• 한국전산유체공학회지
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• 제17권2호
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• pp.93-99
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• 2012

The present study focuses on the case of developing flow with in a channel containing a long array of sinusoidal waves (2a/${\lambda}$=0.1, ${\lambda}$=h, ${\lambda}$ is the wavelength, 2a is the wave height, h is the mean channel depth) at the bottom wall. The Reynolds number defined with channel height, h and the mean velocity, U, is Re=6,700. The channel is sufficiently long such that transition is completed and the flow is fully developed over the downstream half of the channel. For the case of an incoming steady flow with no resolved turbulence, the instantaneous flow fields in the transition region are characterized by the formation of arrays of highly-organized large-scale hairpin vortices whose dimensions scale with that of the roughness elements. The paper explains the mechanism for the formation of these arrays of hairpin vortices and shows these eddies play the primary role in the formation of the large-scale streaks of high and low velocity over the wavy wall region. The presence of resolved turbulence in the incoming flow, reduces the streamwise distance needed for the streaks to develop over the wavy region, but does not affect qualitatively the transition process. In the fully-developed region, isolated and trains of large-scale hairpins play an important role in the dynamics of the streaks over the wavy wall.

• 한국가시화정보학회지
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• 제12권3호
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• pp.15-20
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• 2014

Direct numerical simulation (DNS) dataset of a turbulent boundary layer (TBL) with a step change from smooth to rough surface is analyzed to examine spatially developing flow characteristics. The roughness elements are periodically arranged two-dimensional (2-D) spanwise rods with a streamwise pitch of ${\lambda}=8k$ ($=12{\theta}_{in}$), and the roughness height is $k=15{\theta}_{in}$, where ${\theta}_{in}$ is the inlet momentum thickness. The step change is introduced $80{\theta}_{in}$ downstream from the inlet. For the first time, full images from the DNS data with the step change from the smooth to rough walls is present to get some idea of the geometry of turbulent coherent structures over rough wall, especially focusing on their existence and partial dynamics over the rough wall. The results show predominance of hairpin vortices over the rough wall and their spanwise scale growth mechanism by merging.

• Structural Engineering and Mechanics
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• 제69권3호
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• pp.293-306
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• 2019

In current seismic design codes, various elastic design acceleration spectra are defined considering different seismological and soil characteristics and are widely used tool for calculation of seismic loads acting on structures. Response spectrum analyses directly use the elastic design acceleration spectra whereas time history analyses use acceleration records of earthquakes whose acceleration spectra fit the design spectra of seismic codes. Due to the fact that obtaining coherent structural response quantities with the seismic design code considerations is a desired circumstance in dynamic analyses, the response spectra of earthquake records used in time history analyses had better fit to the design acceleration spectra of seismic codes. This paper evaluates structural response distributions of multi-story reinforced concrete frames obtained from nonlinear time history analyses which are performed by using the scaled earthquake records compatible with various elastic design spectra. Time domain scaling procedure is used while processing the response spectrum of real accelerograms to fit the design acceleration spectra. The elastic acceleration design spectra of Turkish Seismic Design Code 2007, Uniform Building Code 1997 and Eurocode 8 are considered as target spectra in the scaling procedure. Soil classes in different seismic codes are appropriately matched up with each other according to $V_{S30}$ values. The maximum roof displacements and the total base shears of considered frame structures are determined from nonlinear time history analyses using the scaled earthquake records and the results are presented by graphs and tables. Coherent structural response quantities reflecting the influence of elastic design spectra of various seismic codes are obtained.

• 대한원격탐사학회:학술대회논문집
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• 대한원격탐사학회 2002년도 Proceedings of International Symposium on Remote Sensing
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• pp.312-316
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• 2002

Satellite radar interferometry data shows a strong coherent signal on oyster sea farms where artificial structures installed on the bottom. We obtained 21 highly coherent interferograms from eleven JERS-1 SAR data sets despite of large orbital baseline (~2 km) or large temporal baseline (~l year). The phases observed in sea farms are probably induced by double bouncing on sea surface, and consequently reveal a tide height variation. To restore the absolute sea level changes we counted the number of wrapping by exploiting the intensity of backscattering. Backscattering intensity is closely correlated with the change in water surface height, while interferometry gives the detailed variation within the limit of 2$\pi$ (or 15.3 cm). Comparing the radar measurements with the tide gauge records yielded a correlation coefficient of 0.96 and an ms error of 6.0 cm. The results demostrate that radar interferometry is promising to measure sea level.

