• The Journal of the Acoustical Society of Korea
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• v.13 no.1E
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• pp.45-54
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• 1994

Characteristics of a piezoelectric material are evaluated to pick up the optimal crystal cut and propagation direction for a SAW device. For the piezoelectric single crystal $LiTaO_4$, such items are investigated as the Rayleigh wave velocity, the electromechanical coupling factor, the surface permittivity, the frequency-temperature coefficient, the air loading attenuation, the pure mode propagation, the beam steering and the misalignment sensitivity. Theoretical calculations reveal that Y-cut and Z-propagation is the optimal SAW propagation path. The results are confirmed through experiments. The method empolyed in this paper is applicable to other crystals, too, either single or poly crystals.

• Computational Structural Engineering
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• v.11 no.4
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• pp.155-168
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• 1998

보강된 판 및 쉘구조의 안정성 및 후좌굴을 포함하는 기하학적 비선형 해석을 수행하기 위하여, total Lagrangian formulation에 근거한 연속체의 증분평형방정식으로부터 변형된 쉘요소인 유한요소이론을 제시하였다. 쉘구조의 곡률이 불연속적으로 변하거나 쉘부재들이 유한한 각도로 만나는 보강된 판 및 쉘구조의 비선형 해석이 가능하도록 주부재와 보강재 간의 연결점에 대한 일반적인 변환관계를 제시하였으며 좌굴해석 및 기하학적 비선형해석의 경우에 해의 정확성 및 수렴성을 개선시키기 위하여 접선강도행렬 산정시 회전각의 2차항을 포함시켰다. 또한, shear locking 현상을 극복하기 위하여 감차적분을 적용하였고 쉘구조의 좌굴해석에서는 power method를 적용하여 해석의 효율을 높였으며, 후좌굴해석에서는 변위 및 하중증분법을 적절히 결합시켜 보강된 쉘구조의 후좌굴 거동추적이 용이하였다. 또한, 입력자료를 손쉽게 준비하고 좌굴모드 및 후좌굴거동을 효율적으로 분석하기 위하여 전, 후 처리 프로그램을 개발하였고 다양한 해석예제를 통하여 다른 문헌의 해석결과를 비교함으로써 본 연구에서 개발된 유한요소 해석프로그램의 타당성 및 정확성을 입증하였다.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.39 no.7
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• pp.693-700
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• 2015

To estimate the fatigue crack propagation behavior of compact tension shear (CTS) specimen under mixed-mode loads, crack path prediction theories and Tanaka's equation were applied. The stress intensity factor at a newly created crack tip was calculated using a finite element method via ANSYS, and the crack path and crack increment were then obtained from the crack path prediction theories, Tanaka's equation, and the Paris' equation, which were preprogrammed in Microsoft Excel. A new method called the finite element crack tip updating method (FECTUM) was developed. In this method, the finite element method and Microsoft Excel are used to calculate the stress intensity factors and the crack path, respectively, at the crack tip per each crack increment. The developed FECTUM was applied to simulate the fatigue crack propagation of a single-edge notched bending (SENB) specimen under eccentric three-point bending loads. The results showed that the number of cycles to failure of the specimen obtained experimentally and numerically were in good agreement within an error range of less than 3%.

• Journal of the Korean Society for Nondestructive Testing
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• v.20 no.2
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• pp.138-149
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• 2000

Various modeling techniques for ultrasonic wave propagation and scattering problems in finite solid media are presented. Elastodynamic boundary value problems in inhomogeneous multi-layered plate-like structures are set up for modal analysis of guided wave propagation and numerically solved to obtain dispersion curves which show propagation characteristics of guided waves. As a powerful modeling tool to overcome such numerical difficulties in wave scattering problems as the geometrical complexity and mode conversion, the Boundary Element Method(BEM) is introduced and is combined with the normal mode expansion technique to develop the hybrid BEM, an efficient technique for modeling multi mode conversion of guided wave scattering problems. Time dependent wave forms are obtained through the inverse Fourier transformation of the numerical solutions in the frequency domain. 3D BEM program development is underway to model more practical ultrasonic wave signals. Some encouraging numerical results have recently been obtained in comparison with the analytical solutions for wave propagation in a bar subjected to time harmonic longitudinal excitation. It is expected that the presented modeling techniques for elastic wave propagation and scattering can be applied to establish quantitative nondestructive evaluation techniques in various ways.

