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1.
Zonal RANS/LES coupling simulation of a transitional and separated flow around an airfoil near stall
F. Richez I. Mary V. Gleize C. Basdevant 《Theoretical and Computational Fluid Dynamics》2008,22(3-4):305-315
The objective of the current study is to examine the course of events leading to stall just before its occurrence. The stall
mechanisms are very sensitive to the transition that the boundary layer undergoes near the leading edge of the profile by
a so-called laminar separation bubble (LSB). In order to provide helpful insights into this complex flow, a zonal Reynolds-averaged
Navier–Stokes (RANS)/large-eddy simulation (LES) simulation of the flow around an airfoil near stall has been achieved and
its results are presented and analyzed in this paper. LSB has already been numerically studied by direct numerical simulation
(DNS) or LES, but for a flat plate with an adverse pressure gradient only. We intend, in this paper, to achieve a detailed
analysis of the transition process by a LSB in more realistic conditions. The comparison with a linear instability analysis
has shown that the numerical instability mechanism in the LSB provides the expected frequency of the perturbations. Furthermore,
the right order of magnitude for the turbulence intensities at the reattachment point is found.
相似文献
2.
Analysis of DNS and LES of Flow in a Low Pressure Turbine Cascade with Incoming Wakes and Comparison with Experiments 总被引:1,自引:0,他引:1
The flow around a low-pressure turbine rotor blade with incoming periodic wakes is computed by means of DNS and LES. The latter
adopts a dynamic sub-grid-scale model. The computed results are compared with time-averaged and instantaneous measured quantities.
The simulation sreveal the presence of elongated flow structures, stemming from the incoming wake vorticity, which interact
with the pressure side boundary layer. As the wake approaches the upstream half of the suction side, its vortical structures
are stretched and align with the main flow, resulting in an impingement at virtually zero angle of attack. Periodically, in
the absence of impinging wakes, the laminar suction side boundary layer separates in the adverse pressure gradient region.
Flow in the laminar separation bubble is found to undergo transition via a Kelvin–Helmholtz instability. Subsequent impingement
of the wake inhibits separation and thus promotes boundary layer reattachment. LES provides a fair reproduction of the DNS
results both in terms of instantaneous, phase-averaged, and time-averaged flow fields with a considerable reduction in computational
effort.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
3.
The interaction of an oblique shock wave with a turbulent boundary layer under conditions of incipient separation is analyzed
by means of large-eddy simulation (LES) and Reynolds-averaged Navier–Stokes (RANS) turbulence models, with the objective to
explore their predictive capabilities, in particular with respect to the unsteady features of the interaction. Consistent
with earlier direct numerical simulations, we have found that the flow dynamics in the interaction zone is characterized by
strong intermittency associated with the formation of scattered spots of flow reversal near the nominal position of the reflected
shock. Comparison with experimental results (at much larger Reynolds number) show that the qualitative features of the interaction
are predicted reasonably well by both LES and RANS models. RANS models supplemented with a semi-empirical closure are also
found to provide reasonable estimate of the fluctuating pressure loads at the wall. 相似文献
4.
A new turbulent injection procedure dedicated to fully compressible direct numerical simulation (DNS) or large eddy simulation (LES) solvers is proposed. To avoid the appearance of spurious acoustic waves, this method is based on an accurate tracking of the turbulent structures crossing the boundary at the inlet of the domain. A finite difference DNS solver has been coupled with a spectral simulation in which a statistically stationary homogeneous turbulence evolves to provide fluctuating boundary conditions.A new turbulence forcing method, dedicated to spectral solvers, has been developed as well to control the major properties of the injected flow (turbulent kinetic energy, dissipation rate and integral length scale). One-dimensional Navier–Stokes characteristic boundary conditions extended to non-stationary flows are coupled with the injection procedure to evaluate is potential in four various configurations: spatially decaying turbulence, dispersion of vaporizing sprays, propagation of one- and two-phase V-shape turbulent flames. 相似文献
5.
