• Title/Summary/Keyword: Wake flow

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A Study of Generation of Coherent Vortex in Late Wake (잔류내 응집 구조 와류의 생성에 관한 연구)

  • Lee Sungsu
    • Proceedings of the KSME Conference
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    • 2002.08a
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    • pp.443-446
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    • 2002
  • Wake downstream of an object in the stratified flow has been of long-standing interest in fluid dynamics because of its similarity to geophysical flow over topographical terrains and more recently, concerns about the wake left behind a body moving through the ocean thermocline. Decades of studies of geophysical flow have unveiled that the flow downstream of obstacles in stratified flow consists of attached wake and strong internal waves, or separated, fluctuating wake and persistent late wakes, all of which depend on the flow conditions. Among unique and interesting characteristics of the stratified flow past obstacles is the generation of coherent vortex structure in the late wake far downstream of the object. Without the density stratification, the flow field downstream becomes undisturbed after relatively fast diminishing of the near wake. However, no matter how small the stratification is, the flow field downstream self-develops coherent vortex structures even after diminishing of the near wake. This paper present a computational approach to simulate the generation mechanism of the coherent vortex and analysis of the vortical structure.

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Effect of Flow Liners on Ship′s Wake Simulation in a Cavitation Tunnel

  • Lee, Jin-Tae;Kim, Young-Gi
    • Journal of Hydrospace Technology
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    • v.1 no.1
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    • pp.41-56
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    • 1995
  • Flew control devices, such as flow liners, are frequently introduced in a cavitation tunnel in order to reduce the tunnel blockage effect, when a three-dimensional wake distribution is simulated using a complete ship model or a dummy model. In order to estimate the tunnel wall effect and to evaluate the effect of flow liners on the simulated wake distribution, a surface panel method is adopted for the calculation of the flow around a ship model and flow liners installed in a rectangular test section off cavitation tunnel. Calculation results on the Sydney Express ship model show that the tunnel wall effect on the hull surface pressure distribution is negligible for less than 5% blockage and can be appreciable for more than 20% blockage. The flow liners accelerate the flow near the afterbody of the ship model, so that the pressure gradient there becomes more favorable and accordingly the boundary layer thickness would be reduced. Since the resulting wake distribution is assumed to resemble the full scale wake, flow liners can also be used to simulate an estimated full scale wake without modifying the ship model. Boundary taper calculation should be incorporated in order to correlate the calculated wake distribution with the measured one.

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Effects of Secondary Flow on the Turbulence Structure of a Flat Plate Wake (2차유동이 평판후류의 난류구조에 미치는 영향)

  • Kim, Hyeong Soo;Lee, Joon Sik;Kang, Shin Hyung
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.23 no.9
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    • pp.1073-1084
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    • 1999
  • The effects of secondary flow on the structure of a turbulent wake generated by a flat plate was investigated experimentally. The secondary flow was induced In a $90^{\circ}$ curved duct in which the flat plate wake generator was installed. The wake generator was installed in such a way that the wake velocity gradient exists in the span wise direction of the curved duct. Measurements were made in the plane containing the mean radius of curvature where pressure gradient and curvature effects were small compared with the secondary flow effect. All six components of the Reynolds stresses were measured in the curved duct. Turbulence intensities in the curved wake are higher than those in the straight wake due to an increase of the turbulent kinetic energy production by the secondary flow. In the inner wake region, shear stress and strain in the plane containing the velocity gradient of the wake show opposite signs with respect to each other, so that eddy viscosity Is negative in this region. This indicates that gradient-diffusion type turbulence models are not appropriate to simulate this type of flow.

Numerical Study of Coherent Vortex in Late Wake Downstream of a Sphere in Weakly Stratified Fluid (잔류내 응집 와류의 수치 해석)

  • Lee, Sung-Su;Lee, Young-Kyu;Yang, Kyung-Soo
    • Proceedings of the KSME Conference
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    • 2003.04a
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    • pp.1863-1868
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    • 2003
  • Decades of studies of geophysical flow have unveiled that the flow downstream of obstacles in stratified flow consists of attached wake and strong internal waves, or separated, fluctuating wake and persistent late wakes. Among unique and interesting characteristics of the stratified flow past obstacles is the generation of coherent vortex the late wake far downstream of the object. Unlike in homogeneous fluid, the flow field downstream self-develops coherent vortex even after diminishing of the near wake, no matter how small the stratification is. This paper present a computational approach to simulate the generation of the coherent vortex structure in late wake of a moving sphere submerged in weakly stratified fluid. The results are in consistent with several experimental observations and the vortex stretching mechanism is employed to explain the process of coherence.

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Effect of a Turbulent Wake on Two-Dimensional Subsonic Jet (노즐내 물체의 후류가 아음속 이차원 제트구조에 미치는 영향에 관한 연구)

  • Kim, Tae-Ho;Lee, Sang-Chan;Yoon, Bok-Hyun;Oh, Dae-Geun;Kim, Heuy-Dong
    • Proceedings of the KSME Conference
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    • 2003.11a
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    • pp.986-991
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    • 2003
  • A turbulent wake generated by a cylinder in nozzle contraction affects to the jet flow characteristics. In this study, a computational work to investigate the effect of the turbulent wake on two-dimensional subsonic jet was carried out with three different kinds of nozzle. Computations are applied to the two-dimensional unsteady, Navier-Stokes equations. Several kinds of turbulent models and wall functions are employed to validate the computational predictions. It was known that the wake flow enhanced the spread of the jet flow, compared with no wake flow condition. It was also found that the jet core is shortened by the wake flow developed from a control cylinder.

