• Wind and Structures
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• 제30권2호
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• pp.181-198
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• 2020

In the traditional buffeting response analysis method, the spanwise incomplete correlation of buffeting forces is always assumed to be same as that of the incident wind turbulence and the action of the signature turbulence is ignored. In this paper, three typical bridge decks usually adopted in the real bridge engineering, a single flat box deck, a central slotted box deck and a two-separated paralleled box deck, were employed as the investigated objects. The wind induced pressure on these bridge decks were measured via a series of wind tunnel pressure tests of the sectional models. The influences of the wind speed in the tests, the angle of attack, the turbulence intensity and the characteristic distance were taken into account and discussed. The spanwise root coherence of buffeting forces was also compared with that of the incidence turbulence. The signature turbulence effect on the spanwise root coherence function was decomposed and explained by a new empirical method with a double-variable model. Finally, the formula of a sum of rational fractions that accounted for the signature turbulence effect was proposed in order to fit the results of the spanwise root coherence function. The results show that, the spanwise root coherence of the drag force agrees with that of incidence turbulence in some range of the reduced frequency but disagree in the mostly reduced frequency. The spanwise root coherence of the lift force and the torsional moment is much larger than that of the incidence turbulence. The influences of the wind speed and the angle of attack are slight, and they can be ignored in the wind tunnel test. The spanwise coherence function often involves several narrow peaks due to the signature turbulence effect in the high reduced frequency zone. The spanwise coherence function is related to the spanwise separation distance and the spanwise integral length scales, and the signature turbulence effect is related to the deck-width-related reduced frequency.

• 대한기계학회논문집B
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• 제27권12호
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• pp.1681-1690
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• 2003

The effect of roughness is a change in the velocity and turbulence distributions near the surface. Turbulence models with surface roughness effect are applied to the fully developed flow in a two-dimensional, rough wall channel. Modified wall function model, low-Reynolds number k-$\varepsilon$ model, and k-$\omega$ model are selected for comparison. In order to make a fair comparison, the calculation results are compared with the experimental data. The modified wall function model and the low-Reynolds number k-$\varepsilon$ model require further refinement, while the k-$\omega$ model of Wilcox performs remarkably well over a wide range of roughness values.

• 한국연소학회지
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• 제15권4호
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• pp.15-21
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• 2010

The transported probability density function(PDF) model has been applied to simulate the turbulent nonpremixed piloted jet flame. To realistically account for the mixture fraction PDF informations on the turbulent non-premixed jet flame, the present Lagrangian PDF transport approach is based on the joint velocity-composition-turbulence frequency PDF formulation. The fluctuating velocity of stochastic fields is modeled by simplified Langevin model(SLM), turbulence frequency of stochastic fields is modeled by Jayesh-Pope model and effects of molecular diffusion are represented by the interaction by exchange with the mean (IEM) mixing model. To validate the present approach, the numerical results obtained by the joint velocity-composition-turbulence frequency PDF model are compared with experimental data in terms of the unconditional and conditional means of mixture fraction, temperature and species and PDFs.

• International Journal of Aeronautical and Space Sciences
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• 제17권1호
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• pp.8-19
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• 2016

This paper investigates the effects of inlet turbulence conditions and near-wall treatment methods on the heat transfer prediction of gas turbine vanes within the range of engine relevant turbulence conditions. The two near-wall treatment methods, the wall-function and low-Reynolds number method, were combined with the SST and ${\omega}RSM$ turbulence model. Additionally, the RNG $k-{\varepsilon}$, SSG RSM, and $SST_+{\gamma}-Re_{\theta}$ transition model were adopted for the purpose of comparison. All computations were conducted using a commercial CFD code, CFX, considering a three-dimensional, steady, compressible flow. The conjugate heat transfer method was applied to all simulation cases with internally cooled NASA turbine vanes. The CFD results at mid-span were compared with the measured data under different inlet turbulence conditions. In the SST solutions, on the pressure side, both the wall-function and low-Reynolds number method exhibited a reasonable agreement with the measured data. On the suction side, however, both wall-function and low-Reynolds number method failed to predict the variations of heat transfer coefficient and temperature caused by boundary layer flow transition. In the ${\omega}RSM$ results, the wall-function showed reasonable predictions for both the heat transfer coefficient and temperature variations including flow transition onset on suction side, but, low-Reynolds methods did not properly capture the variation of the heat transfer coefficient. The $SST_+{\gamma}-Re_{\theta}$ transition model showed variation of the heat transfer coefficient on the transition regions, but did not capture the proper transition onset location, and was found to be much more sensitive to the inlet turbulence length scale. Overall, the Reynolds stress model and wall function configuration showed the reasonable predictions in presented cases.

