• 대한기계학회논문집B
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• 제29권3호
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• pp.314-321
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• 2005

Discrete Wavelet Transform (DWT) has been applied to the Direct Numerical Simulation (DNS) data of turbulent channel flow. DWT splits the turbulent flow into two orthogonal parts, one corresponding to coherent structures and the other to incoherent background flow. The coherent structure is extracted from not vorticity field but velocity's since the channel flow is not isoropic. By comparing DWT's result of channel flow with that of isotropic flow, it is shown that coherent structure maintains the properties of original channel flow. The velocity field of coherent structures can be represented by few wavelet modes and that these modes are sufficient to reproduce the velocity probability density function (PDF) and the energy spectrum over the entire inertial range. The remaining incoherent background flow is homogeneous, has small amplitude, and is uncorrelated. These results are compared with those obtained for the same compression rate using large eddy simulation (LES) filtering. In contrast to the incoherent background flow of DWT, the LES subgrid scales have a much larger amplitude and are correlated, which makes their statistical modeling more difficult.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2004년도 춘계학술대회
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• pp.1813-1818
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• 2004

Discrete Wavelet Transform (DWT) has been applied to the Direct Numerical Simulation (DNS) data of turbulent channel flow. DWT splits the turbulent flow into two orthogonal parts, one corresponding to coherent structures and the other to incoherent background flow. The coherent structure is extracted from not vorticity field but velocity's since the channel flow is not isotropic. By comparing DWT's result of channel flow with that of isotropic flow, it is shown that coherent structure maintains the properties of original channel flow. The velocity field of coherent structures can be represented by few wavelet modes and that these modes are sufficient to reproduce the velocity probability distribution function (PDF) and the energy spectrum over the entire inertial range. The remaining incoherent background flow is homogeneous, has small amplitude, and is uncorrelated. These results are compared with those obtained for the same compression rate using large eddy simulation (LES) filtering. In contrast to the incoherent background flow of DWT, the LES subgrid scales have a much larger amplitude and are correlated, which makes their statistical modeling more difficult.

• 대한기계학회논문집
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• 제14권6호
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• pp.1661-1668
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• 1990

본 연구에서는 코히런트 와(渦)에 지배되는 난류 예혼합화염의 미세구조를 밝 히기 위해 슐리이렌사진과 온도, 이온전류의 3가지를 동시에 측정하고, 그 변동량을 통계처리, 분석하여, 미시적인 화염구조 모델을 제시하고자 한다.

• 대한기계학회논문집B
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• 제20권3호
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• pp.1028-1039
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• 1996

This study was focused on the examination of the flame structure and the combustion characteristics of diffusion flame which was formed the turbulent shear flow of a double coaxial air jet system. The shear flow was formed by the difference velocity of surrounding air jet(U$\_$s/) and center air jet (U$\_$c/). So experimental condition was divided S-type flame (.lambda. > 1) and C-type flame (.lambda. < 1) by velocity ratio .lambda. (=U$\_$s//U$\_$c/). For examination of the flame structure and the combustion characteristics in diffusion flame, coherent structure was observed in flame by schlieren photograph method. We measured fluctuating temperature and ion current simultaneously and accomplished the statistical analysis of its. According to schlieren photograph, the flame was stabilized in the rim of the direction of lower velocity air jet, coherent eddy was produced and developed by higher velocity air jet. The statistical data of fluctuating temperature and ion current was indicated that reaction was dominated by higher velocity air jet. The mixing state of burnt gas and non-burnt gas was distributed the wide area at Z = 100 mm of C-type flame.

