• 제목/요약/키워드: Phase fluctuation

검색결과 322건 처리시간 0.023초

정지 및 회전하는 원주에 의한 난류후류의 응집구조 (An Investigation of the Coherent Structures in Turbulent Wake Past a Stationary and Rotating Cylinder)

  • 부정숙;이종춘
    • 대한기계학회논문집
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    • 제18권5호
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    • pp.1310-1321
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    • 1994
  • Turbulent coherent structures in the intermediate wake of a stationary and rotating cylinder, spin rate S=0.7, situated in a uniform were experimentally investigated using a conditionalphase average technique. Measurements were carried out at a section of 8.5 diameters downstream form the center of cylinder and a Reynolds number of $Re=6.5{\times}10^{3}.$/TEX> The phase averaged velocity and velocity vector fields, contours of vorticity, turbulent intermittency function and velocity fluctuation energy are presented and discussed in relation to the large scale coherent structures by Karman vortices that shed periodically from the cylinder. Coherent wake structures of the rotating cylinder is almost identical with stationary cylinder, but the lateral displacement and shrinkage of turbulent wake region is occured by rotation. Rotation of the cylinder result in that the deflection of wake center to deceleration region(Y/D${\simeq}-0.3)$ and the decrease of mean velocity defect(10%), vorticity strength of large scale structures(19%), total velocity fluctuation energy(12%).

Experiment and modeling of liquid-phase flow in a venturi tube using stereoscopic PIV

  • Song, Yuchen;Shentu, Yunqi;Qian, Yalan;Yin, Junlian;Wang, Dezhong
    • Nuclear Engineering and Technology
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    • 제53권1호
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    • pp.79-92
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    • 2021
  • Venturi tube is based on turbulent flow, whereby the microbubbles can be generated by the turbulent fragmentation. This phenomenon is common in several venturi bubblers used by the nuclear, aerospace and chemical industries. The first objective of this paper is to study the liquid-phase velocity field experimentally and develop correlations for the turbulent quantities. The second objective is to research velocity field characteristics theoretically. Stereoscopic PIV measurements for the velocity field have been analyzed and utilized to develop the turbulent kinetic energy in the venturi tube. The tracking properties of the tracer particles have been verified enough for us to analyze the turbulence field. The turbulence kinetic energy has a bimodal distribution trend. Also, the results of turbulence intensity along the horizontal direction is gradually uniform along the downstream. Both the mean velocity and the fluctuation velocity are proportional to the Reynolds number. Besides, the distribution trend of the mean velocity and the velocity fluctuation can be determined by the geometric parameters of the venturi tube. An analytical function model for the flow field has been developed to obtain the approximate analytical solutions. Good agreement is observed between the model predictions and experimental data.

Numerical analysis on two-phase flow-induced vibrations at different flow regimes in a spiral tube

  • Guangchao Yang;Xiaofei Yu;Yixiong Zhang;Guo Chen;Shanshan Bu;Ke Zhang;Deqi Chen
    • Nuclear Engineering and Technology
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    • 제56권5호
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    • pp.1712-1724
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    • 2024
  • Spiral tubes are used in a wide range of applications and it is significant to understand the vibration introduced by two-phase flow in spiral tubes. In this paper, the numerical method is used to study the vibration induced by the gas-liquid two-phase flow in a spiral tube with different flow regimes. The pressure fluctuation characteristics at the pipe wall and the solid vibration response characteristics are obtained. The results show that the motion of small bubbles in bubbly flow leads to small pressure fluctuations with low-frequency broadband (0-50 Hz). The motion of the gas plug in the plug flow causes small amplitude periodic pressure fluctuation with a shortened low-frequency broadband (0-15 Hz) compared to the bubbly flow. The motion of the gas slug in the slug flow causes large periodic fluctuations in pressure with a significant dominant frequency (6-7 Hz). The wavy flow is very stable and has a distinct main frequency (1-2 Hz). The vibration regime in the bubbly flow and wave flow are close to the first-order mode, and the vertical vibrating component is dominant. The plug flow and slug flow excite higher-order vibration modes, and the lateral vibration component plays more important part in the vibration response.

