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1.
A resent extension of the nonlinear K–ε model is critically discussed from a basic theoretical standpoint. While it was said in the paper that this model was formulated
to incorporate relaxation effects, it will be shown that the model is incapable of describing one of the most basic such turbulent
flows as is obvious but is described for clarity. It will be shown in detail that this generalized nonlinear K–ε model yields erroneous results for the Reynolds stress tensor when the mean strains are set to zero in a turbulent flow
– the return-to-isotropy problem which is one of the most elementary relaxational turbulent flows. It is clear that K–ε type models cannot describe relaxation effects. While their general formalism can describe relaxation effects, the nonlinear
K–ε model – which the paper is centered on – cannot. The deviatoric part of the Reynolds stress tensor is predicted to be zero
when it actually only gradually relaxes to zero. Since this model was formulated by using the extended thermodynamics, it
too will be critically assessed. It will be argued that there is an unsubstantial physical basis for the use of extended thermodynamics
in turbulence. The role of Material Frame-Indifference and the implications for future research in turbulence modeling are
also discussed.
Received 19 February 1998 and accepted 23 October 1998 相似文献
2.
The elliptic relaxation approach of Durbin (Durbin, P.A., J. Theor. Comput. Fluid. Dyn. 3 (1991) 1–13), which accounts for wall blocking effects on the Reynolds stresses, is analysed herein from the numerical stability point of view, in the form of the $\bar v^2 - f$ . This model has been shown to perform very well on many challenging test cases such as separated, impinging and bluff-body flows, and including heat transfer. However, numerical convergence of the original model suggested by Durbin is quite difficult due to the boundary conditions requiring a coupling of variables at walls. A ‘code-friendly’ version of the model was suggested by Lien and Durbin (Lien, F.S. and Durbin, P.A., Non linear κ ? ε ? υ 2 modelling with application to high-lift. In: Proceedings of the Summer Program 1996, Stanford University (1996), pp. 5–22) which removes the need of this coupling to allow a segregated numerical procedure, but with somewhat less accurate predictions. A robust modification of the model is developed to obtain homogeneous boundary conditions at a wall for both $\bar v^2 $ and f. The modification is based on both a change of variables and alteration of the governing equations. The new version is tested on a channel, a diffuser flow and flow over periodic hills and shown to reproduce the better results of the original model, while retaining the easier convergence properties of the ‘code-friendly’ version. 相似文献
3.
Flow, Turbulence and Combustion - 相似文献
6.
A novel and robust approach has been proposed for the high-order discontinuous Galerkin (DG) discretization of the Reynolds-averaged Navier-Stokes (RANS) equations with the turbulence model of Spalart-Allmaras (SA). The solution polynomials of the SA equation are reconstructed by the Hermite weighted essentially non-oscillatory (HWENO) scheme. Several practical techniques are suggested to simplify and extend a positivity-preserving limiter to further guarantee the positivity of SA working variable. The resulting positivity-preserving HWENO limiting method is compact and easy to implement on arbitrary meshes. Typical turbulent flows are conducted to assess the accuracy and robustness of the present method. Numerical experiments demonstrate that with the increasing grid or order resolution, the limited results of the working variable are getting closer to the unlimited ones. And the most obvious improvement with proposed method is on the computation of the working variable field in wake regions. 相似文献
7.
In this article we build a model for multidimensional flows in soft glassy materials. The construction of the model is based on the ideas of Hébraud and Lequeux, but care is taken to build a frame-indifferent multidimensional model. The main goal of this article is to prove that the methodology we have developed to study the well-posedness and the glass transition for the original Hébraud–Lequeux model can be successfully generalized. Thus, this work may be used as a starting point for more sophisticated studies in the modeling of general flows of glassy materials. 相似文献
8.
In this paper, we report results of a numerical investigation of turbulent natural gas combustion for a jet in a coflow of
lean combustion products in the Delft-Jet-in-Hot-Coflow (DJHC) burner which emulates MILD (Moderate and Intense Low Oxygen
Dilution) combustion behavior. The focus is on assessing the performance of the Eddy Dissipation Concept (EDC) model in combination
with two-equation turbulence models and chemical kinetic schemes for about 20 species (Correa mechanism and DRM19 mechanism)
by comparing predictions with experimental measurements. We study two different flame conditions corresponding to two different
oxygen levels (7.6% and 10.9% by mass) in the hot coflow, and for two jet Reynolds number (Re = 4,100 and Re = 8,800). The
mean velocity and turbulent kinetic energy predicted by different turbulence models are in good agreement with data without
exhibiting large differences among the model predictions. The realizable k- ε model exhibits better performance in the prediction of entrainment. The EDC combustion model predicts too early ignition
leading to a peak in the radial mean temperature profile at too low axial distance. However the model correctly predicts the
experimentally observed decreasing trend of lift-off height with jet Reynolds number. A detailed analysis of the mean reaction
rate of the EDC model is made and as possible cause for the deviations between model predictions and experiments a low turbulent
Reynolds number effect is identified. Using modified EDC model constants prediction of too early ignition can be avoided.
