The Experts below are selected from a list of 4527 Experts worldwide ranked by ideXlab platform
Louis Jezequel - One of the best experts on this subject based on the ideXlab platform.
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the role of Damping and definition of the robust Damping factor for a self exciting mechanism with constant friction
Journal of Vibration and Acoustics, 2007Co-Authors: Jeanjacques Sinou, Guillaume Fritz, Louis JezequelAbstract:This paper presents a linear two-degree-of-freedom model in order to analyze friction-induced instabilities that are governed by modal interaction. The role of Structural Damping on flutter instability is undertaken, and the effects of the Structural Damping Ratio between the stable and unstable modes are investigated in order to clarify and to explain the mechanical process of flutter instability. In certain conditions, it is demonstrated that the merging scenario and the unstable mode may change due to this Structural Damping Ratio. Discussions not only demontrate the role of strutural Damping and the associated mechanical process but also define the robust Damping factor in order to avoid design errors and to reduce flutter instability.
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Mode coupling instability in friction-induced vibRations and its dependency on system parameters including Damping
European Journal of Mechanics - A Solids, 2007Co-Authors: Jeanjacques Sinou, Louis JezequelAbstract:Friction-induced vibRations due to coupling modes can cause severe damage and are recognized as one of the most serious problems in industry. In order to avoid these problems, engineers must find a design to reduce or to eliminate mode coupling instabilities in braking systems. Though many researchers have studied the problem of friction-induced vibRations with experimental, analytical and numerical approaches, the effects of system parameters, and more particularly Damping, on changes in stable-unstable regions and limit cycle amplitudes are not yet fully understood. The goal of this study is to propose a simple non-linear two-degree-of-freedom system with friction in order to examine the effects of Damping on mode coupling instability. By determining eigenvalues of the linearized system and by obtaining the analytical expressions of the Routh-Hurwitz criterion, we will study the stability of the mechanical system's static solution and the evolution of the Hopf bifurcation point as functions of the Structural Damping and system parameters. It will be demonstrated that the effects of Damping on mode coupling instability must be taken into account to avoid design errors. The results indicate that there exists, in some cases, an optimal Structural Damping Ratio between the stable and unstable modes which decreases the unstable region. We also compare the evolution of the limit cycle amplitudes with Structural Damping and demonstrate that the stable or unstable dynamic behaviour of the coupled modes are completely dependent on Structural Damping.
Jiahuang Tu - One of the best experts on this subject based on the ideXlab platform.
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two degree of freedom flow induced vibRations on isolated and tandem cylinders with varying natural frequency Ratios
Journal of Fluids and Structures, 2012Co-Authors: Cheng Huang, Dai Zhou, Jiahuang TuAbstract:Abstract A numerical study is performed on the flow-induced vibRations of isolated and tandem elastically mounted cylinders having two degrees of freedom and a variety of the in-line to the transverse natural frequency Ratio, fnx/fny. The characteristic-based-split finite element method is utilized to obtain the solution of the incompressible flow equations in primitive variables. The Reynolds number, based on the upstream flow velocity U∞ and the diameter of the cylinder D, is fixed at Re=150, and for a tandem arrangement, the centre-to-centre distance between the cylinders is 5.0D. The computation is carried out at a lower reduced mass Ratio of Mr=2.0 and for a wide range of reduced velocities (Ur=3.0–12.0). The Structural Damping Ratio is set to zero to maximize the vortex-induced response of the bodies. In this study, we mainly focused on the effect of the natural frequency Ratio on the characteristics of vortex-induced vibRation (VIV) responses, including wake frequencies, orbital trajectories, response amplitudes, hydrodynamic forces and wake mode patterns. The natural frequency Ratio is varied in the range of fnx/fny=1.0–2.0 with an increment of 0.25. We found that the condition of the occurrence of a dual-resonant response exists over a broad range of tested natural frequency Ratios. A third harmonic frequency component appears in the lift fluctuation, along with additional multi-harmonics, which also interact with the drag frequency. Instead of double response peaks, multiple small peaks occur in the amplitude response of the cylinder. These peaks are distributed over a narrow range of Ur from 4.45 to 5.15, and their magnitudes increase with the increase in Ur. For a tandem arrangement, the response characteristic of the upstream cylinder is similar to that of a single cylinder, whereas that of the downstream cylinder is greatly affected by the upstream wake. For a downstream cylinder, the in-line dynamic response is more sensitive to the natural frequency Ratio than the response in the transverse direction. As the dual-resonance is excited, the isolated cylinder and the upstream cylinder of the tandem arrangement may show a P+S wake pattern, which strongly suppresses the vortex shedding of the downstream cylinder.
