• Title/Summary/Keyword: Turbulent fluxes

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Observation and Analysis of Turbulent Fluxes Observed at Ieodo Ocean Research Station in Autumn 2014 (2014년 가을철 이어도 종합과학기지에서의 난류 플럭스의 관측 및 분석)

  • Yun, Junghee;Oh, Hyoeun;Ha, Kyung-Ja
    • Atmosphere
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    • v.25 no.4
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    • pp.707-718
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    • 2015
  • This study investigates the characteristics of turbulent fluxes observed at Ieodo Ocean Research Station (IORS) in autumn 2014. The 10 Hz IORS data is quality controlled and calculated to be the 30 minutes turbulent fluxes. The quality control consists of five steps: a weather check, Vickers and Mahrt (VM) sequential check, VM parallel check, flag check, and direction check. Since the IORS is an open-sea station with no orographic influence, there are no significant diurnal variations for the turbulent fluxes and 10 m wind speed. According to stabilities, the unstable and semi-unstable states appear more than 28% and 70% in autumn, respectively and they have strong winds of over $10m\;s^{-1}$. In addition, the turbulent fluxes increase with increasing wind speed. In particular, the latent heat flux and its deviations are clearly shown because the latent heat flux is influenced by the change of both the sea surface roughness and wave height induced by the wind. To demonstrate the changes of the turbulent fluxes before and after typhoon, Vongpong (1419), which is the most intense typhoon affecting the Korean Peninsula in 2014, is considered. The turbulent flux fluctuates in accordance with the location of Vongpong. The turbulent fluxes have a large (small) variation when Vongpong approaches (retreats) at the IORS. The overall results represent that the IORS data helps us understand physical processes related to air-sea interaction by providing the valuable and reliable observed data.

Seasonal Characteristics of Turbulent Fluxes Observed at leodo Ocean Research Station (이어도 종합해양과학기지에서 관측된 난류 플럭스의 계절적 특성)

  • Oh, Hyun-Mi;Ha, Kyung-Ja;Shim, Jae Seol;Hyun, Yu-Kyung;Yun, Kyung-Sook
    • Atmosphere
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    • v.17 no.4
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    • pp.421-433
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    • 2007
  • We have investigated the seasonal characteristics of surface turbulent fluxes observed at Ieodo Ocean Research Station from 2005 to 2006. Both 10Hz and 30 minutes flux data are quality controled, and tilt correction is performed in 10Hz data before quality control. The turbulent fluxes of open sea shows clear seasonal variations, though diurnal variations are barely shown. The seasonal ratio of stable and unstable conditions are closely related to the temperature difference between sea surface and air. In stable and semi-stable condition, latent and sensible heat fluxes have very small values without any relationship with wind speed. Though friction velocity shows slightly increasing trend with wind speed, it has many outliers. In unstable condition, turbulent fluxes increased with wind speed. Especially, latent heat flux increased rapidly during DJF. The latent heat flux at high wind speeds is more scatter.

Effects of Different Averaging Operators on the Urban Turbulent Fluxes (평균 방법이 도시 난류 플럭스에 미치는 영향)

  • Kwon, Tae Heon;Park, Moon-Soo;Yi, Chaeyeon;Choi, Young Jean
    • Atmosphere
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    • v.24 no.2
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    • pp.197-206
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    • 2014
  • The effects of different averaging operators and atmospheric stability on the turbulent fluxes are investigated using the vertical velocity, air temperature, carbon dioxide concentration, and absolute humidity data measured at 10 Hz by a 3-dimensional sonic anemometer and an open-path $CO_2/H_2O$ infrared gas analyzer installed at a height of 18.5 m on the rooftop of the Jungnang KT building located at a typical residential area in Seoul, Korea. For this purpose, 7 different averaging operators including block average, linear regression, and moving averages during 100 s, 300 s, 600 s, 900 s, and 1800 s are considered and the data quality control procedure such as physical limit check and spike removal is also applied. It is found that as the averaging interval becomes shorter, turbulent fluxes computed by the moving average become smaller and the ratios of turbulent fluxes computed by the 100 s moving average to the fluxes by the 1800 s moving average under unstable stability are smaller than those under neutral stability. The turbulent fluxes computed by the linear regression are 85~92% of those computed by the 1800 s moving average and nearly the same as those computed by 900 s moving average, implying that the adequate selection of an averaging operator and its interval will be very important to estimate more accurate turbulent fluxes at urban area.

