• International Journal of Naval Architecture and Ocean Engineering
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• 제10권2호
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• pp.212-224
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• 2018

The characteristics of the relationship between the evolution of propeller trailing vortex wake and skew angle are numerically examined based on four different five-blade David Taylor Model Basin (DTMB) model propellers with different skew angles. Numerical simulations are based on Reynolds-averaged Naviere-Stokes (RANS) equations combined with SST $k-{\omega}$ turbulence model. Results show that the contraction of propeller trailing vortex wake can be restrained by increasing skew angle and loading conditions, and root vortices fade away when the propeller skew angle increases. With the increase of the propeller's skew angle, the deformation of the hub vortex and destabilization of the tip vortices are weakening gradually because the blade-to-blade interaction becomes weaker. The transition trailing vortex wake from stability to instability is restrained when the skew increases. Furthermore, analyses of tip vortice trajectories show that the increasing skew can reduce the difference in trailing vortex wake contraction under different loading conditions.

• 한국수자원학회:학술대회논문집
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• 한국수자원학회 2005년도 학술발표회(2)
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• pp.951-951
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• 2005

The characteristics of Boussinesq starting forced plumes were investigated in this study. Two distinct periods in the transient plume penetration were identified, namely the Period of Flow Development (PFD) and Period of Developed Flow (PDF). PFD refers to the time period whereby the penetration rate is governed by the complex vortex dynamics initiated by the exit conditions that can include vortex coalescence, vortex leapfrogging, pinching off of the head vortex from the trailing stem and the eventual reconnection. The pinch-off and reconnection leads to an overshoot of the plume front which is a common observation reported in previous studies. The penetration rate in PDF is more predictable and depends on the continuous feeding of buoyancy and momentum into the head vortex by the trailing buoyant-jet stem. Similarity solutions are developed for PDF to describe the temporal variation of the penetration rate, by incorporating the behavior of an isolated buoyant vortex ring and recent laboratory results on the trailing buoyant jet. In particular, the variations of velocity ratios between the head vortex and trailing buoyant jet are analytically computed. To verify the similarity solutions, experiments were conducted on vertical starting forced plumes using planar laser induced fluorescence (PLIF).

• 대한조선학회논문집
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• 제51권6호
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• pp.480-488
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• 2014

Vortex shedding which is the dominant feature of body wakes and of direct relevance to practical engineering problems, has been intensively studied for flows past a circular cylinder. In contrast, vortex shedding from a hydrofoil trailing edge has been studied to much less extent despite numerous practical applications. The physics of the problem is still poorly understood. The present study deals with $K{\acute{a}}rm{\acute{a}}n$ vortex shedding from a truncated trailing-edge hydrofoil in relatively high Reynolds number flows. The objectives of this paper are twofold. First, we aim to simulate unsteady turbulent flows past a two dimensional hydrofoil through a hybrid particle-mesh method and penalization method. The vortex-in-cell (VIC) method offers a highly efficient particle-mesh algorithm that combines Lagrangian and Eulerian schemes, and the penalization method enables to enforce body boundary conditions by adding a penalty term to the momentum equation. The second purpose is to investigate shedding frequencies of vortices behind a NACA 0009 hydrofoil operating at a zero angle of attack.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2009년도 춘계학술대회 논문집
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• pp.476-476
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• 2009

Periodic vortex separations generate periodic vertical forces acting on a trailing edge of an airfoil. When a natural frequency of the trailing edge of the airfoil is close to a vortex shedding frequency, an amplitude of the edge oscillation becomes maximal; it makes intensive noise called singing. Motion of the trailing edge may also feedback to the vortex shedding so that self-sustained oscillation appear, and a resonant frequency is locked in some interval of the speed of the incident flow. In this study, a theoretical model is proposed and applied for modeling an airfoil singing. Results are compared with experimental measurements which are carried out in an anechoic wind tunnel.

• 한국소음진동공학회논문집
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• 제20권2호
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• pp.115-121
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• 2010

Periodic vortex separations generate periodic vertical forces acting on a trailing edge of an airfoil. When a natural frequency of the trailing edge of the airfoil is close to a vortex shedding frequency, an amplitude of the edge oscillation becomes maximal; it makes intensive noise called singing. Motion of the trailing edge may also feedback to the vortex shedding so that self-sustained oscillation appears, and a resonant frequency is locked in some interval of the speed of the incident flow. In this study, a theoretical model is proposed and applied for modeling an airfoil singing. Results are compared with experimental measurements which are carried out in an anechoic wind tunnel.

