• 제목/요약/키워드: wake region

검색결과 288건 처리시간 0.022초

대형 유조선의 저항 및 추진성능에 대한 축척효과의 수치적 연구 (Computational Study of the Scale Effect on Resistance and Propulsion Performance of VLCC)

  • 최정은;김정훈;이홍기
    • 대한조선학회논문집
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    • 제48권3호
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    • pp.222-232
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    • 2011
  • This article examines the scale effect of the flow characteristics, resistance and propulsion performance on a 317k VLCC. The turbulent flows around a ship in both towing and self-propulsion conditions are analyzed by solving the Reynolds-averaged Navier-Stokes equation together with the application of Reynolds stress turbulence model. The computations are carried out in both model- and full-scale. A double-body model is applied for the treatment of free surface. An asymmetric body-force propeller is used. The speed performances including resistance and propulsion factors are obtained from two kinds of methods. One is to analyze the computational results in model scale through the revised ITTC' 78 method. The other is directly to analyze the computational results in full scale. Based on the computational predictions, scale effects of the resistance and the self-propulsion factors including form factor, thrust deduction fraction, effective wake fraction and various efficiencies are investigated. Scale effects of the streamline pattern, hull pressure and local flow characteristics including x-constant sections, propeller and center plane, and transom region are also investigated. This study presents a useful tool to hull-form and propeller designers, and towing-tank experimenters to take the scale effect into consideration.

뭉뚝한 물체에 작용하는 압력의 효율적인 계산법 개발과 조류중에서의 유연한 유벽의 변형 (Development of an Efficient Calculation Method of Pressure Acting on a Bluff Body and the Deformation of Flexible Oil Fences in Currents)

  • 강관형;이정묵
    • 대한조선학회논문집
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    • 제33권4호
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    • pp.22-31
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    • 1996
  • 임의의 형상을 갖는 몽뚝한 물체에 작용하는 압력을 효율적으로 계산할 수 있는 방법을 확립하여, 조류중 유벽의 부유 성능을 스커트의 변형을 고려해 예측할 수 있는 방법을 개발하였다. 물체 전면의 압력은 반류영역을 박리유선을 경계로한 가상의 강체로 놓고 포텐샬 유동을 해석하여 구하였고, 후면의 압력은 기존의 실험 결과를 바탕으로 결정하였다. 압력 계산 방법의 검증을 위해 몇 가지 뭉뚝한 물체의 무한 수심에서의 압력 저항을 계산하여 기존의 실험 결과와 잘 일치한다는 것을 확인하였다. 유벽의 변형은 유동장과 연계되어 있어서 순차반복법을 통하여 구하였다. 여러 조류속도와 추의 질량에 대한 유벽의 변형된 형상과 홀수의 감소를 구하였다.

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제어판의 정상 및 비정상 토크에 관한 풍동시험 (Wind-Tunnel Experiment for the Steady and Unsteady Torques of a Control Panel)

  • 서민석;고상근;강신형
    • 대한조선학회논문집
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    • 제29권4호
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    • pp.98-103
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    • 1992
  • 고정된 가이드 패널 뒤에 설치된 실제 형상의 3차원 제어판 모델이 균일유동 하에서 작동될 때, 제어판이 받는 토크의 정적 특성과 동적 특성에 대하여 풍동실험을 통해 연구하였다. 제어판이 가이드 패널에 의한 후류에서 작동되면 각도에 따른 토크의 증가는 적으며, 일정한 범위를 지나면 토크계수는 각도에 선형적으로 증가한다. 실험이 수행된 속도 범위 내에서는 토크계수는 속도에 따라서 변화가 거의 없었다. 토크계수가 영이 되는 중립각도의 크기가 영각에 따라서 증가하며, 영각의 절반 정도이다. 비정상 토크계수는 정상상태에 대하여 히스테리시스를 보이며, 풍속이 증가할수록 다소 커진다. 히스테리시스는 각속도가 증가할수록 증가하고, 각속도가 5.5rad/s 보다 적은(reduced frequency, $k={\omega}c/z\;V_0$의 값은 0.035) 경우에는 준정상 상태라고 할 수 있다.

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부가수 질량을 고려한 외팔판의 고유진동 해석 (Natural Frequency Analysis of Cantilever Plates with Added Mass)

  • 장현길;노인식;홍창호;이창섭
    • 대한조선학회논문집
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    • 제50권1호
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    • pp.1-7
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    • 2013
  • The high-skewed and/or composite propellers of current interests to reduce the ship vibration and to increase the acoustic performance are likely to be exposed to the unexpected structural problems. One typical example is that the added mass effect on the propellers working in the non-uniform wake field reduces the natural frequency of the propeller leading to the resonance with the low-frequency excitation of the external forces. To avoid this resonance problem during the design stage, the technique of fluid-structure interaction has been developed, but the higher-order effect of the blade geometry deformation is not yet considered in evaluating the added mass effects. In this paper the fluid boundary-value problem is formulated by the potential-based panel method in the inviscid fluid region with the velocity inflow due to the body deformation, and the structural response of the solid body under the hydrodynamic loading is solved by applying the finite element method which implements the 20-node iso-parametric element model. The fluid-structure problem is solved iteratively. A basic fluid-sturcture interaction study is performed with the simple rectangular plates of thin thickness with various planform submerged in the water of infinite extent. The computations show good correlation with the experimental results of Linholm, et al. (1965).