We give an overview on the usage of computer simulations in industrial turbulent dispersed multiphase flows. We present a few examples of industrial flows: bubble columns and bubbly pipe flows, stirred tanks, cyclones, and a fluid catalytic cracking unit. The fluid catalytic cracking unit is used to illustrate the complexity of the physical phenomena involved, and the possibilities and limitations of the different approaches used: Eulerian–Lagrangian (particle-tracking) and Eulerian–Eulerian (two-fluid). In the first approach, the continuous phase is solved using either RANS simulations (Reynolds-Averaged Navier–Stokes simulations) or DNS/LES (Direct Numerical Simulations/Large-Eddy Simulations), and the individual particles are tracked. In the second approach, the dispersed phase is averaged, leading to two sets equations, which are quite similar to the RANS equations of single-phase flows. The Eulerian–Eulerian approach is the most commonly used in industrial applications, however, it requires a significant amount of modelling. Eulerian–Lagrangian RANS can be simpler to use; in particular in situations involving complex boundary conditions, polydisperse flows and agglomeration/breakup. The key issue for the success of the simulations is to have good models for the complex physics involved. A major weakness is the lack of good models for: the turbulence modification promoted by the particles, the inter-particle interactions, and the near-wall effects. Eulerian–Lagrangian DNS/LES can play an important role as a research tool, in order to get a better physical understanding, and to improve the models used in the RANS simulations (either Eulerian–Eulerian or Eulerian–Lagrangian). 相似文献
6.
Xian-Xiang Li Chun-Ho Liu Dennis Y.C. Leung 《International Journal of Computational Fluid Dynamics》2013,27(1-2):37-49
This article develops a parallel large-eddy simulation (LES) with a one-equation subgrid-scale (SGS) model based on the Galerkin finite element method and three-dimensional (3D) brick elements. The governing filtered Navier–Stokes equations were solved by a second-order accurate fractional-step method, which decomposed the implicit velocity–pressure coupling in incompressible flow and segregated the solution to the advection and diffusion terms. The transport equation for the SGS turbulent kinetic energy was solved to calculate the SGS processes. This FEM LES model was applied to study the turbulence of the benchmark open channel flow at a Reynolds number Reτ = 180 (based on the friction velocity and channel height) using different model constants and grid resolutions. By comparing the turbulence statistics calculated by the current model with those obtained from direct numerical simulation (DNS) and experiments in literature, an optimum set of model constants for the current FEM LES model was established. The budgets of turbulent kinetic energy and vertical Reynolds stress were then analysed for the open channel flow. Finally, the flow structures were visualised to further reveal some important characteristics. It was demonstrated that the current model with the optimum model constants can predict well the organised structure near the wall and free surface, and can be further applied to other fundamental and engineering applications. 相似文献
7.
A new large eddy simulation (LES) approach for particle-laden turbulent flows in the framework of the Eulerian formalism for
inertial particle statistical modelling is developed. Local instantaneous Eulerian equations for the particle cloud are first
written using the mesoscopic Eulerian formalism (MEF) proposed by Février et al. (J Fluid Mech 533:1–46, 2005), which accounts for the contribution of an uncorrelated velocity component for inertial particles with relaxation time larger
than the Kolmogorov time scale. Second, particle LES equations are obtained by volume filtering the mesoscopic Eulerian ones.
In such an approach, the particulate flow at larger scales than the filter width is recovered while sub-grid effects need
to be modelled. Particle eddy-viscosity, scale similarity and mixed sub-grid stress (SGS) models derived from fluid compressible
turbulence SGS models are presented. Evaluation of such models is performed using three sets of particle Lagrangian results
computed from discrete particle simulation (DPS) coupled with fluid direct numerical simulation (DNS) of homogeneous isotropic
decaying turbulence. The two phase flow regime corresponds to the dilute one where two-way coupling and inter-particle collisions
are not considered. The different particle Stokes number (based on Kolmogorov time scale) are initially equal to 1, 2.2 and
5.1. The mesoscopic field properties are analysed in detail by considering the particle velocity probability function (PDF),
correlated velocity power spectra and random uncorrelated velocity moments. The mesoscopic fields measured from DPS+DNS are
then filtered to obtain large scale fields. A priori evaluation of particle sub-grid stress models gives comparable agreement
than for fluid compressible turbulence models. It has been found that the standard Smagorinsky eddy-viscosity model exhibits
the smaller correlation coefficients, the scale similarity model shows very good correlation coefficient but strongly underestimates
the sub-grid dissipation and the mixed model is on the whole superior to pure eddy-viscosity model. 相似文献
8.
