• KSCE Journal of Civil and Environmental Engineering Research
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• v.13 no.4
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• pp.101-109
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• 1993

On the basis of Nikuradse laboratory experiments. two transition flow regimes are defined with respect to the characteristics of boundary layer. One is the transitional turbulent flow which has a transitional characteristics between smooth turbulent flow and rough turbulent flow, and the other may be called as transitional laminar flow which has transitional features between laminar flow and turbulent flow. The laboratory results of Nikuradse are carefully re-examined, and the flow regions are clearly defined. The velocity profile of the transitional turbulent flow is described by newly formulated equation, and the Darcy-Weisbach friction coefficient for the transitional turbulent flow is determined based on the theoretical form of profile equation, which is far better accurate than Colebrook-White equation. Difficulties still arise for the description of velocity profile when the flow undergoes from laminar to turbulent. In this case a linear interpolation procedure is proposed for the estimation of friction coefficient.

• Proceedings of the Korea Water Resources Association Conference
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• 2007.05a
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• pp.438-443
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• 2007

개수로 흐름에 대한 연구는 1700년대 중엽 Chezy에 의해 이론적인 기초를 다졌으며, 광범위하고 조직적인 관측연구는 Darcy(1803-1858)에 의해 약 150년 전에 폭 2m, 길이 600m에 이르는 수로에서 실험관측을 수행하고자 시도하였다. Darcy의 동료이자 후계자인 Bazin은 제한된 조건의 현장관측뿐 아니라 다양한 조건의 수로를 제작하여 실험관측을 수행하였으며 그의 실험자료는 Bazin 자신 뿐만 아니라 Manning이나 Ganguillet와 Kutter 등 여러 연구자들의 경험식 개발에도 이용되었다. Nikuradse(1933)의 균일조도 원형관수로 실험결과로부터 관로 흐름은 층류, 천이층류, 완난류, 천이난류, 전난류 등 다섯 종류의 흐름특성을 가지고 있음을 확인하였고 Bazin(1829-1917)과 varwick(1945)의 실험결과로부터 개수로 흐름에서도 관로 흐름과 마찬가지로 층류, 천이층류, 완난류, 천이난류, 전난류 등 다섯가지의 흐름특성이 존재함을 알 수 있다. 본 연구에서는 기존의 지수형 마찰계수 산정식에서 단순히 조고만의 함수였던 ${\alpha}$에 물의 기본적인 성질인 표면장력, 점성력, 밀도와 자연 하천의 경사, 수심, 수면, 폭, 조고의 영향을 고려한 수심에 관한 무차원수 $Y_h$, 수면 폭에 관한 무차원수 $Y_b$, 조고에 관한 무차원수 $Y_k$를 도입하였다. 따라서 개수로 흐름 해석에 있어서 기존의 마찰계수 산정치보다 여러 영향을 반영하여 정확한 값을 산정할 수 있을 것으로 판단된다.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.13 no.5
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• pp.165-172
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• 1993

On the basis of Nikuradse laboratory experiments conducted in circular pipe with uniform roughness, five flow regimes are defined with respect to the characteristics of boundary layer such as laminar, transition laminar, smooth turbulent, transition turbulent and rough turbulent flows. Two cases are found for the transition laminar flow: one for the transition between laminar flow and smooth turbulent flow and the other for the one between laminar flow and rough turbulent flow. They all can be clearly determined by the relative roughness or the ratio of pipe diameter to the roughness. Explicit functions are developed for the estimation of pipe friction factor for the various flow conditions including turbulent flow regimes, which have excellent agreement with the Nikuradse laboratory data.

• Transactions of the Korean Society of Mechanical Engineers
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• v.16 no.12
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• pp.2368-2375
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• 1992

The transition of turbulent kinetic energy(TKE) balance along the centerline of the flow unit, which is composed of straight-duct, contraction and free-jet, has been investigated by the hot-wire anemometry. It is found that the mean turbulent kinetic energy is balanced by the dissipation in the internal flow region ; by the production and the dissipation, through contraction ; and by the dissipation, in initial region(X〈8D) of free-jet. But in the developing region (8D〈X〈20D) it is balanced by all of the three(ie, diffusion, production and dissipation). Finally, in the downstream of free-jet, the mean TKE is balanced again by dissipation like as the beginning. The decay-laws along the centerline are checked in the region of free jet as well as in the straightduct. After the developing region of free-jet also exist the decay-laws, the exponent of the axial turbulence being bigger than of the radial.