THE EFFECTS OF WALL BOUNDARY CONDITIONS ON MASS TRANSFER IN TURBULENT PIPE FLOW (난류 파이프 유동 내 물질전달에서의 경계조건 영향)

  • Kang, Chang-Woo;Yang, Kyung-Soo
    • Journal of computational fluids engineering
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    • v.17 no.2
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    • pp.42-52
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    • 2012
  • Direct Numerical Simulation(DNS) of turbulent mass transfer in fully developed turbulent pipe flow has been performed to study the effect of wall boundary conditions on the concentration fields at $Re_{\tau}$=180 based on friction velocity and pipe radius. Fully developed turbulent pipe flows for Sc=0.71 are studied with two different wall boundary conditions, namely, constant mass flux and constant wall concentration. The mean concentration profiles and turbulent mass fluxes obtained from the present DNS are in good agreement with the previous numerical results currently available. To investigate the effects of wall boundary condition on the turbulent mass transfer, the mean concentration profile, root-mean-square of concentration fluctuation, turbulent mass fluxes and higher-order statistics(Skewness and Flatness factor) are compared for the two cases. Furthermore, the budgets of turbulent mass fluxes and concentration variance were computed and analyzed to elucidate the effects of wall boundary conditions on the turbulent mass transfer.

Response of Ecosystem Carbon and Water Vapor Exchanges in Evolving Nocturnal Low-Level Jets

  • Hong, Jin-Kyu;Mathieu, Nathalie;Strachan, Ian B.;Pattey, Elizabeth;Leclerc, Monique Y.
    • Asian Journal of Atmospheric Environment
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    • v.6 no.3
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    • pp.222-233
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    • 2012
  • The nocturnal low-level jet makes a significant impact on carbon and water exchanges and turbulent mixing processes in the atmospheric boundary layer. This study reports a case study of nocturnal surface fluxes such as $CO_2$ and water vapor in the surface layer observed at a flat and homogeneous site in the presence of low-level jets (LLJs). In particular, it documents the temporal evolution of the overlying jets and the coincident response of surface fluxes. The present study highlights several factors linking the evolution of low-level jets to surface fluxes: 1) wavelet analysis shows that turbulent fluxes have similar time scales with temporal scale of LLJ evolution; 2) turbulent mixing is enhanced during the transition period of low-level jets; and 3) $CO_2$, water vapor and heat show dissimilarity from momentum during the period. We also found that LLJ activity is related not only to turbulent motions but also to the divergence of mean flow. An examination of scalar profiles and turbulence data reveal that LLJs transport $CO_2$ and water vapor by advection in the stable boundary layer, suggesting that surface fluxes obtained from the micrometeorological method such as nocturnal boundary layer budget technique should carefully interpreted in the presence of LLJs.

Uncertainty Analysis of the Eddy-Covariance Turbulent Fluxes Measured over a Heterogeneous Urban Area: A Coordinate Tilt Impact (비균질 도시 지표에서 측정된 에디 공분산 난류 플럭스의 불확실성 분석: 좌표계 편향 영향)

  • Lee, Doo-Il;Lee, Jae-Hyeong;Lee, Sang-Hyun
    • Atmosphere
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    • v.26 no.3
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    • pp.473-482
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    • 2016
  • An accurate determination of turbulent fluxes over an urban area is a challenging task due to its morphological diversity and associated flow complexity. In this study, an eddy covariance (EC) method is applied over a highly heterogeneous urban area in a small city (Gongju), South Korea to investigate the quantitative influence of 'coordinate tilt' in determining the turbulent fluxes of sensible heat, latent heat, momentum, and carbon dioxide mass. Two widely-used coordinate transform methods are adopted and applied to eight directional sections centered on the site to analyze a 1-year period EC measurement obtained from the urban site: double rotation (DR) and planar fit (PF) transform. The results show that mean streamline planes determined by the PF method are distinguished from the sections, representing morphological heterogeneity of the site. The sectional pitch angles determined by the DR method also compare well with those in the PF method. Both the PF and DR methods show large variabilities in the determined streamline planes at each directional section, implying that flow patterns may form in a complicate way due to the surface heterogeneity. Resulting relative differences of the turbulent fluxes, defined by $(F_{DR}-F_{PF})/F_{DR}$, are found on average +13% in sensible heat flux, +21% in latent heat flux, +37% in momentum flux, and +26% in carbon dioxide mass flux, which are larger values than those reported previously for fairly homogeneous natural sites. The fractional differences depend significantly on wind direction, showing larger differences in northerly winds at the measurement site. It is also found that the relative fractional differences are negatively correlated with the mean wind speed at both stable/unstable atmospheric conditions. These results imply that EC turbulent fluxes determined over heterogeneous urban areas should be carefully interpreted with considering the uncertainty due to 'coordinate tilt' effect in their applications.

