• Title/Summary/Keyword: Boundary breakdown

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Flow Visualization of an Unsteady Airfoil at Low Reynolds Numbers (저 레이놀즈수에서 비정상 에어포일의 흐름 가시화)

  • Kim, Dong-Ha;Chang, Jo-Won
    • Journal of the Korean Society of Visualization
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    • v.4 no.2
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    • pp.51-58
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    • 2006
  • A boundary layer visualization was carried out in order to investigate the influence of Reynolds number on an oscillating airfoil. An NACA 0012 airfoil is sinusoidally pitched at the quarter chord point with oscillation amplitude of ${\pm}6^{\circ}$. A smoke-wire technique was employed to visualize the boundary layer and the near-wake. The freestream velocities are 1.98, 2.83 and 4.03m/s and corresponding chord Reynolds numbers are $2.3{\times}10^4,\;3.3{\times}10^4$, and $4.8{\times}10^4$, respectively. As the reduced frequency of K=0.1 is fixed, the corresponding frequency of an airfoil was adjusted in each case. The results reveal that the point at which the shear stress in an unsteady boundary layer separation disappears does not correspond with the position of the breakdown of the boundary layer, and that the breakdown of the boundary layer occurs further downstream.

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A Linkage Method for the Life Cycle Cost Breakdown Structure through an Analysis of Boundary Conditions (경계조건 분석을 통한 LCCBS 연계방안)

  • Jeong, Jae-Hyuk;Kim, Tae-Hui
    • Journal of the Korea Institute of Building Construction
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    • v.13 no.4
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    • pp.321-332
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    • 2013
  • Costs and expenses are intertwined and incurred throughout an entire construction project, even from the pre-construction phase, and each phase has a different impact on the life cycle cost (LCC). However, the cost breakdown structure (CBS) is different in each phase of a building construction project, which makes it hard to reasonably calculate construction cost. For this reason, the boundary conditions were analyzed in this study based on the life cycle cost break structure (LCCBS). In addition, breakdown factors were analyzed based on the boundary conditions to derive a linkage method. The validity of the linkage method was verified through application to actual construction projects. Through the analysis, it was found that the problem of items being left out was reduced by more than 97.2 percent, and the work was done an average of 6 hours faster compared to the conventional method. It is expected that by applying the new LCC system, LCC will be both reduced and calculated in a more efficient manner.

Analysis of Dielectric Breakdown of Hot SF6 Gas in a Gas Circuit Breaker

  • Kim, Hong-Kyu;Chong, Jin-Kyo;Song, Ki-Dong
    • Journal of Electrical Engineering and Technology
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    • v.5 no.2
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    • pp.264-269
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    • 2010
  • This paper presents the analysis of the dielectric characteristics of a hot $SF_6$ gas in a gas circuit breaker. Hot gas flow is analyzed using the FVFLIC method considering the moving boundary, material properties of real $SF_6$ gas, and arc plasma. In the arc model, the re-absorption of the emitted radiation is approximated with the boundary source layer where the re-absorbed radiation energy is input as an energy source term in the energy conservation equation. The breakdown criterion of a hot gas is predicted using the critical electric field as a function of temperature and pressure. To validate the simulation method, breakdown voltage for a 145kV 40kA circuit breaker was measured for various conditions. Consistent results between the simulation and experiment were confirmed.

Analysis of Boundary Conditions for Cost Breakdown Structure in the Construction and Maintenance phase (시공단계와 유지관리단계 비용분류체계의 경계조건 분석)

  • Jeong, Jae-Hyuk;Shin, Han-Woo;Kim, Tae-Hui
    • Proceedings of the Korean Institute of Building Construction Conference
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    • 2012.05a
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    • pp.21-23
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    • 2012
  • The process of building project for planning, designing, construction, maintenance, and waste disposal are related with each other. However, we have a difficulty for estimating building's LCC due not to be flexible each other. Therefore, we analyzed the boundary condition between the process of construction and maintenance, and analyzed the factor of segment. We also suggested the Link System for flexible relation.

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Effect of Heat Treatment on Properties of Varistors (바리스터의 물성에 미치는 열처리 효과)

  • 홍경진;민용기;오수홍;조재철
    • Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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    • 2001.07a
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    • pp.955-958
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    • 2001
  • The structure characteristics of varistor of Zn oxide to depend on the breakdown voltage has been investigated to annealing condition by additive material of Sb$_2$O$_3$ system. The breakdown voltage that has not doping Sb$_2$O$_3$ was 235[V]. ZnO varistors was shown ohmic properties when it's applied voltage was below critical voltage. It was shown non-ohmic properties over critical voltage, because current was increased with decreasing resistance. High voltage ZnO varistors had high breakdown voltage, but it had bad electrical stability with various surge. Sb$_2$O$_3$was increased non-linear coefficient in ZnO varistors grain boundary.

