• Title/Summary/Keyword: Turbulence structure

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The Effect of Free Stream Turbulence on the Coherent Structures in the near Wake of a Circular Cylinder (원주 후류의 응집구조에 대한 자유흐름 난류강도의 영향)

  • 정양범;양종필
    • Journal of Advanced Marine Engineering and Technology
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    • v.18 no.1
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    • pp.60-72
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    • 1994
  • The effect of free stream tubulence on the coherent structure in the near wake of a circular cylinder was investigated by a conditional sampling technique. The measurements were made from C.T.A. with hot wire I-probe and a Split-film sensor. Contours of phase-averaged velocity and vorticities were presented and discussed. It was found that the value of the vortex strength increased with increasing free stream turbulence which can enhance the roll-up of the shear layer.

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Spanwise Growth of Vortex Structure in Wall Turbulence

  • Adrian, Ronald J.;Balachandar, S.;Liu, Z.C.
    • Journal of Mechanical Science and Technology
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    • v.15 no.12
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    • pp.1741-1749
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    • 2001
  • Recent studies of the structure of wall turbulence have lead to the development of a conceptual model that validates and integrates many elements of previous models into a relatively simple picture based on self-assembling packets of hairpin vortex eddies. By continual spawning new hairpins the packets grow longer in the streamwise direction, and by mutual induction between adjacent hairpins the hairpins are strained so that they grow taller and wider as they age. The result is a characteristic growth angle in the streamwise-wall normal plane. The spanwise growth of individual packets implies that packets must either merge or pass through each other when they come into contact. Direct numerical simulations of the growth and interaction of spanwise adjacent hairpins shows that they merge by the vortex connection mechanism originally proposed by Wark and Nagib (199). In this mechanism the quasi-streamwise legs of two hairpins annihilate each other, by virtue of having opposite vorticity, leaving a new hairpin of approximately double the width of the individuals. PIV measurements in planes parallel to the wall support this picture. DNS of multiple hairpins shows how the spanwise scale doubles when the hairpins form an array.

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The statistics of isothermal turbulence in stratified medium

  • Lee, Hye-Sueng;Ryu, Dong-Su;Kim, Jong-Soo;Cho, Jung-Yeon
    • The Bulletin of The Korean Astronomical Society
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    • v.35 no.1
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    • pp.67.1-67.1
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    • 2010
  • Understanding the nature of MHD turbulence is of fundamental importance in astrophysics. The results of these studies can be used to develop the star formation theory. While most of previous studies have considered turbulence in uniform media, the stratification of media can play an important role. We performed three-dimensional numerical simulations of isothermal, compressible, MHD turbulence with different plasma beta's and different flow Mach numbers in stratified media to study the effects of stratification on turbulence. We found Statistics of turbulent flow in stratified media is different from in uniform media. In this talk, we present the statistical properties of the MHD turbulence, such as the PDF, power spectrum, and structure function.

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Large-Scale Vortical Structures in The Developing Plane Mixing Layer Using LES

  • Seo, Taewon;Kim, Yeung-Chan;Keum, Kihyun
    • International Journal of Aeronautical and Space Sciences
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    • v.2 no.1
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    • pp.12-19
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    • 2001
  • Study of turbulent mixing layers has been a popular subject from the point of view of both practical application and phenomenological importance in engineering field. Turbulent mixing layers can be applied in many fields where rapid transition to turbulence is desirable in order to prevent boundary layer separation or to enhance mixing. The ability to control mixing, structure and growth of the shear flow would obviously have a considerable impact on many engineering applications. In addition to practical applications, free shear flows are one of the simplest flows to understand the fundamental mechanism in the transition process to turbulence. After the discovery of large-scale vortical structure in free shear flows many researchers have investigated the physical mechanism of generation and dissipation processes of the vortical structure. This study investigated the role of the large-scale vortical structures in the turbulent mixing layer using LES(Large-Eddy Simulation). The result shows that the pairing interaction of the vortical structure plays an important role in the growth rate of a mixing layer. It is found that the turbulence quantities depend strongly on the velocity ratio. It is also found that the vorticity in the high-velocity-side can extract energy from the mean flow, while the vorticity in the low-velocity-side lose energy by the viscous dissipation. Finally the results suggest the guideline to obtain the desired flow by control of the velocity ratio.

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Investigation on spanwise coherence of buffeting forces acting on bridges with bluff body decks

  • Zhou, Qi;Zhu, Ledong;Zhao, Chuangliang;Ren, Pengjie
    • Wind and Structures
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    • v.30 no.2
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    • pp.181-198
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    • 2020
  • In the traditional buffeting response analysis method, the spanwise incomplete correlation of buffeting forces is always assumed to be same as that of the incident wind turbulence and the action of the signature turbulence is ignored. In this paper, three typical bridge decks usually adopted in the real bridge engineering, a single flat box deck, a central slotted box deck and a two-separated paralleled box deck, were employed as the investigated objects. The wind induced pressure on these bridge decks were measured via a series of wind tunnel pressure tests of the sectional models. The influences of the wind speed in the tests, the angle of attack, the turbulence intensity and the characteristic distance were taken into account and discussed. The spanwise root coherence of buffeting forces was also compared with that of the incidence turbulence. The signature turbulence effect on the spanwise root coherence function was decomposed and explained by a new empirical method with a double-variable model. Finally, the formula of a sum of rational fractions that accounted for the signature turbulence effect was proposed in order to fit the results of the spanwise root coherence function. The results show that, the spanwise root coherence of the drag force agrees with that of incidence turbulence in some range of the reduced frequency but disagree in the mostly reduced frequency. The spanwise root coherence of the lift force and the torsional moment is much larger than that of the incidence turbulence. The influences of the wind speed and the angle of attack are slight, and they can be ignored in the wind tunnel test. The spanwise coherence function often involves several narrow peaks due to the signature turbulence effect in the high reduced frequency zone. The spanwise coherence function is related to the spanwise separation distance and the spanwise integral length scales, and the signature turbulence effect is related to the deck-width-related reduced frequency.