• Transactions of the Korean Society of Mechanical Engineers
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• v.13 no.6
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• pp.1257-1268
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• 1989

본 연구에서는 이에 천이와 난류모델을 첨가하여 층류 경계층이 천이를 거쳐 난류 경계층으로 발달해가는 전과정을 해석하려고 한다. 또, 이러한 부분 면파의 특성을 needle contact법에 의하여 측정 포물형이나 완전 Navier-Stokes 방정식을 사용함에 있어서 표면곡률항의 중요성을 보이며, 익형의 선단등 표면곡률이 중요한 영역에서도 적용가능한 수치적 방법을 제시한다.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.48 no.1
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• pp.23-33
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• 2020

In this study, the effect of surface roughness distribution and its influence on the inflight icing code was investigated. Previous numerical studies focused on the magnitude of surface roughness, and the effects were only addressed in terms of changes in thermal boundary layers with fully turbulent assumption. In addition, the empirical formula was used to take account the turbulent transition due to surface roughness, which was regarded as reducing the accuracy of ice shape prediction. Therefore, in this study, the turbulent transition model based on the two-equation turbulence model was applied to consider the effects of surface roughness. In order to consider the effect of surface roughness, the transport equation for roughness amplification parameter was applied, and the surface roughness distribution model was implemented to consider the physical properties. For validation, the surface roughness, convective heat transfer coefficient, and ice shape were compared with experimental results and other numerical methodology. As a result, it was confirmed that the excessive prediction of the heat transfer coefficient at the leading edge and the ice horn shape at the bottom of the airfoil were improved accordingly.

• Journal of Energy Engineering
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• v.12 no.2
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• pp.131-138
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• 2003

This paper considered the structural mechanism of transitional boundary layer by the experimental approach. In order to measure the turbulence quantity in the boundary layer, we made a wind tunnel with 400${\times}$190${\times}$2500 mm test section and a flat plate with well fabricated leading edge. Hot wire anemometer was used for acquiring the continuous turbulence signal which is processed by special software. The results of experiment show that the region where turbulence spot is dominant moves from near wall to overall layer and thus the anisotropy of velocity fluctuation shows so large value. Also the turbulence energy originally contained in low frequency band comes up to the high frequency band. Finally the turbulence model needs minimum two length scales to consider the pre-transition region.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.40 no.7
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• pp.435-440
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• 2016

Effects of the increase rate of Reynold number on near-wall turbulent structures are investigated by performing direct numerical simulations of transient turbulent channel flows. The simulations were started with the fully-developed turbulent channel flow at $Re_{\tau}=180$, then temporal accelerations were applied. During the acceleration, the Reynolds number, based on the channel width and the bulk mean velocity, increased almost linearly from 5600 to 13600. To elucidate the effects of flow acceleration rates on near-wall turbulence, a wide range of durations for acceleration were selected. Various turbulent statistics and instantaneous flow fields revealed that the rapid increase of flow rate invoked bypass-transition like phenomena in the transient flow. By contrast, the flow evolved progressively and the bypass transition did not clearly occur during mild flow acceleration. The present study suggests that the transition to the new turbulent regime in transient channel flow is mainly affected by the flow acceleration rate, not by the ratio of the final and initial Reynolds numbers.

• Proceeding of EDISON Challenge
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• 2014.03a
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• pp.533-538
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• 2014

NACA0012형상을 사용하여 천이현상을 고려한 유동해석이 Dynamic Stall에 미치는 영향에 대해 살펴보았다. 천이 현상을 고려한 진동하는 익형의 공력 계수 변화를 비교하기 위해 완전 난류로 가정한 해석 결과와 실험 결과를 비교하였다. NACA0012익형을 사용하였으며 두께 변화에 따른 해석 결과를 비교하기 위해 NACA0009익형도 사용하였다. 정상유동해석의 천이 발생 지점과 비정상 유동해석의 천이 발생 지점을 비교하였으며, 익형주위의 유동결과를 비교하여 천이해석과 완전 난류해석 결과의 차이를 분석하였다.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.39 no.10
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• pp.795-803
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• 2015

The boundary layer developing over the suction surface of a first-stage turbine blade for power generation has been investigated in this study. For three locations selected in the region where local thermal load changes dramatically, mean velocity, turbulence intensity, and one-dimensional energy spectrum are measured with a hot-wire anemometer. The results show that the suction-surface boundary layer suffers a transition from a laminar flow to a turbulent one. This transition is confirmed to be a "separated-flow transition", which usually occurs in the shear layer over a separation bubble. The local minimum thermal load on the suction surface is found at the initiation point of the transition, whereas the local maximum thermal load is observed at the location of very high near-wall turbulence intensity after the transition process. Frequency characteristics of turbulent kinetic energy before and after the transition are understood clearly from the energy spectrum data.