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

• 한국습지학회지
• /
• 제19권2호
• /
• pp.202-210
• /
• 2017

전단응력은 여러 분야에서 사용하는 매우 중요한 역학 인자 중 하나이며, 인공수로의 설계를 위해서 중요하다. 현재 전단응력은 과거에 정해진 계산법을 사용하고 있지만, 사용되는 식에서 바닥전단응력과 에너지경사와 같이 실제로 측정하거나 계산하기 어려운 요소들이 존재한다. 특히, 에너지경사는 산정하기 매우 어려운 인자이며, 전단응력분포를 구하기위해서는 에너지경사가 있어야만 산정할 수 있지만, 경계층의 유속기울기와 유속을 측정하는 것은 현실적으로 어려운 부분이다. 또한 전단응력분포 중 바닥전단응력은 직접 측정하기 매우 어렵고, 유속에 비해 연구가 다소 더딘 실정이다. 전단응력분포를 정확하게 산정할 수 있다면, 바닥전단응력과 에너지경사를 손쉽게 산정할 수 있다. 본 연구에서는 에너지경사를 반영하지 않고 엔트로피 M을 이용하여 평균유속과 전단응력분포를 간단히 산정하는 연구를 진행하였고, 적용한 식의 효용성을 증명하기 위해 기존의 실험실 실측 자료를 사용하였다. 이는 그래프를 통해 응력분포를 나타내어 비교분석을 하였으며, 등류와 부등류에서 각각 결정계수는 0.930-0.998까지로 거의 일치하였다.

• 대한기계학회논문집
• /
• 제13권4호
• /
• pp.789-798
• /
• 1989

본 연구에서는 평균속도구배를 용이하게 변화시킬 수 있는 전단유동 발생기를 제작하였다. 최고속도구배는 38se $c^{-1}$까지 얻을 수 있으며 최대중심선 속도는 15m/sec까지이다. 10개의 채널(두께 2mm인 알루미늄판)로 유동단면을 등분하 였으며, 각 채널의 내부저항조절 방법으로 마름모형태의 막대인 knob을 설치하였다.

• 한국소음진동공학회:학술대회논문집
• /
• 한국소음진동공학회 2002년도 추계학술대회논문초록집
• /
• pp.365.2-365
• /
• 2002

The flow field around a symmetrical airfoil in a uniform flow under the generation of noise was studied by experiments and numerical simulation. The experiments are conducted by visualizing the surface flow over the airfoil with a shear-sensitive liquid-crystal coating and by measuring the instantaneous velocity field around the trailing edge of the airfoil. The numerical simulation was carried out by LES. (omitted)

• 대한기계학회:학술대회논문집
• /
• 대한기계학회 2008년도 추계학술대회A
• /
• pp.1404-1408
• /
• 2008

The migration and proliferation of vascular endothelial cells (VEC), which play an important role in vascular remodeling, are known to be regulated by hemodynamic forces in the blood vessels. When shear stresses of 2, 6, 15 dynes/$cm^2$ are applied on mouse micro-VEC in vitro, cells surprisingly migrate against the flow direction at all conditions. While higher flow rate imposes more resistance against the cells, reducing their migration speed, the horizontal component of the velocity parallel to the flow increases with the flow rate, indicating the higher alignment of cells in the direction parallel to the flow at a higher shear stress. In addition, cells exhibit substrate stiffness and calcium dependent migration behavior, which can be explained by polarized remodeling in the mechanosensitive pathway under shear stress.

• Korea-Australia Rheology Journal
• /
• 제15권1호
• /
• pp.35-41
• /
• 2003

It is expected that particle size distribution of any portion obtained through screening, is of more uniform than that of the original mixture, typically following such as log-normal, Rosin-Rammler distributions and so on. In this study, therefore, a new relation between parameters of the uniform distribution and flow characteristics of the coarse particle suspensions is derived based on the continuous polydisperse model (Ookawara and Ogawa, 2002b), which is derived from the discrete polydisperse model (Ookawara and Ogawa,2002a). The derived model equation predicts a linear increase of viscosity with shear rate, viz., dilutant flow characteristics. Further, the increase of viscosity is expected to be proportional to the square of volume fraction of particles, and to show the linear dependency on density and average diameter of particles. It is also shown that the uniform distribution model includes additional term that expresses the effect of distribution width. For verification of the model, the experimental results of Clarke (1967) are cited as well as in our previous work for the monodisperse model (Ookawara and Ogawa,2000) since most parameters were varied independently in his work. It is suggested that the newly introduced term expands the applicable range compared with the monodisperse model.

