• Wind and Structures
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• 제7권6호
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• pp.421-447
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• 2004

This paper presents a time domain approach for predicting buffeting response of long suspension bridges under skew winds. The buffeting forces on an oblique strip of the bridge deck in the mean wind direction are derived in terms of aerodynamic coefficients measured under skew winds and equivalent fluctuating wind velocities with aerodynamic impulse functions included. The time histories of equivalent fluctuating wind velocities and then buffeting forces along the bridge deck are simulated using the spectral representation method based on the Gaussian distribution assumption. The self-excited forces on an oblique strip of the bridge deck are represented by the convolution integrals involving aerodynamic impulse functions and structural motions. The aerodynamic impulse functions of self-excited forces are derived from experimentally measured flutter derivatives under skew winds using rational function approximations. The governing equation of motion of a long suspension bridge under skew winds is established using the finite element method and solved using the Newmark numerical method. The proposed time domain approach is finally applied to the Tsing Ma suspension bridge in Hong Kong. The computed buffeting responses of the bridge under skew winds during Typhoon Sam are compared with those obtained from the frequency domain approach and the field measurement. The comparisons are found satisfactory for the bridge response in the main span.

• 대한기계학회논문집B
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• 제32권9호
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• pp.659-668
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• 2008

Combustion instability has been considered as very important issue for developing gas turbine and rocket engine. There is a need for fundamental understanding of combustion instability. In this study, combustion instability was numerically and experimentally investigated in a dump combustor with bluff body. The fuel and air mixture had overall equivalence ratio of 0.9 and was injected toward dump combustor. The pressure oscillation with approximately 256Hz was experimentally obtained. For numerical simulation, the standard k-$\varepsilon$ model was used for turbulence and the hybrid combustion model (eddy dissipation model and kinetically controlled model) was applied. After calculating steady solution, unsteady calculation was performed with forcing small perturbation on initial that solution. Pressure amplitude and frequency measured by pressure sensor is nearly the same as those predicted by numerical simulation. Furthermore, it is clear that a combustion instability involving vortex shedding is affected by acoustic-vortex-combustion interaction. The phase difference between the pressure and velocity is $\pi$/2, and that between the pressure and heat release rate is in excitation range described by Rayleigh, which is obvious that combustion instability for the bluff body combustor meets thermoacoustic instability criterion.

• 대한기계학회논문집
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• 제13권3호
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• pp.373-384
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• 1989

본 논문에서는 상기와 같은 문제들을 규명하기 위하여 비교적 긴 원통에 액체를 내장할 수 있도록 회전체를 제작하여 액체의 종류, 점도, 액체의 양 등을 파라미터로 하여 불안정영역의 범위와 진동진폭에 관해서 실험적으로 조사하였고, 비중이 다른 2액체를 동시에 내장한 경우 액체의 자유표면과 경계면에 의한 진동특성을 을 검토하였다.

• International Journal of Fluid Machinery and Systems
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• 제9권1호
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• pp.17-27
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• 2016

Researches on the mechanism of surging and the surge behaviors in the systems of pumps, or fans or compressors, and the effects of flow-paths had been initiated and had made a great progress in Japan in the decades from the nineteen-forties to the nineteen-sixties. In 1947, the essential cause of the surges, i.e., self-excited oscillation nature of the flow-system, was discovered analytically by Professor Sumiji Fujii of Tokyo University, and most of the characteristic behaviors of the phenomena had been explained clearly. Successive studies by many other Japanese researchers continued to prove experimentally the mechanism, to extend the analytical studies, and to attempt preventing surge occurrence, etc. in the following two decades. The historical information on the early surge studies could be helpful to some concerned people. At the same time, the basic and plain ways of discussions and reasoning about the phenomena in the pioneering researches could give us much to be learned even in the present time of high-power computing systems. Regrettably, many of the original research works have been published only in Japanese. The present review introduces very briefly the situations in memories of the pioneering researchers and engineers.

• Wind and Structures
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• 제34권1호
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• pp.73-80
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• 2022

This paper presents a study on amplitude-dependent self-excited aerodynamic forces of a 5:1 rectangular cylinder through free vibration wind tunnel test. The sectional model was spring-supported in a single degree of freedom (SDOF) in torsion, and it is found that the amplitude of the free vibration cylinder model was not divergent in the post-flutter stage and was instead of various stable amplitudes varying with the wind speed. The amplitude-dependent aerodynamic damping is determined using Hilbert Transform of response time histories at different wind speeds in a smooth flow. An approach is proposed to extract aerodynamic derivatives as nonlinear functions of the amplitude of torsional motion at various reduced wind speeds. The results show that the magnitude of A2^*, which is related to the negative aerodynamic damping, increases with increasing wind speed but decreases with vibration amplitude, and the magnitude of A3^* also increases with increasing wind speed but keeps stable with the changing amplitude. The amplitude-dependent aerodynamic derivatives derived from the tests can also be used to estimate the post-flutter response of 5:1 rectangular cylinders with different dynamic parameters via traditional flutter analysis.

