• Title/Summary/Keyword: circular frequency

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Large eddy simulations of the flow around a circular cylinder: effects of grid resolution and subgrid scale modeling

  • Salvatici, E.;Salvetti, M.V.
    • Wind and Structures
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    • v.6 no.6
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    • pp.419-436
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    • 2003
  • Large-eddy simulations of the flow around a circular cylinder at a Reynolds number, based on cylinder diameter and free-stream velocity, $Re_D=2{\times}10^4$ are presented. Three different dynamic subgrid-scale models are used, viz. the dynamic eddy-viscosity model and two different mixed two-parameter models. The sensitivity to grid refinement in the spanwise and radial directions is systematically investigated. For the highest resolution considered, the effects of subgrid-scale modeling are also discussed in detail. In particular, it is shown that SGS modeling has a significant influence on the low-frequency modulations of the aerodynamics loads, which are related to significant changes in the near wake structure.

A Numerically Efficient Full Wave Analysis of Circular Resonators Microbandes Stacked Involving Multimetallisations

  • Chebbara, F.;Fortaki, T.
    • Journal of Electrical Engineering and Technology
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    • v.10 no.1
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    • pp.314-319
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    • 2015
  • The conventional geometry of a plate microstrip resonator is made up of a single metallic patch, which is printed on a monolayer dielectric substrate. Its arrangement is simple and easy to make, but it is limited in its functional abilities. Many searches have been realized to improve the bandwidth and the gain of the microstrip resonators. Among the various configurations proposed in the open literature, the stacked geometry seems to be very promising. By appropriate design, it is able to provide the operation in dual frequency mode, wide bandwidth enough and high gain. The theoretical investigations of structures composed of two stacked anti-reflection coatings, enhanced metallic coatings are available in the literature, however, for the stacked configurations involving three metallic coatings or more, not to exact or approximate analysis was conducted due to the complexity of the structure.

Printed Type Half-wavelength Small Loop Antenna (프린트형 반파장 소형 루프 안테나)

  • Ryu, Hong-Kyun;Woo, Jong-Myung
    • Proceedings of the Korea Electromagnetic Engineering Society Conference
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    • 2005.11a
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    • pp.245-248
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    • 2005
  • In this paper, small sized half-wavelength circular loop antenna which attached beneath a visor of helmet is designed and fabricated at the resonant frequency of 449MHz. To reduce the size of the antenna, double series-shorted stubs are inserted inside the loop for practical use of a vacant space of loop inside. Also, It is designed for the printed type to install the helmet easily. The size of antenna on helmet is reduced to 87.75%(diameter : 70mm, height :.36mm) compare with general type antenna(diameter : 200mm height 101mm). The return loss, -10dB bandwidth and gain are -13.2dB, 17.6MHz(3.9%), and -1.78dBd. And, radiation pattern is omni-directional pattern at H-plane. Therefore, it can be seen that the half-wavelength circular loop antenna using double series-shorted stubs is proper structure for the miniaturization and the installed antenna of the helmet.

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Dual-Polarized Annular Ring Patch Antenna for 2.4 GHz Doppler Radar

  • Kim, Seong-Ho;Yook, Jong-Gwan;Cho, Sung-Ho;Jang, Byung-Jun
    • Journal of electromagnetic engineering and science
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    • v.10 no.3
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    • pp.183-185
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    • 2010
  • A 2.4 GHz dual-polarized antenna for a Doppler radar is studied. The proposed dual-polarized antenna using a stacked annular ring patch with two co-centric gap-coupled feed lines and a $90^{\circ}$ hybrid exhibits fairly good performance of 22 dB isolation at a center frequency of 2.4 GHz. Using a $90^{\circ}$ hybrid, a right-handed circular polarization for the transmitter and a left-handed circular polarization for the receiver are implemented. The gain of the designed antenna is about 0 dBi over operating frequencies. The antenna size including a ground plane is only $40{\times}40\;mm^2$.

Real-Tim Sound Field Effect Implementation Using Block Filtering and QFT (Block Filtering과 QFT를 이용한 실시간 음장 효과구현)

  • Sohn Sung-Yong;Seo Jeongil;Hahn Minsoo
    • MALSORI
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    • no.51
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    • pp.85-98
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    • 2004
  • It is almost impossible to generate the sound field effect in real time with the time-domain linear convolution because of its large multiplication operation requirement. To solve this, three methods are introduced to reduce the number of multiplication operations in this paper. Firstly, the time-domain linear convolution is replaced with the frequency-domain circular convolution. In other words, the linear convolution result can be derived from that of the circular convolution. This technique reduces the number of multiplication operations remarkably, Secondly, a subframe concept is introduced, i.e., one original frame is divided into several subframes. Then the FFT is executed for each subframe and, as a result, the number of multiplication operations can be reduced. Finally, the QFT is used in stead of the FFT. By combining all the above three methods into our final the SFE generation algorithm, the number of computations are reduced sufficiently and the real-time SFE generation becomes possible with a general PC.

