• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2008년 영문 학술대회
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• pp.612-621
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• 2008

For the purpose of improving accuracy in jet noise prediction and investigating its generation mechanism, high subsonic jets were computed by using compressible Large Eddy Simulation(LES), wherein the inflow forcing or disturbance added in the inflow shear layer was incorporated. The far-field Sound Pressure Levels(SPL) as well as the flow field resulted in good agreement with available experimental data by applying only the high azimuthal modes among the inflow forcing parameters. We found that this result was due to an important role of the inflow forcing upon breaking down the axiymmetric vortices that caused high amplitude velocity and pressure fluctuations. In order to examine generation mechanism of the dominant noise component, wavelet transformation was introduced to reveal the presence of a well-organized structure of pressure fluctuations that originated mainly from vortex motions near the end of the jet potential core. This structure took a train of alternately positive and negative wavelet-transformed pressure regions along the jet distance, spreading towards the downstream with advection and propagation. It was concluded that this structure and its dynamic motion are the reason why a high subsonic jet produces the dominant noise with a particular downstream directivity.

• 대한토목학회논문집
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• 제28권5B호
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• pp.547-558
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• 2008

최근 폭증하는 해상물동량으로 인해 선박은 날로 대형화되며 현재 우리나라에서는 이러한 추이에 대응하기 위해 초대형 항만건설이 서둘러 진행되고 있으며 그 중 일부는 이미 부분적으로 완성되어 운영 중이거나 계속적인 확장이 예정되어있다. 초대형 항만의 경우 항 정온도 확보를 위해 대형 직립형 방파제가 수반되기 마련이나 비교적 높은 반사율로 인해 장주기의 파랑에 취약한 구조적 문제를 지닌다. 장주기 파랑의 효과적 제어를 위해 본 고에서는 아치형 curtain wall과 유수실로 구성된 된 장파 제어체 -Eco-breaker 2- 가 제시되며 Eco-breaker 2의 수리특성을 수치적으로 규명하였다. 수치모형은 가장 강건한 파랑모형인 Navier-Stokes Eq.과 SPH(Smoothed Particle Hydrodynamics), LES(Large Eddy Simulation) 기법에 기초하여 구성하였다. 수치모형의 검증을 위한 수리모형실험도 병행하였다. 수치모의 결과 Eco-breaker 2는 유수실 내부에 생성되어 아치형 curtain wall 전면 수역과 유수실을 주기적으로 넘나드는 대규모 구조적 와로 장파를 효과적으로 제어할 수 있었다. 또한 대규모 구조적 와의 생성 영역과 세기는 내습하는 파랑의 주기에 종속하며 유수실 체류시간이 입사파 주기의 반을 하회하는 경우 반사계수는 최대 0.18까지 낮아진다. 이러한 결과는 Eco-breaker 2가 환경친화적 항의 개발에 적극적으로 활용될 수 있음을 시사하는 것으로 이를 위해 그 구체적 제원도 함께 제시하였다.

• 한국전산유체공학회지
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• 제17권2호
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• pp.93-99
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• 2012

The present study focuses on the case of developing flow with in a channel containing a long array of sinusoidal waves (2a/${\lambda}$=0.1, ${\lambda}$=h, ${\lambda}$ is the wavelength, 2a is the wave height, h is the mean channel depth) at the bottom wall. The Reynolds number defined with channel height, h and the mean velocity, U, is Re=6,700. The channel is sufficiently long such that transition is completed and the flow is fully developed over the downstream half of the channel. For the case of an incoming steady flow with no resolved turbulence, the instantaneous flow fields in the transition region are characterized by the formation of arrays of highly-organized large-scale hairpin vortices whose dimensions scale with that of the roughness elements. The paper explains the mechanism for the formation of these arrays of hairpin vortices and shows these eddies play the primary role in the formation of the large-scale streaks of high and low velocity over the wavy wall region. The presence of resolved turbulence in the incoming flow, reduces the streamwise distance needed for the streaks to develop over the wavy region, but does not affect qualitatively the transition process. In the fully-developed region, isolated and trains of large-scale hairpins play an important role in the dynamics of the streaks over the wavy wall.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2003년도 춘계학술대회
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• pp.2233-2238
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• 2003

It is well known that organized vortex rotations swirl and tumble greatly affect the mixing, the combustion and heat transfer processes in engine cylinder. We have developed 3 dimensional numerical simulation codes whose predictions make good agreement with the experimental data. Large eddy simulation based on Smagorinsky subgrid scale model was adopted to describe the turbulence of in-cylinder flows. The tumble motions generated by different inclination angles between valve-port and cylinder head have been calculated. The results show that the angles between direction of induced flow and cylinder walls which the flow collides with play a great role in the formation and generation of tumble motions. Therefore, it is inferred that seat angle and inclination angle are important factors of engine design. In addition, the numerical results of different engine speed -1000 rpm and 3000 rpm are very similar in the flow structure.