• 한국군사과학기술학회지
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• 제24권1호
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• pp.22-30
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• 2021

The ship wake generated by rotation of the propeller yields changes of characteristics of sound wave such as attenuation and scattering. To develope a battle field environment simulator for military purposes, it is very important to understand acoustical properties of ship wake. Existing research results have limitations in direct application because they performed under simple conditions or model ships were applied. In this study, we developed a ship wake generation model based on the ship's geometric wake distribution theory. The model can provide spatial distribution and void fraction with various marine environments as well as ship size. Through the developed model, geometric distribution features of ship wake according to the ship's maneuvering conditions were successfully simulated. In addition, changes of the bubble void fraction with time at any location within the battle field environment were identified. Therefore, the developed model is expected to be used in the development of a simulator to measure the acoustic characteristics of the ship wake.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2003년도 춘계학술대회
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• pp.1795-1800
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• 2003

An experimental study was carried out to investigate near-wake characteristics of an elliptic airfoil oscillating in pitch. The airfoil was sinusoidally pitched about the half chord point between $-5^{\circ}C$ and $+25^{\circ}C$ angles of attack at the freestream velocities of 3.4 and 23.1 m/s The corresponding Reynolds numbers based on the chord length were $3.3{\times}10^4$ and $2.2{\times}10^5$, respectively. A hot-wire anemometer was used to measure the near-wake flow variable at the reduced frequency of 0.1. Ensemble-averaged velocity and turbulence intensity profile were presented to examine the near-wake characteristics depending on the Reynolds number. The axial velocity deficit in the near-wake region tend to decrease with the increase in the Reynolds number a found in many stationary airfoil test . Turbulence intensity in the near-wake region have a tendency to decrease with the increase in the Reynolds number during the pitch-up motion, whereas it shows different feature during the pitch-down motion either the laminar boundary layer or turbulent boundary layer separation.

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

• 대한기계학회논문집
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• 제18권5호
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• pp.1322-1329
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• 1994

The effect of thermal stratification on the stratified flow past a circular cylinder was examined in a wind tunnel. Turbulent intensities, the rms values of temperature and turbulent convective heat flux as well as the velocity and temperature profiles in the cylinder wake with a strong thermal gradient of $200^{\circ}C/m$ were measured by using a hot-wire and cold-wire combination probe. It is found that the temperature field affects as an active contaminant, so that the vertical growth of vortical structure is suppressed and the strouhal number decreases with increasing the extent of stratification. And also, the wake structure can not sustain their symmetricity about the wake centerline and vertical turbulent motion dissipates faster than that of the neutral case 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 of the wake in a stably stratified flow because the turbulent intensities and convective heat flux in the upper half section are larger than those of the lower half of the wake.

• 한국가시화정보학회지
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• 제17권3호
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• pp.46-51
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• 2019

Rotor wake interaction must be considered to understand the quadrotor flight, and the rotor separation distance is an important parameter that affects the rotor wake interaction. In this study, the wake characteristics were investigated with varying the rotor separation distance. The velocity field in the rotor wake was measured using digital PIV for hovering mode at Re = 34,000, and the wake boundaries from the inner and outer rotor tips were quantitatively compared with varying the rotor separation distance. The symmetric rotor-tip vortex shedding about the rotor axis was found at a large rotor separation distance. However, the wake boundary became more asymmetric about the rotor axis with decreasing the rotor separation distance. At the minimum rotor separation distance, in particular, a faster vortex decay was observed due to a strong vortex interaction between adjacent rotors.

• 대한기계학회논문집B
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• 제20권6호
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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.

• 대한조선학회논문집
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• 제45권6호
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• pp.620-630
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• 2008

The characteristics of complicated propeller wake influenced by hull wake are investigated by using a two-frame PIV (Particle Image Velocimetry) technique. As the propeller is significantly affected by the hull wake in a real marine vessel, the measurements of propeller wake under the hull wake would be certainly necessary for more reliable validation and the prediction of numerical simulation with wake modeling. Velocity field measurements have been conducted in a medium-size cavitation tunnel with a hull wake. Generally, the hull wake generated by the boundary layer of ship's hull produces the different loading distribution on the propeller blade in both upper and lower propeller planes. The difference of the propeller wake behaviors caused by the hull wake is discussed in terms of axial velocity, vorticity and turbulence kinetic energy distribution in the present study.

• 수산해양기술연구
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• 제58권1호
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• pp.68-74
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• 2022

This study analyzed the wake characteristics of the rim-driven propeller (RDP) used in an underwater robot. For underwater robots to perform specific missions, not only propulsion characteristics but also wake characteristics must be considered. In this study, a blade was designed based on NAC 0012 with a symmetrical cross-section. The RDP was hubless with three or four blades. The influence of both the free water surface and the bottom was considered, and the wake was measured using a particle image velocimetry in the advance ratio of 0.2 to 1. Model 1 showed symmetrical wakes in the entire advance ratio section. Model 2 showed asymmetric wakes due to the influence of the free water surface and the bottom at low advance ratio.

• 해양환경안전학회:학술대회논문집
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• 해양환경안전학회 2007년도 추계학술발표회
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• pp.177-183
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• 2007

제어봉이 부착된 원관 후류의 유동특성을 관찰하기 위해 입자영상유속계(PIV)를 사용한 계측을 수행하였다. 실험은 소형회류수조에서 유업 유속을 0.lm/s, 0.2m/s, 0.4 m/s 로 하고, 원관(D=50mm) 에 제어봉이 없는 경우와 제어봉이 5mm, 10mm, 20mm 가 부착된 총 l27가지로 정하였고, 속도신호에 따른 주파수특성을 고찰하였다. 실험에서 제어봉의 영향으로 인하여 와류흘림 주파수가 작게 나타났으며, 유동제어의 지배변수로는 d/D와 레이놀즈수이며, 원관 후류의 주파수특성에 큰 영향을 미쳤다. 와류흘림 주파수는 제어봉이 0.20인 경우 가장 작게 나타났다.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2002년도 학술대회지
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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.