• Title/Summary/Keyword: Wake region

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A Study on Estimation of Inflow Wind Speeds in a CFD Model Domain for an Urban Area (도시 지역 대상의 CFD 모델 영역에서 유입류 풍속 추정에 관한 연구)

  • Kang, Geon;Kim, Jae-Jin
    • Atmosphere
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    • v.27 no.1
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    • pp.67-77
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    • 2017
  • In this study, we analyzed the characteristics of flow around the Daeyeon automatic weather station (AWS 942) and established formulas estimating inflow wind speeds at a computational fluid dynamics (CFD) model domain for the area around Pukyong national university using a computational fluid dynamics (CFD) model. Simulated wind directions at the AWS 942 were quite similar to those of inflows, but, simulated wind speeds at the AWS 942 decreased compared to inflow wind speeds except for the northerly case. The decrease in simulated wind speed at the AWS 942 resulted from the buildings around the AWS 942. In most cases, the AWS 942 was included within the wake region behind the buildings. Wind speeds at the inflow boundaries of the CFD model domain were estimated by comparing simulated wind speeds at the AWS 942 and inflow boundaries and systematically increasing inflow wind speeds from $1m\;s^{-1}$ to $17m\;s^{-1}$ with an increment of $2m\;s^{-1}$ at the reference height for 16 inflow directions. For each inflow direction, calculated wind speeds at the AWS 942 were fitted as the third order functions of the inflow wind speed by using the Marquardt-Levenberg least square method. Estimated inflow wind speeds by the established formulas were compared to wind speeds observed at 12 coastal AWSs near the AWS 942. The results showed that the estimated wind speeds fell within the inter quartile range of wind speeds observed at 12 coastal AWSs during the nighttime and were in close proximity to the upper whiskers during the daytime (12~15 h).

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

  • Choi, Jung-Eun;Kim, Jung-Hun;Lee, Hong-Gi
    • Journal of the Society of Naval Architects of Korea
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    • v.48 no.3
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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.

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

  • Kang, Kwan Hyoung;Lee, Choung Mook
    • Journal of the Society of Naval Architects of Korea
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    • v.33 no.4
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    • pp.22-31
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    • 1996
  • An efficient calculation method of pressure acting on a bluff body is developed. The method is applied to assess the containment capability of an oil fence, considering skirt deformation in current. The pressure on the frontal side of the oil fence is determined by analyzing the potential flow, substituting the wake region as a rigid body which has its boundary at the closed separation streamlines. The pressure at the downstream side of the fence, the so called base pressure, is obtained from the existing experimental results. To verify the calculation method, pressure drag of some bluff bodies in an infinite-fluid medium is calculated, which shows good agreement with the experimental results. The deformed shape of the oil fence is obtained by solving, iteratively, the coupled equations governing the flow field and the deformation of the oil fence, respectively. The deformed shape and the decrease of draft of oil fences with different ballast mass in various current conditions are investigated.

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

  • M.S. Suh;S. Kauh;S.H. Kang
    • Journal of the Society of Naval Architects of Korea
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    • v.29 no.4
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    • pp.98-103
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    • 1992
  • The dynamic and static torque characteristics of a three dimensional control panel installed behind a guide panel were investigated in a wind tunnel. The panel was tested for various wind speeds, angles of attack and positions of the panel. The effects of the rotational speed and the amplitude of the sinusoidal motion were also studied. The increasing rate of torque coefficients with the angular position of the panel is small when the panel remains in the wake region, but is linear when it reaches the external stream. In case of a sinusoidal motion of the pannel, a hysterisis appears in the dynamic torque. The hysterisis becomes strong as the wind speed and the angular speed of the panel increase. The unsteady torque is considered quasi-steady when the angular speed is less than 5.5rad/s, i.e. the reduced frequency is less than 0.035.

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

  • Jang, Hyun-Gil;Nho, In Sik;Hong, Chang-Ho;Lee, Chang-Sup
    • Journal of the Society of Naval Architects of Korea
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    • v.50 no.1
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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).

Readeveloping Turbulent Boundary Layer after Separation-Reattachment(I) (박리-재부착 이후의 재발달 난류경계층 I)

  • 백세진;유정열
    • Transactions of the Korean Society of Mechanical Engineers
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    • v.13 no.4
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    • pp.780-788
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    • 1989
  • An experimental study has been performed to investigate the process from nonequilibrium state to equilibrium state in redeveloping turbulent boundary layer beyond separation-reattachment using pitot tube and hot-wire anemometer. The model sued in the experiment has the form of a backward facing step which is assembled by a two-dimensional 4:1 half elipse and a plate. Measurements are carried out up to a distance of about 50 step height downstream of the step, where the reattachment observed at about x/h=6.5. The profiles of the shape factor H the Clauser parameter G and the coefficient of friction $C^{f}$ exhibited the characteristics similar to those of the equilibrium turbulent boundary layer from x/h=25, and the profiles of the trubulent quantities did from x/h=35. However, the wake region of the boundary layer does not seem to recover the equilibrium turbulent boundary layer even at x/h=50. By considering the distributions of the intermittency factor it has been noted that the turbulence structure changes gradually from a mixing layer to a turbulent boundary layer along downstream direction after reattachment. This becomes clearer as we analyse the one-dimensional energy spectra and the dissipation energy spectra which are measured and caculated at various downstream positions after the backward facing step.p.

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

  • Kang, Ki-Jun;Chang, Se-Myong;Ra, In-Ho;Kim, Sun-Woo;Kim, Heung-Tae
    • Smart Media Journal
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    • v.8 no.4
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    • pp.38-45
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    • 2019
  • When there is a spray flow such as from a pesticide nozzle, winds affect the droplet flow of a rotary-wing drone accompanied by a strong wake, with a severe oscillation. Especially, during forwarding flights or when winds come from the side, compare to a simple hovering flight as the droplet is in the effect of aerodynamic drag force, the effect of spraying region becomes even larger. For this reason, the spraying of pesticides using drones may cause a greater risk of scattering or a difference in droplet dispersion between locations, resulting in a decrease in efficiency. Therefore, through proper numerical modeling and its applied simulation, an indication tool is required applicable for the various flight and atmospheric conditions. In this research, we completed both experiment and numerical analysis for the strong downwash from the rotor and flight velocity of the drone by comparing the probability density function of droplet distribution to build a spraying system that can improve the efficiency when spraying droplets in the pesticide spray drone.

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

  • Kodama, Taiki;Watanabe, Toshinori;Himeno, Takehiro;Uzawa, Seiji
    • Proceedings of the Korean Society of Propulsion Engineers Conference
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    • 2008.03a
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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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    • v.36 no.3
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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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    • v.15 no.19
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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.