• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2011년 춘계학술대회논문집
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• pp.414-419
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• 2011

Missile am fighter aircraft have been challenged by low restoring nose-down pitching moment at high angle of attach. The consequence of weak nose-down pitching moment can be resulting in a deep stall condition. Especially, the pressure oscillation has a huge effect on noise generation, structure damage, aerodynamic performance and safety, because the flow has strong unsteadiness at high angle of attack. In this paper, the unsteady aerodynamics coefficients were analyzed at high angle of attack up to 60 degrees around two dimensional NACA0012 airfoil. The two dimensional unsteady compressible Navier-Stokes equation with a LES turbulent model was calculated by OHOC (Optimized High-Order Compact) scheme. The flow conditions are Mach number of 0.3 and Reynolds number of $10^5$. The lift, drag, pressure distribution, etc. are analyzed according to the angle of attack. The results at a low angle of attack are compared with other results before a stall condition. From a certain high angle of attack, the strong vortex formed by the leading edge are flowing downstream as like Karman vortex around a circular cylinder. Unsteady velocity field, periodic vortex shedding, the unsteady pressure distribution on the airfoil surface, and the acoustic fields are analyzed. The effects of these unsteady characteristics in the aerodynamic coefficients are analyzed.

• 한국전산유체공학회지
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• 제18권1호
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• pp.1-6
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• 2013

Missile and fighter aircraft have been challenged by low restoring nose-down pitching moment at high angle of attach. The consequence of weak nose-down pitching moment can be resulting in a deep stall condition. Especially, the pressure oscillation has a huge effect on noise generation, structure damage, aerodynamic performance and safety, because the flow has strong unsteadiness at high angle of attack. In this paper, the unsteady aerodynamics coefficients were analyzed at high angle of attack up to 50 degrees around two dimensional NACA0012 airfoil. The two dimensional unsteady compressible Navier-Stokes equation with a LES turbulent model was calculated by OHOC (Optimized High-Order Compact) scheme. The flow conditions are Mach number of 0.3 and Reynolds number of $10^5$. The lift, drag, pressure, entropy distribution, etc. are analyzed according to the angle of attack. The results of average lift coefficients are compared with other results according to the angle of attack. From a certain high angle of attack, the strong vortex formed by the leading edge are flowing downstream as like Karman vortex around a circular cylinder. The primary and secondary oscillating frequencies are analyzed by the effects of these unsteady aerodynamic characteristics.

• 한국물환경학회지
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• 제29권1호
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• pp.7-18
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• 2013

We compared the structure of fish community and condition of major fish species in Lake Chungpyung and Lake Paldang, which are relatively shallow, meso-eutrophic, cascading dam reservoirs on the North Han River. Two lakes have wide littoral zone in the lakeside providing similarly good habitat for fishes, whereas fishery and water recreational activities such as motorboating, water skiing are allowed in Lake Chungpyung but are prohibited in Lake Paldang. The average lengths of large size fishes in Lake Chungpyung are shorter than those of same species in Lake Paldang, resulting in the slight distortion of generation distribution of those species in Lake Chungpyung, possibly owing to the active fishery such as fixed shore net fishing, gill net fishing and angling. Meanwhile the condition of fishes represented by the length-weight relationship of fish species did not show the significant differences between two lakes and showed normal condition. To evaluate the impact of physical disturbance such as loud noise and turbulent wave from water recreational activities to fishes precisely, further studies including physiological responses to stress an spawing activity should be needed.

• Wind and Structures
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• 제16권6호
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• pp.561-577
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• 2013

Rational Functions are used to express the self-excited aerodynamic forces acting on a flexible structure for use in time-domain flutter analysis. The Rational Function Approximation (RFA) approach involves obtaining of these Rational Functions from the frequency-dependent flutter derivatives by using an approximation. In the past, an algorithm was developed to directly extract these Rational Functions from wind tunnel section model tests in free vibration. In this paper, an algorithm is presented for direct extraction of these Rational Functions from section model tests in forced vibration. The motivation for using forced-vibration method came from the potential use of these Rational Functions to predict aerodynamic loads and response of flexible structures at high wind speeds and in turbulent wind environment. Numerical tests were performed to verify the robustness and performance of the algorithm under different noise levels that are expected in wind tunnel data. Wind tunnel tests in one degree-of-freedom (vertical/torsional) forced vibration were performed on a streamlined bridge deck section model whose Rational Functions were compared with those obtained by free vibration for the same model.

