• Title/Summary/Keyword: Coherent structures

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Near-Infrared Polarization Source Catalog of Northeastern Regions in the Large Magellanic Cloud

  • Kim, Jaeyeong;Jeong, Woong-Seob;Pak, Soojong;Park, Won-Kee;Tamura, Motohide
    • The Bulletin of The Korean Astronomical Society
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    • v.40 no.2
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    • pp.31.2-31.2
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    • 2015
  • We present a near-infrared photometric and polarimetric catalog of sources in the $39^{\prime}{\times}69^{\prime}$ fields on the northeastern part of the Large Magellanic Cloud (LMC), which was observed using SIRPOL, an imaging polarimeter of the Infrared Survey Facility (IRSF). This catalog contains 1,858 sources brighter than 14 mag at H band with polarization signal-to-noise ratio greater than 3 in at least one of J, H, and Ks bands. We examined the polarization structures around the star-forming regions, where coherent polarization position angle distributions are seen. We also estimated magnetic field strengths in some selected fields using Chandrasekhar and Fermi analysis. The magnetic field strengths are estimated to be $3{\sim}25{\mu}G$. The wavelength dependence of polarization degrees indicates that the polarization is most likely to be originated from dichroic extinctions by the local interstellar dusts in the LMC. We found that the polarization patterns are well aligned along the molecular clouds around star-forming regions.

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Experimental investigation on the turbulent elliptic jets by using a 3-D LDV system (3-D LDV 시스템을 이용한 타원제트의 난류특성에 관한 연구)

  • 권영철;이상준
    • Transactions of the Korean Society of Mechanical Engineers
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    • v.15 no.6
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    • pp.2160-2170
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    • 1991
  • Three-dimensional turbulent structures in the near field of elliptic jet were experimentally investigated by using a three-color, three-component Laser Doppler Velocimeter. The Reynolds number based on the nozzle exit velocity and nozzle equivalent diameter(De) was about 4*10$^{4}$. The turbulent characteristics of a sharp-edged elliptic nozzle with aspect ratio of 2 were analyzed along major and minor axis at X/De=2,3,5,7 and along the centerline up to X/De=14. Quantities measured at each point with the 3-D LDV system were three orthogonal velocity components, turbulent intensity, skewness, flatness, and Reynolds shear stress. The nondimensional mean velocities coincided well with the Schlichting's empirical curve with going downstream. Elliptic jet of AR=2 had two switching points at about X/De=2 and 16. The turbulent intensity along the minor axis was distributed widely than that along the major axis. In the near field, X/De<5, the Reynolds shear stresses of the inner part of the elliptic jet had negative value, which indicated the enhancement of entrainment toward the inner part.

An investigation of the Reynolds Number dependence of the Axisymmetric Jet Mixing Layer using the Proper Orthogonal Decomposition

  • Jung, Dae-Han;George, William K.
    • Proceedings of the KSME Conference
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    • 2001.11b
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    • pp.423-425
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    • 2001
  • The Proper Orthogonal Decomposition (POD) technique was applied to investigate the effects of Reynolds number and the characteristics of the organized motions or coherent structures as a function of downstream position from x/D=2 to 6 in a turbulent axisymmetric shear layer at Reynolds numbers of 78,400, 117,600, and 156,800. Data were collected simultaneously using the 138 hot-wire probe used by Citriniti and George (2000). The POD was then applied to a double Fourier transform in time and azimuthal direction of the double velocity correlation tensor. The lowest azimuthal mode for all POD modes, which dominated the dynamics at x=D = 3 in the previous experiments, dies off rapidly downstream. This is consistent with a trend toward homogeneity in the downstream evolution, and suggests that some residual value may control the growth rate of the far jet. On the other hand, for the higher azimuthal modes, the peak shifts to lower mode numbers and actually increases with downstream distance. These mixing layer data, normalized by similarity variables for the mixing layer, collapse at all downstream positions and are nearly independent of Reynolds numbers.

