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Nonlinear vibrations and stability analysis of a rotor on high-static-low-dynamic-stiffness supports using method of multiple scales
, Article Aerospace Science and Technology ; Volume 63 , 2017 , Pages 259-265 ; 12709638 (ISSN) ; Hojjati, M ; Sharif University of Technology
Elsevier Masson SAS
2017
Abstract
This paper presents the vibration and stability analyses of an unbalanced rotor mounted on high-static-low-dynamic-stiffness supports. The stiffness of the supports is modeled as symmetric of cubic order. Then a second-order multiple scales method is used for studying the primary resonance of the system. The types of singular points are investigated and phase-plane of the system is plotted using analytical and numerical methods. The difference between analytical and numerical solutions is less than 2 percent. © 2017 Elsevier Masson SAS
Numerical simulation of the effect of visitor's movement on bacteria-carrying particles distribution in hospital isolation room
, Article Scientia Iranica ; Volume 24, Issue 3 , 2017 , Pages 1160-1170 ; 10263098 (ISSN) ; Abbassi, A ; Saidi, M. H ; Sharif University of Technology
Abstract
The aim of this paper is to simulate numerically the airflow induced by a walking visitor and its effects on the contaminant transport and ventilation system effectiveness. To this end, the following models will be used in this study: the Lagrangian Discrete Random Walk (DRW) model to trace the motion of BCPs, the dynamic mesh method to simulate the visitor movement, and the Reynolds Averaged Navier-Stokes (RANS) model to solve the airflow. The validation results of the numerical method are in full agreement with the available experimental data in the literature. The findings of the present study indicate that the visitor's movement has remarkable effect on the basic airflow, and the...
A new integration scheme for application to seismic hybrid simulation
, Article Earthquake Engineering and Engineering Vibration ; Volume 16, Issue 1 , 2017 , Pages 69-81 ; 16713664 (ISSN) ; Tasnimi, A. A ; Ahmadizadeh, M ; Sharif University of Technology
Institute of Engineering Mechanics (IEM)
2017
Abstract
Hybrid simulation is a powerful test method for evaluating the seismic performance of structural systems. This method makes it feasible that only critical components of a structure be experimentally tested. This paper presents a newly proposed integration algorithm for seismic hybrid simulation which is aimed to extend its capabilities to a wide range of systems where existing methods encounter some limitations. In the proposed method, which is termed the variable time step (VTS) integration method, an implicit scheme is employed for hybrid simulation by eliminating the iterative phase on experimental element, the phase which is necessary in regular implicit applications. In order to study...
A new algorithm to solve sinusoidal steady-state Maxwell's equations on unstructured grids
, Article Scientia Iranica ; Volume 25, Issue 3B , 2018 , Pages 1296-1302 ; 10263098 (ISSN) ; Saidi, M. S ; Sharif University of Technology
Sharif University of Technology
2018
Abstract
A new approach for attaining numerical solution to sinusoidal steady-state Maxwell's equations is developed. This approach is based on Yee's method, and can be applied on unstructured grids. A problem is solved by the method and the results show good agreement with the available analytical solution. This method can be improved to be applicable for general unsteady problems. © 2018 Sharif University of Technology. All rights reserved
Improving the behavior of buckling restrained braces through obtaining optimum steel core length
, Article Structural Engineering and Mechanics ; Volume 65, Issue 4 , February , 2018 , Pages 401-408 ; 12254568 (ISSN) ; Sehat, S ; Nazeryan, M ; Sharif University of Technology
Techno Press
2018
Abstract
Concentric braced frames are commonly used in steel structures to withstand lateral forces. One of the drawbacks of these systems is the possibility that the braces are buckled under compressive loads, which leads to sudden reduction of the bearing capacity of the structure. To overcome this deficiency, the idea of the Buckling Restrained Brace (BRB) has been proposed in recent years. The length of a BRB steel core can have a significant effect on its overall behavior, since it directly influences the energy dissipation capability of the member. In this study, numerical methods have been utilized for investigation of the optimum length of BRB steel cores. For this purpose, BRBs with...
