• Title, Summary, Keyword: Decay Length

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Characterization and correction of bemding deformation in pizeoelectric ceramics displacement (길이변조용 압전소자의 휨 측정과 보정)

  • 김재완;남승희;한재원
    • Korean Journal of Optics and Photonics
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    • v.12 no.4
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    • pp.300-304
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    • 2001
  • We suggest a method to measure and correct bending deformation in piezoelectric ceramics displacement. The angle and direction of the bending deformation are measured by monitoring the position of a laser beam reflected on a mirror which is attached to the piezoelectric ceramics with the uncertainty of the angle measurement of $0.36\mu$rad. We divided the electrode of a piezoelectric ceramic into 3 parts and connected 3 capacitors to each electrode in order to apply different voltage to each electrode with one voltage supplier. The deformation was corrected by adjusting the capacitance of each capacitor and was reduced to 6.3%, comparing to the uncorrected case. By using this corrected piezoelectric ceramic to modulate the length of the ringdown cavity, the fluctuation of the decay time caused by the change in optic axis of the cavity was removed.emoved.

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A Fundamental Study of the Supersonic Microjet Flow (초음속 마이크로 제트 유동에 관한 기초적 연구)

  • 정미선;김현섭;김희동;박종호
    • Journal of the Korean Society of Propulsion Engineers
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    • v.6 no.1
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    • pp.63-70
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    • 2002
  • Computational modeling and simulation can provide an effective predictive capability for the major features of the supersonic microjets. In the present study, computations using the axisymmetic, compressible, Navier-Stokes equations are applied to understand the supersonic microjet flow physics. The pressure ratio of the microjets is changed between 0.2 and 1.25 to obtain both the under- and over-expanded flows at the exit of the micronozzle. and Reynolds number Re is changed between 600 to 40000. For both laminar and turbulent microjet flows, sonic and supersonic microjets are simulated and compared with some experimental results available. Based on computational results, two microjets are discussed in terms of total pressure, jet decay and supersonic core length.

Nanoscale microstructure and magnetic transport in AlN/Co/AlN/Co... discontinuous multilayers

  • Yang, C.J.;Zhang, M.;Zhang, Z.D.;Han, J.S.
    • Proceedings of the Korean Magnestics Society Conference
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    • pp.21-21
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    • 2003
  • Microstructure and magnetic transport phenomina in rf sputtered AlN/Co type ten-layered discontinuous films of nanoscaled [AlN (3 nm)/Co (t nm)]...$\sub$10/ with t$\sub$Co/=1.0∼2.0 nm have been investigated. The microstructure and tunneling magnetic resistance of the samples are strongly dependent on the thickness of Co layer. Negative tunneling magneto-resistance due to the spin-dependent transport has been observed along the current-in-plane configuration in the samples having the Co layers below 1.6 nm thick. When the thickness of Co layer was less than 1,2 nm, randomly oriented granular Co particles were completely isolated and embedded in amorphous AlN matrix, and the films showed the superparamagnetic behavior with a high MR value of Δ$\rho$/$\rho$$\sub$0/=1.8 %. As t$\sub$Co/ increases, a transition from the regime of co-existence of superparamagnetic and ferromagnetic behaviors to ferromagnetic behavior was observed. Tunneling barrier called "decay length for tunneling" for the films having the thickness of Co layer from 1.4 to 1.6 nm was measured to be ranged from 0.004 to 0.021 ${\AA}$$\^$-1/.

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The effects of the Control of Combustion Instabilities in accordance with various Acoustic Cavities (음향공 형상에 따른 연소 불안정 제어 효과)

  • Cha Jung-Phil;Yang Jea-Jun;Seo Ju-Hyoung;Kim Hong-Jip;Ko Young-Sung
    • Proceedings of the Korean Society of Propulsion Engineers Conference
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    • pp.73-76
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    • 2006
  • Acoustic cavity as a stabilization device to control high-frequency combustion instabilities in liquid rocket engine is adopted and its damping capacity is verified in atmospheric temperature. Geometric effects of acoustic cavity on damping characteristics are analyzed and compared quantitatively. Satisfactory agreements have been achieved with linear acoustic analysis and experimental approach. Results show that the acoustic cavity of the largest orifice area or the shortest orifice length was the most effective in acoustic damping of the harmful resonant frequency finally, it is proved that an optimal design process is indispensable for the effective control of combustion instabilities.

