• International Journal of Fluid Machinery and Systems
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• 제4권1호
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• pp.199-208
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• 2011

This work presents numerical results, using OpenFOAM, of the flow in the swirl flow generator test rig developed at Politehnica University of Timisoara, Romania. The work shows results computed by solving the unsteady Reynolds Averaged Navier Stokes equations. The unsteady method couples the rotating and stationary parts using a sliding grid interface based on a GGI formulation. Turbulence is modeled using the standard k-${\varepsilon}$ model, and block structured wall function ICEM-Hexa meshes are used. The numerical results are validated against experimental LDV results, and against design velocity profiles. The investigation shows that OpenFOAM gives results that are comparable to the experimental and design profiles. The unsteady pressure fluctuations at four different positions in the draft tube is recorded. A Fourier analysis of the numerical results is compared whit that of the experimental values. The amplitude and frequency predicted by the numerical simulation are comparable to those given by the experimental results, though slightly over estimated.

• 전기전자학회논문지
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• 제5권1호
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• pp.91-99
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• 2001

공기 갭을 갖는 이방성 매질의 기판과 덮개층 구조에서 사각 마이크로스트립 안테나 공진 주파수의 파수 영역에 대해 해석하였다. 일축성 매질의 특성을 나타내는 텐서의 구성 관계식으로부터 파수영역에서의 다이애딕 그린 함수를 유도하였으며 이 결과를 푸리에 변환을 사용하여 공간에서의 전계 적분 방정식으로 수식화하였다. 캘러킨 모멘트법을 사용하여 적분 방정식을 이산화하였으며 기저함수로는 정현적 함수를 선택하였다. 수치해석 결과의 타당성을 검증하기 위해 기존의 결과와 비교, 일치된 결과를 얻었으며 공기 갭의 두께, 패치 길이 그리고 덮개층의 비유전율과 이방성 덮개층의 이방성 비의 변화에 따른 공진 주파수의 변화가 제시되고 분석되었다.

• 한국군사과학기술학회지
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• 제18권1호
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• pp.15-21
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• 2015

Rifling Force can be described with projectile velocity, gas pressure and rifling angle, etc. Under the same conditions, the character of the rifling angle decisively influences the rifling force. To reduce the harmful effect, locally distinct maximum of rifling force has to be avoided. The optimal design methodology of rifling angle curve had been developed by combined Fourier series and polynomial function. When it was tried newly to design the rifling angle curve, this design trial caused not to produce the lower rifling force than the existing design. Normally, the curve of the rifling angle is designed first, then the rifling force is set according to the rifling angle curve. However during the cause analysis, new design methodology was established to design the ideal rifling force curve before the rifling angle design. With this new methodology, the above optimal design method was analyzed and its limitation was confirmed.

• 한국전기전자재료학회:학술대회논문집
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• 한국전기전자재료학회 2008년도 하계학술대회 논문집 Vol.9
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• pp.379-380
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• 2008

Molecular Dynamics (MD) simulations have been used to model the dynamics of the charge-compensating sodium ions in the non-stoichiometric hollandite Nax$(Ti_{8-x}Cr_x)O_{16}$. These interstitial ions reside in 'tunnels' in the crystal structure and move under the forces of both the ions making up the cage structure and the many body interactions of the other sodium ions in the tunnel. The Velocity Autocorrelation Function (VAF) of the sodium ions is calculated for a range of temperature from 250K to 1000K and converted into the linear ac-conductivity and ac-susceptibility response via Fourier transformation. A peak is found in the conductivity around $6\times10^{12}$ Hz that has some of the character of a Poley absorption. Here it is shown to be due to an harmonically coupled site vibrations of the sodium atoms, which extend only over a limited range. At frequencies below the peak the conductivity tends towards a constant i.e. dc value corresponding to a constant flow of ions through the simulation cell. At high temperatures the conductivity due to this ion transport process behaves like a metal with an insulator to metal transition occurring around a specific temperature.

• Structural Engineering and Mechanics
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• 제31권2호
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• pp.211-223
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• 2009

Transient response analysis can be conducted either in the time domain, or via the frequency domain. Sometimes a frequency domain method (FDM) has advantages over a time domain method. A practical issue in the FDM is to find out an appropriate extended period, which may be affected by several factors, such as the excitation duration, the system damping, the artificial damping, the period of interest, etc. In this report, the extended period of the FDM based on the Duhamel's integral is investigated. This Duhamel's integral based FDM does not involve the unit impulse response function (UIRF) beyond the period of interest. Due to this fact, the ever-lasting UIRF can be simply set as zero beyond the period of interest to shorten the extended period. As a result, the preferred extended period is the summation of the period of interest and the excitation duration. This conclusion is validated by numerical examples. If the extended period is too short, then the front portion of the period of interest is more prone to errors than the rear portion, but the free vibration segment is free of the wraparound error.

