• 한국기계가공학회지
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• 제19권12호
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• pp.49-55
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• 2020

In order to improve the electromagnetic shielding rate of Carbon Fiber (CF), it was produced using the nickel nano-powder impregnating method. Using two types of nickel powder having thicknesses of 50 ㎛ and 100 ㎛, and a thermoplastic elastomer resin, a compound containing 10-20% nickel content was mixed and then manufactured through an extruder. The CF coated with the compound was woven and manufactured using a 1-ply specimen. The final nickel content of the specimen was verified using TGA and the distribution of nickel powder on the CF surface was verified using SEM. The metal shows a high shielding rate in the low-frequency band, but the shielding rate decreases at higher-frequency bands. The CF improves at the higher frequency band, and metals reflect electromagnetic waves while carbon absorbs electromagnetic waves. The study of shielding materials, which are stronger and lighter than metal, by using CF lighter than metal and enabling the shielding rate from low-frequency band to high-frequency band, confirmed that the larger the area coated with nickel nano-powder, the better the electromagnetic shielding performance. In particular, CF coated with a thickness of 100 ㎛ has a shielding rate similar to that of copper and can also be used for EV/HEV automotive cables and other applications in the future.

• 대한조선학회논문집
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• 제31권4호
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• pp.82-98
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• 1994

본 논문에서는 파중에서 조종운동을 하는 선박에 작용하는 유체력에 대한 수학적 모델을 확립하고, 이에 대한 수치해석을 수행하였다. 파중에서 운항하는 선박은 입사파의 주파수에 의존하는 고주파수 운동과 선박의 조종운동에 따른 저주파수 운동을 하게되므로, two time scale expansion 방법을 적용하여 속도포텐셜에 대한 선형경계치 문제를 정립하였으며, 선박의 조종운동에 미치는 파의 영향은 2차차수 저주파수 표류력만을 조종운동방정식에 포함시켜 고려하였다. 속도포텐셜에 대한 적분방정식의 해는 3차원 파넬방법을 사용하여 구하였으며, 선박에 작용하는 힘은 직접적분법을 사용하여 계산하였다.

• Journal of international Conference on Electrical Machines and Systems
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• 제1권2호
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• pp.64-69
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• 2012

This paper presents a hybrid pulse width modulation (PWM) method under low switching frequency conditions based on space vector PWM (SVPWM) and selective harmonic eliminated PWM (SHEPWM), which use asynchronous carrier modulation SVPWM at low frequency, and SHEPWM at high frequency, a square wave after rated conditions. A transitive strategy is proposed to realize a smooth transition of individual modes including SVPWM, SHEPWM and square waves. Experimental results confirm this hybrid modulation method and their transition are reasonable and proper.

• 수산해양기술연구
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• 제33권3호
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• pp.248-258
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• 1997

The analytical solutions of the Fraunhofer Diffraction(FD) theory and the principle for measurement of the dispersion relation of plasma wave is presented. Especially, the method for measurement of low-frequency wave is discussed. The wavenumbers of the density fluctuations are obtained from the curve fitting between the expremental FD profile and theoretical one for each frequency component. In measurement of the wavenumber of the low -frequency region, the information of the wavenumber is easily obtained from the ratio of the intensity at = 0 to the intensity at =0.5. The millimeter wave FD apparatus was designed to measure low-frequency density fluctuations. The determined wavenumbers are in the range of =0.1~ 1.0cm. Thus, the millimeter wave FD method was shown to be useful for the measurement of low-frequency density fluctuations, which are impossible to be measured by using a convention. Thomson scattering. The obtained dispersion relations will be useful information for plasma waves.

• 천문학회보
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• 제37권2호
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• pp.119.2-119.2
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• 2012

Charged particle energization is an outstanding problem in space physics. This paper investigates the nonlinear dynamics of Alfve'n-cyclotron waves accompanying particle heating processes and the drift Alfv'en-cyclotron (or EMIC) instability associated with a current disruption event on 29 January 2008 observed with THEMIS satellite by means of a particle-in-cell simulation. The simulation shows that the drift Alfv'en-cyclotron instabilities are excited in two regimes, a relatively low frequency mode propagating in a quasi-perpendicular direction while the second high-frequency branch propagating in a predominantly parallel propagation direction, which is consistent with observation as well as earlier theories. It is shown that parametric decay processes lead to an inverse cascade of Alfv'en-cyclotron waves and the generation of ion-acoustic waves by decay instability. It is also shown that the nonlinear decay processes are accompanied by small perpendicular heating and parallel cooling of the protons, and a pronounced parallel heating of the electrons.

