• 한국음향학회지
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• 제38권5호
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• pp.543-548
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• 2019

수중 표적의 기어박스 및 보조 장치 등으로부터 방사되는 토널 신호의 주파수 성분은 처리하고자 하는 주파수 대역에 비해 상대적으로 적어 희소신호로 모델링될 수 있다. 근래에 토널 신호의 주파수 희소성을 이용하여 빠른 시간 내에 적은 수의 관측치로 토널 주파수를 복원하는 압축센싱 기반의 연구가 활발히 진행되고 있다. 기존의 방법들은 이산(discrete) 주파수 영역에서 주파수를 검출하기 때문에 이산화로 인한 basis mismatch error가 불가피하다. 본 논문에서는 atomic norm minimization을 이용하여 적은 수의 관측치로 연속(continuous) 주파수 영역에서 토널 주파수를 검출하는 기법을 제안한다. 모의실험을 통해 기존의 기법들에 비해 제안하는 기법의 성능이 정확성과 평균제곱오차 측면에서 우수함을 확인하였다.

• 한국항해항만학회지
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• 제27권5호
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• pp.539-546
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• 2003

유한요소기법을 이용하여 유동해석의 수치모델을 행하였다 공간을 이산화 할 때에는 Galerkin법을 적용하였으며, 시간의 함수를 이산화 할 때에는 많은 수의 요소와 비정상상태의 문제를 다루는데 있어 장점을 가진 2단계 양해법을 이용하였다. 이동경계조건을 고려한 2차원유통모델을 개발하였고, 직사자형 수조에서 개발된 유동모델을 적용하여 검증하였고, 유용성을 확인하였다. 제주항에 개발된 이동경계기법을 적용하여 계산한 결과, 본 이동경계기법의 좋은 적용성을 보여주었다. 본 연구로부터 이동경계처리 방법이 실해역에서의 유동해석에 있어 유용하고 효율적인 방법이라고 결론지을 수 있다.

• Wind and Structures
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• 제17권6호
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• pp.647-670
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• 2013

The paper presents a numerical approach to study of fluid flow characteristics and to predict performance of wind turbines. The numerical model is based on Finite-volume method (FVM) discretization of unsteady Reynolds-averaged Navier-Stokes (URANS) equations. The movement of turbine blades is modeled using moving mesh technique. The turbulence is modeled using commonly used turbulence models: Renormalization Group (RNG) k-${\varepsilon}$ turbulence model and the standard k-${\varepsilon}$ and k-${\omega}$ turbulence models. The model is validated with the experimental data over a large range of tip-speed to wind ratio (TSR) and blade pitch angles. In order to demonstrate the use of numerical method as a tool for designing wind turbines, two dimensional (2-D) and three-dimensional (3-D) simulations are carried out to study the flow through a small scale Darrieus type H-rotor Vertical Axis Wind Turbine (VAWT). The flows predictions are used to determine the performance of the turbine. The turbine consists of 3-symmetrical NACA0022 blades. A number of simulations are performed for a range of approaching angles and wind speeds. This numerical study highlights the concerns with the self-starting capabilities of the present VAWT turbine. However results also indicate that self-starting capabilities of the turbine can be increased when the mounted angle of attack of the blades is increased. The 2-D simulations using the presented model can successfully be used at preliminary stage of turbine design to compare performance of the turbine for different design and operating parameters, whereas 3-D studies are preferred for the final design.

• Steel and Composite Structures
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• 제35권1호
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• pp.77-92
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• 2020

The present paper outlined a procedure for geometrically nonlinear dynamic analysis of functionally graded graphene platelets-reinforced (GPLR-FG) nanocomposite cylinder subjected to mechanical shock loading. The governing equation of motion for large deformation problems is derived using meshless local Petrov-Galerkin (MLPG) method based on total lagrangian approach. In the MLPG method, the radial point interpolation technique is employed to construct the shape functions. A micromechanical model based on the Halpin-Tsai model and rule of mixture is used for formulation the nonlinear functionally graded distribution of GPLs in polymer matrix of composites. Energy dissipation in analyses of the structure responding to dynamic loads is considered using the Rayleigh damping. The Newmark-Newton/Raphson method which is an incremental-iterative approach is implemented to solve the nonlinear dynamic equations. The results of the proposed method for homogenous material are compared with the finite element ones. A very good agreement is achieved between the MLPG and FEM with very fine meshing. In addition, the results have demonstrated that the MLPG method is more effective method compared with the FEM for very large deformation problems due to avoiding mesh distortion issues. Finally, the effect of GPLs distribution on strength, stiffness and dynamic characteristics of the cylinder are discussed in details. The obtained results show that the distribution of GPLs changed the mechanical properties, so a classification of different types and volume fraction exponent is established. Indeed by comparing the obtained results, the best compromise of nanocomposite cylinder is determined in terms of mechanical and dynamic properties for different load patterns. All these applications have shown that the present MLPG method is very effective for geometrically nonlinear analyses of GPLR-FG nanocomposite cylinder because of vanishing mesh distortion issue in large deformation problems. In addition, since in proposed method the distributed nodes are used for discretization the problem domain (rather than the meshing), modeling the functionally graded media yields to more accurate results.