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The Failure Mode Analysis of Machine Tools using Performance Tests (공작기계의 성능시험을 통한 고장모드해석)

  • 이수훈;김종수;박연우;이승우;송준엽;박화영
    • Proceedings of the Korean Society of Precision Engineering Conference
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    • 2002.05a
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    • pp.90-93
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    • 2002
  • In view of reliability assessment, the failure mode analysis by performance tests for machine totals is researched in this study. First, the error analysis with circular movement test data is studied. The various errors and their origins are analyzed by error equations and related parts are investigated. Second, This paper deals with analysis of vibration testing fur machine tools spindle. The various frequency components are classified by FFT and order analysis. The simple measuring devices and error evaluation programs for tests are also developed.

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Circular Diamond Saw with Low Noise Shank (원형 다이아몬드톱의 저소음 생크)

  • 윤승원;지원호;우종혁
    • Proceedings of the Korean Society of Precision Engineering Conference
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    • 2004.10a
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    • pp.729-732
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    • 2004
  • An experimental and numerical approach has been carried out for to see the noise characteristics of a circular diamond saw. The noise level measurement result for the several kinds of shanks shows us that the noise level reduction in high frequency range could be an effective way to control the noise problem. Sandwich type shank whose center part is laminated with a low Young‘s modulus material greatly reduces the noise level. The noise level for the shank where several curve shaped slits are provided in the circumferential direction is reduced considerably too. The response spectrum analysis by the FEM shows us to be an alternative for predicting the noise characteristics of the shank.

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Wave scattering among a large number of floating cylinders

  • Kashiwagi, Masashi
    • Structural Engineering and Mechanics
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    • v.21 no.1
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    • pp.53-66
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    • 2005
  • When a large number of identical cylinders are placed in an array with equal separation distance, near-resonant phenomena may occur between cylinders at critical frequencies, and cause large wave forces on each element of the array. In this paper, 64 truncated circular cylinders arranged in 4 rows and 16 columns are considered to check occurrence of near-resonant phenomena and performance of theoretical predictions based on the potential flow. Experiments are conducted in head waves to measure the wave elevation along the longitudinal centerline of the model, and measured results are compared with numerical ones. Attention is focused on the spatial variation of the wave amplitude around the first near-trapped-mode frequency.

Circular cylinder drag reduction using piezoelectric actuators

  • Orazi, Matteo;Lasagna, Davide;Iuso, Gaetano
    • Advances in aircraft and spacecraft science
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    • v.1 no.1
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    • pp.27-41
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    • 2014
  • An active flow control technique based on "smart-tabs" is proposed to delay flow separation on a circular cylinder. The actuators are retractable and orientable multilayer piezoelectric tabs which protrude perpendicularly from the model surface. They are mounted along the spanwise direction with constant spacing. The effectiveness of the control was tested in pre-critical and in post-critical regime by evaluating the effects of several control parameters of the tabs like frequency, amplitude, height, angular position and plate incidence with respect to the local flow. Measurements of the mean static pressure distribution around the cylinder were used to estimate the pressure drag coefficient. The maximum drag reduction achieved in the pre-critical regime was of the order of 30%, whereas in the post-critical regime was about 10%, 3% of which due to active forcing. Furthermore, pressure fluctuation measurements were performed and spectral analysis indicated an almost complete suppression of the vortex shedding in active forcing conditions.

A Study of the Estimation Method for the Dielectric Properties of Dielectrics in Millimeter Wave Range - Part II (유전체의 밀리미터파대 유전특성 평가방법에 관한 연구 II)

  • 이홍열;전동석;김동영;이상석
    • Journal of the Korean Institute of Electrical and Electronic Material Engineers
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    • v.16 no.12
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    • pp.1087-1090
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    • 2003
  • The circular cavity resonator which can measure the dielectric properties of dielectrics in the V -band(50㎓∼75㎓) frequency range was designed and fabricated. Exciting and detecting of the resonator is performed by WR15 rectangular waveguides using Bethe's small hole coupling. GaAs and PTFE plate samples were used for the verification of the performance of the fabricated circular cavity resonator. In the measurement of GaAs and PTFE using that resonator, the permittivity were measured as I2.87 and 2.14, respectively.