• 한국추진공학회지
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• 제21권5호
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• pp.1-9
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• 2017

초임계 환경에서 와류형 분사기의 극저온 질소 분무 동적 특성 분석을 위하여 3차원 LES 난류 모델을 적용하였다. 초임계 상태에서 질소의 상태량들을 정확하게 예측하기 위해 SRK 실기체 상태방정식을 사용하였고, 점성계수와 열전도도는 Chung의 고압 상태 혼합물에 대한 방정식, 그리고 확산 계수는 Fuller의 이론에 Takahashi의 보정식을 적용하였다. 질소 분무 결과, 분사기 내, 외부에서 유동장과 음향장 사이의 상호작용으로 복잡한 유동구조가 형성된다. 복잡한 유동 현상을 분석하기 위해 FFT, POD 그리고 DMD 기법을 적용하여 해석을 수행하였다. FFT 해석을 수행하여 분사기 내, 외부에서 나타나는 특정 주파수를 파악하였으며, POD와 DMD를 통해 각 주파수가 어떠한 유동 구조를 갖는지에 대한 연구를 수행하였다. 또한, DMD를 통해 각 주파수의 감쇠 계수를 파악하여 이를 실험 결과와 비교하였다.

• 대한토목학회논문집
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• 제26권3B호
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• pp.253-262
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• 2006

본 연구에서는 복단면 개수로 난류흐름에 대한 직접수치모의(DNS)를 수행하였다. DNS 자료를 이용하여 평균흐름 및 난류 구조를 제시하며, 기존의 실험자료 및 수치해석결과와 비교된다. 주수로와 홍수터의 접합부 부근에서 쌍와(雙渦)가 생성되는데, 그 최대크기는 체적평균된 주흐름방향 유속의 약 5%로 나타났다. 쌍와(雙渦)의 영향에 의해 접합부에서 벽전단응력이 최대값을 보였는데, 이는 기존 LES 자료와는 일치하지만 기존 실험연구결과와는 다른 것으로 그 원인에 대해 상세히 논의되었다. 사분면 해석을 통해 접합부 부근에서 레이놀즈 전단응력은 주로 쓸기현상 및 분출현상에 의해 생성되는 것으로 나타났다. 또한 조건부사분면 해석을 통해 지배적인 고유구조의 방향성이 레이놀즈 전단응력 및 쌍와(雙渦)의 생성을 결정한다는 것을 확인하였다.

• Wind and Structures
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• 제1권2호
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• pp.161-174
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• 1998

Wind forces on structures are usually schematized by the sum of their mean static part and a nil mean fluctuation generally treated as a stationary process randomly varying in space and time. The multi-variate and multi-dimensional nature of such a process requires a considerable quantity of numerical procedures to carry out the dynamic analysis of the structural response. With the aim of drastically reducing the above computational burden, this paper introduces a method by means of which the external fluctuating wind forces on slender structures and structural elements are schematized by an equivalent process identically coherent in space. This process is identified by a power spectral density function, called the Generalized Equivalent Spectrum, whose expression is given in closed form.

• Journal of the Optical Society of Korea
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• 제12권1호
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• pp.7-12
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• 2008

Directional emission of light exiting a photonic crystal waveguide by a coherent action of radiative surface modes was recently demonstrated, and subsequently the substantial enhancement of the directional emission was achieved by engineering the surface and adjusting relevant parameters. Here we present the analysis of surface modes causing the enhanced emission by the plane wave expansion method and the finite-difference time-domain method. In particular, surface band structures are calculated for nonradiative and radiative surface modes, respectively, and intensity profiles of some representative modes for nonradiative and radiative cases are given.

• 한국세라믹학회지
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• 제30권9호
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• pp.740-746
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• 1993

TiO2-SnO2 fine powders prepared by coprecipitation from TiCl4-SnCl4 aqueous solution, and their crystal structures were studied. All the TiO2-SnO2 fine powders calcined at 180~$700^{\circ}C$ showed the complete solid solution between TiO2(rutile structure) and SnO2(rutile structure). This crystal structure of TiO2-SnO2 powders is thought to be originated mainly from the heterogeneous nucleation of Ti-hydroxde on the Sn-hydroxide with coherent structure.

• 한국섬유공학회:학술대회논문집
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• 한국섬유공학회 1998년도 가을 학술발표회논문집
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• pp.414-417
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• 1998

Needle punching is a process for converting fiber webs into coherent structures by using barb needles. The results are the mechanical interlocking of fibers into webs. Nonwovens by needle punching is most versatile among the various methods and is widely used in the industry. The original process for the production of mechanical bonds in nonwoven fabrics is that which employs the natural felting characteristics of the wool fiber. (omitted)