• Journal of computational fluids engineering
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• v.17 no.3
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• pp.1-10
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• 2012

Large Eddy Simulation(LES) of turbulent mass transfer in fully developed turbulent pipe flow has been performed to study the effect of Reynolds number on the concentration fields at $Re_{\tau}=180$, 395, 590 based on friction velocity and pipe radius. Dynamic subgrid-scale models for the turbulent subgrid-scale stresses and mass fluxes were employed to close the governing equations. Fully developed turbulent pipe flows with constant mass flux imposed at the wall are studied for Sc=0.71. The mean concentration profiles and turbulent intensities obtained from the present LES are in good agreement with the previous numerical and experimental results currently available. To show the effects of Reynolds number on the turbulent mass transfer, the mean concentration profile, root-mean-square of concentration fluctuations, turbulent mass fluxes, cross-correlation coefficient, turbulent diffusivity and turbulent Schmidt number are presented.

• Journal of the Korean Society of Visualization
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• v.14 no.1
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• pp.46-50
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• 2016

Direct numerical simulations (DNSs) of turbulent pipe flows with temporal deceleration were performed to examine response of the turbulent flows to the deceleration. The simulations were started with a fully-developed turbulent pipe flow at the Reynolds number, $Re_D=24380$, based on the pipe radius and the laminar centerline velocity, and three different constant temporal decelerations were applied to the initial flow with varying dU/dt = -0.001274, -0.00625 and -0.025. It was shown that the mean flows were greatly affected by temporal decelerations with downward shift of log law, and turbulent intensities were increased in particular in the outer layer, compared to steady flows at a similar Reynolds number. The analysis of Reynolds shear stress showed that second- and fourth-quadrant Reynolds shear stresses were increased with the decelerations, and the increase of the turbulence was attributed to enhancement of outer turbulent vortical structures by the temporal decelerations.

• Journal of the Korean Society of Visualization
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• v.15 no.3
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• pp.27-33
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• 2017

Direct numerical simulations of turbulent channel flows with moving wall conditions on the top wall are performed to examine the effects of the moving wall on the turbulent characteristics. The moving wall velocity only applied to the top wall with the opposite direction to the main flow is systematically varied to reveal the sustained-mechanism for turbulence. The turbulence statistics for the Couette-Poiseuille flow, such as mean velocity, root mean square of the velocity fluctuations, Reynolds shear stress and pre-multiplied energy spectra of the velocity fluctuations, are compared with those of canonical turbulent channel flows. The comparison suggests that although the turbulent activity on the top wall increases with increasing the Reynolds number, that on the bottom wall decreases, contrary to the previous finding for the canonical turbulent channel flows. The increase of the turbulent energy on the top wall is attributed to not only the increase of the Reynolds number but also elongation of the logarithmic layer due to increase of the wall layer on the top wall. However, because the logarithmic layer is shortened on the bottom wall due to the decrease of the wall layer, the turbulence energy on the bottom wall decreases despite of the increase of the Reynolds number.

• 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.

• 한국연소학회:학술대회논문집
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• 2004.11a
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• pp.91-96
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• 2004

Simultaneous CH and OH planar laser induced fluorescence(PLIF) and stereoscopic particle image velocimetry (PIV) measurements have been developed to investigate the local flame structure of turbulent premixed flames. The developed simultaneous two radical concentrations and three component velocity measurements on a two-dimensional plane was applied for relatively high Reynolds number turbulent premixed flames in a swirl stabilized combustor. All measurements were conducted for methane-air premixed flames in the corrugated flamelets regime. Strong three-dimensional fluctuation implies that misunderstanding of the flame/turbulent interactions would be caused by the analysis of two-component velocity distribution in a cross section. Furthermore, comparisons of CH-OH PLIF and three-component velocity field show that the burned gases not always have high-speed velocity in relatively high Reynolds number turbulent premixed flame. The Reynolds number dependence of the flame front was clearly captured by the simultaneous CH-OH PLIF and stereoscopic PIV measurements.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.6 no.4
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• pp.325-336
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• 1994

