• Ocean Systems Engineering
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• 제3권3호
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• pp.167-180
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• 2013

Hydrodynamic characteristics of a bluff cylinder oscillating along transverse direction in steady flow were experimentally investigated at Reynolds number of $2{\times}10^5$. The effects of non-dimensional frequency, oscillating amplitude and Reynolds number on drag force, lift force and phase angle are studied. Vortex shedding mechanics is applied to explain the experimental results. The results show that explicit similarities exist for hydrodynamic characteristics of an oscillating cylinder in high and low Reynolds number within subcritical regime. Consequently, it is reasonable to utilize the test data at low Reynolds number to predict vortex induced vibration of risers in real sea state when the Reynolds numbers are in the same regime.

• 소음진동
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• 제7권5호
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• pp.861-869
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• 1997

An approximate analytical method has been developed for estimating hydrodynamic forces acting on oscillating inner cylinder in concentric annulus. When the rigid inner cylinder executes translational oscillation, fluid inertia and damping forces on the oscillating cylinder are generated by unsteady pressure and viscous skin friction. Considering the dynamic-characteristics of unsteady viscous flow and the added mass coefficient of inviscid fluid, these hydrodynamic forces including viscous effect are dramatically simplified and expressed in terms of oscillatory Reynolds number and the geometry of annular configuration. Thus, the viscous effect on the forces can be estimated very easily compared to an existing theory. The forces are calculated by two models developed for relatively high and low oscillatory Reynolds numbers. The model for low oscillatory Reynolds number is suitable for relatively high ratio of the penetration depth to annular space while the model for high oscillatory Reynolds number is applicable to the case of relatively low ratio. It is found that the transient ratio between two models is approximately 0.2~0.25 and the forcea are expressed in terms of oscillatory Reynolds number, explicity. The present results show good agreements with an existing numerical results, especially for high and low penetration ratios to annular gap.

• 유체기계공업학회:학술대회논문집
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• 유체기계공업학회 2006년 제4회 한국유체공학학술대회 논문집
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• pp.309-312
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• 2006

Effective mixing is an important problem in microfluidics for chemical and biomechanical applications. In this study, the influences of the Reynolds number and the oscillating frequency on mixing characteristics of micro-stirrer are studied in a microchannel with single stirrer. The influence of fluid inertial effects in an active mixer is first discussed. It is found that the stirring effects by stirrer oscillation are promptly attenuated at low Reynolds number, which makes greatly difficult the rapid mixing. As the inertial effects are increased, the chaotic advection is generated and then developed. The mixing phase is finally developed some mushroom shaped structure. And the mixing efficiency is also studied as a function of the oscillating frequency. We found that the mixing efficiency does not always increase with higher oscillating frequency of stirrer. Consequently, we found the functional relation between the optimal frequency of a stirrer and the Reynolds number.

• 한국초전도저온공학회:학술대회논문집
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• 한국초전도저온공학회 2003년도 학술대회 논문집
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• pp.21-27
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• 2003

This paper proposes a new model of the pressure drop for more accurate description of oscillating flow through regenerator under pulsating pressure conditions in contrast to an existing model based on steady flow. For the universal uses of the oscillating flow model, non-dimensional parameters, which consist of Reynolds number, Valensi number gas domain length ratio, oscillating flow friction factor and phase angle of pressure drop, are derived from the capillary tube model of the regenerator. Two correlation equations of the model are obtained from the experiments for the twill square screen regenerators under various operating frequencies and inlet mass flow rates. The oscillating friction factor is a function of only the Reynolds number and the phase angle of pressure drop is a function of the Valensi number and the gas domain length ratio. Experiment is also performed to examine the effects of the shape of screens.

