• 한국수자원학회논문집
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• 제49권6호
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• pp.469-480
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• 2016

본 연구는 안드로이드 기반의 스마트폰을 이용한 실시간 표면영상유속계를 개발하는 것이다. 스마트폰이 내장한 카메라, GPS, 방향 센서, CPU를 활용하여, 실시간으로 현장에서 하천의 표면유속을 측정하는 것이다. 먼저, 스마트폰의 GPS를 이용하여 측정 현장의 위치를 파악하고, 경사계(방향 센서)를 활용하여 카메라와 촬영면의 기하적인 관계를 설정한다. 이 때 입력해야 할 유일한 변수는 수면과 카메라의 연직 높이뿐이다. 내장된 카메라로 정해진 시간만큼 동영상을 촬영한다. 촬영된 동영상을 개방 소스의 영상처리 라이브러리인 OpenCV를 이용하여 프레임별로 분할하고, 이를 시공간 영상 분석하여 하천 표면의 2차원 유속장을 추정한다. 시판되는 안드로이드 스마트폰에 적용하여 현장 시험한 결과 약 11초에 1회의 순간유속 측정 (1초간의 평균유속 측정)을 할 수 있어, 현장에서 즉각적으로 하천 수표면의 표면유속을 측정할 수 있었다. 또한 이 순간유속을 수십회 반복한 뒤 평균하여 시간평균유속을 구할 수 있었다. 개발된 시스템을 실험 수로에서 시험한 결과, 측정이 매우 효과적이며 편리하였다. 측정된 결과를 프로펠러 유속계에 의한 측정값과 비교한 결과, 최대 오차 13.9%, 평균적으로 10 % 이내의 오차로 실험 수로의 표면 유속을 측정할 수 있었다.

• 한국태양에너지학회 논문집
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• 제35권1호
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• pp.35-43
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• 2015

A numerical simulation on the wake flow of a wind turbine which is a scaled version of a multi-megawatt wind turbine has been performed. Two different inlet conditions of averaged wind speed including one below and one above the rated wind speed were used in the simulation. Steady-state pitch angles of the blade associated with the two averaged wind speeds were imposed for the simulation. The steady state analysis based on the Reynolds averaged Navier-Stokes equations with the method of frame motion were used for the simulation to find the torque of the rotor and the wake field behind the wind turbine. The simulation results were compared with the results obtained from the wind tunnel testing. From comparisons, it was found that the simulation results on the turbine power are pretty close to the experimental values. Also, the wake results were relatively close to the experimental results but there existed some discrepancy in the shape of velocity deficit. The reason for the discrepancy is considered due to the steady state solution with the frame motion method used in the simulation. However, the method is considered useful for solutions with much reduced calculation time and reasonably good accuracy compared to the transient analysis.

• Ecology and Resilient Infrastructure
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• 제5권4호
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• pp.199-209
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• 2018

하천설계에서 제방과 홍수터의 안정성 확보를 위해서는 바닥전단응력을 고려하는 것이 필수적이다. 본 연구에서는 다양한 하폭과 수심에 따른 복단면의 바닥전단응력을 모의하여 분포 특성을 분석하였다. 바닥전단응력 분포에 지배적인 영향을 주는 이차류를 모의하기 위하여 OpenFOAM의 large eddy simulation (LES)를 적용하였으며 하폭과 수심을 고려하여 12개의 케이스를 모의하였다. 모의 결과를 이용하여 시간 평균 유속 분포, 이차류 분포, 바닥전단응력 분포 등의 특성에 대하여 분석하였다. 홍수터 바닥전단응력 분포는 전체적으로 홍수터 좌안에서 높은 값이 나타나고 우안 방향으로 감소하는 경향을 확인하였으나 이차류에 의해 상당한 국부적인 변화가 나타남을 확인하였다. 홍수터의 수심이 얕은 경우에는 홍수터의 바닥전단응력이 평균전단응력의 0.8배 이하로 낮은 값이 나타나고 있으나 홍수터의 수심이 깊은 경우에는 평균전단응력의 1.2-1.4배의 높은 값이 나타남을 확인하였다. 홍수터의 폭이 좁은 경우에는 홍수터 우안 측벽의 영향으로 국부적으로 높은 값이 나타나는 것도 확인할 수 있었다.