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A Study on the Characteristics of Cylinder Wake Placed in Thermally Stratified Flow (I) (열성층유동장에 놓인 원주후류의 특성에 대한 연구 (1))

  • 김경천;정양범;김상기
    • Transactions of the Korean Society of Mechanical Engineers
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    • v.18 no.3
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    • pp.690-700
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    • 1994
  • The effects of thermal stratification on the flow of a stratified fluid past a circular cylinder were examined in a wind tunnel. In order to produce strong thermal stratifications, a compact heat exchanger type variable electric heater is employed. Linear temperature gradient of up to $250^{\circ}C/m$ can be well sustained. The velocity and temperature profiles in the cylinder wake with a strong thermal gradient of $200^{\circ}C/m$ were measured and the smoke wire flow visualization method was used to investigate the wake characteristics. It is found that the temperature field effects as an active contaminant, so that the mean velocity and temperature profiles can not sustain their symmetricity about the wake centerline when such a strong thermal gradient is superimposed. It is evident that the turbulent mixing in the upper half section is stronger than that of the lower half of the wake in a stably stratified flow.

Unsteady Flow Fields in a Rotor Blade Passage by Wake Passing (회전익 채널내 후류장에 의한 비정상 유동특성에 관한 연구)

  • Kim, Youn J.;Jeon, Y.-R
    • The KSFM Journal of Fluid Machinery
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    • v.2 no.4 s.5
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    • pp.16-23
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    • 1999
  • The characteristic of unsteady flowfields on gas turbine, particularly on a rotor blade surface has been numerically investigated. The unsteady flow in a rotor blade passage as a result of wake/blade interaction is modeled by the inviscid flow approach, and solved by Euler equations using a time accurate marching scheme. Unsteady flow in the blade passage is induced by periodically moving a wake model across the passage inlet. The wake model used in this study is the Gaussian wate model in which the wake flow is assumed to be parallel with uniform static pressure and uniform relative total enthalpy. Numerical results show that for the case of Ps/Pr=1.5, the velocity and pressure distribution on the blade surfaces have much more complex profiles than for the case of Ps/Pr=1.0.

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Formation of Coherent Vortices in Late Wake Downstream of an Object in Weakly Stratified Fluid (약한 밀도 층상류에서 발생하는 물체 후류의 잔류와 응집 와류의 형성)

  • Lee, Sung-Su;Kim, Hak-Sun
    • Journal of the Society of Naval Architects of Korea
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    • v.48 no.5
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    • pp.414-420
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    • 2011
  • Decades of studies of geophysical flow have unveiled that the flow downstream of obstacles in stratified flow consists of attached wake and strong gravity waves, or separated, fluctuating wake and persistent late wakes. Among unique and interesting characteristics of the stratified flow past obstacles is the generation of coherent vortex in the late wake far downstream of the object. Unlike in homogeneous fluid, the flow field downstream self-develops coherent vortex even after diminishing of the near wake, no matter how small the stratification is. This paper present a computational approach to simulate the generation of the coherent vortex structure in the late wake of a moving sphere submerged in weakly stratified fluid. The results are in consistent with several experimental observations and the vortex stretching mechanism is employed to explain the process of coherence.

Influence of Flow Conditions on a Boundary Layer to the Near-Wake of a Flat Plat (평판 경계층 유동조건이 근접후류에 미치는 영향)

  • Kim, D.H.;Chang, J.W.
    • Proceedings of the KSME Conference
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    • 2004.04a
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    • pp.1625-1630
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    • 2004
  • An experimental study was carried out to investigate influence of flow conditions on a boundary layer to the near-wake of a flat plate. The flow condition in the vicinity of trailing edge that is influenced by upstream condition history is an essential factor that determines the physical characteristics of a near-wake. Various tripping wires were used to change boundary layer flow condition of upstream at the freestream velocity of 6.0 m/sec. Measurements of the boundary layer and near-wake according to the change of upstream conditions were conducted by using both I-probe(55P14 for boundary layer) and X-probe(55P61 for wake). Normalized velocity profiles of the boundary layer were shown the flow types such as laminar boundary layer, transition, and turbulent boundary layer at 0.95C from the leading edge. The velocity and turbulence intensity profiles of the near-wake for the case of laminar boundary layer at the flat plate surface exhibited a defect and a double peak showing perfect symmetry, respectively.

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Flow Characteristics of Wake Flow with Relation to a Tip Leakage Vortex at Different Flow Rates in an Axial Flow Fan (유량에 따른 축류홴의 익단누설와류 및 후류 특성)

  • Kim Kwang-Yong;Jang Choon-Man
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.29 no.3 s.234
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    • pp.322-329
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    • 2005
  • The flow characteristics in the blade passage and in the wake region of a low speed axial flow fan have been investigated by experimental analysis using a rotating hot-wire sensor for design and off-design operating conditions. The results show that the tip leakage vortex is moved upstream when flow rate is decreased, thus disturbing the formation of wake flow near the rotor tip. The tip leakage vortex interfaces with blade pressure surface, and results in high velocity fluctuation near the pressure surface. From axial velocity distributions downstream of the fan rotor, large axial velocity decay near the rotor tip is observed at near stall condition, which results in large blockage compared to that at the design condition. Although the wake flow downstream of the rotor blade is clearly measured at all operating conditions, the trough of the high velocity fluctuation due to Karmann vortex street in the wake flow is mainly observed at a higher flow condition than the design flow rate.