• Nuclear Engineering and Technology
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• 제55권10호
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• pp.3874-3897
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• 2023

A high-fidelity computational fluid dynamics (CFD) analysis was performed using the Large Eddy Simulation (LES) model for the lower plenum of the High-Temperature Test Facility (HTTF), a ¼ scale test facility of the modular high temperature gas-cooled reactor (MHTGR) managed by Oregon State University. In most next-generation nuclear reactors, thermal stress due to thermal striping is one of the risks to be curiously considered. This is also true for HTGRs, especially since the exhaust helium gas temperature is high. In order to evaluate these risks and performance, organizations in the United States led by the OECD NEA are conducting a thermal hydraulic code benchmark for HTGR, and the test facility used for this benchmark is HTTF. HTTF can perform experiments in both normal and accident situations and provide high-quality experimental data. However, it is difficult to provide sufficient data for benchmarking through experiments, and there is a problem with the reliability of CFD analysis results based on Reynolds-averaged Navier-Stokes to analyze thermal hydraulic behavior without verification. To solve this problem, high-fidelity 3-D CFD analysis was performed using the LES model for HTTF. It was also verified that the LES model can properly simulate this jet mixing phenomenon via a unit cell test that provides experimental information. As a result of CFD analysis, the lower the dependency of the sub-grid scale model, the closer to the actual analysis result. In the case of unit cell test CFD analysis and HTTF CFD analysis, the volume-averaged sub-grid scale model dependency was calculated to be 13.0% and 9.16%, respectively. As a result of HTTF analysis, quantitative data of the fluid inside the HTTF lower plenum was provided in this paper. As a result of qualitative analysis, the temperature was highest at the center of the lower plenum, while the temperature fluctuation was highest near the edge of the lower plenum wall. The power spectral density of temperature was analyzed via fast Fourier transform (FFT) for specific points on the center and side of the lower plenum. FFT results did not reveal specific frequency-dominant temperature fluctuations in the center part. It was confirmed that the temperature power spectral density (PSD) at the top increased from the center to the wake. The vortex was visualized using the well-known scalar Q-criterion, and as a result, the closer to the outlet duct, the greater the influence of the mainstream, so that the inflow jet vortex was dissipated and mixed at the top of the lower plenum. Additionally, FFT analysis was performed on the support structure near the corner of the lower plenum with large temperature fluctuations, and as a result, it was confirmed that the temperature fluctuation of the flow did not have a significant effect near the corner wall. In addition, the vortices generated from the lower plenum to the outlet duct were identified in this paper. It is considered that the quantitative and qualitative results presented in this paper will serve as reference data for the benchmark.

• 한국농공학회논문집
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• 제58권4호
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• pp.21-35
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• 2016

This study is to evaluate the applicability of SWAT (Soil and Water Assessment Tool) model for multi-purpose dams and multi-function weirs operation in Namhan river basin ($12,577km^2$) of South Korea. The SWAT was calibrated (2005 ~ 2009) and validated (2010 ~ 2014) considering of 4 multi-purpose dams and 3 multi-function weirs using daily observed dam inflow and storage, evapotranspiration, soil moisture, and groundwater level data. Firstly, the dam inflow was calibrated by the five steps; (step 1) the physical rate between total runoff and evapotranspiration was controlled by ESCO, (step 2) the peak runoff was calibrated by CN, OV_N, and CH_N, (step 3) the baseflow was calibrated by GW_DELAY, (step 4) the recession curve of baseflow was calibrated by ALPHA_BF, (step 5) the flux between lateral flow and return flow was controlled by SOL_AWC and SOL_K, and (step 6) the flux between reevaporation and return flow was controlled by REVAPMN and GW_REVAP. Secondly, for the storage water level calibration, the SWAT emergency and principle spillway were applied for water level from design flood level to restricted water level for dam and from maximum to management water level for weir respectively. Finally, the parameters for evapotranspiration (ESCO), soil water (SOL_AWC) and groundwater level fluctuation (GWQMN, ALPHA_BF) were repeatedly adjusted by trial error method. For the dam inflow, the determination coefficient $R^2$ was above 0.80. The average Nash-Sutcliffe efficiency (NSE) was from 0.59 to 0.88 and the RMSE was from 3.3 mm/day to 8.6 mm/day respectively. For the water balance performance, the PBIAS was between 9.4 and 21.4 %. For the dam storage volume, the $R^2$ was above 0.63 and the PBIAS was between 6.3 and 13.5 % respectively. The average $R^2$ for evapotranspiration and soil moisture at CM (Cheongmicheon) site was 0.72 and 0.78, and the average $R^2$ for groundwater level was 0.59 and 0.60 at 2 YP (Yangpyeong) sites.

