• International Journal of Fluid Machinery and Systems
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• v.2 no.2
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• pp.165-171
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• 2009

We developed a 'multi-point vibration acceleration method' for accurately predicting the cavitation intensity in pumps. Pressure wave generated by cavitation bubble collapse propagates and causes pump vibration. We measured vibration accelerations at several points on a casing, suction and discharge pipes of centrifugal and mixed-flow pumps. The measured vibration accelerations scattered because the pressure wave damped differently between the bubble collapse location and each sensor. In a conventional method, experimental constants are proposed without evaluating pressure propagation paths, then, the scattered vibration accelerations cause the inaccurate cavitation intensity. In our method, we formulated damping rate, transmittance of the pressure wave, and energy conversion from the pressure wave to the vibration along assumed pressure propagation paths. In the formulation, we theoretically defined a 'pressure propagation coefficient,' which is a correlation coefficient between the vibration acceleration and the bubble collapse pressure. With the pressure propagation coefficient, we can predict the cavitation intensity without experimental constants as proposed in a conventional method. The prediction accuracy of cavitation intensity is improved based on a statistical analysis of the multi-point vibration accelerations. The predicted cavitation intensity was verified with the plastic deformation rate of an aluminum sheet in the cavitation erosion area of the impeller blade. The cavitation intensities were proportional to the measured plastic deformation rates for three kinds of pumps. This suggests that our method is effective for estimating the cavitation intensity in pumps. We can make a cavitation intensity map by conducting this method and varying the flow rate and the net positive suction head (NPSH). The map is useful for avoiding the operating conditions having high risk of cavitation erosion.

• The Bulletin of The Korean Astronomical Society
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• v.40 no.1
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• pp.49.2-49.2
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• 2015

IntraCluster Medium (ICM) located at the galaxy cluster is in the state of very hot, tenuous, magnetized, and highly ionized X-ray emitting plasmas. High temperature and low density make ICM very viscous and conductive. In addition to the high conductivity, fluctuating random plasma motions in ICM, occurring at all evolution stages, generate and amplify the magnetic fields in such viscous ionized gas. The amplified magnetic fields in reverse drive and constrain the plasma motions beyond the viscous scale through the magnetic tension. Moreover, without the influence of resistivity viscous damping effect gets balanced only with the magnetic tension in the extended viscous scale leading to peculiar ICM energy spectra. This overall collisionless magnetohydrodynamic (MHD) turbulence in ICM was simulated using a hyper diffusivity method. The results show the plasma motions and frozen magnetic fields have power law of $E_V^k{\sim}k^{-3}$, $E_M^k{\sim}k^{-1}$. To explain these abnormal power spectra we set up two simultaneous differential equations for the kinetic and magnetic energy using an Eddy Damped Quasi Normal Markovianized (EDQNM) approximation. The solutions and dimensions of leading terms in the coupled equations derive the power spectra and tell us how the spectra are formed. We also derived the same results with a more intuitive balance relation and stationary energy transport rate.

• Journal of the Korean Society of Propulsion Engineers
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• v.18 no.2
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• pp.16-26
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• 2014

T-Burner tests of an Al/HTPB propellant in conjunction with a Pulsed DB/AB Method were conducted to find an acoustic amplification factor. Aluminum-free and aluminum-heavy propellants were examined. Instant surface ignition was successfully made by the use of a supplementary propellant of fractionally higher reaction rate. With the presence of higher aluminum concentration in the propellants, the pressure perturbations were promptly damped down and the pressure fluctuations were no longer dispersive. Addition of aluminum particles into the propellant was advantageous for stabilizing pressure-coupled unstable waves.

• Korean journal of applied entomology
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• v.36 no.3
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• pp.215-223
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• 1997

A matrix model of rice weevil population based on degree day (DD) was constructed. The basic matrix model predicted on exponential jncrcase of the adult weevil density and the finite rate of increase(h) of the population was estimated to be 2.155/100DD. Adult density simulated by the matrix model including intraspecific competition showed a damped oscillation over time and reached at the stationary level of 530 at 69, 300DD. The experimental population showed similar features to that of the model. But there were some differences in the highest density and period of adult oscillation. The differences could largely be caused by the assumption of the model; resource constancy.

• Korean Journal of Optics and Photonics
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• v.12 no.2
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• pp.109-114
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• 2001

Optical system has some optical and mechanical constraints. The constraints should be preserved in optimization of optical system. For the purpose, the constraints are combined with the merit function by using Lagrange's undetermined multipliers. We propose an active optimization control by using the fact that the errors of constraints are corrected with higher priority than the other errors of the merit function. In this control, the errors which have large contribution to the merit function are converted into constraints. At that time, if the errors are corrected at once, the optimization will be unstable because of their non-linearity. Hence we introduce a target rate which represents a fraction of error to be corrected, and the errors are corrected progressively. An optimization program was developed on the bases of the proposed active control, and applied to design a photographic lens system. By using the active control, we could get better results compared with conventional damped least squares method. ethod.