• Journal of the Korea institute for structural maintenance and inspection
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• v.8 no.2
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• pp.147-158
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• 2004

For stiffened plates composed of composite materials, many researchers have used a finite element method which connected isoparametric plate elements and beam elements. However, the finite element method is difficult to reflect local behavior of stiffener because beam elements are transferred stiffness for nodal point of plate elements, especially the application is limited in case of laminated composite structures. In this paper, for analysis of laminated composite stiffened plates, 3D shell elements for stiffener and plate are employed. Reissner-Mindlin's first order shear deformation theory is considered in this study. But when thickness will be thin, isoparamatric plate bending element based on the theory of Reissner-Mindlin is generated by transverse shear locking. To eliminate the shear locking and virtual zero energy mode, the substitute shear strain field is used. A deflection distribution is investigated for simple supported rectangular and skew stiffened laminated composite plates with arbitrary orientation stiffener as not only variation of slenderness and aspect ratio of the plate but also variation of skew angle of skew stiffened plates.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.33 no.9
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• pp.16-26
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• 2005

The aerodynamic design of a proprotor for the Smart UAV adopting tiltrotor aircraft concept is conducted in this study. Since proprotor of tiltrotor aircraft is operated at both rotary and fixed wing mode with single configuration rotor, the proprotor has to be designed to meet performance requirements for both flight modes. The aerodynamic design of proprotor is accomplished by combining three sources of data - the proprotor performance data, the aerodynamic data of vehicle, and the performance data of engine. The performance analysis code for proprotor is based on the combined momentum and blade element theory and validated by comparison with the TRAM data. In order to design configuration for a proprotor satisfying requirements for both rotary and fixed wing mode, various kind of performance maps are constructed for many performance and configuration parameters. From the analysis the twist angle of 38 degrees and the solidity of 0.118 are decided to be the optimal geometric parameters for both operating conditions.

• The Journal of the Korea Contents Association
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• v.18 no.10
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• pp.11-19
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• 2018

This paper is about multimodal storytelling experience applying Virtual Reality technology in museum. Specifically, this research argues virtual reality in both intuitive understanding of history also multimodal experience in the space. This research investigates cases regarding use of virtual reality in museum sector. As a research method, this paper conducts a literature review regarding multimodal experience and examples applying virtual reality related technologies in museum. Based on the literature review to investigate the concept necessary with its related cases. Based on the investigation, this paper suggests constructing elements for multimodal storytelling based on VR. Ultimately, this paper suggests the elements of building VR storytelling where dynamic audio-visual and interaction mode combines with historical resources for diverse audiences.

• Journal of the Institute of Electronics Engineers of Korea TC
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• v.39 no.5
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• pp.243-250
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• 2002

In the synthesis of the current source distribution function of an array antenna with the arbitrary radiation pattern, the Woodward-Lawson sampling method has been mainly used for the synthesis of an even function lobe pattern. In this paper, the method is extended to the synthesis of the odd function pattern and then the optimum synthesis method for the nonlinear source distribution function is proposed. The proposed method is applied to the design of nonuniform transmission lines with arbitrary reflection responses. The both dispersive impedance profiles of single and coupled nonuniform lines with arbitrary reflection responses are directly synthesized by the sampled values of a reflected spectral pattern which is optimally shaped by a perturbation of its complex null positions, hence removing the conventional step-by-step segmentation process and global optimization routines. The control problem in the case that all of port impedances are identical is also solved. The generality of the proposed method is verified by a filter design with the controlled arbitrary passband

• Proceedings of the Korean Vacuum Society Conference
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• 2014.02a
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• pp.134.1-134.1
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• 2014

