• Title/Summary/Keyword: Cavitation number

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PIV Measurements of Non-cavitating and Cavitating Flow in Wake of Two-dimensional Wedge-shaped Submerged Body (PIV를 이용한 2차원 쐐기형 몰수체 후류의 비공동 및 공동 유동장 계측)

  • Hong, Ji-Woo;Jeong, So-Won;Ahn, Byong-Kwon
    • Journal of Ocean Engineering and Technology
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    • v.33 no.1
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    • pp.26-32
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    • 2019
  • The vortex flow behind a bluff body has been a subject of interest for a very long time because of its engineering applicability such as to vortex induced vibration. In the near wake of a bluff body, vortices are periodically shed in two shear layers, which originate in the trailing edges. The far wake is made up of the classical Karman vortices, which are connected together by streamwise and spanwise vortices. These vortex formations have been studied in many experimental and numerical ways. However, most of the studies considered non-cavitating flow. In this study, we investigated cavitating flow in the wake of a two-dimensional wedge. Experiments were conducted in a cavitation tunnel of Chungnam National University. Using a particle image velocimetry (PIV), we measured the velocity fields under two different flow conditions: non-cavitating and cavitating regimes. We also investigated the vortex shedding frequencies using an absolute pressure transducer mounted on the top of the test window. Throughout the experiments, it was found that the shedding frequency of the vortex was strongly affected by cavitation, and the Strouhal number could exceed its value in the non-cavitating regime.

Generation of emulsions due to the impact of surfactant-laden droplet on a viscous oil layer on water (벤츄리 노즐 출구 형상과 작동 조건에 따른 캐비테이션 기포 발생 특성 연구)

  • Changhoon Oh;Joon Hyun Kim;Jaeyong Sung
    • Journal of the Korean Society of Visualization
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    • v.21 no.1
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    • pp.94-102
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    • 2023
  • Three design parameters were considered in this study: outlet nozzle angle (30°, 60°, 80°), neck length (1 mm, 3 mm), and flow rate (0.5, 0.6, 0.7, 0.8 lpm). A neck diameter of 0.5 mm induced cavitation flow at a venture nozzle. A secondary transparent chamber was connected after ejection to increase bubble duration and shape visibility. The bubble size was estimated using a Gaussian kernel function to identify bubbles in the acquired images. Data on bubble size were used to obtain Sauter's mean diameter and probability density function to obtain specific bubble state conditions. The degree of bubble generation according to the bubble size was compared for each design variable. The bubble diameter increased as the flow rate increased. The frequency of bubble generation was highest around 20 ㎛. With the same neck length, the smaller the CV number, the larger the average bubble diameter. It is possible to increase the generation frequency of smaller bubbles by the cavitation method by changing the magnification angle and length of the neck. However, if the flow rate is too large, the average bubble diameter tends to increase, so an appropriate flow rate should be selected.

Cryogenic Performance Test of LOX Turbopump in Liquid Nitrogen (액체질소를 이용한 산화제펌프의 극저온 성능시험)

  • Kim, Jin-Sun;Hong, Soon-Sam;Kim, Dae-Jin;Choi, Chang-Ho;Kim, Jin-Han
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.34 no.4
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    • pp.391-397
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    • 2010
  • Performance tests of a liquid-oxygen pump were carried out using liquid nitrogen (LN2) as a working fluid in a cryogenic turbopump test facility in Korea Aerospace Research Institute (KARI). The tests were performed at 30-55% of the design rotational speed, and the results were compared with those from a water test. The experimental results confirmed the similarity of the hydraulic performance, which allows the prediction of the pump performance at a design rotational speed of 20,000 rpm. The overall cavitation performance of the pump in the cryogenic environment was better than that in the water environment for all ranges of flow rates and rotational speeds. Critical cavitation number at the design flow rate was determined as 0.012 from the cryogenic test, and as 0.024 from the water test. The improved cavitation performance is due to the thermodynamic effect in cryogenic fluids.

Fundamental Studies for Ventilated Supercavitation Experiments in New High-speed Cavitation Tunnel (신조된 고속 캐비테이션 터널에서 환기 초공동 실험 수행을 위한 기초 연구)

  • Paik, Bu-Geun;Kim, Min-Jae;Jung, Young-Rae;Lee, Seung-Jae;Kim, Kyoung-Youl;Ahn, Jong-Woo;Seol, Han-Shin;Kim, Ki-Sup
    • Journal of the Society of Naval Architects of Korea
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    • v.55 no.4
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    • pp.330-340
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    • 2018
  • In the present works, the High-speed Cavitation Tunnel (HCT) has been designed and manufactured to have the large test section to conduct various supercavitation experiments. The large amount of air ventilated behind a cavitator produces lots of tiny bubbles, which prevent clear observation of supercavitation at the test section. To collect small bubbles effectively, a bubble collecting section of large volume is equipped upstream of the test section. HCT has the test section dimension of $0.3^H{\times}0.3^W{\times}3.0^L\;m^3$ and provides maximum flow speed of 20.4 m/s at the test section. The blockage and Froude effects on the ventilated supercavitation are investigated successfully at the test section. The basic studies such as the supercavitation evolution, drag measurements and cavity shape extraction with air flow rate are also carried out in HCT.