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Organized structure of turbulent boundary layer with rod-roughened wall (표면조도가 난류구조에 미치는 영향)

  • Lee, Jae-Hwa;Lee, Seung-Hyun;Kim, Kyoung-Youn;Sung, Hyung-Jin
    • 한국전산유체공학회:학술대회논문집
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    • 2008.03b
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    • pp.189-192
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    • 2008
  • Turbulent coherent structure near rod-roughened wall are investigated by analyzing the database of direct numerical simulation of turbulent boundary layer. The roughness sublayer id defined as two-point correlations are not independent of streamwise locations around roughness. The roughness sublayer based on the two-point spatial correlation is different from that given by one-point statistics. Quadrant analysis and probability-weighted Reynolds shear stress indicate that turbulent structures are not affected by surface roughness above the roughness sublayer defined by the spatial correlations. The conditionally-averaged flow fields associated with Reynolds shear stress producing Q2/Q4 events show that though turbulent vortices are affected in the roughness sublayer, these are very similar at different streamwise locations above the roughness sublayer. The Reynolds stress producing turbulent vortices in the log layer have almost the same geometrical shape as those in the smooth wall-bounded turbulent flows. This suggests that the mechanism by which the Reynolds stress is produced in the log layer has not been significantly affected by the present surface roughness.

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Low Phase Noise Design and Implementation of X -Band Frequency Synthesizer for Radar Receiver (레이다 수신기용 X-밴드 주파수 합성기의 저 위상잡음설계 및 구현)

  • So, Won-Wook;Kang, Yeon-Duk;Lee, Taek-Kyung
    • Journal of Advanced Navigation Technology
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    • v.2 no.1
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    • pp.22-33
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    • 1998
  • In the coherent-on-receiver radar system using the magnetron source, frequency synthesizer is employed as a STALO(Stable Local Oscillator) to keep the intermediate frequency stable. In this paper, X-band(8.4GHz~9.7GHz) single loop frequency synthesizer is designed and implemented by an indirect frequency synthesis technique. Phase comparison is performed by a digital PLL(Phase-Locked Loop) chip and the loop filter is designed for the low phase noise. The effects of loop component characteristics on the output phase noise are analyzed for single loop structures, and the calculated results are compared with the measured data.

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LES for Turbulent Flow in Hybrid Rocket Fuel Garin (하이브리드 로켓 산화제 난류 유동의 LES 해석)

  • Lee, Chang-Jin;Na, Yang
    • Proceedings of the Korean Society of Propulsion Engineers Conference
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    • 2007.04a
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    • pp.233-237
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    • 2007
  • Recent experimental data shows that an irregular fuel surface pops up during the combustion test. This may contribute to the agitated boundary layer due to blowing effect of fuel vaporization. Blowing effect can be of significance in determining the combustion characteristics of solid fuel within the oxidizer flow. LES was implemented to investigate the flow behavior on the fuel surface and turbulence evolution due to blowing effect. Simple channel geometry was used for the investigation instead of circular grain configuration without chemical reactions. This may elucidate the main mechanism responsible for the formation of irregular isolated spots during the combustion in terms of turbulence generation. The interaction of turbulent flow with blowing mass flus causes to breakup turbulent coherent structures and to form the small scale isolated eddies near the fuel surface. This mechanism attributes to the formation of irregular isolated sopt on the fuel surface.

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Observation of the Domain Structures in Soft Magnetic (Fe97A13)85N15/Al2O3 Multilayers

  • Stobiecki, T.;Zoladz, M.
    • Journal of Magnetics
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    • v.8 no.1
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    • pp.13-17
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    • 2003
  • The longitudinal magnetooptical Kerr effect was used to analyse magnetic domains in soft magnetic ${(Fe_{97}A1_3)}_{85}N_{15}$/$Al_{2}O_{3}$ multilayers in order to get microscopic understanding of interlayer exchange coupling. The measuring system consists of a Kerr microscope, a CCIR camera (with an 8-bit framegrabber), 16 bit digital camera and computer system for real-time image processing and to control external magnetic field and cameras. The strength of ferromagnetic (EM) coupling as a function of the spacer thickness of $Al_2O_3$ was investigated. It was found that strong FM-coupling, strong uniaxial anisotropy and coherent rotation of the magnetization have been observed for the spacer thickness in the range of 0.2 nm $\leq$ t $\leq$ 1 m, however, weak FM-coupling, patch domains and $360^{\circ}$-walls occur for the spacer thickness of t = 2.5 nm. At a spacer thickness of t $\geq$ 5 nm transition takes place from weak FM-coupling to the decoupled state where complex interlayer interactions and different types of the domain walls were observed.

