• 설비공학논문집
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• 제9권4호
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• pp.561-571
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• 1997

In this paper, the fundamental equations are developed for the pulsatile laminar flow generated by changing the oscillatory flow with $0{\leq}f{\leq}48Hz$ into a steady one with $0{\leq}Re{\leq}2500$ in a rigid circular pipe. Analytical solutions for the wave propagation factor k, the axial distributions of cross-sectional mean velocity $u_m$ and pressure p are schematically derived and confirmed experimentally. The axial distributions of centerline velocity and pressure were measured by using Pitot-static tubes and strain gauge type pressure transducers, respectively. The cross-sectional mean velocity was calculated from the centerline velocity by applying the parabolic distribution of the laminar flow and it was confirmed by using the ultrasonic flowmeter. It was found that the axial distributions of cross-sectional mean velocity and pressure agree well with theoretical ones and depend only on the Reynolds number Re and angular velocity $\omega$.

• Advanced Composite Materials
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• 제17권3호
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• pp.191-214
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• 2008

Polypropylene/layered silicate nanocomposites containing maleic anhydride grafted polypropylene were prepared by melt compounding and their rheological behavior was investigated in shear flow. Transient and steady shear flows were simulated numerically by using the K-BKZ integral constitutive equation along with experimentally determined damping functions under dynamic oscillatory and step strain shear flows. Nonlinear shear responses were predicted with the K-BKZ constitutive equation using two different damping functions such as the Wagner and PSM models. It was observed that PP-g-MAH compatibilized PP/layered silicate nanocomposites have stronger and earlier shear thinning and higher steady shear viscosity than pure PP resin or uncompatibilized nanocomposites at low shear rate regions. Strong damping behavior of the PP/layered silicate nanocomposite was predicted under large step shear strain and considered as a result of the strain-induced orientation of the organoclay in the shear flow. Steady shear viscosity of the pure PP and uncompatibilized nanocomposite predicted by the K-BKZ model was in good agreement with the experimental results at all shear rate regions. However, the model was inadequate to predict the steady shear viscosity of PP-g-MAH compatibilized nanocomposites quantitatively because the K-BKZ model overestimates strain-softening damping behavior for PP/layered silicate nanocomposites.

• 한국전자파학회지:전자파기술
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• 제5권4호
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• pp.64-70
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• 1994

전력주파수 자기장 , 펄수 자기장 및 감쇠진동 자기장 내성평가에 대한 규격이 국제전기기술위원회(IEC)에 서 재정되어 전기.전자제품들의 자기장내성에 대한평가가 이루어질 전망이다. 이를 위해 본 연구에서는 IEC규격에서 정하고있는 자기장 내성평가용 유도코일인 정사각형 코일$1m\times1m$, 직사각형 코일 $1m\times2.6m$와 코일사이의 간격이 0.8m 및 0.6m인 이중 정사각형 코일 $1m\times1m$에 대한 자기장의 세기 및 균일도를 계산하여 IEC규격에서 규정한 유도코일의 유호 시험평가 영역을 도출하였다.

• 한국전자파학회논문지
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• 제8권1호
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• pp.83-93
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• 1997

본 논문은 고전압 실험실에서 시변성 전자계의 측정 및 평가에 관한 것으로 전자적 외란은 주로 전력장비의 지락고장 및 개폐조작에 의해 기인되며, 전자회로와 제어장비틀은 전자방해작용에 매우 민감하다. 주어진 전자환 경에 대한 전자방해작용의 레벨을 평가하는 것은 필수적이다. 전계는 40 Hz-2oo MHz 범위의 대역폭을 갖는 전계센서로 관측되었으며, 시변성 자계는 출력이 입사신호에 직접적으로 비례하는 루프형 센서로 측정되었다. 또한 진동성 과도전류와 차단파 임펄스전압에 의해 유도된 전자방해작용의 레벨파 전자 차폐의 외함설계에 관한 정보를 주는 사변성 전자계의 주파수 성분을 고속프리에변환으로 분석하였다.

• 한국해양공학회:학술대회논문집
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• 한국해양공학회 2001년도 춘계학술대회 논문집
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• pp.208-215
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• 2001

An experimental method to investigate the dynamic charasteristics of buoys in extreme environmental condition is established. Because the buoy model requires a resonable size for accurate experiment, the test condition in model basin that satisfies the similarity law is hardly met with capability of test facilities. It is suggested that the linear wave component that is unable to satisy similarity is separated with others. The model experiment can be carried out with mitigated condition for the linear wave components while others including wave drift, current and wind are keeping the similarities. Then the result is extrapolated to give the dynamic behavior of buoys in extreme condition because linear wave component is soley responsible to oscillatory buoy motion and other environmental components are applied as a initial tension. the similarity for current and wind conditions is viewed as equivalence of restoring forces. the validity of proposed method is examined with different types of standard ocean buoys and it indicates that the linearity of measured characteristics is assured with a limitation of resonable distance between test and estimated wave conditions.

