• 한국정밀공학회지
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• 제25권10호
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• pp.41-47
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• 2008

This study aims to obtain machining characteristics during AFM (Atomic force Microscope) machining of silicon wafers and to monitor the machining states using acoustic emission. As in micro scale machining, two distinct regimes of deformation, i. e. ploughing regime and cutting regime were observed. First, the transition between the two regimes are investigated by analyzing the "pile-up" during machining. As far as in process monitoring is concerned, in the ploughing repime, no chips have been formed and related AE RMS values are relatively low, In the mean time, in the cutting regime, the RMS values are significantly higher than the ploughing regime, with apparent chip formation. From the results, we found out that the proposed scheme can be used for the monitoring of nanomachining, especially for the characterization of nanocutting mode transition.

• 대한기계학회논문집B
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• 제24권6호
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• pp.822-833
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• 2000

This experimental study concerns the characteristics of a transitional flow in a concentric annulus with a diameter ratio of 0.52, whose outer cylinder is stationary and inner one rotating. The pressure drops and skin-friction coefficients have been measured for the fully developed flow of water and that of glycerine-water solution (44%) at a inner cylinder rotational speed of $0{\sim}600$ rpm, respectively. The transitional flow has been examined by the measurement of pressure drops and the visualization of flow field, to reveal the relation of the Reynolds and Rossby numbers with the skin-friction coefficients and to understand the flow instability mechanism. The present results show that the skin-friction coefficients have the significant relation with the Rossby numbers, only for laminar regime. The occurrence of transition has been checked by the gradient changes of pressure drops and skin-friction coefficients with respect to the Reynolds numbers. The increasing rate of skin-friction coefficient due to the rotation is uniform for laminar flow regime, whereas it is suddenly reduced for transitional flow regime and, then, it is gradually declined for turbulent flow regime. Consequently, the critical (axial-flow) Reynolds number decreases as the rotational speed increases. Thus, the rotation of inner cylinder promotes the early occurrence of transition due to the excitation of taylor vortices.

• 대한기계학회논문집B
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• 제22권9호
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• pp.1217-1228
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• 1998

In order to elucidate the operational characteristics of rotating heat pipes, the internal flow patterns and heat transfer performance are investigated. Flow patterns and its transition are studied with various rotational speeds by visualizing flows established inside a rotating tube. To verify those results of analysis, 2 heat pipes of the same geometries but fill charge rates of 7, 30% were manufactured and submitted to operating tests. Comparison of experimental results on heat transfer rate show a fairly good agreement with the analytical results. The analysis reveals that the optimum charge ratio is ranged in 4~7% depending on the quantity of thermal loads. but the heat pipe with 7% of fill charge ratio reached dry-out limitation at heat flux of $q^{{\prime}{\prime}}=6.2kW/m^2$ lower than that of analytic results. Transition of flow regime was well related to the correlation by Semena & Khmelev on transient centrifugal Froude Number Frc. But hysteresis phenomenon was observed in transition of flow regime, when the rotational speed was stepwisely changed in the way to undergo 1 cycle.

• 한국대기환경학회지
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• 제21권5호
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• pp.525-535
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• 2005

Using a three-dimensional computational fluid dynamics (CFD) model with the $k-{\varepsilon}$ turbulence closure scheme based on the renormalization group theory, flow regimes in urban street canyons are classified according to the building and street aspect ratios. The transition between skimming flow (SF) and wake interference flow (WIF) is determined with the size of double-eddy circulation generated behind the upwind building. The transition between WIF and isolated roughness flow (IRF) is determined with the flow reattachment distance from the upwind building. The critical aspect ratios at which the flow transition occurs are found and compared with those in previous studies. The results show that the flow-regime classification method used in this study is quite reasonable and that the values of the critical aspect ratios are generally consistent with those in fluid experiments or large-eddy simulation. The regression equation describing a relation between the building and street aspect ratios at the flow-regime transition is presented.

• Nuclear Engineering and Technology
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• 제30권5호
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• pp.396-402
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• 1998

Experiments on the steamside corrosion and hydrogen pick-up of Zircaloy-4 under high pressure up to 10.3MPa are carried out to estimate the pressure effects on the kinetics. Temperature and reaction time are determined to be 37$0^{\circ}C$ and 72hours for the pre-transition test and $700^{\circ}C$ and 210minutes for the post-transition test, respectively. Results show that under 10.3MPa pressure the oxidation reaction is 50% and 100% enhanced in the pre-and the post-transition regime, respectively. Total amount of hydrogen uptake in the reaction is proportionally increased as corrosion weight gain is elevated. However, pick-up fraction is not affected by the high pressure. The fraction is almost twice greater than that in the waterside corrosion. Edges in the specimens play a certain role in the enhancement, especially in the post-transition regime. To identify physical property changes of oxide film such as micro-cracks or micro-pores, careful and thorough examination must be needed with some special techniques.