A numerical simulation of velocity and temperature fields and Nusselt number distributions is performed by using the algebraic stress model (ASM) for the velocity profiles and low Reynolds number ${\kappa}-{\varepsilon}$ model and the algebraic heat flux model(AHFM) for turbulent heat transfer in a $180^{\circ}$ bend with a constant wall heat flux. In the low Reynolds number ${\kappa}-{\varepsilon}$ model, turbulent Prandtl number is modified by considering the streamline curvature effect and the non-equilibrium effect between turbulent kinetic energy production and dissipation rate. Every heat flux term presented in the transport equation of turbulent heat flux is reduced to algebraic expressions in a way similar to algebraic stress model. Also. in the wall region, low Reynods number algebraic heat flux model(AHFM) is applied.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.31 no.8
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• pp.8-18
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• 2003

An experimental study is carried out to investigate the Reynolds number effects on the near-wake of an airfoil oscillating in pitch. An NACA 4412 airfoil is sinusoidally pitched about the quarter chord point between the angle of attack -6$^{\circ}$ and +6$^{\circ}$. A hot-wire anemometer is used to measure the turbulent intensity in the near-wake region of an NACA 4412 airfoil. The freestream velocities of present work are 3.4, 12.4, 26.2 m/s, and the corresponding Reynolds numbers are $5.3{\times}10^4,\;1.9{\times}10^5,\;4.1{\times}10^5$ and the reduced frequency is 0.1. Axial turbulent intensity profiles are presented to show the Reynolds number effects on the near-wake region behind an airfoil oscillating in pitch. All the cases in these measurements show that the turbulent intensities by the change of the Reynolds number are very large at the lowest Reynolds number $R_N=5.3{\times}10^4$; and are small at the other Reynolds number $(R_N=1.9{\times}10^5\;and\;4.1{\times}10^5)$ in the near-wake region. The significant difference of turbulent intensity between $R_N=5.3{\times}10^4,\;and\;1.9{\times}l0^5$ is observed. A critical value of the Reynolds number in the near-wake of an oscillating NACA 4412 airfoil which indicates laminar separation, no separation or turbulent separation exists in the range between $R_N=5.3{\times}10^4\;and\;1.9{\times}10^5$.

• Transactions of the Korean Society of Mechanical Engineers
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• v.17 no.6
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• pp.1596-1608
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• 1993

Low Reynolds number second moment turbulence model which be applicable to the fine gird near the wall region was developed. In this model, turbulence model coefficients in the pressure strain model of the Reynolds stress equation was expressed as functions of turbulence Reynolds number $R_{t}\equivk^{2}/(\nu\varepsilon)).$ In the derivation procedure of the present low Reynolds number algebraic stress model, Laufer's near wall experimental data on Reynolds stresses were curve fitted as functions of R$_{t}$ and the resulting simultaneous equations of the model coefficients were solved by using the boundary conditions at wall and high Reynolds number limiting conditions. Predicted Reynolds stresses and dissipation rate of turbulent kinetic energy etc. in the 2 dimensional parallel, plane channel flow and pipe flow were compared with the preditions obtained by employing the Launder-Shima model, standard algebraic stress model and several experimental data. Results show that all the Reynolds stresses and dissipation rate of turbulent kinetic energy predicted by the present low Reynolds number algebraic stress model agree better with the experimental data than those predicted by other algebraic stress models.

• Wind and Structures
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• v.14 no.5
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• pp.465-480
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• 2011

In this paper, a stabilized large eddy simulation technique is developed to predict turbulent flow with high Reynolds number. Streamline Upwind Petrov-Galerkin (SUPG) stabilized method and three-step technique are both implemented for the finite element formulation of Smagorinsky sub-grid scale (SGS) model. Temporal discretization is performed using three-step technique with viscous term treated implicitly. And the pressure is computed from Poisson equation derived from the incompressible condition. Then two numerical examples of turbulent flow with high Reynolds number are discussed. One is lid driven flow at Re = $10^5$ in a triangular cavity, the other is turbulent flow past a square cylinder at Re = 22000. Results show that the present technique can effectively suppress the instabilities of turbulent flow caused by traditional FEM and well predict the unsteady flow even with coarse mesh.

• 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.