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Experimental study of boundary layer at the entrance of a cavity (공동 입구의 경계층에 관한 실험적 연구)

  • Jung Yong-Wun;Park Seung-O;Lee Duck-Joo
    • Proceedings of the KSME Conference
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    • 2002.08a
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    • pp.775-778
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    • 2002
  • In order to analyse the mechanism of a flow tone around a cavity, the correlations between the flow in the cavity and the boundary layer flow in front of the cavity are studied experimentally in this paper. The instability In the boundary layer forms the vortex at the front edge of the cavity and the flow tone is occurred by the vortex breakdown at the rear edge of the cavity Therefore, the boundary layer measurement is important in the cavity flow control. We measure the velocity of the boundary layer at the entrance of the cavity using hot-wire anemometry and the flow tone around the cavity by microphone. The boundary layer characteristic is changed by the various angle of the flap on the front edge of the cavity, while it is less influenced by the ratio of length and depth of the cavity.

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A Study on the Insulation Breakdown of Mica-Epoxy Composites (Mica-Epoxy 복합재료의 절연파괴에 관한 연구)

  • Kim, Hui-Gon;Kim, Hui-Su
    • Korean Journal of Materials Research
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    • v.7 no.8
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    • pp.650-653
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    • 1997
  • In large generators in power plants, stator winding insulations is exposed to a combination of thermal, electrical, mechanical, environmental stresses in service. These combined stresses cause insulation aging which leads to final insulation breakdown. In order to identify the breakdown mechanism, the stator winding insulation materials which are composed of mica-epoxy is analyzed by the component of materials with EDS, SEM techniques. We concluded that the potassium ions of mica are replaced by hydrogen ions at boundary area of mica-epoxy and/or mica-mica. It is proposed that through these phenomena, the conductive layers of potassium ions enable high voltage fields of multiple stresses to create voids and microcracks.

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Concave Surface Boundary Layer Flows in the Presence of Streamwise Vortices

  • Winoto, Sonny H.;Tandiono, Tandiono;Shah, Dilip A.;Mitsudharmadi, Hatsari
    • International Journal of Fluid Machinery and Systems
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    • v.4 no.1
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    • pp.33-46
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    • 2011
  • Concave surface boundary-layer flows are subjected to centrifugal instability which results in the formation of streamwise counter-rotating vortices. Such boundary layer flows have been experimentally investigated on concave surfaces of 1 m and 2 m radius of curvature. In the experiments, to obtain uniform vortex wavelengths, thin perturbation wires placed upstream and perpendicular to the concave surface leading edge, were used to pre-set the wavelengths. Velocity contours were obtained from hot-wire anemometer velocity measurements. The most amplified vortex wavelengths can be pre-set by the spanwise spacing of the thin wires and the free-stream velocity. The velocity contours on the cross-sectional planes at several streamwise locations show the growth and breakdown of the vortices. Three different vortex growth regions can be identified. The occurrence of a secondary instability mode is also shown as mushroom-like structures as a consequence of the non-linear growth of the streamwise vortices. Wall shear stress measurements on concave surface of 1 m radius of curvature reveal that the spanwise-averaged wall shear stress increases well beyond the flat plate boundary layer values. By pre-setting much larger or much smaller vortex wavelength than the most amplified one, the splitting or merging of the streamwise vortices will respectively occur.

NUMERICAL ANALYSIS OF PRESSURE PERTURBATION OF DELTA WING VORTEX FLOW AT A HIGH ANGLE OF ATTACK (고 받음각 ONERA 70도 삼각날개 와류 유동의 압력 섭동 분석)

  • Son, M.S.;Sa, J.H.;Park, S.H.;Byun, Y.H.
    • Journal of computational fluids engineering
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    • v.20 no.2
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    • pp.73-80
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    • 2015
  • Delayed Detached-Eddy Simulation was conducted to investigate surface pressure coefficient distribution and surface pressure fluctuation over an ONERA 70-degree delta wing at a high angle of attack. Time-averaged surface pressure distribution is directly affected by the primary vortices, whereas the pressure fluctuation is influenced by the unsteady fluctuating boundary layer over the surface. And pressure coefficient, velocity, pressure fluctuation, and turbulent kinetic energy were analyzed along the vortex core in order to investigate the process of vortex breakdown. Consequently, strong pressure fluctuations were found where the vortex breakdown was occurred at x~620 mm. The turbulent kinetic energy abruptly increased and followed after the vortex breakdown.