• 대한기계학회논문집B
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• 제25권10호
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• pp.1417-1426
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• 2001

Turbulent flow characteristics in the near wake of a square cylinder have been studied experimentally by using a Digital PIV method. Experiments are performed at the Reynolds numbers of 1600 and 3900 based on the free-stream velocity and the square height. The ensemble averaged turbulence statistics are acquired from 2030 realizations of instantaneous fluctuating velocity field after the conventional Reynolds decomposition. The differences in turbulent intensity and Reynolds shear stress profiles fur both oases indicate that the effect of Reynolds number seems to be descernible mainly due to the occurrence of transition in the separated shear layer. Because of the periodic nature of vortex shedding process, transverse velocity fluctuations contribute dominantly , to turbulent kinetic energy distribution. A comparison with previous LDV data obtained at much higher Reynolds number shows a fairly good agreement each other. It turns out that the effect of Reynolds number diminishes as increasing Reynolds number, which is a well-known feature of a sharp-edged bluff body wake. The streamwise variation of turbulence intensities are compared with those from a circular cylinder along the centerline at the same Reynolds number. The overall magnitudes and the decay rates of turbulence intensities are quite similar, but some differences are noticeble especially in the transverse intensity variation.

• Wind and Structures
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• 제23권2호
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• pp.127-142
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• 2016

Aerodynamic effects, such as drag force and flow-induced vibration (FIV), on civil engineering structures can be minimized by optimally modifying the structure shape. This work investigates the turbulent wake of a square prism with its faces modified into a sinusoidal wave along the spanwise direction using three-dimensional large eddy simulation (LES) and particle image velocimetry (PIV) techniques at Reynolds number $Re_{Dm}$ = 16,500-22,000, based on the nominal width ($D_m$) of the prism and free-stream velocity ($U_{\infty}$). Two arrangements are considered: (i) the top and bottom faces of the prism are shaped into the sinusoidal waves (termed as WSP-A), and (ii) the front and rear faces are modified into the sinusoidal waves (WSP-B). The sinusoidal waves have a wavelength of $6D_m$ and an amplitude of $0.15D_m$. It has been found that the wavy faces lead to more three-dimensional free shear layers in the near wake than the flat faces (smooth square prism). As a result, the roll-up of shear layers is postponed. Furthermore, the near-wake vortical structures exhibit dominant periodic variations along the spanwise direction; the minimum (i.e., saddle) and maximum (i.e., node) cross-sections of the modified prisms have narrow and wide wakes, respectively. The wake recirculation bubble of the modified prism is wider and longer, compared with its smooth counterpart, thus resulting in a significant drag reduction and fluctuating lift suppression (up to 8.7% and 78.2%, respectively, for the case of WSP-A). Multiple dominant frequencies of vortex shedding, which are distinct from that of the smooth prism, are detected in the near wake of the wavy prisms. The present study may shed light on the understanding of the underlying physical mechanisms of FIV control, in terms of passive modification of the bluff-body shape.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2002년도 학술대회지
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• pp.185-188
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• 2002

The objective of present paper is to apply a stereoscopic PIV(Particle Image Velocimetry) techiique for measuring the 3 dimensional flow structure of turbulent wake behind a marine propeller with 5 blades. It is essential to measure 3-components velocity fields for the investigation of complicated near-wake behind the propeller. The out-of-plane velocity component was measured using the particle images captured by two CCD cameras in the angular displacement configuration.400 instantaneous velocity fields were measured for each of few different blade phases of $0^{\circ},\;18^{\circ},\;36^{\circ}\;and\;54^{\circ}$. They were ensemble averaged to investigate the spatial evolution of the propeller wake in the region ranged from the trailing edge to the region of one propeller diameter(D) downstream. The phase-averaged velocity fields show the viscous wake formed by the boundary layers developed along the blade surfaces. Tip vortices were formed periodically and the slipstream contraction occurs in the near-wake region. The out-of-plane velocity component has large values at the tip and trailing votices. With going downstream, the axial turbulence intensity and the strength of tip vortices were decreased due to the visous dissipation, turbulence diffusion and blade-to-blade interaction. The blade wake traveling at higher speed with respect to the tip vortex overtakes and interacts with tip vortices formed from the previous blade. Tip vortices are separated from the wake and show oscillating trajectory

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 1995년도 추계 학술대회논문집
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• pp.118-125
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• 1995

This paper presents the result of a computational study on the wake characteristics of two tanker models. i.e HSVA and DYNE hull forms. The focus of the study is on the distributions of axial. radial and tangential velocities of the two hull forms in way of the propeller, especially over the propeller disk. The effect of bilge vortices on the velocity distribution is also concerned. For the computation of stern and wake flows of the two hull forms. the incompressible Reynolds-Averaged Navier-Stokes(RANS) equations are numerically solved by the use of a second order finite difference method, which employs a four stage Runge-Kutta scheme with a residual averaging technique and the Baldwin-Lomax model. The calculated pressure distributions on the hull surface and the axial. radial and tangential velocity distributions over the propeller disk are presented for the two hull forms. Finally, the result of wake analysis for the computed wake distribution over the propeller disk is given in comparison with those for the experimental wake distribution for the both hull forms.

