• 대한기계학회논문집B
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• 제25권1호
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• pp.96-107
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• 2001

An unsteady numerical simulation was performed to analyze flow structures behind a local suction/blowing in a flat-plate turbulent boundary layer. The local forcing was given to the boundary layer flow by means of sinusoidally oscillating jet. A version of the unsteady $\kappa$-$\xi$-f(sub)u model (Rhee and Sung 2000) was employed. The Reynolds number based on the momentum thickness was about Re(sub)$\theta$=1700. The forcing frequency was varied in the range 0.011$\leq$f(sup)+$\leq$0.044 with a fixed forcing amplitude A(sub)o=0.4. The predicted results were compared and validated with the experimental data. It was shown that the unsteady locally-forced boundary layer flow is predicted well by the $\kappa$-$\xi$-f(sub)u model. The effect of the pitch angle of local forcing on the reduction of skin friction was also examined.

• 대한기계학회논문집B
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• 제24권3호
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• pp.467-476
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• 2000

An unsteady numerical simulation was performed for locally-forced separated and reattaching flow over a backward-facing step. The local forcing was given to the separated and reattaching flow by means of a sinusoidally oscillating jet from a separation line. A version of the $k-{\varepsilon}-f_{\mu}$ model was employed, in which the near-wall behavior without reference to distance and the nonequilibrium effect in the recirculation region were incorporated. The Reynolds number based on the step height (H) was fixed at $Re_H=33000$, and the forcing frequency was varied in the range $0{\leq}St_H{\leq}2$. The predicted results were compared and validated with the experimental data of Chun and Sung. It was shown that the unsteady locally-forced separated and reattaching flows are predicted reasonably well with the $k-{\varepsilon}-f_{\mu}$ model. To characterize the large-scale vortex evolution due to the local forcing, numerical flow visualizations were carried out.

• Journal of Ship and Ocean Technology
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• 제8권1호
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• pp.42-50
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• 2004

In these ten years, the cavitation and erosion phenomena in the rudder have been increased for high-speed container ships. The cavitation in the rudder blades which is injurious to rudder efficiency is mainly caused by the main flow with a large angle of attack induced by propellers, and the erosion which occurs as a result of repeated blows by shock wave that cavitation collapse may produce was observed in the gap legion of the rudder. However, gap cavitation is not prone to occur in model experiments because of low Reynolds number. So, the viscous effect should be considered for solving the flow of the narrow gap. In order to predict the cavitation phenomena and to improve the performance of the rudder, the analysis of the viscous flow in the rudder gap is positively necessary. In this study, numerical calculation for the solution of the RANS equation is applied to the two-dimensional flow around the rudder gap including horn part and pintle part. The velocity and pressure field are numerically acquired according to Reynolds number and the case that the round bar is installed in the gap is analyzed. For reduced the acceleration that pressure drop can be highly restrained numerically and in model experiment, the cavitation bubbles can be reduced.

• 수산해양기술연구
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• 제23권3호
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• pp.117-126
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• 1987

연구에서는 작동 유체중에 포집되어 있는 불용성 포집 계면층을 통과하는 금속입자 운동에 대한 특성을 조사하기 위하여 상호 불용인 착색 글리세린수용액과 사염화탄소 등의 계면층을 만들고 단일강구를 하강시켜 이 계면을 통과 할 때의 계면의 변화에 의한 유체 이동의 궤적과 그 후연에 생기는 기포체적에 관한 이론해석 및 실험분석을 통하여 다음의 결론을 얻었다. 1. 상층액 내에서의 강구 하강에 의한 유체 이동궤적은 상층액과 하층액의 비중 및 점도가 모두 같은 경우를 제외하고는 상층액의 두께에 영향을 받는다. 2. 하층액의 점도가 상층액의 점도와 같거나 큰 경우의 불용성 계면에서는 용액의 물성치에 관계없이 그 궤적 형상이 일정하고, 하층액의 점도에 의해 지배적이며 그 궤적의 형상은 다음과 같다. (1)/(V-2.35)=0.014+0.038$\mu$l super(-1) + 0.028$\mu$l super(-2). 3. 하층액의 점도가 상층액의 것보다 큰 경우의 유체 이동궤적의 체적은 구의 하강거리와 상하층액의 점도비에 의해 다음과 같다. V(Sr) super(0.34)=7.86a super(-2.08). 4. 후류기포생성체적의 비는 R sub(e)수가 100 이하에서 R sub(e)수에 의존하나 그 이상에서는 M sub(0)수의 영향이 지배적이며 그의 상관식은 다음과 같다. (V sub(w)/V sub(b))M sub(0) super(-0.22)=0.012R sub(e) super(1.53) ; R sub(e)<100. (V sub(w)/V sub(b))M sub(0) super(-0.22)=62.5 ;R sub(e)>100

• 한국항해항만학회지
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• 제41권2호
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• pp.33-38
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• 2017

A marine flapped rudder is designed to improve the effective lift generated by the rudder; this also improves the maneuverability of the ship. The flap is a high lift device installed at the trailing edge of the rudder to augment lift. In this paper, the characteristics of a thick flapped rudder are analyzed at a low Reynolds number with various ratios of flap chord length to total chord length and various aspect ratios, based on the computational fluid dynamics technique. The performance of the rudder with respect to lift, drag, and center of pressure are investigated, and the efficient ratio of flap chord length to total chord length and improved aspect ratio are determined. Ed: highlight - or 'superior'. As a case study, the flow on the flapped rudder of an NACA0021 section shape in free stream condition is simulated. The standard k-epsilon turbulence model is used to model the flow around the flapped rudder. The results indicate that the efficient ratio of the flap chord length to total chord length and aspect ratio are 0.3 and 1.4, respectively.

