• Proceedings of the Korean Society of Machine Tool Engineers Conference
• /
• 2004.04a
• /
• pp.404-409
• /
• 2004

To maximize the productivity in machining molds and dies, machine tools should operate at high speeds. During the high speed operation of moving frames or spindles, vibration problems are apt to occur if the machine tool structures are made of conventional steel materials with inferior damping characteristics. However, self-excited vibration or chatter is bound to occur during high speed machining when cutting speed exceeds the stability limit of machine tool. Chatter is undesirable because of its adverse effect on surface finish, machining accuracy, and tool life. Furthermore, chatter is a major cause of reducing production rate because, if no remedy can be found, metal removal rates have to be lowered until vibration-free performances is obtained. Also, the resonant vibration of machine tools frequently occurs when operating frequency approaches one of their natural frequencies because machine tools have several natural frequencies due to their many continuous structural elements. However, these vibration problems are closely related to damping characteristics of machine tool structures. The polymer concrete has high potential for machine tool bed due to its good damping characteristics with moderate stiffness. This paper presents the use of polymer concrete and sandwich structures to overcome vibration problems. Also, co-cure bonding method for functional part mounting was exhibited experimentally, by which manufacturing time and cost for polymer concrete bed will be remarkably reduced.

• Wind and Structures
• /
• v.7 no.4
• /
• pp.215-234
• /
• 2004

The tuned liquid damper (TLD) is increasingly being used as an economical and effective vibration absorber. It consists of a water tank having the fundamental sloshing fluid frequency tuned to the natural frequency of the structure. In order to perform efficiently, the TLD must possess a certain amount of inherent damping. This can be achieved by placing screens inside the tank. The current study experimentally investigates the behaviour of a TLD equipped with damping screens. A series of shake table tests are conducted in order to assess the effect of the screens on the free surface motion, the base shear forces and the amount of energy dissipated. The variation of these parameters with the level of excitation is also studied. Finally, an amplitude dependent equivalent tuned mass damper (TMD), representing the TLD, is determined based on the experimental results. The dynamic characteristics of this equivalent TMD, in terms of mass, stiffness and damping parameters are determined by energy equivalence. The above parameters are expressed in terms of the base excitation amplitude. The parameters are compared to those obtained using linear small amplitude wave theory. The validity of this nonlinear model is examined in the companion paper.

• Proceedings of the Korean Society of Precision Engineering Conference
• /
• 2003.06a
• /
• pp.113-116
• /
• 2003

To maximize the productivity in machining molds and dies, machine tools should operate at high speeds. During the high speed operation of moving frames or spindles, vibration problems are apt to occur if the machine tool structures are made of conventional steel materials with inferior damping characteristics. However, self-excited vibration or chatter is bound to occur during high speed machining when cutting speed exceeds the stability limit of machine tool. Chatter is undesirable because of its adverse effect on surface finish, machining accuracy, and tool lift. Furthermore, chatter is a major cause of reducing production rate because, if no remedy can be found, metal removal rates have to be lowered until vibration-free performances is obtained. Also, the resonant vibration of machine tools frequently occurs when operating frequency approaches one of their natural frequencies because machine tools have several natural frequencies due to their many continuous structural elements. However, these vibration problems are closely related to damping characteristics of machine tool structures. This paper presents the use of polymer concrete and sandwich structures to overcome vibration problems. The polymer concrete has high potential for machine tool bed due to its good damping characteristics with moderate stiffness. In this study, a polymer concrete bed combined with welded steel structure i.e., a hybrid structure was designed and manufactured for a high-speed gantry-type milling. Also. its dynamic characteristics were measured by modal tests.

• Ocean Systems Engineering
• /
• v.2 no.2
• /
• pp.147-158
• /
• 2012

A time-domain simulation of a land-based Oscillating Water Column (OWC) with various irregular waves as a form of PM spectrum is performed by using a two-dimensional fully nonlinear numerical wave tank (NWT) based on the potential theory, mixed Eulerian-Lagrangian (MEL) approach, and boundary element method. The nonlinear free-surface condition inside the OWC chamber was specially devised to describe both the pneumatic effect of the time-varying pressure and the viscous energy loss due to water column motions. The quadratic models for pneumatic pressure and viscous loss are applied to the air and free surface inside the chamber, and their numerical results are compared with those with equivalent linear ones. Various wave spectra are applied to the OWC system to predict the efficiency of wave-energy take-off for various wave conditions. The cases of regular and irregular waves are also compared.

