Development of a hydrofoil wake oscillator model based on a near-vortex strength for predicting vortex-induced vibration on a hydrofoil

최현규,홍석윤,송지훈,장원석,최원석 대한조선학회 2023 International Journal of Naval Architecture and Oc Vol.15 No.-

Recently, the evaluation of vortex-induced vibration has emerged as a significantly important issue owing to the development of high-speed and lightweight ships and submarines. To derive an accurate vortex-induced vibration response, it is essential to consider the fluid-structure interaction. Moreover, it is necessary to evaluate the generation of the fluid-structure interaction to effectively prevent catastrophic failure in the structures. In this study, a hydrofoil wake oscillator model was developed based on a near-vortex strength that considers the fluid-structure interaction. The near-vortex strength was calculated from the boundary layer on a trailing edge to overcome the empirical parameter of lift fluctuation in conventional wake oscillator models. To predict the vortex-induced vibration on a hydrofoil, procedures for calculating the near-vortex strength and coupling the structural equations and fluid equation were introduced. The vortex-induced vibration derived using the developed hydrofoil wake oscillator model was verified by comparison it against the experimental results. The results reveal that the derived amplitude and lock-in range of the vortex-induced vibration were consistent with the experimental results. In addition, the extent of occurrence of the fluid-structure interaction and its contribution to vortex-induced vibration were evaluated using a non-dimensional wake parameter.

저전력 VLSI 시스템에서 MTCMOS 블록 전원 차단 시의 전원신 잡음을 줄인 파이프라인 전원 복귀 기법

이성주,연규성,전치훈,장용주,조지연,위재경 대한전자공학회 2004 電子工學會論文誌-SD (Semiconductor and devices) Vol.41 No.10

In low-power VLSI systems, it is effective to suppress leakage current by shutting down megablocks in idle states. Recently, multi-threshold voltage CMOS (MTCMOS) is widely accepted to shutdown power supply. However, it requires short wake-up time as operating frequency increases. This causes large current surge during wake-up process, and it often leads to system malfunction due to severe Power line noise. In this paper, a novel wake-up scheme is proposed to solve this problem. It exploits pipelined wake-up strategy in several stages that reduces maximum current on the power line and its corresponding power line noise. To evaluate its efficiency, the proposed scheme was applied to a multiplier block in the Compact Flash memory controller chip. Power line noise in shutdown and wake-up process was simulated and analyzed. From the simulation results, the proposed scheme was proven to greatly reduce the power line noise compared with conventional schemes. VLSI 시스템에서 전력 소모를 줄이기 위해서는 메가블록이 동작하지 않는 동안 전원을 차단하여 누설 전류를 억제하는 방법이 효과적이다. 최근 들어 다중 문턱 전압 CMOS를 사용하여 전원을 차단하는 방법이 널리 연구되고 있으나, 동작 주파수가 증가함에 파라 전원 복귀에 필요한 시간이 짧아지게 되고, 짧은 시간에 전원이 복귀되면서 전원선에 대량의 전류가 순간적으로 흐르게 된다. 이에 따라 매우 큰 전원 잡음이 생겨서 전원 전압이 안정적이지 못하고 흔들리게 되며 이는 많은 경우 시스템의 오동작을 초래하게 된다. 본 논문에서는 이러한 문제점을 해결하기 위하여 새로운 전원 복귀 기법을 제안한다. 제안하는 기법은 메가블록의 전원이 차단되었다가 다시 복귀할 때 한꺼번에 전원을 켜는 것이 아니라 파이프라인 방식으로 몇 단계로 나누어 전원을 켬으로서 전원선에 흐르는 최대 전류 및 이에 따른 전원 잡음을 크게 억제한다. 제안하는 파이프라인 전원 복귀 기법을 검증하기 위해서 컴팩트 플래시 메모리 제어기 칩에 본 기법을 적용하여 곱셈기 블록의 전원을 차단하고 복귀할 때의 전원 잡음을 모의실험하고 분석하였다. 모의실험 결과, 제안하는 기법은 기존의 전원 차단 기법에 비해 전원 잡음을 매우크게 줄일 수 있음을 확인하였다.

