• Wind and Structures
• /
• 제34권1호
• /
• pp.115-125
• /
• 2022

The high-Reynolds number flow around a rectangular cylinder, having streamwise to crossflow length ratio equal to 5 is analyzed in the present paper. The flow is characterized by shear-layer separation from the upstream edges. Vortical structures of different size form from the roll-up of these shear layers, move downstream and interact with the classical vortex shedding further downstream in the wake. The corresponding mean flow is characterized by a recirculation region along the lateral surface of the cylinder, ending by mean flow reattachment close to the trailing edge. The mean flow features on the cylinder side have been shown to be highly sensitive to set-up parameters both in numerical simulations and in experiments. The results of 21 Large Eddy Simulations (LES) are analyzed herein to highlight the impact of the lateral mean recirculation characteristics on the near-wake flow features and on some bulk quantities. The considered simulations have been carried out at Reynolds number Re=DU_∞/ν=40 000, being D the crossflow dimension, U_∞ the freestream velocity and ν the kinematic viscosity of air; the flow is set to have zero angle of attack. Some simulations are carried out with sharp edges (Mariotti et al. 2017), others with different values of the rounding of the upstream edges (Rocchio et al. 2020) and an additional LES is carried out to match the value of the roundness of the upstream edges in the experiments in Pasqualetto et al. (2022). The dimensions of the mean recirculation zone vary considerably in these simulations, allowing us to single out meaningful trends. The streamwise length of the lateral mean recirculation and the streamwise distance from the upstream edge of its center are the parameters controlling the considered quantities. The wake width increases linearly with these parameters, while the vortex-shedding non-dimensional frequency shows a linear decrease. The drag coefficient also linearly decreases with increasing the recirculation length and this is due to a reduction of the suctions on the base. However, the overall variation of C_D is small. Finally, a significant, and once again linear, increase of the fluctuations of the lift coefficient is found for increasing the mean recirculation streamwise length.

• 한국수소및신에너지학회논문집
• /
• 제29권2호
• /
• pp.197-204
• /
• 2018

In the present study, unsteady flow analysis has been conducted to investigate the blade forces and wake flow around a hybrid street-lamp having a vertical-axis small wind turbine and a photovoltaic panel. Uniform velocities of 3, 5 and 7 m/s are applied as inlet boundary condition. Relatively large vortex shedding is formed at the wake region of the photovoltaic panel, which affects the increase of blade torque and wake flow downstream of the wind turbine. It is found that blade force has a good relation to the variation of the angle of attack with the rotation of turbine blades. Variations in the torque on the turbine blade over time create a cyclic fluctuation, which can be a source of turbine vibration and noise. Unsteady fluctuation of blade forces is also analyzed to understand the nature of the vibration of a small wind turbine over time. The detailed flow field inside the turbine blades is analyzed and discussed.

• 대한조선학회논문집
• /
• 제36권4호
• /
• pp.21-27
• /
• 1999

본 논문에서는 교란 포텐셜을 기저로 하는 패널방법을 사용하여 물제트를 분사하는 플랩을 가진 타의 성능해석을 수행하였다. 받음각이 큰 경우, 성능해석의 주요 인자가 되는 후류면의 위치를 구하기 위하여 후류면의 roll-up을 포함시킨 Pyo[1]의 방법을 사용하여 계산하였고, 경계면은 쌍곡면 패널로 이산화하였다. 플랩타의 본체와 플랩 사이의 간극유동을 점성유동인 Couette유동으로 가정하여 계산하였고, 간극 사이에서 분사되는 물제트는 유량변화를 소스로, 모멘텀변화를 떨어져 나가는 보오텍스로 가정하여, 간극의 입구와 출구 패널 위에서 경계조건으로 포함시켜 계산하였다. 본 방법의 수치적 검증을 위해 실험값이 존재하는 경우에 대해 실험값과 프로그램의 수치결과를 비교하였다. 비교 결과, 실험값은 계산값과 좋은 일치를 보임을 확인하였다.

• 항공우주시스템공학회지
• /
• 제16권6호
• /
• pp.114-122
• /
• 2022

본 논문에서는 CFD 기법을 활용하여 전기체 형상의 투척식 무인기 형상에 대해 고속 회전하는 프로펠러와 그로 인해 생성된 후류 간섭 효과를 고려한 비정상 유동해석을 수행하였다. 또한 다양한 투척식 이륙 조건에서 롤 모멘트 평형에 요구되는 에일러론 타각을 정확하게 예측하기 위해 실제 조종면 회전을 고려한 유동해석이 수행되었다. 투척식 소형 무인기의 경우 초기 이륙상태에서 롤 안정성을 증대시키기 위해 적절한 초기 에일러론 설정을 활용하는 것이 유용한 방식이며, 구축된 공력 데이터베이스를 사용하여 다양한 이륙속도와 받음각 조건들에 대해 롤 모멘트를 상쇄시킬 수 있는 에일러론 타각 조건들이 빠르게 예측 가능함을 보였다.

• 대한기계학회논문집
• /
• 제13권1호
• /
• pp.178-189
• /
• 1989

본 연구에서는 우선 승용차의 1/10 모형의 형상을 더욱 단순화시킨 2차원 자동차형 물체를 대상으로 하여 그 주위의 압력, 속도 및 난류량의 분포를 측정하고, 또한 울터프트(wool tuft)를 이용한 유동가시화법으로 후류구조를 파악함으로써 향후 계속될 실험적 및 수치해석적 연구의 기초를 마련하고자 한다.