• 지구물리
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• 제4권1호
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• pp.21-33
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• 2001

한반도의 일차원 지각구조모델 결정을 위하여 1991년부터 1998년까지 한국 기상청 지진관측망에 기록된 국지지진의 P-파의 도달시각자료에 대해 동시역산방법을 적용하였다. 역산에는 29개 지진의 178개 관측자료가 이용되었다. 한반도의 지각과 상부맨틀의 평균속도는 각각 6.3km/sec, 7.9km/sec 이며, 평균 지각 두께는 약 33km로 나타났다. 또한, 하부지각에 저속도층의 가능성을 보여주었다. 동시역산 방법은 진원요소, 관측소 보정치, 속도구조를 동시에 결정하기 위해 적용된다. 11개 관측소에 대한 관측소 보정치는 0～1.2sec 정도의 차이를 보인다. Synthetic data의 분석은 평균 지각 속도구조 뿐만 아니라, 진앙의 위치, 관측소 보정치가 신뢰할 만하다는 것을 보여준다.

• 한국운동역학회지
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• 제26권1호
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• pp.71-82
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• 2016

Objective: The purpose of this study was to investigate kinematic comparisons of Tsukahara vault in gymnastics between a top-level athlete and sublevel collegiate athletes in order to obtain information on key biomechanical points for successful Tsukahara vaults. Methods: An Olympic gold medalist (height, 160 cm; weight, 52 kg; age, 25 years) and five sublevel collegiate gymnasts (height, $168.2{\pm}3.4cm$; weight, $59.6{\pm}3.1kg$; age, $23.2{\pm}1.6years$) participated in this study. They repeatedly performed Tsukahara vaults including one somersault. Fourteen motion-capturing cameras were used to collect the trajectories of 26 body markers during Tsukahara vaults. Event time, displacement and velocity of the center of mass, joint angles, the distance between the two hands on the horse, and averaged horizontal and vertical impact forces were calculated and compared. Results: The top-level athlete showed a larger range of motion (ROM) of the hip and knee joints compared to sublevel collegiate athletes during board contact. During horse contact, the top-level athlete had a narrow distance between the two hands with extended elbows and shoulders in order to produce a strong blocking force from the horse with a shorter contact time. At the moment of horse take-off, reactive hip extension of the top-level athlete enhanced propulsive take-off velocity and hip posture during post-flight phase. Conclusion: Even though a high velocity of the center of mass is important, the posture and interactive action during horse contact is crucial to post-flight performance and the advanced performance of Tsukahara vaults.

• 상하수도학회지
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• 제22권1호
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• pp.93-103
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• 2008

This study was conducted to optimize a unidirectional flushing program in distribution pipes by analysis of water pressure, velocity, quality, and other parameters during flushing. As a result, correlation coefficient between flushed pipe length and the flushing duration was obtained $R^2=0.83$ and the equation $Y_{Time}=0.0571{\cdot}X_{Pipe\;length}+4.7648$ for 10 pipes. The averaged flushing velocity in the pipes, 1.1 m/s, was enough to remove loose deposits on the inner wall of the pipes. 3 of 92 water samples taken during flushing met the National Drinking Water Quality Standard for Fe and Mn, but not for Al. Turbidity less than 1 NTU is suggested for the appropriate criteria to finish flushing in pipes. The coefficient of determination ($R^2$) between turbidity and TSS was 0.95 and the equation was induced as $Y_{TSS}=1.2068{\cdot}X_{Turbidity}$. The amount of removed deposits could be estimated from the turbidity data of discharged water in field because turbidity and TSS in the discharged water is highly correlated.