• 대한기계학회논문집B
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• 제27권8호
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• pp.1023-1032
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• 2003

The effect of local forcing on the separated flow over a backward-facing step is investigated through hot-wire measurements and flow visualization with multi-smoke wires. The boundary layer upstream of the separation point is laminar and the Reynolds number based on the free stream velocity and the step height is 13800. The local forcing is given from a slit located at the step edge and the forcing signal is always defined when the wind tunnel is in operation. In case of single frequency forcing, the streamwise velocity and the reattachment length are measured under forcing with various forcing frequencies. For the range of 0.010〈S $t_{\theta}$〈0.013, the forcing frequency component of the streamwise velocity fluctuation grows exponentially and is saturated at x/h = 0.75 , while its subharmonic component grows following the fundamental and is saturated at x/h = 2.0. However, the saturated value of the subharmonic is much lower than that of the fundamental. It is observed that the vortex formation is inhibited by the forcing at S $t_{\theta}$ = 0.019 . For double frequency forcing, natural instability frequency is adopted as a fundamental frequency and its subharmonic is superposed on it. The fundamental frequency component of the streamwise velocity grows exponentially and is saturated at 0.5 < x/ｈ < 0.75, while its subharmonic component grows following the fundamental and is saturated at x/ｈ= 1.5 . Furthermore, the saturated value of the subharmonic component is much higher than that for the single frequency forcing and is nearly the same or higher than that of the fundamental. It is observed that the subharmonic component does not grow for the narrow range of the initial phase difference. This means that there is a range of the initial phase difference where the vortex parring cannot be enhanced or amplified by double frequency forcing. In addition, this effect of the initial phase difference on the development of the shear layer and the distribution of the reattachment length shows a similar trend. From these observations, it can be inferred that the development of the shear layer and the reattachment length are closely related to the vortex paring.

• Journal of Mechanical Science and Technology
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• 제20권12호
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• pp.2292-2314
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• 2006

Efficient numerical method is developed for the prediction of aerodynamic noise generation and propagation in low Mach number flows such as aeolian tone noise. The proposed numerical method is based on acoustic/viscous splitting techniques of which acoustic solvers use simplified linearised Euler equations, full linearised Euler equations and nonlinear perturbation equations as acoustic governing equations. All of acoustic equations are forced with immersed surface dipole model which is developed for the efficient computation of aerodynamic noise generation and propagation in low Mach number flows in which dipole source, originating from unsteady pressure fluctuation on a solid surface, is known to be more efficient than quadrupole sources. Multi-scale overset grid technique is also utilized to resolve the complex geometries. Initially, aeolian tone from single cylinder is considered to examine the effects that the immersed surface dipole models combined with the different acoustic governing equations have on the overall accuracy of the method. Then, the current numerical method is applied to the simulation of the aeolian tones from twin cylinders aligned perpendicularly to the mean flow and separated 3 diameters between their centers. In this configuration, symmetric vortices are shed from twin cylinders, which leads to the anti-phase of the lift dipoles and the in-phase of the drag dipoles. Due to these phase differences, the directivity of the fluctuating pressure from the lift dipoles shows the comparable magnitude with that from the drag dipoles at 10 diameters apart from the origin. However, the directivity at 100 diameters shows that the lift-dipole originated noise has larger magnitude than, but still comparable to, that of the drag-dipole one. Comparison of the numerical results with and without mean flow effects on the acoustic wave emphasizes the effects of the sheared background flows around the cylinders on the propagating acoustic waves, which is not generally considered by the classic acoustic analogy methods. Through the comparison of the results using the immersed surface dipole models with those using point sources, it is demonstrated that the current methods can allow for the complex interactions between the acoustic wave and the solid wall and the effects of the mean flow on the acoustic waves.

