• 대한기계학회논문집B
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• 제29권7호
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• pp.828-836
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• 2005

Impingement/effusion cooling technique is used for combustor liner or turbine parts cooling in gas turbine engine. In the impingement/effusion cooling system, the crossflow generated in the cooling channel induces an adverse effect on the cooling performance, which consequently affects the durability of the cooling system. In the present study, to reduce the adverse effect of the crossflow and improve the cooling performance, circular pin fins are installed in impingement/effusion cooling system and the heat transfer characteristics are investigated. The pin fins are installed between two perforated plates and the crossflow passes between these two plates. A blowing ratio is changed from 0.5 to 1.5 for the fixed jet Reynolds number of 10,000 and five circular pin fin arrangements are considered in this study. The local heat/mass transfer coefficients on the effusion plate are measured using a naphthalene sublimation method. The results show that local distributions of heat/mass transfer coefficient are changed due to the installation of pin fins. Due to the generation of vortex and wake by the pin fin, locally low heat/mass transfer regions are reduced. Moreover, the pin fin prevents the wall jet from being swept away, resulting in the increase of heat/mass transfer. When the pin fin is installed in front of the impinging let, the blockage effect on the crossflow enhances the heat/mass transfer. However, the pin fin installed just behind the impinging jet blocks up the wall jet, decreasing the heat/mass transfer. As the blowing ratio increases, the pin fins lead to the higher Sh value compared to the case without pin fins, inducing $16\%{\~}22\%$ enhancement of overall Sh value at high blowing ratio of M=1.5.

• 대한기계학회논문집B
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• 제25권5호
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• pp.627-633
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• 2001

Characteristics of a diesel spray impingement with the variation of ambient temperature, wall temperature and ambient pressure were investigated through shadowgraphy method by using high speed camera. The radial penetration of spray was increased with ambient temperature and wall temperature. It is resulted from the decrease of ambient gas density caused by the increase of temperature. The height of spray was also increased with ambient temperature and wall temperature, because the height of stagnate region is noticeably increased, although height of wall jet vortex is decreased. At the same ambient pressure, the area ratio of impinging spray of room temperature environment to high temperature environment was increased, as the temperature difference between room temperature and high temperature increases. And the increment of area ratio was higher at low ambient pressure than high ambient pressure.

• 한국항공우주학회지
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• 제50권11호
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• pp.747-754
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• 2022

유체진동기에서 분출되는 초음속 진동제트를 이용하여 충격파에 의한 경계층 박리유동을 제어하는 실험적 연구가 이루어졌다. 유체진동기의 위치와 제어압력의 변화가 경사충격파에 의하여 발생되는 경계층 박리유동의 특성에 미치는 영향이 관찰되었고, 이를 위하여 고속 슐리렌, 표면유동가시화, 벽압력 측정, 그리고 정밀 피토관 측정 기법이 적용되었다. 본 연구의 초음속 진동제트의 박리유동 제어 특성은 공기제트 와류를 이용한 기존 제어기법과 정량적으로 비교 분석되었다.

• 한국유체기계학회 논문집
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• 제13권2호
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• pp.41-47
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• 2010

The effect of incidence angle on the three-dimensional flow and aerodynamic loss in the tip leakage flow region downstream of a turbine rotor cascade has been investigated for two tip gap-to-chord ratios of h/c=0.0% (no tip gap) and 2.0%. The incidence angle is changed to be $i=-10^{\circ}$, $0^{\circ}$, and $5^{\circ}$. The results show that for $i=5^{\circ}$, secondary flows including the passage vortex are intensified noticeably, and there is a strong interaction between the passage and tip leakage vortices. For $i=-10^{\circ}$, however, the passage vortex is weakened significantly, so that there exists only a strong leakage-jet-like secondary flows near the casing wall. For h/c=0.0% and 2.0%, aerodynamic loss tends to increase with increasing i from $-10^{\circ}$ to $5^{\circ}$. A small increment of i in its positive incidence range results in a remarkable aerodynamic loss increase, while increasing i in the negative incidence range leads to a small change in the aerodynamic loss generation.

• 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.

• 한국수자원학회:학술대회논문집
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• 한국수자원학회 2017년도 학술발표회
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• pp.66-66
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• 2017

하천에 설치된 저낙차 보의 여수로나 위어는 일반적으로 도수가 발생하도록 설계한다. 이러한 수공구조물위를 통과하는 흐름이 갖는 강한 운동에너지를 소산시키는데 도수는 중요한 역할을 한다. 난류흐름과 도수의 수면부근에서 형성되는 롤러 형태의 와류가 이러한 에너지 소산의 대부분을 유발한다. 이 연구에서는 여수로에서 발생하는 수중 도수 현상을 고해상도 3차원 수치모의를 통해서 재현하고 실험실에서 PIV를 이용해서 관측한 실험결과와 비교분석 한다. 아울러 수치모의 결과는 수중의 바닥면을 따라 발생하는 ?의 내부와 외부 전단층 그리고 도수에 의해서 발생하는 제순한 와류의 동적 거동 특성을 제시한다.

