• Journal of Advanced Marine Engineering and Technology
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• v.20 no.4
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• pp.372-372
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• 1996

The torsional vibration of the propulsion shafting system equipped with viscous damper is investigated. The equivalent system is modeled by a two mass softening system with Duffing's oscillator and the vibratory motion is described by non-linear differential equations of second order. The damper casing is fixed at the front-end of crankshaft and the damper's inertia ring floats in viscous silicon fluid inside of the camper casing. The excitation frenquency is proportional to the rotational speed of engine. The steady state response of the equivalent system is analyzed by the computer and for this analyzing, the harmonic balance method is adopted as a non-linear vibration analysis technique. Frequency response curves are obtained for 1st order resonance only. Jump phenomena are explained. The discriminant for the solutions of the steady state response is derived. Both theoretical and measured results of the propulsion shafting system are compared with and evaluated. As a result of comparisions with both data, it was confirmed that Duffing's oscillator can be used in the modeling of the propulsion shafting system attached with viscous damper with non-linear stiffness.

• The Transactions of the Korean Institute of Electrical Engineers C
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• v.50 no.5
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• pp.243-248
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• 2001

In this paper, we present theoretical and experimental study on the viscous damping of the on-substrate torsional micromirrors, oscillating near the silicon substrates. In this theoretical study, we develop theoretical models and test structures for the viscous damping of the on-substrate torsional micromirrors. From a finite element analysis, we estimate the theoretical damping coefficients of the torsional micromirrors. From a finite element analysis, we estimate the theoretical damping coefficients of the torsional micromirrors, fabricated by the surface-micromaching process. From the electrostatic test of the fabricated devices, frequency-dependent rotationalvelocity of the micromirrors has been measured at the atmospheric pressure using devices, frequency-dependent rotational velocity of the micromirrors has been measured at the atmospheric pressure using the Mach-Zehnder interferometer system. Experimental damping coefficients have been extracted from the least square fit of the measured rotational velocity within the filter bandwidth of 150 kHz. We have compared the theoretical values and the experimental results on the dynamic performance of the micromirrors. The theoretical analysis overstimates the resonant frequency in the amount of 15%, while underestimating the viscous damping in the factors of 10%.

• Journal of the Korea Institute of Military Science and Technology
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• v.17 no.1
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• pp.25-32
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• 2014

Recently supercavitating torpedo has been studied because of its high-speed performance as the next generation of underwater weapon systems. In this study, we present a numerical method based on the potential flow. Characteristic features of the shape of supercavities and drag forces are investigated. In addition, we introduce a viscous-potential method to compensate for the effects of viscosity. The results are compared with viscous calculations using a commercial package, FLUENT V13.

• Earthquakes and Structures
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• v.13 no.4
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• pp.337-351
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• 2017

In the present study a capacity spectrum method based on constant ductility inelastic spectra to estimate the seismic performance of structures equipped with elastic viscous dampers is presented. As the definition of the structures' effective damping, due to the damping system, is necessary, an alternative method to specify the effective damping ratio ${\xi}eff$ is presented. Moreover, damping reduction factors (B) are introduced to generate high damping elastic demand spectra. Given the elastic spectra for damping ratio ${\xi}eff$, the performance point of the structure can be obtained by relationships that relate the strength demand reduction factor (R) with the ductility demand factor (${\mu}$). As such expressions that link the above quantities, known as R - ${\mu}$ - Τ relationships, for different damping levels are presented. Moreover, corrective factors (Bv) for the pseudo-velocity spectra calculation are reported for different levels of damping and ductility in order to calculate with accuracy the values of the viscous dampers velocities. Finally, to evaluate the results of the proposed method, the whole process is applied to a four-storey reinforced concrete frame structure and to a six-storey steel structure, both equipped with elastic viscous dampers.

