• Title/Summary/Keyword: Nonlinear Low-Reynolds-Number

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A Nonlinear Low-Reynolds-Number k -$\varepsilon$ Model for Turbulent Separated and Reattaching Flows (난류박리 및 재부착 유동의 해석을 위한 비선형 저레이놀즈수 k -$\varepsilon$ 난류모형의 개발)

  • 박태선;성형진
    • Transactions of the Korean Society of Mechanical Engineers
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    • v.19 no.8
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    • pp.2051-2063
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    • 1995
  • An improved version of nonlinear low-Reynolds-number k-.epsilon. model is developed. In this model, the limiting near-wall behavior and nonlinear Reynolds stress representations are incorporated. Emphasis is placed on the adoption of Ry(.iden. $k^{1}$2/y/.nu.) instead of $y^{[-10]}$ (.iden. $u_{{\tau}/y/{\nu}}$) in the low-Reynolds-number model for predicting turbulent separated and reattaching flows. The non-equilibrium effect is examined to describe recirculating flows away from the wall. The present model is validated by doing the benchmark problem of turbulent flow behind a backward-facing step. The predictions of the present model are cross-checked with the existing measurements and DNS data. The model performance is shown to be generally satisfactory.

Prediction of Fully Developed Turbulent Flow in a Square Duct with Nonlinear Low-Reynolds-Number κ-ε Models (비선형 저레이놀즈수 κ-ε 난류모델에 따른 정사각형 덕트내 완전 발달된 난류유동 예측)

  • Myong, Hyon-Kook,
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.27 no.6
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    • pp.821-827
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    • 2003
  • Fully developed turbulent flow in a square duct is numerically predicted with two nonlinear low-Reynolds-number ${\kappa}-{\varepsilon}$ models. Typical predicted quantities such as axial and secondary velocities, turbulent kinetic energy and Reynolds stresses are compared in detail with each other. It is found that the nonlinear low-Reynolds-number ${\kappa}-{\varepsilon}$ model adopted in a commercial code is unable to predict accurately duct flows involving turbulence-driven secondary motion with the prediction level of secondary flows one order less than that of the experiment.

Development of Low-Reynolds-Number Ssecond Moment Turbulence Closure by DNS Data (DNS 자료에 의한 저레이놀즈수 2차 모멘트 난류모형의 개발)

  • Sin, Jong-Geun;Choe, Yeong-Don
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.20 no.8
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    • pp.2572-2592
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    • 1996
  • A low-Reynolds-number second moment turbulence closure was developed with the aid of DNS data. Model coefficients of nonlinear return to isotropy term were derived by use of Cayley-Hamilton theorem and two component turbulence limit condition as the functions of invariances of anisotropy and turbulent Reynolds number. Launder and Tselepidakis' cubic mean pressure strain model was modified to fit the predicted pressure-strain components to the DNS data. Two component turbulence limit condition was the precondition to be satisfied in developing the second moment turbulence closure for the realizable Reynolds stress prediction. But the satisfactions of Reynolds stress level and pressure-strain level of each component were compromised because the satisfaction of both levels was impossible.

Nonlinear Characteristics of Low-speed Flow Induced Vibration for the Safety Design of Micro Air Vehicle

  • Chang, Tae-Jin;Kim, Dong-Hyun;Lee, In
    • Transactions of the Korean Society for Noise and Vibration Engineering
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    • v.12 no.11
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    • pp.873-881
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    • 2002
  • The fluid induced vibration (FIV) phenomena of an equivalent airfoil system of MAV have been investigated in low Reynolds number flow region. Unsteady flows with viscosity are computed using two-dimensional incompressible Navier-Stokes equations. The present fluid/structure interaction analysis is based on one of the most accurate computational approach with computational fluid dynamics (CFD) and computational structural dynamics (CSD) techniques. The highly nonlinear fluid/structure interaction phenomena due to severe flow separations have been analyzed for the low Reynolds region that has a dominancy of flow viscosity. The effects of Reynolds number and initial angle of attack on the fluid/structure coupled vibration instability are shown and the qualitative trend of FIV phenomenon is investigated.

A Non-linear Low-Reynolds-Number Heat Transfer Model for Turbulent Separated and Reattaching Flows (난류박리 및 재부착 유동에 대한 저레이놀즈수 비선형 열전달 모형의 개발)

  • Rhee, Gwang-Hoon;Sung, Hyung-Jin
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.24 no.2
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    • pp.316-323
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    • 2000
  • A nonlinear low-Reynolds-number heat transfer model is developed to predict turbulent flow and heat transfer in separated and reattaching flows. The $k-{\varepsilon}-f_{\mu}$ model of Park and Sung (1997) is extended to a nonlinear formulation, based on the nonlinear model of Gatski and Speziale (1993). The limiting near-wall behavior is resolved by solving the $f_{\mu}$ elliptic relaxation equation. An improved explicit algebraic heat transfer model is proposed, which is achieved by applying a matrix inversion. The scalar heat fluxes are not aligned with the mean temperature gradients in separated and reattaching flows; a full diffusivity tensor model is required. The near-wall asymptotic behavior is incorporated into the $f_{\lambda}$ function in conjunction with the $f_{\mu}$ elliptic relaxation equation. Predictions of the present model are cross-checked with existing measurements and DNS data. The model preformance is shown to be satisfactory.

