• Title/Summary/Keyword: Momentum flux

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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.

An Analysis of the Wintertime Diurnal Wind Variation and Turbulent Characteristics over Yongpyong Alpine Slope (용평 알파인 경기장에서 겨울철 바람의 일변화 및 난류 특성분석)

  • Jeon, Hye-Rim;Kim, Byung-Gon;Eun, Seung-Hee;Lee, Young-Hee;Choi, Byoung-Cheol
    • Atmosphere
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    • v.26 no.3
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    • pp.401-412
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    • 2016
  • A 3D sonic anemometer has been installed at Yongpyong alpine slope since Oct. 23th 2014 to observe the slope winds and to analyze turbulent characteristics with the change in surface cover (grass and snow) and the synoptic wind strength. Eddy covariance method has been applied to calculate the turbulent quantity after coordinate transformation of a planar-fit rotation. We have carefully selected 3 good episodes in the winter season (23 October 2014 to 28 February 2015) for each category (9 days in total), such as grass and snow covers in case of weak synoptic wind condition, and grass cover of strong synoptic wind. The diurnal variations of the slope winds were well developed like the upslope wind in the daytime and downslope wind in the nighttime for both surface covers (grass and snow) in the weak synoptic forcing, when accordingly both heat and momentum fluxes significantly increased in the daytime and decreased in the nighttime. Meanwhile, diurnal variation of heat flux was not present on the snow cover probably in associated with significant fraction of sunlight reflection due to high albedo especially during the daytime in comparison to those on the grass cover. In the strong synoptic regime, the most dominant feature at Yongpyong, only the southeasterly downslope winds were steadily generated irrespective of day and night with significant increases in momentum flux and turbulent kinetic energy as well, which could suggest that local circulations are suppressed by the synoptic scale forcing. In spite of only one season analysis applied to the limited domain, this kind of an observation-based study will provide the basis for understanding of the local wind circulation in the complex mountain domain such as Gangwon in Korea.

Heat Transfer Coefficient and Shear Factor Subjected to Both Oscillating Flow and Oscillating Pressure in Pulse Tubes (주기적인 유동과 압력의 변화를 수반하는 맥동관의 열전달계수와 전단계수)

  • Jeong, Eun-Soo
    • Korean Journal of Air-Conditioning and Refrigeration Engineering
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    • v.19 no.3
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    • pp.220-227
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    • 2007
  • Heat transfer and momentum transfer under conditions of both oscillating flow and oscillating pressure within pulse tubes show very different behavior from those for steady state conditions. The analytic solutions of axial velocity and temperature of the gas within pulse tubes were obtained by assuming that the variations in pressure and temperature were purely sinusoidal and small. The shear stress and the heat flux at the tube wall obtained from the solutions are expressed in terms of the cross-sectional averaged velocity, the difference between mean temperature and instantaneous cross-sectional averaged temperature and the difference between mean pressure and instantaneous pressure. It is shown that the complex shear factor, which has been applied to momentum transfer of incompressible oscillating flow, and the complex Nusselt number, which has been applied to either heat transfer with oscillating pressure only or heat transfer of incompressible oscillating flow, could also be used for momentum transfer and heat transfer subjected to both oscillating flow and oscillating pressure, respectively.

Prediction of Longshore Current with Set-up/down Effect on a Plane Beach (일정경사 수심단면에서 평균수위의 상승/저하 효과를 고려한 해빈류의 예측)

  • Lee, Cheol-Eung;Kim, Young-Jung;Choi, Han-Kyu
    • Journal of Industrial Technology
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    • v.17
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    • pp.277-289
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    • 1997
  • The numerical model for prediction of longshore current with set-up/down effect on a plane beach is developed using the longshore component of the depth-integrated momentum balance equation. To predict the longshore current, the wave height model should first be formulated because the longshore current depends on the wave height directly. Two wave model, regular wave model and random wave model, are developed based on the energy flux balance equation. Also, the numerical model estimating the set-up inside the shoreline is developed using both the on-offshore momentum equation and the moving boundary technique. The numerical models are verified by the analytical solution, and compared with laboratory data. It is found from the comparison that developed models may be predicted accurately the longshore current with set-up/down effect on a plane beach.

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Study on Heat Transfer around a Circular Jet Ejected into a Supersonic Flow (초음속 유동내에 분사되는 원형 제트 주위에서의 열전달 연구)

  • Yi, Jong-Ju;Yu, Man-Sun;Cho, Hyung-Hee
    • Proceedings of the Korean Society of Propulsion Engineers Conference
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    • 2006.11a
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    • pp.353-356
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    • 2006
  • Convective heat transfer coefficient was measured around a secondary jet ejected into the supersonic flow field. Wall temperature distribution was measured on the surface, which the constant heat flux condition is applied. According to jet to freestream momentum ratio, the secondary flow was penetrated into the supersonic flow field. During the test, two dimensional thermal image of a wall temperature is taken by an infra-red camera. Experiments were performed under the testing condition of freestream Mach number of about 3, stagnation pressure of 630 kPa and Reynolds number of $3.0{\times}10^6$.