Prediction of separation flows around a 6:1 prolate spheroid using RANS/LES hybrid approaches 总被引:2,自引:0,他引:2
Zhixiang Xiao Yufei Zhang Jingbo Huang Haixin Chen Song Fu 《Acta Mechanica Sinica》2007,23(4):369-382
This paper presents hybrid Reynolds-averaged Navier–Stokes (RANS) and large-eddy-simulation (LES) methods for the separated
flows at high angles of attack around a 6:1 prolate spheroid. The RANS/LES hybrid methods studied in this work include the
detached eddy simulation (DES) based on Spalart–Allmaras (S–A), Menter’s k–ω shear-stress-transport (SST) and k–ω with weakly nonlinear eddy viscosity formulation (Wilcox–Durbin+, WD+) models and the zonal-RANS/LES methods based on the
SST and WD+ models. The switch from RANS near the wall to LES in the core flow region is smooth through the implementation
of a flow-dependent blending function for the zonal hybrid method. All the hybrid methods are designed to have a RANS mode
for the attached flows and have a LES behavior for the separated flows. The main objective of this paper is to apply the hybrid
methods for the high Reynolds number separated flows around prolate spheroid at high-incidences. A fourth-order central scheme
with fourth-order artificial viscosity is applied for spatial differencing. The fully implicit lower–upper symmetric-Gauss–Seidel
with pseudo time sub-iteration is taken as the temporal differentiation. Comparisons with available measurements are carried
out for pressure distribution, skin friction, and profiles of velocity, etc. Reasonable agreement with the experiments, accounting
for the effect on grids and fundamental turbulence models, is obtained for the separation flows.
The project supported by the National Natural Science Foundation of China (10502030 and 90505005). 相似文献
9.
Direct numerical simulation of disturbances generated by periodic suction-blowing in a hypersonic boundary layer 总被引:1,自引:0,他引:1
I. V. Egorov A. V. Fedorov V. G. Soudakov 《Theoretical and Computational Fluid Dynamics》2006,20(1):41-54
A numerical algorithm and code are developed and applied to direct numerical simulation (DNS) of unsteady two-dimensional
flow fields relevant to stability of the hypersonic boundary layer. An implicit second-order finite-volume technique is used
for solving the compressible Navier–Stokes equations. Numerical simulation of disturbances generated by a periodic suction-blowing
on a flat plate is performed at free-stream Mach number 6. For small forcing amplitudes, the second-mode growth rates predicted
by DNS agree well with the growth rates resulted from the linear stability theory (LST) including nonparallel effects. This
shows that numerical method allows for simulation of unstable processes despite its dissipative features. Calculations at
large forcing amplitudes illustrate nonlinear dynamics of the disturbance flow field. DNS predicts a nonlinear saturation
of fundamental harmonic and rapid growth of higher harmonics. These results are consistent with the experimental data of Stetson
and Kimmel obtained on a sharp cone at the free-stream Mach number 8. 相似文献
10.
A synthetic turbulence generation (STG) method for subsonic and supersonic flows at low and moderate Reynolds numbers to provide inflow distributions of zonal Reynolds-averaged Navier–Stokes (RANS) – large-eddy simulation (LES) methods is presented. The STG method splits the LES inflow region into three planes where a local velocity signal is decomposed from the turbulent flow properties of the upstream RANS solution. Based on the wall-normal position and the local flow Reynolds number, specific length and velocity scales with different vorticity content are imposed at the inlet plane of the boundary layer. The quality of the STG method for incompressible and compressible zero-pressure gradient boundary layers is shown by comparing the zonal RANS–LES data with pure LES, pure RANS, and direct numerical simulation (DNS) solutions. The distributions of the time and spanwise wall-shear stress, Reynolds stress distributions, and two point correlations of the zonal RANS–LES simulations are smooth in the transition region and in good agreement with the pure LES and reference DNS findings. The STG approach reduces the RANS-to-LES transition length to less than four boundary-layer thicknesses. 相似文献
11.
