• Title, Summary, Keyword: Cavitation Phenomenon(공동현상)

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Analysis of a Hydrodynamic Herringbone-Grooved Journal Bearing in a Small Precision Motor Considering Cavitation (공동현상을 고려한 소형 정밀 모터용 빗살무늬 저널베어링의 해석)

  • 창동일;장건희
    • Transactions of the Korean Society of Mechanical Engineers A
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    • v.24 no.11
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    • pp.2680-2687
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    • 2000
  • The Reynolds equation, incorporating Elrods cavitation algorithm, is discretized on a rectangular grid in computational space through coordinate mapping in order to accurately analyze a herringbone grooved journal bearing of a spindle motor in a computer hard disk drive. The pressure distribution and cavitation area are determined by using the finite volume method. Predicted results are compared to experimental data of previous researchers. It was found that positive pressure is developed within the converging section of the bearing and that a cavity occurs in the diverging section. Cavitation has been neglected in the previous analysis of the herringbone grooved bearing. Load capacity and bearing torque are increased due to the increased of eccentricity and L/D and the decrease of the grooved width ratio. The maximum load capacity was found to occur at a groove angle of 30 degrees while bearing torque remains constant due to the variation of the groove angle. The cavitation region is significantly decreased with the inclusion of herringbone grooves. However, the region increases with the increase of the eccentricity, L/D, groove angle and the rotational speed and the decrease of the grooved width ratio.

NUMERICAL ANALYSIS OF CAVITATING FLOW PAST CYLINDER WITH THREE DIFFERENT CAVITATION MODELS (서로 다른 캐비테이션 모델을 이용한 실린더 주위의 캐비테이션 유동현상 전산해석)

  • Kim, S.Y.;Park, W.G.;Jung, C.M.
    • Journal of computational fluids engineering
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    • v.16 no.1
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    • pp.60-66
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    • 2011
  • Engineering interests of submerged bodies and turbomachinery has led researchers to study various cavitation models for decades. The governing equations used for the present work are the two-phase Navier-Stokes equations with homogeneous mixture model. The solver employed on implicit dual time preconditioning algorithm in curvilinear coordinates. Three different cavitation models were applied to two axisymmetric cylinders and compared with experiments. It is concluded that the Merkle's new cavitation model has successfully accounted for cavitating flows and well captured the re-entrant jet phenomenon over the 0-caliber cylinder.

A Study on the Flow Analysis for KP505 Propeller Open Water Test (KP505 프로펠러의 단독성능 시험을 위한 유동해석에 관한 연구)

  • Lee, Han-Seop;Kim, Min-Tae;Kim, Won-Seop;Lee, Jong-Hoon;Park, Sang-Heup
    • Journal of the Korea Academia-Industrial cooperation Society
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    • v.20 no.9
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    • pp.150-155
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    • 2019
  • Cavitation refers to a phenomenon in which empty spaces occur in a fluid due to changes in pressure and a velocity. When a liquid moves at a high speed, the pressure drops below the vapor pressure, and vapor bubbles are generated in the liquid. This study used CFD to analyze the flow of fluid machinery used in marine and offshore plants. The goals are to ensure the validity of the analysis method for marine propellers in an open water test, to increase the forward ratio, and to use FLUENT to understand the flow pattern due to cavitation. A three-dimensional analysis was performed and compared with experimental data from MOERI. The efficiency was highest at advance ratios of 0.7 - 0.8. Thrust was generated due to the difference between the pressure surface and the suction surface, and it was estimated that bubbles would be generated in the vicinity of the back side surface rather than the face side of the propeller, resulting in more cavitation. The cavitation decreased sharply as the advance ratio increased. The thrust and torque coefficients were comparable to those of the MOERI experimental data except at the advance ratio of 1, which showed a difference of less than 5%. Therefore, it was confirmed that CFD can evaluate an open water propeller test.

