• 대한조선학회논문집
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• 제54권5호
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• pp.406-414
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• 2017

An experimental study in a recirculating water channel was carried out to investigate the effect of large coherent structures to the skin friction on a flat plate. Particle Image Velocimetry (PIV) technique was used to quantify characteristic features of coherent structures growing to the boundary layer. In the PIV measurement, it is difficult to calculate the friction velocity near the wall region due to laser deflection and uncertainty so that Clauser fitting method at the logarithmic region was adopted to compute the friction velocity and compared with the one directly measured by the dynamometer. With changing the free-stream velocity from 0.5 m/s to 1.0 m/s, the activity of coherent structures in the logarithmic region was increased over three times in terms of Reynolds stress. The flow field was separated by Variable Interval Time Averaging (VITA) technique into the weak and the strong structure case depending on the existence large coherent structures in order to validate its effectiveness. The stream-wise velocity fluctuation was scanned through at the boundary thickness whether it had a large deviation from background flow. With coherent structures connected from near-wall to the boundary layer, mean wall shear stress was higher than that of weak structure case. Proper Orthogonal Decomposition (POD) analysis was also applied to compare the energy budget between them at each free-stream velocity.

• 한국추진공학회지
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• 제9권2호
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• pp.97-104
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• 2005

많은 장점에도 불구하고 유동함수를 이용한 수치해석용 격자생성 좌표변환기법의 단점은 저속영역에서의 격자간격이 고속영역에 비해 상대적으로 큼에 따라 수치적 처리에 많은 오차를 내포하고 있다는 점이다. 본 연구에서는 이러한 저속영역에서의 단점을 보완하기 위하여 격자간격을 속도크기 및 영역에 따라 적절히 조절할 수 있도록 수학적으로 변형된 압축성 유동함수를 이용한 좌표변환기법을 제안하고 가스터빈엔진에 주로 적용되는 유동모델로서 동심원상 두개 이상의 난류제트혼합유동에 대해 적용하였으며 해당 실험치, 즉 축 방향 평균속도분포, 난류운동에너지, 그리고 난류전단응력분포와 비교하여 난류운동에너지가 약간 과소평가 된 대칭축을 제외한 혼합경계층 내에서 $3.5\%$ 이내의 신뢰성을 확보하였다. 본 기법은 특히 터보팬엔진에 대한 내부흐름들의 혼합유동을 규명하거나 또는 난류전단응력에 의한 제트소음발생 및 저감방법을 도모하는데 유용하게 활용될 것으로 기대된다.

• 대한기계학회논문집B
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• 제29권5호
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• pp.577-589
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• 2005

The present study has numerically investigated two-dimensional laminar flow over a steadily rotating circular cylinder with a uniform planar shear, where the free-stream velocity varies linearly across the cylinder. It aims to find the combined effect of rotation and shear on the flow. Numerical simulations using the immersed boundary method are performed for the ranges of $-2.5{\le}\alpha{\le}2.5$ and $0{\le}K{\le}0.2$ at a fixed Reynolds number of Re=100, where a and K are respectively the dimensionless rotational speed and velocity gradient. Results show that the positive shear, with the upper side having the higher free-stream velocity than the lower one, favors the effect of the counter-clockwise rotation $(\alpha<0)$ but countervails that of the clockwise rotation $(\alpha>0)$. Accordingly, the absolute critical rotational speed, below which vortex shedding occurs, decreases with increasing K for $(\alpha>0)$, but increases for $\alpha>0$. The vortex shedding frequency increases with increasing \alpha (including the negative) and the variation becomes steeper with increasing K. The mean lift slightly decreases with increasing K regardless of the rotational direction. However, the mean drag and the amplitudes of the lift- and drag-fluctuations strongly depend on the direction. They all decrease with increasing K for $\alpha>0$, but increase for $\alpha<0$. Flow statistics as well as instantaneous flow folds are presented to identify the characteristics of the flow and then to understand the underlying mechanism.

• 대한기계학회논문집
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• 제14권3호
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• pp.725-733
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• 1990

본 연구에서는 Raju등의 연구의 확장으로 일정한 온도로 유지되는 경사진 평 판에 대하여 혼합대류 전 영역에 유효한 부력계수(.xi.)로서 (1+Gr$^{2}$$_{x}$cos.gamma./ Re$^{4}$$_{x}$)$_{-1}$를 적용하여 비상사 경계층 방정식을 유도하였다. 또한 적분 형 경계조건을 가진 편미분 경계층방정식을 유한차분법으로 Keller와 Cebeci의 Box scheme을 6차로 적용하여 수치해석하였다. 부력계수가 큰 영역에서 Mucoglu등의 해 석적 연구결과는 일정한 온도와 45˚의 경사를 유지하는 평판에 대한 Ramachandran의 실험적 연구결과와 약 10%정도 벗어난다. 이에 Ramachandran의 실험적 연구를 해석 적으로 재고찰하였다.하였다.