• Journal of Mechanical Science and Technology
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• 제20권2호
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• pp.262-270
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• 2006

Direct numerical simulations (DNS) of turbulent channel flows up to $Re_{\tau}=1270$ are performed to investigate an elliptic feature and strain rate field on cross sections of coherent fine scale eddies (CFSEs) in wall turbulence. From DNS results, the CFSEs are educed and the strain rate field around the eddy is analyzed statistically. The principal strain rates (i.e. eigenvalues of the strain rate tensor) at the CFSE centers are scaled by the Kolmogorov length $\eta$ and velocity $U_k$. The most expected maximum (stretching) and minimum (compressing) eigenvalues at the CFSE centers are independent of the Reynolds number in each $y^+$ region (i. e. near-wall, logarithmic and wake regions). The elliptic feature of the CFSE is observed in the distribution of phase-averaged azimuthal velocity on a plane perpendicular to the rotating axis of the CFSE $(\omega_c)$. Except near the wall, phase-averaged maximum $(\gamma^{\ast}/\gamma_c^{\ast})$ and minimum $(\alpha^{\ast}/\alpha_c^{\ast})$ an eigenvalues show maxima on the major axis around the CFSE and minima on the minor axis near the CFSE center. This results in high energy dissipation rate around the CFSE.

• 한국수자원학회:학술대회논문집
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• 한국수자원학회 2005년도 학술발표회(1)
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• pp.231-232
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• 2005

• 한국군사과학기술학회지
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• 제17권6호
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• pp.861-867
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• 2014

In the present study, the flow around a circular cylinder at $Re=3.6{\time}10^6$ is numerically simulated using URANS approach. The objective of this study is to evaluate the turbulence models(Realizable k-${\varepsilon}$, RNG k-${\varepsilon}$) through the prediction of the unsteady flow characteristics around the cylinder. The time-averaged drag coefficients and vortex shedding phenomenon in the wake region are compared to available experimental data and other numerical results. The simulation with Realizable k-${\varepsilon}$ model is found to be more dissipative due to large eddy viscosity predicted in the wake region while the simulation with RNG k-${\varepsilon}$ model predicts a complex vortex shedding phenomenon with more coherent structures realistically.

• 대한토목학회논문집
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• 제34권5호
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• pp.1435-1442
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• 2014

본 연구에서는 OpenFOAM에서 제공하는 소스코드를 이용하여 매끄러운 하상의 직사각형 개수로 흐름에 대해 수치모의를 수행하였다. 난류 해석을 위해 LES를 수행하였는데, 비등방성 잔여 응력항을 모델링하기 위해서 Germano et al. (1991)이 제시한 Dynamic Subgrid-scale 모형을 이용하였다. 조직구조를 분석하기 위하여 Lu and Willmarth (1973)가 제시한 uw 사분면기법을 이용하여 순간레이놀즈 응력이 레이놀즈 응력에 미치는 영향을 기여율과 시간비로 나누어 분석하였다. LES 모의 결과를 토대로 기존 실험 및 DNS 모의 결과와 비교하고 분석하였다. 매끈한 하상을 가진 개수로 흐름에서 완충층 이후의 구간에서 분출현상이 쓸기현상에 비해 레이놀즈 응력의 양의 생성에 기여하는 바가 크지만, 분출현상에 비해 쓸기현상의 발생빈도가 큰 것으로 확인되었다.

• Journal of Advanced Marine Engineering and Technology
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• 제13권4호
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• pp.33-39
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• 1989

A turbulent premixed flames of layer formed between burned hot gas and unburned mixture were investigated by means of schlieren photograph with fluctuations of temperature and ion current. The combustion intensity between burned hot gas and shear layer was higher than the intensity between unburned mixture and shear layer. A wrinkled laminar flame and flamelet were appeared at downstream to exist and distributed reaction zone was at upstream as a result of analyzed probability density functions of temperature fluctuation. The initial combustion intensity of reaction zone of eddy between burned hot gas and shear layer was higher than that of final, flowing downstream, and vice versa between unburned mixture and shear layer.

• International Journal of Aeronautical and Space Sciences
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• 제8권2호
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• pp.1-10
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• 2007

A two-dimensional backward facing step flow was comptuted using a Detached Eddy simulation(DES) based on the SST turbulence model. The expansion ratio(ER) was 1.125 and the Reynolds number based on the step height and the mean velocity in the upstream channel was 37,500. The flow condition was the same as with the experimental research[1]. The reattachment length, oscillatory characteristics of the flow and the coherent structures of the present simulation were compared to demonstrate the improtance of spanwise grid spacing.