• 한국항공우주학회지
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• 제41권11호
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• pp.849-854
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• 2013

본 연구에서는 편대비행하는 고속 항공기의 날개 끝 와류의 공력 간섭 효과를 연구하기 위해서 풍동실험기법이 적용되었다. 편대비행에서 앞선 항공기의 날개 끝에서 발생하는 와류가 뒤따르는 항공기의 공력 특성에 영향을 미칠 수 있다. 흐름의 간섭 효과는 앞선 항공기와 뒤따르는 항공기의 날개 끝 사이의 거리에 따라서 변화된다. 본 연구의 실험결과 앞선 항공기에서 발생되는 날개 끝 와류는 뒤따르는 항공기의 공력 및 모멘트 계수를 변화시킴을 확인하였다. 특히, 후방 항공기의 양력계수는 수직 및 수평 위치에 따라 y/b=-0.125, z/b=0.0에서 가장 많이 증가되거나 y/b=-0.5, z/b=0.38에서 가장 크게 감소됨을 확인하였으며, 두 항공기로부터 발생된 날개 끝 와류의 간섭 현상이 자세히 관찰되었다.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2002년도 추계학술대회논문집
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• pp.646-651
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• 2002

The flow field around a symmetrical airfoil in a uniform flow under the generation of noise was studied by experiments and numerical simulation. The experiments are conducted by visualizing the surface flow over the airfoil with a shear-sensitive liquid-crystal coating and by measuring the instantaneous velocity field around the trailing edge of the airfoil. The results indicate that the discrete frequency noise is generated when the separated laminar flow reattaches near the trailing edge of the pressure side and the turbulent boundary layer is formed over the suction side of the airfoil near the trailing edge. The periodic behavior of vortex formation was observed around the trailing edge and it persists further downstream in the wake. The frequency of the vortex formation in the wake was consistent with that of the discrete frequency noise.

• 대한기계학회논문집B
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• 제29권8호
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• pp.911-920
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• 2005

The present study investigates heat/mass transfer and flow characteristics in a ribbed rotating passage with turning region. The duct has an aspect ratio (W/H) of 0.5 and a hydraulic diameter ($D_h$) of 26.67 mm. Rib turbulators are attached in the parallel arrangement on the leading and trailing surfaces of the passage. The ribs have a rectangular cross section of 2 m (e) $\times$ 3 m (w) and an attack angle of $70^{\circ}$. The pitch-to-rib height ratio (p/e) is 7.5, and the rib height-to-hydraulic diameter ratio (e/$D_h$) is 0.075. The rotation number ranges from 0.0 to 0.20 while the Reynolds number is constant at 10,000. To verify the heat/mass transfer augmentation, internal flow structures are calculated for the same conditions using a commercial code FLUENT 6.1. The results show that a pair of vortex cells are generated due to the symmetric geometry of the rib arrangement, and heat/mass transfer is augmented up to $Sh/Sh_0=2.9$ averagely, which is higher than that of the cross-ribbed case presented in the previous study for the stationary case. With the passage rotation, the main flow in the first-pass deflects toward the trailing surface and the heat transfer is enhanced on the trailing surface. In the second-pass, the flow enlarges the vortex cell close to the leading surface, and the small vortex cell on the trailing surface side contracts to disappear as the passage rotates faster. At the highest rotation number ($R_O=0.20$), the turn-induced single vortex cell becomes identical regardless of the rib configuration so that similar local heat/mass transfer distributions are observed in the fuming region for the cross- and parallel-ribbed case.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2000년도 제15회 학술강연회논문초록집
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• pp.41-41
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• 2000

터어빈 블레이드의 경우 제작 또는 설계상의 이유로 뭉툭한 끝단을 가질 수밖에 없게 되는데, 이로 인하여 같은 터보기계인 압축기 블레이드의 경우와는 다르게 블레이드 끝단에서 끝단 와류(Trailing edge vortex)가 발생하게 된다. 이 와류는 블레이드의 손실 증가, 고주파 음파의 생성, 국부적으로 매우 큰 열 전달 및 에너지분산 등 터빈 블레이드의 성능에 좋지 못한 영향을 미치게 된다. 또한 와류와 충격파와의 간섭효과 둥이 존재하는 경우에는 매우 복잡한 유동장을 형성하며 심한 유동 구배가 존재하게 되므로 고해상도의 수치해석 방법이 아니고서는 이를 수치적으로 해석하기가 쉽지 않다.(중략)

• 한국마린엔지니어링학회:학술대회논문집
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• 한국마린엔지니어링학회 2006년도 전기학술대회논문집
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• pp.279-280
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• 2006

In the present study the flow fields around pitching foils have been visualized by using a Schlieren method with a high speed camera in a wind tunnel at low Reynolds number regions. It has been observed that small vortices are shed discretely from the leading and trailing edge and that they stand in line on the integrated streakline of separation shear layer. By counting vortices in the VTR frames it was clarified that the number of vortex shedding from the leading and trailing edge during one pitching cycle strongly depends on the non-dimensional pitching rate.