• Title/Summary/Keyword: Wakes

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Stability Analysis of Reacting Wakes (반응성 후류유동의 불안정성 해석)

  • Shin, D.S.;Hong, S.J.;Park, S.H.
    • Journal of the Korean Society of Combustion
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    • v.2 no.1
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    • pp.9-16
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    • 1997
  • This paper investigates the linear stability of wakes with special emphasis on the difference of velocity and density. Velocity and density profiles for laminar flows have gaussian profiles. Incompressible wakes have two generalized inflection points and two unstable modes-sinuous and varicose modes. Sinuous modes are more unstable than varicose modes irrespective of density variation, which shows wakes will be destabilized by sinuous modes. Large velocity difference and density difference leads to more unstable wakes due to large momentum difference.

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Effect of periodic wakes on separated flows over a NACA0012 airfoil (주기적 통과 후류가 익형위 박리 유동에 미치는 영향)

  • Lee, Hui-Kang;Park, Tae-Choon;Jeon, Woo-Pyung;Kang, Shin-Hyoung
    • Proceedings of the KSME Conference
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    • 2004.04a
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    • pp.1619-1624
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    • 2004
  • Experimental study of separated flow over a NACA0012 airfoil is conducted at $Re=2{\times}10^5$ when periodic wakes pass over the airfoil. The wakes are periodically generated by circular cylinders upstream of the airfoil. The measurement of surface pressure and surface visualization at various angles of attack are carried out without and with passing wakes. Without passing wakes, a separation bubble at the leading edge of the suction surface is formed at an angle of attack, found from a local plateau in the streamwise pressure distribution and two distinct lines in the surface flow visualization. With passing wakes, however, the bubble disappears. Owing to passing wakes, the lift increases at high angle of attack and the angle of stall also increases.

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Study of the Effects of Wakes on Cascade Flow (후류가 익렬유동에 미치는 영향에 대한 실험적 연구)

  • Kim, Hyung-Joo;Joo, Won-Gu;Cho, Kang-Rae
    • 유체기계공업학회:학술대회논문집
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    • 1999.12a
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    • pp.309-314
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    • 1999
  • This paper is concerned with the viscous interaction between rotor and stator The viscous interaction is caused by wakes from upstream blades. The cascade was composed with five blades and cylinders were placed to make wakes and their location was about 50 percent of blade chord upstream. The location of cylinders were varied in the cascade axis with 0, 20, 40, 60 and 80 percent of pitch length. The velocity distribution in the cascade passage were measured using single slanted hot-wire and the ones in the boundary layer using boundary probe. As a result, wakes decay more rapidly at suction surface and more slowly at pressure surface. And the measurement of momentum thickness of cascade shows that the momentum thickness is larger near the blade surface. From measurement of blade boundary layer, turbulent intensity is also larger near the blade surface because wakes collide the boundary layer And wakes make boundary layer thickness smaller and delay flow separation.

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Stability Analysis of Wakes with Chemical Reaction (연소 반응을 가지는 후류 유동의 불안정성)

  • 신동신;홍성제
    • Journal of the Korean Society of Propulsion Engineers
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    • v.2 no.2
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    • pp.30-37
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    • 1998
  • This paper investigates the linear stability of wakes with special emphasis on the effect of chemical reaction. Velocity and density profiles for laminar flows are obtained from analytic profiles as well as from simulation. Wakes have two generalized inflection points and two unstable modes-sinuous and varicose modes. For analytical laminar profiles, sinuous modes are more unstable than varicose modes irrespective of density variation, which shows wakes will be destabilized by sinuous modes. Large velocity difference and density difference lead to more unstable wakes due to large momentum difference. For simulated laminar profiles, chemical reaction with stoichiometric chemistry increases temperature and stabilizes the flow due to increase in compressible reacting wades, flow becomes stable as velocity increases due to viscous dissipation.

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Numerical Analysis of a Turbine Rotor Cascade with Unsteady Passing Wakes (비정상 후류를 지나는 터빈 동익 주위의 유동장 수치해석)

  • Lee, Eun-Seok
    • 유체기계공업학회:학술대회논문집
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    • 2006.08a
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    • pp.153-156
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    • 2006
  • A turbine stage consists of a stator and rotor. A stator provides the required inlet flow conditions so that a rotor can produce the necessary power. Passing wakes generated at the trailing edge of a stator make an interaction with a rotor. In the present study, this interaction flow mechanism is investigated using the numerical analysis. In case of the large gap distance between the stator and rotor, the stator and rotor flow analysis can be separated. First, only the stator flow field is solved. Second, the rotor flow field is solved including the passing wake information from the stator analysis. The passing wake experiences the shearing as it approaches to the rotor leading edge. And it is chopped when it strikes the rotor body. After that, the chopped wakes becomes the prolongation as it goes downstream. Also, the aerodynamic characteristics with the variation of the gap distance between a stator and rotor was investigated. Pressure jumps due to the passing wakes result in the pressure and lift loss and it gets stronger with the closer gap distance. This unsteady effect proves to be directly related to the fatigue and noise in turbomachinery and this study would be helpful to investigate such fields.