• 한국추진공학회지
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• 제7권3호
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• pp.45-52
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• 2003

원추형 초음속 노즐 확산부에 이차유동이 음속으로 분출될 때 나타나는 노즐 내부 유동장에 대한 수치적 연구가 이루어졌다. 대수-난류모델과 $\kappa$-$\varepsilon$ 모델을 사용한 레이놀즈-평균 Navier-Stokes 방정식을 계산함으로서 노즐 내부에서 나타나는 충격파와 경계층의 간섭에 의한 3 차원 유동장을 해석하였다. 얻어진 수치해석의 결과는 동일한 조건에서 수행된 실험결과와 잘 일치하고 있음이 판명되었다. 이차유동의 분출압력 변화가 충격파와 경계층의 간섭과 함께 노즐내부 유동장 구조에 미치는 영향을 평가하였다. 아울러 충격파 간섭 후방에서 나타나는 와류유동 구조와 벽면 압력분포에 관한 정보를 얻었다.

• International Journal of Fluid Machinery and Systems
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• 제2권3호
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• pp.215-222
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• 2009

Jets issuing through small holes in a wall into a freestream has proven effective in the control of flow separation. This technique is known as the vortex generator jet (VGJs) method. If a precursor signal of separation is found, the separation control system using VGJs can be operated just before the onset of separation and the flow field with no separation is always attained. In this study, we measured the flow field and the wall static pressure in a two-dimensional diffuser to find a precursor signal of flow separation. The streamwise velocity measurements were carried out in the separated shear layer and spectral analysis was applied to the velocity fluctuations at some angles with respect to the diffuser. The pattern of peaks in the spectral analysis changes as the divergence angle increases over the angle of which the whole separation occurs. This change in the spectral pattern is related to the enhancement of the growth of shear layer vortices and appears just before the onset of separation. Therefore, the growth of shear layer vortices can be regarded as a precursor signal to flow separation.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2000년도 춘계학술대회논문집B
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• pp.472-478
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• 2000

An experimental study is performed to analyze flow structures behind a local suction/blowing in a flat-plate turbulent boundary layer, The local forcing is given to the boundary layer flow by means of a sinusoidally oscillating jet issuing from a thin spanwise slot at the wall. The Reynolds number based on the momentum thickness is about $Re_{\theta}=1700$. The effects of local forcing are scrutinized by altering the forcing frequency $(0.011{\leq}f^+{\leq}0.044)$. The forcing amplitude is fixed at $A_0=0.4$. It is found that a small local forcing reduces the skin friction, and this reduction increases with the forcing frequency. A phase-averaging technique is employed to capture the coherent structures. Velocity signals are decomposed into a periodic part and a fluctuating part. An organized spanwise vortical structure is generated by the local forcing. The larger reduction of skin friction for the higher forcing frequencies is attributed to the diminished adverse effect of the secondary vortex. An investigation of the random fluctuation components reveals that turbulent energy is concentrated near the center of vortical structures.

• 한국화재소방학회논문지
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• 제20권2호
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• pp.44-53
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• 2006

선박에 설치되는 고정식 $CO_2$ 소화장치의 구성요소 중 하나인 $CO_2$ 소화제분사노즐의 분사각과 분사속도가 유동 및 $CO_2$ 농도분포특성에 미치는 영향을 분석하기 위하여 전산모의실험을 2차원 비정상상태로 수행하였다. 유동장과 $CO_2$ 소화제 농도장을 계산하여 분석하였다. 소화제 분사노즐의 조건에 따라 유동형태의 상이성을 확인할 수 있었으며, 모든 소화제 분사노즐조건에서 와류가 형성되는 영역으로부터 주위로 등농도선대가 확장됨을 알 수 있었다. 소화제 분사노즐각에 따라 계산영역의 밑바닥면을 따르는 벽면제트기류의 강도가 다르게 나타났고, 등농도선대가 확장 또는 축소됨을 예측 가능하였다. $CO_2$ 소화제 분사유량을 일정하게 유지한 상태에서 소화제 분사속도를 증가시키는 것이 감소시키는 것 보다 더 높은 $CO_2$ 등농도선대가 밑바닥면 상에 형성될 것으로 예측되었다.