Volume 17, Issue 2 -

Large eddy simulation (LES) of the neutrally stratified turbulent Ekman layer is performed. In particular, we compare three LES models with direct numerical simulation (DNS), which was validated against existing DNS. The models considered are a standard nondynamic LES model, the Smagorinsky model (SM), a standard dynamic LES model, the stabilized dynamic Smagorinsky model (DSM), and a new linear dynamic model (LDM), which was derived from a realizable stochastic turbulence model. The following conclusions are obtained. The SM does not represent an appropriate model for the flow considered. Mean velocity and turbulence intensities are poorly predicted. With respect to instantaneous fields, the SM provides a tilting of turbulence structures in the opposite direction as seen in DNS. The stabilized DSM also suffers from significant shortcomings. First, its behavior depends on the wall distance. Close to the wall, it produces acceptable turbulence structures. Away from the wall, it suffers from the same shortcomings as the SM. Second, it incorrectly describes the effect of grid coarsening. The new LDM is free from the disadvantages of the SM and stabilized DSM. Its predictions of both mean and instantaneous velocity fields agree very well with DNS. The relevant conclusion is the following. The use of a dynamic LES method represents a mean for correctly simulating large-scale structures (means and stresses), but it does not ensure a correct simultaneous simulation of small-scale structures. Our results indicate that a dynamic method designed in consistency with a realizable stress model can correctly simulate both large-scale and small-scale structures.

A low-Reynolds-number k – ∈ model based on a new turbulent structure parameter a1_NC=−u′v′‾/u′v′‾kk${a_{1\_{\rm{NC}}}}\left({= - \left| {{{\overline {u^' v^'}} \mathord{\left/ {\vphantom {{\overline {u^' v^'}} k}} \right.} k}} \right|} \right)$ and a recalibrated wall-damping function (WDF) fμ${f_\mu}$ is proposed and evaluated. In order to account for the effect of variation of Reynolds number on maximum value of the WDF, a ratio between two different turbulent Reynolds numbers is involved in the WDF. In addition, instead of using a constant ratio between Reynolds shear stress and turbulent kinetic energy, e. g. a 1 = 0.31, the new turbulent structure parameter a 1 _ NC is proposed based on several sets of direct numerical simulation (DNS) data. The deduction of near-wall asymptotic behavior is performed to prove that the new proposed model can yield a correct wall value for turbulent viscosity. The new model is validated with several well-documented flow cases, and the yielding results are in good agreement with experimental data. Moreover, three frequently used turbulence models are also involved into the comparisons and the results indicate that the new model offers remarkable improvement on the nonequilibrium flows, e. g. separated and adverse pressure gradient flows.

Zircaloy is a kind of cladding material for monolithic nuclear fuel plates, which has a strong strain-rate-dependent mechanical constitutive relation at the rolling temperature. In order to model the rolling process of monolithic nuclear fuel plates, it is critical to effectively define the mechanical properties of Zircaloy in the commercial finite element software. Different three-dimensional stress-updating algorithms are built, respectively, including a fully implicit algorithm, a simplified implicit one and an explicit one. The corresponding VUMAT subroutines suitable for explicit dynamic rolling simulation for a Zircaloy plate are developed, and the computation precision and efficiency are compared in ABAQUS. The simplified implicit stress-updating algorithm is determined to be the best choice. The effects of temperature, rolling velocity and reduction on the strain rate distribution, the deformation and rolling force are investigated.

This paper presents direct numerical simulation (DNS) result of the Navier–Stokes equations for turbulent channel flows with blowing and suction effects. The friction Reynolds number is Reτ=394${\rm{R}}{{\rm{e}}_\tau} = 394$ and a range of blowing and suction conditions is covered with different perturbation strengths, i. e. A=0.05,$A = 0.05, $ 0.1, 0.2. While the mean velocity profile has been severely altered, the probability density function (PDF) for (spanwise) vorticity – depending on wall distance (y+)$({y^ +})$ and blowing/suction strength ( A ) – satisfies the generalized hyperbolic distribution (GHD) of Birnir [The Kolmogorov-Obukhov statistical theory of turbulence, J. Nonlinear Sci. (2013a), doi: 10.1007/s00332-012-9164–z; The Kolmogorov-Obukhov theory of turbulence, Springer, New York, 2013b] in the bulk of the flow. The latter leads to accurate descriptions of all PDFs (at y+=40,${y^ +} = 40, $ 200, 390 and A=0.05,$A = 0.05, $ 0.2, for instance) with only four parameters. The result indicates that GHD is a general tool to quantify PDF for turbulent flows under various wall surface conditions.