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Geometric Effects on Damping Characteristics of Acoustic Cavity for the Control of Combustion Instabilities (연소불안정 제어를 위한 음향공의 감쇠에 대한 형상 효과)

  • 차정필;고영성;고영성
    • Journal of the Korean Society for Aeronautical & Space Sciences
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    • v.34 no.6
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    • pp.59-66
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    • 2006
  • Acoustic cavity as a stabilization device to control high-frequency combustion instabilities in liquid rocket engine is adopted and its damping capacity is verified in atmospheric temperature. First, harmful resonant frequency in a modeling chamber can be damped effectively by the installation of properly-tuned acoustic cavity. Besides, geometric effects of acoustic cavity on damping characteristics are analyzed and compared quantitatively. Satisfactory agreements have been achieved with linear acoustic analysis and experimental approach. Results show that the acoustic cavity of the largest orifice area or the shortest orifice length was the most effective in acoustic damping of the harmful resonant frequency. Finally, it is proved that an optimal design process is indispensable for the effective control of combustion instabilities.

Finding Pseudo Periods over Data Streams based on Multiple Hash Functions (다중 해시함수 기반 데이터 스트림에서의 아이템 의사 주기 탐사 기법)

  • Lee, Hak-Joo;Kim, Jae-Wan;Lee, Won-Suk
    • Journal of Information Technology Services
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    • v.16 no.1
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    • pp.73-82
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    • 2017
  • Recently in-memory data stream processing has been actively applied to various subjects such as query processing, OLAP, data mining, i.e., frequent item sets, association rules, clustering. However, finding regular periodic patterns of events in an infinite data stream gets less attention. Most researches about finding periods use autocorrelation functions to find certain changes in periodic patterns, not period itself. And they usually find periodic patterns in time-series databases, not in data streams. Literally a period means the length or era of time that some phenomenon recur in a certain time interval. However in real applications a data set indeed evolves with tiny differences as time elapses. This kind of a period is called as a pseudo-period. This paper proposes a new scheme called FPMH (Finding Periods using Multiple Hash functions) algorithm to find such a set of pseudo-periods over a data stream based on multiple hash functions. According to the type of pseudo period, this paper categorizes FPMH into three, FPMH-E, FPMH-PC, FPMH-PP. To maximize the performance of the algorithm in the data stream environment and to keep most recent periodic patterns in memory, we applied decay mechanism to FPMH algorithms. FPMH algorithm minimizes the usage of memory as well as processing time with acceptable accuracy.

Spatial mapping of screened electrostatic potential and superconductivity by scanning tunneling microscopy/spectroscopy

  • Hasegawa, Yukio;Ono, Masanori;Nishio, Takahiro;Eguchi, Toyoaki
    • Proceedings of the Korean Vacuum Society Conference
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    • pp.12-12
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    • 2010
  • By using scanning tunneling microscopy/spectroscopy (STM/S), we can make images of various physical properties in nanometer-scale spatial resolutions. Here, I demonstrate imaging of two electron-correlated subjects; screening and superconductivity by STM/S. The electrostatic potential around a charge is described with the Coulomb potential. When the charge is located in a metal, the potential is modified because of the free electrons in the host. The potential modification, called screening, is one of the fundamental phenomena in the condensed matter physics. Using low-temperature STM we have developed a method to measure electrostatic potential in high spatial and energy resolutions, and observed the potential around external charges screened by two-dimensional surface electronic states. Characteristic potential decay and the Friedel oscillation were clearly observed around the charges [1]. Superconductivity of nano-size materials, whose dimensions are comparable with the coherence length, is quite different from their bulk. We investigated superconductivity of ultra-thin Pb islands by directly measuring the superconducting gaps using STM. The obtained tunneling spectra exhibit a variation of zero bias conductance (ZBC) with a magnetic field, and spatial mappings of ZBC revealed the vortex formation [2]. Size dependence of the vortex formation will be discussed at the presentation.

