• Transactions of the Korean Society of Mechanical Engineers B
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• v.20 no.4
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• pp.1510-1520
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• 1996

An experimental study on the flow over the axisymmetric backward-facing step was carried out. The purpose of the present study is to investigate the effect of the separating streamline curvature on the reattachment length and to understand the structure of recirculating flows. Local mean and fluctuating velocity components were measured in the separating and reattaching axisymmetric region of turbulent boundary layer on the wall of convex cylinder placed in a water tunnel by using 2-color 4-beam fiber optics laser Doppler velocimetry. The study demonstrates that the reattachment length increases with increasing separating streamline curvature. It is also observed that the reverse flow velocity and turbulent kinetic energy increase with an increase in the separating streamline curvature. In addition, the behavior of maximum turbulent stresses show that the effect of separating streamline curvature is larger in the region of recirculating zone(X/H<2) than in the region of reattachment point.

• Proceedings of the KAIS Fall Conference
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• 2009.12a
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• pp.520-525
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• 2009

본 연구은 해석적인 접근을 통하여 소형엔진 흡기포트 설계인자의 변화에 따른 유량계수를 고찰하였다. 기존모델을 Valve Lift별 유량계수들을 해석하였으며 그 중 유량계수가 일정해지는 구간의 Valve Lift를 기준으로 설계인자 변화에 따른 성능해석을 수행하였다. 결론적으로, 흡기포트 유량계수를 결정하는 중요한 인자는 포트각도와 유선형상임을 확인하였고 이들 값의 최적화를 수행하였다. 흡기포트의 유량계수를 결정하는 설계인자로는 포트각도, 흡기포트의 유선형상, Port Height_Guide, Flange 면적이 있다. 포트각도의 상승이 유량계수를 상승시키지만 그에 따른 굴절 및 Dead Volume의 발생으로 오히려 유량계수 측면에서는 불리한 조건이 되기도 한다. 급격한 형상변화 부분에서 박리현상이 발생되는데 이에 유동을 위한 곡률을 적용하여 급격한 형상변화를 줄여 박리형상을 감쇄시키고, Dead Volume 삭제하여 원활한 유동특성을 만들 수 있다.

• Proceedings of KOSOMES biannual meeting
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• 2005.11a
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• pp.153-158
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• 2005

The turbulent shear flaw around a surface-mounted vertical wall was investigated using the two-frame PIV(CACTUS 3.1) system. From this study, it is revealed that at least 500 instantaneous velocity field data are required for ensemble average to get reliable turbulence statistics, but only 200 field data are sufficient for the time-averaged mean velocity information The flow has an unsteady recirculation region post vertical wall with bottom gap, followed by a slow relaxation to the fiat-plate boundary layer flow. The time-averaged reattachment length estimated from the streamline distribution is about x/H=3H. The large eddy structure in the separated shear layer seems to have signification influence on the development of the separated shear layer and the reattachment process.

• Journal of the Korean earth science society
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• v.22 no.6
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• pp.512-527
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• 2001

Separated flows passed complex geometries are modeled by discrete vortex techniques. The flows are assumed to be rotational and inviscid, and a new techlnique is described to determine the stream functions for linear shear profiles. The geometries considered are the snow cornice and the backward-facing step, whose edges allow for the separation of the flow and reattachment downstream of the recirculation regions. A point vortex has been added to the flows in order to constrain the separation points to be located at the edges, while the conformal mappings have been modified in order to smooth the sharp edges and to let the separation points free to oscillate around the points of maximum curvature. Unsteadiness is imposed to the flow by perturbing the vortex location, either by displacing the vortex from the equilibrium, or by imposing a random perturbation with zero mean to the vortex in equilibrium. The trajectories of passive scalars continuously released upwind of the separation point and trapped by the recirculating bubble are numerically integrated, and concentration time series are calculated at fixed locations downwind of the reattachment points. This model proves to be capable of reproducing the trapping and intermittent release of scalars, in agreement with the simulation of the flow passed a snow cornice performed by a discrete multi-vortex model, as well as with direct numerical simulations of the flow passed a backward-facing step. The results of simulation indicate that for flows undergoing separation and reattachment the unsteadiness of the recirculating bubble is the main mechanism responsible for the intense large-scale concentration fluctuations downstream.

• Journal of the Korean Society of Marine Environment & Safety
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• v.14 no.4
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• pp.333-338
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• 2008

The coastal zone is a delicate and dynamic area in which the majority of a water kinetic energy is dissipated. These processes are subsequent to the transport of the beach materials. In comparison to emerged breakwaters, submerged structures permit the passage of some wave energy and in turn allow for circulation along the shoreline zone. This research aims to examine the beach erosion prevention capability of submerged structure by laboratory model. The flow characteristics behind a submerged obstacle with bottom gap were experimentally investigated at Re = $1.2{\times}10^4$ using the two-frame PIV(CACTUS 2000) system. Streamline curvature field behind the obstacle has been obtained by using the data of time-averaged mean velocity information. And the large eddy structure in the separated shear layer seems to have signification influence on the development of the separated shear layer. As bottom gap size increases, the recirculation occurring behind the obstacle moves toward downstream and its strength is weakened.