• 한국응용과학기술학회지
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• 제23권2호
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• pp.147-152
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• 2006

The accidents occurred by unstable material which is easily exploded or burnt up were caused by heat and collision under the condition of relatively low temperature without oxygen, have been reported frequently. However, the amount of the unstable material is getting higher by development of fine ceramic research area even though its dangerous characteristic is disregarded. This research studied a heat stability and measured boiling point of various carpet material. Carpet has been used in home as well as general indoor usage. Now a day, carpet material which is hardly burnt has been on commercial, but its detailed unstable conditions is not mentioned. This research reports the measurement of the initial temperature of generation heat and heat-radiation change on differential scanning calorimeter (DSC). The DSC data of nylon bulked continuous filament (N-BCF) yam 100%, nylon (NY), poly propylene (PP), and a new material named polytrimethylene terephthalate (PTT) are studied and researched about the effect of them using TGA, furnace, and direct-burning experiment.

• Journal of Construction Engineering and Project Management
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• 제8권2호
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• pp.17-33
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• 2018

This study investigates the stability of professionals' cost variability factor-rankings across different levels of cost-variability and response scenarios. Descriptive statistics are used to examine the stability of factor-ranking for 20 cost variability factors and a Multinomial Logistic (MNL) regression model was implemented to examine the stability of cost variability factors across three cost variability levels. The finding on the descriptive statistics indicated that professionals' factors-rankings are stable only for external factors. The MNL regression results on factor-stability suggested that 8 out of the 20 evaluated factors were unstable determinant of lower cost variability levels. These factors are "risk associated with the project", "personal bias and poor professionalism of the estimators", "limited time available to complete the project", "lack of skills and experience by estimator" "geographical location of projects", "incomplete & rush designs for estimate", "unforeseen or unexpected site constraints", "high class bidders for the contractors". Similarly lack of experience and large size projects were observed to be unstable as well. These observations suggest that professionals' view on pre-tender cost variability factor-ranking yields unstable factor rankings hence should not be relied upon as the only mechanisms to mitigate cost related risks in construction projects.

• 한국강구조학회 논문집
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• 제13권5호
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• pp.587-597
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• 2001

연속체 쉘을 이산화한 구조시스템은 돔형 구조물을 만들기 위해 많이 이용되고 있으며, 이러한 구조물은 하중레벨이 어느 임계값에 도달하면 구조물의 형상에 따라 뜀좌굴(snap-through) 또는 분기좌굴(bifurcation)에 의한 불안정 현상이 일어난다. 케이블 돔의 구조거동 특성은 초기장력 및 외력에 다라 비선형성이 강하게 나타나며 또한 초기조건에 매우 민감하게 반응한다. 본 연구에서는 케이블 돔의 불안정 문제를 파악하기 위해 Geiger 모델을 대상으로 하여 초기 장력도입에 의한 형상결정 문제를 다루고 이를 이용하여 완전형상 및 초기형상 불완전을 도입하여 외력에 의한 불안전성현상을 규명한다.

• 대한기계학회논문집B
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• 제23권2호
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• pp.218-225
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• 1999

Flow visualization study has been conducted to simulate the turbulent dispersion behavior of a crossflow jet physically under the conditions of various thermal stratification in a wind tunnel. A smoke jet with the constant ratio of the jet to freestream velocity is injected normally to the cross flow of the thermally stratified wind tunnel(TSWT) for flow visualization. The typical natures of the smoke dispersion under different thermal stratifications such as neutral, weakly stable, strongly stable, weakly unstable, strongly unstable and inversion layer are successfully reproduced in the TSWT. The Instantaneous velocity and temperature fluctuations are measured by using a cold and hot-wire combination probe. The time averaged dispersion behaviors, the centerline trajectories, the spreading angles and the virtual origins of the cross jet are deduced from the edge detected images with respect to the stability parameter. All the general characteristics of the turbulent dispersion behavior reveal that the definitely different dispersion mechanisms are inherent in both stable and unstable conditions. It is conjectured that the turbulent statistics obtained in the various stability conditions quantitatively demonstrate the vertical scalar flux plays a key role in the turbulent dispersion behavior.

• 천문학회보
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• 제41권2호
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• pp.61.2-61.2
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• 2016

Nuclear rings at centers of barred galaxies exhibit strong star formation activities. They are thought to undergo gravitational instability when sufficiently massive. We approximate them as rigidly-rotating isothermal objects and investigate their gravitational instability. Using a self-consistent eld method, we first construct their equilibrium sequences specified by two parameters: ${\alpha}$ corresponding to the thermal energy relative to gravitational potential energy, and $R_B$ measuring the ellipticity or ring thickness. The density distributions in the meridional plane are steeper for smaller ${\alpha}$, and well approximated by those of infinite cylinders for slender rings. We also calculate the dispersion relations of nonaxisymmetric modes in rigidly-rotating slender rings with angular frequency ${\Omega}$ and central density ${\rho}_c$. Rings with smaller are found more unstable with a larger unstable range of the azimuthal mode number. The instability is completely suppressed by rotation when ${\Omega}$ exceeds the critical value. The critical angular frequency is found to be almost constant at $0.7(G{\rho}_c)^{1/2}$ for ${\alpha}$ > 0.01 and increases rapidly for smaller ${\alpha}$. We apply our results to a sample of observed star-forming rings and confirm that rings without a noticeable azimuthal age gradient of young star clusters are indeed gravitationally unstable.