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A Correlation between the Pressure Oscillation of Combustion Chamber and Thrust Response in a 70 N-class Hydrazine Thruster (70 N급 하이드라진 추력기의 연소실 압력진동 강도와 추력 응답특성의 상관관계)

  • Jung, Hun;Kim, Jeong Soo
    • Journal of the Korean Society of Propulsion Engineers
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    • v.19 no.3
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    • pp.1-8
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    • 2015
  • A ground hot-firing test(HFT) was accomplished to draw a correlation between the pressure oscillation intensity of combustion chamber and thrust response characteristics in a 70 N-class hydrazine thruster which has been developed recently. Monopropellant grade hydrazine was adopted as a propellant for the HFT, and combustion-chamber characteristic length, propellant injection pressure were applied as test parameters. It was confirmed that the decrease of thrust-chamber diameter and injection pressure augmented the pressure oscillation of stagnation chamber in the test condition specified, and the oscillation hampered the pulse response performance of test models.

Experimental validation of simulating natural circulation of liquid metal using water

  • Lee, Min Ho;Jerng, Dong Wook;Bang, In Cheol
    • Nuclear Engineering and Technology
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    • v.52 no.9
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    • pp.1963-1973
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    • 2020
  • Liquid metal-cooled reactors use various passive safety systems driven by natural circulation. Investigating these safety systems experimentally is more advantageous by using a simulant. Although numerous experimental approaches have been applied to natural circulation-driven passive safety systems using simulants, there has been no clear validation of the similarity law. To validate the similarity law experimentally, SINCRO-V experiment was conducted using Wood's metal and water for simulant of the Wood's metal. A pair of SINCRO-V facilities with length-scale ratio of 14.1:1 for identical Bo' was investigated, which was the main similarity parameter in temperature field simulation. In the experimental range of 0.2-1.0% of decay heat, the temperature distribution characteristics of the small water facility were very similar to that of the large Wood's metal facility. The temperature of the Wood's metal predicted by the water experiment showed good agreement with the actual Wood's metal temperature. Despite some error factors like discordance of Gr' and property change along the temperature, the water experiment predicted the Wood's metal temperature with an error of 27%. The validity of the similarity law was confirmed by the SINCRO-V experiments.

Flow Characteristics of Two-Dimensional Turbulent Stepped Wall Jet (2次元 亂流 Stepped Wall Jet 의 流動特性)

  • 부정숙;김경천;박진호;강창수
    • Transactions of the Korean Society of Mechanical Engineers
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    • v.9 no.6
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    • pp.732-742
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    • 1985
  • Measurements of mean velocity and turbulence characteristics are obtained with a linearized constant temperature hot-wire anemometer in a two-dimensional turbulent jet discharging parallel to a flate. Wall static pressure distribution is also measure. The Reynolds number based on the jet nozzle width (D) is about 42,000 and the step height is 2.5D. The reattachment length is found to be 7.5D by using both wool tuft and oil methods. Upstream of the reattachment point, there exist double coherent structures and mean velocity, Reynolds stresses and triple product profiles are asymmetric about jet center line due to the influence of streamline curvature and recirculating flow region. Near the reattachment point, wall static pressure and turbulence quantities change its shape rapidly because of the large eddies by the solid wall. Especially, turbulence intensity has a maximum value in the reattachment regin, then decreases slowly in the redeveloping wall jet ragion. Downstream of X/D=14, a single large scale eddy structure is formed. Far downstream affer the reattachment(X/D.geq.18) mean velocity profile, the decay of maximum velocity and the variation of jet half width are nearly similar to those of plane wall jet, but the Reynolds stresses are higher than those of the latter.