• 한국전산유체공학회지
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• 제19권2호
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• pp.49-57
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• 2014

In the present study, 2-D gas-kinetic flow solver on unstructured meshes was developed for flows from continuum to transitional regimes. The gas-kinetic BGK scheme is based on numerical solutions of the BGK simplification of the Boltzmann transport equation. In the initial reconstruction, the unstructured version of the linear interpolation is applied to compute left and right states along a cell interface. In the gas evolution step, the numerical fluxes are computed from the evaluation of the time-dependent gas distribution function around a cell interface. Two-dimensional compressible flow calculations were performed to verify the accuracy and robustness of the current gas-kinetic approach. Gas-kinetic BGK scheme was successfully applied to two-dimensional steady and unsteady flow simulations with strong contact discontinuities. Exemplary hypersonic viscous simulations have been conducted to analyze the performances of the gas-kinetic scheme. The computed results show fair agreement with other standard particle-based approaches for both continuum part and transitional part.

• International Journal of Aeronautical and Space Sciences
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• 제12권4호
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• pp.318-330
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• 2011

This paper investigates the convergence characteristics of the modified artificial compressibility method proposed by Turkel. In particular, a focus is mode on the convergence characteristics due to variation of the preconditioning factor (${\alpha}_u$) and the artificial compressibility (${\beta}$) in conjunction with an upwind method. For the investigations, a code using the modified artificial compressibility is developed. The code solves the axisymmetric incompressible Reynolds averaged Navier-Stokes equations. The cell-centered finite volume method is used in conjunction with Roe's approximate Riemann solver for the inviscid flux, and the central difference discretization is used for the viscous flux. Time marching is accomplished by the approximated factorization-alternate direction implicit method. In addition, Menter's k-${\omega}$ shear stress transport turbulence model is adopted for analysis of turbulent flows. Inviscid, laminar, and turbulent flows are solved to investigate the accuracy of solutions and convergence behavior in the modified artificial compressibility method. The possible reason for loss of robustness of the modified artificial compressibility method with ${\alpha}_u$ >1.0 is given.

• 대한조선학회논문집
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• 제44권3호
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• pp.258-266
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• 2007

CFD calculations are performed for KRISO 3600TEU container ship(KCS) models with different Reynolds numbers. Numerical calculations of the turbulent flows with the free surface around KCS have been carried out at $Re=0.791{\times}106\;and\;Re=1.4{\times}107$ using a standard Fluent package. In both cases, Froude number is fixed with 0.26 and wave elevation is simulated by using the VOF method. The calculated results at $Re=1.4{\times}107\;and\;Re=0.791{\times}106$ are compared with the experiment data of KRISO towing tank test and RIMS CWC test, respectively. Boundary layer thickness and wake field shows Reynolds number differences. There are some changes in wave pattern behind transom stern.

• 대한조선학회논문집
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• 제44권3호
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• pp.248-257
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• 2007

Turbulent flows around a sailing yacht hull were calculated using CFD technique. Grid system was generated by using Gridgen package and Fluent package was used to calculate flows around a 30ft-class yacht hull designed by MOERI. The drafts at starboard and port sides of a yacht. when the hull was heeled, were determined by adjusting the same displacement in the generated grid system. Pressure distribution on the sailing yacht hull was obtained and the changes of drag and side force by heel and leeway variation were shown. The flow calculations have been carried out both with and without free surface, and keel and rudder were included for both cases. It was found that the calculated results with free surface gives better agreement with experiments than those without free surface, although there are still a room for the improvement in correct prediction of forces.

• 한국항해항만학회지
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• 제28권8호
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• pp.753-757
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• 2004

선미파의 점성 상호작용과 수중고속선의 sub-breaking 현상을 해석하기 위하여 자유표면 유통을 수치적으로 시뮬레이션하였다. Navier-Stokes 방정식을 풀었으며 유한차분법, 물체적합좌표계, 벽법칙, 삼중격자법을 적용하였다. S-103 모형을 대상으로 준쇄파의 수치결과를 실험과 비교하였고 준쇄파 발생조건을 수중 고속선인 3차원 회전체에 대해 적용하였다. 준쇄파 해석결과에 의하면 M/Us 구배가 잠김 깊이에 영향을 미치고 있음을 알 수 있었다. 또한 선수파가 박리에 영향을 미쳐 선미파를 변형시킴을 확인하였다.

• 대한조선학회논문집
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• 제55권3호
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• pp.265-273
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• 2018

Direct numerical and large eddy simulations of transitional flows around studs installed on flat plate and bulbous bow have been performed to investigate an effectiveness of turbulence stimulators on laminar-to-turbulence transition at a very low speed. The flow velocity was determined to be 0.366m/s corresponding to 4 knots of full-scale ship speed when the objective ship was Kriso container ship. The spatial evolution of skin friction coefficient disclosed that a fully development of turbulence was observed behind the second stud installed on flat plate while a rapid transition from laminar to turbulence gave rise to the fully development of turbulence behind the first stud installed on bulbous bow. A comparison of streamwise mean velocity profiles showed that the viscous sublayer and log-layer were in good agreement with previous results although the friction velocity of Smagrosinsky sub-grid scale model was about 10% larger than that of direct numerical simulation. While the turbulence intensities of bulbous bow was similar to those of flat plate in inner region, larger intensities of turbulence were observed in outer region of bulbous bow than those of flat plate.