• 대한기계학회논문집A
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• 제26권4호
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• pp.775-786
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• 2002

A Mixed Volume and Boundary Integral Equation Method is applied for the effective analysis of elastic wave scattering problems and plane elastostatic problems in unbounded solids containing general anisotropic inclusions and voids or isotropic inclusions. It should be noted that this newly developed numerical method does not require the Green's function for anisotropic inclusions to solve this class of problems since only Green's function for the unbounded isotropic matrix is involved in their formulation for the analysis. This new method can also be applied to general two-dimensional elastodynamic and elastostatic problems with arbitrary shapes and number of anisotropic inclusions and voids or isotropic inclusions. In the formulation of this method, the continuity condition at each interface is automatically satisfied, and in contrast to finite element methods, where the full domain needs to be discretized, this method requires discretization of the inclusions only. Finally, this method takes full advantage of the pre- and post-processing capabilities developed in FEM and BIEM. Through the analysis of plane elastostatic problems in unbounded isotropic matrix with orthotropic inclusions and voids or isotropic inclusions, and the analysis of plane wave scattering problems in unbounded isotropic matrix with isotropic inclusions and voids, it will be established that this new method is very accurate and effective for solving plane wave scattering problems and plane elastic problems in unbounded solids containing general anisotropic inclusions and voids/cracks or isotropic inclusions.

• Advances in nano research
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• 제16권3호
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• pp.231-249
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• 2024

When the amplitude of the vibrations is equivalent to that clearance, the vibrations for small amplitudes will really be significantly nonlinear. Nonlinearities will not be significant for amplitudes that are rather modest. Finally, nonlinearities will become crucial once again for big amplitudes. Therefore, the concrete panel system may experience a big amplitude in this work as a result of the high temperature. Based on the 3D modeling of the shell theory, the current work shows the influences of the von Kármán strain-displacement kinematic nonlinearity on the constitutive laws of the structure. The system's governing Equations in the nonlinear form are solved using Kronecker and Hadamard products, the discretization of Equations on the space domain, and Duffing-type Equations. Thermo-elasticity Equations. are used to represent the system's temperature. The harmonic solution technique for the displacement domain and the multiple-scale approach for the time domain are both covered in the section on solution procedures for solving nonlinear Equations. An effective data-driven solution is often utilized to predict how different systems would behave. The number of hidden layers and the learning rate are two hyperparameters for the network that are often chosen manually when required. Additionally, the data-driven method is offered for addressing the nonlinear vibration issue in order to reduce the computing cost of the current study. The conclusions of the present study may be validated by contrasting them with those of data-driven solutions and other published articles. The findings show that certain physical and geometrical characteristics have a significant effect on the existing concrete panel structure's susceptibility to temperature change and GPL weight fraction. For building construction industries, several useful recommendations for improving the thermo-mechanics' behavior of structural concrete panels are presented.

• 한국항해항만학회지
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• 제32권10호
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• pp.777-783
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• 2008

입자영상속도계(PIV)의 해석에 웨이블릿 변환을 적용하여 정성적인 유동정보뿐만 아니라 공간분해능을 갖는 정량적인 속도장 정보를 제공하고 있다 이 기법은 기포유동(bubbly flow)과 같은 다상(multi-phase)의 유동구조를 해석하는 데도 유용하게 살일 수 있다. 본 연구에서는 기체와 액체의 이상유동(two-pase flow)에 PIV기법을 적용하고 이산 웨이블릿 변환을 사용하여 유장해석을 수행함으로써, 기포를 포함한 속도장 특성과 유동특성을 조사한다.