The results are weakly sensitive to the sub-model for laminar viscosity and laminar diffusion fluxes. 相似文献
9.
The long-time asymptotics is analyzed for all finite energy solutions to a model \(\mathbf{U}(1)\)-invariant nonlinear Klein–Gordon equation in one dimension, with the nonlinearity concentrated at a single point: each finite energy solution converges as t→ ± ∞ to the set of all “nonlinear eigenfunctions” of the form ψ( x) e?iω t. The global attraction is caused by the nonlinear energy transfer from lower harmonics to the continuous spectrum and subsequent dispersive radiation.We justify this mechanism by the following novel strategy based on inflation of spectrum by the nonlinearity. We show that any omega-limit trajectory has the time spectrum in the spectral gap [ ? m, m] and satisfies the original equation. This equation implies the key spectral inclusion for spectrum of the nonlinear term. Then the application of the Titchmarsh convolution theorem reduces the spectrum of each omega-limit trajectory to a single harmonic \(\omega\in[-m,m]\).The research is inspired by Bohr’s postulate on quantum transitions and Schrödinger’s identification of the quantum stationary states to the nonlinear eigenfunctions of the coupled \(\mathbf{U}(1)\)-invariant Maxwell–Schrödinger and Maxwell–Dirac equations. 相似文献
10.
In this paper we investigate a subgrid model based on an anisotropic version of the NS- α model using a lid-driven cavity flow at a Reynolds number of 10,000. Previously the NS- α model has only been used numerically in the isotropic form. The subgrid model is developed from the Eulerian-averaged anisotropic
equations (Holm, Physica D 133:215, 1999). It was found that when α
2 was based on the mesh numerical oscillations developed which manifested themselves in the appearance of streamwise vortices
and a ‘mixing out’ of the velocity profile. This is analogous to the Craik–Leibovich mechanism, with the difference being
that the oscillations here are not physical but numerical. The problem could be traced back to the discontinuity in α
2 encountered when α
2 = 0 on the endwalls. A definition of α
2 based on velocity gradients, rather than mesh spacing, is proposed and tested. Using this definition the results with the
model show a significant improvement. The splitting of the downstream wall jet, rms and shear stress profiles are correctly
captured a coarse mesh. The model is shown to predict both positive and negative energy transfer in the jet impingement region,
in qualitative agreement with DNS results. 相似文献
11.
Based on the finite volume method, the flow past a two-dimensional circular cylinder at a critical Reynolds number (Re = 8.5 × 10 5) was simulated using the Navier-Stokes equations and the γ-Re θ transition model coupled with the SST k ? ω turbulence model (hereinafter abbreviated as γ-Re θ model). Considering the effect of free-stream turbulence intensity decay, the SST k ? ω turbulence model was modified according to the ambient source term method proposed by Spalart and Rumsey, and then the modified SST k ? ω turbulence model is coupled with the γ-Re θ transition model (hereinafter abbreviated as γ-Re θ-SR model). The flow past a circular cylinder at different inlet turbulence intensities were simulated by the γ-Re θ-SR model. At last, the flow past a circular cylinder at subcritical, critical and supercritical Reynolds numbers were each simulated by the γ-Re θ-SR model, and the three flow states were analyzed. It was found that compared with the SST k ? ω turbulence model, the γ-Re θ model could simulate the transition of laminar to turbulent, resulting in better consistency with experimental result. Compared with the γ-Re θ model, for relatively high inlet turbulence intensities, the γ-Re θ-SR model could better simulate the flow past a circular cylinder; however the improvement almost diminished for relatively low inlet turbulence intensities The γ-Re θ-SR model could well simulate the flow past a circular cylinder at subcritical, critical and supercritical Reynolds numbers. 相似文献
12.
Euler’s celebrated buckling formula gives the critical load N for the buckling of a slender cylindrical column with radius B and length L as
N/(p3B2)=(E/4)(B/L)2,N/(\pi^3B^2)=(E/4)(B/L)^2, 相似文献
13.
Measuring accurate displacement distributions for large-scale structures is an important issue and a very challenging task.
