• Title/Summary/Keyword: Sphere

Search Result 2,234, Processing Time 0.045 seconds

Active control of flow over a sphere using electro-magnetic actuators (전자석 액츄에이터를 이용한 구 주위의 유동제어)

  • Park, Jin-Il;Choi, Hae-Cheon;Jeon, Woo-Pyung
    • Proceedings of the KSME Conference
    • /
    • 2000.04b
    • /
    • pp.497-501
    • /
    • 2000
  • Flow over a sphere is controlled experimentally at $Re=10^5$ using electro-magnetic actuators. The electro-magnetic actuator developed in this study is composed of the permanent magnet electro-magnet membrane and slot. Eight actuators are placed inside the sphere at equally spaced intervals on a latitudinal plane and the position of the control slot is 76 from the stagnation point. Each actuator generates a periodic blowing and suction through the slot at variable frequencies of $10{\sim}140Hz$ and variable amplitudes by controlling electric signals applied to the electro-magnet. Drag on the sphere measured using a load cell is significantly reduced with control at the forcing frequencies larger than the natural shedding frequency $({\approx}14Hz\;at\;Re=10^5)$, whereas drag is slightly increased at the forcing frequency of 10Hz. It is shown from pressure measurement that the static pressure in the rear surface of the sphere is significantly increased with control, indicating that the separation is delayed due to control. Flow visualizations also show that the detaching shear layer is more attracted to the sphere center with control, the separation bubble size is significantly reduced, and motion inside the bubble is very weak, as compared to the case of uncontrolled flow.

  • PDF

Characteristics of Flow-Induced Noise around a Sphere (구 주위의 유동으로 인해 발생하는 유동 소음의 특성)

  • Yun, Gi-Woong;Choi, Hae-Cheon
    • Proceedings of the KSME Conference
    • /
    • 2003.11a
    • /
    • pp.810-815
    • /
    • 2003
  • Flow-induced noise propagated from flow over a sphere is numerically investigated for laminar flow at Re = 300 and 425, and for turbulent flow at Re = 3700 and $10^4$, where the Reynolds number is based on the freestream velocity and the sphere diameter. The numerical method used for obtaining the flow over a sphere is based on an immersed boundary method in a cylindrical coordinate system. The Curle’s solutions of the Lighthill’s acoustic analogy with and without the far-field and compact-source approximation are used in order to investigate the noise field from flow over a sphere. Since the drag and lift forces change irregularly in time at Re = 425, 3700 and $10^{4}$, the noise propagates in a complicated manner. At Re = 300, 425 and $10^{4}$, the noise from dipole sources is much larger than that from quadrupole sources. On the other hand, at Re = 3700, the quadrupole source becomes dominant. The temporal variation of the flow-induced noise around a sphere is obtained at some observation points, which shows that the peak frequency corresponds to the Strouhal number associated with the wake instability.

  • PDF

Characteristics of the Counter Public Sphere during the Early Modern Era in Korea (조선 후기 대항 공론장의 특성에 관한 연구)

  • Won, Suk-Kyoung;Yoon, Young-Tae
    • Korean journal of communication and information
    • /
    • v.59
    • /
    • pp.92-115
    • /
    • 2012
  • The authors examine the characteristics of the counter public sphere during the late Chosun era based on three dimensions: historical, ideological, and analytical. The counter public sphere during the late Chosun era was a lively communicative sphere where the common people, who resisted the premodern ideas and foreign power, tried to overcome the oppressing social and political environment. Although the counter public sphere did not continue to grow during the Japanese colonization era, it provided a basis of the independence movement and the ideological ground for the democratic movement today.

  • PDF

Pyramid and Half-Sphere Type of Surface Texturing for Si-Solar Cell (실리콘 태양전지의 피라미드와 반구형 표면 조직화)

  • Pyo, Dae-Seong;Jo, Jun-Hwan;Hong, Pyo-Hwan;Lee, Jong-Hyun;Kim, Bonghwan;Cho, Chan-Seob
    • Journal of Sensor Science and Technology
    • /
    • v.22 no.6
    • /
    • pp.433-438
    • /
    • 2013
  • In this paper, we found surface shapes are affected by several parameters of RIE, such as RF power, pressure, temp, and process times. The reflectance of pyramid and half sphere structures show differences among shapes, size, height, and depth of those structures. We made about $1{\mu}m$ pyramid and half sphere shapes of silicon surface with RIE. For comparing the reflectance, pyramid and half sphere structures are fabricated with same height. Pyramid structure cell shows higher cell efficiency of 12.5% by 1.1% than one of half sphere structure of 11.4%. The light absorption is more increased through the pyramid structure than half sphere structure.