콜로이달 리소그래피는 나노미터 크기의 나노구를 자가조립에 의해 정렬시킴으로써, 파장이하 크기의 주기 구조를 저비용으로 쉽게 구현할 수 있는 패터닝 기법이다. 콜로이달 리소그래피나 소프트 리소그래피와 같이 대면적 패터닝이 가능한 공정을 태양전지를 위한 반사방지 및 광 포획 증대 구조에 적용함으로써, 기존 성능을 크게 향상시켰다. 본 연구에서는, 유한차분 시간영역 수치해석법을 이용하여 반사 방지 및 광 포획 증대 구조에 대한 이론적 검증 및 설계를 진행하였고, 콜로이달 리소그래피 및 반도체 공정을 통해 샘플을 제작하였으며, 제작된 샘플의 성능을 적분구를 겸비한 자외선 가시광 근적외선 영역 분광기를 통해 평가하였다. 반사방지 나노섬을 겸비한 나노 원뿔대 언덕형 굴절률 소자를 구현함으로써, 300나노미터 이하의 구조체를 사용하지 않고도 근자외선 영역을 포함하는 태양광 에너지의 손실을 최소화할 수 있는 광대역 방사방지 구조체를 제시하였다. 나노 원뿔대가 격자상수 이상의 파장에 대한 언덕형 굴절률을 제공하고, 4분의 1파장 나노섬 반사방지막이 격자 상수 이하의 근자외선 태양광을 추가적으로 흡수하여, 근자외선 영역에서의 평균 반사율을 3.8% 수준으로 달성 할 수 있었다. 또한, 낮은 양호계수를 갖는 속삭임 회랑 공진기 어레이를 이용하여, 박막 태양전지에 적합한 유전체 기반 광포획 증대 나노구조를 제시하였다. 나노반구, 나노고깔, 나노구, 함몰형 나노구 어레이 형태를 가지며, 500nm의 주기를 갖는 유전체 표면 텍스쳐드 구조를 초박형 비정질 실리콘 필름(100nm) 위에 제작하여 광대역 광 포획 증대 효과를 실험적으로 평가하였다. 구조들 중 함몰형 나노구 어레이가 결합된 비정질 실리콘 박막이 가장 높은 성능을 보였으며, 구조가 없는 경우 대비 약 67.6%의 가중 흡수율 증가를 나타내었다. 특히, 함몰형 나노구 어레이 구조 중 폴리메틸메타아크릴레이트로 제작된 평판형 함몰층은 나노구 비정질 박막 실리콘 사이의 접착력 및 기계적 강성을 향상시켰을 뿐 아니라, 함몰층 내부로 회절되고 산란된 빛들이 도파모드 효과에 의해 부가적인 광 포획 증대를 가져옴으로써, 가장 높은 광 포획 효과를 얻을 수 있었다. 유전체 기반 나노 구조들은 간단하고 저비용이며, 대면적으로 쉽게 제작할 수 있는 자가 조립 기반 콜로이달 리소그래피 및 소프트 리소그래피 기술을 이용하여 제작되었다.

• Journal of Satellite, Information and Communications
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• v.6 no.1
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• pp.86-96
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• 2011

In this paper we propose a Pre-rake system applied with a frequency domain equalizer in TDD/CDMA multi-code transmission. The Pre-rake system has been well known technique in TDD/CDMA to make a receiver simple. However, it still has residual losses of path diversity and signal to noise ratio (SNR). However, gathering all the residual paths demands an additional hardware such as a rake combiner at the receiver. For the reason Pre/Post-rake system has already been proposed at up/downlink correlated channel conditionunder the assumption of noisier channel. There is a trade-off between the first purpose of Pre-rake that makes hardware simple at the receiver and the performance improvement. From the point the frequency domain equalizer (FDE) can be considered in Pre/Post-rake to supply the receiver with the flexible equalizing methods with rather reduced complexity compared with time domain rake combiner or equalizers. Pre-rake itself increases the number of multipath, which results from the convolution of Pre-rake filter and wireless channel, and FDE must be well matched to Pre/Post-rake, while it considers the relationship of hardware complexity and the performance. In this paper, the Pre-rake/Post-FDE system is introduced at TDD/CDMA multi-code transmission. In addition, the cyclic prefix reduction method in the proposed system is introduced, and the theoretical analysis to the proposed system is given by assuming Gaussian approximation, and finally the numerical simulation results are provided.