• 대한수학회보
• /
• 제58권6호
• /
• pp.1563-1567
• /
• 2021

We show that if M is a Utumi module, in particular if M is quasi-continuous, then M = Q ⊕ K, where Q is quasi-injective that is both a square-full as well as a dual-square-full module, K is a square-free module, and Q & K are orthogonal. Dually, we also show that if M is a dual-Utumi module whose local summands are summands, in particular if M is quasi-discrete, then M = P ⊕ K where P is quasi-projective that is both a square-full as well as a dual-square-full module, K is a dual-square-free module, and P & K are factor-orthogonal.

• 대한전자공학회논문지TC
• /
• 제40권3호
• /
• pp.1-9
• /
• 2003

준동기 이동 위성 리턴링크에서 MC-CDMA와 MC-DS/CDMA 시스템의 성능을 분석한다. 각 사용자 단말의 리턴링크간 동기가 수 칩 이내인 준동기를 고려하였다. MC-DS/DMA와 MC-DS/CDMA의 송신 신호는 정포락선 신호가 아니기 때문에 송신 증폭기의 비선형성에 의한 왜곡을 받게 된다. MC-CDMA 시스템은 Walsh 코드를 이용해 주파수 영역 확산을 하기 때문에 증폭기 입력신호레벨이 MC-DS/CDMA 시스템에 비해서 낮게 되어 사용자 수가 1명인 경우 증폭기 입력 backoff가 0dB일 때는 2.3dB, 증폭기 입력 backoff가 6dB일 때는 1dB 성능이 우수하였다. 그러나 준동기 AWGN 채널에서 사용자 수가 증가하게 되면 MC-DS/CDMA 시스템이 많은 수의 부 반송파를 사용하는 MC-CDMA 시스템에 비해서 간섭량이 적어 성능이 매우 우수하였다.

• Structural Engineering and Mechanics
• /
• 제72권5호
• /
• pp.653-673
• /
• 2019

This work investigates a novel quasi-3D hyperbolic shear deformation theory is presented to discuss the buckling of new type of sandwich plates. This theory accounts for both shear deformation and thickness stretching effects by a hyperbolic variation of all displacements through the thickness. The enhancement of this formulation is due to the use of only five unknowns by including undetermined integral terms, contrary to other theories where we find six or more unknowns. It does not require shear correction factors and transverse shear stresses vary parabolically across the thickness. A new type of FGM sandwich plates, namely, both FGM face sheets and FGM hard core are considered. The governing equations and boundary conditions are derived using the principle of virtual displacements. Analytical solutions are obtained for a simply supported plate. The accuracy of the present theory is verified by comparing the obtained results with quasi-3D solutions and those predicted by higher-order shear deformation theories. The comparison studies show that the obtained results are not only more accurate than those obtained by higher-order shear deformation theories, but also comparable with those predicted by quasi-3D theories with a greater number of unknowns.

• Steel and Composite Structures
• /
• 제28권1호
• /
• pp.99-110
• /
• 2018

In this paper, a new size-dependent quasi-3D plate theory is presented for wave dispersion analysis of functionally graded nanoplates while resting on an elastic foundation and under the hygrothermaal environment. This quasi-3D plate theory considers both thickness stretching influences and shear deformation with the variations of displacements in the thickness direction as a parabolic function. Moreover, the stress-free boundary conditions on both sides of the plate are satisfied without using a shear correction factor. This theory includes five independent unknowns with results in only five governing equations. Size effects are obtained via a higher-order nonlocal strain gradient theory of elasticity. A variational approach is adopted to owning the governing equations employing Hamilton's principle. Solving analytically via Fourier series, these equations gives wave frequencies and phase velocities as a function of wave numbers. The validity of the present results is examined by comparing them with those of the known data in the literature. Parametric studies are conducted for material composition, size dependency, two parametric elastic foundation, temperature and moisture differences, and wave number. Some conclusions are drawn from the parametric studies with respect to the wave characteristics.

• Computers and Concrete
• /
• 제24권4호
• /
• pp.347-367
• /
• 2019

This work investigates the effect of Winkler/Pasternak/Kerr foundation and porosity on dynamic behavior of FG plates using a simple quasi-3D hyperbolic theory. Four different patterns of porosity variations are considered in this study. The used quasi-3D hyperbolic theory is simple and easy to apply because it considers only four-unknown variables to determine the four coupled vibration responses (axial-shear-flexion-stretching). A detailed parametric study is established to evaluate the influences of gradient index, porosity parameter, stiffness of foundation parameters, mode numbers, and geometry on the natural frequencies of imperfect FG plates.

• Advances in materials Research
• /
• 제13권5호
• /
• pp.335-353
• /
• 2024

In the present paper, a simple quasi-3D integral higher-order beam theory (HBT) is presented, in which both shear deformation and thickness stretching effects are included for mechanical analysis of advanced composite beams with simply supported boundary conditions, handling mainly bending, buckling, and free vibration problems. The kinematics is based on a novel displacement field which includes the undetermined integral terms and the parabolic function is used in terms of thickness coordinate to represent the effect of transverse shear deformation. The governing equilibrium equations are drawn from the dynamic version of the principle of virtual work; whereas the solution of the problem is obtained by assuming a Navier technique for simply supported advanced composite beams subjected to sinusoidally and uniformly distributed loads. The correctness of the present computational method is checked by comparing the obtained numerical results with quasi-3D solutions found in the literature and with those provided by other shear deformation beam theories. It can be confirmed that the proposed model, which does not involve any shear correction factor, is not only accurate but also simple and useful in solving the static and dynamic response of advanced composite beams.

