• Journal of the Korea Institute of Military Science and Technology
• /
• v.2 no.2
• /
• pp.61-69
• /
• 1999

A dynamic stability test was performed to determine dynamic stability derivatives for the pure pitching motion of air-to-ground missile model in the low speed wind tunnel. The free vibration technique was employed to acquire oscillation characteristics of the model for damping coefficients. Damping coefficients are obtained by the method of logarithmic decrement. Results show good damping effects and stability capability at Mach numbers 0.1 and 0.2, with the angle of attack ranging from -15 to +20 degrees.

• Tribology and Lubricants
• /
• v.13 no.2
• /
• pp.60-67
• /
• 1997

ER(electro-rheological) fluids, which are represented as Bingham fluids, have large and reversible changes in yield shear stresses by application of an electric field. In this paper, ER fluids are employed in a short squeeze film damper. The modified Reynolds equation for an ER short squeeze film damper is theoretically solved to get the approximate solutions of pressure profiles and damping coefficients. The theoretical approximate solutions are compared with numerical ones and both results are coincided very well. Both the direct and cross coupled damping coefficients substantially increase with increasing the yield shear stress of ER fluids. Furthermore, the synchronous response analysis of a rigid rotor supported on ER short squeeze film dampers is performed to show the improved damping capability of an ER short squeeze film damper.

• Structural Engineering and Mechanics
• /
• v.38 no.3
• /
• pp.349-359
• /
• 2011

Input excitation and output response of structure are needed in conventional modal analysis methods. However, input excitation is often difficult to be obtained in the dynamic load test of bridge structures. Therefore, what attracts engineers' attention is how to get dynamic parameters from the output response. In this paper, a structural experimental modal analysis method is introduced, which can be used to conveniently obtain dynamic parameters of the structure from the free decay response. With known damping coefficients, this analysis method can be used to identify the natural frequencies and the mode shapes of MDOF structures. Based on the modal analysis theory, the mathematical relationship of damping ratio and frequency is obtained. By using this mathematical relationship to improve the previous method, an improved experimental modal analysis method is proposed in this paper. This improved method can overcome the deficiencies of the previous method, which can not identify damping ratios and requires damping coefficients in advance. Additionally, this improved method can also identify the natural frequencies, mode shapes and damping ratios of the bridge only from the free decay response, and ensure the stability of identification process by using modern mathematical means. Finally, the feasibility and effectiveness of this method are demonstrated by a numerical example of a simply supported reinforced concrete beam.

• Journal of Ocean Engineering and Technology
• /
• v.31 no.5
• /
• pp.371-377
• /
• 2017

This paper introduces a simulation-based determination method for hydrodynamic derivatives and 6DOF (degrees-offreedom) motion analysis for an underwater vehicle. Hydrodynamic derivatives were derived from second-order modulus expansion and composed of the added mass, and linear and nonlinear damping coefficients. The added mass coefficients were analytically obtained using the potential theory. All of the linear and nonlinear damping coefficients were determined using CFD simulation, which were performed for various cases based on the actual operating condition. Then, the linear and nonlinear damping coefficients were determined by fitting the CFD results, which referred to 6DOF forces and moments acting on an underwater vehicle, with the least square method. To demonstrate the applicability of the current study, 6DOF simulations for three different scenarios (L-, U-, and S-turn) were carried out, and the results were validated on the basis of physical plausibility.