Development of LDV(Laser Doppler Velocimetry) for Measuring Three Dimensional Hull Wake of Ship Model in Large Cavitation Tunnel (대형 캐비테이션 터널 내 선박 모형의 3차원 선체 반류 계측을 위한 레이저 유속계 개발)

  • Paik, Bu-Geun;Ahn, Jong-Woo;Seol, Han-Shin;Park, Young-Ha;Kim, Ki-Sup;Cheon, Ho-Geun
    • Journal of the Society of Naval Architects of Korea
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    • v.54 no.6
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    • pp.515-521
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    • 2017
  • Large Cavitation Tunnel (LCT) of KRISO enables us to conduct cavitation tests of the propeller attached to a ship model. As the ship model tests are done at rather high Reynolds number of 107~108, flow measurement system such as pitot tube cannot be employed because of structural safety problems in its system and difficulties in installing it within the test section. Thus, KRISO has developed new 3-D LDV system used in large test section of LCT. There are several difficulties in using 3-D LDV, which did not allow efficient operation of it. The first trouble was the calibration using the conventional pin hole. To make the focus with same laser-beam waists at the wanted position, the high spatial resolution CCD is utilized in the calibration procedure for 3-D LDV. The off-axis configuration provides two velocity components in the horizontal plane and on-axis configuration gives third velocity component in the vertical plane. The horizontal velocity components are also obtained in the coincidence mode, which prevents any misleading results in the off-axis configuration. The nominal wake of Aframax tanker model is measured by the developed 3-D LDV system. The measured hull wake showed good agreement with that obtained by CFD calculation.

A Study on the Model Test for the Twin Propeller Cavitation Noise (쌍축선 추진기 캐비테이션 소음 모형시험 연구)

  • Park, Cheolsoo;Kim, Gun-Do;Yim, Geun-Tae;Park, Young-Ha;Jang, Hyun-Gil;Jang, Young Hun
    • Journal of the Society of Naval Architects of Korea
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    • v.55 no.1
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    • pp.28-36
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    • 2018
  • The experimental results of the model tests for the twin propeller cavitation noise are presented. The model test was carried out by means of procedure of noise measurement followed by the signal processing and full-scale extrapolation. In order to convert the measured sound pressure level into the sound source level, transfer function measurements for three conditions were performed according to the combination of locations and number of virtual sources. White noise and LFM signal were used as a source signals to examine the influence of the input signal. For the twin propellers, 5 transfer functions were defined and the results were discussed. Cavitation noise measurement tests were performed similarly to the transfer function measurement test. Noise source localization analysis was performed to confirm the test effectiveness. It was confirmed that the source level of the twin propeller can be estimated reliably by using transfer function corrections. Finally, the model test results were converted into full-scale by applying the ITTC '87 model-ship scaling raw, and the validity of the model test was confirmed by comparison with the full-scale measurement result.

Visualization of the Flow in a Diesel Injection Nozzle In case of the Steady Flow Condition (정상류 조건에서의 디젤 연료 분사 노즐내의 유동가시화)

  • 김장헌;송규근
    • Transactions of the Korean Society of Automotive Engineers
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    • v.7 no.6
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    • pp.49-56
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    • 1999
  • The effects of the internal flow in a D.I. Diesel injection nozzle on the atomization of a spray were analyzed experimentally. Flow visualization studies were made using a transparent acrylic model nozzle as a diesel nozzle . Water instead of disel fuel was used as the injection liquid. The geometry of the model nozzle was scaled up 10 times of the actual nozzle and the injection pressure for the model nozzle was adjusted so as to achieve a Reynolds number at the discharge hole that was the same as the actual nozzle. Experimental results show that when the needle lift was small, the high turbulence in the sac chamber generated by the high velocity seat flow made the spread angle of the spray plume large. Cavitation, which arose from the sac chamber, makes the spread angle of the spray plume large but the discharge coefficient small.

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A Study on the Ultrasonic Conditioning for Interlayer Dielectic CMP (층간절연막 CMP의 초음파 컨디셔닝 특성에 관한 연구)

  • 서헌덕;정해도;김형재;김호윤;이재석;황징연;안대균
    • Proceedings of the Korean Society of Precision Engineering Conference
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    • 2000.05a
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    • pp.854-857
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    • 2000
  • Chemical Mechanical Polishing(CMP) has been accepted as one of the essential processes for VLSI fabrication. However, as the polishing process continues, pad pores get to be glazed by polishing residues, which hinder the supply of new slurry. This defect makes removal rate decrease with a number of polished wafer and the desired within-chip planarity, within wafer and wafer-to-wafer nonuniformity are unable to be achieved. So, pad conditioning is essential to overcome this defect. The eletroplated diamond grit disk is used as the conventional conditioner, And alumina long fiber, the .jet power of high pressure deionized water and vacuum compression are under investigation. But, these methods have the defects like scratches on wafer surface by out of diamond grits, subsidences of pad pores by over-conditioning, and the limits of conditioning effect. To improve these conditioning methods. this paper presents the Characteristics of Ultrasonic conditioning aided by cavitation.

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Put Effect of the inducer scale on the suction performance similarity of a turbopump (인듀서의 크기가 터보펌프의 흡입성능 상사에 미치는 영향에 관한 연구)

  • Kang, Byung Yun;Kang, Shin-Hyoung
    • The KSFM Journal of Fluid Machinery
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    • v.17 no.4
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    • pp.47-52
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    • 2014
  • An inducer is forward-attached to an impeller to improve the suction performance. This paper described the experimental and numerical investigations on the concept of NPSH similarity about the inducer scale. As Reynolds number decreased for the same scale inducer, the hydraulic performance is slightly reduced because of the viscosity. The suction performance similarity is in good agreement. For different scale inducers, the NPSH similarity did not follow the conventional rule which is proportional to the square of the inducer diameter. A cavity of two times scale inducer grows faster under cavitation inception, and the head is more drop as the fluid passes blades. Because of the simplified cavitation model and vapor pressure, the NPSH similarity dose not have an accuracy. This study suggested an empirical formula for the NPSH similarity.

Cavitation Compliance in 1D Part-load Vortex Models

  • Dorfler, Peter K
    • International Journal of Fluid Machinery and Systems
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    • v.10 no.3
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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.