Divergent vibration of a test cylinder with a control cylinder at the rear

Sang-Il Kim,Bo-Kyu Kwon 대한기계학회 2022 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.36 No.1

The objective of this study was to experimentally investigate flow-induced vibration occurring in a test cylinder capable of free vibration when there was a control cylinder with a diameter half of that of the test cylinder behind the test cylinder. This paper intensively investigated divergent vibration among flow-induced vibration characteristics of the test cylinder. The mechanism for generating divergent vibration was also determined. To clarify the mechanism, flow-induced vibration characteristics of the test cylinder and wakes were investigated when the control cylinder was located closely to the rear of the test cylinder. Among tests for investigating wakes, a visualization test was also conducted using hydrogen bubble as a dye in a water channel. As a result, it was found that when the control cylinder was in close proximity to the rear of the test cylinder, the divergent vibration that appeared in the test cylinder was divided into three patterns based on vibration amplitude characteristics. Results of wake investigation revealed that the presence of the control cylinder affected the emission frequency, shape, and intensity of the vortex discharged from the test cylinder, which in turn affected vibration characteristics of the test cylinder. As a result of investigating flow-induced vibration characteristics of the test cylinder using different methods by changing the flow velocity, vibration inertia was found to one cause for the divergent vibration of the cylinder.

Suppression of flow-induced vibration of a circular cylinder by means of a flexible sheet

김상일,이승철 대한기계학회 2012 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.26 No.6

In this study, the suppression of flow-induced vibration of an elastically supported circular cylinder by attachment of a flexible sheet was investigated experimentally. In particular, the dependence of flow-induced vibration characteristics of the circular cylinder upon the flow velocity was investigated in detail by axially attaching the flexible poly-ethylene sheet to the cylinder surface. The characteristics of the flow-induced vibration of the cylinder were investigated by changing the attachment angle θ and the length l of the flexible sheet (rectangular type) as experimental parameters in various combinations. The angle θ was set at five different angles, 90°, 45°, 0°, -45° and -90°. The angle’s base point was the back side stagnation point of the cylinder. The length l of the flexible sheet varied from 0.5 to 3.0times of the cylinder’s diameter at the interval of 0.5 times. The width T of the flexible sheet along the span of the cylinder also varied in 7 cases from 1.0L to 0.4L (L is the length of the cylinder) in order to discover the minimum width of the sheet necessary to effectively suppress the flow-induced vibration of the cylinder. Furthermore, the flexible sheet of the minimum width was split into 2 to 5 pieces and attached to the cylinder, and changes in the flow-induced vibration characteristics were investigated. Also, vibration characteristics were investigated for a flexible sheet in the shape of an isosceles triangle. As a result, the optimal length l and minimum width T of the flexible rectangular sheet were found to be 2~2.5D and 0.7L, respectively, to suppress the flow-induced vibration of the cylinder. Most importantly,it was found that the sheet located at the back side stagnation point can suppress the flow-induced vibration generated by any directional flow to strike the front surface of the cylinder.

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>

정밀 연삭된 지대주와 합성수지 지대주를 이용한 보철물의 적합도에 대한 비교 연구

정선희,마장선,정재헌,Jung, Seon-Hee,Ma, Jang-Seon,Chung, Chae-Heon 대한치과보철학회 1999 대한치과보철학회지 Vol.37 No.6

Recently, various implant cylinders were supplied. especially received gold cylinders, cast cylinders produced from premade gold and plastic cylinders and plastic cylinders. This study measured and compared precise fit produced when using conventional gold and plastic cylinders. The comparative fit of lapped and non-lapped castings made from plastic pattern was examined. The implant/abutment interface fit was evaluated by scanning electron microscopy(SEM) for each of four cylinders. The following results were obtained: 1. The case of plastic cylinder showed $9.67{\pm}1.50{\mu}m$ gap when Steri-Oss fixture was connected. 2. The case of lapped casting made from plastic cylinder showed $3.01{\pm}2.81{\mu}m$ gap when Steri-Oss fixture was connected. 3 The case of gold/plastic cylinder showed $9.80{\pm}1.68{\mu}m$ gap when Steri-Oss fixture was connected. 4. The case of gold cylinder showed ${5.47{\pm}2.43\mu}m$ gap when Steri-Oss fixture was connected. 5. In case of each cylinder which was connected with Steri-Oss fixture, the size of gap was showed less in the order of the case of gold/plastic cylinder, the case of plastic cylinder, the case of gold cylinder and the case of lapped casting made from plastic cylinder As results of this study, the use of premade gold cylinder offers an advantage over plastic pat-terns in precise fit. When plastic patterns are used, polishing of implant cylinder components should provide precise fit.

Forced convection heat transfer from the helically twisted elliptic cylinder inspired by a daffodil stem

Yoon, Hyun Sik,Kim, Hyo Ju,Wei, Duck Jae Pergamon Press 2018 International journal of heat and mass transfer Vol. No.

