• 한국가시화정보학회지
• /
• 제18권3호
• /
• pp.109-115
• /
• 2020

Five layers in mean flow are proposed by using the direct numerical simulation data of turbulent pipe flow up to Reτ = 3008. Viscous sublayer, buffer layer, mesolayer, log layer and core region are investigated. In the buffer layer, the viscous force is counterbalanced by the turbulent inertia from the streamwise mean momentum balance, and a log law occurs here. The overlap layer is composed of the mesolayer and the log layer. Above the buffer layer, the non-negligible viscous force causes the power law, and this region is the mesolayer, where it is the lower part of the overlap layer. At the upper part of the overlap layer, where the viscous force itself becomes naturally negligible, the log layer will appear due to that the acceleration force of the large-scale motions increases as the Reynolds number increases. In the core region, the velocity-defect form is satisfied with the power-law scaling.

• 한국전산유체공학회지
• /
• 제20권3호
• /
• pp.94-103
• /
• 2015

The turbulent flow characteristics in the channel flow are investigated using large eddy simulation(LES) of FDS code, built in NIST(USA), in which the near-wall flow is solved by Werner-Wengle wall function. The periodic flow condition is applied in streamwise direction to get the fully developed turbulent flow and symmetric condition is applied in lateral direction. The height of the channel is H=1m, and the length of the channel is 6H, and the lateral length is H. The total grid is $32{\times}32{\times}32$ and $y^+$ is kept above 11 to fulfill the near-wall flow requirement. The Smagorinsky model is used to solve the sub-grid scale stress. Smagorinsky constant $C_s$ is 0.2(default in FDS). Three cases of Reynolds number(10,700, 26,000, 49,000.), based on the channel height, are analyzed. The simulated results are compared with direct numerical simulation(DNS) and particle image velocimetry(PIV) experimental data. The linear low-Re eddy viscosity model of Launder & Sharma and non-linear low-Re eddy viscosity model of Abe-Jang-Leschziner are utilized to compare the results with LES of FDS. Reynolds normal stresses, Reynolds shear stresses, turbulent kinetic energys and mean velocity flows are well compared with DNS and PIV data.

• 한국전산유체공학회지
• /
• 제17권3호
• /
• pp.59-67
• /
• 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. The effects of Reynolds number on the turbulent mass transfer are identified in the higher-order statistics(Skewness and Flatness factor) and instantaneous concentration fields. The budgets of turbulent mass fluxes and concentration variance were computed and analyzed to elucidate the effect of Reynolds number on turbulent mass transfer. Furthermore, to understand the correlation between near-wall turbulence structure and concentration fluctuation, we present an octant analysis in the vicinity of the pipe wall.

• 대한기계학회논문집B
• /
• 제30권5호
• /
• pp.438-445
• /
• 2006

We performed the simulation of the unsteady three dimensional flow over a square cylinder in a wind tunnel in moderate Reynolds number range, $100{\sim}2500$ by using LBM. SGS model was applied for the turbulent flow. Frist of all we compared LBM(Lattice Boltzmann Method) solution of Poiseuille flow applied Farout and bounce back boundary conditions with the analytical and FOAM solutions to verify the applicability of the boundary conditions. For LBM simulation the calculation domain was formed by structured grids and prescribed uniform velocity and density inlet and Farout boundary conditions were imposed on the in-out boundaries. Bounceback and wind tunnel boundary conditions were applied to the cylinder walls and the boundaries of calculation domain respectively. The maximum Strouhal number of the vortex shedding is 0.2025 at Re = 750. and the number maintains the constant value of 0.18 when Re>1000. We also predicted that the critical reynolds number of the turbulent flow is in the range of $250{\sim}500$.