• Journal of the Society of Naval Architects of Korea
• /
• v.48 no.2
• /
• pp.107-112
• /
• 2011

The aim of this study is to analyze the hydrodynamic properties of a 2D floating body with moon pool using a 2D fully nonlinear Numerical Wave Tank(NWT). This NWT was developed based on the Boundary Element Method(BEM) with potential theory and fully nonlinear free surface boundary conditions. Free surface elevations in the moon pool were calculated in the time domain for various frequencies of forced body motions. The added-mass and damping coefficients of the heaving body were also obtained. The present numerical results were compared with the analytic and experimental results and their accuracy was verified.

• Bulletin of the Society of Naval Architects of Korea
• /
• v.11 no.1
• /
• pp.1-8
• /
• 1974

Hydrodynamic forces and moments produced by the swaying oscillation on the free surface were exactly calculated by Ursell-Tasai method for the cylinders with Kim's chine form sections and the sway responses of the cylinders of those chine form sections among the regular beam sea were also calculated. The results of the computation were compared with those of Lewis form sections. It is concluded that the effects of the section form on the added mass, and damping are small, if the section forms had same beam-draft ratio and sectional area coefficient in the case of sway motion. It is also known that the above little effects of section shapes on the basic hydrodynamic forces do not effect on the sway motion responses of cylinderical bodies among the regular beam sea. The sway motion responses of cylinderical bodies are varied linearly with the wave numbers.

• Bulletin of the Society of Naval Architects of Korea
• /
• v.24 no.2
• /
• pp.47-54
• /
• 1987

Linear and nonlinear hydrodynamic force, which acts on submerged circular and eilliptic cylinders in oscillations as well as in advancing motion, are investigated as an initial-boundary value problem using a numerical method, which makes use of the source distribution on the body surface and the spectral method for treating the free surface waves. In the numerical code developed here, the boundary condition at the body surface is linearized. Using the numerical code so attained, nonlinear effects for different forward speeds and of the large-amplitude motion are computed. One of the major findings is that, when the forward speed is large, the added mass has its minimum and the damping force change rapidly around the frequency corresponding to the speed-frequency parameter, $\tau$=0.25, Compared to the result of Grue's [10], who used linear theory to get abrupt changes in values of the added mass and the damping force at the frequency corresponding to $\tau$=0.25, the present study, which takes nonlinear effects into account, shows much smoother variations near the frequency.

• Water for future
• /
• v.29 no.2
• /
• pp.221-233
• /
• 1996

To investigate the hydrodynamic characteristics of a two-dimensional buoyant surface jet, the most important factors of the numerical analysis are the evaluation of the free surface and the turbulence transportation under the stratification. In present study, a numeriacal simulation model used with the semi-implicit method for pressure-linked equations (SIMPLE), the non-hydrostatic approximation and the algebraic stress model (ASM) is applied to investigate the vertical structure of internal flow hydrodynamically. The ASM enables to take account of anisotropy of turbulence, the damping effects of the density interface, and the free surface on the turbulence structure accurately. The ASM tested produces better agreement than the $\kappa-\varepsilon$ model with measurements by Nakatsuji (1984) on the flow development and turbulence structure. Applicability of the ASM to a two-dimensional buoyant surface jet is examined through comparison with experimental data.

• Journal of Ocean Engineering and Technology
• /
• v.29 no.1
• /
• pp.1-8
• /
• 2015

An analytical model of liquid sloshing is developed to consider the energy-loss effect through a partially submerged porous baffle in a horizontally oscillating rectangular tank. The nonlinear boundary condition at the porous baffle is derived to accurately capture both the added inertia effects and the energy-loss effects from an equivalent non-linear drag law. Using the eigenfunction expansion method, the horizontal hydrodynamic force (added mass, damping coefficient) on both the wall and baffle induced by the fluid motion is assessed for various combinations of porosity, submergence depth, and the tank's motion amplitude. It is found that a negative value for the added mass and a sharp peak in the damping curve occur near the resonant frequencies. In particular, the hydrodynamic force and free surface amplitude can be largely reduced by installing the proper porous baffle in a tank. The optimal porosity of a porous baffle is near P=0.1.

• Steel and Composite Structures
• /
• v.28 no.4
• /
• pp.461-470
• /
• 2018

In this study, the effect of polyurethane foam filler, in addition to surface layer thickness and core material thickness, on vibration characteristics of sandwich structures was investigated. The manufacturing process was carried out according to the Taguchi method. The natural frequencies and damping ratios of the produced samples were determined experimentally for fixed-free boundary conditions. In addition, solid models were developed for test samples and their finite element analyses were performed with $ANSYS^{(R)}$ to obtain their natural frequencies and mode shapes. An acceptably good agreement was found with the comparison of experimental results with the numerically obtained ones. The most effective parameters on the vibration characteristics of the sandwich structure were determined by the Taguchi method.