<P><B>Abstract</B></P> <P>The present study is an original research for the forced convection heat transfer around a helically twisted elliptic (<I>HTE</I>) cylinder inspired by a daffodil stem. Also, this study is an initial investigation to find the effect of the Reynolds number (<I>Re</I>) in the laminar flow regime. We carried out numerical simulations to investigate the flow and heat transfer around the <I>HTE</I> cylinder in 60 ⩽ Re ⩽ 150 and Prandtl number (<I>Pr</I>) of 0.7. The circular cylinder is considered for the purpose of the comparison. The drag and lift fluctuation for the HTE cylinder are much smaller than those of the smooth cylinder. The decreasing rate of Strouhal number ( St ) for the <I>HTE</I> cylinder to that for the smooth cylinder is about 4–7% in the present Reynolds number range. The smaller St of the <I>HTE</I> cylinder than the smooth cylinder associate a longer vortex formation length of the <I>HTE</I> cylinder. The three-dimensional (3D) geometry of the HTE cylinder formed the spanwise variation of the Nusselt number (<I>Nu</I>), resulting in the sinusoidal profiles. This spanwise variation of the Nusselt number is identified by the flow structures and the isotherms distribution. The time- and total surface-averaged Nusselt number of the <I>HTE</I> cylinder decreases from about 1.2% to 2.8% in the present <I>HTE</I> configuration and <I>Re</I> range, compared to the smooth cylinder.</P> <P><B>Highlights</B></P> <P> <UL> <LI> A helically twisted elliptic (<I>HTE</I>) cylinder based on biomimetic is considered. </LI> <LI> The forced convection around the <I>HTE</I> cylinder is originally investigated. </LI> <LI> The effect of <I>Re</I> on the flow around the <I>HTE</I> cylinder is initially researched. </LI> <LI> The local Nu identifies the effect of the <I>HTE</I> geometry on the heat transfer. </LI> </UL> </P>

Two-dimensional Stokes flow around a circular cylinder in a microchannel

정재택,윤석현 대한기계학회 2014 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.28 No.2

Two-dimensional Stokes flow around a circular cylinder in a microchannel is investigated based on Stokes approximation. The cylinderwith arbitrary radius translates along the centerline of the channel, and plane Poiseuille flow exists upstream and downstream fromthe cylinder. The translating velocity of the cylinder and the magnitude of the Poiseuille flow are arbitrary. The Stokes flow is examinedanalytically using Papkovich-Fadle eigenfunction expansion and least square method. The stream function and the pressure distributionof the flow field are obtained and shown for some typical cases. The force exerted on the cylinder and the pressure drop due to the cylinderare calculated as functions of the radius of the cylinder. For a small radius of the cylinder, the results of the force are coincident withprevious asymptotic expressions for the force. For a given average velocity of the Poiseuille flow in the channel, translational drift velocityof the cylinder is determined as a function of blockage factor. The drift velocity is slightly lower than the mean velocity of thePoiseuille flow component projected by the cylinder. The induced pressure drop due to the drifting cylinder in the Poiseuille flow is quitesmall. When the cylinder translates in the stagnant channel, a series of Moffatt eddies appears far from the cylinder in the channel, asexpected. The size of the primary eddy increases with the radius of the cylinder.

큰에디모의 모형을 이용한 높은 레이놀즈 수에서의 사각 기둥 후면의 와열 분석: 풍향과 풍속, 기둥 너비의 영향

한범순,곽경환,백종진 한국기상학회 2017 대기 Vol.27 No.4

This study investigates turbulent flow around a square cylinder mounted on a flat surface at high Reynolds number using a large-eddy simulation (LES) model, particularly focusing on vortex streets behind the square cylinder. Total 9 simulation cases with different inflow wind directions, inflow wind speeds, and cylinder widths in the x- and y-directions are considered to examine the effects of inflow wind direction, speed, and cylinder widths on turbulent flow and vortex streets. In the control case, the inflow wind parallel to the x-direction has a maximum speed of 5 m s−1 and the width and height of the cylinder are 50 m and 200 m, respectively. In all cases, downdrafts in front of the cylinder and updrafts, wakes, and vortex streets behind the cylinder appear. Low-speed flow below the cylinder height and high-speed flow above it are mixed behind the cylinder, resulting in strong negative vertical turbulent momentum flux at the boundary. Accordingly, the magnitude of the vertical turbulent momentum flux is the largest near the cylinder top. In the case of an inflow wind direction of 45o, the height of the boundary is lower than in other cases. As the inflow wind speed increases, the magnitude of the peak in the vertical profile of mean turbulent momentum flux increases due to the increase in speed difference between the low-speed and high-speed flows. As the cylinder width in the y-direction increases, the height of the boundary increases due to the enhanced updrafts near the top of the cylinder. In addition, the magnitude of the peak of the mean turbulent momentum flux increases because the low-speed flow region expands. Spectral analysis shows that the non-dimensional vortex generation frequency in the control case is 0.2 and that the cylinder width in the y-direction and the inflow wind direction affect the non-dimensional vortex generation frequency. The non-dimensional vortex generation frequency increases as the projected width of the cylinder normal to the inflow direction increases.

