• Transactions of the Korean Society of Mechanical Engineers B
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• v.26 no.9
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• pp.1284-1291
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• 2002

Combustion instabilities are an important concern associated with lean premixed combustion. Laboratory-scale dump combustor was used to understand the underlying mechanisms causing combustion instabilities. Experiments were conducted at atmospheric pressure and sound level meter was used to track the pressure fluctuations inside the combustor. Instability maps and phase-resolved OH chemiluminescence images were obtained at several conditions to investigate the mechanism of combustion instability and relations between pressure wave and heat release rate. It showed that combustion instability was susceptible to occur at higher value of equivalence ratio (>0.6) as the mean velocity was decreased. Instabilities exhibited a longitudinal mode with a dominant frequency of ∼341.8 Hz, which corresponded to a quarter wave mode of combustor. Heat release and pressure waves were in-phase when instabilities occurred. Rayleigh index distribution gave a hint about the location where the strong coherence of pressure and heat release existed. These results also give an insight to the control scheme of combustion instabilities. Emission test revealed that NOx emissions were affected by not only equivalence ratio but also combustion instability.

• The Bulletin of The Korean Astronomical Society
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• v.38 no.2
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• pp.35.1-35.1
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• 2013

By probing nuclear regions and overall properties of AGN hosts as a function of their environments, we aim to observationally examine how AGN activities are related to their surroundings. We have selected eight representative AGN hosts in the Virgo cluster, including seven cluster members (M49, M60, M84, M87, NGC 4435, NGC 4526, NGC 4636) and one galaxy that is likely to be background (NGC 4261) but still close enough to be studied in high resolution. The selected galaxies are located in a range of density regions showing various morphology in 1.4 GHz continuum. High resolution observations with the KVN allow us to access the inner region of the AGN without suffering from dust extinction and synchrotron self-absorption. Since half of our targets are weak to be detected at K-band within its coherence time, we applied phase referencing (fast antenna position switching) to calibrate fast atmospheric phase fluctuations. We successfully detected relatively bright AGNs, such as M87, M84 and NGC4261, but no detection signature was found to the other members of the sample. In this talk, we will present our first results from our KVN observations, while we will discuss in detail the applied technique and our immediate future plans.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.28 no.4
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• pp.425-432
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• 2004

The objective of this study is a qualitative comparison between line-integrated OH chemiluminescence(OH$\^$*/) image and its Abel inverted image to investigate the flame structure at different phase of the oscillating pressure field. PIV(Particle Image Velocimetry) measurements were conducted under non-reacting conditions to see the global flow structure and NOx emission was measured to investigate the effect of fuel-air premixing on combustion instability and emission characteristics. Experiments were carried out in an atmospheric pressure, laboratory-scale dump combustor operating on natural gas. Combustion instabilities in present study exhibited a longitudinal mode with a dominant frequency of ∼341.8㎐, which corresponded to a quarter wave mode of combustor. Heat release and pressure waves were in-phase when instability occurred. Results gave an insight about the location where the strong coherence of pressure and heat release existed. Also an additional information on active control to suppress the combustion instabilities was obtained. For lean premixed combustion, strong correlation between OH$\^$*/ and NOx emissions was expected largely due to the exponential dependence of thermal NOx mechanism on flame temperature.

• Proceedings of the KSME Conference
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• 2003.11a
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• pp.356-361
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• 2003

The objective of this study is a qualitative comparison between line-integrated OH chemiluminescence ($OH{\ast}$) image and its Abel inversion image at different phase of the oscillating pressure field. PIV(Particle Image Velocimetry) measurements were conducted under non-reacting conditions to see the global flow structure. Also NOx emission was measured to investigate the effect of fuel-air premixing on combustion instability and emission characteristics. Experiments were carried out in an atmospheric pressure, laboratory-scale dump combustor operating on natural gas. Combustion instabilities in present study exhibited a longitudinal mode with a dominant frequency of ${\sim}341.8$ Hz, which corresponded to a quarter wave mode of combustor. Heat release and pressure waves were in-phase when instability occurred. Results gave an insight about the location where the strong coherence of pressure and heat release existed. Also an additional information on active control to suppress the combustion instabilities was obtained. For lean premixed combustion, strong correlation between $OH{\ast}$ and NOx emissions was expected largely due to the exponential dependence of thermal NOx mechanism on flame temperature.

