Synthetic Turbulence Generators for RANS-LES Interfaces in Zonal Simulations of Aerodynamic and Aeroacoustic Problems

The paper presents the detailed formulation and validation results of simple and robust procedures for the generation of synthetic turbulence aimed at providing artificial turbulent content at the RANS-to-LES interface within a zonal Wall Modelled LES of attached and mildly separated wall-bounded flows. There are two versions of the procedure. The aerodynamic version amounts to a minor modification of a synthetic turbulence generator developed by the authors previously, but the acoustically adapted version is new and includes an internal damping layer, where the pressure field is computed by “weighting” of the instantaneous pressure fields from LES and RANS. This is motivated by the need to avoid creating spurious noise as part of the turbulence generation. In terms of pure aerodynamics, the validation includes canonical shear flows (developed channel flow, zero pressure gradient boundary layer, and plane mixing layer), as well as a more complex flow over the wall-mounted hump with non-fixed separation and reattachment, with emphasis on a rapid conversion from modeled to resolved Reynolds stresses. The aeroacoustic applications include the flow past a trailing edge and over a two-element airfoil configuration. In all cases the methodology ensures a very acceptable accuracy for the mean flow, turbulent statistics and, also, the near- and far-field noise.     

Formation of nanodomain ensembles during polarization reversal in Sr<Subscript>0.61</Subscript>Ba<Subscript>0.39</Subscript>Nb<Subscript>2</Subscript>O<Subscript>6</Subscript>: Ce single crystals

The results of the study of nano- and microdomain structure evolution in single crystals of relaxor ferroelectric strontium barium niobate Sr_0.61Ba_0.39Nb₂O₆ doped by cerium are presented. It was shown that the initial nanodomain structure represents a self-similar three-dimensional maze. The fractal dimension and average period were revealed. It was demonstrated that application of series of alternating electric field pulses makes it possible to produce a single-domain state in the surface layer. The features of the growth and “merging,” as well as the shape of ensembles of isolated nanodomains formed during switching from the single-domain state, were investigated. The formation of the nanodomain ensembles was considered as a result of the self-organized discrete switching controlled by determined nucleation.     

Deciding context equivalence of binary overlap-free words in linear time

We study the structure of the language of binary overlap-free words, which is one of the classical objects in combinatorics of words and formal languages. In a natural way, this study requires a solution to the word problem for the syntactic monoid of the language. In this paper we present an algorithm that efficiently solves this word problem. Namely, the time complexity of the algorithm is linear in the total length of compared words.     

Coordination chemistry of anticrowns. Synthesis and structure of the binuclear complex of perfluorinated three-mercury anticrown (o-F6Hg)3 with 1,2-dichloroethane

Russian Chemical Bulletin - The reaction of perfluorinated three-mercury anticrown (o-C6F4Hg)3 (1) with 1,2-dichloroethane (DCE) affords the complex {[(o-C6F4Hg)3]2(ClCH2CH2Cl)}(2) containing two...     

Large-Eddy and Direct Numerical Simulations of the Bachalo-Johnson Flow with Shock-Induced Separation

The Bachalo-Johnson experiment on an axisymmetric bump has been a primary validation case for turbulence models in shock-boundary-layer interactions since the 1980’s. In the present work, Wall-Modelled Large-Eddy Simulations (WMLES) of this flow were conducted using Improved Delayed Detached-Eddy Simulation (IDDES) as the sub-grid-scale (SGS) and wall model, with a synthetic turbulence generator, expecting close enough agreement with experiment. However, the WMLES results are disappointing, even in terms of the shock position, even though the results from two grids with 4.7 × 10⁸ and 1.6 × 10⁹ cells respectively agree well with each other. This failure of grid refinement to warn of an inaccurate simulation is of great concern, and the reasons for it are explored. We then conducted a Direct Numerical Simulation (DNS) embedded in the LES over a reduced domain, with 8 × 10⁹ grid cells. The DNS has a far more accurate shock position and overall pressure distribution. The skin friction in the favourable pressure gradient is also much higher than in the LES; thus, wide differences appear upstream of the shock wave, most probably caused by the rapid acceleration which leads to atypical shear-stress profiles. Other SGS models were tried, and performed worse than IDDES. The DNS essentially fulfils the initial expectations although in a reduced domain and provides data for turbulence-modelling studies, for instance by extracting an effective eddy viscosity from it. The most noticeable remaining disagreement with experiment is over the Reynolds shear stress.     

Growth of power-free languages: Numerical and asymptotic bounds

We consider power-free languages over finite alphabets. That is, the words which are powers with the exponent greater than a given constant cannot be factors of words in these languages. We give numerical bounds of the exponential growth rate for a wide range of such languages. These bounds are obtained using the algorithms proposed by the author. On the basis of these bounds we suggest the asymptotic behaviour of the growth rate as a function of the exponent and the size of the alphabet, and give some theorems about this behaviour.     

Synthesis of nanocomposite with a core—shell structure based on Fe3O4 magnetic nanoparticles and iron glycerolate

Russian Chemical Bulletin - An original method for preparation of nanoparticles with a core—shell structure based on Fe3O4 magnetic nanoparticles (MNPs) and iron glycerolate was proposed. The...     

Diffraction of Light on a Regular Domain Structure with Inclined Walls in MgO:LiNbO3

JETP Letters - Features of the Bragg diffraction of a Gaussian light beam on a regular domain structure with inclined 180° domain walls in a 5% MgO:LiNbO3 crystal have been studied...     

Formation of nanodomain structures in lithium niobate as a result of pulsed laser irradiation

The surface nanodomain structures formed in lithium niobate single crystals as a result of pulsed irradiation of polar surfaces with UV and IR lasers have been investigated. The structures formed have been classified. The main features of the domain kinetics during one-dimensional growth at strong deviation from equilibrium have been revealed. A mechanism of polarization switching is proposed and discussed.