The near wake of three circular cylinders in an equilateral triangular arrangement at a low Reynolds number Re=100

Honglei Bai,Yufeng Lin,Md. Mahbub Alam 한국풍공학회 2020 Wind and Structures, An International Journal (WAS Vol.30 No.5

Two-dimensional numerical simulations are conducted at a low Reynolds number Re = 100 to investigate the near wake of three identical circular cylinders that are arranged in an equilateral triangular configuration. The incident angle of the three-cylinder configuration with respect to incoming flow is varied from  = 0o to 60o, while the spacing between adjacent cylinders (L) covers a wide range of L/D = 1.25-7.0, where D is diameter of the cylinder. Typical flow structures in the near wake of the three-cylinder configuration are identified, including a single Karman vortex street, bistable flip-flopping near wake, anti-phase and/or in-phase vortex shedding, shear layer reattachment, and vortex impingement, depending on the configuration (L/D, ). The behavior of Strouhal number (St) is discussed in detail, echoing the distinct structures of near wake. Furthermore, fluid forces on the individual cylinders are examined, which, though highly depending on (L/D, ), exhibit a close correlation to the near wake behavior.

DGS Rectenna를 이용한 Wake-Up 수신기 시스템 설계

최태민(Taemin Choi),이석재(Seok-Jae Lee),이희종(Heejong Lee),임종식(Jongsik Lim),안달(Dal Ahn),한상민(Sang-Min Han) 한국전자파학회 2012 한국전자파학회논문지 Vol.23 No.3

본 논문에서는 저전력 센서를 위한 평면형 구조의 Rectenna 시스템을 설계하여 수신 센서의 wake-up 회로에 응용함으로써, 유효 주파수 신호의 입사시에만 동작되는 수신기 시스템을 제안하였다. 2.4 GHz 대역의 평면형 집적이 가능한 Rectenna 시스템 설계를 위해 기존 여파기를 DGS 형태로 설계하여 다이오드에 의한 고조파 성분인 4.8 GHz, 7.2 GHz 신호를 제거와 DC-path를 위한 2.4 GHz 주 신호원의 제거를 수행하였다. 설계된 Rectenna의 변환 출력 전압 분석을 통해 수신시 시스템의 공급 전원의 스위칭 회로 구동에 활용함으로써 저전력 센서 수신 시스템의 동작 여부를 평가하였다. 제안된 시스템은 수신 센서의 신호 수신 여부에 따른 저 잡음 증폭기 동작 점검에 의해 wake-up 성능이 평가되었으며, 실험 결과 우수한 동작 성능을 나타내었다. In this paper, a new design of a planar rectenna system and its application to a wake-up receiver operating for incoming signal with a specified frequency are proposed for low-power sensor system applications. The planar and integrable rectenna system is designed with DGSs(Defected Ground Structures) at 2.4 GHz. The DGSs reject harmonic components of 4.8 and 7.2 GHz and eliminate 2.4 GHz fundamental frequency for DC-path filtering. The rectenna system has been evaluated for the conversion output voltages, and applied to the switching of a power supply at the low-power sensor receivers. The proposed system has been evaluated for the wake-up performance by testing a lownoise amplifier operation. From the experimental results, the proposed receiver system presents excellent operation performances.

수직 하강 비행 시 로터-로터 상호작용이 UAV의 후류 구조에 미치는 영향

채석봉(Seokbong Chae),이승철(Seungcheol Lee),김주하(Jooha Kim) 대한기계학회 2021 대한기계학회 춘추학술대회 Vol.2021 No.11

In the present study, we experimentally investigate the effect of rotor-rotor interaction on the rotor wake structure in axial descent. A twin rotor configuration system with various rotor separation distances were installed in the closed-circuited wind tunnel to observe wake structures of rotors in axial descent. Velocity fields around twin rotors were measured by using DPIV(digital particle image velocimetry). At a range of descent rate of V<SUB>d</SUB>/V<SUB>h</SUB> = 1.08 – 1.52 and a high rotor separation distance (s/R = 2.5), the flow around rotor enters VRS(vortex ring state), which form a large recirculation zone near the rotor tip. However, a decrease in s/R induces a stronger effect of rotor-rotor interaction on the rotor wake, resulting in the change in the location of a vortex ring core. At a very low s/R of 0.25, rotor-rotor interactions disrupt the vortex ring structure and make synthetic jets with the adjacent rotor wake. At various rotor separation distances, the frequency of vortex ring state was analyzed through the correlation analysis. At V<SUB>d</SUB>/V<SUB>h</SUB> = 1.08, the frequency of vortex ring decreases gradually as the rotor separation distance decreases and reach less than half at s/R = 1.5.