• 대한조선학회논문집
• /
• 제49권2호
• /
• pp.174-181
• /
• 2012

In this paper, the Flow Control Fin(FCF) optimization has been carried out using computational fluid dynamics(CFD) techniques. This study focused on evaluation for the performance of the FCF attached in the stern part of the ship. The main advantage of FCF is to enhance the resistance performance through the lift generation with a forward force component on the foil section, and the propulsive performance by the uniformity of velocity distribution on the propeller plane. This study intended to evaluate these functions and to find optimized FCF form for minimizing viscous resistance and equalizing wake distribution. Four parameters of FCF are used in the study, which were angle and position of FCF, longitudinal location, transverse location, and span length in the optimization process. KRISO 300K VLCC2(KVLCC2) was chosen for an example ship to demonstrate FCF for optimization. The optimization procedure utilized genetic algorithms (GAs), a gradient-based optimizer for the refinement of the solution, and Non-dominated Sorting GA-II(NSGA-II) for Multiobjective Optimization. The results showed that the optimized FCF could enhance the uniformity of wake distribution at the expense of viscous resistance.

• 대한기계학회논문집B
• /
• 제22권6호
• /
• pp.849-859
• /
• 1998

The heat generated in an induction motor is mostly dissipated through the frame. The study on the heat transfer characteristics of a newly manufactured finless TEFC(Totally Enclosed Fan Cooled) induction motor showed/that it had an unsuitable structure in view of the heat transfer. The angle of the cooling air flow was very large and the ribs disturbed the air flow and partially generated the wake region on the frame. In the wake region the temperature was very high. Thus the heat transfer coefficients were lower than those of the frame with fins. Also was investigated the heat transfer characteristics of the motor frame by installing various guide vanes in the fan-side end cap. An optimum heat transfer case was found and the average heat transfer coefficient of the frame was 70% higher and the average coil temperature measured by the resistance method was 9 deg. C lower than that of the frame which had no guide vanes.

• Journal of Advanced Marine Engineering and Technology
• /
• 제31권7호
• /
• pp.842-851
• /
• 2007

Oscillating foil propulsion, the engineering application of fish-like movement of a hydrofoil, has received in recent decades as a possible competitor for propellers. The oscillating foil produces an effective angle of attack, resulting in a normal force vector with thrust and lift components, and it can be expected to be a new highly effective propulsion system. We have explored propulsion hydrodynamics as a concept in wake flow pattern. The present study has been examined various conditions such as oscillating frequencies and amplitudes in NACA0010 profile. Flow visualizations showed that high thrust was associated with the generation of moderately strong vortices, which subsequently combine with trailing-edge vorticity leading to the formation of a reverse $K\acute{a}rm\acute{a}n$ vortex street. Vortex generation was inherent to jet production and playeda fundamental role in the wake dynamics. And it was shown that the strong thrust coefficient obtained as the Strouhal number was larger.

• Wind and Structures
• /
• 제12권1호
• /
• pp.49-61
• /
• 2009

A method is presented to estimate the form drag and the base pressure on a triangular cylinder in the presence of blockage effect. The Strouhal number, which is found to increase with the flow constriction experimentally by Ramamurthy & Ng (1973), may be decoupled from the blockage effect when re-defined by using the velocity at flow separation and a theoretical wake width. By incorporating this wake width into the momentum equation by Maskell (1963) for the confined flow, a relationship between the form drag and the base pressure is derived. Independently, the experimental data of surface pressure from Ramamurthy & Lee (1973) are found to be independent of the blockage effect when expressed in terms of a modified pressure coefficient involving the pressure at separation. Using the potential flow model by Parkinson & Jandali (1970) and its subsequent development in Yeung & Parkinson (2000) for the unconfined flow, a linear relation between the pressure at separation and the form drag is formulated. By solving the two equations simultaneously with a specified blockage ratio and an apex angle of the triangular cylinder, the predictions of the drag and the base pressure are in reasonable agreement with experimental data. A new theoretical relationship for the Strouhal number, pressure drag coefficient and base pressure proposed in this study allows the confinement effect to be appropriately taken into consideration. The present approach may be extended to three-dimensional bluff bodies.

• International Journal of Fluid Machinery and Systems
• /
• 제8권2호
• /
• pp.73-84
• /
• 2015

Aerating hydroturbines have recently been proposed as an effective way to mitigate the problem of low dissolved oxygen in the discharge of hydroelectric power plants. The design of such a hydroturbine requires a precise understanding of the dependence of the generated bubble size distribution upon the operating conditions (viz. liquid velocity, air ventilation rate, hydrofoil configuration, etc.) and the consequent rise in dissolved oxygen in the downstream water. The purpose of the current research is to investigate the effect of location of air injection on the resulting bubble size distribution, thus leading to a quantitative analysis of aeration statistics and capabilities for two turbine blade hydrofoil designs. The two blade designs differed in their location of air injection. Extensive sets of experiments were conducted by varying the liquid velocity, aeration rate and the hydrofoil angle of attack, to characterize the resulting bubble size distribution. Using a shadow imaging technique to capture the bubble images in the wake and an in-house developed image analysis algorithm, it was found that the hydrofoil with leading edge ventilation produced smaller size bubbles as compared to the hydrofoil being ventilated at the trailing edge.