• Publications of The Korean Astronomical Society
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• v.30 no.2
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• pp.629-631
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• 2015

In this proceedings, preliminary results of the KVN Source-Frequency Phase-Referencing (SFPR) observation of 3C 66A and 3C 66B are presented. The motivation of this work is to measure the core-shift of these 2 sources and study the temporal evolution of the jet opacity. Two more sources were observed as secondary reference calibrators and each source was observed at 22, 43, and 86 GHz simultaneously. Our preliminary results show that after using the observations at the lower frequency to calibrate those at the higher frequency of the same source, the residual visibility phases for each source at the higher frequencies became more aligned, and the coherence time became much longer; also, the residual phases for different sources, within 10 degrees angular separations, follow similar trends. After reference to the nearby calibrator, the SFPRed maps were obtained as well as the astrometric measurements, i.e. the combined coreshift. The measurements were found to be affected by structural blending effects because of the large beamsize of KVN, but this can be corrected with higher resolution maps (e.g. KAVA maps).

• Women's Health Nursing
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• v.11 no.1
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• pp.58-66
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• 2005

Purpose: The purpose of this study was to identify the patterns of activities of daily living (ADL) functional status and to examine the relationships between sense of coherence(SOC), depression, and uncertainty in gynecologic cancer patients who were receiving chemotherapy. Method: A prospective, longitudinal design with repeated measures was utilized. Women reported depression, SOC, and uncertainty at the beginning of the first cycle of chemotherapy, and they recorded ADL functional status everyday for two consecutive treatment cycles. the The instruments used were the Karnofsky Performance Status Index, Orientation to Life Questionnaire, Beck Depression Inventory, and Mishel Uncertainty in Illness Scale-Adult Form. Result: Data from 42 women showed that the ADL functional status during the second cycle was better than that of the first cycle with significant improvement each week. However, it did not completely recover to the baseline level even three weeks after the treatment ended in both cycles. SOC was correlated with depression(r=-.64, p<.001) and uncertainty(r=-.62, p<.001). Uncertainty was related to depression (r=.66, p<.001) and to functional status during the second cycle(r=-.45, p<.05), while the scores of the functional status during the two cycles were not related. Conclusion: Changing patterns and level of functional status during the treatment phase would be useful information for cancer patients to prepare coping strategies for positive health outcomes.

• Transactions of the Korean Society of Mechanical Engineers
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• v.15 no.6
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• pp.1948-1966
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• 1991

Two-phase cross-flow exists in many shell-tube heat exchangers such as condensers, reboilers and nuclear steam generators. To avoid problems due to excessive vibration, information on vibration excitation in two-phase cross-flow is required. Fluid-elastic instability is discussed in this paper. Four tube bundle configurations were subjected to increasing flow up to the onset of fluid-elastic instability. The tests were done on bundles with one flexible tube surrounded by rigid tubes. The fluid-elastic instability behavior is different for intermittent flows than for bubbly flows. For bubbly flows, the observed instabilities satisfy the relationship V/fd=K(2.pi..zeta. m/rho. $d^{21}$)$^{0.51}$ in which the minimum instability factor K was found to be 2.3 for bundles of p/d=1.22. The lowest critical velocities for fluid-elastic instability were experienced with parallel-triangular tube bundles. For intermittent flow, the observed instabilities did not follow the forgoing relation-ship. Significantly lower flow velocities were required for instability..