• Tribology and Lubricants
• /
• v.30 no.2
• /
• pp.77-85
• /
• 2014

This study set out to predict the load capacity and rotordynamic coefficients of tilting-pad journal bearings, taking the pivot stiffness into account. The analysis uses rocker-back (cylindrical) and ball in socket (spherical) pivot models, both of which are based on Hertzian contact stress theory. The models ascertain the non-linear elastic deformation of the pivots according to the applied load, pivot geometry, and material properties. At present, the Reynolds equation for an isothermal, isoviscous, and incompressible fluid is used to calculate the film pressure by using the finite-element method, after which the Newton-Raphson method is used to simultaneously find the journal center location, pad angles, and pivot deflections. The bearing analysis, excluding the pivot models, is validated using predictions those are readily available in the literature. As the rotor speed increases, the predicted journal eccentricity and damping coefficients decrease, but the stiffness coefficients increase, as expected. Most importantly, the implementation of the pivot models increases the journal eccentricity but significantly decreases the stiffness and damping coefficients of the tilting-pad journal bearings.

• Tribology and Lubricants
• /
• v.17 no.1
• /
• pp.64-71
• /
• 2001

This paper presents dynamic responses of disk flutter and bump in HDD slider. The slider is modeled for three degree-of-freedom systems, which are capable of lifting, pitching, and rolling motions. In numerical analysis, loads from air pressure, preload and static moments from the slider, and stiffness and damping coefficients of the suspension are considered for investigating the dynamic characteristics analysis. The numerical results are presented as functions of typical parameters such as a disk velocity, stiffness and damping coefficients of the suspension, and skew angle.

• Proceedings of the KIEE Conference
• /
• 1998.07c
• /
• pp.1147-1149
• /
• 1998

This paper presents the result of an analysis on effects of phase compensation on power system stability in the PSS parameter tuning. Synchronizing and damping coefficients are induced from lineal model for generator with PSS. Synchronizing and damping coefficients corresponding to time constants of phase compensation control block are calculated on a single machine, infinite bus test system. The Parameter tuning concepts, basic function, structural elements and performance criteria of PSS are introduced.

• Journal of Ocean Engineering and Technology
• /
• v.23 no.3
• /
• pp.6-13
• /
• 2009

Based on the boundary element method, the motion responses and transmission coefficients of a moored floating breakwater were investigated in oblique waves. To satisfy the outgoing radiation condition in the far field, the fluid domain was divided into inner and outer regions. The complete solution could be obtained by applying the matching conditions between the eigenfunction-based outer solution and BEM-based inner solution. Using the developed predictive tools, the wave exciting forces, added mass, damping coefficients, motion responses, and transmission coefficients were assessed for various combinations of breakwater configuration, wave heading, mooring cables properties, and wave characteristics. It was found that the transmission coefficient for a moored floating breakwater was closely dependent on the motion responses, which were greatly amplified at the resonant frequencies.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
• /
• 2001.11b
• /
• pp.1026-1031
• /
• 2001

In this paper. the dynamic characteristics of a super high-speed tilting-pad air bearing(TPGB) used in a turbo expander with high expansion ratio are analyzed. The dynamic behavior and stability of a rotary system supported by two journal air bearings are investigated numerically. The transient response of the shaft is obtained by simultaneously solving the equation of motion of the shaft and the dynamic Reynolds equation. The stiffness and damping coefficients of the bearing are calculated from the loading coefficients of the bearing are calculated from the loading capacity. shaft velocity and displacement by using a curve fitting method. The natural frequencies of the 1st and 2nd rigid modes can be calculated from these coefficients. The theoretical method of a rigid rotor system is verified by experimentsut.

• International Journal of Naval Architecture and Ocean Engineering
• /
• v.7 no.3
• /
• pp.540-558
• /
• 2015

Maneuvering oblique towing test is simulated in a Computational Fluid Dynamic (CFD) environment to obtain the linear and nonlinear velocity dependent damping coefficients for a DTMB 5512 model ship. The simulations are carried out in freely accessible OpenFOAM library with three different solvers, rasInterFoam, LTSInterFoam and interDyMFoam, and two turbulence models, $k-{\varepsilon}$ and SST $k-{\omega}$ in presence of free surface. Turning and zig-zag maneuvers are simulated for the DTMB 5512 model ship using the calculated damping coefficients with CFD. The comparison of simulated results with the available experimental shows a very good agreement among them.