• 대한기계학회논문집B
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• 제25권3호
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• pp.305-314
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• 2001

A modified model is proposed for calculation of transitional boundary layer flows. In order to develop the eddy viscosity model for the problem, the flow is divided into three regions; namely, pre-transition region, transition region and fully turbulent region. The pre-transition eddy-viscosity is formulated by extending the mixing length concept. In the transition region, the eddy-viscosity model employs two length scales, i.e., pre-transition length scale and turbulent length scale pertaining to the regions upstream and the downstream, respectively, and a universal model of stream-wise intermittency variation is used as a function bridging the pre-transition region and the fully turbulent region. The proposed model is applied to calculate three benchmark cases of the transitional boundary layer flows with different free-stream turbulent intensity (1%∼6%) under zero-pressure gradient. It was found that the profiles of mean velocity and turbulent intensity, local maximum of velocity fluctuations, their locations as well as the stream-wise variation of integral properties such as skin friction, shape factor and maximum velocity fluctuations are very satisfactorily predicted throughout the flow regions.

• 한국신재생에너지학회:학술대회논문집
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• 한국신재생에너지학회 2009년도 추계학술대회 논문집
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• pp.478-481
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• 2009

As a first step for aerodynamic analysis of vertical axis wind turbine, dynamic stall characteristics of airfoil was investigated. Dynamic stall of wind turbine airfoil is caused by severe variation of angle of attack and relative velocity of flow around airfoil. Angle of attack and relative velocity can be expressed with tip speed ratio. Variation of angle of attack is strongly dependent on the tip speed ratio. For tip speed ratio, 1.4 and free stream velocity, 15m/s, dynamic stall characteristics of wind turbine airfoil is compared with those of oscillating airfoil having same angle of attack variation.

• 대한조선학회논문집
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• 제53권3호
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• pp.210-216
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• 2016

Direct numerical simulations of a spatially developing turbulent boundary layer on a flat plate have been performed to verify the applicability of OpenFOAM and adapted mesh with prism layers to turbulent numerical simulation with high fidelity as well as provide a guideline on numerical schemes and parameters of OpenFOAM. Reynolds number based on a momentum thickness at inlet and a free-stream velocity was Re_θ=300. Time dependent inflow fields with near-wall turbulent structures were generated by a method of Lund et al. (1998), which was to extract instantaneous velocity fields from an auxiliary simulation with rescaled and recycled velocities at inlet. To ascertain the statistical characteristics of turbulent boundary layer, the mean profiles of streamwise velocity and turbulent intensities obtained from structured and adapted meshes were compared with the previous data.

• 한국수자원학회논문집
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• 제56권spc1호
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• pp.1037-1048
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• 2023

하천에 설치된 댐, 보 등과 같은 횡단구조물은 수생태계에 영향을 미치며 종적 연결성을 단절하기 때문에 어류의 이동을 확보하기 위하여 어도가 설치된다. 그러나 어도 내부의 흐름 특성 및 어종에 따라 어류의 통과효율의 차이가 발생한다. 본 연구에서는 3차원 RANS 모형과 자유수면 해석을 위한 VOF (volume of fluid)기법을 적용한 수치모형을 활용하여 계단식 어도에서 발생하는 난류흐름을 수치모의 하였다. 계단식 어도의 pool의 길이 변화 및 상류 수위 변동을 고려하였으며 이들의 변화에 따라 평균유속 및 난류운동에너지 분포를 분석하였다. 표면류 및 잠입류 특성을 잘 재현하였으며 pool의 길이가 증가하면서 표면류에서 잠입류로 변화하였고 상류 수위가 증가함에 따라 표면류 특성이 명확하게 나타났다. 어도 내 어류의 이동과 관련된 수리학적 인자는 유속 및 난류운동에너지 등이 있으며 이를 바탕으로 어류의 소상 가능성을 검토하였다.

• Journal of Mechanical Science and Technology
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• 제16권5호
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• pp.720-731
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• 2002

Flow with moving free surfaces is analyzed with an the Eulerian coordinate system. This study proposes a semi-implicit filling algorithm using VOF in which the PLIC (Piecewise Linear Interface Calculation) -type interface reconstruction method and the donor-acceptor-type front advancing scheme are adopted. Also, a new scheme using extrapolation of the stream function is proposed to find the velocity of the node that newly enters the computational domain. The effect of wall boundary conditions on the flow field and temperature field is examined by numerically solving a two-dimensional casting process.

• 한국해양공학회지
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• 제23권5호
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• pp.1-8
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• 2009

This paper is a continuation of the recent development on Hermite-based divergence free element method and deals with a non-isothermal fluid flow thru the buoyancy driven flow in a square enclosure with temperature difference across the two sides. The basis functions for the velocity field consist of the Hermite function and its curl while the basis functions for the temperature field consists of the Hermite function and its gradients. Hence, the number of degrees of freedom at a node becomes 6, which are the stream function, two velocities, the temperature and its x and y derivatives. This paper presents numerical results for Ra = 105, and compares with those from a stabilized finite element method developed by Illinca et al. (2000). The comparison has been done on 32 by 32 uniform elements and the degree of approximation of elements used for the stabilized finite element are linear (Deg. 1) and quadratic (Deg. 2). The numerical results from both methods show well agreements with those of De vahl Davi (1983).