• 대한기계학회:학술대회논문집
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• 대한기계학회 2003년도 춘계학술대회
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• pp.1754-1759
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• 2003

Two-dimensional flow over a pair of circular cylinders in side-by-side arrangements at low Reynolds numbers has been numerically investigated in this study. Numerical simulations are performed, using the immersed boundary method, in the ranges of $40{\leq}Re{\leq}160$ and $g^{\ast}<5$, where Re and $g^{\ast}$ are, respectively, the Reynolds number and the spacing between the two cylinder surfaces divided by the cylinder diameter. Results show that total six kinds of wake patterns are observed over the ranges: antiphase-synchronized, inphase-synchronized, flip-flopping, single bluff-body, deflected, and steady wake patterns. It is found that the characteristics of the flow significantly depends both on the Reynolds number and gap spacing, with the latter much stronger than the former. Instantaneous flow fields, time traces, flow statistics and so on are presented to identify the wake patterns and then to understand the underlying mechanism. It is remarkable that, for the deflected wake pattern, the gap flow is deflected invariably to the cylinder of higher drag coefficient and the deflection way does not change at all. Moreover, the bifurcation phenomena where either of two wake patterns can occur are found at certain flow conditions.

• 대한전기학회:학술대회논문집
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• 대한전기학회 1994년도 하계학술대회 논문집 C
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• pp.1530-1532
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• 1994

This paper describes the discharge characteristics of liquid nitrogen for plane-plane, needle-plane electrode at variation of gap spacing at atmospheric pressure. The important results obtained from this study are as follows. (1) Breakdown voltage of $LN_2$ for needle-plane electrode is higher than that of for plane-plane electrode and discharge duration tine is longer with increase of gap spacing at atmospheric pressure. (2) The formation of bubbles by evaporation is observed in spite of non-applying source at atmospheric pressure and the creation of corona confirmed for plane-plane electrode results from the bubbles. (3) The applied voltage-discharge magnitude( V-Q) characteristics of $LN_2$ showed hysteresis and the discharge magnitude decreasing and corona voltage increasing proportional to the pulse per second at atmospheric pressure.

• Wind and Structures
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• 제28권6호
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• pp.355-367
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• 2019

Control of horseshoe vortex in the circular cylinder-plate juncture using vortex generator (VG) was studied at $Re_D$(where D is the diameter of the cylinder) = $2.05{\times}10^5$. Impact of a number of parameters e.g., the shape of the VG's, number of VG pairs (n), spacing between the VG and the cylinder leading edge (L), lateral gap between the trailing edges of a VG pair (g), streamwise gap between two VG pairs ($S_{VG}$) and the spacing between the two VG's in parallel arrangement ($Z_{VG}$) etc. were investigated on the horseshoe vortex control. The study is conducted using surface oil flow visualization and surface pressure measurements in low speed wind tunnel. It is observed that all the parameters studied have significant control effect, either by reduction in separation region or by lowering the adverse pressure along the symmetric axis upstream of the juncture.

• 한국입자에어로졸학회지
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• 제17권4호
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• pp.81-89
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• 2021

Understanding particle-laden flow around cylindrical bodies is essential for the better design of various applications such as filters. In this study, laminar flows around two tandem cylinders and the motions of particles in the flow are numerically investigated at low Reynolds numbers. We aim to reveal the effects of the spacing between cylinders, Reynolds number and particle Stokes number on the characteristics of particle trajectories. When the cylinders are placed close, the unsteady flow inside the inter-cylinder gap at Re = 100 shows a considerable modification. However, the steady recirculation flow in the wake at Re = 10 and 40 shows an insignificant change. The change in the flow structure leads to the variation of particle dispersion pattern, particularly of small Stokes number particles. However, the dispersion of particles with a large Stokes number is hardly affected by the flow structure. As a result, few particles are observed in the cylinder gap regardless of the cylinder spacing and the Reynolds number. The deposition efficiency of the upstream cylinder shows no difference from that of a single cylinder, increasing as the Stokes number increases. However, the deposition on the downstream cylinder is found only at Re = 100 with large spacing. At this time, the deposition efficiency is generally small compared to that of an upstream cylinder, and the deposition location is also changed with no deposited particles near the stagnation point.

• 대한전기학회:학술대회논문집
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• 대한전기학회 1992년도 하계학술대회 논문집 B
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• pp.1247-1249
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• 1992

Oscillations with radio-frequencies are observed in a cesiated variable spacing thermionic diode having a parallel-plane geometry under certain conditions. The emitter is operated over the temperature range from 1300 to 2000 K and collector from 300 to 1050 K. The cesium vapor pressures are in the range of $10^{-3}$ to 3 torr. The interelectrode gap is between 1 mm to 0.08 mm. The oscillations are only observed in two nonoverlapping regimes. The low temperature oscillations (1300 to 1550 K) are inversely proportional to both the cesium vapor pressure and the square of the interelectrode gap. The high temperature oscillations is inversely proportional to the interelectrode gap. However, all these observed oscillations are frequencies of the plasma ion oscillation.

