• 한국환경기술학회지
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• 제19권6호
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• pp.528-535
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• 2018

본 연구에서는 전산유체역학(CFD) 모델을 이용하여, 건물 외관비에 따른 건물협곡에서의 소용돌이 현상을 재현하고 정량적인 해석을 시도하였다. 이를 위하여 건물협곡의 폭(W)을 기준으로 건물의 길이(L) 및 높이(H) 증가에 따른 민감도 실험을 수행하였으며, 건물협곡에서의 바람 벡터장과 2차 역회전 소용돌이의 형성 등을 분석하였다. 수평소용돌이의 경우에는 건물의 길이 증가에 따라 성장하다가 L/W=2.5부터 건물협곡의 중앙부에서 벡터의 크기 약화 및 방향 변화 등의 조짐이 보이기 시작하였고, L/W=3.0 이상에서 흐름이 분리되어 1차 소용돌이는 약화되고 건물협곡의 안쪽에서 2차 역회전 소용돌이가 형성되었다. 연직소용돌이의 경우에는 건물의 높이 증가에 따라 성장하다가 H/W=2.5부터 건물협곡의 하부에서 벡터의 방향전환 현상이 나타나기 시작하였고, H/W=3.5 이상의 조건에서 1차 소용돌이는 약화되고 2차 역회전 소용돌이가 형성되었다.

• 대한기계학회논문집B
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• 제21권3호
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• pp.350-357
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• 1997

The transient incompressible flow behind the axisymmetric bluff body is numerically simulated using the random vortex method(RVM). Based on the vorticity formulation of the unsteady Navier-Stokes equations, the Lagrangian approach with a stochastic simulation of diffusion using random walk technique is employed to account for the transport processes of the vortex elements. The numerical solutions for 2-dimensional recirculating flow behind a backward-facing step in the laminar range of Reynolds number are compared with experimental data. The present simulation focuses on the transitional flow regime where the recirculation zone behind the bluff body becomes highly unsteady and large-scale vortex eddies are shed from the bluff body wake due to intrinsic shear layer instabilities. The unsteady vertical flow structures and the mixing characteristics behind the bluff body are discussed in detail.

• Journal of Advanced Marine Engineering and Technology
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• 제29권7호
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• pp.769-778
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• 2005

The velocity and pressure fields of a ship's Weis-Fogh type propulsion mechanism are studied in this paper using an advanced vortex method. The wing (NACA0010 airfoil) and channel are approximated by source and vortex panels. and free vortices are introduced away from the body surfaces. The viscous diffusion of fluid is represented using the core-spreading model to the discrete vortices. The velocity is calculated on the basis of the generalized Biot-Savart law and the pressure field is calculated from an integral, based on the instantaneous velocity and vorticity distributions in the flow field. Two-dimensional unsteady viscous flow calculations of this propulsion mechanism are shown. and the calculated results agree qualitatively with the measured thrust and drag due to un-modeled large fluctuations in the measured data.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2004년도 추계학술대회
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• pp.1407-1412
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• 2004

The velocity and pressure fields of a ship's propulsion mechanism of Weis-Fogh type are studied by advanced vortex method. The wing of NACA0010 type and the channel are approximated by a finite of source and vortex panels, and the free vortices are introduced from the surface of their bodies. The viscous diffusion of fluid is represented by the core-spreading method. The velocity field is calculated on the basis of Biot-Savart law and the pressure field is calculated from the integration equation formulated by Uhlman. The flow fields of this propulsion mechanism are unsteady and complex, but the flow fields are clarified by numerical simulation.

• 대한기계학회논문집B
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• 제22권8호
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• pp.1041-1051
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• 1998

The fluid dynamic properties 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 by 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.