Sure Che - One of the best experts on this subject based on the ideXlab platform.
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flutter reliability analysis of suspension bridges
Journal of Wind Engineering and Industrial Aerodynamics, 2005Co-Authors: Ji Cheng, Ru-cheng Xiao, C S Cai, Sure CheAbstract:A reliability analysis method is proposed in this paper through a combination of the advantages of the response surface method (RSM), finite element method (FEM), first-order reliability method (FORM) and the importance sampling updating method. The method is especially applicable for the reliability evaluation of complex structures of which the limit state surfaces are not known explicitly. After the accuracy and efficiency of the method are demonstrated through numerical examples, the method is used to estimate the flutter reliability of a suspension bridge. The uncertainties such as material properties, geometric parameters, Structural Damping Ratio, flutter derivatives and extreme wind velocity at the bridge site are considered. The example suspension bridge is the Jiang Yin Bridge with a main span length of 1385 m built in China. The results show that the proposed method based on an empirical formula in which the limit state function is explicitly represented as a function of variables overestimates the flutter reliability of suspension bridges. The actual flutter reliability should be more accurately analyzed using the proposed method based on the deterministic finite element method in which the limit state function is implicitly represented as a function of variables. Finally, the most influential random variables on flutter reliability of suspension bridges are identified by using a sensitivity analysis.
Leonardo Dueñas-osorio - One of the best experts on this subject based on the ideXlab platform.
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Effects of liquefiable soil and bridge modelling parameters on the seismic reliability of critical Structural components
Structure and Infrastructure Engineering, 2010Co-Authors: Jamie E. Padgett, Jayadipta Ghosh, Leonardo Dueñas-osorioAbstract:This study investigates the sensitivity of seismic fragility estimates for bridge components to variation in Structural and liquefiable soil modelling parameters. A rigorous sensitivity analysis is conducted to evaluate the relative importance of 13 random variables that reflect uncertainty in the seismic performance assessment of bridges in regions with liquefiable soils. The results indicate that the fixed and expansion bearings and bent piles tend to be sensitive to the greatest number of modelling parameters for the case study system, while the abutments are less sensitive. The most significant modelling parameters affecting the seismic fragility include such parameters as undrained shear strength of soil, Structural Damping Ratio, soil shear modulus, gap between deck and abutment, ultimate capacity of soil and fixed and expansion bearing coefficients of friction. The 5% and 95% confidence intervals reveal wide bounds on the seismic fragility curves, particularly for more vulnerable bridge components ...
Hernández Santiago - One of the best experts on this subject based on the ideXlab platform.
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Inclusion of mechanical dampers in the multimodal flutter analysis of slender structures
'WITPRESS LTD.', 2020Co-Authors: Quintela Suárez Juan, Jurado, José Ángel, Rapela Ponte Carlos, Álvarez Naveira, Antonio José, Hernández SantiagoAbstract:[Abstract:] Sometimes slender structures are reinforced with mechanical dampers to reduce the vibRations caused by aeroelastic phenomena like flutter. However the formulation of flutter analysis only considers the classical Damping Ratio to take into account the Structural Damping. This paper explains the procedure used for adding mechanical dampers with a known constant to the analysis software FLAS. This code was developed at Universidade da Coruña to calculate the critical wind speed for flutter instability. An example of a solar tracker with two rows of flat panels is shown. In this slender structure two mechanical dampers are used to reduce the vibRations caused by the wind in structure interaction. The solar tracker has been studied for five different positions of the angle of attack. Results of flutter speed for several values of the dampers constant and global Structural Damping Ratio are presented.Ministerio de Ciencia e Innovación; PID2019-110786GB-I00Xunta de Galicia; ED431C2017/7