• 한국전산유체공학회지
• /
• 제21권4호
• /
• pp.1-10
• /
• 2016

The analysis of characteristics of turbulent flow and thermal boundary layer for natural convection caused by fire along vertical wall is performed. The 4m-high vertical copper plate is heated and kept at a uniform surface temperature of $60^{\circ}C$ and the surrounding fluid (air) is kept at $16.5^{\circ}C$. The flow and temperature is solved by large eddy simulation(LES) of FDS code(Ver.6), in which the viscous-sublayer flow is calculated by Werner-Wengle wall function. The whole analyzed domain is assumed as turbulent region to apply wall function even through the laminar flow is transient to the turbulent flow between $10^9$<$Gr_z$<$10^{10}$ in experiments. The various grids from $7{\times}7{\times}128$ to $18{\times}18{\times}128$ are applied to investigate the sensitivity of wall function to $x^+$ value in LES simulation. The mean velocity and temperature profiles in the turbulent boundary layer are compared with experimental data by Tsuji & Nagano and the results from other LES simulation in which the viscous-sublayer flow is directly solved with many grids. The relationship between heat transfer rate($Nu_z$) and $Gr_zPr$ is investigated and calculated heat transfer rates are compared with theoretical equation and experimental data.

• 대한기계학회논문집B
• /
• 제29권3호
• /
• pp.314-321
• /
• 2005

Discrete Wavelet Transform (DWT) has been applied to the Direct Numerical Simulation (DNS) data of turbulent channel flow. DWT splits the turbulent flow into two orthogonal parts, one corresponding to coherent structures and the other to incoherent background flow. The coherent structure is extracted from not vorticity field but velocity's since the channel flow is not isoropic. By comparing DWT's result of channel flow with that of isotropic flow, it is shown that coherent structure maintains the properties of original channel flow. The velocity field of coherent structures can be represented by few wavelet modes and that these modes are sufficient to reproduce the velocity probability density function (PDF) and the energy spectrum over the entire inertial range. The remaining incoherent background flow is homogeneous, has small amplitude, and is uncorrelated. These results are compared with those obtained for the same compression rate using large eddy simulation (LES) filtering. In contrast to the incoherent background flow of DWT, the LES subgrid scales have a much larger amplitude and are correlated, which makes their statistical modeling more difficult.

• 대한기계학회:학술대회논문집
• /
• 대한기계학회 2004년도 춘계학술대회
• /
• pp.1813-1818
• /
• 2004

Discrete Wavelet Transform (DWT) has been applied to the Direct Numerical Simulation (DNS) data of turbulent channel flow. DWT splits the turbulent flow into two orthogonal parts, one corresponding to coherent structures and the other to incoherent background flow. The coherent structure is extracted from not vorticity field but velocity's since the channel flow is not isotropic. By comparing DWT's result of channel flow with that of isotropic flow, it is shown that coherent structure maintains the properties of original channel flow. The velocity field of coherent structures can be represented by few wavelet modes and that these modes are sufficient to reproduce the velocity probability distribution function (PDF) and the energy spectrum over the entire inertial range. The remaining incoherent background flow is homogeneous, has small amplitude, and is uncorrelated. These results are compared with those obtained for the same compression rate using large eddy simulation (LES) filtering. In contrast to the incoherent background flow of DWT, the LES subgrid scales have a much larger amplitude and are correlated, which makes their statistical modeling more difficult.

• Journal of Advanced Marine Engineering and Technology
• /
• 제30권3호
• /
• pp.369-375
• /
• 2006

Large eddy simulation has been applied to simulate turbulent flow around a cubic obstacle mounted on a channel surface for a Reynolds number of 40000(based on the incoming bulk velocity and the obstacle height) using a Smagorinsky model and a Lagrangian dynamic model. In order to develop the LES to the practical engineering application, the effect of upwind scheme, turbulent sub-grid scale model were investigated. The computed velocities. turbulence quantifies, separation and reattachment length were evaluated by compared with the previous experimental results.

• Journal of Mechanical Science and Technology
• /
• 제14권4호
• /
• pp.441-447
• /
• 2000

Direct numerical simulation has been used to study turbulent boundary layers with convex curvature. A direct numerical simulation program has been developed to solve incompressible Navier-Stokes equations in generalized coordinates with the finite volume method. We considered two boundary layer thicknesses. When the curvature effect is small, mean velocity statistics show little difference with those of a plane channel flow. Turbulent intensity decreases as curvature increases. Contours suggest that streamwise vorticities are strong where large pressure fluctuations exist.

• Wind and Structures
• /
• 제14권5호
• /
• pp.465-480
• /
• 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.