• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2007년도 춘계 학술대회논문집
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• pp.72-78
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• 2007

To develop the aerodynamic performance, there are widely two group of studies are achieved. The first one is about design of the vehicles geometry and the second one is about aerodynamic devices. Geometry design is highly credible and stable method. But it is not flexible and each parts are related interactively. So if one part geometry are modified, the other parts are required to be redesigned. The other hand, flow control by aerodynamic device is flexible and modulized method. Though it needs energy, relatively little input makes far advanced aerodynamic performance. Synthetic Jet is one of the second group method. The device repeats suction and blowing motion in constant frequency. According to the performance, the flow which are near the flight surface are served momentum. This mechanism can reduce the aerodynamic loss by boundary layer and separated flow. Synthetic jet actuator has several parameters, that influence the flow control. This study focus the parameters effects of the synthetic jet - orifice geometry, frequency, jet speed and etc.

• 대기
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• 제18권4호
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• pp.485-492
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• 2008

This study confirms that a decadal variation of the SST (Sea Surface Temperature) in the WNP (Western North Pacific) has an influence on the genesis and passage ofa Tropical Cyclone. The decadal mode was obtained by calculating the SST anomaly on the domain $150^{\circ}E-190^{\circ}E$, and $5^{\circ}S-5^{\circ}N$. Such decadal variation was subsequently analyzed to confirm that it is a dominant mode in central Pacific region. Next, after classifying the years into relatively positive years and relatively negative years, the characteristics of Tropical Cyclone in each year, such as a genesis and passage frequency, were investigated. Compared to the relatively negative years, during the relatively positive years, the location of Tropical Cyclone genesis was biased toward South-Eastern region, while the characteristics of the cyclone were more distinct during late season of the year trom September to December than in mid season from June to August. Examining the movement passage through the observation of passage fiequency, there was a significant difference between positive year and negative year in their passages at a 90% confidence level. Moreover, the number of Tropical Cyclone, maximum wind, and life time also showed higher values in positive years than in negative years. These features were confirmed by examining the 850hPa cyclonic flow field, vorticity field, and vertical wind shear field, all of which contribute to the genesis of a Tropical Cyclone.

• International Journal of Naval Architecture and Ocean Engineering
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• 제4권4호
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• pp.447-459
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• 2012

The present study numerically investigates the effect of the wavy leading edge on hydrodynamic characteristics for the flow of rectangular wings with the low aspect ratio of 1.5. Five different wave lengths at fixed wavy amplitude have been considered. Numerical simulations are performed at a wide range of the angle of attack ($0^{\circ}{\leq}{\alpha}{\leq}40^{\circ}$) at one Reynolds number of $10^6$. The wavy wings considered in this study did not experience enough lift drop to be defined as the stall, comparing with the smooth wing. However, in the pre-stall region, the wavy wings reveal the considerable loss of the lift, compared to the smooth wing. In the post-stall, the lift coefficients of the smooth wing and the wavy wings are not much different. The pressure coefficient, limiting streamlines and the iso-surface of the spanwise vorticity are also highlighted to examine the effect of the wave length on the flow structures.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 1999년도 춘계 학술대회논문집
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• pp.155-161
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• 1999

The three-dimensional turbulent cascade flows with and without endwall fences are numerically investigated by solving the incompressible Navier-Stokes equations with a high-Reynolds number $k-{\varepsilon}$ turbulence closure model. A projection method based algorithm is used in the finite-volume formulation, with the second order upwind-differencing scheme for the convective terms. First, assessments on accuracy of the present method are made by comparing the static pressure distributions at the mid-span of the cascade with measured data, and also by confirming the experimental observations on the choice of an optimal fence height for the secondary flow control. In understanding the three-dimensional nature of the secondary flow in turbine cascade, the limiting streamline patterns and the static pressure contours at the suction surface of the blade as well as on the cascade endwall are employed to visualize the effectiveness of the endwall fence for the secondary flow control. Analysis on the streamwise vorticity contour maps along the cascade with the three-dimensional representation of their iso-surfaces reveals the strucuture of the complicated vortical flow in the turbine cascade with endwall fence, and also leads to an understanding on formation of the counter-rotating streamwise vortex over the endwall fence, in explaining the mechanisms of controlling the secondary flow and also for the proper selection of an optimal fence height.

• 대한기계학회논문집B
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• 제24권5호
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• pp.733-743
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• 2000

We solve the integral representation of the Navier-Stokes equations in a lagrangian view by tracking the particles, which have vortex strengths. We simulate the unsteady viscous flow around an impulsively started cylinder using the vortex particle method. Particles are advanced via the Biot-Savart law for a lagrangian evolution of particles. The particle strength is modified based on the scheme of particle strength exchange. The solid boundary satisfies the no-slip boundary condition by the vorticity generation algorithm. We newly modify the diffusion scheme and the boundary condition for simulating an unsteady flow efficiently. To save the computation time, we propose the mixed scheme of particle strength exchange and core expansion. We also use a lot of panels to ignore the curvature of the cylinder, and not to solve the evaluation of the surface density. Results are compared to those from other theoretical and experimental works.