• 대한기계학회논문집
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• 제19권9호
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• pp.2328-2341
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• 1995

An experimental study on the flow over the axisymmetric backward-facing step was carried out. The purposes of the present study are to investigate the effect of the free stream turbulence intensity on the reattachment length and to understand the turbulence structure of the recirculating flows. Local mean and fluctuating velocity components were measured in the separated and reattaching axisymmetric turbulent boundary layer over the wall of convex cylinder placed in a water tunnel by using 2-color 4-beam fiber optics laser Doppler velocimetry. As the free stream turbulence intensity increased, the reattachment length became shorter due to the enhanced mixing in the separated shear layer. It was also observed that the reverse flow velocity and turbulent kinetic energy increase with increasing free stream turbulence intensity. Spectral data and flow visualization showed that low-frequency motions occur in the separated flow behind a backward-facing step. These motions have a significant effect on the time-averaged turbulence data.

• Wind and Structures
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• 제26권5호
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• pp.317-329
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• 2018

Wake characteristics of the flow over a finite square prism at different incidence angles were experimentally investigated using an open-loop wind tunnel. A finite square prism with a width D = 15 mm and a height H = 7D was vertically mounted on a horizontal flat plate. The Reynolds number was varied from $6.5{\times}10^3$ to $28.5{\times}10^3$ and the incidence angle ${\alpha}$ was changed from $0^{\circ}$ to $45^{\circ}$. The ratio of boundary layer thickness to the prism height was about ${\delta}/H=7%$. The time-averaged velocity, turbulence intensity and the vortex shedding frequency were obtained through a single-component hotwire probe. Power spectrum of the streamwise velocity fluctuations revealed that the tip and base vortices shed at the same frequency as that ofspanwise vortices. Furthermore, the results showed that the critical incidence angle corresponding to the maximum Strouhal number and minimum wake width occurs at ${\alpha}_{cr}=15^{\circ}$ which is equal to that reported for an infinite prism. There is a reduction in the size of the wake region along the height of the prism when moving away from the ground plane towards the free end.

• 대한기계학회논문집B
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• 제23권2호
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• pp.218-225
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• 1999

Flow visualization study has been conducted to simulate the turbulent dispersion behavior of a crossflow jet physically under the conditions of various thermal stratification in a wind tunnel. A smoke jet with the constant ratio of the jet to freestream velocity is injected normally to the cross flow of the thermally stratified wind tunnel(TSWT) for flow visualization. The typical natures of the smoke dispersion under different thermal stratifications such as neutral, weakly stable, strongly stable, weakly unstable, strongly unstable and inversion layer are successfully reproduced in the TSWT. The Instantaneous velocity and temperature fluctuations are measured by using a cold and hot-wire combination probe. The time averaged dispersion behaviors, the centerline trajectories, the spreading angles and the virtual origins of the cross jet are deduced from the edge detected images with respect to the stability parameter. All the general characteristics of the turbulent dispersion behavior reveal that the definitely different dispersion mechanisms are inherent in both stable and unstable conditions. It is conjectured that the turbulent statistics obtained in the various stability conditions quantitatively demonstrate the vertical scalar flux plays a key role in the turbulent dispersion behavior.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2002년도 학술대회지
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• pp.771-774
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• 2002

Highly sweep leading edge extensions(LEX) applied to delta wings have greatly improved the subsonic maneuverability of contemporary fighters. In this study, systematic approach by PIV experimental method within a circulating water channel was adopted to study the fundamental characteristics of induced vortex generation, development and its breakdown appearing on a delta wing model with or without LEX in terms of four angles of attack($15^{\circ},\;20^{\circ},\;25^{\circ},\;30^{\circ}$) and six measuring sections of chord length($30{\%},\;40{\%},\;50{\%},\;60{\%},\;70{\%},\;80{\%}$). Sideslip effect in case of the LEX was also studied for two sideslip(yaw) angles($5^{\circ},\;10^{\circ}$) at one angle of attack(20). Distribution of time-averaged velocity vectors and vorticity over the delta wing model were compared along the chord length direction. Quantitative comparison of the maximum vorticity featuring the induced pressure distribution were also conducted to clarify the significance of the LEX existence. Animation presentation in velocity distribution was also implemented to reveal the effect of LEX with wing vortex interaction.