• 한국지반공학회논문집
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• 제37권4호
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• pp.19-26
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• 2021

원자력발전소에서 발생되는 고준위폐기물은 지하 수백 미터 깊이의 암반에 처분된다. 이러한 고준위폐기물은 인체에 유해하기에 공학적방벽시스템에 의해 안전하게 처분되어야 하며, 공학적방벽시스템은 처분용기, 뒤채움재, 완충재 등으로 구성된다. 고준위폐기물처분장에 이러한 공학적방벽시스템의 구성요소를 설치하게 되면, 처분용기 및 완충재 사이, 완충재와 자연 암반 사이 등에 갭이 존재하게 된다. 이러한 갭의 존재로 인해 공학적방벽재의 차수능과 열전달 효율이 떨어질 수 있기에, 갭 공간의 크기 및 갭채움재 특성 평가 등의 연구가 반드시 필요하다고 할 수 있다. 해외에 비해 국내 처분시스템을 고려한 갭 공간 및 갭채움재에 대한 연구는 아직 진행되고 있지 않는 상황이기에, 본 연구에서는 수치해석을 통해 국내 처분시스템을 고려한 갭 공간이 공기로 채워져 있는 조건하에서 갭의 크기에 따른 벤토나이트 완충재의 첨두 온도를 도출하였다. 처분용기와 완충재 사이의 갭 공간이 완충재의 첨두 온도에 미치는 영향은 미미하였으나, 완충재와 자연 암반 사이의 갭 공간에 따른 완충재의 첨두 온도는 최고 약 40%의 차이를 나타냈다.

• Transactions on Electrical and Electronic Materials
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• 제6권5호
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• pp.229-232
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• 2005

Direct characterization of band alignment at chemical bath deposition $(CBD)-CdS/Cu_{0.93}(In_{1-x}Ga_x)Se_2$ has been carried out by photoemission spectroscopy (PES) and inverse photoemission spectroscopy (IPES). Ar ion beam etching at the condition of the low ion kinetic energy of 400 eV yields a removal of surface contamination as well as successful development of intrinsic feature of each layer and the interfaces. Especially interior regions of the wide gap CIGS layers with a band gap of $1.4\~1.6\;eV$ were successfully exposed. IPES spectra revealed that conduction band offset (CBO) at the interface region over the wide gap CIGS of x = 0.60 and 0.75 was negative, where the conduction band minimum of CdS was lower than that of CIGS. It was also observed that an energy spacing between conduction band minimum (CBM) of CdS layer and valance band maximum (VBM) of $Cu_{0.93}(In_{0.25}Ga_{0.75})Se_2$ layer at interface region was no wider than that of the interface over the $Cu_{0.93}(In_{0.60}Ga_{0.40})Se_2$ layer.

• 대한전기학회:학술대회논문집
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• 대한전기학회 1996년도 하계학술대회 논문집 C
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• pp.1816-1818
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• 1996

A micro-size nonthermal plasma generator, using a $SiO_2$ film as a dielectric barrier, has been studied experimentally for a high frequency ac voltage in 2LPM oxygen gas fed. The $SiO_2$ film as a micro-size dielectric barrier was made by the wet oxidation of n-type Si wafer($220[{\mu}mt]$). It can be generated ozone, as a nonthermal plasma intensity parameter, at very low level of applied voltage about 1[kV] by using the micro-size dielectric barrier. As a result, in case that have no air gap spacing i.e. surface discharge case shows relatively higher ozone concentration rather than that case of the micro-airgap spacing.

• Journal of Mechanical Science and Technology
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• 제18권10호
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• pp.1869-1879
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• 2004

The present study has numerically investigated two-dimensional flow over three circular cylinders in an equidistant side-by-side arrangement at a low Reynolds number. For the study, numerical simulations are performed, using the immersed boundary method, in the range of g* < 5 at Re= 100, where g* is the spacing between two adjacent cylinder surfaces divided by the cylinder diameter. Results show that the flow characteristics significantly depend on the gap spacing and a total of five kinds of wake patterns are observed over the range: modulation-synchronized (g* (equation omitted) 2), inphase-synchronized (g* (equation omitted) 1.5) , flip-flopping (0.3 < g* (equation omitted) 1.2) , deflected (g* (equation omitted) 0.3), and single bluff-body patterns (g* < 0.3). Moreover, the parallel and symmetric modes are also observed depending on g* in the regime of the flip-flopping pattern. The corresponding flow fields and statistics are presented to verify the observations.

• Wind and Structures
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• 제29권1호
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• pp.1-13
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• 2019

A numerical investigation on forces and flow around three square cylinders in side-by-side arrangement is conducted at a Reynolds number Re = 150 with the cylinder center-to-center spacing ratio L/W = 1.1 ~ 9.0, where W is the cylinder side width. The flowat this Re is assumed to be two-dimensional, incompressible, and Newtonian. The flow simulation is conducted by using ANSYS-Fluent. The flow around the three side-by-side cylinders entails some novel flow physics, involving the interaction between the gap and free-stream side flows as well as that between the two gap flows. An increase in L/W from 1.1 to 9.0 leads to five distinct flow regimes, viz., base-bleed flow (L/W < 1.4), flip-flopping flow (1.4 < L/W < 2.1), symmetrically biased beat flow (2.1 < L/W < 2.6), non-biased beat flow (2.6 < L/W < 7.25) and weak interaction flow (7.25 < L/W < 9.0). The gap flow behaviors, time-averaged and fluctuating fluid forces, time-averaged pressure, recirculation bubble, formation length, and wake width in each flow regime are discussed in detail.