• 제목/요약/키워드: Turbulence stimulation

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평판 경계층에 대한 스터드의 난류촉진 영향 연구 (A Study on Turbulence Stimulation Effect of Studs for Boundary Layer Over a Flat Plate)

  • 이준형;정소원;황승현
    • 대한조선학회논문집
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    • 제59권1호
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    • pp.18-28
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    • 2022
  • The turbulence stimulation effect of studs for boundary layer over a flat plate was investigated through the flow measurement in KRISO cavitation tunnel. For the test, Laser Doppler Velocimetry (LDV) and three flat plate models were used: (1) flat plate without studs; (2) flat plate with one stud row; (3) flat plate with two stud rows. The dimension and location of stud rows and the inflow speed were selected considering test conditions for standard-sized model ships in KRISO towing tank. The boundary layer characteristics of test models were analyzed and compared in terms of mean velocity profiles, turbulence intensity profiles, boundary layer thickness, and shape factor. In the case of the flat plate without studs, transition from laminar to turbulent flow occurred around Rex=3.83 ~ 5.19 × 105. In the case of flat plates with stud rows, the flow rapidly changed into turbulent flow right after passing the first stud row. In the state where turbulence was already developed, the second stud row slightly increased the turbulence intensity near the top of the stud, but did not significantly affect the boundary layer characteristics such as mean velocity distribution, boundary layer thickness, and shape factor.

난류 촉진기 주위 천이 유동의 대형 와 모사를 위한 격자 테스트 (Grid Tests for Large Eddy Simulation of Transitional Flows around Turbulence Stimulators)

  • 이상봉;박동우;백광준
    • 해양환경안전학회지
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    • 제23권1호
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    • pp.112-121
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    • 2017
  • 평판에 설치된 스터드 주위의 천이 유동에 있어 격자 크기의 영향을 알기 위해 대형 와 모사를 수행하였다. 스터드에서 야기되는 주 유동 방향의 와 구조가 스터드 후류의 천이에 미치는 영향이 매우 크기 때문에 주 유동 방향, 벽면 수직 방향 그리고 횡 방향으로 격자 크기를 ${\sqrt{2}}$ 배씩 증가시키거나 감소시키면서 스터드 후류에서 주 유동 방향의 와도를 비교하였다. 그 결과 스터드 후류에서 발달하는 주 유동 방향의 와도는 횡 방향 격자 크기에 매우 큰 영향을 받는 것을 알 수 있었으며, 이러한 결과를 바탕으로 ${\Delta}x^+{_{min}}=7.6$, ${\Delta}x^+{_{max}}=41$, ${\Delta}y^+{_{wall}}=0.25$ and ${\Delta}z^+=7.6$의 격자 크기를 결정하였다. 이러한 격자 구성에 있어 모든 방향으로 격자 크기를 동시에 ${\sqrt{2}}$ 배씩 증가시키거나 감소시키면서 스터드에 작용하는 힘의 변화를 비교하여 격자 검증을 실시한 결과 평균 압력 계수와 항력 계수의 비보정 불확실성이 각각 21.6 %와 2.8 % 정도로 추정되었으며, 보정 불확실성은 각각 2 %와 0.3 %로 추정되었다.

Computational simulations of transitional flows around turbulence stimulators at low speeds

  • Lee, Sang Bong;Seok, Woochan;Rhee, Shin Hyung
    • International Journal of Naval Architecture and Ocean Engineering
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    • 제13권1호
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    • pp.236-245
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    • 2021
  • In this study, direct numerical and large eddy simulations of transitional flows around studs were conducted to investigate the effectiveness of turbulence stimulators at very low speeds for the minimum propulsion power condition of four knots. For simplicity, the studs were assumed to be installed on a flat plate, while the wake was observed up to 0.23 m downstream behind the second stud. For applicability to a model ship, we also studied the flow characteristics behind the first and second studs installed on a curved plate, which was designed to describe the geometry of a bulbous bow. A laminar-to-turbulent transition was observed in the wake at ReD ≥ 921 (U≥0.290 m/s), and the wall shear stress at ReD = 1162 (U = 0.366 m/s) in the second wake was similar to that of the fully developed turbulent boundary layer after a laminar-to-turbulent transition in the first wake. At ReD = 581 (U = 0.183 m/s), no turbulence was stimulated in the wake behind the first and second studs on the flat plate, while a cluster of vortical structures was observed in the first wake over the curved plate. However, a cluster of vortical structures was revealed to be generated by the reattachment process of the separated shear layer, which was disturbed by the first stud rather than directly initiated by the first stud. It was quite different from a typical process of transition, which was observed at relatively high ReD that the spanwise scope of the turbulent vortical structures expanded gradually as it went downstream.

Development of UAV Teleoperation Virtual Environment Based-on GSM Networks and Real Weather Effects

  • AbdElHamid, Amr;Zong, Peng
    • International Journal of Aeronautical and Space Sciences
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    • 제16권3호
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    • pp.463-474
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    • 2015
  • Future Ground Control Stations (GCSs) for Unmanned Aerial Vehicles (UAVs) teleoperation targets better situational awareness by providing extra motion cues to stimulate the vestibular system. This paper proposes a new virtual environment for long range Unmanned Aerial Vehicle (UAV) control via Non-Line-of-Sight (NLoS) communications, which is based on motion platforms. It generates motion cues for the teleoperator for extra sensory stimulation to enhance the guidance performance. The proposed environment employs the distributed component simulation over GSM network as a simulation platform. GSM communications are utilized as a multi-hop communication network, which is similar to global satellite communications. It considers a UAV mathematical model and wind turbulence effects to simulate a realistic UAV dynamics. Moreover, the proposed virtual environment simulates a Multiple Axis Rotating Device (MARD) as Human Machine Interface (HMI) device to provide a complete delay analysis. The demonstrated measurements cover Graphical User Interface (GUI) capabilities, NLoS GSM communications delay, MARD performance, and different software workload. The proposed virtual environment succeeded to provide visual and vestibular feedbacks for teleoperators via GSM networks. The overall system performance is acceptable relative to other Line-of-Sight (LoS) systems, which promises a good potential for future long range, medium altitude UAV teleoperation researches.

A Study on Export Financing Policy for Korean SMEs - Focusing on Korea EXIM'S Bank -

  • Bae, Sang Mok;Park, Se Hun
    • 무역상무연구
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    • 제60권
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    • pp.177-194
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    • 2013
  • According to the 2013 statistics, the small/medium-sized enterprises (SMEs) accounted for 99.9% of the Korean economy with 87.8% of employment. This simple figures also indicate SMEs export extension is a key task for the national economy stimulation and job creation. The SMEs export plunge has been affected by the European financial turbulence, US financial instability, developing countries like China entering foreign markets, a weak yen, etc. EXIM Bank, in this context, will need to take a more proactive attitude to provide trade finance by, for example, reviewing the business feasibility for SMEs with a lower credit rank or investigating importers' credit status, etc. Moreover EXIM Bank provides factoring service mostly to large companies and should lower its threshold for service provision to SMEs. Finally EXIM Bank should play more than a primary forfeiting market entity. It also needs to facilitate the secondary forfeiting market.

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