Sharif Digital Repository

This work studies the nonlinear oscillations of an elastic rotating shaft with acceleration to pass through the critical speeds. A mathematical model incorporating the Von-Karman higher-order deformations in bending is developed and analyzed to investigate the nonlinear dynamics of rotors. A flexible shaft on flexible bearings with springs and dampers is considered as rotor system for the present work. The shaft is modeled as a beam with a circular cross-section and the Euler Bernoulli beam theory is applied. The kinetic and strain energies of the rotor system are derived and Lagrange method is then applied to obtain the coupled nonlinear differential equations of motion for 6° of freedom.... 

The paper investigates the dynamics of simple and complex geometry rotors of various configurations with permanent bow, mounted on nonlinear bearings, in regards to nonlinear stability, quality of motion, and bifurcation set. Permanent shaft bow is a fault existing in most slender rotors in thermal machines, e.g. gas turbines, steam turbines, and turbopumps, and other applications of rotating machinery like transmission shafts and geared shafts. In this work, finite element models of multi-segment rotors are implemented, including permanent bow, and a generic rotor system of a uniform rotor with three masses is studied first on its stability characteristics for various bow values, and for... 

This work studies the nonlinear oscillations of an elastic rotating shaft with acceleration to pass through the critical speeds. A mathematical model incorporating the Von-Karman higher-order deformations in bending is developed and analyzed to investigate the nonlinear dynamics of rotors. A flexible shaft on flexible bearings with springs and dampers is considered as rotor system for the present work. The shaft is modeled as a beam with a circular cross-section and the Euler Bernoulli beam theory is applied. The kinetic and strain energies of the rotor system are derived and Lagrange method is then applied to obtain the coupled nonlinear differential equations of motion for 6° of freedom.... 

Capture of discrete nature of nanomaterials is central to predict their behavior in high precision. To this end, various augmented continuum theories have been presented. However, their accuracy is strongly dependent on the recognition of accurate values of associated characteristic length scale parameter, and many ambiguities and uncertainties still remain about them. As main purpose of this study, the accurate values of elasticity tensor elements in the framework of first strain gradient theory are calculated for graphene with trigonal crystal system. A promising way to achieve them could be through molecular mechanics model. So that, the energy, including strain and kinetic energy, based... 

This research aims to investigate the dynamic response of a dual rotor-bearing system for passing through critical speeds. The Newmark-β method for solving the nonlinear lateral vibration of rotor-bearing systems is approached and the effects of various parameters including shaft crack, unbalanced mass, shaft bow, and support parameters are shown. A model, including shaft cracks, rotor unbalance, and shaft bow, is established using the finite element method with six degrees of freedom per node. The results provide a detailed feature analysis of the support parameters in a dual rotor-bearing system when it is coupled with various faults to pass through the critical speeds. © The Author(s)... 

This research aims to investigate the dynamic response of a dual rotor-bearing system for passing through critical speeds. The Newmark-β method for solving the nonlinear lateral vibration of rotor-bearing systems is approached and the effects of various parameters including shaft crack, unbalanced mass, shaft bow, and support parameters are shown. A model, including shaft cracks, rotor unbalance, and shaft bow, is established using the finite element method with six degrees of freedom per node. The results provide a detailed feature analysis of the support parameters in a dual rotor-bearing system when it is coupled with various faults to pass through the critical speeds. © The Author(s)... 

Carbon nanoropes (CNRs) are of interest for a wide variety of nanotechnological applications. Since little attention has been paid to mechanical properties of CNRs, their axial elastic modulus is explored herein. Molecular dynamics (MDs) simulations are adopted for analysis of Young's modulus of CNRs. It is also shown that increase in the initial helical angle decreases Young's modulus; however, by increase in the number of CNTs and strands, different influence on Young's modulus emerges. Therefore, the highest value of Young's modulus obtained at the lower value of initial helical angle and consequently, Young's modulus of bundle of straight CNTs is higher than CNRs with hierarchical... 

The aim of the present study is to improve the efficiency of a rotor bearing system for passing through critical speed. The current study assesses the influences of support parameters on the cracked rotor response for passing through critical speeds. The actual breathing mechanism of the transverse crack is addressed herein. The Finite element model is developed for more realistic analysis. The Newmark-β method to solve the nonlinear lateral vibration of rotor-bearing systems is approached and the effects of various parameters are shown. © 2024, The Author(s), under exclusive license to Springer Nature Switzerland AG  

In the present study, the mechanical properties of vertically aligned carbon nanotube (VACNT) arrays in the most general case of anisotropic elasticity is studied. To quantify the elastic stiffness coefficients, continuum mechanics is employed, in which the constitutive relation is formulized in terms of the interatomic potential obtained via molecular dynamics (MDs) simulations. Detailed investigations into elastic stiffness coefficients with different geometrical parameters are performed and, consequently, their sensitivity on length and radius parameters is hereby studied. With such study it would be possible to tune the mechanical properties of VACNTs. © 2019, Società Italiana di Fisica... 

