• International Journal of Precision Engineering and Manufacturing
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• 제6권4호
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• pp.44-48
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• 2005

As mechanical components are miniaturized, the demands on micro die/mold are increasing. Micro mechanical components usually have high hardness and good conductivity. Micro electrical discharge machining (MEDM) can thus be an effective way to machine those components. In micro cavity fabrication using MEDM, it is observed that the bottom surface of the cavity is distorted. Electric charges tend to be concentrated at the sharp edge, and debris cannot be drawn off easily at the center of the bottom surface. These two phenomena make the bottom surface of electrode and workpiece distort. As machining depth increases, the distorted shape of the electrode approaches hemisphere. This process is affected by both capacitance and the size of electrode. By using a smaller electrode than the desired cavity size and appropriate tool movement, bottom shape distortion can be prevented.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2008년도 제31회 추계학술대회논문집
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• pp.341-344
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• 2008

Numerical simulations have been carried out for a supersonic two-dimensional flow over open, rectangular cavities (length-to-depth ratios are L/D = 1.0) in order to investigate the effect of non-equilibrium condensation of moist air on supersonic flows around the cavity for the flow Mach number 1.83 at the cavity entrance. In the present computational investigation, a condensing flow was produced by an expansion of moist air in a Laval nozzle. The results obtained showed that in the case with non-equilibrium condensation for L/D = 1.0, amplitudes of oscillation in the cavity became smaller than those without the non-equilibrium condensation. Furthermore, the occurrence of the non-equilibrium condensation reduced the peaks of power spectrum density and the frequency of the flow field oscillation increased in comparison with the case of $S_0$ = 0.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2009년도 춘계학술대회 논문집
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• pp.119-122
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• 2009

본 연구에서는 2차원 초음속 공동유동에서 발생하는 압력진동을 제어하기 위하여 수치해석적 연구를 수행하였다. 본 계산에서는 압력진동을 제어하기 위하여 보조공동의 형상을 변화시켰으며, 유동의 마하 수를 1.50, 1.83 그리고 2.50로 변화시켰다. 그 결과, 보조공동은 압력진동을 상당히 감소시켰으며, 압력진동의 제어효과는 유동의 마하수와 보조공동의 상세형상에 크게 의존함을 알았다.

• 폴리머
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• 제36권6호
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• pp.685-692
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• 2012

사출성형공정은 충전, 보압, 냉각, 이형 및 취출 순서로 이루어진다. 이러한 공정 중 보압단계에서 캐비티 내에 가장 큰 압력이 주어진다. 따라서 캐비티 내압은 보압전환 시점과 보압의 크기에 가장 크게 영향을 받는다. 캐비티의 큰 내압은 금형에 큰 응력을 집중시켜 금형을 손상시킬 우려가 있으므로 캐비티 내압을 관찰하고 조절하는 것은 매우 중요하다. 본 연구에서는 이론해석과 실험을 통하여 보압전환 시점과 보압의 크기에 따라 캐비티 내압을 분석하였다. 보압전환 시점이 늦어짐에 따라서 내압이 증가하였다. 또한 보압전환이 늦어지면서 충전시간이 길게 되어 전체적으로 압력을 받는 시간이 길어져 보압 이후 냉각이 끝난 후에도 잔여압력이 존재하였다. 캐비티 내의 압력은 보압크기가 커질수록 비례적으로 증가하는 경향을 보였다. 역시 보압의 크기가 클수록 냉각 후 잔여압력이 증가하였다. 결과적으로 보압전환 시점이 늦고 보압크기가 크면 캐비티 내에 높은 압력이 형성되고 냉각이 끝난 후에도 잔여압력이 존재함을 알 수 있었다. 실험과 해석을 비교해 보았을 때 전체적인 경향은 매우 유사하였으나 해석에서는 잔여압력을 예측하지 못하였다. 캐비티 내압 조절을 위해서는 보압조건의 설정이 중요하며 CAE 해석을 통하여 최적 조건 설정이 가능함을 알 수 있었다.

• Advances in aircraft and spacecraft science
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• 제9권2호
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• pp.131-148
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• 2022

The paper presents the surface-modified NACA 2412 airfoil performance with variable cavity characteristics such as size, shape and orientation, by numerically investigated with the pre-validation study. The study attempts to improve the airfoil aerodynamic performance at 30 m/s with a variable angle of attack (AOA) ranging from 0° to 20° under Reynolds number (R_e) 4.4×10⁵. Through passive surface control techniques, a boundary layer control strategy has been enhanced to improve flow performance. An intense background survey has been carried out over the modifier orientation, shape, and numbers to differentiate the sub-critical and post-critical flow regimes. The wall-bounded flows along with its governing equations are investigated using Reynolds Average Navier Strokes (RANS) solver coupled with one-equational transport Spalart Allmaras model. It was observed that the aerodynamic efficiency of cavity airfoil had been improved by enhancing maximum lift to drag ratio ((l/d) max) with delayed flow separation by keeping the flow attached beyond 0.25C even at a higher angle of attack. Detailed investigation on the cavity distribution pattern reveals that cavity depth and width are essential in degrading the early flow separation characteristics. In this study, overall general performance comparison, all the cavity airfoil models have delayed stalling compared to the original airfoil.