• Wind and Structures
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• 제11권3호
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• pp.221-240
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• 2008

The spanwise flow structure around a rigid smooth circular cylinder model in cross-flow has been investigated based on the experimental data obtained from a series of wind tunnel tests. Surface pressures were collected at five spanwise locations along the cylinder over a Reynolds number range of $1.14{\times}15^5$ to $5.85{\times}10^5$, which covered sub-critical, single-bubble and two-bubble regimes in the critical range. Separation angles were deduced from curve fitted to the surface pressure data. In addition, spanwise correlations and power spectra analyses were employed to study the spatial structure of flow. Results at different spanwise locations show that the transition into single-bubble and two-bubble regimes could occur at marginally different Reynolds numbers which expresses the presence of overlap regions in between the single-bubble regime and its former and later regimes. This indicates the existence of three-dimensional flow around the circular cylinder in cross-flow, which is also supported by the observed cell-like surface pressure patterns. Relatively strong spanwise correlation of the flow characteristics is observed before each transition within the critical regime, or formation of first and second separation-bubbles. It is also noted that these organized flow structures might lead to greater overall aerodynamic forces on a circular cylinder in cross-flow within the critical Reynolds number regime.

• 의정연구
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• 제23권1호
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• pp.5-36
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• 2017

이 논문은 민주화 이후 출현한 87년체제의 특성을 밝히고 그 진화 과정을 비판적으로 분석함으로써 이에 대한 혁신의 방향과 내용을 모색하는 논의의 출발점을 제공하고자 한다. 이를 위해 우선 한국이 현재 세계사의 거시적 흐름속에서 어떤 국면에 처해 있는지 파악해 본 다음 87년체제가 현대한국정치의 큰 흐름 속에서 어떻게 출현했으며 그 부정적 특성은 어떻게 형성되었는지 분석한다. 그리고 민주화 이후 87년체제의 진화 과정을 분석한 다음 현재 파국에 이른 87년체제 극복을 위한 혁신의 단초를 제시한다.

• Nuclear Engineering and Technology
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• 제52권5호
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• pp.925-936
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• 2020

To ascertain the characteristics of pool sloshing behavior that might be encountered during a core disruptive accident of sodium-cooled fast reactors, in our earlier work several series of experiments were conducted under various scenarios including the condition with mono-sized solid particles. It is found that under the particle-bed condition, three typical flow regimes (namely the bubble-impulsion dominant regime, the transitional regime and the bed-inertia dominant regime) could be identified and a flow-regime model (base model) has been even successfully established to estimate the regime transition. In this study, aimed to further understand this behavior at more realistic particle-bed conditions, a series of simulated experiments is newly carried out using mixed-size particles. Through analyses, it is verified that for present scenario, by applying the area mean diameter, our previously-developed base model can provide the most appropriate predictive results among the various effective diameters. To predict the regime transition with a form of extension scheme, a correction factor which is based on the volume-mean diameter and the degree of convergence in particle-size distribution is suggested and validated. The conducted analyses in this work also indicate that under certain conditions, the potential separation between different particle components might exist during the sloshing process.

• International Journal of Fluid Machinery and Systems
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• 제5권2호
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• pp.60-64
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• 2012

The helical flow regime was investigated by using DPIV when the rotating Reynolds number is small. The wall slits were azimuthally located along the inner wall of outer cylinder and the slits number of each model was 9 and 18, another plain wall model was also studied for comparison purpose. The helical vortex flow regime can be observed in all the three models. The negative temperature gradients determine the direction of the rotation and movement of the helical vortex. But the helical wavy vortex flow can only be found in the plane and 9-slit models. And the result showed that the existence of slit wall accelerated the transition process.

• 과학기술학연구
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• 제15권1호
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• pp.109-143
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• 2015

최근 혁신의 지속과 성공을 위해 기술혁신시스템을 어떻게 바꿀지가 중요한 논제로 등장했다. 실천적, 이론적 차원에서 혁신시스템의 변화에 관심이 높아지고 있는 시점에서 네덜란드의 전환은 큰 주목을 받고 있다. 네덜란드의 전환정책은 시스템의 근본적 변화를 추구하면서도 기존의 변화와는 상당히 다른 접근과 방식을 취하고 있기 때문이다. 대표적 전환으로는 지속가능한 에너지 전환, 생물다양성과 자연자원의 지속가능한 활용으로의 전환, 지속가능한 농업으로의 전환, 지속가능한 교통 등이 있다. 장기적 변화라는 점에서 네덜란드의 전환은 여전히 초기단계지만 전환에 대한 공감 확산, 적대적 행위자 간 연합 형성, 다양한 전환 네트워크의 형성이라는 긍정적 효과가 나타나고 있다. 하지만 여전히 해결이 필요한 문제도 있다. 이론과 실제의 연계, 전환 개념의 추상성과 모호성 극복, 장기적 비전과 단기적 행위의 연결, 폭넓은 민주적 참여 확보, 현존 레짐의 저항 극복, 집행 문제, 정부 내에서 전환에 대한 지지 확산의 문제 등이다. 이런 문제에도 불구하고 네덜란드의 전환은 시스템의 근본적 변화가 필요한 시점에서 많은 교훈을 주고 있다. 따라서 기존 시스템과 레짐에 의한 고착화를 해결하고 구조적 변화를 이끌어 내기 위해 네덜란드의 전환이론과 정책에서 배울 필요가 있다.