• Wind and Structures
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• 제21권1호
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• pp.1-23
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• 2015

Wind tunnel testing of a low-rise building with openings (holes) of different sizes and shapes on a roof corner is conducted to measure the internal and external pressures from the building model. Detailed analysis of the testing data is carried out to investigate the characteristics of the internal and external pressures of the building with different openings' configurations. Superimposition of the internal and external pressures makes the emergence of positive net pressures on the roof. The internal pressures demonstrate an overall uniform distribution. The probability density function (PDF) of the internal pressures is close to the Gaussian distribution. Compared with the PDF of the external pressures, the non-Gaussian characteristics of the net pressures weakened. The internal pressures exhibit strong correlation in frequency domain. There appear two humps in the spectra of the internal pressures, which correspond to the Helmholtz frequency and vortex shedding frequency, respectively. But, the peak for the vortex shedding frequency is offset for the net pressures. Furthermore, the internal pressure characteristics indirectly reflect that the length of the front edge enhances the development of the conical vortices.The objective of this study aims to further understanding of the characteristics of internal, external and net pressures for low-rise buildings in an effort to reduce wind damages to residential buildings.

• 대한기계학회논문집B
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• 제28권8호
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• pp.953-960
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• 2004

In this study, the effects of freestream turbulence intensity on laminar-turbulent transition of separated shear layers in the wake of a circular cylinder are investigated using an immersed boundary method and LES. It is shown that the present numerical results without freestream turbulence for Re=3,900 based on bulk mean velocity and the cylinder diameter are in good agreement with other authors' experimental observations and numerical results, verifying our numerical methodology. Then a 'prescribed power spectrum' method is imposed to generate isotropic turbulence at the inlet of the computational domain at each time step. The principal effects of freestream turbulence intensity on flow statistics are investigated for Re=3,900. Statistical study reveals that the Reynolds stresses in the near-wake region gradually increase, and transition occurs further upstream, as the turbulence intensity increases. On the other hand, the bubble size behind the cylinder decreases as the turbulence intensity increases, which indicates that the freestream turbulence helps mean velocity be quickly recovered.

• Wind and Structures
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• 제19권2호
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• pp.219-232
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• 2014

The characteristics of turbulent boundary layers over hilly terrain depend strongly on the hill slope and upstream condition, especially inflow turbulence. Numerical simulations are carried out to investigate the neutrally stratified turbulent boundary layer over two-dimensional hills. Two kinds of hill shape, a steep one with stable separation and a low one without stable separation, two kinds of inflow condition, laminar turbulent, are considered. An auxiliary simulation, based on the local differential quadrature method and recycling technique, is performed to simulate the inflow turbulence be imposed at inlet boundary of the turbulent inflow, which preserves very well in the computational domain. A large separation bubble is established on the leeside of the steep hill with laminar inflow, while reattachment point moves upstream under turbulent inflow condition. There is stable separation on the side of low hill with laminar inflow, whilw not turbulent inflow. Besides increase of turbulence intensity, inflow can efficiently enhance the speedup around hills. So in practice, it is unreasonable to study wind flow over hilly terrain without considering inflow turbulence.

• 한국세라믹학회지
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• 제50권6호
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• pp.518-522
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• 2013

This study investigatesthe coloration mechanism by identifying the factor that affects thered coloration of copper red glazesin traditional Korean ceramics. The characteristics of the reduction-fired copper red glaze's sections are analyzed using an optical microscope, Raman spectroscopy, scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDX). The sections observed using an optical microscope are divided into domains of surface, red-bubble, and red band. According to the Raman micro spectroscopy analysis results, the major characteristic peak is identified as silicate in all three domains, and the intensity of $Cu_2O$ increases toward the red band. In addition, it is confirmed that the most abundant CuO exists in the glaze bubbles. Moreover, CuO and $Cu_2O$ exist as fine particles in a dispersed state in the surface domain. Thus, Cu combined with oxygen is distributed evenly throughout the copper red glaze, and $Cu_2O$ is more concentrated toward the interface between body and glaze. It is also confirmed that CuO is concentrated around the bubbles. Therefore, it is concluded that the red coloration of the copper red glaze is revealed not only through metallic Cu but also through $Cu_2O$ and CuO.

• Nuclear Engineering and Technology
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• 제48권4호
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• pp.847-858
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• 2016

An overview is given on the work of the Laboratory of Nuclear Energy Systems at ETH, Zurich (ETHZ) and of the Laboratory of Thermal Hydraulics at Paul Scherrer Institute (PSI), Switzerland on tight-lattice bundles. Two-phase flow in subchannels of a tight triangular lattice was studied experimentally and by computational fluid dynamics simulations. Two adiabatic facilities were used: (1) a vertical channel modeling a pair of neighboring sub-channels; and (2) an arrangement of four subchannels with one subchannel in the center. The first geometry was equipped with two electrical film sensors placed on opposing rod surfaces forming the subchannel gap. They recorded 2D liquid film thickness distributions on a domain of $16{\times}64$ measuring points each, with a time resolution of 10 kHz. In the bubbly and slug flow regime, information on the bubble size, shape, and velocity and the residual liquid film thickness underneath the bubbles were obtained. The second channel was investigated using cold neutron tomography, which allowed the measurement of average liquid film profiles showing the effect of spacer grids with vanes. The results were reproduced by large eddy simulation + volume of fluid. In the outlook, a novel nonadiabatic subchannel experiment is introduced that can be driven to steady-state dryout. A refrigerant is heated by a heavy water circuit, which allows the application of cold neutron tomography.

• 한국음향학회지
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• 제28권8호
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• pp.740-754
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• 2009

실제 해양에서 음파는 해수면/해저면의 거친 경계면이나 기포층/어군과 같이 불규칙적으로 분포된 체적에 의해 산란 되며, 잔향음 신호는 이러한 산란 신호의 합으로 형성된다. 실측된 잔향음 신호를 정확하게 모의하기 위해서는 각 산란 메커니즘에 적합한 산란 모델을 음파 전달 손실 모델과 결합시켜야 한다. 본 논문에서는 기존의 산란 모델과 결합이 용이한 음선 이론을 기반으로 잔향음 모델을 개발하였다. 개발된 잔향음 모델은 (1) 해수면에 대한 산란 신호로 실험 기반의 Chapman-Harris 식과 이론 기반의 APL-UW/SSA 모델, (2) 해저면에 대해서는 실험 기반의 Lambert 법칙과 이론 기반의 APL-UW/SSA 모델을 선택적으로 사용하도록 한다. 개발된 잔향음 모델의 타당성을 검증하기 위해서 정상 모드법 기반으로 개발된 Ellis 모델 결과와 2006 잔향음 공동웍크�乍【� 발표된 여러 잔향음 모델 결과와 비교하였다. 모델간의 비교를 통해 검증된 잔향음 모델을 이용하여 한국 근해의 중주파수 대역 잔향음 신호를 모의하고, 이를 실측 데이터와 시간 영역에서 직접 비교하였다. 이러한 비교를 통해 각 해역의 해양 환경의 특성에 따라 상호 다른 잔향음 신호 경향을 고찰 할 수 있으며, 나아가 각 해역 특성을 반영하는 산란 강도 함수를 본 잔향음 모델을 통해 선정할 수 있다.