• 대한기계학회논문집B
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• 제27권4호
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• pp.415-423
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• 2003

The hydrodynamic instability of the three-dimensional boundary-layer over a rotating disk has been numerically investigated for these flows; Ro = -1, -0.5, and 0, using linear stability theory. Detailed numerical values of the disturbance wave number. wave frequency. azimuth angle. radius (Reynolds number, Re) and other characteristics have been calculated for the pre-swirl flows. On the basis of Ekman and Karman boundary layer theory, the instability of the pre-swirl flows have been investigated for the unstable criteria. The disturbance will be relatively fast amplified at small fe and within wide bands of wave number compared with previously known Karman boundary-layer results. The flow (Ro =-0.5) is found to be always stable for a disturbance whose dimensionless wave number is greater than 0.9. It has a larger range of unstable interval than Karman boundary layer and can be unstable at smaller Re.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2007년도 제29회 추계학술대회논문집
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• pp.377-380
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• 2007

스월 난류연소기에서의 혼합기 유입온도와 스월세기에 따른 연소불안정 발생 메커니즘 알아보기 위하여 Large Eddy Simulation을 수행하였다. 스월각 45도 경우 연료공기 혼합기의 온도를 600K에서 660K으로 증가시켰을 경우 화염분기(Bifurcation)현상이 관찰되었고, 스월 강도가 변할 경우 온도와 관계없이 화염분기가 일어나거나 그렇지 않음을 확인하였다. 벽면근처의 혼합가스 유동속도와 화염속도간의 상관관계는 화염분기현상의 발생에 주요한 인자임을 확인하였다.

• 제어로봇시스템학회논문지
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• 제13권9호
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• pp.843-850
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• 2007

This paper proposes a somatosensory stimulation system for the improvement of postural stability using vibration as somatosensory stimulation. This system consists of vibratory stimulation and postural response measurement. To evaluate this system, the center of pressure(COP) was closely observed in turn with simultaneous or separate mechanical vibratory stimulations to flexor ankle muscles (tibialis anterior, triceps surae) and two plantar zones on both feet while standing on a stable and an unstable support. The simultaneous vibratory stimulations cleared influenced postural stability and the effects of vibrations were higher with the unstable support. In separate vibratory stimulations, the extent of the COP sway reduced when the direction of the vibratory stimulations and that of the inclination of body coincided for flexor ankle muscle stimulations. In the contrary, the extent of the COP sway increased when the direction of the stimulations and that of body inclination coincided for plantar zone stimulations. These results can be useful for the development of rehabilitation systems that utilizes somatosensory inputs for postural balance.

• 한국광학회지
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• 제5권1호
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• pp.51-59
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• 1994

네 개의 평면 반사경과 두 개의 볼록 렌즈로 구성된 Newtonian 망원경 형의 negative branch confocal unstable ring (NBCUR) 공진기를 이용한 진행파형 Nd:YAG 레이전 발진기를 설계 제작하였다. 출력경으로 scraper mirror를 사용하여 원고리형 출력광을 얻었다. 이 레이저 발진기는 22개의 광학면을 가지고 있으며 이의 광학적 시준은 등경사 간섭법에 의해 간섭무늬를 관측함으로서 해결하였다. 일방향 진행파형 레이저 발진을 이룩하기 위하여 고리형 공진기 내에 자체 설계 제작된 영구 자석형 Faraday isolator를 삽입하였고, 이에 의한 발진 효과를 검토 하였다. 전기 입력 에너지 70J에서 레이저 출력에너지 약 80mJ 이었으며, BDN 색소에 의한 수동형 Q 스위칭에 의하여 첨두 출력 0.5 MW를 얻었다.

• 농촌계획
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• 제23권1호
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• pp.97-112
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• 2017

Stability of labor market in rural areas was analyzed in this paper with categories of industrial group, employment scale, surviving period, and founder group. The stability of each classified labor market was compared with each other to figure out the stable business class and the unstable class in rural areas. The results of rural areas were compared with those of urban areas. The stability was analyzed with average and coefficient of variation (C.V.) of annual total employees' change rates. It was revealed that labor market of 'primary industry', including agriculture, is unstable. Especially, labor market of 'mid-size' and 'primary industry' businesses founded as 'incorporated company' in rural areas is vulnerable. While labor market of 'large-size' is proved to be unstable, it is confirmed that 'small-size' or 'mid-size', and 'over-ten-year survived' businesses have positive contribution to the stable labor market in rural and urban areas. The results show that the stability of labor market is different in each category of business and in each region of rural or urban area. It is expected that the results can be utilized for the regional development policies, of labor and industry part.