• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 춘계학술대회논문집E
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• pp.253-258
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• 2001

Control of drag force on a circular cylinder using a detached splitter plate is numerically studied for laminar flow. A splitter plate with the same length as the cylinder diameter(d) is placed horizontally in the wake region. Its position is described by the gap ratio(G/d), where G represents the gap between the cylinder base point and the leading edge of the plate. The drag varies with the gap ratio; it has the minimum value at a certain gap ratio for each Reynolds number. The drag sharply increases past the optimum gap ratio; this seems to be related to the sudden change in the bubble size in the wake region. This trend is consistent with the experimental observation currently available in case of turbulent flow. It is also found that the net drag coefficient significantly depends on the variation of base suction coefficient.

• 대한기계학회논문집B
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• 제38권10호
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• pp.797-805
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• 2014

본 연구의 목적은 해수 흡입구를 고려한 초공동 수중운동체 캐비테이터의 항력과 양력특성 및 해수 흡입유로의 입구에서 압력손실에 대해 예측하는 것이다. 흡입구 직경과 유로에서의 속도, 흡입구의 곡률반경 및 캐비테이터의 받음각이 미치는 영향에 대해 유동해석을 수행하였다. 연구 결과 직경비가 커지면, 항력계수와 압력손실계수가 감소하며, 속도비가 증가할 때 항력계수와 양력계수는 감소하고 압력손실계수는 증가한다. 해수 흡입구에 곡률을 주면 항력계수와 양력계수에는 영향을 미치지 않지만, 압력손실계수가 크게 감소한다. 캐비테이터의 받음각은 항력계수와 압력손실계수에 미소한 영향만을 주나, 양력계수를 크게 변화시킨다. 초공동 수중운동체 설계 시 본 연구 결과를 반영할 수 있다.

• Wind and Structures
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• 제34권1호
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• pp.137-149
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• 2022

In this paper, the fluctuating lift and drag forces on 5:1 rectangular cylinders with two different geometric scales in three turbulent flow-fields are investigated. The study is particularly focused on understanding the influence of the ratio of turbulence integral length scale to structure characteristic dimension (the length scale ratio). The results show that both fluctuating lift and drag forces are influenced by the length scale ratio. For the model with the larger length scale ratio, the corresponding fluctuating force coefficient is larger, while the spanwise correlation is weaker. However, the degree of influence of the length scale ratio on the two fluctuating forces are different. Compared to the fluctuating drag, the fluctuating lift is more sensitive to the variation of the length scale ratio. It is also found through spectral analysis that for the fluctuating lift, the change of length scale ratio mainly leads to the variation in the low frequency part of the loading, while the fluctuating drag generally follows the quasi-steady theory in the low frequency, and the slope of the drag spectrum at high frequencies changes with the length scale ratio. Then based on the experimental data, two empirical formulas considering the influence of length scale ratio are proposed for determining the lift and drag aerodynamic admittances of a 5:1 rectangular cylinder. Furthermore, a simple relationship is established to correlate the turbulence parameter with the fluctuating force coefficient, which could be used to predict the fluctuating force on a 5:1 rectangular cylinder under different parameter conditions.

• Journal of Mechanical Science and Technology
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• 제15권10호
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• pp.1434-1441
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• 2001

A computational study has been performed to determine the effects of divergent trailing edge (DTE) modification to a supercritical airfoil in transonic flow field. For this, the computational result with the original DLBA 186 supercritical airfoil was compared to that of the modified DLBA 283. A wavier-Stokes code, Fluent 5. 1, was used with Spalart-Allmaras's one-equation turbulence model. Results in this study showed that the reduction in drag due to the DTE modification is associated with weakened shock and delayed shock appearance. The decrease in drag due to the DTE modification is greater than the increase in base drag. The effect of the recirculating flow region on lift increase was also observed. An airfoil with DTE modification achieved the same lift coefficient at a lower angle of attack while giving a lower drag coefficient. Thus, the lift-to-drag ratio increases in transonic flow conditions compared to the original airfoil. The lift coefficient increases considerably whereas the lift slope increases just a little due to DTE modification.

