• Title/Summary/Keyword: Two-dimensional backward facing step flow

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Buoyancy-Affected Separated Laminar Flow over a Vertically Located, Two-Dimensional Backward-Facing Step (수직으로 놓인 후향계단위를 흐르는 유체유동에 미치는 부력의 영향에 관한 연구)

  • 백병준;박복춘;김진택
    • Transactions of the Korean Society of Mechanical Engineers
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    • v.17 no.5
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    • pp.1253-1261
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    • 1993
  • Numerical analysis and measurements of the velocity and temperature distributions in buoyancy assisting laminar mixed convection flow over a vertically located, two-dimensional backward-facing step are reported. Laser-Doppler Velocimeter and Constant Temperature Anemometer operated in constant current were used to measure simultaneously the velocity and temperature distributions in the recirculation region downstream of the step. The reattachment length was measured by using flow visualization technique for different inlet velocities, wall temperatures and step heights. While the reattachment length $X_r$ increases as the inlet velocity or step height increase, it decreases as the buoyancy force increases, causing the size of the recirculation region to decrease. For the experimental range of $Gr_s$/$Re_{s}^{2}$$\times$$10^3$<17, a correlation equation for the reattachment length can be given by $X_{r}=1.05(2.13+0.021 Re_{s})exp$ $(-33.7_s^{-0.186}/Gr_{s}/Re_{s}^2).$ The Nusselt number is found to increase and the location of its maximum value moves closer to the step as the buoyancy force increases. The location of the maximum Nusselt number occurs downstream of the reattachment point, and distance between the reattachment point and the location of the maximum Nusselt mumber increases as the buoyancy force increases. Computational prediction agrees favorably well with measured results.

Prediction of Turbulent Flows with Separation and Swirl Using the RNG K-$\varepsilon$ Turbulence Model (RNG k-$\varepsilon$ 난류모델을 이용한 유동박리 및 선회를 가지는 난류유동의 예측)

  • 김성구;오군섭;김용모;이창식
    • Transactions of the Korean Society of Automotive Engineers
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    • v.4 no.5
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    • pp.119-129
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    • 1996
  • This study is concerned with the critical evaluation of predicative capability of a k-$\varepsilon$ turbulence model using the Renormalization Group(RNG) theory. The present numerical model for solution of the Navier-Stokes System is based on the modified PISO algorithms. Computations have been performed with the RNG-based K-$\varepsilon$ model for the two-dimensional flow over a backward-facing step, a confined coaxial jet, and a swirling flow in a swirl combustor. Numerical results are compared with experimental data in terms of mean flow velocities, turbulent kinetic energy, and turbulent stresses. Numerical results clearly indicate that the RNG-based K-$\varepsilon$ turbulence model shows a significant improvement over a standard K-$\varepsilon$ model in predicting the turbulent flows with flow separation and swirl.

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Preconditioned Compressible Navier- Stokes Algorithm for Low Mach Number Flows (예조건화 압축성 알고리즘에 의한 저마하수 유동장 해석기법)

  • Ko Hyun;Yoon Woong-Sup
    • 한국전산유체공학회:학술대회논문집
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    • 1998.05a
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    • pp.35-42
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    • 1998
  • Time marching algorithms applied to compressible Navier-Stokes equation have a convergence problem at low Mach number. It is mainly due to the eigenvalue stiffness and pressure singularity as Mach number approaches to zero. Among the several methods to overcome the shortcomings of time marching scheme, time derivative preconditioning method have been used successfully. In this numerical analysis, we adopted a preconditioner of K.H. Chen and developed a two-dimensional, axisymmetric Navier-Stokes program. The steady state driven cavity flow and backward facing step flow problems were computed to confirm the accuracy and the robustness of preconditioned algorithm for low Mach number flows. And the transonic and supersonic flows insice the JPL axisymmetric nozzle internal flow is exampled to investigate the effects of preconditioning at high Mach number flow regime. Test results showed excellent agreement with the experimental data.

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Calculation of the incompressible Navier-stokes equations in generalized nonorthogonal body fitted coordinate system (일반 비직교 표면좌표계에서의 비압축성 Navier-Stokes방정식의 수치해석)

  • Gang, Dong-Jin;Bae, Sang-Su
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.20 no.3
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    • pp.1015-1027
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    • 1996
  • In this paper, a numerical procedure for the calculation of the incompressible Navier-Stokes equations in a generalized nonorthogonal body fitted coordinate system is proposed and is validated through three test problems. Present numerical procedure derives the pressure equation by using the pressure substitution method on the regular grid system, and discretized momentum equations are based on the covariant velocity components. Cavity flow, backward facing step flow, and two dimensional channel flow with a sinusoidal wavy wall are chosen as three test problems. Numerical solutions obtained by present procedure shows a good agreement with previous numerical and/or experimental results. Convergence rate is also satisfactory.

