• Structural Engineering and Mechanics
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• 제61권5호
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• pp.563-576
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• 2017

This paper is a continuation of the investigations started in the paper by Akbarov, S.D., Guliyev, H.H and Yahnioglu, N. (2016) "Natural vibration of the three-layered solid sphere with middle layer made of FGM: three-dimensional approach", Structural Engineering and Mechanics, 57(2), 239-263, to the case where the three-layered sphere is a hollow one. Three-dimensional exact field equations of elastodynamics are employed for investigation and the discrete-analytical method is employed for solution of the corresponding eigenvalue problem. The FGM is modelled as inhomogeneous for which the modulus of elasticity, Poison's ratio and density vary continuously through the inward radial direction according to power law distribution. Numerical results on the natural frequencies are presented and discussed. These results are also compared with the corresponding ones obtained in the previous paper by the authors. In particular, it is established that for certain harmonics and for roots of certain order, the values of the natural frequency obtained for the hollow sphere can be greater (or less) than those obtained for the solid sphere.

• 한국소음진동공학회논문집
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• 제12권3호
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• pp.213-220
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• 2002

Modifying effects of the rectangular forward step for suppressing the unsteady pressure fluctuation during interaction between the upstream vortical flow and the edge are studied numerically. The vertical flow is modeled by a point vortex, and the unsteady pressure coefficient is obtained from the velocity and the potential fields. To investigate the effects of the edge shape the rectangular forward step is chamfered wish various angles. Calculation shows that the pressure peaks become decreased by increasing the vortex height as well as the chamfering angle. The pressure amplitudes are very sensitive to the change of the initial vertex height and its strength. From this study we can find out that the chamfered edge has two effects; the one is that it suppresses the pressure amplitude generated from the edge, and the other is that it decreases the time variation of unsteady pressure fluctuation. These modifying concepts can be applied to attenuate the self-sustained oscillation mechanism at the open cavity flow.

• 한국전자파학회논문지
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• 제13권1호
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• pp.60-67
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• 2002

본 논문에서는 발진부 귀환경로에 스위칭소자를 이용하여 최소의 소자을 사용한 소형화된 새로운 방식의 이동통신 단말기용 듀얼대역 VCO(Voltage Control Oscillator)를 제안하였다. VCO에 공진기의 Q(Quality factor)값을 정확히 모델링하기 위하여 공진기를 등가 모델하고, 이론 수치 해석하여 최적의 공진기를 설계하였다. 본 논문에서 제안된 방식을 적용하여 GSM(Global System for Mobile), DCS(Digital Cordless System) 대역의 동작 주파수를 갖는 듀얼대역 전압제어 발진기를 설계하였으며, 논문의 객관성을 입증하기 위해 제작 및 실험을 통하여 새로 제안된 방식의 듀얼대역 전압제어 발진기의 실용화가 가능함을 확인하였고, 앞으로 차세대 이동통신 기기의 고주파 부품 등에 응용될 수 있을 것이다.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2008년 영문 학술대회
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• pp.364-370
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• 2008

Spinning detonations propagating in a circular tube were numerically investigated with a one-step irreversible reaction model governed by Arrhenius kinetics. The time evolution of the simulation results was utilized to reveal the propagation mechanism of single-headed spinning detonation. The track angle of soot record on the tube wall was numerically reproduced with various levels of activation energy, and the simulated unique angle was the same as that of the previous reports. The maximum pressure histories of the shock front on the tube wall showed stable and unstable pitch modes for the lower and higher activation energies, respectively. The shock front shapes and the pressure profiles on the tube wall clarified the mechanisms of two modes. The maximum pressure history in the stable pitch remained nearly constant, and the single Mach leg existing on the shock front rotated at a constant speed. The high and low frequency pressure oscillations appeared in the unstable pitch due to the generation and decay of complex Mach interaction on the shock front shape. The high frequency oscillation was self-induced because the intensity of the transverse wave was changed during propagation in one cycle. The high frequency behavior was not always the same for each cycle, and therefore the low frequency oscillation was also induced in the pressure history.

• Ocean Systems Engineering
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• 제11권4호
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• pp.353-371
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• 2021

Sloshing behavior of liquid within containers represents one of the most fundamental fluid-structure interactions. Liquid in partially filled tanks tends to slosh when subjected to external disturbances. Sloshing is a vicious resonant fluid motion in a moving tank. To understand the effect of baffle positioned at L/3 and 2L/3 location, a shake table experiments was conducted for different fill volumes of aspect ratio 0.163, 0.325 and 0.488. For a fixed amplitude of 7.5 mm, the excitation frequencies are varied between 0.457 Hz to 1.976 Hz. Wave probes have been located at both tank ends to capture the surface elevation. The experimental parameters such as sloshing oscillation and energy dissipation are discussed here. Comparison is done for with baffles and without baffles conditions. For both conditions, the results showed that aspect ratio of 0.163 gives better surface elevation and energy dissipation than obtained for aspect ratio 0.325 and 0.488. Good agreement is observed when numerical analysis is compared with the experiments results.

• Computers and Concrete
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• 제32권6호
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• pp.553-565
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• 2023

This work is the first to apply nonlocal theory and a variety of deformation plate theories to study the issue of forced vibration and buckling in organic nanoplates, where the effect of the drag parameter inside the structure has been taken into consideration. Whereas previous research on nanostructures has treated the nonlocal parameter as a fixed value, this study accounts for its effect, and finds that its value fluctuates with the thickness of each layer. This is also a new point that no works have mentioned for organic plates. On the foundation of the notion of potential movement, the equilibrium equation is derived, the buckling issue is handled using Navier's solution, and the forced oscillation problem is solved using the finite element approach. Additionally, a set of numerical examples exhibiting the forced vibration and buckling response of organic nanoplates are shown. These findings indicate that the nonlocal parameter and the drag parameter of the structure have a substantial effect on the mechanical responses of organic nanoplates.

