• Title, Summary, Keyword: Eddy region

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COARSE GRID LARGE-EDDY SIMULATION OF FLOW OVER A HEAVY VEHICLE (화물차 주위 유동의 성긴 격자 큰에디모사)

  • Lee, S.;Kim, M.;You, D.;Kim, J.J.;Lee, S.J.
    • Journal of computational fluids engineering
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    • v.21 no.1
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    • pp.30-35
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    • 2016
  • In order to investigate effects of grid resolution on large-eddy simulation of flow over a heavy vehicle, large-eddy simulations over the vehicle with coarse grid and fine grid are conducted. In addition, comparison of drag coefficients with the experimental data obtained by a wind tunnel experiment is conducted. Both of the drag coefficients of coarse grid and fine grid large-eddy simulation show good agreement with the experimental data. Flow fields obtained by the coarse and the fine grid large-eddy simulation are compared in the vehicle frontal-face region, the vehicle rear wheel region, and the vehicle base region. Coarse grid large-eddy simulation shows good agreement with the fine grid large-eddy simulation in the vehicle front face region and the vehicle rear wheel region, since the flow over the present vehicle is dominated by flow separation which is geometrically pre-determined, not by the skin friction which is known to be sensitive to grid resolution.

DETACHED EDDY SIMULATION OF BASE FLOW IN SUPERSONIC MAINSTREAM (초음속 유동장에서 기저 유동의 Detached Eddy Simulation)

  • Shin, J.R.;Won, S.H.;Choi, J.Y.
    • 한국전산유체공학회:학술대회논문집
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    • pp.104-110
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    • 2008
  • Detached Eddy Simulation (DES) is applied to an axisymmetric base flow at supersonic mainstream. DES is a hybrid approach to modeling turbulence that combines the best features of the Reynolds-averaged Navier-Stokes (RANS) and large-eddy simulation (LES) approaches. In the Reynolds-averaged mode, the model is currently based on either the Spalart-Allmaras (S-A) turbulence model. In the large eddy simulation mode, it is based on the Smagorinski subgrid scale model. Accurate predictions of the base flowfield and base pressure are successfully achieved by using the DES methodology with less computational cost than that of pure LES and monotone integrated large-eddy simulation (MILES) approaches. The DES accurately resolves the physics of unsteady turbulent motions, such as shear layer rollup, large-eddy motions in the downstream region, small-eddy motions inside the recirculating region. Comparison of the results shows that it is necessary to resolve approaching boundary layers and free shear-layer velocity profiles from the base edge correctly for the accurate prediction of base flows. The consideration of an empirical constant CDES for a compressible flow analysis may suggest that the optimal value of empirical constant CDES may be larger in the flows with strong compressibility than in incompressible flows.

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DETACHED EDDY SIMULATION OF BASE FLOW IN SUPERSONIC MAINSTREAM (초음속 유동장에서 기저 유동의 Detached Eddy Simulation)

  • Shin, J.R.;Won, S.H.;Choi, J.Y.
    • 한국전산유체공학회:학술대회논문집
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    • pp.104-110
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    • 2008
  • Detached Eddy Simulation (DES) is applied to an axisymmetric base flow at supersonic mainstream. DES is a hybrid approach to modeling turbulence that combines the best features of the Reynolds-averaged Navier-Stokes RANS) and large-eddy simulation (LES) approaches. In the Reynolds-averaged mode, the model is currently based on either the Spalart-Allmaras (S-A) turbulence model. In the large eddy simulation mode, it is based on the Smagorinski subgrid scale model. Accurate predictions of the base flowfield and base pressure are successfully achieved by using the DES methodology with less computational cost than that of pure LES and monotone integrated large-eddy simulation (MILES) approaches. The DES accurately resolves the physics of unsteady turbulent motions, such as shear layer rollup, large-eddy motions in the downstream region, small-eddy motions inside the recirculating region. Comparison of the results shows that it is necessary to resolve approaching boundary layers and free shear-layer velocity profiles from the base edge correctly for the accurate prediction of base flows. The consideration of an empirical constant CDES for a compressible flow analysis may suggest that the optimal value of empirical constant CDES may be larger in the flows with strong compressibility than in incompressible flows.