Control of the flow past a sphere in a turbulent boundary layer using O-ring

  • Okbaz, Abdulkerim;Ozgoren, Muammer;Canpolat, Cetin;Sahin, Besir;Akilli, Huseyin
    • Wind and Structures
    • /
    • v.35 no.1
    • /
    • pp.1-20
    • /
    • 2022
  • This research work presents an experimental study's outcomes to reveal the impact of an O-ring on the flow control over a sphere placed in a turbulent boundary layer. The investigation is performed quantitatively and qualitatively using particle image velocimetry (PIV) and dye visualization. The sphere model having a diamater of 42.5 mm is located in a turbulent boundary layer flow over a smooth plate for gap ratios of 0≤G/D≤1.5 at Reynolds number of 5 × 103. Flow characteristics, including patterns of instantaneous vorticity, streaklines, time-averaged streamlines, velocity vectors, velocity fluctuations, Reynolds stress correlations, and turbulence kinetic energy (), are compared and discussed for a naked sphere and spheres having O-rings. The boundary layer velocity gradient and proximity of the sphere to the flat plate profoundly influence the flow dynamics. At proximity ratios of G/D=0.1 and 0.25, a wall jet is formed between lower side of the sphere and flat plate, and velocity fluctuations increase in regions close to the wall. At G/D=0.25, the jet flow also induces local flow separations on the flat plate. At higher proximity ratios, the velocity gradient of the boundary layer causes asymmetries in the mean flow characteristics and turbulence values in the wake region. It is observed that the O-ring with various placement angles (𝜃) on the sphere has a considerable alteration in the flow structure and turbulence statistics on the wake. At lower placement angles, where the O-ring is closer to the forward stagnation point of the sphere, the flow control performance of the O-ring is limited; however, its impact on the flow separation becomes pronounced as it is moved away from the forward stagnation point. At G/D=1.50 for O-ring diameters of 4.7 (2 mm) and 7 (3 mm) percent of the sphere diameter, the -ring exhibits remarkable flow control at 𝜃=50° and 𝜃=55° before laminar flow separation occurrence on the sphere surface, respectively. This conclusion is yielded from narrowed wakes and reductions in turbulence statistics compared to the naked sphere model. The O-ring with a diameter of 3 mm and placement angle of 50° exhibits the most effective flow control. It decreases, in sequence, streamwise velocity fluctuations and length of wake recovery region by 45% and 40%, respectively, which can be evaluated as source of decrement in drag force.

AN EMBEDDED 2-SPHERE IN IRREDUCIBLE 4-MANIFOLDS

  • Park, Jong-Il
    • Bulletin of the Korean Mathematical Society
    • /
    • v.36 no.4
    • /
    • pp.683-691
    • /
    • 1999
  • It has long been a question which homology class is represented by an embedded 2-sphere in a smooth 4-manifold. In this article we study the adjunction inequality, one of main results of Seiberg-Witten theory in smooth 4-manifolds, for an embedded 2-sphere. As a result, we give a criterion which homology class cannot be represented by an embedded 2-sphere in some cases.

  • PDF

Study on the thermal and flow characteristics around a sphere submerged in the fluid (유체속에 잠긴 구주위에서의 열 및 유동특성에 관한 연구)

  • ;;Lim, Kyung Bin
    • Transactions of the Korean Society of Mechanical Engineers
    • /
    • v.5 no.3
    • /
    • pp.238-243
    • /
    • 1981
  • An empirical formula for the mass flow rate in natural convection flume above a spherical heated body which is submerged in a finite fluid contained in a tank was determined. The ratio of depth of submergence to diameter of the sphere for which the sphere sensed as infinite medium was found to be bigger than 7. A dimensionless heat transfer correlation for the natural convection from a sphere was determined for Gr.Prq etween 7x10$\^$7/ and 2x10$\^$8/ also.

Three-dimensional potential flow due to the montion of a sphere touching a plane wall (평면벽에 접한 구의 운동에 의한 3차원의 Potential 흐름)

  • ;Kim, Moon-Uhn
    • Transactions of the Korean Society of Mechanical Engineers
    • /
    • v.3 no.4
    • /
    • pp.181-184
    • /
    • 1979
  • Three-dimensional potential flow due to the translation of a sphere touching a rigid plane wall or a free wall is investigated by use of tangent sphere coordinates. Exact expressions for the velocity potential are derived in integral formes. Added mass and lift force on the sphere are also calculated.

Convergence of Fuzzy Spheres (퍼지구의 수렴성)

  • Kim Mi-Hye;Kim Tae-Soo
    • Proceedings of the Korea Contents Association Conference
    • /
    • 2003.11a
    • /
    • pp.338-342
    • /
    • 2003
  • The concept of a circularity function is used in defining fuzzy spheres The circularity function of a fuzzy sphere converses to one of crisp sphere as the fuzzy sphere shapes itself more and more like a crisp sphere.

  • PDF