Large Eddy Simulations Using the Subgrid-Scale Estimation Model and Truncated Navier–Stokes Dynamics
J. Andrzej Domaradzki Kuo Chieh Loh Patrick P. Yee 《Theoretical and Computational Fluid Dynamics》2002,15(6):421-450
We describe a procedure for large eddy simulations of turbulence which uses the subgrid-scale estimation model and truncated
Navier–Stokes dynamics. In the procedure the large eddy simulation equations are advanced in time with the subgrid-scale stress
tensor calculated from the parallel solution of the truncated Navier–Stokes equations on a mesh two times smaller in each
Cartesian direction than the mesh employed for a discretization of the resolved quantities. The truncated Navier–Stokes equations
are solved through a sequence of runs, each initialized using the subgrid-scale estimation model. The modeling procedure is
evaluated by comparing results of large eddy simulations for isotropic turbulence and turbulent channel flow with the corresponding
results of experiments, theory, direct numerical simulations, and other large eddy simulations. Subsequently, simplifications
of the general procedure are discussed and evaluated. In particular, it is possible to formulate the procedure entirely in
terms of the truncated Navier–Stokes equation and a periodic processing of the small-scale component of its solution.
Received 27 April 2001 and accepted 16 December 2001 相似文献
12.
Fujihiro Hamba 《Theoretical and Computational Fluid Dynamics》2001,14(5):323-336
Large eddy simulation (LES) is combined with the Reynolds-averaged Navier–Stokes (RANS) equation in a turbulent channel-flow
calculation. A one-equation subgrid-scale model is solved in a three-dimensional grid in the near-wall region whereas the
standard k–ε model is solved in a one-dimensional grid in the outer region away from the wall. The two grid systems are overlapped to
connect the two models smoothly. A turbulent channel flow is calculated at Reynolds numbers higher than typical LES and several
statistical quantities are examined. The mean velocity profile is in good agreement with the logarithmic law. The profile
of the turbulent kinetic energy in the near-wall region is smoothly connected with that of the turbulent energy for the k–ε model in the outer region. Turbulence statistics show that the solution in the near-wall region is as accurate as a usual
LES. The present approach is different from wall modeling in LES that uses a RANS model near the wall. The former is not as
efficient as the latter for calculating high-Reynolds-number flows. Nevertheless, the present method of combining the two
models is expected to pave the way for constructing a unified turbulence model that is useful for many purposes including
wall modeling.
Received 11 June 1999 and accepted 15 December 2000 相似文献
13.
We compare the space-time correlations calculated from direct numerical simulation(DNS) and large-eddy simulation(LES) of turbulent channel flows.It is found from the comparisons that the LES with an eddy-viscosity subgrid scale(SGS) model over-predicts the space-time correlations than the DNS.The overpredictions are further quantified by the integral scales of directional correlations and convection velocities.A physical argument for the overprediction is provided that the eddy-viscosity SGS model alone does not includes the backscatter effects although it correctly represents the energy dissipations of SGS motions.This argument is confirmed by the recently developed elliptic model for space-time correlations in turbulent shear flows.It suggests that enstrophy is crucial to the LES prediction of spacetime correlations.The random forcing models and stochastic SGS models are proposed to overcome the overpredictions on space-time correlations. 相似文献
14.