Recently, a simple and accurate phase measurement technique called sampling moiré method [Exp Mech 50–4:501–508, ( 2010)] has been developed for small-displacement distribution measurements. In this method, the phase distribution of moiré fringes
can be analyzed from a single grating image by simultaneously performing down-sampling image processing and intensity-interpolation
to generate multiple phase-shifted moiré fringe images. In addition, the phase of the original grating can also be obtained
from the phase of the moiré fringe by adding the phase of the sampling grating. In this study, the measurement accuracy of
the sampling moiré method was analyzed through computer simulations and a displacement measurement experiment. Four factors
of the sampling moiré method were investigated, including the sampling pitch, the order of the intensity-interpolation, random
noise, and the form of grating. The results show that determining the optimal sampling pitch is an important factor for obtaining
better results but it is not critical. In addition, a practical application of the sampling moiré method is presented that
involves a deflection measurement on a 10-meter-long crane. The experimental results demonstrate that submillimeter deflections
of the crane can be successfully detected. 相似文献
14.
We show here the global, in time, regularity of the three dimensional viscous Camassa–Holm (Navier–Stokes-alpha) (NS- ) equations. We also provide estimates, in terms of the physical parameters of the equations, for the Hausdorff and fractal dimensions of their global attractor. In analogy with the Kolmogorov theory of turbulence, we define a small spatial scale,
, as the scale at which the balance occurs in the mean rates of nonlinear transport of energy and viscous dissipation of energy. Furthermore, we show that the number of degrees of freedom in the long-time behavior of the solutions to these equations is bounded from above by ( L/
) 3, where L is a typical large spatial scale (e.g., the size of the domain). This estimate suggests that the Landau–Lifshitz classical theory of turbulence is suitable for interpreting the solutions of the NS- equations. Hence, one may consider these equations as a closure model for the Reynolds averaged Navier–Stokes equations (NSE). We study this approach, further, in other related papers. Finally, we discuss the relation of the NS- model to the NSE by proving a convergence theorem, that as the length scale
1 tends to zero a subsequence of solutions of the NS- equations converges to a weak solution of the three dimensional NSE. 相似文献
15.
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
16.
We consider the 3D quantum many-body dynamics describing a dilute Bose gas with strong confinement in one direction. We study the corresponding Bogoliubov-Born-Green-Kirkwood-Yvon (BBGKY) hierarchy, which contains a diverging coefficient as the strength of the confining potential tends to ∞. We find that this diverging coefficient is counterbalanced by the limiting structure of the density matrices and we establish the convergence of the BBGKY hierarchy. Moreover, we prove that the limit is fully described by a 2D cubic nonlinear Schrödinger equation (NLS) and we obtain the exact 3D to 2D coupling constant. 相似文献
18.
Nonlinear Dynamics - 相似文献
19.
Soil water evaporation plays a critical role in mass and energy exchanges across the land–atmosphere interface. Although much is known about this process, there is no agreement on the best modeling approaches to determine soil water evaporation due to the complexity of the numerical modeling scenarios and lack of experimental data available to validate such models. Existing studies show numerical and experimental discrepancies in the evaporation behavior and soil water distribution in soils at various scales, driving us to revisit the key process representation in subsurface soil. Therefore, the goal of this work is to test different mathematical formulations used to estimate evaporation from bare soils to critically evaluate the model formulations, assumptions and surface boundary conditions. This comparison required the development of three numerical models at the REV scale that vary in their complexity in characterizing water flow and evaporation, using the same modeling platform. The performance of the models was evaluated by comparing with experimental data generated from a soil tank/boundary layer wind tunnel experimental apparatus equipped with a sensor network to continuously monitor water–temperature–humidity variables. A series of experiments were performed in which the soil tank was packed with different soil types. Results demonstrate that the approaches vary in their ability to capture different stages of evaporation and no one approach can be deemed most appropriate for every scenario. When a proper top boundary condition and space discretization are defined, the Richards equation-based models (Richards model and Richards vapor model) can generally capture the evaporation behaviors across the entire range of soil saturations, comparing well with the experimental data. The simulation results of the non-equilibrium two-component two-phase model which considers vapor transport as an independent process generally agree well with the observations in terms of evaporation behavior and soil water dynamics. Certain differences in simulation results can be observed between equilibrium and non-equilibrium approaches. Comparisons of the models and the boundary layer formulations highlight the need to revisit key assumptions that influence evaporation behavior, highlighting the need to further understand water and vapor transport processes in soil to improve model accuracy. 相似文献
20.
In 1968 V.E. Zakharov derived the Nonlinear Schrödinger equation for the two-dimensional water wave problem in the absence of surface tension, that is, for the evolution of gravity driven surface water waves, in order to describe slow temporal and spatial modulations of a spatially and temporarily oscillating wave packet. In this paper we give a rigorous proof that the wave packets in the two-dimensional water wave problem in a canal of finite depth can be approximated over a physically relevant timespan by solutions of the Nonlinear Schrödinger equation. 相似文献
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