Exploration of structural, thermal and spectroscopic properties of self-activated sulfate Eu2(SO4)3 with isolated SO4 groups

  • Denisenko, Yu.G.;Aleksandrovsky, A.S.;Atuchin, V.V.;Krylov, A.S.;Molokeev, M.S.;Oreshonkov, A.S.;Shestakov, N.P.;Andreev, O.V.
    • Journal of Industrial and Engineering Chemistry
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    • v.68
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    • pp.109-116
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    • 2018
  • $Eu_2(SO_4)_3$ was synthesized by chemical precipitation method and the crystal structure was determined by Rietveld analysis. The compound crystallizes in monoclinic space group C2/c. In the air environment, $Eu_2(SO_4)_3$ is stable up to $670^{\circ}C$. The sample of $Eu_2(SO_4)_3$ was examined by Raman, Fourier-transform infrared absorption and luminescence spectroscopy methods. The low site symmetry of $SO_4$ tetrahedra results in the appearance of the IR inactive ${\nu}_1$ mode around $1000cm^{-1}$ and ${\nu}_2$ modes below $500cm^{-1}$. The band intensities redistribution in the luminescent spectra of $Eu^{3+}$ ions is analyzed in terms of the peculiarities of its local environment.

Microstructures in Blue Feathers of the Common Kingfisher

  • Lee, Eunok;Bae, Haejin;Jeon, Deok-Jin;Ji, Seungmuk;Yeo, Jong-Souk;Kim, Jinhee
    • Proceedings of the National Institute of Ecology of the Republic of Korea
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    • v.2 no.1
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    • pp.21-25
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    • 2021
  • We measured the sizes of microstructures and the reflectance of blue feathers in the Common Kingfisher (Alcedo atthis). The colors were mainly produced in the barbs, which were composed of keratin sponge layers with air spaces and melanin rods. The reflectance spectra of back and tail feathers of the Common Kingfisher showed a peak with a broad plateau in the visible wavelength, whereas those of the wing feathers showed peaks in ultraviolet and visible and short-wavelengths. Moreover, the reflectance of back and tail feathers was higher than that of wing feathers. The blue color of the feathers comes from the keratin sponge layer due to coherent scattering. The back and tail feathers are composed of the keratin sponge layer only, and the wing feathers are composed of the keratin sponge layer and the keratin honeycomb structure. Due to the difference in these structures, it supposed that the reflectance is different. Determining why the reflectance spectra of the back and tail feathers were flattened will require further study.

Experimental investigation on flow field around a flapping plate with single degree of freedom

  • Hanyu Wang;Chuan Lu;Wenhai Qu;Jinbiao Xiong
    • Nuclear Engineering and Technology
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    • v.55 no.6
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    • pp.1999-2010
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    • 2023
  • Undesirable flapping motion of discs can cause the failure of swing check valves in nuclear passive safety systems. Time-resolved particle image velocimetry (PIV) was employed to investigate the flow characteristics around a free-to-rotate plate and the motion response, with the Reynolds numbers, based on the hydraulic diameter of the channel, from 1.32 × 104 to 3.95 × 104. Appreciable flapping motion (±3.52°) appeared at the Reynolds number of 2.6 × 104 with the frequency of 5.08 Hz. In the low-Reynolds-number case, the plate showed negligible flapping. In the high-Reynolds-number case, the deflection angle increased with reduced flapping amplitude. The torque from the fluid determined the flapping amplitude. In the low-Reynolds-number case, Karman vortices were absent. With increasing Reynolds numbers, Karman vortices developed behind the plate with larger deflection angles. Strong interaction between the wake flow from the leading and trailing edge of the plate was observed. Based on power spectrum density (PSD) analysis, the vortex shedding frequency coincided with the flapping frequency, and the amplitude was positively correlated to the strength of the vortices. Proper orthogonal decomposition (POD) modes evince that, in the case of appreciable motion, coherent structures exhibited a larger spatial scale, enhancing the magnitude of the external torque on the plate.