• 대한기계학회논문집B
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• 제29권8호
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• pp.887-894
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• 2005

This paper describes effects of incidence angle on the wake turbulent flow of a high-turning turbine rotor blade. For three incidence angles of -5, 0 and 5 degrees, energy spectra as well as profiles of mean velocity magnitude and turbulence intensity at mid-span are reported in the wake. Vortex shedding fiequencies are obtained from the energy spectra. The result shows that as the incidence angle changes from -5 to 5 degrees, the suction-side wake tends to be widened and the deviation angle is increased. Strouhal numbers based on the shedding fiyequencies have a nearly constant value, regardless of the tested incidence angles.

• 대한기계학회논문집
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• 제18권10호
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• pp.2779-2786
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• 1994

This study presents an experimental investigation of the effect of the wake on mass transfer about a circular cylinder in crossflow. The flat-plate wake was generated by merging two mirror images of turbulent boundary layers that were well developed along the both sides of flat plate with a sharp trailing edge. The velocity field was measured by a hot-wire system and the mass transfer rate by a naphthalene sublimation method. The mixing and developing stages of the wake were addressed to identify flow conditions. The mass transfer effects of different developing stages of the wake was discussed in detail. It is noted that a local maximum appears not at the front stagnation point but at a point a little downstream when the cylinder is located in the nearwake region and much more elevated mass transfer rate is obtained compared to effect of free-stream turbulence.

• Journal of Hydrospace Technology
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• 제2권2호
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• pp.1-13
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• 1996

This paper presents the result of a computational study on the wake characteristics of two tanker models, i.e. HSVA and Mystery hull forms. The focus of the study is on the distributions of axial, radial and tangential velocities of the two hull forms in way of the propeller, especially over the propeller disk. The effect of bilge vortices on the velocity distribution is also concerned. For the computation of stern and wake flows of the two hull farms, the incompressible Reynolds-Averaged Wavier-Stokes(RANS) equations are numerically solved by the second order finite difference method, which employs a four stage Runge-Kutta scheme with a residual averaging technique and the Baldwin-Lomax model. The calculated pressure distributions on the hull surface and the axial, radial and tangential velocity distributions over the propeller disk are presented for the two hull forms. Finally, the result of wake analysis for the computed wake distribution over the propeller disk is given in comparison with those for the experimental wake distribution fur the both hull forms.

• 한국항공우주학회지
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• 제32권10호
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• pp.1-11
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• 2004

평판의 근접 후류에서 상류 경계층의 영향을 조사하기 위한 실험적 연구가 수행되었다. 트리핑 와이어(tripping wire)의 여러 부착 위치가 경계층의 유동조건을 유발시키기 위하여 선택되었다. 평판 앞전에서부터 0.98C 위치에서의 층류, 천이, 난류경계층이 대칭, 비대칭 후류의 발달과정을 조사하기 위하여 부과되었다. X형태의 열선 프로브(55P61)를 이용한 측정은 근접 후류의 8위치에서 수행되었고, 측정된 평균속도분포는 기존의 후류 상사변수로 무차원화 되어 제시되었다. 대칭 후류는 후류의 중앙부분에서 기존의 상사곡선과 일치하지만, 비대칭 후류는 측정된 범위에서 기존의 상사 곡선과 일치하지 않았다.

• 한국항행학회논문지
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• 제14권4호
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• pp.479-488
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• 2010

본 논문은 활주로 방향에 수직인 바람이 불어올 때 이로 인하여 활주로에 발생되는 유동현상과 받음각 변화량을 3차원 수치해석을 통하여 연구하였다. 3차원 수치해석결과 활주로 주변 시설물에서 발생된 후류로 인하여 활주로 상에서 발생되는 받음각 변화량의 최대진폭은 $6^{\circ}$이고 그 지속 시간은 약 1초 이내로 나타났으며 전체적으로 보면 불규칙적인 비주기적 성향으로 나타난다. 특히 시설물 사이를 통과한 빠른 흐름으로 인하여 발생된 후류가 병합되면서 더욱 강한 후류영역을 발생시켜 활주로 상에서 심각한 이착륙 불안정성을 발생시킬 것으로 예상된다.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2002년도 학술대회지
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• pp.467-470
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• 2002

The offects of a small control rod (d=3mm) located near a main circular cylinder on the drag reduction and wake structure modification were investigated. The location of the small control rod mounted on a rod-like spring is self-adjusting according to the wake structure far optimal control of the flow around the main cylinder. The experiments were carried out at the Reynolds numbers based on the cylinder diameter (D=50mm) in the range $Re_{D}=1{\times}10^4{\~}6{\times}10^4$. Mean velocity and turbulent statistics were measured with varying the angle along the cylinder circumference ${\Theta}=15^{\circ},\;30^{\circ},\;45^{\circ}$ and the distance between the main and control rods L =0.7, 1. Compared with the bare cylinder, the main circular cylinder with the fixed and self-adjusting rods reduced drag coefficient by $10{\%}$ at the angle of ${\Theta}=45^{\circ}$. For the main cylinder with self-adjusting rot as the Reynolds numbers increase, the streamwise mean velocity is increased, however, the turbulence intensity is decreased. In addition, the control rods tested in this study are effective at higher Reynolds number than at lower Reynolds number.