• 대한기계학회:학술대회논문집
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• 대한기계학회 2000년도 춘계학술대회논문집B
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• pp.508-513
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• 2000

An experimental investigation has been carried out for two circular cylinders having different groove configurations(U and V-shape). The results were compared with those for the smooth circular cylinder. The drag force, mean velocity and turbulent intensity profiles of wake behind the cylinders were measured with varying the Reynolds number $Re=8000{\sim}14,000$ based on the cylinder diameter. As a results, the U-groove circular cylinder was found to be most effective riblet shape with reducing the drag up to 21%. As the Reynolds number increases, the vortex shedding frequency of the grooved cylinders becomes a little larger, compared to the smooth cylinder. The flow visualization using the smoke-wire technique was also carried out to see the flow structure qualitatively.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 추계학술대회논문집B
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• pp.306-311
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• 2001

Effect of boundary layer thickness on the flow characteristics around a rectangular prism has been investigated by using a PIV(Particle Image Velocimetry) technique. Three different boundary layers(thick, medium and thin)were generated in the Atmospheric Boundary Layer Wind Tunnel at Pusan National University. The thick boundary layer having 670mm thickness was generated by using spires and roughness elements. The medium thickness of boundary layer$(\delta=270mm)$ was the natural turbulent boundary layer at the test section with fully long developing length(18m). The thin boundary layer with 36.5mm thickness was generated by on a smooth panel elevated 70cm from the wind tunnel floor. The Reynolds number based on the free stream velocity and the height of the model was $7.9{\times}10^3$. The mean velocity vector fields and turbulent kinetic energy distribution were measured and compared. The effect of boundary layer thickness is clearly observed not only in the length of separation bubble but also in the reattachment points. The thinner boundary layer thickness, the higher turbulent kinetic energy peak around the model roof. It is strongly recommended that the height ratio between model and approaching boundary layer thickness should be a major parameter.

• 소음진동
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• 제11권2호
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• pp.226-233
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• 2001

It is essential to analyze structural vibrations due to turbulent wall pressure fluctuations over a body surface which moves through a fluid, because the vibrations can be a severe source of noise affecting to passengers in airplanes and SONAR performance. Generally, this kind of problems have been solved for very simplified models, e.g. plates, which can be applied to the wavenumber domain analysis. In this paper, a finite element modeling of the walt pressure fluctuations is investigated, which can be applied to those over arbitrary smooth surfaces. It is found that the modeled wall pressure fluctuation at nodes becomes uncorrelated at higher frequencies and at lower flow speeds, and the response is over-estimated due to the aliased power. Then the frequency range available for uncorrelated loading model and two power correction schemes are presented.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2000년도 춘계학술대회논문집
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• pp.817-824
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• 2000

It is essential to analyze structural vibrations due to turbulent wall pressure fluctuations over a body surface which moves through a fluid, because the vibrations can be a severe source of noise affecting to passengers in airplanes and SONAR performance. Generally, this kind of problems have been solved for very simplified models, e.g. plates, which can be applied to the wavenumber domain analysis. In this paper, a finite element modeling of the wall pressure fluctuations over arbitrary smooth surfaces is investigated. It is found that the modeled wall pressure fluctuation at nodes becomes uncorrelated at higher frequencies and at lower flow speeds, and the response is over-estimated due to the aliased power. Finally, the frequency range available for uncorrelated loading model and two power correction schemes are presented.

• 한국콘텐츠학회논문지
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• 제16권4호
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• pp.614-623
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• 2016

스마트워터그리드의 중요한 요소기술로써 그리드(관망)의 설계는 압력, 마찰계수, 마찰속도, 수두손실, 그리고 에너지 경사와 같은 수리학적 매개변수를 추정하는 것이 필수적이다. 특히, 그리드의 마찰속도는 에너지 경사, 마찰계수, 압력, 수두손실 등의 결합에 있어 매우 중요한 인자이다. 그러나 마찰 수두손실, 마찰속도 등 마찰인자를 정확히 산정하는 것은 매우 어려우며, 경험적 마찰 인자는 여전히 약 100년 전에 개발된 공식과 이론을 사용함으로써 산정된다. 따라서, 본 논문에서는 최대유속과 마찰속도 사이의 새로운 공식을 Darcy-Weisbach의 마찰수두 손실공식과 Schlichting 공식 사이의 적분관계 및 회귀분석을 통하여 개발하였다. 개발된 공식을 증명하기 위하여 매끄러운 관 자료가 사용되었으며 제안한 공식은 관측 자료와 비교하여 높은 정확성을 보여준다. 이 연구의 결과는 안정성 향상과 그리드 설계에 사용이 가능할 것으로 판단된다.

• 대한기계학회논문집B
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• 제27권3호
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• pp.319-325
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• 2003

We investigate the spectra and the pseudospectra in plane Poiseuille flow, plane Couette flow and Blasius flow. At subcritical Reynolds number, the spectra are lied strictly inside the stable complex half-plane, but the pseudospectra are lied in the unstable half-plane, reflecting the large linear transient growth that certain perturbations may excite. It means that the smooth flows may become to turbulent even though all the eigenmodes decay monotonically. We found that pseudospectra is one reason that causes subcritical transition in plane Poiseuille flow and plane Couette flow and bypass transition in Blasius flow.