• Journal of Astronomy and Space Sciences
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• 제36권3호
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• pp.105-113
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• 2019

Spectroscopic observations of barium star ${\zeta}$ Capricornus (HD204075) obtained at the 8.2 m telescope of the European Southern Observatory, with a spectral resolving power R = 80,000 and signal to noise ratio greater than 300, were used to refine the atmospheric parameters. We found new values for effective temperature ($T_{eff}=5,300{\pm}50K$), surface gravity ($log\;g=1.82{\pm}0.15$), micro-turbulent velocity ($v_{micro}=2.52{\pm}0.10km/s$), and iron abundance ($log\;N(Fe)=7.32{\pm}0.06$). Previously published abundances of chemical elements in the atmosphere of HD204075 were analyzed and no correlations of these abundances with the second ionization potentials of these elements were found. This excludes the possible influence of accretion of hydrogen and helium atoms from the interstellar or circumstellar environment to the atmosphere of this star. The accretion of nuclear processed matter from the evolved binary companion was primary cause of the abundance anomalies. The young age of HD204075 allows an estimation of the time-scale for the creation of the abundance anomalies arising from accretion of interstellar hydrogen and helium as is the case of stars with low magnetic fields; which we estimate should exceed $10^8$ years.

• 한국소음진동공학회논문집
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• 제14권6호
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• pp.528-535
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• 2004

Ultrasonic Vibrator is designed to achieve the maximum vibration amplitude at 30 kHz by in-cluding a horn (diameter, 40 mm), mechanical vibration amplifier at the top of the ultrasonic vibrator in the system and making the complete system resonate. In addition, it is experimentally visualized by particle imaging velocimetry (PIV) that the acoustic streaming velocity in the gap is at maximum when the gap between the ultrasonic vibrator and stationary plate agrees with the multiples of half-wavelength of the ultrasonic wave. This fact results from the resonance of the sound wave and the theoretical analysis of that is also accomplished and verified by experiment. It is observed that the magnitude of the acoustic streaming dependent upon the gap between the ultrasonic vibrator and stationary plate possibly changes due to the measurement of the average velocity fields of the acoustic streaming induced by the ultrasonic vibration at resonance and non-resonance. There exists extremely small average velocity at non-resonant gaps while the relatively large average velocity exists at resonant gaps compared with non-resonant gaps. It also reveals that there should be larger axial turbulent intensity at the hub region of the vibrator and at the edge of it in the resonant gap where the air streaming velocity is maximized and the flow phenomena is conspicuous than that at the other region. Because the variation of the acoustic streaming velocity at resonant gap is more distinctive than that at non-resonant gap, shear stress increases more in the resonant gap and is also maximized at the center region of the vibrator except the local position of center (r〓0). At the non-resonant gap there should be low values of vorticity distribution, but in contrast to the non-resonant gap, high and negative values of it exist at the center region of the vibrator with respect to the radial direction and in the vicinity of the middle region with respect to the axial direction. Acoustic streaming is noise-free due to the ultrasonic vibration and maintenance-free because of the absence of moving parts. Moreover, the proposed method by acoustic streaming can be utilized to the nano and micro-electro mechanical systems as a driving mechanism in addition to the augmentation of the streaming velocity.

• 대한토목학회논문집
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• 제29권5B호
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• pp.473-482
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• 2009

본 연구에서는 대부분이 수조실험에 의한 추정되어 온 해중의 방파제나 호안 등의 연직벽체에 작용하는 undular 및 turbulent bore에 의한 단파파력을 수치적으로 추정하기 위하여 Navier-Stokes운동방정식에 수면형상의 추적에 VOF법(Hirt and Nichols, 1981)을 채용하고 있는 CADMAS-SURF(CDIT, 2001)를 적용한다. 적용에서는 소스코드를 본 연구의 목적에 부합하도록 일부 수정하였다. 얻어진 원수치데이터에는 급격한 파력의 증감을 나타내는 스파이크현상이 강하게 표현되었으며, 이에 수치필터를 적용하여 5Hz 이상의 고주파수성분을 필터링하였다. 수치해석결과의 신뢰성을 확보하기 위하여 Matsutomi(1991) 및 Ramsden(1996)의 수조실험결과와 비교 검토하였으며, 이로부터 매우 좋은 일치성과 유용성을 확인할수 있었고, 단파성지진해일의 작용하에 있는 구조물의 설계에 도입될 수 있을 것으로 판단된다. 그리고, 본 2차원수조내에서 단파의 변형을 수위의 시 공간변화로부터 추정함과 동시에 전파속도의 변화특성을 나타내었다. 단파의 전파속도는 전파과정에서 변화되는 것을 확인할 수 있었다.