• Restorative Dentistry and Endodontics
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• 제24권1호
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• pp.136-146
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• 1999

The purpose of this study was to estimate the changes of marginal adaptation to the cavity floor of light-cured glass ionomer cement base after application of a composite restoration. Eighty non-carious extracted human molars were used in the present study. Circular cavities were prepared on the center of the exposed dentin surface to 0.5mm, 1.0mm, 1.5mm, 2.0mm in depth and the prepared cavities were pretreated with Dentin conditioner and filled with Fuji II LC(GC Int. Co., Japan). They randomly assigned into 3 groups according to the difference in application of a composite restoration; Group 1(control group): only glass ionomer base, Group 2: The application of a composite restoration surrounded by dentin with class I cavity over glass ionomer base after conventional dentin bonding to the exposed dentin and glass ionomer base, Group 3: The application of composite restoration not-surrounded by dentin over glass ionomer base after conventional dentin bonding to the exposed dentin and glass ionomer base. To examine the interface between cavity floor and light-cured glass ionomer cement base, each groups were sectioned vertically through the center of restorations with diamond saw and the gap size(${\mu}m$) of interface measured by SEM. The results were analyzed by using One Way ANOVA. The results were as follows: 1. Good adaptation between glass ionomer cement base and cavity floor was showed in specimens with 0.5mm, 1.0mm depth base of control group. But in specimens with 1.5mm, 2.0mm depth base of control group, the gap was measured about $15{\mu}m$, $40{\mu}m$ respectively. 2. Gap size in group 2 was significantly higher than that in control group(P<0.05). 3. Gap size in group 3 was significantly higher than that in control group and group 2(P<0.05). 4. It was possible to observe the good adaptation between glass ionomer cement base and dentin which was intermediated with 4-10${\mu}m$ hybrid layer in specimens with 0.5mm, 1.0mm depth base of control group. Cohesive fracture within cement base was observed in all specimens which had the gap between glass ionomer cement base & dentin. 5. It was possible to observe the gap formation between cement base and bonding agent and between composite resin and dentin in all specimens of group 2.

• 한국지반신소재학회논문집
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• 제17권1호
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• pp.45-54
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• 2018

본 연구에서는 최근 도심지에서 발생하고 있는 지반함몰에 대하여 도로의 붕괴 가능성을 높이는 지하공동 영향인자에 대한 수치해석적 분석을 수행하였다. 지하공동으로 인한 지반함몰 영향 인자는 서울시에서 제안한 공동관리 등급제를 참고하여 아스팔트 포장층의 두께, 토피고, 공동 폭을 고려하였고, 본 연구에서 추가로 공동의 높이를 선정하였다. 이들 영향 인자들을 다양한 범위 내에서 변화시킨 조건에서 지하공동이 있는 아스팔트 포장층 상단에 집중하중과 등분포하중을 각각 변위제어하여 도로 파괴하중을 분석하였다. 분석된 파괴하중을 토대로 서울형 공동관리 등급제에 대한 적용성을 평가하였다. 해석 결과 공동의 높이가 파괴에 미치는 영향은 크지 않았으며 이외의 다른 인자에 따라 복합적으로 파괴하중이 결정되었다. 또한 파괴하중을 기준으로 한 공동분류 방법은 공동 조건을 기준으로 한 분류 방법보다 더 세밀한 분류가 가능하며 파괴 시 물리적인 현상을 반영할 수 있어 더 합리적인 방법이다.

• 대한기계학회논문집B
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• 제20권9호
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• pp.2944-2952
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• 1996

In this paper the CSCM type upwind flux difference splitting Navier-Stokes method has been applied to study the ARL-SL19 transoni $c^ersonic compressor cascade flow. First, the general characteristics of baseline cascade flow were analyzed. At freestream Mach n.1.612 and exit/inlet pressure ratio 2.15, the results from current laminar flow were compared well in suction surface with the experiment; however, not well in pressure surface. Second, numerical study of the transoni $c^ersonic compressor cascade flow demonstrated the effectiveness of a passive control by the various size cavities. A cavity under the shock foot point at the suction surface of the blades was used as a passive control. The passive control of shock-boundary layer interaction by a cavity reduced total pressure losses. The effect of cavity length and depth was studied. The total pressure loss was reduced by about 10% and the isentropic efficiency was improved slightly. The effect of cavity depth in current study(d/l = 0.05, 0.02) was not found strong. Further adequate turbulence modeling and TVD schemes would help to capture the shock more accurately and increase the effectiveness of the current shock-boundary layer interaction study using upwind flux difference splitting computational methods.thods.

• Journal of Mechanical Science and Technology
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• 제14권1호
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• pp.65-71
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• 2000

Mold cavities of gears should be made larger than the product specification since plastics shrink when changing from a molten to a solid state. For injection molded spur gears, two design methods for the compensation of shrinkage are widely used. One is the module correction method and the other is the pressure angle correction method. Both methods are based on the assumption that shrinkage occurs toward the center of a molded gear. This paper deals with the shrinkage rate and proposes a method of designing gear cavity derived from the measured shrinkage rates which govern the outside diameter, the tooth depth and the tooth thickness of a molded gear. The proposed method imposes no restriction on the shrinkage direction and provides a cavity with all of the fundamental gear design parameters.

• 천문학회지
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• 제20권1호
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• pp.27-47
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• 1987

In the present study a two-mode, separately concurring resonant cavity model is proposed for theoretical interpretation of the 3 minute umbral oscillation. The proposed model has been investigated by calculating the transmission coefficients of the waves propagating through the umbral photosphere (photospheric weak-field cavity) and chromosphere (chromospheric strong-field cavity) into the corona, for 3 different umbral model atmospheres by Staude (1982), Beebe et al. (1982) and Avrett (1981). In computing the transmission coefficients we made use of multi-layer approximation by representing the umbra] atmosphere by a number of separate layers with (1) temperature varying linearly with depth and (2) temperature constant within each layer. The medium is assumed to be compressible, non-viscous, perfectly conducting under gravity. The computed resonant periods, transmission spectra, phase spectra, and kinetic energy density of the waves associated with the oscillations are presented in comparison with the observations and their model dependent characteristics are discussed.