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A V Nikolaev - One of the best experts on this subject based on the ideXlab platform.
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ab initio based description of the unusual increase of the electric field gradient with temperature at ti sites in rutile tio 2
Physical Review B, 2020Co-Authors: A V Nikolaev, N M Chtchelkatchev, A V Bibikov, D A Salamatin, A V TsvyashchenkoAbstract:Combining a precise ab initio electron band structure calculation of the ${\mathrm{TiO}}_{2}$ rutile structure with the temperature evolution of the Ti mean-square displacements, we reproduce a puzzling temperature increase of the electric field gradient at Ti sites in ${\mathrm{TiO}}_{2}$, observed experimentally. Our method employs a procedure of averaging two quadrupole electron density Components ($L=2$) inside a sphere vibrating with the Ti nucleus at its center, where the key factor introducing the temperature dependence is the square root of the Debye-Waller factor. Although the Debye-Waller factor always reduces the corresponding Fourier Component, in ${\mathrm{TiO}}_{2}$ due to the interplay between terms of opposite signs, it results in a net increase of the whole sum with temperature, leading to the growth of the electric field gradient. Quantitatively, we find that the increase of electric field gradient is only half of the experimental value, which we ascribe to anharmonic effects or a strong oxygen position influence. In addition, our method reproduces the unusual temperature dependence of the asymmetry parameter $\ensuremath{\eta}$, which first decreases with temperature, goes to zero, and then increases.
M E Goldstein - One of the best experts on this subject based on the ideXlab platform.
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effect of free stream turbulence and other vortical disturbances on a laminar boundary layer
Journal of Fluid Mechanics, 1999Co-Authors: S J Leib, David W Wundrow, M E GoldsteinAbstract:This paper is concerned with the effect of free-stream turbulence on the pretransitional flat-plate boundary layer. It is assumed that either the turbulence Reynolds number or the downstream distance (or both) is small enough so that the flow can be linearized. The dominant disturbances in the boundary layer, which are of the Klebanoff type, are governed by the linearized unsteady boundary-region equations, i.e., the Navier Stokes equations with the streamwise derivatives neglected in the viscous and pressure-gradient terms. The turbulence is represented as a superposition of vortical free-stream Fourier modes, and the corresponding individual Fourier Component solutions to the boundary-region equations are obtained numerically. The results are then superposed to compute the root mean square of the fluctuating streamwise velocity in the boundary layer produced by the actual free-stream turbulence. The calculated boundary-layer disturbances are in good quantitative agreement with the experimentally observed Klebanoff modes when strong low-frequency anisotropic effects are included in the free-stream turbulence spectrum. We discuss some additional effects that may need to be accounted for in order to obtain a complete description of the Klebanoff modes.
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effect of free stream turbulence and other vortical disturbances on a laminar boundary layer
Journal of Fluid Mechanics, 1999Co-Authors: S J Leib, David W Wundrow, M E GoldsteinAbstract:This paper is concerned with the effect of free-stream turbulence on the pretransitional flat-plate boundary layer. It is assumed that either the turbulent Reynolds number or the downstream distance (or both) is small enough that the flow can be linearized. The dominant disturbances in the boundary layer, which are of the Klebanoff type, are governed by the linearized unsteady boundary-region equations, i.e. the linearized Navier–Stokes equations with the streamwise derivatives neglected in the viscous and pressure-gradient terms. The turbulence is represented as a superposition of vortical free-stream Fourier modes and the corresponding Fourier Component solutions to the boundary-region equations are obtained numerically. The results are then superposed to compute the root mean square of the fluctuating streamwise velocity in the boundary layer produced by the actual free-stream turbulence. It is found that the disturbances computed with isotropic free-stream turbulence do not reach the levels measured in experiments. However, good quantitative agreement is obtained with the relatively low turbulent Reynolds number data of Kendall when the measured strong anisotropy of the low-frequency portion of his spectrum is accounted for. Data at higher turbulent Reynolds numbers are affected by nonlinearity, which manifests itself through the generation of small spanwise length scales. We attempt to model this within the context of the linear theory by choosing a free-stream spectrum whose energy is concentrated at larger transverse wavenumbers and achieve very good agreement with the data. The results suggest that even small deviations from pure isotropy can be an important factor in explaining the large amplitudes of the Klebanoff modes in the pre-transitional boundary layer, and also point to the importance of nonlinear effects. We discuss some additional effects that may need to be accounted for in order to obtain a complete description of the Klebanoff modes.
S J Leib - One of the best experts on this subject based on the ideXlab platform.
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effect of free stream turbulence and other vortical disturbances on a laminar boundary layer
Journal of Fluid Mechanics, 1999Co-Authors: S J Leib, David W Wundrow, M E GoldsteinAbstract:This paper is concerned with the effect of free-stream turbulence on the pretransitional flat-plate boundary layer. It is assumed that either the turbulence Reynolds number or the downstream distance (or both) is small enough so that the flow can be linearized. The dominant disturbances in the boundary layer, which are of the Klebanoff type, are governed by the linearized unsteady boundary-region equations, i.e., the Navier Stokes equations with the streamwise derivatives neglected in the viscous and pressure-gradient terms. The turbulence is represented as a superposition of vortical free-stream Fourier modes, and the corresponding individual Fourier Component solutions to the boundary-region equations are obtained numerically. The results are then superposed to compute the root mean square of the fluctuating streamwise velocity in the boundary layer produced by the actual free-stream turbulence. The calculated boundary-layer disturbances are in good quantitative agreement with the experimentally observed Klebanoff modes when strong low-frequency anisotropic effects are included in the free-stream turbulence spectrum. We discuss some additional effects that may need to be accounted for in order to obtain a complete description of the Klebanoff modes.