• 한국추진공학회지
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• 제4권4호
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• pp.59-69
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• 2000

2차인 압축성/비압축성 Navier-Stokes 방정식은 이용하여 DCA 압푹기 익렬의 수치 해석을 수행하고 실험치와 비교 검토하였다. SIMPLE 알고리즘을 적용한 비압축성 코드는 대류항의 이산화에 하이브리드 도식을 진동해를 방지하기 위해 집중격자 기법을 사용하였다. 압축성 코드는 예조건화 기법 을 적용하였으며 공간 이산화출 위해 풍상 차분법을, 시간 적분을 위해서는 LU-SGS 기법을 사용하였다. 또한 난류 점성 유동장을 해석하기 위해 Baldwin-Lomax, standard $\kappa$ -$\varepsilon$, $\kappa$ -$\varepsilon$ Lam. Bremhorst, standard $\kappa$-$\omega$, $\kappa$ -$\omega$ SST 모델 등의 난류 모델을 적용하여 각 모델들의 특성을 살펴보았다.

• 대한설비공학회지:설비저널
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• 제12권4호
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• pp.264-274
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• 1983

Two-dimensional jet flows into a couple of confined rectangular enclosures such as an Ondol flue channel and their flow distributions were analyzed by numerical graphics : rectangular space in one enclosure is vacated and the other has 8 rectangular small posts. Both enclosures have a protruded inlet nozzle and on outlet on its center line. Steady state incompressible laminar viscous flow was assumed. The primitive forms of Navier-Stokes equations and continuity equation in a cartesian coordinate system were solved numerically by the Marker and Cell method for Reynolds numbers of 5, 10, 20, 30 and 40. From the numerical graphics it was found that the flow regions in both enclosures were devided into tow parts ; one part was the jet flow localized in a narrow center region of the enclosure and the other part was the very slow recirculating flow occupying the rest of the flow region in the enclosures. However there were a little differences in the shapes of jet flow in both enclosures for Reynolds numbers of 5 and 10 and also in the shapes of recirculating flows in both enclosures for all Reynolds number. Also it was found that waving flow appeared right before the outlet at Reynolds number of 20 and more.

• Advances in aircraft and spacecraft science
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• 제9권5호
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• pp.415-431
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• 2022

Secondary flows have a huge impact on losses generation in modern low pressure gas turbines (LPTs). At design point, the interaction of the blade profile with the end-wall boundary layer is responsible for up to 40% of total losses. Therefore, predicting accurately the end-wall flow field in a LPT is extremely important in the industrial design phase. Since the inlet boundary layer profile is one of the factors which most affects the evolution of secondary flows, the first main objective of the present work is to investigate the impact of two different inlet conditions on the end-wall flow field of the T106A, a well known LPT cascade. The first condition, labeled in the paper as C1, is represented by uniform conditions at the inlet plane and the second, C2, by a flow characterized by a defined inlet boundary layer profile. The code used for the simulations is based on the Discontinuous Galerkin (DG) formulation and solves the Reynolds-averaged Navier-Stokes (RANS) equations coupled with the Spalart Allmaras turbulence model. Secondly, this work aims at estimating the influence of viscosity and turbulence on the T106A end-wall flow field. In order to do so, RANS results are compared with those obtained from an inviscid simulation with a prescribed inlet total pressure profile, which mimics a boundary layer. A comparison between C1 and C2 results highlights an influence of secondary flows on the flow field up to a significant distance from the end-wall. In particular, the C2 end-wall flow field appears to be characterized by greater over turning and under turning angles and higher total pressure losses. Furthermore, the C2 simulated flow field shows good agreement with experimental and numerical data available in literature. The C2 and inviscid Euler computed flow fields, although globally comparable, present evident differences. The cascade passage simulated with inviscid flow is mainly dominated by a single large and homogeneous vortex structure, less stretched in the spanwise direction and closer to the end-wall than vortical structures computed by compressible flow simulation. It is reasonable, then, asserting that for the chosen test case a great part of the secondary flows details is strongly dependent on viscous phenomena and turbulence.

• 대한기계학회논문집B
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• 제26권1호
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• pp.159-169
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• 2002

The present study investigates in detail the combined effects of the Coriolis and centrifugal farce on the development of laminar flows in a square-sectioned U-bend rotating about an axis parallel to the center of bend curvature. When a viscous fluid flows through a rotating curved region, two types of secondary flow occur. One is caused by the Coriolis force due to the rotation of U-bend and the other by the centrifugal farce due to the curvature of U-bend. When the values of Rossby number and curvature ratio are large, the flow field in a rotating U-bend can be represented by two dimensionless parameters ; the Dean number K$\_$LC/=Re/√λ and a body ratio F=λ/Po. For positive rotation, where the rotation is in the same direction as that of the main flow, both the Coriolis force and the centrifugal force act radially outwards, the directions of the two secondary flows are the same. Therefore, the flow structure is qualitatively similar to that observed in a stationary curved duct with a larger f7c. On the other hand, in case of negative rotation, where two farces act in opposite direction, more complex flow fields can be observed depending on the relative magnitudes of the forces.