Stability preservation analysis for frequency-based methods in numerical simulation of fractional order systems
, Article SIAM Journal on Numerical Analysis ; Volume 47, Issue 1 , 2008 , Pages 321-338 ; 00361429 (ISSN) ; Haeri, M ; Bolouki, S ; Siami, M ; Sharif University of Technology
2008
Abstract
In this paper, the frequency domain-based numerical methods for simulation of fractional order systems are studied in the sense of stability preservation. First, the stability boundary curve is exactly determined for those methods. Then, this boundary is analyzed and compared with an accurate (ideal) boundary in different frequency ranges. Also, the critical regions in which the stability does not preserve are determined. Finally, the analytical achievements are confirmed via some numerical illustrations. © 2008 Society for Industrial and Applied Mathematics
Evaluation of the equivalent mechanical properties in a novel composite cruciform honeycomb using analytical and numerical methods
, Article Composite Structures ; Volume 275 , 2021 ; 02638223 (ISSN) ; Ashrafian, M. M ; Gharehbaghi, H ; Nazari, R ; Sharif University of Technology
Elsevier Ltd
2021
Abstract
In the present study, a novel theoretical model is developed, based on the energy method, to predict the equivalent mechanical properties of a new morphing structure with zero Poisson's ratio, which is composed of continuous fiber reinforced composite struts. Due to the employing glass fiber in fabricating the proposed cruciform honeycomb, higher strength than the structures made of pure isotropic materials is obtained. The use of cells with a zero Poisson's ratio also increases the flexural strength of the structure. In the continuation of the paper, by examining the geometric effects on the equivalent properties, a parametric study is performed. Then, using the appropriate failure...
Theoretical study of diffusional release of a dispersed solute from a hollow cylindrical polymeric matrix
, Article Scientia Iranica ; Volume 28, Issue 3 , 2021 , Pages 1428-1435 ; 10263098 (ISSN) ; Tajsoleiman, T ; Abdekhodaie, M. J ; Sharif University of Technology
Sharif University of Technology
2021
Abstract
The present study proposes an exact solution for the release kinetic of a solute from inside a hollow cylindrical polymeric matrix into an infinite medium when the initial concentration of the solute (A) is greater than the solubility limit (Cs). A combination of analytical and numerical methods was used to calculate the solute concentration profile and the release rate. The model was developed for two different conditions including: (1) The release medium was flowing through the hollow cylinder in which the boundary layer may be neglected, and (2) The release medium inside the hollow cylinder was stagnant where the boundary layer needed to be considered. The results indicated that the...
Determination of Forming Limit Diagram with Existence of Normal Stress and Investigation on the Methods of Calibration in this Diagram
, M.Sc. Thesis Sharif University of Technology ; Assempour, Ahmad (Supervisor)
Abstract
Theoretical studies in the analysis of sheet metal failures are referred to obtain the Forming Limit Diagrams (FLDs). Since extracting the Forming Limit Diagrams experimentally is cost and time consuming, many researches have been accomplished for obtaining these diagrams theoretically. However, in many of these researches, the assumption of plane stress state has been used to obtain the FLD, which is not correct in some processes such as hydroforming because of high level of normal stress. In this study, determination of the forming limit diagram with existence of the normal stress is based on the Marciniak and Kuczynski model. Newton-Raphson method has been used to find the set of...
Scattering from Objects in Presence of Rough Surfaces
, M.Sc. Thesis Sharif University of Technology ; Shishegar, Amir Ahmad (Supervisor) ; Rejaei, Behzad (Co-Advisor)
Abstract
Electromagnetic wave scattering from objects is one of the most important problems in applied electromagnetics. Usually the electromagnetic scattering characteristic from an object is analyzed with the assumption that the object is in free space. If the object is not near other objects or surfaces this is acceptable. However if the object is near other objects or surfaces then the interactions between the scattered wave from the object and surroundings can significantly alter the scattering characteristic from the object.In this thesis we have proposed a method for the analysis of electromagnetic scattering from an object in the presence of rough surfaces. This method is based on the...
