• 대한기계학회논문집B
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• 제32권10호
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• pp.754-760
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• 2008

Computation of moving interface by the level set method typically requires the reinitialization of level set function. An inaccurate estimation of level set function $\phi$ results in incorrect free-surface capturing and thus errors such as mass gain/loss. Therefore, an accurate and robust reinitialization process is essential to the simulation of free-surface flows. In the present paper, we pursue further development of the reinitialization process, which evaluates level set function directly using a normal vector on the interface without solving there-distancing equation of hyperbolic type. The Taylor-Galerkin approximation and P1P1 splitting/SUPG (Streamline Upwind Petrov-Galerkin) FEM are adopted to discretize advection equation of the level set function and the incompressible Navier-Stokes equation, respectively. Advection equation and re-initialization process of free surface capturing are validated with benchmark problems, i.e., a broken dam flow and timereversed single vortex flow. The simulation results are in good agreement with the existing results.

• 대기
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• 제27권2호
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• pp.177-188
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• 2017

The development mechanisms of an unusual heavy snowfall event, which occurred in the coast of Jeju Island on 23 January 2016 were investigated through a thermodynamic approach. The formation of heavy snowfall was attributed to the enhanced thermal convection in two ways. First, the convection was enhanced by the air-sea temperature difference between the cold air advection in low-troposphere associated with the strengthening of the Siberian High and abnormal warm sea surface temperature, which is $1{\sim}2^{\circ}C$ higher than normal year over the Yellow Sea (YS). Second, the convective instability was increased by the vertical temperature gradient between the 7 days-sustained cold air advection in low-troposphere and the abrupt cold air intrusion in mid-troposphere induced by the southward shift of a cold cut-off vortex ($-45^{\circ}C$) at the formation stage. Compared to the twelve hours prior to the formation, the low-level moisture increased by 5% through the moisture supply from the YS, and the air-sea temperature difference increased from $18.5^{\circ}C$ to $28.5^{\circ}C$. Furthermore, the upward sensible (latent) heat flux increased 1.5 (1.2) times over the YS before the twelve hours prior to the formation. Thereafter, the sustained moisture supply and upward turbulent heat flux helped to maintain the snowstorm.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 1997년도 추계 학술대회논문집
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• pp.49-54
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• 1997

Rice's monotone streamline upwind finite element method, which was proposed to treat convection-dominated flows, is applied to the linear triangular element. An alignment technique of unstructured grids with given velocity fields is used to prevent the interpolation error produced in evaluating the convection term in the upwind method. The alignment of grids is accomplished by optimizing a target function defined with the inner-product of a properly chosen side vector in the element with the velocity field. Two pure advection problems are considered to demonstrate the superiorities of the present approach in solving the convection-dominated flow on the unstructured grid. Solutions obtained with aligned grids are much closer to the exact solutions than those with initial regular grids. The capability of the present approach in predicting the appearance of the secondary vortex in the laminar confined jet impingement is shown by comparing streamlines to those produced by SIMPLE on a highly stretched grid toward the impingement plate.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2004년도 제22회 춘계학술대회논문집
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• pp.341-346
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• 2004

Motion of a bubble inside narrow tube is numerically studied. The numerical code assumes axi-symmetric incompressible flow field. The surface of the bubble is captured by VOF (Volume Of Fluid) method, and it is advected by MARS (Multiphase Advection and Reconstruction Scheme). Air bubble inside water is first studied, and it was found that a strong vortex, which is induced by the pressure difference caused by the surface tension, is formed at the rear part of the bubble. Then flow parameters are parametrically varied to understand the correlation between the bubble shape, the bubble velocity, and the flow parameters. The parametric study revealed that the aspect ratio of the bubble mainly depends on We number, and the oscillation of the bubble speeds is dependent on Re number.

• Wind and Structures
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• 제13권2호
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• pp.169-189
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• 2010

This paper describes the use of coupled Computational Fluid Dynamics (CFD) and Rigid Body Dynamics (RBD) in modelling the aerodynamic behaviour of wind-borne plate type objects. Unsteady 2D and 3D Reynolds Averaged Navier-Stokes (RANS) CFD models are used to simulate the unsteady and non-uniform flow field surrounding static, forced rotating, auto-rotating and free-flying plates. The auto-rotation phenomenon itself is strongly influenced by vortex shedding, and the realisable k-epsilon turbulence modelling approach is used, with a second order implicit time advancement scheme and equal or higher order advection schemes for the flow variables. Sequentially coupling the CFD code with a RBD solver allows a more detailed modelling of the Fluid-Structure Interaction (FSI) behaviour of the plate and how this influences plate motion. The results are compared against wind tunnel experiments on auto-rotating plates and an existing 3D analytical model.

• 대한기계학회논문집B
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• 제27권3호
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• pp.347-356
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• 2003

The characteristics of near wakes of circular cylinders with serrated fins are investigated experimentally using a hot-wire anemometer for various freestream velocities. Near wake structures of the fin tubes are observed using a phase average technique. With increasing fin height and decreasing fin pitch. oscillation of streamwise velocity increases. It file oscillation of lateral velocity decreases. The time averaged V-component velocity distribution of the finned tube is contrary to that of the circular cylinder due to the different strength of entrainment flow. This strength is affected by the distance of (equation omitted) = 1.0 contour lines. (equation omitted) = 1.0 contour line approaches to the wake center line when the fin density is increased. When the distance between (equation omitted) = 1.0 contour lines comes close the shear force should be increased and the flow toward the wake center line can be more strengthened because of the shear force. Factors related to the velocity recovery in the near wake of the finned tube are attributed to tile turbulent intensity, the boundary layer thickness. the position and strength of entrainment process.

• 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.