Wake-induced vibration of the hanger of a suspension bridge: Field measurements and theoretical modeling

Shouying Li,Yangchen Deng,Xu Lei,Teng Wu,Zheng Qing Chen 국제구조공학회 2019 Structural Engineering and Mechanics, An Int'l Jou Vol.72 No.2

The underlying mechanism of the wind-induced vibration of the hangers of the suspension bridges is still not fully understood at present and hence is comprehensively examined in this study. More specifically, a series of field measurements on the No. 2 hanger of the Xihoumen Bridge was first carefully conducted. Large amplitude vibrations of the hanger were found and the oscillation amplitude of the leeward cable was obviously larger than that of the windward cables. Furthermore, the trajectory of the leeward cable was close to an ellipse, which agreed well with the major characteristics of wake-induced vibration. Then, a theoretical model for the wake-induced vibration based on a 3-D continuous cable was established. To obtain the responses of the leeward cable, the finite difference method (FDM) was adopted to numerically solve the established motion equation. Finally, numerical simulations by using the structural parameters of the No. 2 hanger of the Xihoumen Bridge were carried out within the spatial range of 4≤X≤10 and 0≤Y≤4 with a uniform interval of ΔX=ΔY=0.25. The results obtained from numerical simulations agreed well with the main features obtained from the field observations on the Xihoumen Bridge. This observation indicates that the wake-induced vibration might be one of the reasons for the hanger oscillation of the suspension bridge. In addition, the effects of damping ratio and windward cable movement on the wake-induced vibration of the leeward cable were numerically investigated.

부유식 풍력단지 성능 및 하중에 대한 후류영향 분석

손재훈(J. H. Son),나요한(Y. H. Ra),남윤수(Y. S. Nam),백인수(I. S. Paek) 한국해양환경·에너지학회 2013 한국해양환경·에너지학회 학술대회논문집 Vol.2013 No.11

본 연구는 풍력발전기로 불어오는 바람이 풍력발전기를 지남에 따라 생성되는 후류효과에 대해 분석하였다. 4기의 2MW 상용 풍력발전기로 구성된 작은 부유식 풍력단지로 연구를 진행하였다. 모든 풍력발전기는 부유체의 모서리에 설치된다고 가정한다. 사각형 바지선 형태의 부유체의 치수는 200m ? 200m 로 고려되었다. 부유식 풍력단지의 주 풍향에 따른 출력의 영향은 제주도의 서쪽해안에 있는 두 개의 서로 다른 기후 요소에 대해 조사하였다. 고려된 풍황에 대해 최대 전력 생산을 위한 최적 풍력단지의 방향이 결정되었다. In this study, the effect of wake created by upstream wind turbines on the downstream wind turbines is analyzed. A small floating wind farm consisting of four 2 MW commercial wind turbines were studied. All the wind turbines were assumed to be installed at the corners of the floating structure. The dimension of the square barge-type floating structure was considered to be 200 m ? 200 m. The effect of orientation of the floating wind farm on the power output was investigated for two different climatologies representing the climatology in the west coast of Jeju island, Korea. The optimal orientation of the wind farm for maximum power output was obtained for the climatologies considered.

