• Title/Summary/Keyword: Near Wake

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PIV Measurements of Hull Wake behind a Container Ship Model with Varying Loading Condition and Reynolds number (선박 모형의 하중 (loading)조건 및 Reynolds 수의 변화에 따른 선미 반류의 PIV 속도장 측정)

  • Lee Jung-Yeop;Paik Bu-Geun;Lee Sang-Joon
    • 한국가시화정보학회:학술대회논문집
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    • 2005.12a
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    • pp.54-57
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    • 2005
  • Flow characteristics of hull wake behind a container ship model were investigated experimentally with varying loading condition and Reynolds number. Large-scale bilge vortices of nearly the same strength are formed in the near-wake region. They are symmetric and counter-rotating with respect to the wake centerline for all loading conditions tested. With going downstream for both design and ballast loading conditions, the strength of the bilge vortices decreases and the wake region expands due to diffusion and viscous dissipation. Under the design loading condition, the bilge vortices start to appear at St=0.363 transverse plane above the propeller-boss. For the ballast loading condition, however, the bilge vortices start to appear at St=0.591 below the propeller-boss. They move upward as the hull wake goes downstream and Reynolds number increases. These wake characteristics, under the ballast loading condition, may weaken the propulsion and cavitation performances of the propeller, which are usually optimized for the design loading condition.

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PIV analysis of free surface effects on flow around a rotating propeller with varying water depth (자유표면과 수심깊이가 회전하는 프로펠러 주위 유동에 미치는 영향에 대한 PIV 해석)

  • Paik Bu Geun;Lee Jung Yeop;Lee Sang Joon
    • 한국가시화정보학회:학술대회논문집
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    • 2004.11a
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    • pp.40-43
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    • 2004
  • The effects of free surface on wake behind a rotating propeller were investigated experimentally in a circulating water channel with the variation of water depth. Instantaneous velocity fields were measured using two-frame PIV technique at tow different blade phases and ensemble-averaged to investigate the phase-averaged flow structure in the wake region. For an isolated propeller, the flow behind the propeller is influenced by the propeller rotation and the free surface. The phase-averaged mean velocity fields show that the potential wake and the viscous wake are formed by the boundary layers developed on the blade surfaces. The interaction between the tip vortices and the slipstream causes the oscillating trajectory of tip vortices. Tip vortices are generated periodically and the slipstream contracts in the near-wake region. The presence of free surface affects the wake structure largely, when the water depth is less than 0.6D. The free surface modifies the vortex structure, especially the tip and trailing vortices and flow structure in slipstreams of the propeller wake behind X/D = 0.3.

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Wind Tunnel Study on Flow Characteristics around KRISO 300K VLCC Double-body Model (KRISO 300K VLCC 이중모형선의 유동특성에 대한 풍동실험 연구)

  • Hak-Rok Kim;Sang-Joon Lee
    • Journal of the Society of Naval Architects of Korea
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    • v.36 no.3
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    • pp.15-21
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    • 1999
  • The flow characteristics around KRISO 300K VLCC double-body model have been experimentally investigated in a closed-type subsonic wind tunnel. The local mean velocity and turbulence statistics including turbulent intensity. Reynolds shear stress and turbulent kinetic energy were measured using a x-type hot-wire probe. The measurements were carried out at several transverse stations of the stern and near wake regions. The surface flow was visualized using on oil-film technique to see the flow pattern qualitatively. The flow in the stern and near wake region revealed complicated three-dimensional flow characteristics. The VLCC model shows a hook-shaped wake structure behind the propeller boss in the main longitudinal vortex region. The thin boundary layer at midship was increased gradually in thickness over the stern and evolved into a full three-dimensional turbulent wake.