다중파 전파전파환경에서의 이동화상통신의 동기시간 변동량해석에 관한 연구 (An Analysis of the SYNC Timing Fluctuations in Mobile Visual Communication Urder Urban Multipath Propagation Environments)

  • 하덕호
    • 한국통신학회논문지
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    • 제14권5호
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    • pp.472-485
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    • 1989
  • 광대역 화상정보신호(TV방송파)의 이동수신에 의한 화상품질영화는 고정수신의 경우와는 전혀 다른 상황을 타나내며, 이동체의 이동과 더불어 시시각각 복잡하게 변화한다. 본 논문에서는 야외 및 실내에서의 기초적 실험에 의해 텔레비 이동수신에서 Fluttering ghost가 화상품질열화원인중 가장 중요하다고 생각하여, 그 발생원인과 특성을 조사했다. Fluttering ghost는 텔레비 수상기에서 재생되는 수평동기펄스의 동기시각이 다중파간섭 페이딩에 의해 불안정하게 변동하므로서 발생한다는 것을 알았다. 따라서, H-sync pulse timing의 다중파 조건에 대한 변화를 이론적으로 해석하고 그 특성을 밝혀 동기시간변동의 조건과 원인을 밝혔다.

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액적변동을 고려한 액체로켓의 연소실 내 비정상 분무연소 해석 (Spray Combustion Analysis for Unsteady State in Combustion Chamber of Liquid Rocket Engine Considering Droplet Fluctuation)

  • 정대권;노태성
    • 한국추진공학회:학술대회논문집
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    • 한국추진공학회 2006년도 제27회 추계학술대회논문집
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    • pp.175-178
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    • 2006
  • 인젝터의 특성을 고려한 분무연소 현상을 해석하기 위하여 연소실에 주입되는 연료와 산화제의 액적에 변동을 가해 연소실내 분무연소 현상을 수치적으로 해석하였다. 2차원 비정상 상태의 유동장을 QUICK Scheme과 SIMPLER Algorithm을 사용하여 계산하였고, 분무모델로는 DSF 모델과 Euler-Lagrange방법을 사용하였다. 연료와 산화제의 액적 변동은 사인 함수를 이용하여 모델링 하였고, 액적과 가스상의 커플효과와 가스상과 증발된 기체상의 커플효과는 PSIC 모델을 사용하여 계산하였다.

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액티브형 직접메탄올연료전지 시스템의 메탄올 농도 변동이 성능에 미치는 영향성에 대한 수치적 연구 (A Numerical Investigation of Effects of Methanol Concentration Fluctuation in Active-type Direct Methanol Fuel Cell (DMFC) Systems)

  • 곽건희;고요한;이수원;이진우;백동현;정두환;주현철
    • 한국수소및신에너지학회논문집
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    • 제24권6호
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    • pp.495-509
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    • 2013
  • In this study, we develop a one-dimensional (1-D), two-phase, transient-thermal DMFC model to investigate the effect of methanol concentration fluctuation that usually occurs in active-type direct methanol fuel cell (DMFC) systems. 1-D transient simulations are conducted and time-dependent behaviors of DMFCs are analyzed under various DMFC operating conditions such as anode/cathode stoichiometry, cell temperature, and cathode inlet humidification. The simulation results indicate that the effect of methanol concentration fluctuation on DMFC performance can be mitigated by proper control of anode/cathode stoichiometry, providing a guideline to optimize operating conditions of active DMFC systems.