• 천문학회보
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• 제45권1호
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• pp.37.1-37.1
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• 2020

Cosmic-ray protons (CRp) are efficiently produced at starburst galaxies (SBGs), where the star formation rate (SFR) rate is high. In this talk, we present estimates of gamma-ray and neutrino emissions from nearby SBGs, M82, NGC253, and Arp220. Inside the starburst nucleus (SBN), CRp are accelerated at supernova remnant (SNR) shocks as well as at stellar wind (SW) termination shocks, and their transport is governed by the advection due to starburst-driven wind and diffusion mediated by turbulence. We here model the momentum distributions of SNR and SW-produced CRp with single or a double power-law forms. We also employ two different diffusion models, where CRp are resonantly scattered off large-scale turbulence in SBN or self-excited waves driven by CR streaming instability. We then calculate gamma-ray/neutrino fluxes. The observed gamma-ray fluxes by Fermi-LAT, Veritas, and H.E.S.S are well reproduced with double power-law distribution for SNR-produced CRp and the CRp diffusion by self-excited turbulence. The estimated neutrino fluxes are <~10-3 of the atmospheric neutrino flux in the energy range of Eneutrino <~100 GeV and <~10-1 of the IceCube point source sensitivity in the energy range of Eneutrino >~60 TeV.

• 반도체디스플레이기술학회지
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• 제4권2호
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• pp.11-14
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• 2005

In plasma processing reactors, it is common practice to control plasma density and ion bombardment energy by manipulating excitation voltage and frequency. In this paper, a dually excited capacitively coupled rf plasma reactor is self-consistently simulated with a three moment model. Effects of phase differences between primary and secondary voltage waves, simultaneously modulated at various combinations of commensurate frequencies, on plasma properties are investigated. The simulation results show that plasma potential and density as well as primary self-dc bias are nearly unaffected by the phase lag between the primary and the secondary voltage waves. The results also show that, with the secondary frequency substantially lower than the primary frequency, secondary self·do bias remains constant regardless of the phase lag. As the secondary frequency approaches to the primary frequency, however, the secondary self-dc bias becomes greatly altered by the phase lag, and so does the ion bombardment energy at the secondary electrode. These results demonstrate that ion bombardment energy can be more carefully controlled through plasma simulation.

• Bulletin of the Korean Chemical Society
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• 제35권5호
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• pp.1511-1515
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• 2014

The multiconfiguration second-order perturbation theory (CASPT2) and complete active space self-consistent field (CASSCF) methods were employed to calculate the geometries and energy levels for the low-lying electronic states of 1,3,5-$C_6H_3Cl{_3}^+$ ion. The CASPT2 values for the 1,3,5-$C_6H_3Cl{_3}^+$ ion were in reasonable agreement with the available experimental values. The current calculations augmented previous theoretical investigations on the ground state and assigned the low-lying excited electronic states of the 1,3,5-$C_6H_3Cl{_3}^+$ ion. The Jahn-Teller distortion in the excited electronic state for the 1,3,5-$C_6H_3Cl{_3}^+$ ion were reported for the first time.

• 전기의세계
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• 제18권4호
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• pp.7-15
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• 1969

In this method the condenser excite winding has the phase angle of 90 electrical degree, with the load winding in stator. The condenser excite wing is connected with the condenser while the load winding is with the full rectifer. Direct and quardrature axis components of rotating field winding are composed, of balanced two phase winding, and each one of them is connected with half wave rectifiers. Initically, small amount of lead current can be induced at the condenser excite winding by residual magnetism of rotor. The induced lead current forces the rotating field winding to be excited by synchronous alternating magnetic field. The speed electromotive force, there for, induced in rotating field winding shall electro magnetize the rotating field pole by rotating half wave rectifiers. In the case of the charging generator directly coupled with engines at the operation of wide range speed, the generated region, such as vehicles, aircraft, ships etc, is occured. In conclusion, we can take the advantage of, omitting of voltage regurator and current limiter for charging load and reducing the consumption of fuel using the generated region which can be devided in to Impossible generated region, Generated region, and suspension generated region.

• 한국지진공학회논문집
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• 제21권5호
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• pp.237-244
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• 2017

This study proposes a numerical model to explain the closely placed double modes in the vibration of a layered stone pagoda system. The friction surface between the stones is modelled as the Timoshenko finite element while each stone layer is modelled as a rigid body. It is assumed that the irregular asperity on the friction surface enables the stone to be excited. This results in the closely placed modes that are composed of natural modes and self-excited modes. To examine the validity of the proposed model, a set of modal testing and analysis for a layered stone pagoda mock-up model has been conducted and a set of closely placed double modes are extracted. Applying the extended sensitivity-based system identification technique, the various system parameters are identified so that the modal parameters of the proposed numerical model are the same with those of the experimental mock-up. For a horizontal impulse excitation, the simulated acceleration responses are compared with measurements.