• Title/Summary/Keyword: Second-moment Closure

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Development of a New Correlation for the Heat Transfer Coefficient of Turbulent Supercritical Carbon Dioxide Flow (초임계 상태 이산화탄소 난류유동의 새로운 열전달계수 상관식 개발)

  • 임홍영;최영돈;김용찬;김민수
    • Korean Journal of Air-Conditioning and Refrigeration Engineering
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    • v.15 no.4
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    • pp.274-286
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    • 2003
  • Numerical simulations are performed to investigate the turbulent convective heat transfer of the supercritical carbon dioxide flows in vertical and horizontal square ducts. The gas cooling process at the supercritical state experiences a sudden change in thermodynamic and transport properties. This results in the extraordinary variations of the heat transfer coefficients in the supercritical state, which are much different from those of single or two phase flows. Algebraic second moment closure which can include the effects of large thermophysical property variations of carbon dioxide and of buoyancy is employed to model the Reynolds stresses and turbulent heat fluxes in the governing equations. The previous correlations for the turbulent heat transfer coefficient for the supercritical carbon dioxide flows couldn't reflect the buoyancy effect. The present results are used to establish a new heat transfer coefficient correlation including the effects of large thermophysical property variation and buoyancy on in-duct cooling process of supercritical carbon dioxide.

Turbulent Flow through a Square Straight and Curved Duct with Reynolds Stress Models (정사각 직관과 $180^{\circ}$ 곡관내 난류유동의 레이놀즈응력모형 적용)

  • Chun, Kun-Ho;Choi, Young-Don
    • Proceedings of the KSME Conference
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    • 2000.04b
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    • pp.771-776
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    • 2000
  • Fine grid calculations are reported for the developing turbulent flow in a straight duct and a curved duct of square cross-section with a radius of curvature to hydraulic diameter ratio ${\delta}=R_c/H_H=3.357$ and a bend angle of 180 deg. A sequence of modeling refinements is introduced; the replacement of wall function by a fine mesh across the sublayer and a low Reynolds number second moment closure up to the near wall sublayer in which the non-linear return to isotropy model and the cubic-quasi-isotropy model for the pressure strain are adopted; and the introduction of a multiple source model for the exact dissipation rate equation. Each refinement is shown to lead to an appreciable improvement in the agreement between measurement and computation.

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Assessment of Reynolds Stress Model for the Analysis of Floating Flames in Stagnating Flows (정체유동에서의 난류 부상 화염 해석을 위한 Reynolds 응력 모델의 검증)

  • Im, Yong-Hoon;Huh, Kang-Yul
    • Journal of the Korean Society of Combustion
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    • v.7 no.2
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    • pp.49-61
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    • 2002
  • Numerical simulation is performed for stagnating turbulent flows of impinging and countercurrent jets by the Reynolds stress model(RSM). Results are compared with those of the ${\kappa}-{\varepsilon}$ model and available data to assess the flow characteristics and turbulence modes. Three variants of the RSM tested are those of Gibson and Launder(GL), Craft and Launder(GL-CL) and Speziale, Sarkar and Gatski(SSG). As well known, the ${\kappa}-{\varepsilon}$ model overestimates turbulent kinetic energy near the wall significantly. Although the RSM is superior to the ${\kappa}-{\varepsilon}$ model, it shows considerable difference according to how the redistributive pressure-strain term is modeled. Results of the RSM for countercurrent jets are improved with the modified coefficients for the dissipation rate, $C_{{\varepsilon}1}\;and\;C_{{\varepsilon}2}$ suggested by Champion and Libby. The performance of the three variants of the RSM model for stagnating flows are assessed.

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Influence of Internal Resonance on Responses of a Spring-Pendulum System under Broad Band Random Excitation (광대역 불규칙 가진력을 받는 탄성진자계의 내부공진효과)

  • 이원경;조덕상
    • Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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    • 1997.04a
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    • pp.86-94
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    • 1997
  • An investigation into the modal interaction of an autoparametric system under broad-band random excitation is made. The specific system examined is a spring-pendulum system with internal resonance, which is known to be a good model for a variety of engineering systems, including ship motions with nonlinear coupling between pitching and rolling motions. By means of the Gaussian closure method the dynamic moment equations explaining the random response of the system are reduced to a system of autonomous ordinanary differential equations of the first and second moments. In view of equilibrium solutions of this system and their stability we examine the system responses. The stabilizing effect of system damping is also examined.

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A Study on the y+ Effects on Turbulence Model of Unstructured Grid for CFD Analysis of Wind Turbine (풍력터빈 전산유체역학해석에서 비균일 그리드 무차원 연직거리의 난류모델에 대한 영향특성)

  • Lee, Kyoung-Soo;Ziaul, Huque;Han, Sang-Eul
    • Journal of Korean Association for Spatial Structures
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    • v.15 no.1
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    • pp.75-84
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    • 2015
  • This paper presents the dimensionless wall distance, y+ effect on SST turbulent model for wind turbine blade. The National Renewable Energy Laboratory (NREL) Phase VI wind turbine was used for the study, which the wind tunnel and structural test data has publicly available. The near wall treatment and turbulent characteristics have important role for proper CFD simulation. Most of the CFD development in this area is focused on advanced turbulence model closures including second moment closure models, and so called Low-Reynolds (low-Re) number and two-layer turbulence models. However, in many cases CFD aerodynamic predictions based on these standard models still show a large degree of uncertainty, which can be attributed to the use of the $\epsilon$-equation as the turbulence scale equation and the associated limitations of the near wall treatment. The present paper demonstrates the y+ definition effect on SST (Shear Stress Transport) turbulent model with advanced automatic near wall treatment model and Gamma theta transitional model for transition from lamina to turbulent flow using commercial ANSYS-CFX. In all cases the SST model shows to be superior, as it gives more accurate predictions and is less sensitive to grid variations.

Unsteady RANS Analysis of the Hydrodynamic Response for a Ship with Forward Speed in Regular Wave (규칙파중 전진하는 선박의 유체역학적 응답에 대한 비정상 수치해석)

  • Park, Il-Ryong;Kim, Kwang-Soo;Kim, Jin;Van, Suak-Ho
    • Journal of the Society of Naval Architects of Korea
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    • v.45 no.1
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    • pp.29-41
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    • 2008
  • The present paper provides a CFD analysis of diffraction problem for a ship with forward speed using an unsteady RANS simulation method, a WAVIS code. The WAVIS viscous solver adopting a finite volume method has second order accuracy in time and field discretizaions for the RANS equations. A two phase level-set method and a realizable ${\kappa}-{\varepsilon}$ turbulence model are adopted to compute the free surface and to meet the turbulence closure, respectively. To validate the capability of the present numerical methods for the simulation of an unsteady progressive regular wave, computations are performed for three grid sets with refinement ratio of ${\sqrt{2}}$. The main simulation is performed for a DTMB5512 model with a forward speed in a regular head sea condition. Validation of the present numerical method is carried out by comparing the present CFD results with available unsteady experimental data published in the 2005 Tokyo CFD Workshop: resistance, heave force, pitch moment, unsteady free surface elevations and velocity fields.