• Title/Summary/Keyword: Cylinder Wake

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Flow structure of wake behind a finite circular cylinder (자유단이 있는 원주의 후류 유동특성에 관한 실험적 연구)

  • Lee, Sang-Jun;Jeong,Yong-Sam
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.20 no.6
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    • pp.2014-2022
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    • 1996
  • Flow characteristics of the wake behind a finite circular cylinder(FC) mounted on a flat plate was experimentally investigated. Three finite cylinder models having aspect ratio (length to diameter ratio, L/D) of 6,10 and 13 were tested in this study. Wake velocity was measured by a hot-wire anemometry at Reynolds number of 20,000, and the results were compared with those of two-dimensional circular cylinder. As a result, the free-end effect on the wake structure becomes more dominant with decreasing the aspect ratio(L/D) of the finite cylinder. Invisid flow entrained into the wake region decreases the turbulence intensity and periodicity of the vortex shedding due to existence of the free end. From spectral analysis and cross correlation of the velocity signals, vortices having 24Hz frequency characteristics are found in the down wash flow just behind the free end. There exists very complicated flow near the free end due to interaction between the entrained flow and streamwise vortices. Vortex formation region is destroyed significantly in the near wake and shows quite different wake structures from those of 2-D cylinder.

The effect of Reynolds number on the elliptical cylinder wake

  • Shi, Xiaoyu;Alam, Md. Mahbub;Bai, Honglei;Wang, Hanfeng
    • Wind and Structures
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    • v.30 no.5
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    • pp.525-532
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    • 2020
  • This work numerically investigates the effects of Reynolds number ReD (= 100 - 150), cross-sectional aspect ratio AR = ( 0.25 -1.0), and attack angle α (= 0° - 90°) on the forces, Strouhal number, and wake of an elliptical cylinder, where ReD is based on the freestream velocity and cylinder cross-section height normal to the freestream flow, AR is the ratio of the minor axis to the major axis of the elliptical cylinder, and α is the angle between the cylinder major axis and the incoming flow. At ReD = 100, two distinct wake structures are identified, namely 'Steady wake' (pattern I) and 'Karman wake followed by a steady wake (pattern II)' when AR and α are varied in the ranges specified. When ReD is increased to 150, an additional wake pattern, 'Karman wake followed by secondary wake (pattern III)' materializes. Pattern I is characterized by two steady bubbles forming behind the cylinder. Pattern II features Karman vortex street immediately behind the cylinder, with the vortex street transmuting to two steady shear layers downstream. Inflection angle αi = 32°, 37.5° and 45° are identified for AR = 0.25, 0.5 and 0.75, respectively, where the wake asymmetry is the greatest. The αi effectively distinguishes the dependence on α and AR of force and vortex shedding frequency at either ReD. In Pattern III, the Karman street forming behind the cylinder is modified to a secondary vortex street. At a given AR and α, ReD = 150 renders higher fluctuating lift and Strouhal number than ReD = 100.

Effect of cylinder aspect ratio on wake structure behind a finite circular cylinder located in an atmospheric boundary layer (대기경계층 내에 놓인 자유단 원주의 형상비가 후류유동에 미치는 영향에 관한 연구)

  • Park, Cheol-Woo;Lee, Sang-Joon
    • Proceedings of the KSME Conference
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    • 2001.06e
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    • pp.247-252
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    • 2001
  • The flow around free end of a finite circular cylinder(FC) embedded in an atmospheric boundary layer has been investigated experimentally. The experiments were carried out in a closed-return type subsonic wind tunnel with varying aspect ratio of the finite cylinder mounted vertically on a flat plate. The wake structures behind a 2-D cylinder and a finite cylinder located in a uniform flow were also measured for comparison. Reynolds number based on the cylinder diameter was about Re=20,000. A hot-wire anemometer was employed to measure the wake velocity and the mean pressure distributions on the cylinder surface were also measured. The flow past the FC free end shows a complicated three-dimensional wake structure and flow phenomenon is quite different from that of 2-D cylinder. The three-dimensional flow structure was attributed to the downwashing counter rotating vortices separated from the FC free end. As the FC aspect ratio decreases, the vortex shedding frequency is decreased and the vortex formation length is increased compared to that of 2-D cylinder. Due to the descending counter-rotating twin-vortex, in the region near the FC free end, regular vortex shedding from the cylinder is suppressed and the vortex formation region is hardly established. In the wake center region, the mean velocity for the FC located in atmospheric boundary layer has large velocity deficit, compared to that of uniform flow.

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A Study on Frequency Characteristics of Wake Flow a Circular Cylinder with Control Cylinder (제어봉이 부착된 원관 후류의 주파수특성에 관한 연구)

  • Choe, Sang-Bom;Han, Won-Hui;Cho, Dae-Hwan;Gim, Ok-Sok
    • Proceedings of KOSOMES biannual meeting
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    • 2007.11a
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    • pp.177-183
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    • 2007
  • Flow characteristics of the cylinder wake controlled with a small control cylinder were experimentally investigated by the PN (Particle Image Velocimetry) technique. Flow visualization of the flow around a circular cylinder was conducted in the Circulating Water Channel. The control cylinder having diameter of d=5mm, 10mm and 20mm was installed behind a circular cylinder of D=50mm. And the Reynolds number were $Re=4.9{\times}10^3,\;Re=9.9{times}10^3$ and $Re=1.9{\times}10^4$. In this study, the frequency characteristics of the controlled wake were analyzed by using spectral analysis of the measured wake velocity signals. As a result, the controlled wake had smaller vortex shedding frequency than that of circular cylinder wake by the effect of the control cylinder. Governing parameters of the flow control were d/D, and Reynolds number and they largely influenced the frequency characteristics of the cylinder wake. And vortex shedding frequency appeared most lowly at d=0.2D