• 한국건설관리학회논문집
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• 제18권4호
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• pp.3-16
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• 2017

초고층 개발사업의 불확실한 리스크 요인을 고려한 재무타당성을 분석하기 위해 확률론적 접근방법을 통한 몬테카를로 시뮬레이션을 적용하기 위해서는 먼저 타당성 분석 시 고려되어야 하는 리스크 인자에 대한 식별이 이루어 져야 한다. 이 부분은 선행연구에서 식별된 초고층 개발사업의 단계별 리스크 요인들을 고려하고자 하며, 추출된 리스크 인자 중에서 개발사업의 수익을 저하시키는 리스크 인자들을 2차 식별하여 각 인자별 데이터의 특성을 잘 표현할 수 있는 확률분포를 추정하여 몬테카를로 시뮬레이션을 적용하였다. 이렇게 도출된 시뮬레이션 결과를 통하여 현실에서 사업타당성 검토 중인 여러 초고층 개발사업의 추후 운영 시 수익성을 예측하는 자료로 적용 가능한 대안을 찾아보고자 한다. 분석 결과, 아파트를 포함한 통합 현금흐름으로 추정한 초고층부의 평균 NPV와 IRR은 흑자 수익구조를 가지고 있다. 이는 아파트 분양수익으로 호텔 및 오피스에서 오는 적자를 상쇄시켜 주기 때문으로 보인다. 초고층부 시설 중 사업성에 미치는 영향력은 전망대 1차년도 매출액이 가장 크며, 다음으로 운영기간에 따른 오피스 공실률이 큰 영향을 미치고 있음을 알 수 있었다. 또한 초고층 개발사업의 수익성을 높이기 위한 최적의 포트폴리오 구성은 매우 중요함을 알 수 있다.

• 한국방재안전학회논문집
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• 제14권4호
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• pp.47-60
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• 2021

최근 기후변동성으로 인하여 집중호우의 발생빈도 및 강우강도 증가로 노후화된 여수로 바닥슬래브 표면에서의 손상이 발생하여 잦은 보수·보강이 필요한 실정이다. 이를 위해 현장조사, 수리모형 실험 및 수치모형 실험을 통하여 여수로 방류에 따른 손상발생 원인 검토에 관한 연구가 많이 진행되어 왔다. 그러나 대부분의 연구는 일반적으로 여수로의 흐름특성 및 압력분포에 대한 검토를 수행하였을 뿐 손상의 근본적인 발생원인 규명에 관한 연구는 미비한 실정이다. 이에 본 연구에서는 여수로 바닥슬래브 손상발생 원인을 도출하기 위해 공동침식 및 수력잭킹(hydraulic jacking)으로 인한 콘크리트 탈락관점에서 3차원 수치모형인 FLOW-3D와 COMSOL Multiphysics를 사용하여 검토하였다. 또한 공동지수를 산정하고 압력분포로 인하여 구조물이 받는 응력과 콘크리트의 인장·굽힘강도를 비교하여 공동침식 및 수력잭킹으로 인한 콘크리트 탈락 발생 가능성을 확인하였다. 수문 완전개도 조건에서 여수로 방류에 따른 공동침식 및 수력잭킹에 대하여 수치모의를 수행한 결과, 여수로 하류부에서 공동지수가 0.3 미만으로 공동침식 발생 가능성을 확인하였고, 공동부 및 균열부에서 압력분포에 따라 콘크리트가 받는 응력은 4.6~5.0 MPa로 콘크리트 인장강도와 굽힘강도와 비교를 통하여 지속적인 압력변동으로 인한 콘크리트의 피로파괴 또는 휨파괴 가능성을 확인하였다. 따라서 여수로 고유속 흐름에 의한 공동 현상 및 수력잭킹이 여수로 바닥슬래브 손상발생의 다양한 원인 중 하나로 판단하였다. 그러나 본 연구는 다양한 형상 조건 및 방류 시나리오를 적용하고 유체-구조물 상호작용(Fluid-Structure Interaction, FSI)모의를 수행하지 못하였다는 한계점이 있다. 이에 향후에는 한계점을 보완하여 검토한다면 보다 효율적이고 효과적인 여수로 유지관리 방안 도출이 가능할 것으로 기대한다.

