• Title/Summary/Keyword: Turbulence model

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Galloping of overhead transmission lines in gusty wind

  • Ohkuma, Takeshi;Marukawa, Hisao
    • Wind and Structures
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    • v.3 no.4
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    • pp.243-253
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    • 2000
  • To develop galloping suppression devices, it is important to understand the effects of wind turbulence on galloping and to establish an evaluation method which takes 'large conductor deformations' into account. This paper introduces some findings on galloping in gusty wind obtained by numerical simulation using a model based on the Mogami Test Line of the Tokyo Electric Power Co. The equations of motion of the conductor are based on the Lagrangian formulations by Simpson, and they are made discrete in accordance with a finite element method.

A STUDY ON THE IMPROVEMENT OF κ-εTURBULENCE MODEL FOR PREDICTION OF THE RECIRCULATION FLOW (재순환유동 예측을 위한 κ-ε 난류모델 개선에 대한 연구)

  • Lee, Y.M.;Kim, C.W.
    • Journal of computational fluids engineering
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    • v.21 no.2
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    • pp.12-24
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    • 2016
  • The standard ${\kappa}-{\varepsilon}$ and realizable ${\kappa}-{\varepsilon}$ models are adopted to improve the prediction performance on the recirculating flow. In this paper, the backward facing step flows are used to assess the prediction performance of the recirculation zone. The model constants of turbulence model are obtained by the experimental results and they have a different value according to the flow. In the case of an isotropic flow situation, decaying of turbulent kinetic energy should follow a power law behavior. In accordance with the power law, the coefficients for the dissipation rate of turbulent kinetic energy are not universal. Also, the other coefficients as well as the dissipation coefficient are not constant. As a result, a suitable coefficients can be varied according to each of the flow. The changes of flow over the backward facing step in accordance with model constants of the ${\kappa}-{\varepsilon}$ models show that the reattachment length is dependent on the growth rate(${\lambda}$) and the ${\kappa}-{\varepsilon}$ models can be improved the prediction performance by changing the model constants about the recirculating flow. In addition, it was investigated for the curvature correction effect of the ${\kappa}-{\varepsilon}$ models in the recirculating flow. Overall, the curvature corrected ${\kappa}-{\varepsilon}$ models showed an excellent prediction performance.

Numerical Analysis of Thermal and Flow affected by the variation of rib interval and Pressure drop Characteristics (리브 간격 변화에 따른 열.유동 수치해석 및 압력 저하 특성)

  • Chung, Han-Shik;Lee, Gyeong-Wan;Shin, Yong-Han;Choi, Soon-Ho;Jeong, Hyo-Min
    • Journal of Advanced Marine Engineering and Technology
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    • v.35 no.5
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    • pp.616-624
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    • 2011
  • The flow characteristics and heat transfer augment on the periodically arranged semi-circular ribs in a rectangular channel for turbulent flow has been investigated numerically. The aspect ratio of the rectangular channel was AR=5, the rib height to hydraulic diameter ratio were 0.07 and rib height to channel height ratio was set as e/H=0.117 for various PR(rib pitch-to-rib height rate) between 8~14, respectively. The SST k-${\omega}$ turbulence model and v2-f turbulence model were used to find out the heat transfer and the flow characteristics of near the wall which are suited to obtain realistic phenomena. The numerical analysis results show turbulent flow characteristics, heat transfer enhancement and friction factor as observed experimentally. The results predict that turbulent kinetic energy(k) is closely relative to the diffusion of recirculation flow. and v2-f turbulence model simulation results have a good agreement with experimental values.

Numerical Simulation and PIV Measurement on the Internal Flow in a Centrifugal Mini Pump at Low Flow Rate Conditions

  • Yuan, Hui-Jing;Shao, Jie;Cao, Guang-Jun;Liu, Shu-Hong;Wu, Yu-Lin
    • Proceedings of the Korean Society of Propulsion Engineers Conference
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    • 2008.03a
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    • pp.775-780
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    • 2008
  • This paper reports on the internal flow of a centrifugal mini pump working at the low flow rate operating conditions. The RNG $\kappa-\varepsilon$ turbulence model was employed to simulate the three-dimensional turbulent flow in the pump. To examine and certify the simulation results, a transparent acrylic centrifugal mini pump model which is suitable for PIV measurement has been developed. The tongue region and the passages region between blades were investigated using PIV. In order to eliminate the effect of refraction on the area closed to the wall and increase the measurement accuracy, the fluorescent particles were scatted into the working fluid with the tracing particles. It is found from the calculation and PIV measurement results that there is a large area of recirculation flow near the tongue at low flow rate operating conditions. The computationally predicted water head using the $\kappa-\varepsilon$ turbulence model at low flow rate operating conditions are in very good agreement with the experimentally measured water head and the mean velocity distributions at investigation area obtained by PIV and calculation showed a satisfactory agreement as well. Meanwhile, the results of PIV measurements show that the flow status in one passage is different to another. And for capturing the internal flow detail information, the $\kappa-\varepsilon$ turbulence model is not very suitable.