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A Wind Tunnel Study on Influences of ILS Tower on Wind Speed Measurement (계기착륙장치 타워가 풍속관측에 미치는 영향에 관한 풍동실험연구)

  • Choi, Cheol-Min;Kim, Kye-Hwan;Kim, Young-Chul;Kwon, Kybeom
    • Atmosphere
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    • v.23 no.4
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    • pp.513-517
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    • 2013
  • In this study, it is first intended to simulate the vertical profile of atmospheric flow in a short wind tunnel. In order to accomplish it, proper devices are designed properly to reduce freestream flow momentum and it is confirmed from the measured velocity profile using hot-wire anemometer that momentum flux of the tunnel free stream can be reduced and desired atmospheric boundary can be created. Second, experiments are performed to identify influences of a surrounding structure measuring correct wind velocity by an anemometer, which are located nearby due to area limitation in actual airport and correction factors are proposed from experimental results. One of findings is that in order to limit the velocity attenuation due to a nearby structure under 10%, wind velocity measuring equipment should be installed at least 6 times of the structure height away from the structure of concern.

Turbulent Heat Transfer in Rough Concentric Annuli With Heating Condition of Constant Wal Heat Flux (일정벽면열유속의 가열조건의 갖는 거친 동심환형관내의 난류열전달)

  • 손유식
    • Journal of Advanced Marine Engineering and Technology
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    • v.23 no.1
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    • pp.54-61
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    • 1999
  • The fully developed turbulent momentum and heat transfer induced by the roughness elements on the outer wall surfaces in concentric annuli are analytically studied on the basis of a modified turbu-lence model. The resulting momentum and heat transfer are discussed in terms of various parame-ters such as the radius ratio the roughness density Reynolds number and Prandtl number accord-ing to the heating condition. The study shows that certain artificial roughness elements may be used to enhance heat transfer rates with advantage from the overall efficiency point of view.

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Study on the Surface Heat Transfer Around a Circular Secondary Jet in the Supersonic Flow (초음속 유동내 원형 2차분사 제트 주변에서의 표면 열전달 현상 연구)

  • Yi, Jong-Ju;Yu, Man-Sun;Cho, Hyung-Hee
    • Journal of the Korean Society of Propulsion Engineers
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    • v.11 no.2
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    • pp.47-53
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    • 2007
  • Convective heat transfer coefficient was measured around a circular secondary jet ejected into the supersonic flow field. The wall temperature measurement around a injection nozzle was conducted using infra-red camera. The constant heat flux is applied to the wall around a secondary nozzle. According to jet to freestream momentum ratio, the injection flow penetrates into the supersonic flow field. The measured temperature is used to calculate the convective heat transfer coefficient.

A Numerical Study on the Smoke Behavior by Solar Radiation through Ceiling Glass in Atrium Fires

  • Jeong, Jin-Yong
    • International Journal of Air-Conditioning and Refrigeration
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    • v.10 no.3
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    • pp.117-128
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    • 2002
  • This paper describes the smoke filling process of a fire field model based on a self-deve-loped SMEP (Smoke Movement Estimating Program) code to the simulation of fire induced flows in the two types of atrium space containing a ceiling heat flux. The SMEP using PISO algorithm solves conservation equations for mass, momentum, energy and species, together with those for the modified k-$\varepsilon$ turbulence model with buoyancy production term. Also it solves the radiation equation using the discrete ordinates method. Compressibility is assumed and the perfect gas law is used. Comparison of the calculated upper-layer average tempera-ture and smoke layer clear height with the zone models has shown reasonable agreement. The zone models used are the CFAST and the NBTC one-room. For atrium fires with ceiling glass the ceiling heat flux by solar heat causes a high smoke temperature near the ceiling. However, it has no effect on the smoke movement such as the smoke layer clear heights that are important in fire safety. In conclusion, the smoke layer clear heights that are important in evacuation activity except the early of a fire were not as sensitive as the smoke layer tem-perature to the nature of ceiling heat flux condition. Thus, a fire sensor in atrium with ceiling glass has to consider these phenomena.

Characteristics of SMD and Volume Flux of Two-phase Jet Injected into Cross-flow with Various Gas-liquid Ratio and Reynolds Number (횡단 유동장의 기액비 및 레이놀즈수 변화에 따른 외부혼합형 이상유체 제트의 액적크기 및 체적유속 특성)

  • Kim, Jong-Hyun;Lee, Bong-Soo;Koo, Ja-Ye
    • Transactions of the Korean Society of Automotive Engineers
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    • v.17 no.2
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    • pp.75-81
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    • 2009
  • A study was performed to investigate the characteristics of two-phase jet injected into subsonic cross-flow using the external mixed gas blast two-phase nozzle. The shadowgraph method was adopted for the cross-flow jet visualization and PDPA system was used to measure droplet size, velocity, and volume flux. The atomization of two-phase jet is initially determined according to gas to liquid mass flow-rate ratio and the Reynolds number of cross-flows. The highest penetration trajectories of two-phase jet injected into cross-flow are governed by the momentum ratio at subsonic cross-flow. As GLR of two-phase jet injected into cross-flow increases, the droplet size decreases and the distribution area of volume flux increases. The distribution of volume flux that influenced by the counter vortex pair at the downstream of cross-flow is symmetric in shape of horseshoe.