• Journal of the Korean Society for Industrial and Applied Mathematics
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• 제19권2호
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• pp.197-215
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• 2015

Modeling water flow in variably saturated, porous media is important in many branches of science and engineering. Highly nonlinear relationships between water content and hydraulic conductivity and soil-water pressure result in very steep wetting fronts causing numerical problems. These include poor efficiency when modeling water infiltration into very dry porous media, and numerical oscillation near a steep wetting front. A one-dimensional finite element formulation is developed for the numerical simulation of variably saturated flow systems. First order backward Euler implicit and second order Crank-Nicolson time discretization schemes are adopted as a solution strategy in this formulation based on Picard and Newton iterative techniques. Five examples are used to investigate the numerical performance of two approaches and the different factors are highlighted that can affect their convergence and efficiency. The first test case deals with sharp moisture front that infiltrates into the soil column. It shows the capability of providing a mass-conservative behavior. Saturated conditions are not developed in the second test case. Involving of dry initial condition and steep wetting front are the main numerical complexity of the third test example. Fourth test case is a rapid infiltration of water from the surface, followed by a period of redistribution of the water due to the dynamic boundary condition. The last one-dimensional test case involves flow into a layered soil with variable initial conditions. The numerical results indicate that the Crank-Nicolson scheme is inefficient compared to fully implicit backward Euler scheme for the layered soil problem but offers same accuracy for the other homogeneous soil cases.

• 한국수자원학회논문집
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• 제34권1호
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• pp.57-65
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• 2001

개수로에서 발생하는 천이류의 해석을 위해 개발한 수치모형을 여러 형태의 수로에 적용하였다. 그 동안 개발된 천이류 해석 모형은 주로 균일하도나 가상하도를 대상으로 개발되어 다양한 형태의 하도에는 적용하기 곤란한 점이 있었다. 본 연구에서는 2차 정확도 음해적 ENO 기법을 하상 및 하폭이 변화하는 다양한 형태의 비균일 하도에서 발생하는 천이류에 적용하여 모형의 정확도와 안전성을 검증하였다. 또한 정상류 상태의 천이류 뿐만 아니라 비정상류 상태에서 발생하는 천이류에도 적용하여 모형을 검증하였다. 모형의 적용결과 수치진동의 발생없이 전반적으로 수위와 유속 등 흐름을 정확하게 계산하였으며 특히 도수의 발생위치, 불연속 구간의 계산 등에서도 좋은 결과를 나타내어 고정확도 기법으로서의 정확도와 음해법으로서의 안정성을 검증할 수 있었다.

• 유변학
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• 제10권4호
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• pp.202-216
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• 1998

기존의 'Orthotropic' 근사모델의 개선된 형태인 ORW를 새로운 유동 자료를 이용하여 수치적으로 구하였다. 기존의 'Orthotropic' 근사모델인 ORF나 ORL은 특히 전단유동 하에서 상호작용상수 $C_1$<0.001인 경우 비물리적 진동특성을 나타낸다. 물론 center-gated disk와 같은 비균일 유동하에서도 비물리적 진동특성을 나타내고 'Distribution Function Calculation'과 비교하여 배향 상태를 낮게 예측한다. 이런 현상들은 바로 least-square 최적화 시 사용된 유동 자료에 기인한 것을 알 수 있었다. 작은 상호작용계수의 균일 유동 자료를 이용하여 최적화를 한 ORW의 경우 비물리적 진동특성도 나타나지 않았고 균일 및 비균일 유동하에서 모두 정성적으로 잘 일치함을 확인할 수 있었다. 최적화 시 사용된 함수의 선택은 근사모델을 발전시키는데 그다지 영향을 미치지 못하였다. 하지만, 모든 배향 텐서의 eigenvalue들을 고려하면 보다 정량적으로 발전시킬 수 있지만 이들의 함수모양 선택은 중요하고 어려운 문제다. 비교를 위하여 ORW와 다른 여러 가지 근사모델을 이용하여 Film-gated strip과 Center-gated disk에 대한 연계효과 및 평면속도구배를 포함한 사출성형 충전공정의 수치모사를 수행하였다. ORW가 'Distribution Function Calculation' 과 비교하여 정량적으로도 거의 비슷한 결과를 예측함을 보여주지만 실제 실험자료와 비교하였을 때 약간의 차이가 있음을 확인하였다. 따라서 좀 더 정확히 섬유의 배향도를 예측하기 위해서는 섬유들의 상호작용을 나타내는 항의 모델링의 변화가 요구된다.

• 대한기계학회논문집
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• 제17권8호
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• pp.2122-2130
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• 1993

A numerical and experimental study has been performed on the flow in a shallow rectangular tank accompanying a vortex shedding. The model is composed of a rectangular tank with a vertical plate with a length half the width of the tank. The tank is subject to a horizontal sinusoidal oscillation. The numerical analysis shows that the pattern of vortex shedding changes considerably when the Reynolds number $R_e$ is varied from 500 to 7500. It is symmetric for $R_e$ <1500 and asymmetric for $R_e$ > 1500. The kinetic energies of the right-hand and left-hand sides of the vertical plate are used to quantify the degree of the asymmetry. Experimental visualization is carried out at $R_e$ = 3876 and 52000. The development of the streamline pattern at $R_e$ = 3876 is in closer agreement with the numerical result at $R_e$ = 1000 than that at $R_e$ =3876. The asymmetric pattern is observed at $R_e$ = 52000.