• The Bulletin of The Korean Astronomical Society
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• v.36 no.2
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• pp.133.2-133.2
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• 2011

KVN phase Calibrator Survey(KVNCS)는 VLBI 관측 시 대기의 불규칙한 수증기 분포로 인한 visibility 위상의 불규칙한 변화를 보정하기 위해 도입되는 phase-referencing 기법 등에서 필수요소인 위상보정 calibrator를 얻기 위한 연구이다. Phase-referencing 기법을 이용하여 위상을 보정하기 위해서는 대상 천체의 근접한 곳에 비교적 compact한 calibrator가 존재해야 한다. 또한 Asaki et al.(1996)에 의하면 대기의 coherence structure가 유지되기 위해서는 두 천체가 적어도 $5^{\circ}$ 이내의 분리각을 가져야 한다. 위상보정 calibrator에 대한 연구는 주로 2, 8GHz 대역에서 진행되어 왔고 최근에는 22GHz에서 VLBI 관측이 진행되고 있지만 천구상의 특정 영역에 국한되거나 calibrator들 간의 분리각이 여전히 크다. KVNCS는 천구상에서 calibrator의 분포를 좀 더 고르게 하고 더 많은 calibrator를 얻어 적어도 $5^{\circ}$ 이내의 분리각을 구현하고자 한다. 먼저, 단일경을 이용하여 KVNCS의 대상을 확보하기위하여 이들의 플럭스를 정확히 측정하였다. 2, 8GHz 대역에서 관측된 VLBA(Very Long Baseline Array) Calibrator Survey(VCS) 목록을 기초로 power-law를 가정하여 22GHz에서 100mJy 이상일 것으로 예상되는 천체 2503개를 KVNCS 단일경 연구의 후보로 선정하였다. KVN 연세와 울산 전파망원경경을 이용하여 2009년 12월부터 2011년 3월까지 2298개의 플럭스 측정 관측을 진행하여 22GHz에서 약 77%, 43GHz에서 약 23%의 검출률을 얻었다. 또한 이 천체들의 공간분포도 $5^{\circ}$의 분리각을 만족하는 것을 확인하였다. 앞으로 KVNCS 단일경 결과를 활용하여 KVN 각 사이트의 위치 정보를 비롯하여 22GHz KVN VLBI 관측을 통해 KVN 위상보정 calibrator를 확보 할 계획이다.

• Current Optics and Photonics
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• v.8 no.4
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• pp.406-415
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• 2024

The major error factors that degrade the efficiency of coherent beam combining (CBC) are numerically studied in a comprehensive manner, paying particular attention to phase, tip-tilt, polarization angle, and beam quality. The power in the bucket (PIB), normalized to the zero-error PIB, is used as a figure of merit to quantify the effect of each error factor. To maintain a normalized PIB greater than or equal to 95% in a 3-channel CBC configuration, the errors in phase, tip-tilt, and polarization angle should be less than 1.06 radians, 1.25 ㎛, and 1.06 radians respectively, when each of the three parameters is calculated independently with the other two set to zero. In a worst-case scenario of the composite errors within the parameter range for the independent-95%-normalized-PIB condition, the aggregate effect would reduce the normalized PIB to 83.8%. It is noteworthy that the PIB performances of a CBC system, depending on phase and polarization-angle errors, share the same characteristic feature. A statistical approach for each error factor is also introduced, to assess a CBC system with an extended number of channels. The impact of the laser's beam-quality factor M² on the combining efficiency is also analyzed, based on a super-Gaussian beam. When M² increases from 1 to 1.3, the normalized PIB is reduced by 2.6%, 11.8%, 12.8%, and 13.2% for a single-channel configuration and 3-, 7-, and 19-channel CBC configurations respectively. This comprehensive numerical study is expected to pave the way for advances in the evaluation and design of multichannel CBC systems and other related applications.

• Journal of Biomedical Engineering Research
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• v.9 no.2
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• pp.171-178
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• 1988

In this study, we proposed a test to explore the function of Vestibulo-ocular reflex (VOR) which subjected to an angular head acceleration using pseudo random binary stimulus. Resultant eye movements(horizontal vestibular nystagmus) were digitized, filtered and transformed into the frequency domain. At first we evaluated the transfer function of V.0.R(gain and phase) and the coherence function between stimulus and response by linear frequency methods in view of the quantitative analysis since the vestibulo-ocular reflex can be considered as a linear system. at least, in normals. Secondly. with the proposed test, we showed a direct possibility that we could interpret the pathological situation quantitatively as an illustration of clinical application.