• 한국해안·해양공학회논문집
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• 제29권5호
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• pp.217-227
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• 2017

경기만 염하수로에서 시 공간적으로 변화하는 총수송량과 잔차류를 산정하고자 소조기와 대조기에 염하수로의 하류(정선-1), 염하수로 중간 지점(정선-2)에서 13시간 동안 단면 유속을 관측하였다. 총수송량은 Eulerian flux와 Stokes drift의 합인 Lagrange flux로 계산하였고, 잔차류는 최소자승법을 이용하여 구하였다. 총수송량과 잔차류의 계산은 관측 시간별, 수평 수직 sigma 좌표계로 변환하여 수행하였다. 변환된 sigma 좌표체계는 z-level 좌표 체계와 비교하였을 때 주 방향 유속 오차가 3~5% 내외로 자료 분석에 무리가 없는 것으로 판단되었다. 분석결과 단면 잔차류는 정선-1에서는 대조기에 주 수로 방향에서 북향, 수로 양 끝 단에서 남향하였으며, 소조기에는 수직적으로 표층에서는 창조, 저층에서는 낙조하는 이층흐름 구조를 보였다. 반면 정선-2에서는 대조, 소조 모두 남향(낙조)하였다. 한편 총수송량은 정선-1에서는 대조 시와 소조 시에 각각 $359m^3s^{-1}$, $248m^3s^{-1}$로 북향(창조), 정선-2에서 대조 시와 소조 시에 각각 $576m^3s^{-1}$, $67m^3s^{-1}$로 남향(낙조)하였다. 정선 별 공간 수송량 차이로 영종도와 강화도 사이의 조간대 지역의 순 유출량을 추정하였으며, 크기는 대조기와 소조기에 각각 $935m^3s^{-1}$, $315m^3s^{-1}$로 나타났다. 이처럼 대 소조기와 공간적 특성에 따라 잔차류와 순 수송량이 변화되는 주된 요인은 순압력구배와 Stokes drift가 복합적으로 작용한 결과이다.

• 헤리티지:역사와 과학
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• 제53권1호
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• pp.52-63
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• 2020