농약 살포 드론의 노즐 유동 특성 및 시뮬레이션 (Nozzle Flow Characteristics and Simulation of Pesticide Spraying Drone)

  • 강기준;장세명;나인호;김선우;김흥태
    • 스마트미디어저널
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    • 제8권4호
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    • pp.38-45
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    • 2019
  • 회전익 드론의 후류에서 발생하는 강한 유동과 살포되는 농약 노즐에서의 스프레이 유동이 동반될 경우, 극심한 기체의 요동과 함께 액적 유동은 바람의 영향을 크게 받게 된다. 특히 액적은 공기역학적인 항력의 영향을 받기 때문에 단순한 제자리 비행과는 달리 전진 비행을 할 경우 또는 측풍을 받고 있을 경우, 살포 영역에 큰 변화가 발생한다. 이로 인하여 드론을 활용한 농약살포에는 보다 큰 비산의 위험성이나 위치간의 낙하분산에 차이가 커져 효율성이 떨어질 수 있는 문제가 생긴다. 따라서 적절한 수치 모델링과 이를 적용한 전산 시뮬레이션을 통하여 다양한 비행 및 대기 조건을 적용할 수 있는 예측 도구가 필요하다. 본 연구에서는 로터에서 나오는 강한 하풍과 드론의 비행속도에 따른 액적 유동 특성에 대해 실험 및 수치해석을 진행하였으며, 액적이 분포하는 확률밀도함수를 구하여 서로 비교함으로써 농약살포용 드론에서 액적을 살포할 시 효율성을 증진시킬 수 있는 분사 시스템을 구축하고자 한다.

Reynolds Number Effects on Aerodynamic Characteristics of Compressor Cascades for High Altitude Long Endurance Aircraft

  • Kodama, Taiki;Watanabe, Toshinori;Himeno, Takehiro;Uzawa, Seiji
    • 한국추진공학회:학술대회논문집
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    • 한국추진공학회 2008년 영문 학술대회
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    • pp.195-201
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    • 2008
  • In the jet engines on the aircrafts cruising at high altitude over 20 km and subsonic speed, the Reynolds number in terms of the compressor blades becomes very low. In such an operating condition with low Reynolds number, it is widely reported that total pressure loss of the air flow through the compressor cascades increases dramatically due to separation of the boundary layer and the secondary-flow. But the detail of flow mechanisms causes the total pressure loss has not been fully understood yet. In the present study, two series of numerical investigations were conducted to study the effects of Reynolds number on the aerodynamic characteristics of compressor cascades. At first, the incompressible flow fields in the two-dimensional compressor cascade composed of C4 airfoils were numerically simulated with various values of Reynolds number. Compared with the corresponding experimental data, the numerically estimated trend of total pressure loss as a function of Reynolds number showed good agreement with that of experiment. From the visualized numerical results, the thickness of boundary layer and wake were found to increase with the decrease of Reynolds number. Especially at very low Reynolds number, the separation of boundary layer and vortex shedding were observed. The other series, as the preparatory investigation, the flow fields in the transonic compressor, NASA Rotor 37, were simulated under the several conditions, which corresponded to the operation at sea level static and at 10 km of altitude with low density and temperature. It was found that, in the case of operation at high altitude, the separation region on the blade surface became lager, and that the radial and reverse flow around the trailing edge become stronger than those under sea level static condition.

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LES study of flow field and aerodynamic forces on a circular cylinder at Re=3900 with focus on grid resolution

  • Hongmiao Jing;Jitao Zhang;Qingkuan Liu;Yangxue Wang
    • Wind and Structures
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    • 제36권3호
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    • pp.175-200
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    • 2023
  • The large eddy simulation (LES) of the flow around a circular cylinder is not only affected by the sub-grid scale (SGS) model but also by the grid resolution of the computational domain. To study the influence of different grids on the LES results, the LES simulations of the flow around a circular cylinder with different grids at Reynolds number (Re) = 3900 was performed. A circular computational domain with different radial growth rates and circumferential and spanwise grid numbers was adopted for the simulations. Meanwhile, the aerodynamic forces, wind pressure coefficients, mean and instantaneous flow fields, and the effect of grid resolution on them were comprehensively analyzed. The results indicate that the lift coefficient, wind pressure coefficient, and recirculation length are significantly affected by the radial growth rate of the grid and the circumferential grid number. The spanwise grid number has a significant influence on the three-dimensionality of the flow and plays an important role in velocity fluctuations in the wake region. Nevertheless, the aerodynamic coefficients and recirculation length are not sufficiently sensitive to the grid number in the spanwise direction. By comparing the results, it can be concluded that suitable and reliable LES results can be obtained when the radial growth rate is 1.03 or 1.05, the circumferential grid number is 160, 200, or 240, and the spanwise grid number is 64. A radial growth rate 1.05, circumferential grid number 160, and spanwise grid number 64 are recommended to reduce the grid amount and further improve the efficiency.