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Lock-on Characteristics of wake behind a Rotationally Oscillating Circular Cylinder (주기적으로 회전진동하는 원주 후류의 공진특성)

  • Lee Jung Yeop;Lee Sang Joon
    • 한국가시화정보학회:학술대회논문집
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    • 2004.11a
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    • pp.18-21
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    • 2004
  • Lock-on characteristics of the flow around a circular cylinder performing a rotationally oscillation with a relatively high forcing frequency have been investigated experimentally using flow visualization and hot-wire measurements. Dominant parameters are Reynolds number (Re), amplitude of oscillation $(\theta_A)$, and frequency ratio $F_R=f_f\;/\;f_n$, where $f_f$ is the forcing frequency and if is the natural frequency of vortex shedding. Experiments were carried out under the conditions of $Re=4.14\times10^3,\;\pi/15\leq\theta_A\leq\pi/3$, and $F_R=1.0$. The effects of this active control technique on the lock-on flow regime of the cylinder wake were evaluated through wake velocity measurements and spectral analysis of hot-wire signals. The rotary oscillation modified the flow structure of near wake significantly. The lock-on phenomenon was found to occur in the range of frequency encompassing the natural vortex shedding frequency. In addition, when the amplitude of oscillation is less than a certain value, the lock-on phenomenon was occurred only at $F_R=1.0$. The lock-on range expanded and vortex formation length decreased as the amplitude of oscillation increases. The rotary oscillation generated small-scale vortex structure just near the cylinder surface.

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Investigation of vortex core identification method for wind turbine wake (터빈 후류를 관찰하기 위한 와류 코어 식별 기법 연구)

  • Ko, Seungchul;Na, Jisung;Lee, Joon Sang
    • Journal of the Korean Society of Visualization
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    • v.15 no.1
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    • pp.19-24
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    • 2017
  • In this study, we conduct a numerical experiment of the single 5MW NREL wind turbine and compare the performance of various vortex core identification for the wake behind the wind turbine. In the kinetic analysis of wind turbine, 20% velocity deficit at 200 s is observed, showing wake which contains tip vortex near blade tip and rotor vortex at the center of the wind turbine. Time series of velocity and turbulent intensity show numerical simulation converge to a quasi-steady state near 200 s. In the comparison between methods for vortex identification, ${\lambda}_2$-method has good performance in terms of tip vortex, rotor vortex, vortex during its cascade process compared to vorticity magnitude criteria, ${\Delta}$-method. We conclude that ${\lambda}_2$-method is suitable for vortex identification method for wake visualization.

On the Thick Axisymmetric Boundary Layer and Wake Around the Body of Revolution (몰수분의 두꺼운 경계층 및 반류해석)

  • Gang, Sin-Hyeong;Hyeon, Beom-Su;Lee, Yeong-Gil
    • 한국기계연구소 소보
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    • s.9
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    • pp.141-151
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    • 1982
  • An iterative procedure for the calculation of the thick axisymmetric boundary layer and wake near the stern of a body of revolution is presented. Procedure consists of the potential flow calculation by a method of the integral equation of first kind and the calculation of boundary layer and wake by a differential me¬thod of the boundary layer theory. Additionally, higher order terms are included in the conventional momentum equations and continuity equation for the consider¬ation of the characteristics of axisymmetric flow different from the one of two dimentional flow and the thick boundary layer. These solutions are matched at the edge of boundary layer and wake. The results obtained by the present me¬thod are compared with the experimental data and it is found that the nominal wake distribution at the propeller plane of a axisymmetric body is in good agree¬ment with the experiment.

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PIV Velocity Field Measurements of Flow around a Ship with Rotating Propeller (PIV를 이용한 선박 프로펠러 후류의 속도장 계측)

  • 이상준;백부근
    • Journal of the Society of Naval Architects of Korea
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    • v.40 no.5
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    • pp.17-25
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    • 2003
  • Velocity field behind a container ship model with a rotating propeller has been investigated using PIV (particle image velocimetry) system. Four hundred instantaneous velocity fields were measured at 4 different blade phases and ensemble-averaged to investigate the spatial evolution of vortical structure of near wake within one propeller diameter downstream. The phase-averaged mean velocity fields show the potential wake and the viscous wake formed due to the boundary layers developed on the blade surfaces. The interaction between bilge vortex developed along the hull surface and the tangential velocity component of incoming flow causes to have asymmetric flow structure in the transverse plane.