Comparison of flow structures behind rigid and flexible finite cylinders

Jung, Sung Yong,Kim, Jeong Jae,Park, Han Wook,Lee, Sang Joon Elsevier 2018 International journal of mechanical sciences Vol.142 No.-

<P><B>Abstract</B></P> <P>Flows over bluff bodies are closely related to aero- or hydro-dynamic forces on structures. In bio-mimetic applications, flexible structures are usually employed, and quantitative flow information is essential for optimizing structural design. In this study, the flow behind a flexible cylinder is compared with the flow behind a rigid cylinder using flow visualization, time-resolved particle image velocimetry (PIV), and proper orthogonal decomposition (POD) modal analysis. The spectral analysis demonstrates that the oscillatory motion of a flexible cylinder strongly influences the flow behind the cylinder. The flows behind the rigid and flexible cylinders are clearly different near the free end of the cylinder. The downwash flow from the free end of the flexible cylinder is relatively weak compared to the rigid cylinder. Behind the tip of the flexible cylinder, large-scale vortices are shed and they are propagated downstream. However, such flow phenomena are not observed behind the rigid cylinder. These results could be used as basic data in the design process of bio-inspired structural applications.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Wake flow characteristics behind the finite rigid and flexible cylinders are compared. </LI> <LI> The spanwise vortex propagates downstream for the flexible cylinder. </LI> <LI> The vortices from the rigid cylinder only influence the nearby adjacent flow region. </LI> <LI> Oscillatory cylinder motion strongly influences flows behind the flexible cylinder. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

Development of a novel vibro-wind galloping energy harvester with high power density incorporated with a nested bluff-body structure

Sun, Wan,Guo, Feng,Seok, Jongwon Elsevier 2019 Energy Conversion and Management Vol. No.

<P><B>Abstract</B></P> <P>A two degrees of freedom (DOF) vibro-wind galloping energy harvester with a nested structure is proposed to improve wind energy harvesting efficiency. Unlike traditional wind energy harvesters configured in a single bluff body-beam structure, the proposed system comprises outer and inner square cross-sectional bluff bodies in tandem, connected with elastic cantilevered beams to allow separate oscillations. The generated power performance of a conventional single-DOF wind galloping energy harvester and the proposed system is compared through wind tunnel tests. Aerodynamic interactions between the two oscillating bluff bodies are analyzed by varying the gap distances and the coupling mechanism is determined by analyzing their power spectral density (PSD). Experimental results show that the outer bluff body has a dominant influence on the inner bluff body at low wind speed, which is a secondary cause of inner body vibration after the plain galloping directly induced by the incoming wind. The proposed system exhibits excellent performance with a significant power density increase of 27.8%. Moreover, the interference effects between the two bluff bodies can be reinforced by reducing their gap distance. We conclude that the inner bluff body plays a key role in enhancing output power, specifically at relatively high wind speed.</P> <P><B>Highlights</B></P> <P> <UL> <LI> A two-DOF vibro-wind galloping energy harvester with a nested structure was developed. </LI> <LI> High power density is achieved by incorporating the bluff bodies in a nested structure. </LI> <LI> The inner bluff body plays a significant role in improving the harvester’s performance. </LI> <LI> An aerodynamic interaction between the outer and inner bluff bodies was identified. </LI> </UL> </P>

초고층 건축물의 공력학적 설계에 의한 풍진동 저감 효과

홍성일(Hong Sung-Il),김무환(Kim Mu-Hwan),이옥진(Lee Ok-Jin),조강표(Cho Kang-Pyo) 대한건축학회 2009 대한건축학회 학술발표대회 논문집 - 계획계/구조계 Vol.29 No.1(구조계)

Modern tall buildings having multiplex functions are increasing in height for the powerful use of land by the development of construction skills, the progress of structural analysis techniques, the concentration of population, and industrial and economic development of modern society. Wind-induced vibration of tall buildings are mainly due to buffeting, vortex and wake. Also, wind-induced responses of buildings are different according to the geometry of buildings, structural characteristics such as stiffness, mass and damping, and turbulent characteristics of wind. In this paper, wind-tunnel experiments are performed using variables such as the plane and elevation shapes and aerodynamic apparatus of buildings. The square, equilateral triangular and circular sections were selected as plane shapes of buildings, and the setback and tapered types were tried for the circular section. Also, wind-induced responses were investigated for a total of nine models having equivalent effective slenderness ratios by attaching ribs and spirals to the circular model, the setback type model, and the tapered type model. From the results of the experiments, wind-induced response was the least for the circular section, and the wind-induced response for the setback type was much smaller than that of the tapered type. The ribs and spirals as aerodynamic apparatus had the similar damping effects in the cross-wind direction.