• Wind and Structures
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• 제35권1호
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• pp.1-20
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• 2022

This research work presents an experimental study's outcomes to reveal the impact of an O-ring on the flow control over a sphere placed in a turbulent boundary layer. The investigation is performed quantitatively and qualitatively using particle image velocimetry (PIV) and dye visualization. The sphere model having a diamater of 42.5 mm is located in a turbulent boundary layer flow over a smooth plate for gap ratios of 0≤G/D≤1.5 at Reynolds number of 5 × 10³. Flow characteristics, including patterns of instantaneous vorticity, streaklines, time-averaged streamlines, velocity vectors, velocity fluctuations, Reynolds stress correlations, and turbulence kinetic energy (), are compared and discussed for a naked sphere and spheres having O-rings. The boundary layer velocity gradient and proximity of the sphere to the flat plate profoundly influence the flow dynamics. At proximity ratios of G/D=0.1 and 0.25, a wall jet is formed between lower side of the sphere and flat plate, and velocity fluctuations increase in regions close to the wall. At G/D=0.25, the jet flow also induces local flow separations on the flat plate. At higher proximity ratios, the velocity gradient of the boundary layer causes asymmetries in the mean flow characteristics and turbulence values in the wake region. It is observed that the O-ring with various placement angles (𝜃) on the sphere has a considerable alteration in the flow structure and turbulence statistics on the wake. At lower placement angles, where the O-ring is closer to the forward stagnation point of the sphere, the flow control performance of the O-ring is limited; however, its impact on the flow separation becomes pronounced as it is moved away from the forward stagnation point. At G/D=1.50 for O-ring diameters of 4.7 (2 mm) and 7 (3 mm) percent of the sphere diameter, the -ring exhibits remarkable flow control at 𝜃=50° and 𝜃=55° before laminar flow separation occurrence on the sphere surface, respectively. This conclusion is yielded from narrowed wakes and reductions in turbulence statistics compared to the naked sphere model. The O-ring with a diameter of 3 mm and placement angle of 50° exhibits the most effective flow control. It decreases, in sequence, streamwise velocity fluctuations and length of wake recovery region by 45% and 40%, respectively, which can be evaluated as source of decrement in drag force.

• 한국수자원학회논문집
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• 제33권6호
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• pp.815-825
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• 2000

Einstein(1942)의 부유사량 산정식 유도과정에 있어 몇 가지 문제점들을 개선하였다. 수류조건을 판별하여 완난류 조건에서 유속의 영점높이를 산정하는 방법을 제시하고 부유사량 산정식을 일반화할 수 있는 무차원수를 도입하였다. 또한, 기준점 농도 산정의 정확성을 개선하기 위하여 최근에 개발된 소류사량 산정식으로부터 관련치를 추정하였으며, 수치적으로 또는 도표를 이용하여 해를 구해야 하는 수심적분치의 근사식을 개발하였다. Einstein의 기존 부유사량 산정식과 개선된 부유사량 산정식의 정확도를 검토하여 Brownlie(1981)가 수집한 자료에 대한 산정결과를 제시하였다.

• 동력기계공학회지
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• 제4권1호
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• pp.26-32
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• 2000

Single-phase heat transfer performance and pressure drop for internally grooved tubes with angles were studied. Experiments were carried out in a counter flow heat exchanger with water as a working fluid. Two commercially available internally grooved tubes and smooth tube were tested. The internal diameter of the smooth tube was 16.5mm and the internal diameters of grooved tubes were 15.4mm, 14.9mm, 15.0mm, 16.7mm, respectively. Grooved angles in the tubes were $37^{\circ},\;43^{\circ},\;45^{\circ},\;50^{\circ}$, respectively. An experimental device to measure the friction factor and heat transfer coefficient was constructed. The experimental results were obtained for the fully developed turbulent flow of water in tube on the condition of uniform heat flux. As the increase of flow rate, Reynolds number, numbers of groove and grooved angle led to the increase of pressure drop. Also this paper showed that heat transfer rate increased with increasing numbers of groove and grooved angle. An empirical relation taken from this study represented most of the data within ${\pm}25%$.

• 한국수자원학회논문집
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• 제33권2호
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• pp.195-205
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• 2000

도달시간 산정에 있어 고려되어야 할 요소에는 유역형상과 흐름상태, 유출특성 등이 있다. 한편 Singh(1976)은 지표면흐름을 Kinematic Wave 이론으로 해석하였으며 유역형상을 평면형상과 수렴형상, 발산형상으로 분류하여 유역형상에 따른 도달시간 산정식을 제시하였다. 본 논문에서는 Singh이 제시한 평면형상에서의 도달시간 산정식에 기초하여 불투수 지표면 유출에서의 도달시간 산정을 다루었다. 이를 위한 이론식 유도는 흐름을 개수로 흐름(층류, 완난류, 전난류)으로 파악하여 수행하였고 각 개수로 흐름상태에 대한 이론식을 제시하였다. 또한 강우강도를 주요 인자로 고려하여 재현기간별 강우강도를 포함한 근사식 즉 복합형 도달시간 산정식을 개발하였다.