• International Journal of Railway
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• v.2 no.3
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• pp.118-123
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• 2009

Railway track alignment Standards set a minimum lenght value for straight and circular alignments (art. 5.2.9.), in order to ensure passenger ride comfort in railway vehicles of which dynamic oscillations will thus have to be limited. The transitions between alignments can cause abrupt changes (usually called discontinuities or singular points of the alignment) of curvature, of rate of change of curvature or of rate of change of cant. A passenger is likely to experience effects due to the excitation of the elastic suspension of the vehicle which generates oscillations that are damped as the vehicle moves away from the singularity. The amplitude of these oscillations should be adequately attenuated by the damping of the suspension system within the interval between two successive singular points, especially to avoid resonances. Therefore minimum lengths between two successive singular points are stated in alignment standards. Nevertheless, these nonnative values can be overly conservative in some cases. As an alternative, track alignment designers could try to assess how much the excitation has been attenuated between two successive singular points and thus assess at which point a new singularity may be present without affecting ride comfort. Although such assessment can be made with commercial SW packages which simulate the dynamic behavior of a vehicle considered as a set of rigid bodies interconnected with elastic elements simulating the suspension systems (such as SIMPACK, ADAMS or VAMPIRE), a simplified and user-friendly computation method (based upon the analytical solution of differential equations governing the phenomenon) is made available in this paper to track design engineers, not always used to working with full dynamic models.

• Journal of the Korean Society of Propulsion Engineers
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• v.9 no.2
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• pp.32-39
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• 2005

The effectiveness of the pogo suppression device (PSD) on the response of the piping system simulating the fuel (or oxidizer) supply lines of the rocket engines was investigated. The system response defined as the ratio of the flow rate to the pressure in the main tube was obtained for various PSD gas volumes $((0\~2)\times10^{-3}m^3)$ and three different baffle hole diameters (5, 50, 115mm). Existence of a gas volume in the PSD reduced the system resonance frequency. With a larger gas volume, the resonance frequency became lower, but only slightly, while the fluctuations of the main tube pressure and the flow rate damped down considerably. The resonance frequency decreased with the increase of the PSD inlet restriction (or the decrease of the baffle hole diameter), though slightly. However, with a larger inlet restriction, the PSD pressure wave showed a delayed response with the smaller amplitude compared to the pressure variation in the main tube.

• The Journal of the Acoustical Society of Korea
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• v.37 no.5
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• pp.338-342
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• 2018

In order to solve the floor impact sound problem in the upper and lower floors, the Ministry of Land, Transport and Maritime Affairs also notifies the physical properties of the resilient material affecting the floor impact sound level. The dynamic modulus of elasticity and the loss factor before and after heating are most related to the floor impact noise, especially for the cushioning material. Therefore, in this study, the rate of change with respect to the dynamic modulus and loss factor with temperature change was examined by increasing $10^{\circ}C$ by $10^{\circ}C$ from the temperature condition of $70^{\circ}C$ specified in the standard. The dynamic modulus of elasticity and the loss modulus were measured by using the pulse excitation method for eight kinds of samples. The calculation method was calculated by the time series analysis method using the damped vibration waveform.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.34 no.6
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• pp.59-66
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• 2006

Acoustic cavity as a stabilization device to control high-frequency combustion instabilities in liquid rocket engine is adopted and its damping capacity is verified in atmospheric temperature. First, harmful resonant frequency in a modeling chamber can be damped effectively by the installation of properly-tuned acoustic cavity. Besides, geometric effects of acoustic cavity on damping characteristics are analyzed and compared quantitatively. Satisfactory agreements have been achieved with linear acoustic analysis and experimental approach. Results show that the acoustic cavity of the largest orifice area or the shortest orifice length was the most effective in acoustic damping of the harmful resonant frequency. Finally, it is proved that an optimal design process is indispensable for the effective control of combustion instabilities.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.11 no.3
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• pp.479-485
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• 2007

In this paper, a hierarchical simulation with an approximate implicit method is proposed in order to efficiently and plausibly animate mass-spring based cloth models. The proposed hierarchical simulation method can generate realistic motion of extremely fine mesh in interactive rate. The proposed technique employs a fast and stable simulation method which approximates the implicit integration. Although the approximate method is efficient, it is extremely inaccurate and shows excessively damped behavior. The hierarchical simulation technique proposed in this paper constructs multi-level mesh structure in order to represent the realistic appearance of cloth model and performs simulation on each level of the mesh with constraints that enforce some of the mass-points of current level to follow the movement of the previous level. This hierarchical method efficiently generates a plausible movement of a cloth model composed of large number of mass points. Moreover, this hierarchical method enables us to generate realistic wrinkles on the cloth, and the wrinkle pattern on the cloth model can be easily controlled because we can specify different contraction resistance force of springs according to their hierarchical level.