Numerical Investigation of Laminar Premixed Flame Response to Acoustic Excitation
, M.Sc. Thesis Sharif University of Technology ; Farshchi, Mohammad (Supervisor)
Abstract
Flame Response to acoustic excitation which plays an important role in combustion chamber stability state may be found through numerical simulation. A finite volume code of higher order accuracy developed for compressible flows was adopted. Species equations along with a single-step irreversible reaction model were added to this non-reacting program. Changes were also made to the energy equation. Premixed methane/air flame in tubes was studied and the same problem was solved with FLUENT software. It was found that whether the Lewis number is considered constant or temperature dependent has a strong effect on flame structure and shape and that using mean values for gas properties yields out...
Numerical Investigation on Offshore wind Turbine Feasibility and Function
, M.Sc. Thesis Sharif University of Technology ; Abbaspour Tehrani Fard, Madjid (Supervisor)
Abstract
This thesis consists of three main chapters. The first chapter is about describing principles about offshore wind turbine, principal concepts. This chapter also explains about advantages and disadvantages of installing wind turbine at offshore fields, and economical aspect of this kind of wind turbine farms. In the second chapter,anumerical method is used to investigate a SeaStar tension leg platform offshore wind turbine response behavior in parked condition. This code considers nonlinearities due to changes in tension of tethers. Wave forces are calculated adopting the Airy wave theory. Furthermore, shear turbulent wind is exerted on tower, with maximum speed 30 m/s on the hub. Since surge...
Modeling of Wellbore Stability in Shales including Proelsatic, Chemical and Thermal Effects
, M.Sc. Thesis Sharif University of Technology ; Ghotbi, Sirous (Supervisor) ; Pishvaie, Mahmoud Reza (Supervisor)
Abstract
Wellbore instability is one of the serious problems during drilling. This can cause delay in drilling, increasing drilling costs and in some cases well abandonment. The present project investigates the effects of non- isothermal drilling and the difference in salt concentration between formation fluid and drilling fluid on the stability of wellbores drilled in the saturated porous shale formations subjected to an arbitrary stress field. This project also focuses on the analysis of wellbore stability in shale formations in which the thermal diffusivity is higher than fluid invasion and the mechanism of heat transfer is dominated by conduction. The governing equations used in this dissertation...
Numerical Methods in Simulation and Implementation of Proportional-Weighted Integral (PWI) Controllers
, M.Sc. Thesis Sharif University of Technology ; Tavazoei, Mohammad Saleh (Supervisor)
Abstract
The main effort of this thesis is to present a numerical method to simulate and implement a special type of controllers, called proportional-weighted integral (PWI) controllers. For this purpose, at first mathematical equations of the fractional order proportional-integral controllers have been investigated in the time domain, and then by generalization of these controllers, the PWI controllers are achieved. Next the discretization of these controllers are carried out and the performance of the controllers are simulated by means of this discretization and comparison between these controllers and the conventional PI controllers has been held. After that a method for estimation of these...
Non-hydrostatic layered flows over a sill
, Article Fluid Dynamics Research ; Volume 45, Issue 2 , February , 2013 ; 01695983 (ISSN) ; Sharif University of Technology
2013
Abstract
This work takes a new approach to solving non-hydrostatic equations of layered flows over bottom topography. A perturbation technique is used to find explicit expressions for a flow for different regimes of single- and two-layer flows over a sill. Excellent agreement with previous solutions and experimental data is obtained, and more details of the non-hydrostatic flow over a sill are revealed. The proposed method is simple and compact and removes the need for complex numerical techniques to solve the non-hydrostatic equations. It is shown that in the approach-controlled regime of two-layer flow over a sill, the flow upstream and farther downstream the sill crest can be described by the...