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Mean Meridional Circulation-Eddy Interaction in Three Reanalysis Data Sets during the Boreal Winter (세 가지 재분석 자료에서의 겨울철 북반구 평균 자오면 순환-에디 상호작용)

  • Moon, Hyejin;Ha, Kyung-Ja
    • Atmosphere
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    • v.25 no.3
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    • pp.543-557
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    • 2015
  • The present study examines an interaction between the eddy and mean meridional circulation (MMC) comparing the results in three reanalysis data sets including ERA-Interim, NCEP2, and JRA-55 during the boreal winter in the Northern Hemisphere. It is noteworthy that the JRA-55 tends to produce stronger MMC compared to those of others, which is mainly due to the weak eddy flux. ERA-Interim represents the ensemble averages of MMC. The MMC-eddy interaction equation was adopted to investigate the scale interaction of the eddy momentum flux (EMF), eddy heat flux (EHF), and diabatic heating (DHT) with MMC. The EMF (EHF) shows a significant correlation coefficient with streamfunction under (above) 200 hPa-level. The perturbation (time mean) part of each eddy is dominant compared to another part in the EMF (EHF). The DHT is strongly interacted with streamfunction in the region between the equator and extra-tropical latitude over whole vertical column. Thus, the dominant term in each significant region modulates interannual variability of MMC. The inverse (proportional) relationship between MMC and pressure (meridional) derivative of the momentum (heat) divergence contributions is well represented in the three reanalysis data sets. The region modulated interannual variability of MMC by both EMF and DHT (EHF) is similar in ERA-Interim and JRA-55 (ERA-Interim and NCEP2). JRA-55 shows a lack of significant region of EHF due to the high resolution, compared to other data sets.

Application of the E-$\varepsilon$turbulence numerical model to a flow and dispersion around triangular ridge( I ) (E-$\varepsilon$모델을 이용한 삼각 봉우리 주변의 유동과 확산 수치해석(I))

  • 정상진
    • Journal of Korean Society for Atmospheric Environment
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    • v.10 no.2
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    • pp.116-123
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    • 1994
  • The E- $\varepsilon$ turbulence numerical model was applied to a flow around triangular ridge in neutral boundary layer. Scale of cavity region, mean velocity, Reynolds stress and eddy diffusivity were investigated. The height of cavity region was in satifactory agreement with the wind tunnel data while the length of cavity region was underestimated. The man wind velocities outside the cavity region were well Predicted by the model, however in cavity region the mean wind velocities of wind tunnel data were larger than the model results Reynolds stress of cavity region was overestimated by the model. The eddy diffusivity of wake region was strongly modified under the influence of triangular ridge. The local minimum of the eddy diffusivity was occured in the lee of the ridge top.

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A New k-$\varepsilon$ Model for Prediction of Transitional Boundary-Layer Under Zero-Pressure Gradient (압력 구배가 없는 평판 천이 경계층 유동을 예측하기 위한 k-$\varepsilon$모형의 개발)

  • Baek, Seong-Gu;Im, Hyo-Jae;Jeong, Myeong-Gyun
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.25 no.3
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    • pp.305-314
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    • 2001
  • A modified model is proposed for calculation of transitional boundary layer flows. In order to develop the eddy viscosity model for the problem, the flow is divided into three regions; namely, pre-transition region, transition region and fully turbulent region. The pre-transition eddy-viscosity is formulated by extending the mixing length concept. In the transition region, the eddy-viscosity model employs two length scales, i.e., pre-transition length scale and turbulent length scale pertaining to the regions upstream and the downstream, respectively, and a universal model of stream-wise intermittency variation is used as a function bridging the pre-transition region and the fully turbulent region. The proposed model is applied to calculate three benchmark cases of the transitional boundary layer flows with different free-stream turbulent intensity (1%∼6%) under zero-pressure gradient. It was found that the profiles of mean velocity and turbulent intensity, local maximum of velocity fluctuations, their locations as well as the stream-wise variation of integral properties such as skin friction, shape factor and maximum velocity fluctuations are very satisfactorily predicted throughout the flow regions.