• 한국우주과학회:학술대회논문집(한국우주과학회보)
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• 한국우주과학회 2009년도 한국우주과학회보 제18권2호
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• pp.44.1-44.1
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• 2009

Identification of wave characteristics during current disruption events in the near-Earth geomagnetic tail region (~ 10 RE) is important to understand the substorm onset mechanism. In this paper, linear stability analysis in the ion-cyclotron grequency range, considering temperature anisotropy and cross-field flow is presented. It is found that the ion-cyclotron drift waves propagating in a quasi-perpendicular direction with respect to the ambient magnetic field are characterized by low frequencies ($\omega$ < $0.5{\Omega}ci$), while quasi-parallel waves have frequencies close to the ion-cyclotron frequency ($\omega{\sim}{\Omega}ci$). This finding is consistent with the observation by THEMIS spacecraft of a current disruption event in which a similar high- and low-frequency band structure is also present [A. T. Y. Lui, et al., J. Geophys. Res. 113, A00C06 (2008)]. It is also found that the quasi-perpendicular mode is excited by the ion cross-field flow.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2003년도 춘계학술대회논문집
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• pp.393-396
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• 2003

Conventional method for designing and installing anechoic chambers is to utilize porous wedges for the sound absorbers. As cutoff frequency lowers down such as 63Hz or 50Hz, the corresponding long wedges diminish the free field area of the chamber. In this study, a new broadband compact absorber(BCA) is introduced which absorbs acoustic energy down to 50Hz. Most prominent is that it measures only 250mm thick. A freely vibrating panel between the non-fibrous absorbers allows tuned absorption at the low frequency region in addition to the high frequency absorption resulted from the conventional absorber installed at the front. Standing waves at low frequency range are suppressed as the BCA modules which are tuned to the corresponding modes absorb sound energy effectively, resulting in anechoic condition. Not only the low frequency performances, but the high frequency absorption is measured to meet adequate conditions for the anechoic chamber. Realized BCA chambers are presented.

• Wind and Structures
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• 제12권2호
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• pp.103-120
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• 2009

Articulated tower platforms due to its compliant nature are more susceptible to the dynamic effects of wind than conventional fixed platforms. Dynamic response analysis of a double hinged articulated tower excited by low frequency wind forces with random waves is presented in this paper. The exposed super structure of the platform, housing the drilling and production facilities is subjected to mean and fluctuating wind loads, while the submerged portion is acted upon by wind driven waves. The fluctuating component of the wind velocity is modeled by Emil Simiu's spectrum, while the sea state is characterized by Pierson-Moskowitz spectrum. Nonlinearities in the system due to drag force, added mass, variable submergence and instantaneous tower orientation are considered in the analysis. To account for these nonlinearities, an implicit time integration scheme (Newmark's-${\beta}$) has been employed which solves the equation of motion in an iterative fashion and response time histories are obtained. The power spectra obtained from random response time histories show the significance of low frequency responses.

• 한국해안해양공학회지
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• 제8권1호
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• pp.37-43
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• 1996

본 논문에서는 2차원 수조의 임의 지점에서 쇄파를 만드는 실험방법에 대하여 논하였다. 쇄파를 만들기 위해서 임의의 지점에 각 성분의 파정이 모이도록 위상차를 갖는 선형파를 합성하여 조파기를 작동시켰다. 또한 조파판의 운동진폭을 조절하여 다양한 파기울기에 대하여 실험을 수행하였다. 실험 결과 운동진폭이 너무 작은 경우에는 쇄파현상이 일어나지 않았고 불규칙한 파형만이 나타났으며, 최대파고가 H=0.0113g/T$^2$ 이상이어야 쇄파 현상이 나타남을 확인하였다. 쇄파의 모양은 대부분 경우에 spilling 형태였고, 특정한 운동진폭일 때 plunging 형태의 쇄파를 관찰할 수 있었다. 한편 비점성, 비압축성 유체로 가정하여 얻어진 경계적분방정식을 고차경계요소로 이산화하여 수치계산을 수행하였다. 자유표면의 처리를 위해서 Mixed Euler-Lagrangian 기법을 이용하였다 수치결과는 자유표면에서 주파수간의 상호간섭의 영향을 제대로 반영하고 있으며, 실험에서 계측한 파형을 제대로 모사하고 있음을 확인할 수 있었다.

• 한국음향학회:학술대회논문집
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• 한국음향학회 1993년도 학술논문발표회 논문집 제12권 1호
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• pp.68-72
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• 1993

A method to measure the distortions of the loudspeaker at low frequencies without an anechoic chamber is proposed. This method utilizes the fact that the n-th harmonic distortion outside the enclosure is boosted by 40log n dB compared to that inside the enclosure. By compensating for the effect of standing waves occuring inside the enclosure, it is possible to predict the distortions for wide frequency ranges below the first break-up frequency of the diaphragm. The method is applicable to the intermodulation distortion as well.