CAVITATION ANALYSIS IN A CENTRIFUGAL PUMP USING VOF METHOD (VOF기법을 이용한 원심펌프 내의 공동현상에 관한 유동해석)

  • Lee, W.J.;Lee, J.H.;Hur, N.;Yoon, I.S.
    • Journal of computational fluids engineering
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    • v.20 no.4
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    • pp.1-6
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    • 2015
  • Centrifugal pumps consume considerable amount of energy in various industrial applications. Therefore, improvement of the efficiency of these machines has become a major challenge. Cavitation is a phenomenon which decreases the pump efficiency and even causes structural demage. Hence, the goal of this paper is to investigate the cavitation problem in the single-stage and double-stage centrifugal pumps. The Volume of Fraction (VOF) method has been used for the numerical simulations together with Rayliegh-Plesset model for the gas-liquid two-phase flow inside the pump. In order to capture the turbulent phenomena, the standard k-${\varepsilon}$ turbulence model has been adopted, and the simulations have been done as unsteady cases. In addition, the motion of the rotating parts has been simulated using Multi Reference Frame(MRF) method. The results are presented and compared in terms of hydraulic head and NPSH for both the single-stage and double-stage pumps. The H-Q curves show the effects of cavitation on decreasing the pumps performances.

Experimental Verification on the Effect of the Gap Flow Blocking Devices Attached on the Semi-Spade Rudder using Flow Visualization Technique (유동가시화를 이용한 혼-타의 간극유동 차단장치 효과에 관한 실험적 검증)

  • Shin, Kwangho;Suh, Jung-Chun;Kim, Hyochul;Ryu, Keuksang;Oh, Jungkeun
    • Journal of the Society of Naval Architects of Korea
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    • v.50 no.5
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    • pp.324-333
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    • 2013
  • Recently, rudder erosion due to cavitation has been frequently reported on a semi-spade rudder of a high-speed large ship. This problem raises economic and safety issues when operating ships. The semi-spade rudders have a gap between the horn/pintle and the movable wing part. Due to this gap, a discontinuous surface, cavitation phenomenon arises and results in unresolved problems such as rudder erosion. In this study, we made a rudder model for 2-D experiments using the NACA0020 and also manufactured gap flow blocking devices to insert to the gap of the model. In order to study the gap flow characteristics at various rudder deflection angles($5^{\circ}$, $10^{\circ}$, $35^{\circ}$) and the effect of the gap flow blocking devices, we carried out the velocity measurements using PIV(Particle Image Velocimetry) techniques and cavitation observation using high speed camera in Seoul National University cavitation tunnel. To observe the gap cavitation on a semi-spade rudder, we slowly lowered the inside pressure of the cavitation tunnel until cavitation occurred near the gap and then captured it using high-speed camera with the frame rate of 4300 fps(frame per second). During this procedure, cavitation numbers and the generated location were recorded, and these experimental data were compared with CFD results calculated by commercial code, Fluent. When we use gap flow blocking device to block the gap, it showed a different flow character compared with previous observation without the device. With the device blocking the gap, the flow velocity increases on the suction side, while it decreases on the pressure side. Therefore, we can conclude that the gap flow blocking device results in a high lift-force effect. And we can also observe that the cavitation inception is delayed.

A study on the identification of underwater propeller singing phenomenon (수중 프로펠러 명음 현상의 규명에 관한 연구)

  • Kim, Taehyung;Lee, Hyoungsuk
    • The Journal of the Acoustical Society of Korea
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    • v.37 no.2
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    • pp.92-98
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    • 2018
  • This paper is a study on the generation mechanism of propeller singing based on the cavitation tunnel test, underwater impact test, finite element analysis and computational flow analysis for the model propeller. A wire screen mesh, a propeller and a rudder were installed to simulate ship stern flow, and occurrence and disappearance of propeller singing phenomenon were measured by hydrophone and accelerometer. The natural frequencies of propeller blades were predicted through finite element analysis and verified by contact and non-contact impact tests. The flow velocity and effective angle of attack for each section of the propeller blades were calculated using RANS (Reynolds Averaged Navier-Stokes) equation-based computational fluid analysis. Using the high resolution analysis based on detached eddy simulation, the vortex shedding frequency calculation was performed. The numerical predicted vortex shedding frequency was confirmed to be consistent with the singing frequency and blade natural frequency measured by the model test.