• 한국항공우주학회지
• /
• 제44권5호
• /
• pp.399-406
• /
• 2016

대형 풍력터빈은 지상 전단 흐름 내에서 회전하면서 주기적인 유입속도의 변동 조건 하에 운용된다. 수직 전단흐름에 의해서 경계층 내의 유입 속도는 최고점에서 속도가 최대가 되고 최저점에서 속도가 최소가 된다. 이러한 공간적인 풍속 분포는 풍력터빈 로터의 허브와 저속회전축에서 6분력 하중에 대한 주기적인 진동을 야기한다. 본 연구에서는 수직 전단 흐름 효과를 무시한 균일 흐름장과 지상 전단 흐름효과를 고려한 두 가지 경우에 대한 공력 하중을 비교분석하였다. 계산 결과로부터 허브에서의 추력과 굽힘모멘트, LSS의 굽힘모멘트가 크게 변동하는 결과를 보여주었다. 따라서 지상 전단흐름 효과를 반영한 공력 해석이 피로 해석을 위해서 반드시 필요함을 확인하였다.

• 한국전산유체공학회:학술대회논문집
• /
• 한국전산유체공학회 2009년 추계학술대회논문집
• /
• pp.88-92
• /
• 2009

When exposed to uniform magnetic fields externally applied, paramagnetic particles acquire dipole moments and the induced moments interacting with each other lead to the formation of chainlike structures or clusters of particles aligned with the field direction. A direct simulation method, based on the Maxwell stress tensor and a fictitious domain method, is applied to solve flows with magnetic chains in simple shear flow. We assumed that the particles constituting the chains are paramagnetic, and inertia of both flow and magnetic particles is negligible. The numerical scheme enables us to take into account both hydrodynamic and magnetic interactions between particles in a fully coupled manner, enabling us to numerically visualize breakup and reformation of the chains by the combined effect of the external field and the shear flow. Simple shear flow with suspended magnetic chains is solved in a periodic domain for a given magnetic field. Dynamics of interacting magnetic chains is found to be significantly affected by a dimensionless parameter called the Mason number, the ratio of the viscous force to the magnetic force in the shear flow. The effect of particle area fraction on the chain dynamics is investigated as well.

• 한국전산유체공학회지
• /
• 제12권4호
• /
• pp.20-27
• /
• 2007

Two-dimensional stagnation flow toward a plane wall coated with magnetic fluid of uniform thickness is investigated. The flow field is represented as a similarity solution of the Navier-Stokes equation for this incompressible laminar flow. The resulting third order ordinary differential equation is solved numerically by using the shooting method and by determining two shooting parameters so as to satisfy the boundary and interface conditions. Features of the flow including streamline patterns are investigated for the varying values of density ratio, viscosity ratio, and Reynolds number. An adverse flow with double eddy pair in magnetic fluid region is found to emerge as the Reynolds number becomes higher than a threshold value. The results for the interface velocity, interface and wall shear stress, and boundary layer and displacement thickness are also presented.

• Advances in Computational Design
• /
• 제5권2호
• /
• pp.111-125
• /
• 2020

The flow behavior of polymer melts within a slit die is an important consideration when designing a die geometry. The quality of the extruded polymer product can be determined through an evaluation of the flow homogeneity, wall shear rate and pressure drop across the central height of the die. However, mathematical formulations cannot fully determine the behavior of the flow due to the complex nature of fluid dynamics and the nonlinear physical properties of the polymer melts. This paper examines two slit die geometries in terms of outlet velocity uniformity, shear rate uniformity at the walls and pressure drop by using the licensed computational fluid dynamics package, Ansys POLYFLOW, based on the finite element method. The Carreau-Yasuda viscosity model was used for the rheological properties of the polypropylene. Comparative analysis of the simulation results will conclude that the modified die design performs better in all three aspects providing uniform exit velocity, uniform wall shear rates, and lower pressure drop.