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Flow Structure of the Wake behind an Elliptic Cylinder Close to a Free Surface

  • Daichin;Lee, Sang-Joon
    • Journal of Mechanical Science and Technology
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    • v.15 no.12
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    • pp.1784-1793
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    • 2001
  • The flow fields around an elliptic cylinder of axis ratio AR=2 adjacent to a free surface were investigated experimentally using a water channel. The main objective is to understand the effect of the free surface on the flow structure in the near-wake. The flow fields were measured by varying the depth of cylinder submergence, for each experimental condition, 350 velocity fields were measured using a single-frame PIV system and ensemble-averaged to obtain the spatial distribution of turbulent statics. For small submergence depths a large-scale eddy structure was observed in the near-wake, causing a reverse flow near the free surface, downstream of the cylinder. As the depth of cylinder submergence was increased, the flow speed in the gap region between the upper surface of the cylinder and the free surface increased and formed a substantial jet flow. The general flow structure of the elliptic cylinder is similar to previous results for a circular cylinder submerged near to a free surface. However, the width of the wake and the angle of downward deflection of the shear layer developed from the lower surface of the elliptic cylinder are smaller tan those for a circular 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.

The near wake of three circular cylinders in an equilateral triangular arrangement at a low Reynolds number Re=100

  • Bai, Honglei;Lin, Yufeng;Alam, Md. Mahbub
    • Wind and Structures
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    • v.30 no.5
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    • pp.451-463
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    • 2020
  • Two-dimensional numerical simulations are conducted at a low Reynolds number Re = 100 to investigate the near wake of three identical circular cylinders that are arranged in an equilateral triangular configuration. The incident angle of the three-cylinder configuration with respect to incoming flow is varied from θ = 0° to 60°, while the spacing between adjacent cylinders (L) covers a wide range of L/D = 1.25-7.0, where D is diameter of the cylinder. Typical flow structures in the near wake of the three-cylinder configuration are identified, including a single Karman vortex street, bistable flip-flopping near wake, anti-phase and/or in-phase vortex shedding, shear layer reattachment, and vortex impingement, depending on the configuration (L/D, θ). The behavior of Strouhal number (St) is discussed in detail, echoing the distinct structures of near wake. Furthermore, fluid forces on the individual cylinders are examined, which, though highly depending on (L/D, θ), exhibit a close correlation to the near wake behavior.

Experimental Study on the Near Wake Behind a Circular Cylinder with Helical Surface Protrusions (나선형의 표면돌출물이 부착된 원주의 근접후류에 관한 실험적 연구)

  • Gwon, Gi-Jeong;Kim, Hyeong-Beom
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.20 no.8
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    • pp.2601-2610
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    • 1996
  • Surface protrusions have been attached on a cylinder surface to reduce the flow-induced structural vibration by controlling the wake flow. Wind tunnel tests on the near wake of a circular cylinder with surface protrusions were carried out to investigate the flow characteristics of the controlled wake. Three experimental models were used in this experiment; one plain cylinder of diameter D and two cylinders wrapped helically by three small wires of diameter d=0.075D with pitches of 5D and 10D, respectively. Free stream velocity was ranged to have Reynolds number from 5000 to 50,000. Streamwise and vertical velocity components of the wake were measured by a hot-wire anemometry. The spanwise velocity component measured by a one-component fiber optic LDV revealed that time-averaged wake field has a nearly two-dimensional structure. It was found that the surface protrusions elongate the vortex formation region, which decrease the vortex shedding frequency. The suppression of vortices caused by the surface protrusions increases the velocity deficit in the center of wake region.

PIV measurements of near wake behind a sinusoidal cylinder

  • Zhang W.;Daichin Daichin;Lee S. J.
    • 한국가시화정보학회:학술대회논문집
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    • 2003.11a
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    • pp.59-62
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    • 2003
  • The near wake behind a sinusoidal cylinder at Re=5200 has been investigated using DPIV system. The velocity fields, streamlines and vorticity contours of the mean flow were compared at the nodal, saddle and middle planes with those of a right circular cylinder. For the sinusoidal cylinder, the vortex core moves downstream and the vortex formation region is expanded in streamwise direction while suppressed in transverse direction at the nodal plane. At the saddle and the middle plane the vortex spread in both streamwise and transverse directions, forming the maximum vortex region at the saddle plane.

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A Control of Two-Dimensional Subsonic Diffuser Flow Using the Turbulent Wake Caused by a Cylinder (실린더 후류를 이용한 2 차원 아음속 디퓨저 유동의 제어에 관한 연구)

  • Kim, Tae-Ho;Lee, Sang-Chan;Yoon, Bok-Hyun;Kim, Heuy-Dong
    • Proceedings of the KSME Conference
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    • 2003.11a
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    • pp.980-985
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    • 2003
  • The present study addresses a computational work to investigate the influence of a turbulent wake flow on the pressure recovery of a subsonic diffuser. The turbulent wake is generated by a cylinder with a small diameter, which is installed at the inlet of a 2-dimensional diffuser. Computation are applied to three-dimensional steady Navier-Stokes equations. The fully implicit finite volume scheme is used to discretize the governing equations. The computational results are qualitatively well compared to the experimental results. The results show that the pressure recovery of the subsonic diffuser is dependent on the diameter and location of cylinder. It is found that a certain diameter and location of the cylinder to generate the turbulent wake give a better pressure recovery, compared with no cylinder flow.

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