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effect of free stream turbulence and other vortical disturbances on a laminar boundary layer
Journal of Fluid Mechanics, 1999Co-Authors: S J Leib, David W Wundrow, M E GoldsteinAbstract:This paper is concerned with the effect of free-stream turbulence on the pretransitional flat-plate boundary layer. It is assumed that either the turbulent Reynolds number or the downstream distance (or both) is small enough that the flow can be linearized. The dominant disturbances in the boundary layer, which are of the Klebanoff type, are governed by the linearized unsteady boundary-region equations, i.e. the linearized Navier–Stokes equations with the streamwise derivatives neglected in the viscous and pressure-gradient terms. The turbulence is represented as a superposition of vortical free-stream Fourier modes and the corresponding Fourier Component solutions to the boundary-region equations are obtained numerically. The results are then superposed to compute the root mean square of the fluctuating streamwise velocity in the boundary layer produced by the actual free-stream turbulence. It is found that the disturbances computed with isotropic free-stream turbulence do not reach the levels measured in experiments. However, good quantitative agreement is obtained with the relatively low turbulent Reynolds number data of Kendall when the measured strong anisotropy of the low-frequency portion of his spectrum is accounted for. Data at higher turbulent Reynolds numbers are affected by nonlinearity, which manifests itself through the generation of small spanwise length scales. We attempt to model this within the context of the linear theory by choosing a free-stream spectrum whose energy is concentrated at larger transverse wavenumbers and achieve very good agreement with the data. The results suggest that even small deviations from pure isotropy can be an important factor in explaining the large amplitudes of the Klebanoff modes in the pre-transitional boundary layer, and also point to the importance of nonlinear effects. We discuss some additional effects that may need to be accounted for in order to obtain a complete description of the Klebanoff modes.
A Airapetian - One of the best experts on this subject based on the ideXlab platform.
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effects of transversity in deep inelastic scattering by polarized protons
Physics Letters B, 2010Co-Authors: A Airapetian, N Akopov, Z Akopov, E C Aschenauer, W Augustyniak, R Avakian, A Avetissian, E Avetisyan, Alessandro BacchettaAbstract:Abstract Single-spin asymmetries for pions and charged kaons are measured in semi-inclusive deep-inelastic scattering of positrons and electrons off a transversely nuclear-polarized hydrogen target. The dependence of the cross section on the azimuthal angles of the target polarization ( ϕ S ) and the produced hadron (ϕ) is found to have a substantial sin ( ϕ + ϕ S ) modulation for the production of π + , π − and K + . This Fourier Component can be interpreted in terms of non-zero transversity distribution functions and non-zero favored and disfavored Collins fragmentation functions with opposite sign. For π 0 and K − production the amplitude of this Fourier Component is consistent with zero.
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single spin asymmetries in semi inclusive deep inelastic scattering on a transversely polarized hydrogen target
Physical Review Letters, 2005Co-Authors: A Airapetian, N Akopov, Z Akopov, M Amarian, A Andrus, E C Aschenauer, W Augustyniak, R Avakian, A Avetissian, E AvetissianAbstract:Single-spin asymmetries for semi-inclusive electroproduction of charged pions in deep-inelastic scattering of positrons are measured for the first time with transverse target polarization. The asymmetry depends on the azimuthal angles of both the pion ($\ensuremath{\phi}$) and the target spin axis (${\ensuremath{\phi}}_{S}$) about the virtual-photon direction and relative to the lepton scattering plane. The extracted Fourier Component $\mathbf{⟨}\mathrm{sin}(\ensuremath{\phi}+{\ensuremath{\phi}}_{S}){\mathbf{⟩}}_{UT}^{\ensuremath{\pi}}$ is a signal of the previously unmeasured quark transversity distribution, in conjunction with the Collins fragmentation function, also unknown. The Component $\mathbf{⟨}\mathrm{sin}(\ensuremath{\phi}\ensuremath{-}{\ensuremath{\phi}}_{S}{\mathbf{⟩}}_{UT}^{\ensuremath{\pi}}$ arises from a correlation between the transverse polarization of the target nucleon and the intrinsic transverse momentum of quarks, as represented by the previously unmeasured Sivers distribution function. Evidence for both signals is observed, but the Sivers asymmetry may be affected by exclusive vector meson production.
A V Tsvyashchenko - One of the best experts on this subject based on the ideXlab platform.
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ab initio based description of the unusual increase of the electric field gradient with temperature at ti sites in rutile tio 2
Physical Review B, 2020Co-Authors: A V Nikolaev, N M Chtchelkatchev, A V Bibikov, D A Salamatin, A V TsvyashchenkoAbstract:Combining a precise ab initio electron band structure calculation of the ${\mathrm{TiO}}_{2}$ rutile structure with the temperature evolution of the Ti mean-square displacements, we reproduce a puzzling temperature increase of the electric field gradient at Ti sites in ${\mathrm{TiO}}_{2}$, observed experimentally. Our method employs a procedure of averaging two quadrupole electron density Components ($L=2$) inside a sphere vibrating with the Ti nucleus at its center, where the key factor introducing the temperature dependence is the square root of the Debye-Waller factor. Although the Debye-Waller factor always reduces the corresponding Fourier Component, in ${\mathrm{TiO}}_{2}$ due to the interplay between terms of opposite signs, it results in a net increase of the whole sum with temperature, leading to the growth of the electric field gradient. Quantitatively, we find that the increase of electric field gradient is only half of the experimental value, which we ascribe to anharmonic effects or a strong oxygen position influence. In addition, our method reproduces the unusual temperature dependence of the asymmetry parameter $\ensuremath{\eta}$, which first decreases with temperature, goes to zero, and then increases.