• 한국전산유체공학회지
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• 제15권4호
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• pp.1-8
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• 2010

This article describes the numerical investigation of turbulent blood flow in the stenosed artery bifurcation under periodic acceleration of the human body. Numerical analyses for turbulent blood flow were performed with different magnitude of periodic accelerations using a modified turbulence model which was considering drag reduction of non-Newtonian fluid. The blood was considered to be a non-Newtonian fluid which was based on the power-law viscosity. In order to validate the modified $k-{\varepsilon}$ model, numerical simulations were compared with the standard $k-{\varepsilon}$ model and the Malin's low Reynolds number turbulence model for power-law fluid. As results, the modified $k-{\varepsilon}$ model represents intermediate characteristics between laminar and standard $k-{\varepsilon}$ model, and the modified $k-{\varepsilon}$ model showed good agreements with Malin's verified power law model. Moreover, the computing time and computer resource of the modified $k-{\varepsilon}$ model were reduced about one third than low Reynolds number model including Malin's model.

• 대한토목학회논문집
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• 제33권1호
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• pp.93-100
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• 2013

본 연구에서는 트윈박스 단면인 이순신대교의 레이놀즈수 변화에 따른 공기력의 영향을 살펴보는데 그 목적이 있다. 이를 위하여 1/30 대축척 모형을 제작하여 공군사관학교 아음속 중형 풍동에서 최대 풍속 70m/s까지 풍속을 증가시켜가면서 공기력을 측정하여, 전북대학교 소형풍동에서 수행한 저레이놀즈수 풍동실험 결과와 비교하였다. 본 연구 대상 교량 단면은 레이놀즈수의 영향을 받는 것으로 나타났으며, 고레이놀즈수 실험 결과 기존 저레이놀즈수 실험보다 항력계수는 약 23%정도 낮은 수준인걸로 나타났다. 또한 경계층 촉진장치를 효과적으로 이용하면 기존의 저레이놀즈수 풍동실험 조건에서 고레이놀즈수 모사 실험이 가능한 것으로 판단된다.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2003년도 추계학술대회
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• pp.804-809
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• 2003

In this study, 2D computations of the Aeolian tones for some obstacles (circular cylinder, square cylinder and NACA0012 airfoil) are simulated. First of all, we calculate the flow noise generated by a uniform flow around a two-dimensional circular cylinder at Re=150 are simulated by applying the finite difference lattice Boltzmann method (FDLBM). The third-order-accurate up-wind scheme (UTOPIA) is used for the spatial derivatives, and the second-order-accurate Runge-Kutta scheme is applied for the time marching. The results show that we successively capture very small acoustic pressure fluctuation with the same frequency of the Karman vortex street compared with the pressure fluctuation around a circular cylinder. The propagation velocity of the acoustic waves shows that the points of peak pressure are biased upstream due to the Doppler effect in the uniform flow. For the downstream, on the other hand, it is faster. To investigate the effect of the lattice dependence, furthermore, simulations of the Aeolian tones at the low Reynolds number radiated by a square cylinder and a NACA0012 airfoil with a blunt trailing edge at high incidence are also investigated.

• International Journal of Aeronautical and Space Sciences
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• 제18권1호
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• pp.8-16
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• 2017

Most of small air vehicles with moving wing fly at low Reynolds number condition and the reduced frequency of the moving wing ranges from 0.0 to 1.0. The physical phenomena over the wing dramatically vary with the reduced frequency. This study examines experimentally the effect of the reduced frequency at low Reynolds number. The NACA0012 airfoil performs sinusoidal pitching motion with respect to the quarter chord with the four reduced frequencies of 0.1, 0.2, 0.4 and 0.76 at the Reynolds number $2.3{\times}10^4$. Smoke-wire flow visualization, unsteady surface pressure measurement, and unsteady force calculation are conducted. At the reduced frequency of 0.1 and 0.2, various boundary layer events such as reverse flow, discrete vortices, separation and reattachment change the amplitude and the rotation direction of the unsteady force hysteresis. However, the boundary layer events abruptly disappear at the reduced frequency of 0.4 and 0.76. Especially at the reduced frequency of 0.76, the local variation of the unsteady force with respect to the angle of attack completely vanishes. These results lead us to the conclusion that the unsteady aerodynamic characteristics of the reduced frequency of 0.2 and 0.4 are clearly distinguishable and the unsteady aerodynamic characteristics below the reduced frequency of 0.2 are governed by the boundary layer events.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2005년도 추계 학술대회논문집
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• pp.249-253
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• 2005

The turbulence noise generated from blunt trailing-edge is numerically predicted by using the hydrodynamic/acoustic splitting method at the Reynolds number based on thickness of flat plate, $Re_h=1000$, and the freestream Mach number $M_o=0.2$. The turbulent flow field is simulated by incompressible large-eddy simulation and the acoustic field is predicted efficiently with the linearized perturbed compressible equations (LPCE) recently proposed by the authors. The turbulent flow characteristics are validated with the results of the previous experimental study and direct numerical simulation. The acoustic properties predicted from LPCE are compared with the solutions of analytical formulations.