A Consensus Plan for Action to Improve Access to Cancer Care in the Association of Southeast Asian Nations (ASEAN) Region

  • Woodward, Mark
    • Asian Pacific Journal of Cancer Prevention
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    • 제15권19호
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    • pp.8521-8526
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    • 2014
  • In many countries of the Association of Southeast Asian Nations (ASEAN), cancer is an increasing problem due to ageing and a transition to Western lifestyles. Governments have been slow to react to the health consequences of these socioeconomic changes, leading to the risk of a cancer epidemic overwhelming the region. A major limitation to motivating change is the paucity of high-quality data on cancer, and its socioeconomic repercussions, in ASEAN. Two initiatives have been launched to address these issues. First, a study of over 9000 new cancer patients in ASEAN - the ACTION study - which records information on financial difficulties, as well as clinical outcomes, subsequent to the diagnosis. Second, a series of roundtable meetings of key stakeholders and experts, with the broad aim of producing advice for governments in ASEAN to take appropriate account of issues relating to cancer, as well as to generate knowledge and interest through engagement with the media. An important product of these roundtables has been the Jakarta Call to Action on Cancer Control. The growth and ageing of populations is a global challenge for cancer services. In the less developed parts of Asia, and elsewhere, these problems are compounded by the epidemiological transition to Western lifestyles and lack of awareness of cancer at the government level. For many years, health services in less developed countries have concentrated on infectious diseases and mother-and-child health; despite a recent wake-up call (United Nations, 2010), these health services have so far failed to allow for the huge increase in cancer cases to come. It has been estimated that, in Asia, the number of new cancer cases per year will grow from 6.1 million in 2008 to 10.6 million in 2030 (Sankaranarayanan et al., 2014). In the countries of the Association of Southeast Asian Nations (ASEAN), corresponding figures are 770 thousand in 2012 (Figure 1), rising to 1.3 million in 2030 (Ferlay et al., 2012). ASEAN consists of Brunei Darussalam, Cambodia, Indonesia, Lao, Malaysia, Myanmar, the Philippines, Singapore, Thailand and Viet Nam. It, thus, includes low- and middle-income countries where the double whammy of infectious and chronic diseases will pose an enormous challenge in allocating limited resources to competing health issues. Cancer statistics, even at the sub-national level, only tell part of the story. Many individuals who contract cancer in poor countries have no medical insurance and no, or limited, expectation of public assistance. Whilst any person who has a family member with cancer can expect to bear some consequential burden of care or expense, in a poor family in a poor environment the burden will surely be greater. This additional burden from cancer is rarely considered, and even more rarely quantified, even in developed nations.

선박 추진시스템 유동 소음원 상대적 기여도 분석 (Investigation on relative contribution of flow noise sources of ship propulsion system)

  • 하준범;구가람;정철웅;설한신;정홍석;정민석
    • 한국음향학회지
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    • 제41권3호
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    • pp.268-277
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    • 2022
  • 본 논문에서는 KVLCC2 선체 축소모형에 설치된 추진시스템의 세부 구성품별 유동 소음원을 분석하였으며, 각각의 소음원이 수중방사소음에 미치는 영향에 대해 정량적으로 분석하였다. 수치 해석 영역은 실험 결과와의 비교를 위하여 선박해양플랜트연구소 대형 캐비테이션 터널의 시험부와 동일하게 설정하였다. 먼저 유동장내 소음원을 정확하게 모사하기 위하여 고정밀 해석기법인 비압축성 다상 Delayed Detached Eddy Simulation 방법을 적용하였고, 유동해석 결과를 기반으로 Ffowcs Williams and Hawkings 적분방정식을 사용하여 수중방사소음을 예측하였으며, 터널 실험결과와의 비교를 통해 해석절차의 유효성을 확인하였다. 추진시스템의 유동 소음원별 영향을 정량적으로 비교하기 위하여 추진기 날개 끝-와류 공동, 날개 표면 그리고 방향타 표면을 소음원 영역으로 선정하였으며, 음압과 파워 스펙트럼 밀도, 음향 파워를 비교하였다. 공동에 의한 홀극 소음원의 기여도가 추진기 날개 및 방향타에 의한 쌍극 소음원에 비해 수중방사소음에 크게 기여하였으며, 추진기 후류의 영향으로 방향타에 의한 기여도가 추진기 보다 더 크게 발생함을 확인하였다.