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A novel approach of ship wakes target classification based on the LBP-IBPANN algorithm

  • Bo, Liu;Yan, Lin;Liang, Zhang
    • Ocean Systems Engineering
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    • v.4 no.1
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    • pp.53-62
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    • 2014
  • The detection of ship wakes image can demonstrate substantial information regarding on a ship, such as its tonnage, type, direction, and speed of movement. Consequently, the wake target recognition is a favorable way for ship identification. This paper proposes a Local Binary Pattern (LBP) approach to extract image features (wakes) for training an Improved Back Propagation Artificial Neural Network (IBPANN) to identify ship speed. This method is applied to sort and recognize the ship wakes of five different speeds images, the result shows that the detection accuracy is satisfied as expected, the average correctness rates of wakes target recognition at the five speeds may be achieved over 80%. Specifically, the lower ship's speed, the better accurate rate, sometimes it's accuracy could be close to 100%. In addition, one significant feature of this method is that it can receive a higher recognition rate than the nearest neighbor classification method.

Experimental Study on the Effects of Upstream Periodic Wakes on Aerofoil-Boundary Layer and Loss (주기적 상류 후류의 익 경계층과 손실에 미치는 영향에 대한 실험적 연구)

  • Rim, In-Won;Cho, Kang-Rae;Joo, Won-Gu
    • Proceedings of the KSME Conference
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    • 2001.06e
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    • pp.661-667
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    • 2001
  • This paper is concerned with the effects of periodically approaching upstream wakes on cascade-flow and loss. The reduced frequency of the periodic wakes was varied in the narrow range from 0.5 to 0.7. According to a wake-passing through the cascade, two velocity deficits appear near the boundary layer contour in the downstream from about 60% chord-length. The first velocity deficit is caused by a periodic wake and the second one appears after some delayed time. The second velocity deficit may be interpreted as the results of reattachment of flow-separation. The higher reduced frequency decreases the duration time of separation appearance and the lesser loss of aerofoil is resulted.

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Experimental Study on the Effects of Upstream Periodic Wakes on Aerofoil-Boundary Layer and Loss (주기적 상류 후류의 익 경계층과 손실에 매치는 영향에 대한 실험적 연구)

  • Im, In-Won;Jo, Gang-Rae;Ju, Won-Gu
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.26 no.2
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    • pp.219-226
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    • 2002
  • This paper is concerned with the effects of periodically approaching upstream wakes on cascade-flow and loss. The reduced frequency of the periodic wakes was varied in the narrow range from 0.5 to 0.7 Corresponding to a wake-passing through the cascade, two velocity deficits appeared near the boundary layer contour in the downstream from about 60% chord-length. The first velocity deficit was caused by a periodic wake and the second one appeared after some delayed time. The second velocity deficit was interpreted as the results of reattachment of flow-separation. The higher reduced frequency decreased the duration time of separation appearance and the lesser losses of blade were resulted.

Study of the Effects of Wakes on Cascade Flow (후류가 익렬 유동에 미치는 영향에 대한 실험적 연구)

  • Kim, Hyung-Joo;Cho, Kang-Rae;Joo, Won-Gu
    • Proceedings of the KSME Conference
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    • 2000.04b
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    • pp.561-567
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    • 2000
  • This paper is concerned with the viscous interaction between rotor and stator. The viscous interaction is caused by wakes from upstream blades. The rotor cascade in the experiment was composed with five blades, and cylinders were placed to make the stator wakes and their locations were about 50 percent upstream of blade chord. The locations of cylinders were varied in the direction of cascade axis with 0, 12.5, 25, 50, and 75 percent of pitch length. The static pressure distributions on the blade surfaces and the velocity distributions in the cascade flow were measured. From the experimental result it was found that the value of velocity defect by a cylinder wake might vary depending on the wake position within the cascade but the value at the cascade exit approached to some constant value regardless of the difference of wake locus. The momentum defect at the downstream from the cascade and the pressure distribution on the blade surfaces showed that the wake flowing near the blade surfaces caused the decrease of lift and the increase of drag regardless of the disappearance of flow separation.

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Comparison of Cavitation Patterns between Model Scale Observations using Model and Full-Scale Wakes and Full Scale Observations for a Propeller of Crude Oil Carrier (원유운반선 프로펠러의 모형 및 실선 축척 반류에서의 공동현상과 실선에서 관측된 공동현상의 비교)

  • Choi, Gil-Hwan;Chang, Bong-Jun;Hur, Jae-Wook;Cho, Dae-Seung
    • Journal of the Society of Naval Architects of Korea
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    • v.48 no.1
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    • pp.15-22
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    • 2011
  • In this paper, cavitation patterns of model tests were compared with those of full-scale measurement for a propeller of crude oil carrier which was suffered from erosions on suction side of blade tip region. Cavitation tests were performed at design and ballast draft using model and full scale nominal wakes. A model ship and wire mesh method was used for the simulation of wake patterns of model nominal wakes. For the prediction of full-scale wake patterns, a RANS solver(Fluent 6.3) was used and wire mesh method was used for the simulation of the full scale wakes. Comparison results show that cavitation patterns using predicted full-scale wake patterns are closer to cavitation patterns of full-scale measurement at ballast draft condition. Also, cloud cavitations were observed on the position of eroded area at both full-scale measurement and cavitation tests using simulated full-scale wake patterns.