Two new conjugate gradient methods based on modified secant equations
, Article Journal of Computational and Applied Mathematics ; Volume 234, Issue 5 , 2010 , Pages 1374-1386 ; 03770427 (ISSN) ; Ghanbari, R ; Mahdavi Amiri, N ; Sharif University of Technology
2010
Abstract
Following the approach proposed by Dai and Liao, we introduce two nonlinear conjugate gradient methods for unconstrained optimization problems. One of our proposed methods is based on a modified version of the secant equation proposed by Zhang, Deng and Chen, and Zhang and Xu, and the other is based on the modified BFGS update proposed by Yuan. An interesting feature of our methods is their account of both the gradient and function values. Under proper conditions, we show that one of the proposed methods is globally convergent for general functions and that the other is globally convergent for uniformly convex functions. To enhance the performance of the line search procedure, we also...
Efficiency measurement in data envelopment analysis in the presence of ordinal and interval data
, Article Neural Computing and Applications ; 2016 , Pages 1-12 ; 09410643 (ISSN) ; Tavana, M ; Rahmani, M ; Santos Arteaga, F. J ; Sharif University of Technology
Springer London
2016
Abstract
Several methods have been proposed in data envelopment analysis (DEA) for measuring efficiency in problems with interval or ordinal data. In this study, we review the weaknesses and drawbacks of these methods and show how converting ordinal or interval data into precise data can lead to violations of established DEA axioms. One of the axioms violated by these conversion processes is the inclusion of observations axiom, which requires a consistent definition of the production possibility set. We describe the special properties of ordinal and interval data together with their effect on the DEA-based rankings using a theorem and an example. We also propose a new algorithm and apply random...
Efficiency measurement in data envelopment analysis in the presence of ordinal and interval data
, Article Neural Computing and Applications ; Volume 30, Issue 6 , 2018 , Pages 1971-1982 ; 09410643 (ISSN) ; Tavana, M ; Rahmani, M ; Santos Arteaga, F. J ; Sharif University of Technology
Springer London
2018
Abstract
Several methods have been proposed in data envelopment analysis (DEA) for measuring efficiency in problems with interval or ordinal data. In this study, we review the weaknesses and drawbacks of these methods and show how converting ordinal or interval data into precise data can lead to violations of established DEA axioms. One of the axioms violated by these conversion processes is the inclusion of observations axiom, which requires a consistent definition of the production possibility set. We describe the special properties of ordinal and interval data together with their effect on the DEA-based rankings using a theorem and an example. We also propose a new algorithm and apply random...
Phase-I robust parameter estimation of simple linear profiles in multistage processes
, Article Communications in Statistics: Simulation and Computation ; 2019 ; 03610918 (ISSN) ; Akhavan Niaki, T ; Sharif University of Technology
Taylor and Francis Inc
2019
Abstract
This paper addresses the problem of robust parameter estimation of simple linear profiles in multistage processes in the presence of outliers in Phase I. In this regard, two robust approaches, namely the Huber’s M-estimator and the MM estimator, are proposed to estimate the parameters of the process in Phase I in the presence of outliers in historical data. In addition, the U statistic is applied to the robust parameter estimates to remove the effect of the cascade property in multistage processes and as a result, to obtain adjusted robust estimates of the parameters of simple linear profiles. The performance of the proposed methods is evaluated under weak and strong autocorrelations...
Phase-I robust parameter estimation of simple linear profiles in multistage processes
, Article Communications in Statistics: Simulation and Computation ; 2019 ; 03610918 (ISSN) ; Akhavan Niaki, S. T ; Sharif University of Technology
Taylor and Francis Inc
2019
Abstract
This paper addresses the problem of robust parameter estimation of simple linear profiles in multistage processes in the presence of outliers in Phase I. In this regard, two robust approaches, namely the Huber’s M-estimator and the MM estimator, are proposed to estimate the parameters of the process in Phase I in the presence of outliers in historical data. In addition, the U statistic is applied to the robust parameter estimates to remove the effect of the cascade property in multistage processes and as a result, to obtain adjusted robust estimates of the parameters of simple linear profiles. The performance of the proposed methods is evaluated under weak and strong autocorrelations...