Friedel oscillations responsible for stacking fault of adatoms: The case of Mg(0001) and Be(0001)

We perform a first-principles study of Mg adatom and adislands on the Mg(0001) surface, and Be adatom on Be(0001), to obtain further insights into the previously reported energetic preference of the fcc faulty stacking of Mg monomers on Mg(0001). We first provide a viewpoint on how Friedel oscillations influence ionic relaxation on these surfaces. Our three-dimensional charge-density analysis demonstrates that Friedel oscillations have maxima which are more spatially localized than what one-dimensional average density or two-dimensional cross sectional plots could possibly inform: The well-known charge-density enhancement around the topmost surface layer of Mg(0001) is strongly localized at its fcc hollow sites. The charge accumulation at this site explains the energetically preferred stacking fault of the Mg monomer, dimer and trimer. Yet, larger islands prefer the normal hcp stacking. Surprisingly, the mechanism by which the fcc site becomes energetically more favorable is not that of enhancing the surface-adatom bonds but rather those between surface atoms. To confirm our conclusions, we analyze the stacking of Be adatom on Be(0001) - a surface also largely influenced by Friedel oscillations. We find, in fact, a much stronger effect: The charge enhancement at the fcc site is even larger and, consequently, the stacking-fault energy favoring the fcc site is quite large, 44 meV.Comment: Submitted to Physical Review

Evidences of persisting thermal structures in Couette flows

[EN] DNS of passive thermal turbulent Couette flow at several friction Reynolds numbers (180, 250, and 500), and the Prandtl number of air are presented. The time averaged thermal flow shows the existence of long and wide thermal structures never described before in Couette flows. These thermal structures, named CTFS (Couette Thermal Flow Superstructures), are defined as coherent regions of hot and cold temperature fluctuations. They are intrinsically linked to the velocity structures present in Couette flows. Two different 2D symmetries can be recognized, which get stronger with the Reynolds number. These structures do not affect the mean flow or mean quantities as the Nusselt number. However, turbulent intensities and thermal fluxes depend on the width of the structures, mainly far from the walls. Since the width of the structures is related to the channel width, the statistics of thermal Couette flow are to some point box-dependent.This work was supported by the MINECO/FEDER, under project ENE2015-71333-R. The computations of the new simulations were made possible by a generous grant of computing time from the Barcelona Supercomputing Centre, reference FI-2018-1-0037. FAA is partially funded by GVA/FEDER project ACIF2018. We are very grateful for the advices and revision provided by one of the referees of the article, as it has helped to enrich its content.Alcántara-Ávila, F.; Gandía-Barberá, S.; Hoyas, S. (2019). Evidences of persisting thermal structures in Couette flows. International Journal of Heat and Fluid Flow. 76:287-295. https://doi.org/10.1016/j.ijheatfluidflow.2019.03.001S2872957

Ab initio lattice dynamics and electron-phonon coupling of Bi(111)

We present a comprehensive ab initio study of structural, electronic, lattice dynamical and electron-phonon coupling properties of the Bi(111) surface within density functional perturbation theory. Relativistic corrections due to spin-orbit coupling are consistently taken into account. As calculations are carried out in a periodic slab geometry, special attention is given to the convergence with respect to the slab thickness. Although the electronic structure of Bi(111) thin films varies significantly with thickness, we found that the lattice dynamics of Bi(111) is quite robust and appears converged already for slabs as thin as 6 bilayers. Changes of interatomic couplings are confined mostly to the first two bilayers, resulting in super-bulk modes with frequencies higher than the optic bulk spectrum, and in an enhanced density of states at lower frequencies for atoms in the first bilayer. Electronic states of the surface band related to the outer part of the hole Fermi surfaces exhibit a moderate electron-phonon coupling of about 0.45, which is larger than the coupling constant of bulk Bi. States at the inner part of the hole surface as well as those forming the electron pocket close to the zone center show much increased couplings due to transitions into bulk projected states near Gamma_bar. For these cases, the state dependent Eliashberg functions exhibit pronounced peaks at low energy and strongly deviate in shape from a Debye-like spectrum, indicating that an extraction of the coupling strength from measured electronic self-energies based on this simple model is likely to fail.Comment: 30 pages, 11 figure