• 한국해양공학회지
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• 제7권1호
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• pp.13-24
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• 1993

해양오염은 환경파괴의 주요 인자이다. 해양바닥에 가라않은 오염물질을 근본적으로 제거하는 문제와는 별도로, 파동(wave)에 의해 그것이 자동적으로확산될 수가 있다. 파문(ripple)으로 덮혀진 해저(sea bottom)에서 표면의 중력파에 의한 물의 수평방향 요동운동은 와류(vrotices)를 발생시칸다. 이런한 유동장은 해저 침전물을 부유시켜 멀리까지 화가신시키는 작용을 한다.파문주위의 유동장을 살펴보면 모서리(crest)에서 발생된 와류로 인해 정상유동성분이 존재하며 이런한 정상유동은 파문의 주기적 형상으로인해 다분히 순환적이다. 이ㅔ 파동에 의한 요동운동이 가세하면 Taylor 와류와 같은 효과를 보여 줄 것이다. 해저부근에서의 이러한 확산효과를 보기 위하여, 해양유동을 단순화하여 최근 널리 이용되고 있는 혼돈이론을 가미시켰다. 아주 단순한 유동이라도 복잡한 입자의 궤적을 나타내며 입자의 확산과 연관됨을 수치해석을 이용하여 보여준다.

• 대한전기학회논문지:전기기기및에너지변환시스템부문B
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• 제53권4호
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• pp.213-223
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• 2004

In the machine tool industry, direct drive linear motor technology is of increasing interest as a means to achieve high acceleration, and to increase reliability. The advantages of such a motor are that it has a good linearity and has no need of such mechanical energy conversion parts, which change rotary motion into linear motion, as screws, gears, chains etc In this paper, two structures of LOA are analyzed. One is the moving-coil type LOA and the other is moving-magnet type LOA. Two types of LOA are analyzed, with reference to the following parameters as variables: magnetic field, flux linkage, motor thrust and back emf. These variables are derived by the use of analytical method in terms of two-dimensional rectangular coordinate system. The maximum values of thrust according to such design parameters as air-gap length and magnet height for each model is also represented. The results are validated extensively by comparison with finite element method. In particular, we experiment moving-coil LOA which is already manufactured and confirm that the experimental results are shown in good agreement with analysis through the comparison of between analytical and experimental results

• 한국전산유체공학회지
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• 제16권3호
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• pp.29-36
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• 2011

The natural convection in a horizontal enclosure heated from the bottom wall, cooled at the top wall, and having a square adiabatic body at its centered area was studied. Three different Prandtl numbers (0.01, 0.7 and 7) were considered for an effect of the Prandtl number on natural convection. A two-dimensional solution for unsteady natural convection was obtained, using Chebyshev spectral methodology for different Rayleigh numbers varying over the range of $10^4$ to $10^6$. It had been experimentally and numerically reported [1,2] that the heat transfer mode becomes oscillatory when Pr is out of a specific Pr band beyond the critical Ra. In this study, we reproduced this phenomenon numerically. The variation of time- and surface-averaged Nusselt numbers on the hot and cold walls for different Rayleigh numbers and Prandtl numbers was presented to show the overall heat transfer characteristics in the system. And also, the isotherms and streamline distributions were presented in detail to compare the physics related to their thermal behavior.

• 대한기계학회논문집B
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• 제25권3호
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• pp.381-388
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• 2001

Presented are heat data which describe the effect of interaction between bulk flow pulsations and a vortex embedded in a turbulent boundary layer. The pulsation frequencies are 3 Hz, 15 Hz and 30 Hz. A half delta wing with the same height as the boundary layer thickness is used to generate the vortex flow. The convection heat transfer coefficients on a constant heat-flux surface are measured by embedded 77 T-type thermocouples. Spanwise profiles of convection heat transfer coefficients show that upwash region of vortex flow is influenced by bulk flow pulsations. The local heat transfer coefficient increases approximately by 7 percent. The increase in the local change of convection heat transfer coefficient is attributed to the spanwise oscillatory motion of vortex flow especially at the low Strouhal number and to the periodic change of vortex size.

• 대한기계학회논문집B
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• 제32권6호
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• pp.413-420
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• 2008

The unsteady heat release characteristics play a significant role in combustion instabilities observed in low emissions gas turbine combustors. Such combustion instabilities are often caused by coupling mechanisms between unsteady heat release rates and acoustic perturbations. A generalized model of the turbulent flame response to acoustic perturbations is analytically formulated by considering a distributed heat release along a curved mean flame front and using the flame's kinematic model that incorporates the turbulent flame development. The effects of the development of flame speed on the flame transfer functions are examined by calculating the transfer functions with a constant or developing flame speed. The flame transfer function due to velocity fluctuation shows that, when a developing flame speed is used, the transfer function magnitude decreases faster with Strouhal number than the results with a constant flame speed at low Strouhal numbers. The flame transfer function due to mixture ratio fluctuation, however, exhibits the opposite results: the transfer function magnitude with a developing flame speed increases faster than that with a constant flame speed at low Strouhal numbers. Oscillatory behaviors of both transfer function magnitudes are shown to be damped when a developing flame speed is used. Both transfer functions also show similar behaviors in the phase characteristics: The phases of both transfer functions with a developing flame speed increase more rapidly than those with a constant flame speed.