Simulation and Modelling of Turbulent Trailing-Edge Flow 总被引:1,自引:0,他引:1
Computations of turbulent trailing-edge flow have been carried out at a Reynolds number of 1000 (based on the free-stream
quantities and the trailing-edge thickness) using an unsteady 3D Reynolds-Averaged Navier–Stokes (URANS) code, in which two-equation
(k–ε) turbulence models with various low-Re near wall treatments were implemented. Results from a direct numerical simulation
(DNS) of the same flow are available for comparison and assessment of the turbulence models used in the URANS code. Two-dimensional
URANS calculations are carried out with turbulence mean properties from the DNS used at the inlet; the inflow boundary-layer
thickness is 6.42 times the trailing-edge thickness, close to typical turbine blade flow applications. Many of the key flow
features observed in DNS are also predicted by the modelling; the flow oscillates in a similar way to that found in bluff-body
flow with a von Kármán vortex street produced downstream. The recirculation bubble predicted by unsteady RANS has a similar
shape to DNS, but with a length only half that of the DNS. It is found that the unsteadiness plays an important role in the
near wake, comparable to the modelled turbulence, but that far downstream the modelled turbulence dominates. A spectral analysis
applied to the force coefficient in the wall normal direction shows that a Strouhal number based on the trailing-edge thickness
is 0.23, approximately twice that observed in DNS. To assess the modelling approximations, an a priori analysis has been applied using DNS data for the key individual terms in the turbulence model equations. A possible refinement
to account for pressure transport is discussed.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
15.
亚格子(SGS)应力建模在湍流大涡模拟(LES)中有着极为重要的作用. 传统亚格子应力模型存在相对误差较大、耗散过强等问题. 近年来, 计算机技术的发展使得人工神经网络(ANN)等机器学习方法逐渐成为亚格子应力建模型的新研究范式. 本文着重考虑滤波宽度及雷诺数影响, 在不可压缩槽道湍流中建立了亚格子应力的ANN模型. 该模型以滤波后的直接数值模拟(fDNS)流场物理量及滤波尺度为输入信息, 相应滤波尺度下的亚格子应力为输出量. 通过对不同滤波尺度及不同雷诺数数据的训练, ANN模型能够给出与直接数值模拟(DNS)高度吻合的亚格子应力. 此外, 模型在亚格子耗散等非ANN建模量上也有着优异的预测性能, 与基于DNS获得的对应物理量的相关系数大都在0.9以上, 较梯度模型及Smagorinsky模型有明显提升. 在后验测试中, ANN模型对流向平均速度剖面的预测同样优于梯度模型、Smagorinsky模型及隐式大涡模拟(ILES)等传统LES模型. 在脉动速度均方根预测方面, 除了某些法向位置外ANN模型的性能整体上相对其他3个模型有所提升. 然而, 随着网格尺度的增大ANN模型预测的结果与fDNS结果的偏差逐渐增大. 总之, ANN方法在发展高精度亚格子应力模型上具有很大的潜力. 相似文献
16.
Finite Element-Based Characterization of Pore-Scale Geometry and Its Impact on Fluid Flow 总被引:1,自引:0,他引:1
We present a finite element (FEM) simulation method for pore geometry fluid flow. Within the pore space, we solve the single-phase
Reynold’s lubrication equation—a simplified form of the incompressible Navier–Stokes equation yielding the velocity field
in a two-step solution approach. (1) Laplace’s equation is solved with homogeneous boundary conditions and a right-hand source
term, (2) pore pressure is computed, and the velocity field obtained for no slip conditions at the grain boundaries. From
the computed velocity field, we estimate the effective permeability of porous media samples characterized by section micrographs
or micro-CT scans. This two-step process is much simpler than solving the full Navier–Stokes equation and, therefore, provides
the opportunity to study pore geometries with hundreds of thousands of pores in a computationally more cost effective manner
than solving the full Navier–Stokes’ equation. Given the realistic laminar flow field, dispersion in the medium can also be
estimated. Our numerical model is verified with an analytical solution and validated on two 2D micro-CT scans from samples,
the permeabilities, and porosities of which were pre-determined in laboratory experiments. Comparisons were also made with
published experimental, approximate, and exact permeability data. With the future aim to simulate multiphase flow within the
pore space, we also compute the radii and derive capillary pressure from the Young–Laplace’s equation. This permits the determination
of model parameters for the classical Brooks–Corey and van-Genuchten models, so that relative permeabilities can be estimated. 相似文献
17.
