• Structural Engineering and Mechanics
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• 제75권4호
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• pp.435-444
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• 2020

Ultrasonic guided wave testing is a very promising non-destructive testing method for rails, which is of great significance for ensuring the safe operation of railways. On the basis of the semi-analytical finite element (SAFE) method, a analytical model of 59R2 grooved rail was proposed, which is commonly used in the ballastless track of modern tram. The dispersion curves of ultrasonic guided waves in free rail and supported rail were obtained. Sensitivity analysis was then undertaken to evaluate the effect of rail elastic modulus on the phase velocity and group velocity dispersion curves of ultrasonic guided waves. The optimal guided wave mode, optimal excitation point and excitation direction suitable for detecting rail integrity were identified by analyzing the frequency, number of modes, and mode shapes. A sinusoidal signal modulated by a Hanning window with a center frequency of 25 kHz was used as the excitation source, and the propagation characteristics of high-frequency ultrasonic guided waves in the rail were obtained. The results show that the rail pad has a relatively little influence on the dispersion curves of ultrasonic guided waves in the high frequency band, and has a relatively large influence on the dispersion curves of ultrasonic guided waves in the low frequency band below 4 kHz. The rail elastic modulus has significant influence on the phase velocity in the high frequency band, while the group velocity is greatly affected by the rail elastic modulus in the low frequency band.

• 한국지구물리탐사학회:학술대회논문집
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• 한국지구물리탐사학회 2006년도 공동학술대회 논문집
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• pp.41-50
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• 2006

본 연구에서는 표면파의 군속도 분산 분석을 이용하여 한반도에 설치된 광대역 관측소 사이의 지각 및 상부맨틀의 지진파 속도 구조를 연구하였다. 지진파 잡음의 상호상관기법(Campillo and Paul, 2003; Shapiro et al., 2005)을 이용하여 단주기 표면파의 군속도 분산을 계산하였다. 기존의 잡음 상호상관 기법과는 다르게 지진파 신호를 화이트닝하여 분석에 사용하였다. 그린 함수에서 MFT(multiple filter technique) 방법을 이용하여 0.5 - 20 초 주기의 표면파의 군속도를 계산하였다. 두 관측소 경로에 대하여 얻어진 표면파 군속도 자료를 한반도에 대해 공간적으로 이메이징하기 위하여 표면파 토모그래피 역산을 하였다. 역산 결과 한반도의 남동부에 위치한 경상 분지의 낮은 군속도를 확인할 수 있었다.

• Journal of the Optical Society of Korea
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• 제18권3호
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• pp.207-216
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• 2014

In this work, we report detailed numerical analysis of the near-elliptic core index-guiding triangular-lattice and square-lattice photonic crystal fiber (PCFs); where we numerically characterize the birefringence, single mode, cut-off behavior and group velocity dispersion and effective area properties. By varying geometry and examining the modal field profile we find that for the same relative values of $d/{\Lambda}$, triangular-lattice PCFs show higher birefringence whereas the square-lattice PCFs show a wider range of single-mode operation. Square-lattice PCF was found to be endlessly single-mode for higher air-filling fraction ($d/{\Lambda}$). Dispersion comparison between the two structures reveal that we need smaller lengths of triangular-lattice PCF for dispersion compensation whereas PCFs with square-lattice with nearer relative dispersion slope (RDS) can better compensate the broadband dispersion. Square-lattice PCFs show zero dispersion wavelength (ZDW) red-shifted, making it preferable for mid-IR supercontinuum generation (SCG) with highly non-linear chalcogenide material. Square-lattice PCFs show higher dispersion slope that leads to compression of the broadband, thus accumulating more power in the pulse. On the other hand, triangular-lattice PCF with flat dispersion profile can generate broader SCG. Square-lattice PCF with low Group Velocity Dispersion (GVD) at the anomalous dispersion corresponds to higher dispersion length ($L_D$) and higher degree of solitonic interaction. The effective area of square-lattice PCF is always greater than its triangular-lattice counterpart making it better suited for high power applications. We have also performed a comparison of the dispersion properties of between the symmetric-core and asymmetric-core triangular-lattice PCF. While we need smaller length of symmetric-core PCF for dispersion compensation, broadband dispersion compensation can be performed with asymmetric-core PCF. Mid-Infrared (IR) SCG can be better performed with asymmetric core PCF with compressed and high power pulse, while wider range of SCG can be performed with symmetric core PCF. Thus, this study will be extremely useful for designing/realizing fiber towards a custom application around these characteristics.