• 대한기계학회논문집B
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• 제28권5호
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• pp.553-559
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• 2004

Stratified flow past a three-dimensional obstacle such as a sphere has been a long-lasting subject of geophysical, environmental and engineering fluid dynamics. In order to investigate the effect of the stratification on the near wake, in particular, the unsteady vortex formation behind a sphere, numerical simulations of stratified flows past a sphere are conducted. The time-dependent Navier-Stokes equations are solved using a three-dimensional finite element method and a modified explicit time integration scheme. Laminar flow regime is considered, and linear stratification of density is assumed under Bossiness approximation. The computed results include the characteristics of the near wake and the unsteady vortex shedding. With a strong stratification, the separation on the sphere is suppressed and the wake structure behind the sphere becomes planar, resembling that behind a vertical cylinder.

• Journal of Advanced Marine Engineering and Technology
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• 제27권3호
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• pp.365-372
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• 2003

The Flow patterns around two cylinders in various arrangements were studied by a discrete vortex method. The flow for the surface of each cylinder was represented by arranging bound vortices at adequate intervals. The viscous diffusion of fluid was represented by the random walk method. The vortex distributions. streaklines. timelines and velocity vectors around two cylinders were calculated for centre-to-centre pitch ratios of P/D=1.5 and 2.5, attack ang1es of $a=0^{circ}, 30^{circ}, 60^{circ} and 90^{\circ}$. and Reynolds number of Re= 1200. The results of simulation correspond to the photographs by flow visualization and the flow intereference between two cylinders in various arrangements were clearly visualized by a numerical simulation.

• 한국자동차공학회논문집
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• 제4권3호
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• pp.130-138
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• 1996

The transient incompressible flow behind the widely-spaced co-axial jet is numerically simulated using the random vortex method(RVM). This numerical approach is based on the Lagrangian approach for the vorticity formulation of the unsteady Navier-Stokes equations, utilizing vortex elements to account for the convection and diffusion processes. The effects of the mass flow rate of an annular air jet and a central fuel jet on the co-axial jet flow dynamics is investigated. To validate the present procedure, the numerical results are compared with the available experimental data the present procedure, the numerical results are compared with the available experimental data in terms of the centerline and off-centerline profiles of the mean axial velocity. Discrepancies between the RVM results and the measurements are discussed in detail.

• Journal of Ship and Ocean Technology
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• 제8권1호
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• pp.10-30
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• 2004

High-fidelity RANS simulations are presented for a ducted marine propulsor, including verification ＆ validation (V＆V) using available experimental fluid dynamics (EFD) data. CFDSHIP-IOWA is used with $\textsc{k}-\omega$ turbulence model and extensions for relative rotating coordinate system and Chimera overset grids. The mesh interpolation code PEGASUS is used for the exchange of the flow information between the overset grids. Intervals V＆V for thrust, torque, and profile averaged radial velocity just downstream of rotor tip are reasonable in comparison with previous results. Flow pattern displays interaction and merging of tip-leakage and trailing edge vortices. In interaction region, multiple peaks and vorticity are smaller, whereas in merging region, better agreement with EFD. Tip-leakage vortex core position, size, circulation, and cavitation patterns for $\sigma=5$ also show a good agreement with EFD, although vortex core size is larger and circulation in interaction region is smaller.

• Wind and Structures
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• 제1권2호
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• pp.127-144
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• 1998

In this paper, the wake structures behind a square cylinder at the Reynolds number of 22,000 are simulated using the large eddy simulation, and the main features of the wake structure associated with unsteady vortex-shedding are investigated. The Smagorinsky model is used for parametrization of the subgrid scales. The finite element method with isoparametric linear elements is employed in the computations. Unsteady computations are performed using the explicit method with streamline upwind scheme for the advection term. The time integration incorporates a subcycling strategy. No-slip condition is enforced on the wall surface. A comparative study between two-and three-dimensional computations puts a stress on the three-dimensional effects in turbulent flow simulations. Simulated three-dimensional wake structures are compared with numerical and experimental results reported by other researchers. The results include time-averaged, phase-averaged flow fields and numerically visualized vortex-shedding pattern using streaklines. The results show that dynamics of the vortex-shedding phenomenon are numerically well reproduced using the present method of finite element implementation of large eddy simulation.