A Low-Reynolds-Number 4-Equation Model for Turbulent Separated and Reattaching Flows (난류박리 및 재부착 유동의 해석을 위한 저레이놀즈수 4-방정식 난류모형의 개발)

  • 이광훈;성형진
    • Transactions of the Korean Society of Mechanical Engineers
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    • v.19 no.8
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    • pp.2039-2050
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    • 1995
  • The nonlinear low-Reynolds-number k..epsilon. model of park and Sung is extended to predict the turbulent heat transports in separated and reattaching flows. The equations of the temperature variance( $k_{\theta}$ and its dissipation rate(.epsilon.$_{\theta}$ are solved, in concert with the equations of the turbulent kinetic energy(k) and its dissiation rate(.epsilon). In the present model, the near-wall effect and the non-equilibrium effect are fully taken into consideration. The validation of the model is then applied to the turbulent flow behind a backward-facing step and the flow over a blunt body. The predicted results of the present model are compared and evaluated with the relevant experiments.

Numerical computation of turbulent flow in a square sectioned $180^{\circ}$ bend by low-Reynolds-number second moment turbulence closure (저레이놀즈수 2차 모멘트 난류모형에 의한 정사각단면의 $180^{\circ}$ 곡덕트 난류유동의 수치해석)

  • Sin, Jong-Geun;Choe, Yeong-Don
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.20 no.8
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    • pp.2650-2669
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    • 1996
  • A new low Reynolds number nonlinear second moment turbulence closure was introduced to analyze a square sectioned 180.deg. bend flow. Inclusion of nonlinear return to isotropy term and cubic mean pressure strain term has brought out a marked improvement in the level of agreement with measured velocity profiles. Optimization of present closure was performed by comparison of computed velocity profiles with the experimental ones with variation of nonlinear return to isotropy term and quadratic and cubic pressure-strain model. Progressive vortex breakdown due to the interaction of primary and secondary flows was well captured by using the optimized second moment turbulence closure.

Nonlinear Characteristics of Flow Separation Induced Vibration at Low-Speed Using Coupled CSD and CFD technique (전산구조진동/전산유체 기법을 연계한 저속 유동박리 유발 비선형 진동특성 연구)

  • Kim, Dong-Hyun;Chang, Tae-Jin;Kwon, Hyuk-Jun;Lee, In
    • Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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    • 2002.05a
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    • pp.140-146
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    • 2002
  • The fluid induced vibration (FIV) phenomena of a 2-D.O.F airfoil system have been investigated in low Reynolds number incompressible flow region. Unsteady flows with viscosity are computed using two-dimensional incompressible Navier-stokes code. To validate developed Navier-Stokes code, steady and unsteady flow fields around airfoil are analyzed. The present fluid/structure interaction analysis is based on the most accurate computational approach with computational fluid dynamics (CSD) and computational structural dynamics (CSD) techniques. The highly nonlinear fluid/structure interaction phenomena due to severe flow separations have been analyzed fur the low Reynolds region (R$_{N}$ =500~5000) that has a dominancy of flow viscosity. The effect of R$_{N}$ on the fluid/structure coupled vibration instability of 2-DOF airfoil system is presented and the effect of initial angle of attack on the dynamic instability are also shown.own.

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VORTEX-INDUCED VIBRATION SIMULATION OF MULTIPLE CIRCULAR CYLINDERS IN LOW REYNOLDS NUMBER FLOWS USING CARTESIAN MESHES (직교 격자를 이용한 저 레이놀즈 수 유동장내 다중 배치된 실린더의 와유기 진동 해석)

  • Han, Myung-Ryoon;Ahn, Hyung-Teak
    • Journal of computational fluids engineering
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    • v.16 no.1
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    • pp.73-82
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    • 2011
  • In this paper, the vortex-induced vibration of circular cylinders is studied using the immersed boundary method on the Cartesian mesh. The Reynolds numbers considered is from 100 to 200. Using the configuration of tendemly arranged multiple circular cylinders, the vortex shedding behind of the cylinders and their flow-induced motion are investigated. The staggered MAC grid arrangement, which is the typical grid system for the incompressible flow on the Cartesian meshes, is utilized. Pressure correction method is applied for solving the divergence-free incompressible velocity field. The body motion is described by immersed boundary technique that has advantages for moving object on the fixed computational domain. It is also discussed for the computational noise in hydrodynamic forces when body motion is represented by the immersed boundary method. The Predictor/Corrector method is used for simulating the nonlinear response of the elastically mounted cylinder excited by vortex-shedding.

Application of a near-wall turbulence model to the flows over a step with inclined wall (경사진 계단유동의 해석을 위한 벽면근접 난류모형의 적용)

  • An, Jong-U;Park, Tae-Seon;Seong, Hyeon-Jin
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.21 no.6
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    • pp.735-746
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    • 1997
  • A nonlinear low-Reynolds-number k-.epsilon. model of Park and Sung was extended to predict the flows over a step with inclined wall, where a boundary layer flow without separation and a separated and reattaching flow coexist. For a better prediction of the flows, a slight modification was made on the function of the wall damping( $f_{\mu}$) and the model constant ( $C_{{\epsilon}1}$) in the .epsilon.-equation. The model performance was validated by comparing the model predictions with the experiment. It was shown that the flows over a step with inclined wall are simulated successfully with the present model.ent model.