• 한국항행학회논문지
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• 제16권3호
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• pp.440-448
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• 2012

그룹 속도 분산 (GVD; group velocity dispersion)과 비선형 효과의 상호 작용에 의해 왜곡되는 파장 분할 다중 (WDM; wavelength division multiplexing) 신호를 보상하기 위해 광 위상 공액기 (OPC; optical phase conjugator)와 분사 제어 (DM; dispersion management)가 적용된 광전송 링크에서의 전체 잉여 분산 (NRD; net residual dispersion)의 최적치와 유효 입사 전력 범위를 도출하였다. 실제 광전송 시스템의 융통적 설계를 위해 한 쪽 반 전송 구획에서의 단일 모드 광섬유 (SMF; single mode fiber) 중계 구간(span) 당 잉여 분산 (RDPS; residual dispersion per span)이 랜덤하게 분포하는 광전송 링크를 고려하였다. precompensation으로 전체 분산을 조절하는 링크의 최적 NRD는 10 ps/nm이고 이 경우 유효 입사 전력 범위는 -8~1 dBm인 것을 확인하였다. 또한 postcompensation으로 전체 분산을 조절하는 링크의 최적 NRD는 -10 ps/nm이고, 유효 입사 전력 범위는 -7.5~1 dBm인 것을 확인하였다.

• Composites Research
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• 제18권1호
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• pp.38-44
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• 2005

본 연구에서는 단일방향 탄소섬유강화복합재료 판에서 $S_o$ 대칭모드의 군속도 분선곡선을 전파 방향을 변화시키먼서 계산하였다. 그리고 속노 분산 곡선에서 첫 번째 단절주파수 이하인 0.2 MHz-mm에 해당하는 $S_o$ 대칭모드 군속도를 가지고서 위상속도 곡선을 얻고 slowness 곡선을 도입하여 군속도 곡선으로 교정하였다. 단일방향 섬유강화복합재료 판에서 $S_o$ 대칭모드의 속도를 전파 방향을 변화시키면서 측정하였고 교정된 군속도 곡선과 비교하였다. 측정된 속도는 섬유 방향 근처의 cusp 영역을 제외하고는 교정된 군속도 곡선과 잘 일치하였다. 이것은 단일방향 탄소섬유강화복합재료 판에서 속도가 빠른 방향으로 에너지를 더 잘 전파하기 때문에 전파 방향이 주축방향과 일치하지 않을 때 군속도 방향은 섬유 방향 쪽으로 기울어지는 결과로 나타난 것이다.

• 한국항행학회논문지
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• 제16권5호
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• pp.801-809
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• 2012

분산 제어 (DM; dispersion management)은 파장 분할 다중 (WDM; wavelength division multiplexed) 채널의 고품질 전송을 위해 광섬유의 그룹 속도 분산 (GVD; group velocity dispersion)과 비선형 효과의 상호 작용에 의해 왜곡되는 신호의 보상할 수 있는 대표적 기술이다. DM이 적용된 광전송 링크의 유연한 구성에 필요한 중계 구간 (fiber span)을 구성하는 단일 모드 광섬유 (SMF; single mode fiber)의 길이와 중계 구간 당 잉여 분산 (RDPS; residual dispersion per span)을 랜덤하게 분포시키는 구조와 인위적으로 분포시키는 구조에서의 최적 전체 잉여 분산 (NRD; net residual dispersion)의 최적치와 유효 입사 전력 범위를 도출하였다. 고려한 SMF의 길이와 RDPS 분포 패턴 모두 precompensation으로 NRD를 조절하는 경우에서는 +10 ps/nm, postcompensation으로 NRD를 조절하는 경우에서는 -10 ps/nm이 최적 NRD라는 것을 알 수 있었다. 그리고 NRD가 상기 값으로 설정된 경우 중계 구간이 증가할수록 SMF 길이를 감소시키고, RDPS를 증가시키는 인위적인 분포의 전송 링크에서의 시스템 성능이 가장 양호하여 균일한 분포의 링크에 비해 유효 입사 전력 범위가 2 dB 정도 신장되는 것을 확인하였다.