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Numerical and wind tunnel simulation of pollutant dispersion in the near wake of buildings

  • Wang, X.;McNamara, K.F.
    • Wind and Structures
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    • v.8 no.6
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    • pp.427-442
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    • 2005
  • Numerical and wind tunnel simulations of pollutant dispersion around rectangular obstacles with five aspect ratios have been conducted in order to identify the effects of flow patterns induced by buildings on plume dispersion in the near wake of buildings. An emission from a low source located upwind of obstacles was used in this simulation. The local flow patterns and concentrations around a cubical obstacle were initially investigated using three RANS turbulence models, (the standard $k-{\varepsilon}$, Shear Stress Transport (SST), Reynolds-Stress RSM turbulence model) and also using Large-eddy simulation (LES). The computed concentrations were compared with those measured in the wind tunnel. Among the three turbulence models, the SST model offered the best performance and thus was used in further investigations. The results show, for normal aspect ratios of width to height, that concentrations in the near wake are appreciably affected because of plume capture by the horseshoe vortex and convection by the vertical vortex pairs. These effects are less important for high aspect ratios. Vertical vortex pairs present a strong ability to exchange mass vertically and acts efficiently to reduce ground-level concentrations in the near wake.

Numerical Simulation of Particle Deposition on a Wafer Surface (웨이퍼 표면상의 입자침착에 관한 수치 시뮬레이션)

  • 명현국;박은성
    • Transactions of the Korean Society of Mechanical Engineers
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    • v.17 no.9
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    • pp.2315-2328
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    • 1993
  • The turbulence effect of particle deposition on a horizontal free-standing wafer in a vertical flow has been studied numerically by using the low-Reynolds-number k-.epsilon. turbulence model. For both the upper and lower surfaces of the wafer, predictions are made of the averaged particle deposition velocity and its radial distribution. Thus, it is now possible to obtain local information about the particle deposition on a free-standing wafer. The present result indicates that the particle deposition velocity on the lower surface of wafer is comparable to that on the upper one in the diffusion controlled deposition region in which the particle sizes are smaller than $0.1{\mu}m$. And it is found in this region that, compared to the laminar flow case, the averaged deposition velocity under the turbulent flow is about two times higher, and also that the local deposition velocity at the center of wafer is high equivalent to that the wafer edge.

NUMERICAL SIMULATION OF SUPERSONIC FLOW USING POROUS AND ROUGH WALL BOUNDARY CONDITIONS (다공성 벽면(porous-wall)과 거칠기가 있는 벽면(rough-wall)에 과한 경계조건을 이용한 초음속 흐름의 수치모사)

  • Kwak, E.K.;Yoo, I.Y.;Lee, S.
    • 한국전산유체공학회:학술대회논문집
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    • 2009.04a
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    • pp.104-111
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    • 2009
  • The existing code which solves two-dimensional RANS(Reynolds Averaged Navier-Stokes) equations and 2-equation turbulence model equations was modified to enable numerical simulation of various supersonic flows. For this, various boundary conditions have been implemented to the code. Bleed boundary condition was incorporated into the code for calculating wall mean flow quantities. Furthermore, the boundary conditions for the turbulence quantities along rough surfaces as well as porous walls were applied to the code. The code was verified and validated by comparing the computational results against the experimental data for the supersonic flows over bleed region on a flat plate. Using the newly modified code, numerical simulations were performed and compared with other computational results as well as the experimental data for the supersonic flows over an oblique shock with a bleed region.

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Incompressible/Compressible Flow Analysis over High-Lift Airfoils Using Two-Equation Turbulence Models (2-방정식 난류모델을 이용한 고양력 익형 주위의 비압축성/압축성 유동장 해석)

  • Kim C. S.;Kim C. A.;Rho O. H.
    • Journal of computational fluids engineering
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    • v.4 no.1
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    • pp.53-61
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    • 1999
  • Two-dimensional, unsteady, incompressible and compressible Navier-Stokes codes are developed for the computation of the viscous turbulent flow over high-lift airfoils. The compressible code involves a conventional upwind-differenced scheme for the convective terms and LU-SGS scheme for temporal integration. The incompressible code with pseudo-compressibility method also adopts the same schemes as the compressible code. Three two-equation turbulence models are evaluated by computing the flow over single and multi-element airfoils. The compressible and incompressible codes are validated by predicting the flow around the RAE 2822 transonic airfoil and the NACA 4412 airfoil, respectively. In addition, both the incompressible and compressible code are used to compute the flow over the NLR 7301 airfoil with flap to study the compressible effect near the high-loaded leading edge. The grid systems are efficiently generated using Chimera overlapping grid scheme. Overall, the κ-ω SST model shows closer agreement with experiment results, especially in the prediction of adverse pressure gradient region on the suction surfaces of high-lift airfoils.

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Analysis of a Rim-Seal with a Semicircular Rib or Groove (반원형 리브 혹은 그루브가 부착된 림실에 대한 연구)

  • Lee, Chung-Suk;Moon, Mi-Ae;Kim, Kwang-Yong
    • The KSFM Journal of Fluid Machinery
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    • v.16 no.3
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    • pp.39-47
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    • 2013
  • Effect of semicircular rib or groove on the performance of a rim-seal was evaluated using three-dimensional Reynolds-averaged Navier-Stokes equations. The turbulence was modeled using the one-equation Spalart-Allmaras turbulence closure model. Reynolds number based on the axial chord of the turbine blade was 500,000 at the mainstream outlet. The numerical results for a sealing effectiveness was validated in comparison with experimental data. To examine the effect of the semicircular rib or groove on sealing effectiveness of the rim-seal, location and diameter of the rib or groove were selected as the parameters to be performed. The rim-seals with the semicircular groove showed a higher sealing effectiveness than that with the semicircular rib. The semicircular groove installed on the stator side showed best sealing effectiveness among the numerical simulation results.