• Proceedings of the Korea Water Resources Association Conference
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• 2006.05a
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• pp.594-597
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• 2006

조.세립상(組.細粒床) 대상(帶狀)연속구조를 갖는 개수로 흐름의 부유사 농도분포를 수치모의 하였다. 흐름의 지배방정식인 Navier-Stokes 방정식은 레이놀즈응력모형을 이용하여 해석되었으며, 와점성계수 개념을 이용하여 부유사 수송 방정식을 해석하였다. 기존의 실험결과와 비교하여 모형의 검증을 실시하였으며, 동일한 흐름에 대하여 부유사 농도 분포를 계산하였다. 부유사량의 계산 결과 매끈한 하상 위의 노도분포가 거친하상 위 보다 크게 나타났다. 또한 수치모의로부터 산정된 와확산계수를 Wang and Cheng(2005)이 제안한 해석적 방법과 비교 하였다.

• Journal of computational fluids engineering
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• v.2 no.2
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• pp.9-17
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• 1997

The performance of several turbulence models in computing an axisymmetric supersonic base flow is investigated. A compressible Navier-Stokes code, which incorporates k-ε, k-ω model and Reynolds stress closure with three kinds of pressure-strain correlation model, has been developed using implicit LU-SGS algorithm with second-order upwind TVD scheme. Numerical computations have been carried out for Herrin and Dutton's base flow. It is observed that the two-equation models give large backward axial velocity approaching to the base and somewhat larger variation of base pressure distribution than the Reynolds stress model. It is also found that the Reynolds stress model with third order pressure-strain model in the anisotropy tensor predicts most accurate mean flow field.

• Transactions of the Korean Society of Mechanical Engineers
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• v.17 no.6
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• pp.1596-1608
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• 1993

Low Reynolds number second moment turbulence model which be applicable to the fine gird near the wall region was developed. In this model, turbulence model coefficients in the pressure strain model of the Reynolds stress equation was expressed as functions of turbulence Reynolds number $R_{t}\equivk^{2}/(\nu\varepsilon)).$ In the derivation procedure of the present low Reynolds number algebraic stress model, Laufer's near wall experimental data on Reynolds stresses were curve fitted as functions of R$_{t}$ and the resulting simultaneous equations of the model coefficients were solved by using the boundary conditions at wall and high Reynolds number limiting conditions. Predicted Reynolds stresses and dissipation rate of turbulent kinetic energy etc. in the 2 dimensional parallel, plane channel flow and pipe flow were compared with the preditions obtained by employing the Launder-Shima model, standard algebraic stress model and several experimental data. Results show that all the Reynolds stresses and dissipation rate of turbulent kinetic energy predicted by the present low Reynolds number algebraic stress model agree better with the experimental data than those predicted by other algebraic stress models.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.34 no.3
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• pp.813-820
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• 2014

This study presents a numerical modeling of mean flow and turbulence structures of partly-vegetated open-channel flows. For this, Reynolds-averaged Navier-Stokes equations with vegetation drag terms are solved numerically using the non-linear k-${\varepsilon}$ model. The numerical model is applied to laboratory experiments of Nezu and Onitsuka (2001), and simulated results are compared with data from measurement and computations by Kang and Choi's (2006) Reynolds stress model. The simulation results indicate that the proposed numerical model simulates the mean flow well. Twin vortices are found to be generated at the interface between vegetated and non-vegetated zones, where turbulence intensity and Reynolds stress show their maximums. The model simulates the pattern of the Reynolds stress well but under-predicts the intensity of Reynolds stress slightly.

• Journal of Korea Water Resources Association
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• v.43 no.9
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• pp.823-833
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• 2010

Open-channel flows with submerged vegetation show two distinct flow structures in the vegetation and upper layers. That is, the flow in the vegetation layer is featured by relatively uniform mean velocity with suppressed turbulence from shear, while the flow in the upper layer is akin to that in the plain open-channel. Due to this dual characteristics, the flow has drawn many hydraulic engineers' attentions. This study compares layer-averaged models for flows with submerged vegetation. The models are, in general, classified into two-layer and three-layer models. The two-layer model divides the flow depth into vegetation and upper layers, while the three-layer model further divides the vegetation layer into inner and outer vegetation layers depending on the influence of the bottom roughness. This study compares the two-layer model and the three layer-model. It is found that the two-layer model predicts better the average value of the velocity and the prediction by the three-layer model is sensitive to Reynolds shear stress. In the three-layer model, the mean flow in the inner vegetation layer does not affect the flow seriously, which motivates the proposal of the modified two-layer model. The two-layer model, capable of predicting non-uniform mean velocity, is based on the Reynolds stress which is linear and of power form in the upper and vegetation layers, respectively. Application results reveal that the modified two-layer model predicts the mean velocity at an accuracy similar to the two- and three-layer models, but it predicts poorly in the case of very low vegetation density.