Near-Wall Modelling of Turbulent Heat Fluxes by Elliptic Equation (타원방정식에 의한 벽면 부근의 난류열유속 모형화)

  • Shin, Jong-Keun;An, Jeong-Soo;Choi, Young-Don
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.28 no.5
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    • pp.526-534
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    • 2004
  • A new second-moment closure model for turbulent heat fluxes is proposed on the basis of the elliptic equation. The new model satisfies the near-wall balance between viscous diffusion, viscous dissipation and temperature-pressure gradient correlation, and also has the characteristics of approaching its respective conventional high Reynolds number model far away from the wall. The predictions of turbulent heat transfer in a channel flow have been carried out with constant wall heat flux and constant wall temperature difference boundary conditions respectively. The velocity field variables are supplied from the DNS data and the differential equations only fur the mean temperature and the scalar flux are solved by the present calculations. The present model is tested by direct comparisons with the DNS to validate the performance of the model predictions. The prediction results show that the behavior of the turbulent heat fluxes in the whole region is well captured by the present model.

REYNOLDS NUMBER EFFECTS ON MASS TRANSFER IN TURBULENT PIPE FLOW: PART I. MEAN CONCENTRATION FIELD AND LOW-ORDER STATISTICS (난류 파이프 유동 내 물질전달에 대한 레이놀즈 수 영향: Part I. 평균 농도장 및 저차 난류통계치)

  • Kang, Chang-Woo;Yang, Kyung-Soo
    • Journal of computational fluids engineering
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    • v.17 no.3
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    • pp.1-10
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    • 2012
  • Large Eddy Simulation(LES) of turbulent mass transfer in fully developed turbulent pipe flow has been performed to study the effect of Reynolds number on the concentration fields at $Re_{\tau}=180$, 395, 590 based on friction velocity and pipe radius. Dynamic subgrid-scale models for the turbulent subgrid-scale stresses and mass fluxes were employed to close the governing equations. Fully developed turbulent pipe flows with constant mass flux imposed at the wall are studied for Sc=0.71. The mean concentration profiles and turbulent intensities obtained from the present LES are in good agreement with the previous numerical and experimental results currently available. To show the effects of Reynolds number on the turbulent mass transfer, the mean concentration profile, root-mean-square of concentration fluctuations, turbulent mass fluxes, cross-correlation coefficient, turbulent diffusivity and turbulent Schmidt number are presented.

REYNOLDS NUMBER EFFECTS ON MASS TRANSFER IN TURBULENT PIPE FLOW: PART II. INSTANTANEOUS CONCENTRATION FIELD, HIGHER-ORDER STATISTICS AND MASS TRANSFER BUDGETS (난류 파이프 유동 내 물질전달에 대한 레이놀즈 수 영향: Part II. 순간농도장, 고차 난류통계치 및 물질전달수지)

  • Kang, Chang-Woo;Yang, Kyung-Soo
    • Journal of computational fluids engineering
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    • v.17 no.3
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    • pp.59-67
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    • 2012
  • Large Eddy Simulation(LES) of turbulent mass transfer in fully developed turbulent pipe flow has been performed to study the effect of Reynolds number on the concentration fields at $Re_{\tau}=180$, 395, 590 based on friction velocity and pipe radius. Dynamic subgrid-scale models for the turbulent subgrid-scale stresses and mass fluxes were employed to close the governing equations. Fully developed turbulent pipe flows with constant mass flux imposed at the wall are studied for Sc=0.71. The mean concentration profiles and turbulent intensities obtained from the present LES are in good agreement with the previous numerical and experimental results currently available. The effects of Reynolds number on the turbulent mass transfer are identified in the higher-order statistics(Skewness and Flatness factor) and instantaneous concentration fields. The budgets of turbulent mass fluxes and concentration variance were computed and analyzed to elucidate the effect of Reynolds number on turbulent mass transfer. Furthermore, to understand the correlation between near-wall turbulence structure and concentration fluctuation, we present an octant analysis in the vicinity of the pipe wall.

A Hybrid Turbulence Model for Prediction of Buoyancy-Driven Turbulent Thermal Convection Flow (부력에 의한 난류 열대류의 혼성 난류모델)

  • 김태규;성형진
    • Transactions of the Korean Society of Mechanical Engineers
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    • v.17 no.8
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    • pp.2069-2078
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    • 1993
  • The buoyancy-driven turbulent thermal convection is predicted using an anisotropic hybrid turbulence model, which is incorporated with a low Reynolds k-.epsilon. turbulence model and an anisotropic buoyant part of algebraic stress model(ASM). The numerical predictions are compared with the Davidson's model,(1) the full ASM and the experimental results of Cheesewright et al.(2) All the models are shown to predict good agreements with the experiments for the averaged turbulence quantities. It is found that the effect of an anisotropic part on the Reynolds stress and the turbulent heat fluxes is substantial. In this study, the present hybrid model gives a fairly reasonable prediction in terms of the computational accuracy, convergence and stability. The contribution of an anisotropic buoyant part to turbulent heat fluxes are also scrutinized over the range of Rayleigh numbers $(4.79{\times}10^{10}{\le}Ra{\le}7.46{\times}10^{10}).$