• 한국전산유체공학회지
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• 제22권1호
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• pp.81-87
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• 2017

The characteristics of flow past a square cylinder submerged under the free surface have been numerically studied. An immersed boundary method was adopted for implementation of the cylinder cross-section in a Cartesian grid system. Also, a level-set method was used to capture the interface of the two fluids. The case for Reynolds number 150 was examined. At the specific Reynolds number, by varying the gap ratio(0.25, 0.40, 0.55, 0.70, 1.00, 1.50, 2.50, 5.00) the effects of the free surface on the force coefficients and Strouhal number of vortex shedding were identified. The presence of the free surface very close to the cylinder significantly affects the shedding pattern, resulting in considerable deviation of the force coefficients and Strouhal number from those of the single-phase flow. In addition, the influence of Froude number was considered in this study. By increasing Froude number(0.2-0.4), flow topology change was identified at the specific gap ratios(0.40, 0.70, 1.50, 5.00).

• 대한기계학회논문집B
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• 제26권2호
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• pp.253-260
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• 2002

It is known that frost formation on surfaces of the heat exchanger seriously affects the performance of the refrigeration system. Accordingly, defrosting should follow, and effective defrosting is possible only when both analytic tools and comprehensive experimental data on frost formation are assailable. An experimental investigation was undertaken to characterize the effect of environmental parameters on frost formation on a horizontal cylinder in cross uniform flow. Several experiments were carried out with various environmental parameters such as inlet air temperature, inlet air humidity, air velocity and cooling surface temperature. Frost thickness, mass, surface temperature and cylinder inner and outer temperature were measured at front and rear positions of the cylinder. Thickness, mass, density, and effective thermal conductivity of the frost layer were obtained from measured data and effects of environmental parameters on the frost formation were analyzed. Data from experiments were correlated using dimensionless variables.

• Structural Engineering and Mechanics
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• 제71권6호
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• pp.713-738
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• 2019

In the paper the dynamics of the oscillating moving load acting in the interior of the hollow cylinder surrounded with elastic medium is studied within the scope of the exact field equations of 3D elastodynamics. It is assumed that the oscillating load act on the certain arc of the internal circle of the cylinder's cross section and this load moves with constant velocity along the cylinder's axis. The corresponding 3D dynamic problem is solved by employing moving coordinate system, the exponential Fourier transform and the presentation these transforms with the Fourier series. The expressions of the transforms are determined analytically, however their originals are found numerically. Under the investigations carried out in the paper the main attention is focused on the so-called "gyroscopic effect", according to which, the influence of the vibration frequency on the values of the critical velocity and interface stresses are determined. Numerical results illustrated this effect are presented and discussed. In particular, it is established how the non-axisymmetricity of the problem acts on the influence of the load oscillation on its critical velocity and on the interface stresses.

• Wind and Structures
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• 제34권1호
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• pp.101-113
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• 2022

We experimentally investigate the high-Reynolds flow around a rectangular cylinder of aspect ratio 5:1. This configuration is the object of the international BARC benchmark. Wind tunnel tests have been carried out for the flow at zero angle of attack and a Reynolds number, based on the crossflow cylinder length and on the freestream velocity, equal, to 40 000. Velocity measurements are obtained by using hot-wire anemometry along 50 different cross-flow traverses on the cylinder side and in the near wake. Differential pressure measurements are acquired on multiple streamwise sections of the model. The obtained measurements are in a good agreement with the state-of-the-art experiments. For the first time among the several contributions to the BARC benchmark, detailed flow measurements are acquired in the region near the cylinder side and in the near-wake flow. The edges and the thickness of the shear layers detaching from the upstream edges are derived from velocity measurements. Furthermore, we compute the flow frequencies characterizing the roll-up of the shear layers, the evolution of vortical structures near the cylinder side and the vortex shedding in the wake.

• 한국안광학회지
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• 제16권2호
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• pp.167-170
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• 2011

목적: 등가구면 시력을 통한 피검자의 난시량 예측이 가능한지를 알아보고자 하였다. 방법: 평균연령 23.3세의 남녀 54명(108안)을 대상으로 굴절이상 완전교정 후 크로스실린더의 도수를 ${\pm}$0.25D씩 증가시키면서 크로스실린더의 (-)축이 $180^{\circ}$ 일 때, $90^{\circ}$ 일 때, 그리고 $45^{\circ}$ 일 때의 원거리(5m) 시력변화를 각각 측정하였다. 결과: 크로스실린더의 도수가 증가될수록 시력은 반비례하여 감소되었다. 크로스실린더 ${\pm}$2.50D ($180^{\circ}$와 $90^{\circ}$)과 ${\pm}$2.25D ($45^{\circ}$)에서 측정 최소값인 시력 0.05로 나타났다. 결론: 각각의 등가구면 시력에 대응하는 난시량에 대한 관계를 추정할 수 있었다.