• 한국초전도ㆍ저온공학회논문지
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• 제5권2호
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• pp.51-57
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• 2003

This paper proposes a new oscillating flow model of the pressure drop through the regenerator under pulsating pressure. In this oscillating flow model. pressure drop is expressed by the amplitude and the phase angle with respect to the inlet mass flow rate. In order to generalize the oscillating flow model. non-dimensional parameters, which are Reynolds number, Valensi number, gas domain length ratio, oscillating flow friction factor and phase angle of pressure drop, are derived from the capillary tube model of the regenerator. Correlations for the oscillating flow friction factor and the phase angle are obtained from the experiments for the twill-square screen regenerators under various operating frequencies and inlet mass flow rates. The oscillating friction factor is a function of the Reynolds number alone and the phase angle of pressure drop is a function of the Valensi number and the gas domain length ratio. Experiment is also performed to examine the effect of the weave style of screen. Experimental data demonstrate the superiority of the oscillating flow model over the previous steady flow model.

• 한국항공우주학회지
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• 제31권7호
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• pp.15-25
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• 2003

진동하는 에어포일의 근접후류 특성을 조사하기 위한 실험적 연구가 수행되었다. NACA 4412에어포일은 1/4 시위 지점을 중심으로 조화적으로 피칭운동을 하고, 순간받음각이 +6$^{\circ}$에서 -6$^{\circ}$까지 진동하도록 하였다. 진동하는 에어포일의 근접후류에서의 평균속도를 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 나타내기 위하여 축방향 위상평균 속도분포를 제시하였다. 본 측정에서 모든 경우에 속도결손은 $R_N$=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 관찰 할 수 있었다. 이와 같이 위상평균속도의 커다란 차이는 $R_N$=5.3${\times}10^4$과 1.9${\times}10^5$ 사이에 있다는 것을 관찰하였다. 따라서 본 연구는 진동하는 에어포일의 근접후류에서의 레이놀즈수의 임계값이 5.3${\times}10^4$에서 1.9${\times}10^5$ 범위에 존재한다는 것을 보여준다.

• 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.

• 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.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2003년도 춘계학술대회
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• pp.1795-1800
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• 2003

An experimental study was carried out to investigate near-wake characteristics of an elliptic airfoil oscillating in pitch. The airfoil was sinusoidally pitched about the half chord point between $-5^{\circ}C$ and $+25^{\circ}C$ angles of attack at the freestream velocities of 3.4 and 23.1 m/s The corresponding Reynolds numbers based on the chord length were $3.3{\times}10^4$ and $2.2{\times}10^5$, respectively. A hot-wire anemometer was used to measure the near-wake flow variable at the reduced frequency of 0.1. Ensemble-averaged velocity and turbulence intensity profile were presented to examine the near-wake characteristics depending on the Reynolds number. The axial velocity deficit in the near-wake region tend to decrease with the increase in the Reynolds number a found in many stationary airfoil test . Turbulence intensity in the near-wake region have a tendency to decrease with the increase in the Reynolds number during the pitch-up motion, whereas it shows different feature during the pitch-down motion either the laminar boundary layer or turbulent boundary layer separation.

• 대한기계학회논문집B
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• 제27권3호
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• pp.334-346
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• 2003

An experimental study was carried out to investigate near-wake characteristics of an elliptic airfoil oscillating in pitch. The airfoil was sinusoidally pitched about the half chord point between -5$^{\circ}$and +25$^{\circ}$angles of attack at the freestream velocities of 3.4 and 23.1 m/s. The corresponding Reynolds numbers based on the chord length were 3.3$\times$10$_{4}$ and 2.2$\times$10$^{5}$ , respectively. A hot-wire anemometer was used to measure the near-wake flow variables at the reduced frequency of 0.1. Ensemble-averaged velocity and turbulence intensity profiles were presented to examine the near-wake characteristics depending on the Reynolds number. The axial velocity deficit in the near-wake region tends to decrease with the increase in the Reynolds number as found in many stationary airfoil tests. Turbulence intensity in the near-wake region have a tendency to decrease with the -increase in the Reynolds number during the pitch-up motion, whereas it shows different feature during the pitch-down motion according to the separation characteristics.