analysis of a turbomachinery cascade with Magnetorheological Elastomer (MRE) based sandwich blades. At first, bending vibration of the rotary blades equipped with an MRE core based on the three-layered sandwich beam theory have been studied in the flexural and edgewise directions. Further, the torsional vibration is investigated and the effects of different parameters such as the intensity of the magnetic field, rotating speed and the MRE layer thickness on the natural frequencies and loss factors of the system are studied. On the basis of the modeling obtained for the structural dynamics and based on the Whitehead's unsteady... 

This thesis examines the modal characteristics of the payload after connecting to the carrier via the adapter. Basically, the modal characteristics of a payload change after connection to the carrier due to the different stiffness of the coupling mechanism. In this report, it is attempted to split and model the assembly structural system into two substructures interconnected by the V band clamp mechanism. Then, each of the substructures analyze individually and the modal characteristics of the integrated system are extracted using the infrastructure coupling methods. The coupling method is the "Component Mode synthesis" (CMS) method and input data, dynamic and modal characteristics required... 

This research investigates the possibility of using the component mode synthesis (CMS) method in experimentation. It should be noted that the original idea for this project originated from an industrial problem. The main issue in this problem was the lack of sufficient equipment to conduct tests on very large structural modes and the high computational cost for their analysis. Therefore, a test platform was constructed using a scaled-down sample (approximately 1:25) with connections that closely resembled real connections. This assembly was then tested separately, and the results were compared with analytical analyses. After updating and correcting the model in several stages, the structure... 

In this thesis, the high frequency random vibrations analysis of a reinforced plate with a cutout is carried out using Energy Finite Element Analysis (EFEA) and the final results are validated with experimental results. EFEA method uses the average energy density as the primary variable to form the governing differential equations of energy flow in the structure. In this method the equations of space and time averaged energy density are derived in the Far Field and the energy intensity is considered as continues variable over the structure. In this work, at first for simple beam and plate an EFEA code is developed. Then the relationships for joints are derived using structural dynamics. Some... 

The present research aims to achieve two main objectives in the Rotordynamic. The first one is, study and calculate the linear dynamic coefficients of a tapered gas seal, and the second one is, analysis the dynamic behavior and stability of the Timoshenko rotor supported with tapered gas seal. Classical Reynolds equation is used to calculate the dynamic coefficients of tapered gas seal. At first, using the perturbation method, the perturbed equations for gas film thickness and pressure are obtained. The finite difference method (FDM) is utilized to transfer the obtained PDEs to the algebraic system of equations. The algebraic system of equations then solved, and the zeroth and first-order... 

Adaptive vibration control of a structure under different condition of exciting forces or structural response is the main scope of this research.Using a combination of the pole placement and online Empirical Mode Decomposition (EMD) methods, a new algorithm is proposed for adaptive active control of structural vibration. For this purpose, by structural response which is evaluated from Hilbert-Huang Transform (HHT) and using prior knowledge for corresponding conditions, proper and optimum control forces are applied to structure. Hence, error sources of EMD method in the HHT such as end effects error, mode mixing problem and decomposition resolution are being studied. A modified method based...