M. Klein 《Flow, Turbulence and Combustion》2005,75(1-4):131-147
While methods for assessing the uncertainty of Reynolds–Averaged–Navier–Stokes (RANS) simulations have been well established
in the past, the verification of Large Eddy Simulations (LES) is more difficult. One reason is that the numerical discretization
error as well as the subgrid scale model contribution depend on the grid resolution and that both terms interact. In the present
paper the accuracy of single-grid estimators to assess the amount of the unresolved turbulent kinetic energy is studied first.
In the second part of the paper the sensitivity of the simulation results on the modeling error as well as the numerical error
will be investigated in the context of LES with implicit filtering. This will be achieved by performing a systematic grid
and model variation. The analysis is applied to an isothermal, turbulent, plane jet and a turbulent channel flow. 相似文献
18.
Zhihua Wang Pei He Yu Lv Junhu Zhou Jianren Fan Kefa Cen 《Flow, Turbulence and Combustion》2010,84(4):669-686
Direct numerical simulation(DNS) of spatially developing round turbulent jet flow with Reynolds number 4,700 was carried out.
Over 20 million grid points were used in this simulation. Fully compressible three-dimensional Navier–Stokes equations were
solved. High order explicit spatial difference schemes and Runge–Kutta time integration scheme were used to calculate derivatives
and time marching, respectively. Non-reflecting boundary conditions and exit zone techniques were adopted. Some refined computational
grids were used in order to capture the smallest turbulent structures near the centerline of the jet. Low level disturbance
were imposed on the jet inflow velocity to trigger the developing of turbulence. Turbulent statistics such as mean velocity,
Reynolds stresses, third order velocity moments were obtained and compared with experimental data. One-dimensional velocity
autospectra was also calculated. The inertial region where the spectra decays according to the k − 5/3 was observed. The quantitative profiles of mean velocity and all of the third order velocity moments which were difficult
to measure via experimental techniques were presented here in detail. The jet flow was proven to be close to fully self-similar
around 19 jet diameters downstream of jet exit. The statistic data and revealed flow feature obtained in this paper can provide
valuable reference for round turbulent jet research. 相似文献
19.
The application of large-eddy simulation (LES) to particle-laden turbulence raises such a fundamental question as whether the LES with a subgrid scale (SGS) model can correctly predict Lagrangian time correlations (LTCs). Most of the currently existing SGS models are constructed based on the energy budget equations. Therefore, they are able to correctly predict energy spectra, but they may not ensure the correct prediction on the LTCs. Previous researches investigated the effect of the SGS modeling on the Eulerian time correlations. This paper is devoted to study the LTCs in LES. A direct numerical simulation (DNS) and the LES with a spectral eddy viscosity model are performed for isotropic turbulence and the LTCs are calculated using the passive vector method. Both a priori and a posteriori tests are carried out. It is observed that the subgrid;scale contributions to the LTCs cannot be simply ignored and the LES overpredicts the LTCs than the DNS. It is concluded from the straining hypothesis that an accurate prediction of enstrophy spectra is most critical to the prediction of the LTCs. 相似文献
20.
The flow field resulting from a transverse injection through a slot into supersonic flow is numerically simulated by solving
Favre-averaged Navier–Stokes equations with κ − ω SST turbulence model with corrections for compressibility and transition. Numerical results are compared to experimental
data in terms of surface pressure profiles, boundary layer separation location, transition location, and flow structures at
the upstream and downstream of the jet. Results show good agreement with experimental data for a wide range of pressure ratios
and transition locations are captured with acceptable accuracy. κ − ω SST model provides quite accurate results for such a complex flow field. Moreover, few experiments involving a sonic round
jet injected on a flat plate into high-speed crossflow at Mach 5 are carried out. These experiments are three-dimensional
in nature. The effect of pressure ratio on three-dimensional jet interaction dynamics is sought. Jet penetration is found
to be a non-linear function of jet to free stream momentum flux ratio. 相似文献