• 기계저널
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• 제16권1호
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• pp.49-53
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• 1976

Groundwater flow in a two-layer aquifer is studied theoretically and experimentally. Nonlinear differential equations are independently linearized for the upper and the lower layer, and approximate solutions are obtained. Fine sands and coarse sands are used in experiments. For steady flow, theoretical solutions agrees excellently with experimental results For unsteady flow, however, agreement is not so good, because the experimental conditions do not match with theoretical assumptions.

• Journal of Mechanical Science and Technology
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• 제15권11호
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• pp.1548-1554
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• 2001

Flow patterns and dynamic properties of two-stage Weis-Fogh type ship propulsion mechanism are studied by a discrete vortex method. To study mutual interference between two wings, two cases are con sidered - wing motions with the same and reverse phases. The predicted flow patterns correspond to the available flow visualization results. Time histories of thrust and drag coefficients are also calculated, and the interference between the two wings are numerically clarified.

• 대한기계학회논문집B
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• 제41권3호
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• pp.191-197
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• 2017

본 연구는 Weis-Fogh형 수차모델의 비정상 유동장을 PIV를 이용해 가시화한 것이다. 실험은 비교적 효율이 높은 날개의 열림각 ${\alpha}=40^{\circ}$ 및 날개의 이동 속도에 대한 일정류의 속도비 U/V=1.5~2.5 범위 내에서 진행했다. 유동장은 각 실험 파라메터에 대해 열리는 과정, 병진운동의 과정 및 닫히는 과정으로 나누어 고찰되었으며, 그 결과를 요약하면 다음과 같다. 열리는 과정에서는 날개와 벽 사이에 유체가 흡입되며, 그 유입속도는 열림각이 클수록, 속도비가 클수록 증가했다. 병진운동의 과정에서 날개 압력면의 유체는 날개의 이동방향으로 움직였으며, 배면에서의 경계층의 두께는 속도비 2.0일 때 가장 작았다. 닫히는 과정에서는 날개와 벽 사이에서 유체가 분출되며, 그 분출속도는 열림각이 작을수록 증가했지만, 속도비와는 관계가 없었다.

• International Journal of Fluid Machinery and Systems
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• 제7권1호
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• pp.7-15
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• 2014

In the present study, a computational analysis of the flow in a centrifugal blower is carried out to predict a performance and to explain noise characteristics of the blower. Unsteady, 3D Navier-Stokes equations were solved with k-${\varepsilon}$ turbulence model using CFX software. CFD results were compared with the experimental data that is acquired from an experiment conducted with the same blower. The pressure fluctuation in the blower was transformed into the frequency domain by Fourier decomposition to find the relationship between flow behaviors and noise characteristics. Sound pressure level (SPL) which is obtained from wall pressure fluctuation at impeller outlet represents relative overall sound level of the blower well. Sound spectra show that there are some specific peak frequencies at each mass flow rate and it can be explained by flow pattern.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2008년도 춘계학술대회논문집
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• pp.13-16
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• 2008

After the collapse of the Tacoma bay bridge at Tacoma Washington, the accurate prediction of aerodynamics became crucial to the sound design of bridges. CFD(Computational Fluid Dynamics) becomes important tool for the prediction on wind effects on the bridge due to the recent development of CFD. The usage of CFD is further prompted by the advantages in using CFD, such as low-cost and fast feed-back of design. In this paper, an unsteady compressible Reynolds averaged Navier-Stokes code is used for the computation of the flow over bridges. Coakley's ��q-${\omega}$ �� two-equation turbulence model is used for the turbulent eddy viscosity. For accurate and stable computations, the local preconditioning method is adapted to the code. Aerodynamic characteristics of a couple bridges are presented to show the validity and the accuracy of the method.

• Journal of Advanced Marine Engineering and Technology
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• 제28권8호
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• pp.1258-1269
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• 2004

The fluid dynamic characteristics of a circular arc type sea anchor were calculated by a discrete vortex method. The flow for the surface of the sea anchor was represented by arranging bound vortices at adequate intervals. The simulations were performed by assuming that the separations occur at edges. With time, the drag coefficient was almost constant but the lift coefficient oscillated in a cycle due to von Karman's vortex street. As the camber ratios increase, the drag coefficient and Strouhal number were almost constant but the oscillating amplitude of the lift coefficient increased largely.

