• 한국자동차공학회논문집
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• 제4권3호
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• pp.1-9
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• 1996

This study investigated torque characteristics for Couette flow experimentally under circumstaces that ferrofluids were between two coaxial spheres. Torque measurement was obtained for the situation where the inner sphere was rotating while the outer sphere was kept stationary. The magnetic field was imposed on the fluid, using a bar magnet which was inserted in the inner sphere. In the laminar flow region the torque increase when the magnetic field is applied and the critical Reynolds number is increased. However, in the transition regime, the effect of the magnetic field on the torque characteristics decrease as Reynolds number increases. The value of torque were the same as those of glycerine solution beyond the cirtical Reynolds number. We also made experimental equation which could obtain coefficient of torque within critical Reynolds number in terms of sphere spacing Reynolds number and magnetic properties of ferrofluid.

• 대한기계학회논문집B
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• 제35권9호
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• pp.891-896
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• 2011

본 연구에서는 사각 마이크로 채널의 열전달 특성을 연구하기 위한 실험을 수행하였다. 실험에 사용된 시료의 채널 수력직경은 $700{\mu}m$이며, 채널의 개수는 20개이다. 작동유체는 물이며, 작동유체의 입구 온도는 $20^{\circ}C$ 이다. 실험 변수는 Reynolds 수 400 ~ 800 및 열 유속 35 ~ 85 kW/$m^2$ 이다. 결과로, Reynolds 수가 큰 경우일수록 대류 열전달 계수가 증가하는 것으로 나타났으며, 열적으로 완전히 발달 된 영역에 대하여 대류 열전달 계수는 약 4.6 ~ 6.4 kW/$m^2^{\circ}C$로 나타났다. 또한, 사각 마이크로 채널에서의 열적 입구길이는 Reynolds 수가 커지는 경우일수록 길어지는 것을 알 수 있었으나, 열 유속의 변화는 입구길이에 영향을 미치지 않는 것으로 나타났다. 본 연구의 결과로 완전히 발달된 유동영역에 대하여 사각 마이크로 채널의 열적 특성을 나타내기 위한 Nusselt 수 상관식을 제안하였다.

• 대한기계학회논문집B
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• 제26권1호
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• pp.27-38
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• 2002

The effects of Reynolds number on the non-nulling calibrations of a cone-type ave-type probe in low-speed flows have been investigated at Reynolds numbers of 2.04$\times$10$^3$, 4.09$\times$10$^3$and 6.13$\times$10$^3$. The calibration is conducted at the pitch and yaw angles in ranges between -35 degrees and 35 degrees with an angle interval of 5 degrees. In addition to the calibration coefficients, reduced pitch and yaw angles, static and total pressures, and velocity magnitude are obtained through a typical non-nulling reduction procedure. The result shows that each calibration coefficient, in general, is a function of both the pitch and yaw angles, so that the pre-existing calibration data in a nulling mode are not enough in accounting far the full non-nulling calibration characteristics. Due to interference of the probe stem, the calibration coefficient are more sensitive to Reynolds number at positive pitch angles than at negative ones. The calibration data reduced in this study may serve as a guide line in the estimation of uncertainty intervals resulted from the Reynolds number effects at low Reynolds numbers.

• 한국항공우주학회지
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• 제31권8호
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• pp.8-18
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• 2003

진동하는 에어포일의 근접후류에서의 레이놀즈수 효과를 조사하기 위한 실험적 연구가 수행되었다. NACA 4412에어포일은 1/4 시위 지점을 중심으로 조화적으로 피칭운동을 하고, 순간받음각은 +6$^{\circ}$에서 -6$^{\circ}$까지 진동되도록 하였다. 진동하는 에어포일의 근접후류에서의 난류강도를 측정하기 위하여 열선풍속계를 사용하였다. 본 연구에서 자유류의 속도는 3.4, 12.4, 26.2 m/s이다. 이러한 자유류 속도에 따른 시위 레이놀즈수는 $R_N=5.3{\times}10^4$, $1.9{\times}10^5$, $4.1{\times}10^5$이고, 무차원 진동수는 K=0.1이다. 레이놀즈수가 진동하는 에어포일의 근접후류에 미치는 영향을 나타내는 축방향 난류강도 분포를 제시하였다. 본 측정에서 모든 경우에 난류 강도는 $R_N=5.3{\times}10^4$인 경우에 아주 크고, $R_N=1.9{\times}10^5$과 $4.1{\times}10^5$인 경우에는 작다는 것을 관찰할 수 있었다. 진동하는 에어포일의 근접후류에서 레이놀즈수의 임계값은 층류분리인 경우, 분리가 발생하지 않거나 난류분리인 경우로 구분되며, 그 값은 $R_N=5.3{\times}10^4\;{\sim}\;1.9{\times}10^5$사이에 존재한다.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2009년 추계학술대회논문집
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• pp.3-7
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• 2009