Advanced Wastewater Treatment using Sludge Solubilization by the Cavitation and PGA addition (Cavitation에 의한 슬러지 가용화와 PGA를 이용한 하수고도처리에 관한 연구)

  • KIM, Dongha
    • Journal of Korean Society of Water and Wastewater
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    • v.22 no.4
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    • pp.449-454
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    • 2008
  • Some pretreatment methods have been proposed to enhance the biodegradability and to shorten the hydrolysis reaction time. By means of efficient pretreatment the suspended solids (SS) can be made of better accessible for the anaerobic bacteria. There are several ways how this can be accomplished, which include biological, mechanical, thermal, and chemical methods. For the sludge solubilization using the cavitation phenomenon, we have tried to develop a pretreatment process consisted of a reactor and pumps. The objectives of this study were to develop a advanced wastewater treatment consisted of IABR and the cavitation with PGA. The most effective removal for organic matter and nutrients were occured when both cavitation pretreatment and ${\gamma}$-PGA were applied at the IABR process. Only small portion of ${\gamma}$-PGA at a rate of 1.38mg/L, was enough to improve sedimentation ability, SS removal efficiencies, and sludge volume reduction. After the sludge solubilization by the cavitation, SCOD increased to 193% and SS decreased to 36%. The removal ratio of BOD was 94.5%, T-N removal ratio was 85.5% and T-P removal ratio was 84.9%. The combination process of the IABR with the cavitation and PGA addition seems to be very effective alternative wastewater treatment process.

Numerical Analysis for the Flow Field past a Two-Staged Conical Orifice (이단 원추형 오리피스를 지니는 유동장에 대한 수치해석)

  • Kim, Yeon-Soo;Kim, You-Gon
    • Proceedings of the KSME Conference
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    • pp.499-505
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    • 2001
  • The objective of the paper was to measure the pressure drop and to investigate the recirculation region of the conical orifices used in Kwang-yang Iron & Steel Company. The flow field with water used as a working fluid was the turbulent flow for Reynolds number of $2{\times}10^4$. The effective parameters for the pressure drop and the recirculation region were the conical orifice's inclined angle (${\theta}$) against the wall, the interval(L) between orifices, the relative angle of rotation(${\alpha}$) of the orifices, the shape of the orifice's hole(circle, rectangle, triangle) having the same area. It was found that the shape of the orifice's hole affected the pressure drop and the flow field a lot, But the other parameters did not make much differences to the pressure drop. The PISO algorithm with FLUENT code was employed.

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Nonlinear Behaviors of a Gas-filled Bubble Oscillator with Large Amplitude of Excitation (큰 압력 진폭에 의해 구동되는 기포진동체의 비선형 거동 특성)

  • 김동혁
    • Journal of Advanced Marine Engineering and Technology
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    • v.26 no.1
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    • pp.116-124
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    • 2002
  • The bubble model by Keller and Prosperetti is adapted to solve the nonlinear oscillation of a gas bubble. This formulation leads to accurate results since it introduces the energy equation instead of the polytropic assumption for the bubble interior. The numerical method used in this study is stable enough to handle large amplitude of bubble oscillation. The numerical results show some interesting nonlinear phenomena fur the bubble oscillator. The excitation changes the natural frequency of the bubble and makes some harmonic resonances at $f/f_0=1/2, 1/3$ and so on. The natural frequency of a bubble oscillator decreases compared with the linear case result, which means that the nonlinear bubble oscillation system is a "softening"system. In addition, the frequency response curve jumps up or down at a certain frequency. It is also found that there exist multi-valued regions in the frequency response curve depending on the initial conditions of bubble. The dependency of the bubble motion on the initial condition can generate extremely large pressure and temperature which might be the cause of the acoustic cavitation and the sonoluminescence.inescence.

Numerical Study on Cavitation Reduction in Velocity-Control Trim of Valve with High Pressure Drop (고차압 밸브의 속도제어형 트림에서 케이테이션 억제에 관한 수치적 연구)

  • Kim, Dae Kwon;Sohn, Chae Hoon
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.37 no.9
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    • pp.863-871
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    • 2013
  • Flow characteristics of velocity-control trim in a valve is investigated numerically with high pressure drop. A basic trim widely used for a valve in domestic powerplants is selected and designed for a baseline of velocity-control trim. The numerical analysis is focused on flow rate and cavitation with the basic trim. For a condition of high-pressure drop, pressure drop between inlet and outlet and fluid temperature are selected to be 18.1 MPa and $160^{\circ}C$, respectively, which are typical ones considering operating conditions adopted in powerplants. With this baseline model and condition, design changes are made for improvement of flow rate and cavitation phenomenon. For re-design, trim is divided into three zones in radial direction and design parameters of flow area, stage, and flow direction are considered in each zone. With these combined parameters applied to each zone, 4 models with design changes are proposed and their flow rates and cavitation areas are investigated. From comparison with those in the baseline model of a basic trim, proposed models show better performance in both flow rate and cavitation.