• 대한기계학회논문집B
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• 제27권8호
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• pp.1165-1173
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• 2003

The effects of (1) phonon dispersion on thermal conductivity model and (2) differentiation of group velocity and phase velocity are examined for germanium. The results show drastic change of thermal conductivity regardless of the same relaxation time model. Also the contribution of transverse acoustic (TA) phonon and longitudinal acoustic (LA) phonon on the thermal conductivity at high temperatures is reassessed by considering more rigorous dispersion model. Holland model, which is commonly used for modeling thermal conductivity, underestimates the scattering rate for TA phonon at high frequency. This leads the conclusion that TA is dominant heat transfer mode at high temperatures. But according to the rigorous consideration of phonon dispersion, the reduction of thermal conductivity is much larger than the estimation of Holland model, thus the TA at high frequency is expected to be no more dominant heat transfer mode. Another heat transfer mechanism may exist at high temperatures. Two possible explanations we the roles of (1) Umklapp scattering of LA phonon at high frequency and (2) optical phonon.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2003년도 춘계학술대회
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• pp.1627-1632
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• 2003

The effect of (1) phonon dispersion in thermal conductivity model and (2) the differentiation of group velocity and phase velocity for Ge is examined. The results show drastic change of thermal conductivity regardless of using same relaxation time model. Also the contribution of transverse acoustic (TA) phonon and longitudinal acoustic (LA) phonon is changed by considering more rigorous dispersion model. Holland model underestimates the scattering rate for high frequency TA, so misleading conclusion, i.e. TA is dominant heat transfer mode at high temperature. But the actual reduction of thermal conductivity is much larger than the estimation by Holland model and high frequency TA is no more dominant heat transfer mode. Another heat transfer mechanism may exist for high temperature. Two possible explanations are (1) high frequency LA by Umklapp scattering and (2) optical phonon.

• Journal of the Optical Society of Korea
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• 제9권4호
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• pp.140-144
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• 2005

In this paper we have investigated the guiding properties of photonic crystal fibers (PCFs) with a square-lattice of air-holes in the cladding. We have shown numerical results of PCFs with various air hole sizes and hole-to-hole spacings over a wide wavelength range. The group velocity dispersion, effective area and effective refractive index of PCF have been calculated numerically. The waveguide dispersion has greatly affected the group velocity dispersion when hole-to-hole spacing is about $1{\mu}m$. The effective area is quite flat over the wide spectral range whether the hole-to-hole spacing is large or ratio of diameter to pitch is large. From the field distribution, we found that the field is tightly confined within the core region of PCF when the pitch is $3{\mu}m$ and the air-filling fraction is 0.9.

• 한국공작기계학회논문집
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• 제10권1호
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• pp.133-140
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• 2001

Recently, to assure the integrity of a structural components such as piping pressure vessels and thinning structure, Lamb wave inspection technique has been used in material evaluation. It is very important to select the optimal Lamb wave mode and to analyze the signal accurately because of its unique dispersion properties grnerating several modes within the speci-men. It this study, the feasibility of material evaluation applications using wavelet analysis of Lamb wave has been veir-fied experimentally. These results show as follows; 1)dispersion characteristic of each mode in dispersion curve is demon-strated that A0 mode propagating material surface is useful mode having the lest energy loss and not sensitive to surface condition. 2) it can be detected even the micro defect ($1\times2mm$) fabricated in ultrasonic probe flaw distance (290mm) to axis direction. 3) the wavelet transform which is called "time-frequency analysis" shows the Lamb wave propagation due to the change of materials characterization can be evaluated at each frequency and experimental group velocity of Lamb wave agrees quite well with that of simulated dispersion curve.ion curve.