DNS of thermal channel flow up to Re-tau=2000 for medium to low Prandtl numbers

[EN] Direct Numerical Simulations of turbulent heat transfer in a channel flow are presented for three different Reynolds numbers, namely Re-tau = 500, 1000 and 2000. Medium and low values of the molecular Prandtl number are studied, ranging from 0.71 (air), down to 0.007 (molten metals), in order to study its effect on the thermal flow. Mixed boundary conditions at both walls are used for the thermal flow. Mean value and intensities of the thermal field were obtained. Two different behaviors were observed, depending on the Prandtl and Peclet numbers. The expected logarithmic behavior of the thermal flow completely disappears for Prandtl below 0.3. This is a direct effect of the thicker viscous thermal layer generated as the Prandtl number is reduced. Von Karman constant was computed for cases above this Prandtl, and turbulent Prandtl and Nusselt numbers were obtained for all the cases. Finally, the turbulent budgets for heat fluxes, temperature variance and its dissipation rate are presented. As a general result, there is a scaling failure near the wall in very cases studied, which is accentuated for lower Prandtl numbers. The statistics of all simulations can be downloaded from the web page of our group. (C) 2018 Elsevier Ltd. All rights reserved.This work was supported by MINECO/FEDER, under project ENE2015-71333-R. The computations of the new simulations were made possible by a generous grant of computing time from the Barcelona Supercomputing Centre, reference FI-2018-1-0037. We are grateful to Messiers Kawamura, Pirozzoli, Bernardini and Orlandi for providing us with copies of their original data.Alcántara-Ávila, F.; Hoyas, S.; Pérez Quiles, MJ. (2018). DNS of thermal channel flow up to Re-tau=2000 for medium to low Prandtl numbers. International Journal of Heat and Mass Transfer. 127:349-361. https://doi.org/10.1016/j.ijheatmasstransfer.2018.06.149S34936112

Stratification effect on extreme-scale rolls in plane Couette flows

[EN] The existence of the large-scale structures appearing in turbulent Couette flows is studied by means of a direct numerical simulation data set of active thermal Couette flows for different friction Richardson numbers, at the Prandtl number of air. The existence of these structures is linked to the nonexistence of an active thermal flow. As soon as the Richardson number is greater than 1.5, the structures are less energetic, and for a value of only 3, the structures have vanished. This is due to the reorganization of the intense Reynolds stress events. Thus, large-scale structures will hardly appear in real-life Couette flows of air with a stable wall-normal gradient of temperature.This work was supported by Grant No. RTI2018-102256-B-I00 of MINECO/FEDER. The computations of the new simulations were made possible by a generous grant of computing time from the Barcelona Supercomputing Centre, Grant No. AECT-2020-2-0005. F.A.A. is partially funded by Generalitat Valenciana, GVA/FEDER project ACIF2018.Gandía-Barberá, S.; Alcántara-Ávila, F.; Hoyas, S.; Avsarkisov, V. (2021). Stratification effect on extreme-scale rolls in plane Couette flows. Physical Review Fluids. 6(3):1-18. https://doi.org/10.1103/PhysRevFluids.6.0346051186

A Code for Simulating Heat Transfer in Turbulent Channel Flow

[EN] One numerical method was designed to solve the time-dependent, three-dimensional, incompressible Navier-Stokes equations in turbulent thermal channel flows. Its originality lies in the use of several well-known methods to discretize the problem and its parallel nature. Vorticy-Laplacian of velocity formulation has been used, so pressure has been removed from the system. Heat is modeled as a passive scalar. Any other quantity modeled as passive scalar can be very easily studied, including several of them at the same time. These methods have been successfully used for extensive direct numerical simulations of passive thermal flow for several boundary conditions.This work was supported by RTI2018-102256-B-I00 of MINECO/FEDER. The computations of the new simulations were made possible by several generous grants of computing time from the Barcelona Supercomputing Centre, references FI-2018-1-0037, FI-2018-2-0021, FI-2018-3-0032, FI-2019-1-0025, IM-2019-2-2016, AECT-2020-1-0024, AECT-2020-2-0005.Lluesma-Rodríguez, F.; Alcántara-Ávila, F.; Pérez Quiles, MJ.; Hoyas, S. (2021). A Code for Simulating Heat Transfer in Turbulent Channel Flow. Mathematics. 9(7):1-12. https://doi.org/10.3390/math9070756S1129