• Coupled systems mechanics
• /
• 제9권3호
• /
• pp.237-264
• /
• 2020

This article investigates the static behaviour of functionally graded (FG) plates sometimes declared as advanced composite plates by using a simple and accurate quasi-3D and 2D hyperbolic higher-order shear deformation theories. The properties of functionally graded materials (FGMs) are assumed to vary continuously through the thickness direction according to exponential law distribution (E-FGM). The kinematics of the present theories is modeled with an undetermined integral component and satisfies the free transverse shear stress conditions on the top and bottom surfaces of the plate; therefore, it does not require the shear correction factor. The fundamental governing differential equations and boundary conditions of exponentially graded plates are derived by employing the static version of principle of virtual work. Analytical solutions for bending of EG plates subjected to sinusoidal distributed load are obtained for simply supported boundary conditions using Navier'is solution procedure developed in the double Fourier trigonometric series. The results for the displacements and stresses of geometrically different EG plates are presented and compared with 3D exact solution and with other quasi-3D and 2D higher-order shear deformation theories to verify the accuracy of the present theory.

• 한국연소학회지
• /
• 제4권1호
• /
• pp.131-139
• /
• 1999

A Numerical Analysis of NOx production in Hydrogen-Air flame is performed using the quasi-laminar reaction modelling. As results, in low global strain rate region, $U_F/D_F\;{\leq}\;50,000$, the quasi-laminar reaction modelling reproduces the experimentally observed EINOx half power scaling that the ratio of EINOx and flame residence time, $L_f^3(D_F^2U_F)$, is proportional to the square root of global strain rate. Thus, it suggests that turbulence-chemistry interaction has a minor impact on the trend of NOx production in low global strain rate region. However, the quasi-laminar reaction modelling predicts the higher temperature and NOx than experimentally observed. This overprediction may be due to the lack of radiation and quasi-laminar reaction modelling.

• 한국항행학회논문지
• /
• 제28권2호
• /
• pp.232-237
• /
• 2024

본 논문에서는 4×2 미앤더라인 배열 구조를 이용하여 8 dBi 이득을 유지하는 광대역 이중 다이폴 준-야기 안테나에 대하여 연구하였다. 4×2 미앤더라인 배열 구조는 미앤더라인 도체 모양의 단위 셀로 구성되며, 이중 다이폴 준-야기 안테나의 두 번째 다이폴 안테나 위에 배치하였다. 일반적으로 많이 사용하는 여러 개의 스트립 도파기를 사용한 경우와 동일한 크기의 FR4 기판에 설계하였고, 입력 반사 계수와 이득 특성을 비교하였다. 비교 결과, 기존의 다중 스트립 도파기를 사용하였을 때 보다 주파수 대역폭이 6.3% 증가하였고, 이득이 8dBi 이상인 주파수 대역폭은 10.1% 증가하고 평균 이득도 조금 증가하였다. 제작된 안테나의 전압 정재파비 (VSWR; voltage standing wave ratio)가 2 이하인 대역은 1.548-2.846 GHz (59.1%)이고, 1.6-2.8 GHz 대역에서 이득이 8 dBi 이상으로 측정되었다.

• Steel and Composite Structures
• /
• 제51권2호
• /
• pp.185-202
• /
• 2024

In this research paper, and for the first time, wave propagations in sigmoidal imperfect functionally graded material plates are investigated using a simplified quasi-three-dimensionally higher shear deformation theory (Quasi-3D HSDTs). By employing an indeterminate integral for the transverse displacement in the shear components, the number of unknowns and governing equations in the current theory is reduced, thereby simplifying its application. Consequently, the present theories exhibit five fewer unknown variables compared to other Quasi-3D theories documented in the literature, eliminating the need for any correction coefficients as seen in the first shear deformation theory. The material properties of the functionally graded plates smoothly vary across the cross-section according to a sigmoid power law. The plates are considered imperfect, indicating a pore distribution throughout their thickness. The distribution of porosities is categorized into two types: even or uneven, with linear (L)-Type, exponential (E)-Type, logarithmic (Log)-Type, and Sinus (S)-Type distributions. The current quasi-3D shear deformation theories are applied to formulate governing equations for determining wave frequencies, and phase velocities are derived using Hamilton's principle. Dispersion relations are assumed as an analytical solution, and they are applied to obtain wave frequencies and phase velocities. A comprehensive parametric study is conducted to elucidate the influences of wavenumber, volume fraction, thickness ratio, and types of porosity distributions on wave propagation and phase velocities of the S-FGM plate. The findings of this investigation hold potential utility for studying and designing techniques for ultrasonic inspection and structural health monitoring.