이 연구는 국내 대표적인 석회 동굴인 대금굴, 온달동굴, 성류굴의 내부 온도 변동의 특성과 기온이 동굴 내부 온도에 미치는 영향을 평가하기 위하여 수행되었다. 2019년 4월 13일부터 2019년 6월 25일까지 대금굴, 온달동굴 및 성류굴의 각 세 지점에서 매시간 온도를 측정하였다. 또한, 같은 기간에 동굴 주변의 기온 자료를 기상청에서 수집하였다. 수집된 자료의 기초 통계, 시간에 따른 변동 특성, 시계열 분석을 수행하였다. 대금굴, 온달동굴 및 성류굴에서 동굴 입구, 동굴수 유입 지점, 중간 지점 순으로 온도 변화가 크게 나타났다. 연구 기간 동안에 동굴 온도는 점차 증가하는 경향을 보였다. 대금굴은 외부로부터 약 150m에 위치한 모노레일 정류장에서 온도 변화가 크게 나타났고, 중간 지점 이후부터는 거의 일정하게 유지되었다. 정류장에서는 관람객의 영향으로 인해 기온의 영향이 크게 나타나지는 않았지만, 동굴 입구에서 내부로 갈수록 기온의 영향이 점차 감소하였다. 온달동굴은 입구에서 기온의 영향으로 온도가 넓은 범위를 보였지만 내부는 거의 일정하게 유지되었다. 그러나 동굴 내 관람 구간 가장 안쪽의 동굴수가 유입되는 지점에서는 유입되는 동굴수의 수온이 동굴 온도에 영향을 주었다. 성류굴은 입구와 중간 지점에서 뚜렷한 온도 변화를 보이고 내부는 거의 일정하게 유지되었다. 입구는 기온의 영향으로 온도 변화가 나타났지만, 중간지점은 기온보다는 호수의 수온에 영향을 더 많이 받은 것으로 추정되었다. 그러나 이번 연구에서는 이를 명확하게 설명하지 못하였다. 시계열 분석 결과 모든 동굴에서 기온이 약 1시간이 지난 후에 동굴 내부 온도에 영향을 주는 것으로 평가되었다. 동굴의 규모, 내부 구조, 동굴수, 동굴 입구의 크기와 형태, 공기의 흐름, 관람객 등이 동굴의 온도에 영향을 줄 수 있으므로 동굴 온도 변동 특성을 명확하게 이해하기 위해서는 이러한 요소들도 함께 고려되어야 한다.

• 생태와환경
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• 제41권4호
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• pp.547-553
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• 2008

In recent years, attention on the inhabitation environments of animals and plants which coexist with humans is growing more and more, and relevant research is being activated. In habitats of rivers, a lot of factors are interacting, even among them, some elements especially such hydraulic factors as water velocity and water depth, and such geological shapes as gravels, sand and mud are being considered as primary elements. In this study, various field investigations are carried out to determine the relationship between the river habitats of fishes and hydraulic primary elements using high-tech equipments. Furthermore numerical experiments to classify such habitats according to topographical spaces are carried out. In detail, hydraulic field investigations performed in this study can be summarized as topographical survey, discharge measurement, water level fluctuation monitoring and so on. In numerical experiments, the RMA2 model of the commercial program, Surface-Water Modeling System (SMS), which is widely used in conducting a two-dimensional analysis of the flow behavior of a river is utilized. In conclusion, as a result of field investigation, the relationship between water velocity and water depth is obtained. And the relationship between water velocity and water temperature is identified, too. Finally, using above obtained results, the inhabitation environment was classified into Riffle, Glide, Run, Pool, and E.D.Z according to the relationship between water velocity and water depth.

• International Journal of Control, Automation, and Systems
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• 제5권6호
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• pp.643-651
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• 2007

The various blood pressure simulators have been proposed to evaluate and improve the performance of the automatic sphygmomanometer. These have some problems such as the deviation of the actual blood pressure waveform, limitation in the blood pressure condition of the simulator, or difficulty in displaying the blood flow. An improved simulator using disturbance observer is proposed to supplement the current problems of the blood pressure simulator. The proposed simulator has an artificial arm model capable of feeding appropriate fluids that can generate the blood pressure waveform to evaluate the automatic sphygmomanometer. A controller was designed and thereafter, simulation was performed to control the output signal with respect to the reference input in the fluid dynamic model using the proposed proportional control valve. To minimize the external fluctuation of pressure applied to the artificial arm, a disturbance observer was designed on the plant. A hybrid controller combined with a proportional controller and feed-forward controller was fabricated after applying a disturbance observer to the control plant. Comparison of the simulations between the conventional proportional controller and the proposed hybrid controller indicated that even though the former showed good control performance without disturbance, it was affected by the disturbance signal induced by the cuff. The latter exhibited an excellent performance under both situations.