Diffusion of the Cu monomer and dimer on Ag(111): Molecular dynamics simulations and density functional theory calculations

We present results of molecular dynamics (MD) simulations and density functional theory (DFT) calculations of the diffusion of Cu adatom and dimer on Ag(111). We have used potentials generated by the embedded-atom method for the MD simulations and pseudopotentials derived from the projected-augmented-wave method for the DFT calculations. The MD simulations (at three different temperatures: 300, 500, and 700 K) show that the diffusivity has an Arrhenius behavior. The effective energy barriers obtained from the Arrhenius plots are in excellent agreement with those extracted from scanning tunneling microscopy experiments. While the diffusion barrier for Cu monomers on Ag(111) is higher than that reported (both in experiment and theory) for Cu(111), the reverse holds for dimers [which, for Cu(111), has so far only been theoretically assessed]. In comparing our MD result with those for Cu islets on Cu(111), we conclude that the higher barriers for Cu monomers on Ag(111) results from the comparatively large Ag-Ag bond length, whereas for Cu dimers on Ag(111) the diffusivity is taken over and boosted by the competition in optimization of the Cu-Cu dimer bond and the five nearest-neighbor Cu-Ag bonds. Our DFT calculations confirm the relatively large barriers for the Cu monomer on Ag(111)-69 and 75 meV-compared to those on Cu(111) and hint a rationale for them. In the case of the Cu dimer, the relatively long Ag-Ag bond length makes available a diffusion route whose highest relevant energy barrier is only 72 meV and which is not favorable on Cu(111). This process, together with another involving an energy barrier of 83 meV, establishes the possibility of low-barrier intercell diffusion by purely zigzag mechanisms

Wall turbulence at high friction Reynolds numbers

[EN] A new direct numerical simulation of a Poiseuille channel flow has been conducted for a friction Reynolds number of 10000, using the pseudospectral code LISO. The mean streamwise velocity presents a long logarithmic layer, extending from 400 to 2500 wall units, longer than it was thought. The maximum of the intensity of the streamwise velocity increases with the Reynolds number, as expected. Also, the elusive second maximum of this intensity has not appeared yet. In case it exists, its location will be around y(+) approximate to 120, for a friction Reynolds number extrapolated to approximately 13 500. The small differences in the near-wall gradient of this intensity for several Reynolds numbers are related to the scaling failure of the dissipation, confirming this hypothesis. The scaling of the turbulent budgets in the center of the channel is almost perfect above 1000 wall units. Finally, the peak of the pressure intensity grows with the Reynolds number and does not scale in wall units. If the pressure at the wall is modeled as an inverse quadratic power of Re-tau, then p(infinity)'(+) approximate to 4.7 at the limit of infinite Reynolds number.The authors gratefully acknowledge computing time provided by the Gauss Centre for Supercomputing e.V. on the GCS Supercomputer SuperMUC-NG at Leibniz Supercomputing Centre under Project No. pr92la, on the supercomputer Lichtenberg II at TU Darmstadt under Project No. project00072, and on the supercomputer CLAIX-2018 at RWTH-Aachen, Project No. bund0008. We are thankful to Mr. Monkewitz for providing us a copy of his model. S.K. and M.O. acknowledge funding by the German Research Foundation (DFG) through the Project No. OB96/39-1 and OB96/48-1. S.H. and F.A.A. were supported by Contract No. RTI2018-102256-B-I00 of MINECO/FEDER. F.A.A. is partially funded by GVA/FEDER Project No. ACIF2018. Finally, the authors thank Paul Hollmann for corrections with Latex.Hoyas, S.; Oberlack, M.; Alcántara-Ávila, F.; Kraheberger, SV.; Laux, J. (2022). Wall turbulence at high friction Reynolds numbers. Physical Review Fluids. 7(1):1-10. https://doi.org/10.1103/PhysRevFluids.7.0146021107