• International Journal of Naval Architecture and Ocean Engineering
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• 제9권1호
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• pp.77-99
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• 2017

Offshore oscillating water columns (OWC) represent one of the most promising forms of wave energy converters. The hydrodynamic performance of such converters heavily depends on their interactions with ocean waves; therefore, understanding these interactions is essential. In this paper, a fully nonlinear 2D computational fluid dynamics (CFD) model based on RANS equations and VOF surface capturing scheme is implemented to carry out wave energy balance analyses for an offshore OWC. The numerical model is well validated against published physical measurements including; chamber differential air pressure, chamber water level oscillation and vertical velocity, overall wave energy extraction efficiency, reflected and transmitted waves, velocity and vorticity fields (PIV measurements). Following the successful validation work, an extensive campaign of numerical tests is performed to quantify the relevance of three design parameters, namely incoming wavelength, wave height and turbine damping to the device hydrodynamic performance and wave energy conversion process. All of the three investigated parameters show important effects on the wave-pneumatic energy conversion chain. In addition, the flow field around the chamber's front wall indicates areas of energy losses by stronger vortices generation than the rear wall.

• Wind and Structures
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• 제17권5호
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• pp.515-535
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• 2013

The present study aims to investigate characteristics of the flow structures around the Ahmed body by using both experimental and numerical methods. Therefore, 1/4 scale Ahmed body having $25^{\circ}$ slant angle was employed. The Reynolds number based on the body height, H and the free stream velocity, U was $Re_H=1.48{\times}10^4$. Investigations were conducted in two parts. In the first part of the study, Large Eddy Simulation (LES) method was used to resolve the flow structures around the Ahmed body, numerically. In the second part of the study the particle image velocimetry (PIV) technique was used to measure instantaneous velocity fields around the Ahmed body. Time-averaged and instantaneous velocity vectors maps, streamline topology and vorticity contours of the flow fields were presented and discussed in details. Comparison of the mean and turbulent quantities of the LES results and the PIV results with the results of Lienhart et al. (2000) at different locations over the slanted surface and in the wake region of the Ahmed body were also given. Flow features such as critical points and recirculation zones in the wake region downstream of the Ahmed body were well captured. The spectra of numerically and experimentally obtained stream-wise and vertical velocity fluctuations were presented and they show good consistency with the numerical result of Minguez et al. (2008).

• 대한조선학회논문집
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• 제30권3호
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• pp.62-74
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• 1993

균일한 정상유동을 하는 유체 안에 가로놓인 원주로부터 일어나는 vortex유출을 이산 vortex 방법을 써서 vortex cloud모형으로 simulation했다. 원주표면에서 생겨나는 와도를 매 시간 간격마다 미리 정해진 많은 갯수의 이산화된 초생 vortex로 나타낸 후 기 유출된 vortex들에 합류시켜 누적된 vortex들의 운동을 탐색함으로서 vortex 분포 상태의 진화를 알아낸다. vortex들의 이동은 cloud-in-cell 기법을 써서 추적 하였고 항력 계수와 양력 계수는 Sarpkaya의 식과 Lee의 식을 써서 계산하여 실험치와 비교하였다. 계산 인수 사이의 상호 연관 관계를 논고하였으며 부분적이나마 적정값 선정 원칙을 제시하였다.

• International Journal of Naval Architecture and Ocean Engineering
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• 제12권1호
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• pp.918-927
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• 2020

Flows induced by hybrid CRP pod propulsion systems (CRP-POD) are fundamentally characterized by unsteadiness. This work presents a numerical study on the unsteady flow of a CRP-POD at behind-hull condition based on CFD (Computational Fluid Dynamics). Unsteady RANS method is adopted, coupled with SST k-u turbulence model and sliding mesh method. The propeller thrusts and torques obtained by CFD is validated by model tests and acceptable agreements are obtained. The time histories of shingle-blade loads and pressures near the hull surface are recorded for the analysis of unsteady flow features. The cases of forward propeller alone and aft propeller alone are also computed to distinguish the hull-propeller interaction and propeller-propeller interaction. The results show the blade loads of both forward and aft propellers strongly fluctuate with phase angles. For the forward propeller, the blade load fluctuation is mainly governed by the hull-propeller interaction, while the aft blade load is remarkably affected by the propeller-propeller interaction in terms of the load average and fluctuation pattern. The fields of pressure, vorticity and velocity are also analyzed to reveal the unsteady flow features.

• 대기
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• 제26권4호
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• pp.635-647
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• 2016

The effects of sea surface temperature (SST) gradient induced by the previous typhoon on the following typhoon motion over East Asia have been investigated using Weather Research and Forecasting (WRF) model for the previous Typhoon Bolaven (1215) and following Typhoon Tembin (1214). It was observed that Typhoon Bolaven remarkably reduced SST by about $7^{\circ}C$ at Yellow Sea buoy (YSbuoy). Using the WRF experiments for the imposed cold wake over West of Tembin (WT) and over East of Tembin (ET), this study demonstrates that the effects of eastward SST gradient including cold wake over WT is much significant rather than that over ET in relation to unexpected Tembin's eastward deflection. This difference between two experiments is attributed to the fact that cold wake over WT increases the magnitude of SST gradient under the eastward SST gradient around East Asia and the resultant asymmetric flow deflects Typhoon Tembin eastward, which is mainly due to the different atmospheric response to the SST forcing between ET and WT. Therefore, it implies that the enhanced eastward SST gradient over East Asia results in larger typhoon deflection toward the region of warmer SST according to the location of the cold wake effect. This result can contribute to the improvement of track prediction for typhoons influencing the Korean Peninsula