• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2007년도 춘계 학술대회논문집
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• pp.130-133
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• 2007

A two-phase method in CFD has been developed and is applied to model the cavitation flow. The governing equation system is two-phase Navier-Stokes equation, comprised of the mixture mass, momentum and liquid-phase mass equation. It employs an implicite, dual time, preconditioned algorithm using finite difference scheme in curvilineal coordinates and Chien ${\kappa}-{\varepsilon}$ turbulence equation. The experimental cavitating flows around ogive-cylinder and venturi type objects are employed to test the solver. To prove the capabilities of the solver, several three-dimentional examples are presented.

• Journal of Advanced Marine Engineering and Technology
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• 제36권5호
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• pp.618-626
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• 2012

Suppression of abnormal flow phenomena in the Francis hydro turbine is very important to improve the turbine performance. Especially, as cavitation and cavitation surge makes serious problems when the turbine is operated in the range of partial flow rate, optimum method of suppressing the abnormal flow characteristics is required necessarily. Moreover, as swirl flow in the draft tube of the Francis turbine decreases pressure at the inlet of the draft tube, suppression of the swirl flow can be an useful method of suppressing the occurrence of cavitation. In order to clarifying the possibility of suppressing the swirl flow by J-Groove in the draft tube, a series of CFD analysis has been conducted in the range of partial load, designed condition and excessive flow rate of a Francis turbine. A kind of J-Groove is designed and applied to the draft tube of the Francis hydro turbine model. The pressure contours, circumferential velocity vectors and vortex core regions in the draft tube are compared by the conditions with or without J-Groove. In addition, a group of data about the velocity in the draft is presented to show the influence of J-Groove.

• 대한의용생체공학회:의공학회지
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• 제22권2호
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• pp.205-210
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• 2001

결석 파쇄에 ES지 장치가 이용되기 시작한 이래, 장치의 성능 및 장치에서 발생시킨 충격파가 생체에 미치는 영향등에 관한 연구가 다수 행해지고 있다. 그 중의 하나가 충격파가 전달되는 곳에는 항상 거론되는 캐비테이션 문제이다. 본 연구에서는 압전식 ESWL 장치로 대상물을 파쇄한다는 가정하에, 그 때의 매질을 캐비테이션이 미치는 영향과 관련지어 둘로 구분한다. 그것에 따라 각 매질에서의 방사음을 관측하고, 매질에 따른 파쇄효율과의 관계를 분석한 결과를 제시한다. 본 논문의 결과들은 캐비테이션 기포의 발생율이 작은 탈기수쪽이 방사음에 있어서도 분산성이 작고, 파쇄효율면에 있어서도 안정된 점을 확실히 나타내고 있다.

• 한국해양공학회지
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• 제31권1호
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• pp.8-13
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• 2017

Cavitation is used in various fields. This study examined the drag reduction of an underwater vehicle using cavitation. In this study, the natural partial cavitation analysis results were verified using CFD code with the Navier-Stokes equation based on a mixture model. The momentum and continuity equations in the mixture phase were separately solved in the liquid and vapor phases. The solver employs an implicit preconditioning algorithm in curvilinear coordinates. The results of a computational analysis showed good agreement with the experiment. A computational analysis was also performed on the supercavity. The study investigated the cavity characteristics and drag of an underwater vehicle and studied the speed required to achieve a supercavity. Finally, a 1DOF analysis was carried out to investigate the thrust system for a supercavity. As a result, one of the methods for determining a suitable thrust system for a supercavitating underwater vehicle was presented.

• Journal of Advanced Marine Engineering and Technology
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• 제19권2호
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• pp.56-66
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• 1995

Recently, with theraped advancement in th oceanology such an ocean-going vessel and oceanic structures, there is a need to study the cavitation erosion-corrosion control of pump impeller, the partial element of ocean machinery, for more effective operation. Especially, the cathodic protection (impressed current method & Al-sacrificial anode method) was applied to sea water, and Cu-alloy material mixed Zn & Al was used as a control method of cavitation erosion-corrosion. In this study, used the piezoelectric vibrator with 20KHz, 24.mu.m to cavity generation apparatus, and investigated the weight loss, weight loss rate, electrode potential & current density etc. under this condition. According to test result, thos describes how to indentify an influence of the cathodic protection and Al & Zn addition in material development for the control of cavitation erosion-corrosion, and those will serve as fundamental data on the cavitation erosion-corrosion control of oceanic centrifugal pump.