In this study, two and three dimensional low Reynolds number flows are compared. For the two dimensional flow, an airfoil was considered and for the three dimensional low wing and full-body aircraft were considered. Because a flight condition of the aircraft is in a low Reynolds number flow, itl requires reflecting flow transition. In the two dimensional analysis, transition is predicted using en method. In the three dimensional flow, the effect of transition is included using k-w SST turbulence models.

• 한국전산유체공학회지
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• 제10권4호통권31호
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• pp.18-23
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• 2005

Numerical simulations of two-dimensional steady incompressible lid-driven flow in a square cavity are presented by a new solution code(PowerCFD) which adopts an unstructured cell-centered method. Solutions are obtained for configurations with a Reynolds number as high as 10,000 with both rectangular and hybrid types of unstructured grid mesh in order to validate the code's independency of grid type. Interesting features of the flow are presented in detail and comparisons are made with benchmark solutions found in the literature. It is found that the code is capable of producing accurately the nature of the lid-driven cavity flow at high Reynolds numbers with no grid type dependency.

• 한국전산유체공학회지
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• 제18권2호
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• pp.1-8
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• 2013

In nuclear power plant, the reactor cooling system has maintained high-Reynolds-number flow above 1E+07 to cool a heat generated by the reactor. To minimize uncertainty for flow calibration, it is necessary to simulate the high Reynolds' number flow. Y-connection is selected to connect four (4) parallel high flow circulation pumps for maintaining the high flow rate. This paper describes the characteristics for Y-connection by computer flow simulation. It was confirmed through the results that the pressure loss of the Y-connection was lower than that of T-connection. Also as the connection angle of Y-connection was small, as the pressure loss was low.

• 한국전산유체공학회지
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• 제18권3호
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• pp.34-41
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• 2013

We develop a novel immersed boundary (IB) method based on implicit direct forcing scheme for incompressible flows. The proposed IB method is based on an iterative procedure for calculating the direct forcing coupled with the momentum equations in order to satisfy no-slip boundary conditions on IB surfaces. We perform simulations of two-dimensional flows over a circular cylinder for low and moderate Reynolds numbers. The present method shows that the errors for estimated velocities on IB surfaces are significantly reduced even for low Reynolds number with a fairly large time step while the previous methods based on direct forcing failed to provide no-slip boundary conditions on IB surfaces.

• 한국가시화정보학회지
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• 제4권2호
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• pp.51-58
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• 2006

A boundary layer visualization was carried out in order to investigate the influence of Reynolds number on an oscillating airfoil. An NACA 0012 airfoil is sinusoidally pitched at the quarter chord point with oscillation amplitude of ${\pm}6^{\circ}$. A smoke-wire technique was employed to visualize the boundary layer and the near-wake. The freestream velocities are 1.98, 2.83 and 4.03m/s and corresponding chord Reynolds numbers are $2.3{\times}10^4,\;3.3{\times}10^4$, and $4.8{\times}10^4$, respectively. As the reduced frequency of K=0.1 is fixed, the corresponding frequency of an airfoil was adjusted in each case. The results reveal that the point at which the shear stress in an unsteady boundary layer separation disappears does not correspond with the position of the breakdown of the boundary layer, and that the breakdown of the boundary layer occurs further downstream.

• Journal of Advanced Marine Engineering and Technology
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• 제20권3호
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• pp.101-109
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• 1996

Two-dimensional lid-driven closed flows within square cavity were studied numerically for four Reynolds numbers : $10^4$, 3$\times10^4$, 5$\times10^4$ and 7.5$\times10^4$. A convective difference scheme to maintain the same spatial accurary by irregular grid correction is adopted by applying the interior division principle. Grid number is $80\times80$and its minimum size is about 1/400 of the cavity height. At Re=$10^4$, periodic migration of small eddies appearing in corner separation region and its temporal sinusoidal fluctuation are represented. At three higher Reynolds numbers(3$\times10^4$, 5$\times10^4$ and 7.5$\times10^4$), an organizing structure of four consecutive vorticles at two lower corners is revealed from time-mean flow patterns. But, instantaneous flow characteristics show very random unsteady fluctuation mainly due to the interaction between rotating shed vortices and stationary eddies within the corners.