• Transactions of the Korean Society of Mechanical Engineers
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• v.19 no.8
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• pp.2051-2063
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• 1995

An improved version of nonlinear low-Reynolds-number k-.epsilon. model is developed. In this model, the limiting near-wall behavior and nonlinear Reynolds stress representations are incorporated. Emphasis is placed on the adoption of Ry(.iden. $k^{1}$2/y/.nu.) instead of $y^{[-10]}$ (.iden. $u_{{\tau}/y/{\nu}}$) in the low-Reynolds-number model for predicting turbulent separated and reattaching flows. The non-equilibrium effect is examined to describe recirculating flows away from the wall. The present model is validated by doing the benchmark problem of turbulent flow behind a backward-facing step. The predictions of the present model are cross-checked with the existing measurements and DNS data. The model performance is shown to be generally satisfactory.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.29 no.1B
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• pp.73-81
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• 2009

This paper presents a numerical investigation of the impact of the secondary currents on solute transport in open-channel flows. The RANS model with Reynolds stress model is used for flow modeling, and the GGDH(generalized gradient diffusion hypothesis) model is used to close the scalar transport equation. Using the developed model, the impact of secondary currents on solute transport in open channel flows over smooth-rough strip is investigated. Through numerical experiments, the secondary currents are found to affect the solute spreading, leading a movement of the position of the peak concentration and a skewed distribution of solute concentration. Due to the lateral flow of secondary currents near the free surface, the concentration at the rough strip is found to be larger than that at the smooth strip bed. The solute at the rough strip is more rapidly transported than smooth bed. A magnitude analysis of the solute transport rate in scalar transport equation is also carried out to investigate the effect of secondary currents and scalar flux on the concentration distribution.

• Proceedings of the Korea Water Resources Association Conference
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• 2019.05a
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• pp.93-93
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• 2019

식생이 있는 개수로 흐름의 분석이 중요한 이유는 식생이 흐름 특성과 유사이송 및 하상변동에 영향을 주어 복잡한 흐름 현상을 만들어 내기 때문이다. 식생 밀도, 식생 길이, 식생이 식재된 형태에 따라 흐름의 현상이 달라지며, 흐름 특성의 변화로 인해 하상 및 주변 하천구조물에 영향을 줄 수 있다. 하천의 식생은 대부분 폭과 길이가 제한된 모습으로 존재하는 경우가 많으며, 식생이 존재하는 영역에서는 식생항력에 의해 유속이 감소하고 식생이 없는 영역에서 유속이 증가한다. 식생항력에 의해 변화된 흐름 특성은 일정 거리를 지나면 평형상태에 도달하게 되며, 식생의 경계면에서는 전단층이 발달하여 모멘텀의 교환이 활발하게 발생하는 현상을 보인다. 식생 흐름의 특성을 분석하기 위해 수치모형으로는 식생 흐름의 강한 비등방성을 모의할 수 있는 비등방성 수치모형이 적합하다. 본 연구에서는 부분 식생이 식재된 개수로에서 RSM (Reynolds stress model)을 이용하여 식생과 식생 주변의 흐름 특성 및 난류량을 분석하였다. 흐름의 변화에 따라 크게 3 구간으로 나누어 흐름 특성을 파악할 수 있으며, 식생의 밀도와 식생 영역의 폭의 길이에 따라 각 구간의 길이가 달라지는 것을 확인하였다.

• Transactions of the Korean Society of Mechanical Engineers
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• v.18 no.5
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• pp.1203-1217
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• 1994

In the present study, in order to analyze a turbulent flow in a square sectiond 180.deg. bend, Kim's low Reynolds number second moment turbulence closure is adopted. In this model, turbulence model constants in the wall region are modified as functions of turbulent Reynolds number by use of near wall turbulent universal properties based on Laufer's experimental results of Reynolds stress distriburions. Algebraic stress model and Reynolds stress equation model are used to verify the low Reynolds number second moment closure. The application of the present low Reynolds number algebraic stress model to the prediction of a square sectioned 180.deg. bend flow gives improved velocities and Reynolds stresses profiles compared with those obtained by using the van Driest mixing length model and present low Reynolds number Reynolds stress equation model.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.20 no.8
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• pp.2572-2592
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• 1996

A low-Reynolds-number second moment turbulence closure was developed with the aid of DNS data. Model coefficients of nonlinear return to isotropy term were derived by use of Cayley-Hamilton theorem and two component turbulence limit condition as the functions of invariances of anisotropy and turbulent Reynolds number. Launder and Tselepidakis' cubic mean pressure strain model was modified to fit the predicted pressure-strain components to the DNS data. Two component turbulence limit condition was the precondition to be satisfied in developing the second moment turbulence closure for the realizable Reynolds stress prediction. But the satisfactions of Reynolds stress level and pressure-strain level of each component were compromised because the satisfaction of both levels was impossible.