• Ocean Systems Engineering
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• 제10권2호
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• pp.201-226
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• 2020

A systematic numerical comparative study of the performance of semicircular and rectangular submerged breakwaters interacting with solitary waves is the basis of this paper. To accomplish this task, Nwogu's extended Boussinesq model equations are employed to simulate the interaction of the wave with breakwaters. The finite difference technique has been used to discretize the spatial terms while a fourth-order predictor-corrector method is employed for time discretization in our numerical model. The proposed computational scheme uses a staggered-grid system where the first-order spatial derivatives have been discretized with fourth-order accuracy. For validation purposes, five test cases are considered and numerical results have been successfully compared with the existing analytical and experimental results. The performances of the rectangular and semicircular breakwaters have been examined in terms of the wave reflection, transmission, and dissipation coefficients (RTD coefficients) denoted by K_R, K_T, K_D. The latter coefficient K_D emerges due to the non-energy conserving K_R and K_T. Our computational results and graphical illustrations show that the rectangular breakwater has higher reflection coefficients than semicircular breakwater for a fixed crest height, but as the wave height increases, the two reflection coefficients approach each other. un the other hand, the rectangular breakwater has larger dissipation coefficients compared to that of the semicircular breakwater and the difference between them increases as the height of the crest increases. However, the transmission coefficient for the semicircular breakwater is greater than that of the rectangular breakwater and the difference in their transmission coefficients increases with the crest height. Quantitatively, for rectangular breakwaters the reflection coefficients K_R are 5-15% higher while the diffusion coefficients K_D are 3-23% higher than that for the semicircular breakwaters, respectively. The transmission coefficients K_T for rectangular breakwater shows the better performance up to 2.47% than that for the semicircular breakwaters. Based on our computational results, one may conclude that the rectangular breakwater has a better overall performance than the semicircular breakwater. Although the model equations are non-dissipative, the non-energy conserving transmission and reflection coefficients due to wave-breakwater interactions lead to dissipation type contribution.

• 한국지반환경공학회 논문집
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• 제15권3호
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• pp.41-48
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• 2014

준설토에 대한 연구는 주로 준설토의 1차원 침강 및 자중압밀 특성을 파악하는 실험적 연구가 진행되었다. 하지만 양질의 준설지반 확보를 위한 효과적인 투기장의 설계와 배출수에 의한 환경오염을 최소화하기 위해서는 준설토의 투기에 의한 유동특성의 체계적인 연구가 필요하다. 본 연구에서는 준설토 투기장의 펌핑에 의한 토사의 유동 형상을 모사하기 위하여 준설토사를 단일상으로 가정하고 연속 방정식을 유도하여 좌표축에 따른 힘 평형 방정식을 유도하였다. 준설토장의 3차원 거동 해석을 위한 컴퓨터 연산 부하와 모델링 소요시간을 최적화하기 위하여, 토체의 깊이 방향으로 적분을 수행하는 깊이 적분 방법을 지배 방정식에 적용하여, 3차원적 지형조건을 고려할 수 있도록 하였다. 지배 방정식의 보간함수를 이용한 공간분할에서 Petrov-Galerkin 수식화 기법을 적용하였다. 일반화된 사다리꼴 법칙으로 시간적분을 수행하고 Newton의 반복과정을 이용할 수 있도록 근사화시켰다. 가중행렬은 DG과 CDG 기법을 적용하였으며, 준설토 유동해석에서 가중행렬에 따른 수치적인 안정성을 평가하기 위하여 사각형 기둥 슬럼프 시뮬레이션을 수행하였다. 수치기법에 대한 비교 분석 결과는 DG 기법을 적용한 SU/PG 수식화가 유사진동을 최소화시키는 가장 안정적인 수치해석결과를 보여주는 것으로 나타났다.

• 전기전자학회논문지
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• 제19권3호
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• pp.342-348
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• 2015

광대역 통신 모뎀이나 초고해상도 비디오 코덱 등과 같이 높은 데이터율을 갖는 시스템을 하드웨어로 구현할 때에는 디지털 필터의 고속 구현이 필수적이다. 디지털 필터의 임계경로는 대부분 MAC (multiplication and accumulation) 연산 회로이므로 필터 계수의 0이 아닌 비트의 갯수가 희소하다면 하드웨어 비용이 적은 덧셈기로도 디지털 필터를 고속으로 구현할 수 있다. 압축센싱은 신호의 희소 표현이나 희소 신호의 복원에 우수한 성능을 보임이 최근 연구에서 보고되고 있다. 본 논문에서는 압축센싱에 기반한 디지털 FIR 필터의 CSD (canonic signed digit) 계수를 찾는 방법을 제안한다. 주어진 주파수 응답과의 오차를 최소하면서 탐욕적 방법으로 희소한 0이 아닌 부호자리수를 찾고 잘못 선택되었던 부호자리수는 제거하는 과정을 반복한다. 설계 예를 통해 제안된 방법으로 희소한 0이 아닌 CSD 계수의 FIR 필터를 설계할 수 있음을 보인다.