• Structural Engineering and Mechanics
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• 제66권3호
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• pp.329-341
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• 2018

This paper considers the smooth receding contact problem between a homogeneous half-plane and a composite laminate composed of an inhomogeneously coated elastic layer. The inhomogeneity of the elastic modulus of the coating is approximated by an exponential function along the thickness dimension. The three-component structure is pressed together by either a concentrated force or uniform pressures applied at the top surface of the composite laminate. Both semianalytical and finite element analysis are performed to solve for the extent of contact and the contact pressure. In the semianalytical formulation, Fourier integral transformation of governing equations and boundary conditions leads to a singular integral equation of Cauchy-type, which can be numerically integrated by Gauss-Chebyshev quadrature to a desired degree of accuracy. In the finite element modeling, the functionally graded coating is divided into homogeneous sublayers and the shear modulus of each sublayer is assigned at its lower boundary following the predefined exponential variation. In postprocessing, the stresses of any node belonging to sublayer interfaces are averaged over its surrounding elements. The results obtained from the semianalytical analysis are successfully validated against literature results and those of the finite element modeling. Extensive parametric studies suggest the practicability of optimizing the receding contact peak stress and the extent of contact in multilayered structures by the introduction of functionally graded coatings.

• 한국분무공학회지
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• 제9권4호
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• pp.17-23
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• 2004

Spray characteristics of a twin-hole air shrouded nonle designed for gasoline injectors was investigated by using laser diffraction particle analyzer (LDPA) and tomography reconstruction- A confined spray chamber which is optically accessible through a pair of glass windows was made to simulate the fuel injection condition in intake manifold of gasoline engine. The measurement was applied to the twin hole injector with and without an air shroud. It demonstrates that for the case with an air shroud, fine atomization is achieved and there exists a large number of fine droplets between the region of the main spray streams, which conforms with the spray visualization. The drop size distribution was investigated as a function of elapse time after fuel injection. The distribution was greatly affected by the measurement position from the injector exit. Also, the spatially resolved spray volume fraction and Sauter Mean Diameter (SMD) from line-of-sight data of the LDPA are tomographically reconstructed by Convolution Fourier transformation under the steady injection condition.

• 충청수학회지
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• 제25권1호
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• pp.65-72
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• 2012

Let T > 0 be given. Let $(C[0,T],m_{\varphi})$ be the analogue of Wiener measure space, associated with the Borel proba-bility measure ${\varphi}$ on ${\mathbb{R}}$, let $(L_{2}[0,T],\tilde{\omega})$ be the centered Gaussian measure space with the correlation operator $(-\frac{d^{2}}{dx^{2}})^{-1}$ and ${\el}_2,\;\tilde{m}$ be the abstract Wiener measure space. Let U be the space of all sequence $<c_{n}>$ in ${\el}_{2}$ such that the limit $lim_{{m}{\rightarrow}\infty}\;\frac{1}{m+1}\;\sum{^{m}}{_{n=0}}\;\sum_{k=0}^{n}\;c_{k}\;cos\;\frac{k{\pi}t}{T}$ converges uniformly on [0,T] and give a set function m such that for any Borel subset G of $\el_2$, $m(\mathcal{U}\cap\;P_{0}^{-1}\;o\;P_{0}(G))\;=\tilde{m}(P_{0}^{-1}\;o\;P_{0}(G))$. The goal of this note is to study the relationship among the measures $m_{\varphi},\;\tilde{\omega},\;\tilde{m}$ and $m$.

• Wind and Structures
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• 제14권4호
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• pp.337-357
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• 2011

The collection of wind speed time series by means of digital data loggers occurs in many domains, including civil engineering, environmental sciences and wind turbine technology. Since averaging intervals are often significantly larger than typical system time scales, the information lost has to be recovered in order to reconstruct the true dynamics of the system. In the present work we present a simple algorithm capable of generating a real-time wind speed time series from data logger records containing the average, maximum, and minimum values of the wind speed in a fixed interval, as well as the standard deviation. The signal is generated from a generalized random Fourier series. The spectrum can be matched to any desired theoretical or measured frequency distribution. Extreme values are specified through a postprocessing step based on the concept of constrained simulation. Applications of the algorithm to 10-min wind speed records logged at a test site at 60 m height above the ground show that the recorded 10-min values can be reproduced by the simulated time series to a high degree of accuracy.

• 한국재료학회지
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• 제19권12호
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• pp.686-691
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• 2009

The effects of thermal properties on the crystallization behavior of $CaMgSi_2O_6$ glass-ceramics were investigated as a function of sintering temperature from 800$^{\circ}C$ to 900$^{\circ}C$. The crystallization behavior of the specimens depended on the sintering temperature, which could be evaluated from the differential thermal analysis, X-ray diffraction and Fourier transform infrared spectroscopy. With increasing sintering temperature, the thermal conductivity of the sintered specimens increased, while the coefficient of thermal expansion (CTE) of the sintered specimens decreased. These results could be attributed to the increase of crystallization, confirmed from the estimation by density measurements. Also, the thermal diffusivity and specific heat capacity of the sintered specimens were discussed with relation to the sintering temperature. Typically, a thermal conductivity of 3.084 $W/m^{\circ}C$, CTE of 8.049 $ppm/^{\circ}C$, thermal diffusivity of 1.389 $mm^2/s$ and specific heat capacity of 0.752 $J/g^{\circ}C$ were obtained for $CaMgSi_2O_6$ specimens sintered at 900$^{\circ}C$ for 5 h.