Phase-Locked Three-Dimensional Structures in the Cylinder Wake Observed from Cinematic PIV Data (Cinematic PIV에 의한 실린더 후류의 위상평균된 3차원 구조)

  • Sung, Jae-Yong;Park, Kang-Kuk;Yoo, Jung-Yul
    • Proceedings of the KSME Conference
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    • 2000.04b
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    • pp.661-666
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    • 2000
  • Near-wake flow field of a circular cylinder is studied by means of a cinematic PIV system with high sampling rate and large internal memory block. Experiments are conducted in a closed-cycle water tunnel system and a cross-correlation algorithm in conjunction with FFT (Fast Fourier Transform) analysis and an offset correlation technique is used for vector processing. With the help of very high sampling frequency compared to the shedding frequency, it is possible to obtain phase-averaged information of the three-dimensional wake, even though the shedding is not forced but natural. Phase-locked vortical structures observed simultaneously from the spanwise and cross-stream planes are displayed in the wake-transition regime where fine-scale secondary vortices have a spanwise wavelength or around one diameter. Spatial relations and temporal evolutions of the primary Karman vortex and the secondary vortex are also discussed schematically.

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Near-wake Measurements of an Oscillating NACA 0012 Airfoil (진동하는 NACA 0012 에어포일의 근접후류 측정)

  • Kim, Dong-Ha;Kim, Hak-Bong;Jang, Jo-Won
    • Journal of the Korean Society for Aeronautical & Space Sciences
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    • v.34 no.12
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    • pp.1-8
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    • 2006
  • An experimental study was carried out in order to investigate the influence of Reynolds number on the near-wake of an oscillating airfoil. An NACA 0012 airfoil was sinusoidally pitched at the quarter chord point, and is oscillated over a range of instantaneous angles of attack of $\pm$6$^{\circ}$. An X-type hot-wire probe was employed to measure the near-wake of an oscillating airfoil, and the smoke-wire visualization technique was used to examine the flow properties of the boundary layer. The free-stream velocities were 1.98, 2.83 and 4.03 m/s and the corresponding chord Reynolds numbers were 2.3${\times}10^4$, 3.3$\times$104 and 4.8${\times}10^4$, respectively. The frequency of airfoil oscillation was adjusted to fix a reduced frequency of K=0.1. The results show that the properties of the boundary layer and the near-wake can dramatically be distinguished in the range of Reynolds numbers between 2.3${\times}10^4$ and 3.3${\times}10^4$, on the other hand, it is similar in the cases of Re=3.3$\times$104 and 4.8$\times$104. This is caused by that the unsteady separation point is dramatically delayed in case of Re= 2.3${\times}10^4$.

Study on the Three Dimensional Flow Characteristics of the Propeller Wake Using PIV Techniques (PIV 기법을 이용한 프로펠러 후류의 3차원 유동 특성 연구)

  • Paik, Bu-Geun;Kim, Jin;Kim, Kyung-Youl;Kim, Ki-Sup
    • Journal of the Society of Naval Architects of Korea
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    • v.44 no.3 s.153
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    • pp.219-227
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    • 2007
  • A stereo-PIV (particle image velocimetry) technique is used to investigate the vortical structure of the wake behind a rotating propeller in the present study. A four bladed propeller is tested in a cavitaion tunnel without any wake screen. Hundreds of instantaneous velocity fields are phase-averaged to reveal the three dimensional spatial evolution of the flow behind the propeller. The results of conventional 2-D PIV are also compared with those of the stereo-PIV to understand the vortical structure of propeller wake deeply. The variations of radial and axial velocities in the 2-D PIV results seem to be affected by the out-of-plane motion. generating a little perspective error in the in-plane velocity components of the slipstream. The strong out-of-plane motion around the hub vortex also causes the perspective error to vary the axial velocity component a little at the near wake region. The out-of-plane velocity component had the maximum value of about 0.3U0 in the tip vortices and continued its magnitude in the wake region.