• 천문학회보
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• 제36권2호
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• pp.108.1-108.1
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• 2011

Turbulence is ubiquitous in astrophysical fluids such as the interstellar medium(ISM) and the intracluster medium(ICM). There are many driving mechanisms which can inject energy into the fluid in variety driving scales, But the plausible driving scale of ISM/ICM turbulence are yet unknown. Therefore, understanding different statistical properties between turbulence with single driving scale and turbulence with double driving scale is required. In this work, we performed 3-dimensional isothermal compressible, magnetohydrodynamic(MHD) turbulence simulations. We drive turbulence in the Fourier space in two ranges, 2

• 대한토목학회논문집
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• 제31권2A호
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• pp.97-104
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• 2011

본 연구에서는 교량 현장에서 계측한 풍속 스펙트럼을 풍동에서 구현하고, 콘크리트 사장교를 대상으로 공력 어드미턴스함수를 측정하였다. 그리고 현장 측정한 난류 특성을 바탕으로 콘크리트 사장교의 3차원 버페팅 해석을 수행하였다. 본 연구의 결과를 보면 적절히 공력 어드미턴스함수를 고려할 경우에 고려하지 않은 경우보다 버페팅 응답이 절반 가까이 줄어드는 것으로 나타났다. 그리고 격자난류를 사용할 경우에 저주파 영역에서 공력 어드미턴스 함수가 낮아서 풍하중을 과소평가할 가능성이 있는 것으로 나타났다. 공력 어드미턴스 함수가 버페팅 응답에 미치는 영향이 상당하므로 향후 교량의 버페팅 해석시 플러터계수나 능동난류로부터 추출한 공력 어드미턴스 함수를 사용할 것을 추천한다.

• Wind and Structures
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• 제12권5호
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• pp.441-455
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• 2009

Studying the spatial distribution in coherent fields such as turbulence and turbulence-induced force is important to model and evaluate turbulence-induced forces and response of structures in the turbulent flows. Turbulence field-based coherence function is commonly used for the spatial distribution characteristic of the turbulence-induced forces in the frequency domain so far. This paper will focus to study spectral coherent structure of the turbulence and induced forces in not only the frequency domain using conventional Fourier transform-based coherence, but also temporo-spectral coherence one in the time-frequency plane thanks to wavelet transform-based coherence for better understanding of the turbulence and force coherences and their spatial distributions. Effects of spanwise separations, bluff body flow, flow conditions and Karman vortex on coherent structures of the turbulence and induced pressure, comparison between turbulence and pressure coherences as well as intermittency of the coherent structure in the time-frequency plane will be investigated here. Some new findings are that not only the force coherence is higher than the turbulence coherence, the coherences of turbulence and forces depend on the spanwise separation as previous studies, but also the coherent structures of turbulence and forces relate to the ongoing turbulence flow and bluff body flow, moreover, intermittency in the time domain and low spectral band is considered as the nature of the coherent structure. Simultaneous measurements of the surface pressure and turbulence have been carried out on some typical rectangular cylinders with slenderness ratios B/D=1 (without and with splitter plate) and B/D=5 under the artificial turbulent flows in the wind tunnel.

• 천문학회보
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• 제35권1호
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• pp.67.1-67.1
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• 2010

Understanding the nature of MHD turbulence is of fundamental importance in astrophysics. The results of these studies can be used to develop the star formation theory. While most of previous studies have considered turbulence in uniform media, the stratification of media can play an important role. We performed three-dimensional numerical simulations of isothermal, compressible, MHD turbulence with different plasma beta's and different flow Mach numbers in stratified media to study the effects of stratification on turbulence. We found Statistics of turbulent flow in stratified media is different from in uniform media. In this talk, we present the statistical properties of the MHD turbulence, such as the PDF, power spectrum, and structure function.

• Journal of the Optical Society of Korea
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• 제19권3호
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• pp.237-240
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• 2015

We derived the average bit error rate (BER) of coherent free-space optical (FSO) systems with digital binary phase shift keying (BPSK) modulations over atmospheric turbulence channels with a gamma-gamma distribution. To obtain a generalized derivation in a closed-form expression, we used special integrals and transformations of the Meijer G function. Furthermore, we numerically analyzed and simulated the average BER behavior according to the average SNR for different turbulence strengths. Simulation results are demonstrated to confirm the analytical results.

• 한국전산유체공학회지
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• 제21권4호
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• pp.78-84
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• 2016

In this paper, turbulence models for considering roughness in the open source code(OpenFOAM) was investigated. Wall function in the RANS(Reynolds-averaged Navier - Stokes) turbulence model was modified considering roughness on the flat plate by using roughness function. Correlation between the first layer height in the CFD model and roughness height of the plate was observed, and the most proper roughness function, and the first layer height from the plate wall in the CFD analysis was suggested in this paper.