• 한국가시화정보학회:학술대회논문집
• /
• 한국가시화정보학회 2005년도 추계학술대회 논문집
• /
• pp.54-57
• /
• 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.

• 한국군사과학기술학회지
• /
• 제4권2호
• /
• pp.215-224
• /
• 2001

An experimental investigation was conducted on the interaction of vortices over a delta wing with the leading edge extension for three angles of attack($16^{\circ},\; 24^{\circ} \;and\; 28^{\circ}$) at Reynolds number of $1.76{\times}10^6.$ The experimental data included total pressure contours and velocity vectors using 5-hole probe measurements. Constant total pressure coefficient contours show the LEX vortex moves downward and outboard, while the wing vortex exhibited an inboard and upward migration. At near the trailing edge, these vortices reveal a direct interaction between the wing and LEX vortex, featuring a coiling of vortex cores about each other. The combined effect of the interaction of these two vortices and proximity to the wing surface results in the increase of the suction peak. This is in contrast to the result obtained on the delta wing alone configuration, where the effect of the vortex breakdown was manifested. The interaction of the wing and LEX vortices is more pronounced at higher AOA.

• 한국가시화정보학회:학술대회논문집
• /
• 한국가시화정보학회 2004년도 추계학술대회 논문집
• /
• pp.40-43
• /
• 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.

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

• 대한조선학회논문집
• /
• 제48권3호
• /
• pp.233-237
• /
• 2011

In the present experiments, vortical structures behind the hydrofoil trailing edge are visualized and analyzed as an elementary study for propeller singing phenomena. Two sorts of hydrofoil are selected for the measurement of shedding vortices. One was KH45 hydrofoil section and the other is KH45 with the truncated trailing edge that is positioned at X/C = 0.9523(C=chord length). Assuming the Strouhal number of 0.23, the shedding frequencies of vortices are extracted by analyzing the boundary layer thickness and the flow speed. The frequency distribution of shedding vortices is obtained with the variation of angle-of-attack while the flow speed is fixed to 8m/s. The truncation of the trailing edge makes the frequency of shedding vortices about 120Hz lower than that of original trailing edge and makes the vorticity value higher than the original trailing edge.

• 한국가시화정보학회:학술대회논문집
• /
• 한국가시화정보학회 2004년도 Proceedings of 2004 Korea-Japan Joint Seminar on Particle Image Velocimetry
• /
• pp.169-175
• /
• 2004

The characteristics of flow around a rotating propeller were investigated using PIV technique. For each of four different blade phases of $0^{\circ},\;18^{\circ},\;36^{\circ}\;and\;54^{\circ}$four hundred instantaneous velocity fields were ensemble averaged to investigate the spatial evolution of the flow around a propeller. The phase-averaged mean velocity fields show that the viscous wake formed by the boundary layers developed on the blade surfaces and the slipstream contraction in the near-wake region. The out-of-plane velocity component and strain rate had large values at the locations of the tip and trailing vortices. The boundary layer developed along the ship hull bottom surface of the ship stern provides a strong turbulent shear layer, affecting the vortex structure in the propeller near-wake. As the flow develops in the downstream direction, the trailing vortices formed behind the propeller hub move upward slightly due to the presence of the hull wake and free surface. The turbulence intensity has large values around the tip and trailing vortices. As the wake moves downstream, the strength of the vorticity diminishes and the turbulence intensity increases due to turbulent diffusion and active mixing between the tip vortices and adjacent wake flow.

• 한국추진공학회:학술대회논문집
• /
• 한국추진공학회 2007년도 제28회 춘계학술대회논문집
• /
• pp.335-338
• /
• 2007

공대공미사일의 경우 초음속 하에서 고 기동성을 얻기 위하여 고받음각 능력이 요구되어진다. 옆 미끄러짐이 없는 대칭형 세장형 몸체의 경우라도 비대칭 와류는 생성된다. 이러한 비대칭 와류는 불필요한 측력 및 요잉모멘트를 발생시키고 이는 곧 방향 안전성을 저하시킨다. 본 연구는 전산해석을 통하여 초음속유동하에서 세장형 몸체 주변에 발생되는 비대칭와류 모사를 실시하였으며 비대칭 와류의 모사를 위해 선두부에 Bump를 장착하였다. 전산해석 결과 세장형 몸체 주변에 발생하는 비대칭와류를 모사 할 수 있었다.

• Wind and Structures
• /
• 제5권1호
• /
• pp.25-48
• /
• 2002

Wind flow and pressure on the roof of the Texas Tech Experimental Building are studied along with the incident wind in an effort to understand the wind-structure interaction and the mechanisms of roof pressure generation. Two distinct flow phenomena, cornering vortices and separation bubble, are investigated. It is found for the cornering vortices that the incident wind angle that favors formation of strong vortices is bounded in a range of approximately 50 degrees symmetrical about the roof-corner bisector. Peak pressures on the roof corner are produced by wind gusts approaching at wind angles conducive to strong vortex formation. A simple analytical model is established to predict fluctuating pressure coefficients on the leading roof corner from the knowledge of the mean pressure coefficients and the incident wind. For the separation bubble situation, the mean structure of the separation bubble is established. The role of incident wind turbulence in pressure-generation mechanisms for the two flow phenomena is better understood.

• Wind and Structures
• /
• 제22권6호
• /
• pp.663-684
• /
• 2016

Conical vortices generated at the corner regions of large-span flat roofs have been investigated by using the Particle Image Velocimetry (PIV) technique. Mean and instantaneous vector fields for velocity, vorticity, and streamlines were measured at three visual planes and for two different flow angles of $15^{\circ}$. The results indicated that conical vortices occur when the wind is not perpendicular to the front edge. The location of the leading edge corresponding to the negative peak vorticity and maximum turbulent kinetic energy was found at the center of the conical vortex. The wind pressure reaches the maximum near the leading edge roof corner, and a triangle of severe suctions zone appears downstream. The mean pressure in uniform flow is greater than that under turbulent flow condition, while a significant increase in the fluctuating wind pressure occurs in turbulent streams. From its emergence to stability, the shape of the vortex cross-section is nearly elliptical, with increasing area. The angle that forms between the vortex axis and the leading edge is much smaller in turbulent streams. The detailed flow structures and characteristics obtained through FLUENT simulation are in agreement with the experimental results. The three dimensional (3-D) structure of the conical vortices is clearly observed from the comprehensive arrangement of several visual planes, and the inner link was established between the vortex evolution process, vortex core position and pressure distribution.

• 대한기계학회논문집B
• /
• 제23권7호
• /
• pp.918-927
• /
• 1999

In the fully developed internal flow fields, there are complex transition flows caused by interaction of the cross flow and jet when jet is Injected Into the flow. These interactions are studied by means of the flow visualization methods. An instantaneous laser tomographic method is used to reveal the physical mechanism and the structure of vortices formation in the branch pipe flow. The velocity range of cross flow of the pipe is 0.7m/s and the corresponding Reynolds number $R_{cf}$, based on the duct height is $5.6{\times}10^3$, diameter/height ratios(d/H) 0.14 and velocity ratios 3.0. Oil mist with the size of $10{\mu}m$ diameter is used for the scattering particle. The instantaneous topological features of the vortex ring roll-up of the jet shear layer and characteristics of this flow are studied in detail by performing flow visualization in rectangular duct flow. It is found that the formation and roll-up of ring vortices is a periodic phenomenon. The detailed topology of the vortices in the near field of a cross -flow jet and the mechanism associated with them give enforced hints of vortex breakdown within the vortex system due to the interaction of the jet and the cross-flow.