Control of Turbulent Recirculating Flow by Local Forcing (국소교란에 의한 난류 재순환유동의 제어)

  • 전경빈;성형진
    • Transactions of the Korean Society of Mechanical Engineers
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    • v.18 no.2
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    • pp.446-455
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    • 1994
  • An experimental study is conducted for the turbulent recirculating flow behind a backward-facing step when the oscillating jet is issued sinusoidally through a thin slit at the separation edge. Two key parameters are dealt with in the present experiment, i.e., the amplitude of forcing and the forcing frequency. The Reynolds number based on the step height is varied from 25,000 to 35,000. In order to investigate the effect of local forcing, turbulent structures are scrutinized for both the flow of forcing and the flow of no forcing. The growth of shear layer with a local forcing is larger than that of no forcing. The influence of a local forcing brings forth the decrease of reattachment length and the particular frequency gives a minimum reattachment length. The most effective frequency depends on the non-dimensional frequency, St/sub .theta./, based on the momentum thickness at the separation point. A comparative study leads to the conclusion that the large-scale vortical structure is strongly associated with the forcing frequency and the natural flow instability.

An Imprevement of the Approximate-Factorization Scheme and Its Application to the Analysis of Incompressible Viscous Flows (근사인자화법의 개량과 비압축성 유동해석에의 응용)

  • 신병록
    • Transactions of the Korean Society of Mechanical Engineers
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    • v.19 no.8
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    • pp.1950-1963
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    • 1995
  • A modification of the approximate-factorization method is made to accelerate the convergency rate and to take sufficiently large Courant number without loss of accuracy. And a stable implicit finite-difference scheme for solving the incompressible Navier-Stokes equations employed above modified method is developed. In the present implicit scheme, the volume fluxes with contravariant velocity components and the pressure formulation in curvilinear coordinates is adopted. In order to satisfy the continuity condition completely and to remove spurious errors for the pressure, the Navier-Stokes equations are solved by a modified SMAC scheme using a staggered gird. The upstream-difference scheme such as the QUICK scheme is also employed to the right hand side. The implicit scheme is unconditionally stable and satisfies a diagonally dominant condition for scalar diagonal linear systems of implicit operator on the left hand side. Numerical results for some test calculations of the two-dimensional flow in a square cavity and over a backward-facing step are obtained using both usual approximate-factorization method and the modified one, and compared with each other. It is shown that the present scheme allows a sufficiently large Courant number of O(10$^{2}$) and reduces the computing time.

Effects of Injection Configuration on Mixing in Supersonic Combustor

  • Sakamoto, Hayato;Matsuo, Akiko;Mitani, Tohru
    • Proceedings of the Korean Society of Propulsion Engineers Conference
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    • 2004.03a
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    • pp.48-54
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    • 2004
  • The effects of injector spacing s and injector diameter d on mixing are numerically investigated in supersonic combustor with perpendicular injection behind a backward-facing step. Simulations are reported for airstream Mach number of 2.4. Parameters are changed on following 4 cases to investigate the effects of injector configuration on mixing efficiency $\eta_m$. In the case of varying d or s, dynamic pressure ratio $Rq(=(pu^2)_j/(pu^2)_a)$ is also varied to keep bulk equivalence ratio $\Phi({\oe})Rq.d^2/s)$ constant. (l) Injector spacing s is varied at constant $\Phi$=0.5, 1, 2 for injector diameter d=6mm. In the case of $\Phi$=1, $\eta_m$ has its maximum value at s=24mm. The reason is that increase of $\eta_m$. , by widening spacing at Rq=constant competes with decrease of $\eta_m$ by increasing Rq at s=constant. When spacing is narrow, the flow field of vicinity of injector becomes two-dimensional because adjacent jets interferes each other. By widening spacing, air is easily entrained by three-dimensional effect. This mechanism also appears in the case of $\Phi$=0.5, 2 for d=6mm, and $\eta_m$. reaches its maximum value at s=24mm for $\Phi$=0.5 and at s=42mm for $\Phi$=2. (2) In the case of injector diameter d varied at $\Phi$=1 for s=30mm, $\eta_m$. has its maximum value at d=3mm. The reason is that decrease of $\eta_m$ by increasing injector diameter competes with increase of $\eta_m$ by decreasing Rq at d=constant.(3) In the case of s varied at $\Phi$=0.5, 1,2 for d=3mm, the injector spacing at which mixing efficiency has its maximum value is s= 18mm for $\Phi$=0.5, s=24mm for $\Phi$=1, s=24mm for $\Phi$=2. Therefore it is found that d=3mm and s=24mm can be optimum configuration over a range of $\Phi$=0.5~2.(4) The effect of h on the optimum spacing is investigated. s is varied for d=6mm at step height h=4, 6, 8mm. The simulation results do not show significant change on the step height.

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A Passive Control of Cavity-Induced Pressure Oscillations Using Sub-Cavity System (보조공동계를 이용한 공동 유기 압력진동의 피동제어)

  • Kang, M.S.;Kwon, J.K.;Lee, J.S.;Kim, H.D.;Setoguchi, T.
    • 한국전산유체공학회:학술대회논문집
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    • 2008.03b
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    • pp.452-455
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    • 2008
  • A new passive control technique of cavity-induced pressure oscillations has been investigated numerically for a supersonic two-dimensional flow over open rectangular cavities at Mach number 1.83 just upstream of a cavity, in which a sub-cavity system is installed on the backward-facing step of the main cavity. A third-order TVD (Total Variation Diminishing) finite difference scheme with MUSCL is used to discretize the spatial derivatives in the unsteady compressible Navier-Stokes equations. The results obtained show that the present sub-cavity system is very effective in reducing cavity-induced pressure oscillations. The results also showed that the resultant amount of attenuation of cavity-induced pressure oscillations was dependent on the length and thickness of the flat plate, and also on the depth of the sub-cavity used as an oscillation suppressor.

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