액체질소 분사 안정화를 통한 극저온가공 품질 향상 (Improvement of the Quality of Cryogenic Machining by Stabilization of Liquid Nitrogen Jet Pressure)

  • 강명구;민병권;김태곤;이석우
    • 한국정밀공학회지
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    • 제34권4호
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    • pp.247-251
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    • 2017
  • Titanium alloy has been widely used in the aerospace industry because of its high strength and good corrosion resistance. During cutting, the low thermal conductivity and high chemical reactivity of titanium generate a high cutting temperature and accelerates tool wear. To improve cutting tool life, cryogenic machining by using a liquid nitrogen (LN2) jet is suggested. In cryogenic jet cooling, evaporation of LN2 in the tank and transfer tube could cause pressure fluctuation and change the cooling rate. In this work, cooling uniformity is investigated in terms of liquid nitrogen jet pressure in cryogenic jet cooling during titanium alloy turning. Fluctuation of jet spraying pressure causes tool temperature to fluctuate. It is possible to suppress the fluctuation of the jet pressure and improve cooling by using a phase separator. Measuring tool temperature shows that consistent LN2 jet pressure improves cryogenic cooling uniformity.

Active and Reactive Power Control of ESS in Distribution System for Improvement of Power Smoothing Control

  • Shin, Seong-Su;Oh, Joon-Seok;Jang, Su-Hyeong;Cha, Jae-Hun;Kim, Jae-Eon
    • Journal of Electrical Engineering and Technology
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    • 제12권3호
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    • pp.1007-1015
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    • 2017
  • This paper proposes a new control technique of energy storage system (ESS) for smoothing the active power of renewable energy sources (RES) such as photovoltaic and wind turbine generation. As the penetration of RES into grid increases, it is difficult to maintain the permissible level of power quality, that is, voltage and frequency fluctuation in power systems. To solve this problem, ESS control methods using low pass filter (LPF) have been proposed for mitigating the fluctuation of RES output. However, those have a lot of drawbacks which need to be supplemented. Hence, this paper presents the improved active power control with additional reactive power control for maintaining power quality properly. The proposed method minimizes the capacity of ESS to be required for smoothing RES output fluctuation through mitigation of phase delay problem in LPF. In addition, the voltage regulation improves by using additional reactive power control. The proposed method was verified through simulation analysis using PSCAD/EMTDC.

Phase Behavior of Binary and Ternary Blends Having the Same Chemical Components and Compositions

  • Yoo, Joung-Eun;Kim, Yong;Kim, Chang-Keun;Lee, Jae-Wook
    • Macromolecular Research
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    • 제11권5호
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    • pp.303-310
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    • 2003
  • The phase behavior of binary blends of dimethylpolycarbonate-tetramethyl polycarbonate (DMPCTMPC) copolycarbonates and styrene-acrylonitrile (SAN) copolymers has been examined and then compared with that of DMPC/TMPC/SAN ternary blends having the same chemical components and compositions except that the DMPC and TMPC were present in the form of homopolymers. Both binary and ternary blends were miscible at certain blends compositions, and the miscible blends showed the LCST-type phase behavior or did not phase separated until thermal degradation temperature. The miscible region of binary blends is wider than that of the corresponding ternary blends. Furthermore, the phase-separation temperatures of miscible binary blends are higher than those of miscible ternary blends at the same chemical compositions. To explain the destabilization of polymer mixture with the increase of the number of component, interaction energies of binary pairs involved in these blends were calculated from the phase separation temperatures using lattice-fluid theory and then the phase stability conditions for the polymer mixture was analyzed with volume fluctuation thermodynamics.

Phase Resonance in a Centrifugal Compressor

  • Nishiyama, Yumeto;Suzuki, Takayuki;Yonezawa, Koichi;Tanaka, Hiroshi;Doerfler, Peter;Tsujimoto, Yoshinobu
    • International Journal of Fluid Machinery and Systems
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    • 제4권3호
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    • pp.324-333
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    • 2011
  • Phase resonance in a centrifugal compressor was experimentally observed and simulated with a commercial CFD code. It was found that pressure fluctuation at the volute outlet becomes the maximum when the rotational speed of the modes caused by the rotor-stator interaction agrees with the sound velocity. A simple one-dimensional theory is presented to explain the phase resonance in turbomachinery.