CFD Simulation of Axial Turbulent Flow in a Triangular Rod Bundle

  • In W.K.;Chun T. H.;Myong H. K;Ko K
    • 한국전산유체공학회:학술대회논문집
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    • 2003.10a
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    • pp.71-73
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    • 2003
  • A CFD analysis has been made for fully developed turbulent flows in a triangular bare rod bundle with pitch to diameter ratio (P/D) of 1.123. The nonlinear turbulence models predicted the turbulence­driven secondary flow in the triangular subchannel. The nonlinear quadratic $\kappa-\omega$ models by Speziale and Myong-Kasagi predicted turbulence structure in the rod bundle fairly well. The nonlinear quadratic and cubic $\kappa-\omega$ models by Shih et al. and Craft et al. showed somewhat weaker anisotropic turbulence. The differential Reynolds stress model appeared to overpredict the turbulence anisotropy in the rod bundle.

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CFD Analysis of Turbulent Heat Transfer in a Heated Rod Bundle (가열 봉다발의 난류 열전달에 대한 전산유체역학 해석)

  • In, Wang-Kee;Oh, Dong-Seok;Chun, Tae-Hyun
    • Proceedings of the KSME Conference
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    • 2003.11a
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    • pp.598-603
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    • 2003
  • A CFD analysis has been performed to investigate turbulent heat transfer in a triangular rod bundle with a pitch-to-diameter ratio(P/D) of 1.06. Anisotropic turbulence models predicted the turbulence-driven secondary flow in the triangular subchannel and the distributions of time mean velocity and temperature showing significantly improved agreement with the measurements over the linear standard ${\kappa}-{\varepsilon}$. The anisotropic turbulence models predicted turbulence structure in large flow region fairly well but could not predict the very high turbulent intensity of azimuthal velocity observed in narrow flow region(gap).

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An Experiment on the Effects of Free Stream Turbulence Intensity on the Backward-Facing Step Flow (자유흐름 난류강도가 후향계단유동에 미치는 영향에 대한 실험)

  • 김사량;유정열
    • Transactions of the Korean Society of Mechanical Engineers
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    • v.19 no.9
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    • pp.2297-2307
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    • 1995
  • An experimental study on the structure of a separated shear layer downstream of the backward-facing step has been performed by examining mean flow and turbulent quantities in terms of free stream turbulence. When free stream turbulence exists, the entrainment rate of the separated shear layer and the flow rate in the recirculation region are enhanced, resulting in shorter reattachment length. The production and diffusion terms in the turbulent kinetic energy balance are shown to increase more than the dissipation term does. Rapid decrease of the pressure-strain term in the shear stress balance implies the enhancement of the three-dimensional motion by free stream turbulence.

Turbulence Enhancement Characteristics Analysis of Inclined-Tumbles for Various SCV Configurations (SCV형상별 경사텀블유동의 난류증가 특성 해석)

  • Lee, J.W.;Kang, K.Y.;Choi, S.H.;Park, S.C
    • Transactions of the Korean Society of Automotive Engineers
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    • v.6 no.6
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    • pp.234-242
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    • 1998
  • It has been demonstrated that the in-cylinder turbulence is enhanced by inclined swirl with a SCV(swirl control valve). The inclined-tumble flow measurement and analysis were performed for various types of intake systems that generated several different combinations of swirl ratio and tumble ratio in the cylinder. Experiments were conducted in a 4-valve optically accessed transparent research engine using a backward-scatter LDV mode under motoring condition at 1,000rpm. The influence of swirl/tumble levels on the characteristics of turbulence was analysed. This study presents experimental results of the inclined-tumble flow structure, including the flow motion phenomena, angular momentum and turbulence intensity.

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Numerical Analysis of the Turbulent Flow and Heat Transfer in a Heated Rod Bundle

  • In Wang-Kee;Shin Chang-Hwan;Oh Dong-Seok;Chun Tae-Hyun
    • Nuclear Engineering and Technology
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    • v.36 no.2
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    • pp.153-164
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    • 2004
  • A computational fluid dynamics (CFD) analysis has been performed to investigate the turbulent flow and heat transfer in a triangular rod bundle with pitch-to-diameter ratios (P/D) of 1.06 and 1.12. Anisotropic turbulence models predicted the turbulence-driven secondary flow in a triangular subchannel and the distributions of the time mean velocity and temperature, showing a significantly improved agreement with the measurements from the linear standard $k-{\epsilon}$ model. The anisotropic turbulence models predicted the turbulence structure for a rod bundle with a large P/D fairly well, but could not predict the very high turbulent intensity of the azimuthal velocity observed in the narrow flow region (gap) for a rod bundle with a small P/D.