• International Journal of Naval Architecture and Ocean Engineering
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• 제3권4호
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• pp.263-273
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• 2011

The diffuse interface model of Saurel et al. (2008) is used for the computation of compressible cavitating flows around underwater missiles. Such systems use gas injection and natural cavitation to reduce drag effects. Consequently material interfaces appear separating liquid and gas. These interfaces may have a really complex dynamics such that only a few formulations are able to predict their evolution. Contrarily to front tracking or interface reconstruction method the interfaces are computed as diffused numerical zones, that are captured in a routinely manner, as is done usually with gas dynamics solvers for shocks and contact discontinuity. With the present approach, a single set of partial differential equations is solved everywhere, with a single numerical scheme. This leads to very efficient solvers. The algorithm derived in Saurel et al. (2009) is used to compute cavitation pockets around solid bodies. It is first validated against experiments done in cavitation tunnel at CNU. Then it is used to compute flows around high speed underwater systems (Shkval-like missile). Performance data are then computed showing method ability to predict forces acting on the system.

• 대한기계학회논문집B
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• 제33권2호
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• pp.69-78
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• 2009

Cavitating flow simulation is of practical importance for many hydraulic engineering systems, such as pump, turbine, nozzle, injector, etc. In the present work, a solver for cavitating flow has been developed and applied to simulate the flows past axisymmetric cylinders. Governing equations are the two-phase Navier-Stokes equations, comprised of continuity equation of liquid and vapor phase. The momentum equation is in the mixture phase. The solver employed an implicit, dual time, preconditioned algorithm in curvilinear coordinates. Computations were carried out for three axisymmetric cylinders: hemispherical, ogive, and caliber-0 forebody shape. Then, the present calculations were compared with experiments and other numerical results to validate the present solver. Also, the code has shown its capability to accurately simulate the re-entrant jet phenomena and ventilated cavitation. Hence, it has been found that the present numerical code has successfully accounted for cavitating flows past axisymmetric cylinders.

• International Journal of Fluid Machinery and Systems
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• 제10권3호
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• pp.197-208
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• 2017

When Francis turbines operate at partial load, residual swirl in the draft tube causes low-frequency pulsation of pressure and power output. Scale effects and system response may bias the prediction of prototype behavior based on laboratory tests, but could be overcome by means of a 1D analytical model. This paper deals with the two most important features of such a model, the compliance and the source of excitation. In a distributed-parameter version, compliance should be represented as an exponential function of local pressure. Lack of similarity due to different Froude number can thus be compensated. The normally unknown gas content in the vortex cavity has significant influence on the pulsation, and should therefore be measured and considered as a test parameter.

• Korean Chemical Engineering Research
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• 제54권2호
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• pp.145-151
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• 2016

유체의 흐름에 의한 부분적 진공현상인 캐비테이션이 재료 표면에 충격을 주어 철강류 표면이 마모되는 현상에 대응하기 위해 판상형 glass-flake를 적용한 내캐비테이션용 도료를 개발하였다. 특히, 가소성이 좋고 내수성이 뛰어난 특성의 NBR (Acrylonitrile-butadiene rubber) 변성 에폭시 수지에 판상형 glass-flake를 필러로 상용하여 유 무기 복합 세라믹 코팅도료를 개발하였다. 특히 glass-flake는 두께는 100~200 nm 정도로 박막형이며, 길이는 $20{\sim}30{\mu}m$ 정도의 판상으로 종횡비가 약 200~300배에 달해 마모 및 내식성에 우수한 성능을 나타낸다. 본 도료로 도막형성 후 접착강도, 인장강도, 연신률, 내캐비테이션 성능을 평가한 결과, 인장강도 $4.8{\sim}6N/mm^2$ 이상, 파단연신률 30%이상, 마식속도 $10mm^2/h$이하, 복합사이클 내식성시험에서 모두 우수한 성능을 보였다. 특히 내 캐비테이션 성능 평가에서 기존 선진 외국 제품대비 2배 이상의 우수한 성능을 나타내었다.

• 대한조선학회논문집
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• 제51권1호
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• pp.88-98
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• 2014

A supercavitation is modern technology that can be used to reduce the frictional resistance of the underwater vehicle. In the process of reaching the supercavity condition which cavity envelops whole vehicle body, a vehicle passes through transition phase from fully-wetted to supercaviting operation. During this phase of flight, unsteady hydrodynamic forces and moments are created by partial cavity. In this paper, analytical and numerical investigations into the dynamics of supercavitating vehicle in transition phase are presented. The ventilated cavity model is used to lead rapid supercavity condition, when the cavitation number is relatively high. Immersion depth of fins and body, which is decided by the cavity profile, is calculated to determine hydrodynamical effects on the body. Additionally, the frictional drag reduction associated by the downstream flow is considered. Numerical simulation for depth tracking control is performed to verify modeling quality using PID controller. Depth command is transformed to attitude control using double loop control structure.