• Structural Engineering and Mechanics
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• v.55 no.6
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• pp.1241-1260
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• 2015

Any rational approach to define the configuration and size of viscous fluid dampers in a structure should be based on the dynamic properties of the system with the dampers. In this paper we propose an alternative representation of the complex eigenvalues of multi degree of freedom systems with dampers to calculate new equivalent natural frequencies. Analytical expressions for the dynamic properties of a two-story building model with a linear viscous damper in the first floor (i.e. with a non-proportional damping matrix) are derived. The formulas permit to obtain the equivalent damping ratios and equivalent natural frequencies for all the modes as a function of the mass, stiffness and damping coefficient for underdamped and overdamped systems. It is shown that the commonly used formula to define the equivalent natural frequency is not applicable for this type of system and for others where the damping matrix is not proportional to the mass matrix, stiffness matrix or both. Moreover, the new expressions for the equivalent natural frequencies expose a novel phenomenon; the use of viscous fluid dampers can modify the vibration frequencies of the structure. The significance of the new equivalent natural frequencies is expounded by means of a simulated free vibration test. The proposed approach may offer a new perspective to study the effect of viscous dampers on the dynamic properties of a structure.

• Earthquakes and Structures
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• v.17 no.2
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• pp.191-204
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• 2019

Near-field earthquake records including long-period high-amplitude velocity pulses can cause large isolation system displacements leading to buckling or rupture of isolators. In such cases, providing supplemental damping in the isolation system has been proposed as a solution. However, it is known that linear viscous dampers can reduce base displacements in case of near-field earthquakes but at the potential expense of increased superstructure response in case of far-field earthquakes. But can non-linear dampers with different levels of non-linearity offer a superior seismic performance? In order to answer this question, the effectiveness of non-linear viscous dampers in reducing isolator displacements and its effects on the superstructure response are investigated. A comparison with linear viscous dampers via time history analysis is done using a base-isolated benchmark building model under historical near-field and far-field earthquake records for a wide range of different levels of non-linearity and supplemental damping. The results show that the non-linearity level and the amount of supplemental damping play important roles in reducing base displacements effectively. Although use of non-linear supplemental dampers may cause superstructure response amplification in case of far-field earthquakes, this negative effect may be avoided or even reduced by using appropriate combinations of non-linearity level and supplemental damping.

• Proceedings of the Korea Concrete Institute Conference
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• 2010.05a
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• pp.133-134
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• 2010

An experiment was carried out to evaluate energy dissipation capacity and velocity dependent characteristics of the viscous damper. From the experiment, it was found that the viscous damper showed velocity dependent characteristics and excellent energy dissipation capacity.

• Publications of The Korean Astronomical Society
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• v.18 no.1
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• pp.21-24
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• 2003

The problem of the collapse of a self gravitating disk is here considered. We show self-similar solutions for the above problem under a modified viscous parameter. Surface density depends on r$^m$ in the inner region, where m is -1.6. Therefore growing central mass goes on without mass inflow to the system.

• 한국전산유체공학회:학술대회논문집
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• 1995.10a
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• pp.113-117
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• 1995

Viscous analysis on incompressible flows is performed using unstructured triangular meshes. A two-dimensional and axisymmetric incompressible Navier-Stokes equations are solved in time-marching form by artificial compressibility method. The governing equations are discretized by a cell-centered based finite-volume method. and a centered scheme is used for inviscid and viscous fluxes with fourth order artificial dissipation. An explicit multi-stage Runge-Kutta method is used for the time integration with local time stepping and implicit residual smoothing. Convergence properties are examined and solution accuracies are also validated with benchmark solution and experiment.

• Journal of applied mathematics & informatics
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• v.19 no.1_2
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• pp.385-395
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• 2005

A finite element scheme is considered for the viscous Cahn-Hilliard equation with the nonconstant gradient energy coefficient. The scheme inherits energy decay property and mass conservation as for the classical solution. We obtain the corresponding error estimate using the extended Lax-Richtmyer equivalence theorem.