Purpose: Different studies have analyzed the relationship between organizational learning (OL) and innovation performance (IP). However, the question of how innovation culture (IC) affects the relationship between OL and IP remains unexplored. This study aims to examine the impact of IC on the relationship between OL and various dimensions of IP, including product, process and objective innovation. Design/methodology/approach: A research model was developed and performed based on the relevant literature in the field of OL, IC and IP. The hypotheses are tested with the data collected from companies operating in an intensive knowledge-based industry. Findings: Based on the results of 625... 

Purpose: The purpose of this research is to propose a framework for developing business intelligence systems (BIS). Design/methodology/approach: Following a qualitative research method, a comprehensive framework at organizational and individual level, as well as team level, was developed. Findings: The findings revealed that team-level influential factors play a more significant role in BIS development success. The results also indicate that BIS development relies on suppliers, customers and their relationships, whereas the supplier side has a more significant impact on determining successful development. Drawing on these findings, embedded knowledge, knowledge management process capability... 

This paper investigates the influence of residual bow on the stability of a rotating system consisting of an unbalanced Jeffcott rotor with anisotropy and two nonlinear journal bearing supports. The motion equations of the rotor system are formulated and analyzed considering most common faults and features including coupling effects of unbalance, anisotropy and residual shaft bow, by applying Lagrangian dynamics and Floquet theory. Time varying system properties of stiffness and damping are introduced into the equations of motion affecting the rotor behavior, and the influence is studied with respect to the extent of the bearing nonlinearity and the rotor anisotropy. Increasing area of... 

Comparison of the traditional linear heating method of TPD with an original stepwise heating scheme was reported for the first time. Stepwise heating TPD was carried out by keeping the temperature constant as soon as ammonia desorption signal rises until the signal returns to the baseline. More ammonia desorption peaks on a SAPO-34 catalyst were identified using TPD with stepwise heating. The effect of temperature ramp on desorption peak broadening in TPD curve was also addressed. The more distinct ammonia desorption peaks in stepwise TPD indicates that ammonia adsorbs in about five or six different ways on SAPO-34, and attribution of different adsorptions may be explained considering some... 

Most part of Iran is dominated by arid and semi-arid areas due to low annual rainfall. The need for production of fresh water from brackish water is considerably high, especially in dry regions. In this study 1 month daily-based experimental data from a solar still site has been reported. The technical and operational problems of this site which finally contributed to the total cease of the site are described. Then a detailed analysis is investigated on progress of a prototype which constructed in order to solve the site's problems. The results of 1 month of experimental data of the final design showed that the last prototype could be used to solve the current problems of the site. The... 

Hydrocracking is one of the most versatile petroleum refining processes for production of valuable products including gasoline, gas oil, and jet fuel. In this paper, a five-parameter continuous lumping model was used for kinetic modeling of hydrocracking of vacuum gas oil (VGO). The model parameters were estimated from industrial data obtained from a fixed bed reactor operating at an average temperature of 400°C and residence time of 0.3 h. Product distributions were obtained in terms of the weight fraction of various boiling point cuts. The model parameters were estimated using the Nelder-Mead optimization procedure and were correlated with temperature. Comparison of experimental and... 

Purpose - The purpose of this paper is to present an improved methodology for design of custom-made hip prostheses, through integration of advanced image processing, computer aided design (CAD) and additive manufacturing (AM) technologies. Design/methodology/approach - The proposed methodology for design of custom-made hip prostheses is based on an independent design criterion for each of the intra-medullary and extra-medullary portions of the prosthesis. The intra-medullar part of the prosthesis is designed using a more accurate and detailed description of the 3D geometry of the femoral intra-medullary cavity, including the septum calcar ridge, so that an improved fill and fit performance... 

In this letter, we propose a method for real-time high density impulse noise suppression from images. In our method, we first apply an impulse detector to identify the corrupted pixels and then employ an innovative weighted-average filter to restore them. The filter takes the nearest neighboring interpolated image as the initial image and computes the weights according to the relative positions of the corrupted and uncorrupted pixels. Experimental results show that the proposed method outperforms the best existing methods in both PSNR measure and visual quality and is quite suitable for real-time applications