Natural convection in a square enclosure with different positions and inclination angles of an elliptical cylinder Part I: A vertical array of one elliptical cylinder and one circular cylinder

Park, Seong Hyun,Seo, Yeong Min,Ha, Man Yeong,Park, Yong Gap Elsevier 2018 INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER - Vol.126 No.2

<P><B>Abstract</B></P> <P>This paper numerically investigates the two-dimensional natural convection in a square enclosure with a vertical array of a hot elliptical cylinder and a hot circular cylinder with Rayleigh numbers in the range of <SUP> 10 4 </SUP> ⩽ Ra ⩽ <SUP> 10 6 </SUP> . An immersed boundary method was used to capture the wall boundary of the cylinders. The effects of the position and inclination angle ϕ of the elliptical cylinder were investigated. When the Rayleigh number increases to <I>Ra</I> = 10<SUP>6</SUP>, the numerical solutions reach an unsteady state for all cases of the lower elliptical cylinder and the cases of the upper elliptical cylinder except at ϕ = 90 ° . The transition of the flow regime from unsteady state to steady state depends on the flow direction and the space between the upper cylinder and top wall of the enclosure due to the changes in the inclination angle of the elliptical cylinder. At <I>Ra</I> = 10<SUP>6</SUP> in the case of upper elliptical cylinder at ϕ = 0 ° , the time- and surface-averaged Nusselt numbers for the walls of the enclosure increase by about 1.99% compared to the case of two circular cylinders. The thermal performance and flow stability were influenced by the position and inclination angle of the elliptical cylinder.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Two-dimensional numerical simulations were conducted for the natural convection. </LI> <LI> The Immersed Boundary Method was used, based on the Finite Volume Method. </LI> <LI> The transition of the flow regime was investigated at <I>Ra</I> = 10<SUP>6</SUP> according to <I>ϕ</I>. </LI> <LI> The effects of <I>ϕ</I> on the heat transfer characteristics are highlighted. </LI> </UL> </P>

가상경계 격자볼쯔만법을 이용한 벽면에 근접하여 이동하는 실린더주위의 유동해석

김형민(Hyung Min Kim) 대한기계학회 2008 大韓機械學會論文集B Vol.32 No.12

Immersed boundary method (IBM) is the most effective method to overcome the disadvantage of LBM (Lattice Boltzmann Method) related to the limitation of the grid shape. IBM also make LBM possible to simulate flow over complex shape of obstacle without any treatment on the curved boundary. In the research, IBLBM was used to perform LBM simulation of a flow over a moving circular cylinder to determine the flow feature and aerodynamics characteristic of the cylinder. To ascertain the applicability of IBLBM on the moving obstacle near the wall, it was first simulated for the case of the flow over a fixed circular cylinder in a channel and the results were compared against the solution of moving cylinder in the channel using IBLBM. The simulations were performed in a moderate range of Reynolds number at each moving cylinder to identify the flow feature and aerodynamic characteristics of circular cylinder in a channel. The drag and lift coefficients of the cylinder were calculated from the simulation results. We have numerically confirmed that the critical Reynolds number for vortex shedding is Re=50 and the result is the same as the case of fixed cylinder. As the cylinder approaching to a wall (γ < 2.5), the 2nd vortex is developed by interacting with the wall boundary-layer vorticity. When the cylinder is very closed to the wall, γ < 0.6, the cylinder acts like blockage to block the flow between the cylinder and wall so that the vortex developed on the upper cylinder elongated and time averaged lifting and drag coefficients abruptly increase.

우리나라 황해의 해산 돌말류 Ditylum brightwellii (West) Grunow의 형태학적 특징에 의한 두 유형

이진환,윤석제,Lee, Jin-Hwan,Youn, Seok-Jea 한국조류학회(藻類) 2007 ALGAE Vol.22 No.4

This study deals with the taxonomy of the marine diatom Ditylum brightwellii (West) Grunow: shape, size and ratio, marginal ridge, labiate process, areolation, and seasonal distribution are all considered. Ditylum brightwellii were divided into two type; prism- and cylinder-shaped. Comparisons between specimens from the Yellow Sea were made on, (1) shape- prism types are prismatic-shaped and/or regular square or right-angled tetragon, cylinder types are cylinder-shaped and/or long right-angled tetragon in the girdle view, respectively; (2) size and ratiodiameters of prism types are 82.1 ± 2 μm, while those of cylinder types are 21.2 ± 5 μm and ratio of pervalvar axis/diameter- prism types cells are 1.0-1.5, but cylinder types are 3.9-5.5; (3) marginal ridges- prism types have dotted, while those of the cylinder-type cells have piece, slotted or fimbriated on the marginal ridge; (4) labiate process- both prism- and cylinder-types are central part in valve face, but length of labiate processes of prism types are longer than those of cylinder types; (5) cell margin- the girdle margin show many shallow furrows in prism type and smooth in cylinder type; (6) areolation- both prism- and cylinder-types are a radial pattern in the valve center, but areolae of prism types are larger than those of cylinder types; (7) seasonal distribution- cylinder types are dominated in summer and prism types in the other seasons.