• 항공우주시스템공학회지
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• 제14권3호
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• pp.17-23
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• 2020

본 연구는 Chebyshev collocation operator를 지배 방정식의 시간 미분항에 적용하여 비정상 유동해석을 해석할 수 있는 기법을 개발한 논문이다. 시간적분으로 유속항은 내재적으로 처리하였으며 시간 미분항은 Chebyshev collocation operator을 적용하여 원천항 형태로 외재적으로 처리하여 부분 내재적 시간적분법을 적용하였다. 본 연구의 방법을 검증하기 위해 1차원 비정상 burgers 방정식과 2차원 진동하는 airfoil에 적용하였으며 기존의 비정상 유동 주파수 해석기법과 시험 결과를 비교하여 나타내었다. Chebyshev collocation operator는 주기적인 문제와 비주기적인 문제에 대해서 시간 미분항을 처리할 수 있으므로 추후 비주기적인 문제에 적용할 예정이다.

• 대한기계학회논문집B
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• 제26권9호
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• pp.1218-1225
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• 2002

Large-scale vortical structure of a turbulent separation bubble affected by unsteady wake is essential to understand flow mechanisms in various fluid devices. A spoked-wheel type of wake generator provides unsteady wake, which modifies the turbulent separation bubble significantly by changing rotation directions and passing frequencies. A detailed mechanism of vortex shedding from the separation bubble with unsteady wake is analyzed by taking a conditional average with spatial box filtering, which spatially integrates measured signals at pre-determined wavelength. A convecting nature of the large-scale vortical structure is analyzed carefully. Spatial evolution of the large-scale vortical structure with frequency variance is also exemplified.

• Journal of Astronomy and Space Sciences
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• 제19권3호
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• pp.207-214
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• 2002

인공위성 추진시스템의 액체연료(Hydrazine) 비정상유동 해석을 통해 연료공급 시스템내 유압특성이 유도된다. 정상상태 연소의 경우 연료유동량은 일정하나, 추력기밸브가 갑자기 닫히면 배관내 압력은 초기 탱크압력보다 높아진다. 결국 배관내 유압은 비정상상태가 되며, 유압 및 유량은 맥동현상을 보인다. 만약 상승압력이 너무 크게 되면, 추진제(연료)가 폭발분해를 일으키며, 추력기밸브 기능에 손상을 입힐 수 있고, 하이드라진 연료의 초음속 연소현상이 발생할 가능성이 있다. 또한 반사된 충격파로 인해 압력변환기의 감도저하 및 오작동을 유발하기도 한다. 위성의 추진시스템 설계시 비정상연료의 해석이 선행되어야 하며, 본 논문에서는 여러 설계인자에 대한 연료배관내 유압특성을 MOC 유동해석을 통해 제시하였다.

• Nuclear Engineering and Technology
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• 제28권4호
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• pp.405-414
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• 1996

In this paper, the unsteady 2-dimensional turbulent flow model for thermal stratification in a pressurizer surge line of PWR plant is proposed to numerically investigate the heat transfer and flow characteristics. The turbulence model is adapted to the low Reynolds number K-$\varepsilon$ model (Davidson model). The dimensionless governing equations are solved by using the SIMPLE (Semi-Implicit Method for Pressure Linked Equations) algorithm. The results are compared with simulated experimental results of TEMR Test. The time-dependent temperature profiles in the fluid and pipe nil are shown with the thermal stratification occurring in the horizontal section of the pipe. The corresponding thermal stresses are also presented. The numerical result for thermal stratification by the outsurge during heatup operation of PWR shows that the maximum dimensionless temperature difference is about 0.83 between hot and cold sections of pipe well and the maximum thermal stress is calculated about 322MPa at the dimensionless time 28.5 under given conditions.