• 대한기계학회논문집
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• 제16권5호
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• pp.924-932
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• 1992

본 연구에서는 풍동내에 놓여진 탄성지지 실린더의 와유출 특성과 진동 응답 을 동시에 측정하고 분석하여 유입속도 변화에 따른 이들의 상호관계를 확인하고 아울 러 실린더가 통과하는 풍동측정부에 있는 틈새 간격변화가 와유출과 실린더의 진동특 성에 미치는 영향에 대해 조사하였다.

• Wind and Structures
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• 제34권5호
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• pp.437-450
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• 2022

Vortex-induced vibration (VIV) is a significant concern when designing slender structures with square cross sections. VIV strongly depends on structural dynamics and flow states, which depend on the conditions of the approaching flow and shape of a structure. Therefore, the effects of the angle of attack on the coupling effects of VIV for a square cylinder are expected to be significant in practice. In this study, the aerodynamic forces for a fixed and elastically mounted square cylinder were measured using wind pressure tests. Aerodynamic forces on the stationary cylinder are firstly discussed by comparisons of variation of statistical aerodynamic force and wind pressure coefficient with wind angle of attack. The coupling effect between the aerodynamic forces and the motion of the oscillating square cylinder by VIV is subsequently investigated in detail at typical wind angels of attack with occurrence of three typical flow regimes, i.e., leading-edge separation, separation bubble (reattachment), and attached flow. The coupling effect are illustrated by discussing the onset of VIV, characteristics of aerodynamic forces during VIV, and interaction between motion and aerodynamic forces. The results demonstrate that flow states can be classified based on final separation points or the occurrence of reattachment. These states significantly influence coupling effects of the oscillating cylinder. Vibration enhances vortex shedding, which creates strong fluctuations in aerodynamic forces. However, differences in the lock-in range, aerodynamic force, and interaction process for angles of attack smaller and larger than the critical angle of attack revealed noteworthy characteristics in the VIV of a square cylinder.

• 한국안광학회지
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• 제20권3호
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• pp.349-354
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• 2015

목적: 크로스실린더를 이용한 난시정밀검사에서 망막 상의 최소착란원 위치가 난시교정값에 미치는 영향을 알아보고자 하였다. 방법: 평균나이 $22.24{\pm}2.48$세의 62명(115안)을 대상으로 검사하였다. 방사선시표를 이용한 난시검사 후 최대교정시력이 얻어진 구면굴절력(MPMVA) 상태에서 크로스실린더를 이용한 정밀검사를 실시하였다. 그런 다음 S+0.75 D, S+0.50 D, S+0.25 D, S-0.25 D, S-0.50 D, 그리고 S-0.75 D의 렌즈를 각각 더한 상태(최소착락원의 위치이동)에서 동일한 정밀검사를 실시하고, 얻어진 검사값을 MPMVA 상태에서 얻어진 검사값과 비교하였다. 결과: MPMVA 상태의 검사값과 비교하여 더해준 (+)구면렌즈와 (-)구면렌즈가 증가할수록 축 변화량은 증가하였다. 동일한 검사상태에서 난시굴절력이 높을수록 축 변화량은 감소하였다(p<0.05). MPMVA 상태의 검사값과 비교하여 더해준 (+)구면렌즈가 증가할수록 난시굴절력은 낮게 나타났고(p<0.05), (-)구면렌즈가 증가할수록 높게 나타났다. (+)구면렌즈를 더해준 경우, 동일한 검사상태에서 난시굴절력이 높을수록 검사값의 차이는 더 크게 나타났다. 결론: 올바른 난시교정값을 얻기 위해서 크로스실린더를 이용한 난시정밀검사 전 최소착락원 위치를 정확히 조정하여야 한다.

• 대한조선학회지
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• 제22권2호
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• pp.27-34
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• 1985

In this paper, the second order time mean forces acting on the circular cylinder floating on the free surface of a finite water depth are calculated. Under the assumption that fluid is idea and the wave the linear gravity wave, the velocity potential is calculated by the source distribution method, and the second order time mean lateral and vertical drifting forces are calculated by the direct integration of fluid pressures over the immersed body surface. The comparison of the lateral drifting forces with Rhee's results by momentum theorem shows good agreements. And it is shown that the second order time sinkage forces of a floating circular cylinder cross zero for all water depths.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2000년도 춘계학술대회논문집
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• pp.1386-1392
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• 2000

This paper presents a new prediction method of radiated noise from grille of the airconditioning appliance. Laminar vortex sheddings behind a circular cylinder are simulated by solving two dimensional unsteady incompressible Navier-Stokes equation. The Finite Elements Method(FEM) and unstructured grid generation technique are applied to solve, the unsteady lift/drag coefficients are obtained to compute far-field noise using Lighthill's acoustic analogy. Grille is divided into some cylinder segments, and radiated noise from grille is obtained by summing noise generated from each segment. The effects of changing cross section of cylinder and grille geometry are studied. And sound pressure levels radiated from typical H-type grille are measured in KAIST anechoic wind tunnel at various inflow conditions and compared with numerical predictions.