• 대한기계학회논문집B
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• 제32권5호
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• pp.342-350
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• 2008

The effects of combustion instability on flow structure and flame dynamics with the inlet configurations in a model gas turbine combustor were investigated using large eddy simulation (LES). A G-equation flamelet model was employed to simulate the unsteady flame behaviors. As a result of mean flow field, the change of divergent half angle($\alpha$) at combustor inlet results in variations in the size and shape of the central toroidal recirculation (CTRZ) as well as the flame length by changing corner recirculation zone (CRZ). The case of ${\alpha}=45^{\circ}$ show smaller size and upstream location of CTRZ than those of $90^{\circ}$ and $30^{\circ}$ by the development of higher swirl velocity. The flame length in the case of ${\alpha}=45^{\circ}$ is shorter than other cases, while the case of ${\alpha}=30^{\circ}$ yields the longest flame length due to the decrease of effective reactive area with the absence of CRZ. Through the analysis of pressure fluctuation, it was identified that the case of ${\alpha}=45^{\circ}$ shows the largest damping effect of pressure oscillation in all configurations and brings in the noise reduction of 2.97dB, compared to that of ${\alpha}=30^{\circ}$ having the largest pressure oscillation. These reasons were discussed in detail through the analysis of unsteady phenomena related to recirculation zone and flame surface. Finally the effects of flame-acoustic interaction were evaluated using local Rayleigh parameter.

• Journal of Astronomy and Space Sciences
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• 제38권2호
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• pp.135-143
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• 2021

We report, for the first time, the afternoon (i.e., from noon to sunset time) observations of the northern mid-latitude E-region field-aligned irregularities (FAIs) made by the very high frequency (VHF) coherent backscatter radar operated continuously since 29 December 2009 at Daejeon (36.18°N, 127.14°E, 26.7°N dip latitude) in South Korea. We present the statistical characteristics of the mid-latitude afternoon E-region FAIs based on the continuous radar observations. Echo signal-to-noise ratio (SNR) of the afternoon E-region FAIs is found to be as high as 35 dB, mostly occurring around 100-135 km altitudes. Most spectral widths of the afternoon echoes are close to zero, indicating that the irregularities during the afternoon time are not related to turbulent plasma motions. The occurrence of afternoon E-regional FAI is observed with significant seasonal variation, with a maximum in summer and a minimum in winter. Furthermore, to investigate the afternoon E-region FAIs-Sporadic E (E_s) relationship, the FAIs have also been compared with E_s parameters based on observations made from an ionosonde located at Icheon (37.14°N, 127.54°E, 27.7°N dip latitude), which is 100 km north of Daejeon. The virtual height of E_s (h'E_s) is mainly in the height range of 105 km to 110 km, which is 5 km to 10 km greater than the bottom of the FAI. There is no relationship between the FAI SNR and the highest frequencies (f_tE_s) (or blanket frequencies (f_bE_s)). SNR of FAIs, however, is found to be related well with (f_tE_s-f_bE_s).

• 천문학회보
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• 제44권2호
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• pp.68.1-68.1
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• 2019

Turbulence plays a crucial role in controlling star formation as it produces density fluctuation as well as non-thermal pressure against gravity. Therefore, turbulence controls the mode and tempo of star formation. However, despite a plenty of previous studies, the properties of turbulence remain poorly understood. As part of the Taeduk Radio Astronomy Observatory (TRAO) Key Science Program (KSP), "mapping Turbulent properties In star-forming MolEcular clouds down to the Sonic scale (TIMES; PI: Jeong-Eun Lee)", we mapped the Orion A and the Ophiuchus clouds, in three sets of lines (13CO 1-0/C18O 1-0, HCN 1-0/HCO+ 1-0, and CS 2-1/N2H+ 1-0) with a high-velocity resolution (~0.1 km/s) using the TRAO 14-m telescope. The mean Trms for the observed maps are less than 0.25 K, and all these maps show uniform Trms values throughout the observed area. These homogeneous and high signal-to-noise ratio data provide the best chance to probe the nature of turbulence in two different star-forming clouds, the Orion A and Ophiuchus clouds. We present comparisons between the line intensities of different molecular tracers as well as the results of a Principal Component Analysis (PCA).