• 대한기계학회논문집B
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• 제25권11호
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• pp.1632-1639
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• 2001

Control of drag farce on a circular cylinder using a detached splitter plate is numerically studied for laminar flow. A splitter plate with the same length as the cylinder diameter(d) is placed horizontally in the wake region. Its position is described by the gap ratio(G/d), where G represents the gap between the cylinder base point and the leading edge of the plate. The drag varies with the gap ratio; it has the minimum value at a certain gap ratio for each Reynolds number. The drag sharply increases past the optimum gap ratio; this seems to be related to the sudden change in bubble size in the wake region. This trend is consistent with the experimental observation currently available in case of turbulent flow. It is also found that the net drag coefficient significantly depends on the variation of base suction coefficient.

• 대한기계학회논문집B
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• 제31권8호
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• pp.688-693
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• 2007

To improve aerodynamic efficiency of the Smart Un-manned Aerial Vehicle(SUAV), vortex generator was applied along the wing upper surface during SUAV tests. Vortex generator, initially used in TR-S2 configuration to enhance lift characteristic, increased lift coefficient. Meanwhile vortex generator produced excessive drag and eventually reduced lift-to-drag ratio. To examine the effect of vortex generator's height, three different heights of vortex generator were used for various SUAV configuration. Vortex generator of 3mm height used in TR-S4 configuration produced 3.1% increase in maximum lift coefficient and 1.5% reduction in lift-to-drag ratio.

• International Journal of Naval Architecture and Ocean Engineering
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• 제6권4호
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• pp.965-980
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• 2014

This paper presents a new estimation method of full scale propulsive performance for the pulling type podded propeller. In order to estimate the drag of pod housing, a drag velocity ratio, which includes the effects of podded propeller loading and Reynolds number, is presented and evaluated through the comparison of model test and numerical analysis. By separating the thrust of propeller blade and the drag of pod housing, extrapolation method of pod housing drag to full scale is deduced, and correction method of propeller blade thrust and torque to full scale is presented. This study utilized the drag coefficient ratio of the pod housing as a measure for expanding it to full scale, but in order to increase the accuracy of performance evaluation, additional study is necessary on the method for the full scale expansion via separating the drag of pod body, strut and fin which consist the pod housing.

• 대한기계학회논문집B
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• 제30권3호
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• pp.278-286
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• 2006

Experiments were conducted to estimate the performance of drag force type vertical axis wind turbine with an opening-shutting rotor. It was operated by the difference in drag force generated on both sides of the blades. The rotational speed was measured by a tachometer in a wind tunnel and the tunnel wind speed was measured by using a pilot-static tube and a micro manometer. The performance test for a prototype was accomplished by calculating power, power coefficient, torque coefficient from the measurement of torque and rpm by a dynamometer controller. Various design parameters, such as the number of blades(B), blade aspect ratio(W/R), angle of blades$(\alpha)$ and drag coefficient acting on a blade, were considered for optimal conditions. At the experiment of miniature model, maximum efficiency was found at N=15, $\alpha=60^{\circ}$ and W/R=0.32. The measured test variables were power, torque, rotational speed, and wind speeds. The data presented are in the form of power and torque coefficients as a function of tip-speed ratio V/U. Maximum power was found in case of $\Omega=0.33$, when the power and torque coefficient were 0.14 and 0.37 respectively. Comparing model test with prototype test, similarity law by advance ratio for vertical axis wind turbine was confirmed.

• 대한기계학회논문집B
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• 제20권1호
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• pp.369-376
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• 1996

Variations of the drag and lift forces of a circular cylinder in a planar turbulent jet were experimentally investigated. The force was directly measured using the load cell and estimated by integrating the pressure distribution on the cylinder. As the cylinder moves outward from the center of the jet, the direction of lift force changes and the drag force decreases. Reynolds number, the ratio of cylinder's diameter to half width of jet had effect on maximum drag coefficient and the location where the direction of lift changes.

• 항공우주기술
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• 제8권1호
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• pp.197-206
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• 2009

스마트 무인기의 공력특성을 향상시키기 위하여 주익에는 와류생성기(vortex generator), 주익의 끝단에는 유동펜스(flow fence)를 적용하였다. 와류생성기는 SUAV의 최대양력계수와 실속각을 지연시키는 효과가 있었지만 높은 항력증가를 초래하여, 결국에는 양항비가 줄어들었다. 이를 개선하기 위하여 L-형태와 높이가 3mm와 5mm인 와류생성기를 적용하였다. 유동펜스는 나셀 틸팅각이 증가함에 따라 나셀에서 발생하는 박리에 의하여 주익성능이 감소하는 현상을 방지하기 위하여 사용하였다. 두 가지 유동제어 장치를 사용함에 따라 스마트 무인기의 공력특성들이 어떻게 변화하였는지를 정리하였다.