Development of k-$\epsilon$ model for prediction of transition in flat plate under free stream with high intensity (고난류강도 자유유동에서 평판 경계층 천이의 예측을 위한 난류 모형 개발)

  • Baek, Seong Gu;Lim, Hyo Jae;Chung, Myung Kyoon
    • 유체기계공업학회:학술대회논문집
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    • pp.337-344
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    • 2000
  • A modified k-$\epsilon$ model is proposed for calculation of transitional boundary layer flows. In order to develop the eddy viscosity model for the problem, the flow is divided into three regions; namely, pre-transition region, transition region and fully turbulent region. The pre-transition eddy-viscosity is formulated by extending the mixing Length concept. In the transition region, the eddy-viscosity model employs two length scales, i.e., pre-transition length scale and turbulent length scale pertaining to the regions upstream and the downstream, respectively, and a university model of stream-wise intermittency variation is used as a function bridging the pre-transition region and the fully turbulent region. The proposed model is applied to calculate three benchmark cases of the transitional boundary layer flows with different free-stream turbulent intensity ( $1\%{\~}6\%$ ) under zero-pressure gradient. It was found that the profiles of mom velocity and turbulent intensity, local maximum of velocity fluctuations, their locations as well as the stream-wise variation of integral properties such as skin friction, shape factor and maximum velocity fluctuations are very satisfactorily Predicted throughout the flow regions.

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Large-scale structure of circular jet in transitional region at reynolds number of ${10}^{4}$ (Reynolds수 ${10}^{4}$일때 천이영역에서의 왼형제트의 Large-Scale 구조에 관한 연구)

  • 이택식;최은수
    • Transactions of the Korean Society of Mechanical Engineers
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    • v.10 no.6
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    • pp.823-829
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    • 1986
  • The Large-scale structure of the circular jet in the transition region, which influences the subsequent flow in the turbulent region, was studied experimentally. Measuring equipments are composed of the two channel hot-wire anemometer, the computer controlled two-directional traverse mechanism, the data acquisition system, and FFT-analyzer. The circular jet has 50mm diameter. The mean velocity distribution, the velocity fluctuation, the auto 'cross correlations and the power spectra were acquired at moderate Reynolds number of 10$^{4}$. And the VITA method was used to measure the convection velocity of Large-scale eddy. The phase of u'is in advance of that of v'in all regions. .over bar. $R_{u}$(.tau.=0) is approximately zero in the potential core region, but a small regular deviation is observed. At a position in the mixing layer region the convection velocity is different along the part of the eddy, and in this experiment the convection velocity of the inner region is larger than the outer region. The averge convection velocity of the eddy along y/D=0 was approximately constant in the transition region.D=0 was approximately constant in the transition region.

Analysis of Multiple Factor of the Eddy Current Brake for Railway Application (철도차량용 와전류 브레이크의 다중 인자 분석)

  • Lee, Chang-Mu;Park, Hyun-Jun;Cho, Sooyoung;Lee, Ju;Lee, Hyung-Woo
    • The Transactions of The Korean Institute of Electrical Engineers
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    • v.64 no.9
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    • pp.1385-1390
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    • 2015
  • This paper is analysis of multiple factor that should be considered in the design of an eddy current brake used as auxiliary brake system. The eddy current brake is a brake that generates a braking torque in a rotational direction opposite to the direction of the rotor by using a time-varying magnetic flux. The eddy current brake has the advantage of being able to take high current densities because this is used for a short period of time. Also, the eddy current brake is influenced by multiple factor such as number of slots, teeth width, coating thickness, air-gap length and so on. Therefore the eddy current brake was designed for use in railway application in consideration of the operation region and critical parameters.

Detached Eddy Simulation of Base Flow in Supersonic Mainstream (초음속 유동에서 기저유동의 Detached Eddy Simulation)

  • Shin, Jae-Ryul;Moon, Sung-Young;Won, Su-Hee;Choi, Jeong-Yeol
    • Journal of the Korean Society for Aeronautical & Space Sciences
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    • v.37 no.10
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    • pp.955-966
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    • 2009
  • DES method is applied to an axisymmetric base flow at supersonic mainstream. The model is based on the Spalart-Allmaras (S-A) turbulence model in the RANS mode, and is based on the subgrid scale model in the Large-eddy simulation (LES) mode. Accurate predictions of the base flowfield and base pressure are successfully achieved by using the DES methodology which is less expensive than LES. Flow properties at the edge of base, such as boundary layer thickness, momentum thickness and skin fraction are compared with Dutton et al [experimental data to proper prediction of base flowfiled. From the present results, The DES accurately resolves the physics of unsteady turbulent motions, such as shear layer rollup, large-eddy motions in the downstream region and small eddy motions inside the recirculating region. Moreover, The present results of using an empirical constant $C_{DES}$ of 1.2 shows good agreement with experimental data than conventional empirical constant $C_{DES}$ of 0.65.