• Coupled systems mechanics
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• v.9 no.4
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• pp.311-323
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• 2020

This paper deals with the effect of the mode shapes on the dynamic response of a multi-storey frame subjected to moving train loads which are modelled as loads of constant intervals with constant velocity using the finite element method. The multi-storey frame is modelled as a number of Bernoulli-Euler beam elements. First, the first few modes of the multi-storey frame are determined. Then, the effects of force span length to beam length ratio and velocity on dynamic magnification factor (DMF) are evaluated via 3D velocity-force span length to beam length ratio-DMF graphics and its 2D projections. By using 3D and 2D graphics, the directions of critical speeds that force the structure under resonance conditions are determined. Last, the mode shapes related to these directions are determined by the time history and frequency response graphs. This study has been limited by the vibration of the frame in the vertical direction.

• 유체기계공업학회:학술대회논문집
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• 2002.12a
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• pp.453-458
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• 2002

The feasibility study on supporting a turbocharger rotor on air foil bearing is investigated. Based on finite difference method and Newton-Raphson method, the static equilibrium position of a turbocharger rotor is predicted. And using finite difference method and perturbation method, dynamic characteristics of air foil bearings are calculated. Rotordynamic analysis is performed by finite element method, with collaboration of calculated stiffness and damping of foil bearing. The effect of compliance and clearance of bump foil bearing on the oil-free turbocharger is investigated in terms of rotordynamics. And the critical speeds, eccentricity ratio, vibration amplitude, and stability are considered. It is demonstrated that foil bearings offer a rlausible replacement for oil-lubricated bearings in turbocharger.

• The KSFM Journal of Fluid Machinery
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• v.6 no.1 s.18
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• pp.51-56
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• 2003

The feasibility study on the oil-free turbocharger supported by air foil bearings is investigated. Using the perturbation method, dynamic characteristics of air foil bearings are calculated based on the static equilibrium position of a turbocharger rotor is predicted. With collaboration of calculated stiffness and damping of foil bearing, rotordynamic analysis is performed using the finite element method. The effects of bump compliance and bearing clearance on rotordynamic characteristics of the oil-free turbocharger such as the critical speeds, eccentricity ratio, vibration amplitude and stability are investigated.

• The KSFM Journal of Fluid Machinery
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• v.16 no.2
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• pp.21-26
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• 2013

Super-cavitating vehicle which is operating under water speeds up to 100m/s. In this process super-cavitation around body reduces frictional resistance. This paper introduces experimental study on different width of suction path as there is a warhead in the torpedo. Hydraulic performances of turbopump at non-cavitating condition does not display a significant differences depending on different width of suction path. However, cavitation performance of each model shows obvious differences in the same condition of experience. In case of radial inlet, the value of critical NPSH(which indicates 3% head drop) increases about 20% in comparison of axial inlet.

• Journal of KSNVE
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• v.8 no.6
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• pp.1069-1077
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• 1998

A dynamic model of turbo compressor having helical gear pairs is developed. The model accounts for the shaft and bearing flexibilities, gyroscopic effects and the force couplings among the transverse, torsion. and axial motions due to gearings. For the mode analysis of turbo compressor, a transfer matrix method is used. The excitation sources caused by the mass unbalances of the rotors and misalignment of the shafts, the transmitted errors of the gearings. and the vane passing frequencies of the Impeller are studied qualitatively. By introducing the perturbation method, the generated forcing frequencies are defined and devided into three groups. With the field data, two critical speeds are analytically found and the corresponding modal characteristics are examined.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2004.10a
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• pp.212-216
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• 2004

Bearingless motors are the rotational electric machine which utilize a common magnetic structure for rotation and magnetic suspension. Since the bearing function is combined with the motor, the shaft length can be shortened resulting in higher critical speeds. Relationship between suspension force and current of bearingless motor is clearly derived by prior research. However, relationship between displacement of rotor and suspension force is not precisely defined. In this paper, we present model of bearingless motor describing the radial force variation due to the movement of the rotor. Using a distributed magnetic circuit and maxwell stress tensor, we derived a mathematical expression for the radial force. For a slotless bearingless motor, we are able to find an analytical model presented in the form of stiffness. For a slotted motor, we can compute the stiffness by semi-analytical analysis. This model is validated by a finite-element-analysis.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 1998.04a
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• pp.206-210
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• 1998

A study on dynamic characteristics of rotating disks in magneto optical disk drives is presented. Natural frequencies of rotating disks are investigated experimentally and numerically. The frequency response and critical speeds of the ASMO disk are discussed. The characteristics of airflow around the disk and their effects on disk vibrations are also investigated. It is found that the numerical calculation of the natural frequencies of rotating disks agrees well with the experimental results. The airflow around the disk in the cartridge affects the characteristics of the disk vibrations to reduce the modal frequencies of the disk. The experiment shows that negative vertical offsets of the disk in the cartridge possibly increase the vibration amplitudes. As being influenced by the geometry of the cartridge, the rotation of the disk causes an asymmetric airflow in the presence of window.

• Tribology and Lubricants
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• v.11 no.1
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• pp.5-11
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• 1995

This paper addresses the lubrication analysis of a short squeeze film damper operating with electro-rheological (ER) fluids which have large and reversible changes in yield shear stresses with respect to an applied electric field. The ER fluids are assumed to be modeled as Bingham fluids. The governing lubrication equation for the ER short squeeze film damper is developed on the basis of a Bingham fluid model, and the equation is subsequently solved in order to investigate the effects of the ER fluids on the damping capability of the damper. It is shown that a substantial increase in damping (both direct and cross coupled) is accomplished by increasing the yield shear stress of the ER fluids. This significant improvement of the damping capability suggests that the ER short squeeze film damper could be very effective for reducing the vibration and controlling the critical speeds of a rotor system.

• Smart Structures and Systems
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• v.17 no.3
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• pp.391-411
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• 2016

This study investigates the mode identifiability of a multi-span cable-stayed bridge in terms of a benchmark study using stabilization diagrams of a system model identified using stochastic subspace identification (SSI). Cumulative contribution ratios (CCRs) estimated from singular values of system models under different wind conditions were also considered. Observations revealed that wind speed might influence the mode identifiability of a specific mode of a cable-stayed bridge. Moreover the cumulative contribution ratio showed that the time histories monitored during strong winds, such as those of a typhoon, can be modeled with less system order than under weak winds. The blind data Acc 1 and Acc 2 were categorized as data obtained under a typhoon. Blind data Acc 3 and Acc 4 were categorized as data obtained under wind conditions of critical wind speeds around 7.5 m/s. Finally, blind data Acc 5 and Acc 6 were categorized as data measured under weak wind conditions.

• Journal of the Korean Institute of Intelligent Systems
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• v.10 no.5
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• pp.478-486
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• 2000

This paper proposes a design method that can minimize the power dissipation of CMOS digital circuits without affecting their optimal operation speeds. The proposed method is based on genetic algorithms(GAs) combined to the retiming technique, a circuit transformation technique of repositioning flip-flops. The proposed design method consists of two phases: the phase of retiming for optimizing clock periods and the phase of GA retiming for minimizing power dissipation. Experimental results using Synopsys Design Analyzer show that the proposed design method can reduce the critical path delay of example circuits by about 30-50% and improve the dynamic power performance of the circuits by about 1.4~18.4%.