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The physical mechanisms of frost formation have been widely studied, yet much empirism is still needed in numerical approaches. Indeed, accurate simulations of frost growth can be reached by setting up a specific combination of the model empirical inputs while using a method to accurately track the frost-air interface. This paper presents a finite volume ALE method which captures the air-frost interface using dynamic meshes. It is divided into two main sections. First, the search of a valid set of empirical correlations to correctly emulate frost growth under certain experimental conditions. An assessment of seven reference cases is carried out by comparing solutions using different empirical correlations against experimental data. As a result, a discussion on the performance of such parameters is made, emphasizing the fact of using diffusion resistance factors above 1.0 in order to capture the frost growth. Second, a 2D numerical test consisting of a duct flow with a non-homogeneously cooled lower boundary is performed. Aspects related to the frost thickness and growth rate are analysed, proving the method to be a valid candidate to simulate frost growth.

The physics of semiconductor quantum rings near the band edge is often well described considering decoupled bands. There are however instances where band coupling leads to relevant changes in the electronic structure and derived properties. In this chapter we analyze two such cases. First, we focus on the heavy hole-light hole band mixing in self-assembled InAs/GaAs quantum rings, which is important for current endeavour to develop quantum information science using the spin of holes. In InAs/GaAs quantum dots, the hole ground state is known to be mainly formed by the heavy hole subband. However, there is a finite spin-orbit coupling with the light-hole subband which is critical in determining the hole spin properties. Based on k\(\cdot \)p theory, in this chapter we study the influence of hole subband mixing in quantum rings. It is shown that the inner cavity of the ring enhances the light hole component of the ground state. As the quasi-1D limit is approached, the light-hole character becomes comparable to that of the heavy hole. Strain reduces the coupling, but it is still larger than in quantum dots. Second, we study the electronic structure of monolayer MoS\(_2\) quantum dots subject to a magnetic field. Here, the coupling between conduction and valence band gives rise to mid-gap topological states which localize near the dot edge. These edge states are analogous to those of 1D quantum rings. We show they present a large, Zeeman-like, linear splitting with the magnetic field, anticross with the delocalized Fock-Darwin-like states of the dot, give rise to Aharonov-Bohm-like oscillations of the conduction (valence) band low-lying states in the K (K\(^{\prime }\)) valley, and modify the strong-field Landau levels limit form of the energy spectrum.

The aim of this work is to delve into the numerical analysis of viscoplastic-type non-Newtonian fluid flows. Specifically, improvements in the spatial discretization schemes and the temporal integration methods have been proposed to overcome the numerical problems introduced by the transpose diffusive term and associated with the velocity field discontinuity, the artificial viscous diffusion, and the transpose viscous coupling. The resulting knowledge may be useful, among many other reasons, to improve the corresponding numerical simulations and gain insight into the underlying physics of this class of non-Newtonian fluid flows. The aim of this work is to delve into the numerical analysis of viscoplastic-type non-Newtonian fluid flows. Specifically, improvements in the spatial discretization schemes and the temporal integration methods have been proposed to overcome the numerical problems introduced by the transpose diffusive term and associated with the velocity field discontinuity, the artificial viscous diffusion, and the transpose viscous coupling. The resulting knowledge may be useful, among many other reasons, to improve the corresponding numerical simulations and gain insight into the underlying physics of this class of non-Newtonian fluid flows.

We address theoretically differences and similarities on the electronic structure of CdSe/CdS dot-in-dot nanocrystals (NCs) with wurtzite/wurtzite (WZ/WZ), zinc-blende/zinc-blende (ZB/ZB) and polytype ZB/WZ crystalline phases, as they are currently being synthesized and used in optoelectronic devices. We show that the electronic structure of polytypic CdSe/CdS NCs closely resembles that of WZ or ZB NCs with regard to quantum confinement and strain, resulting in similar single-exciton wave functions. The main differences arise in the nature and magnitude of built-in electric fields. We predict that these fields are stronger in polytypes than in pure WZ or ZB NCs due to the sharp spontaneous polarization mismatch between the cubic core and the hexagonal shell lattices. Polarization in NCs is currently believed to be screened by several surface effects. In polytypical structures, however, the polarization mismatch at the interface may create effective charges that are sufficiently far from the outer surface to be quenched. To make a definitive assessment on this controversial issue, we propose experiments in polytypic ZB/WZ NCs where both core and shell are made of CdSe. In such a case, band offsets are small, strain is absent, and our calculations predict pyroelectricity should become the driving force, inducing transitions from type-I to type-II excitons with increasing core or shell size. We thank I. Moreels and P. Guyot-Sionnest for useful discussions. Support from MINECO project CTQ2014-60178-P, UJI project P1-1B2014-24 is acknowledged.

In this article, the three-dimensional turbulent forced convection in a matrix of wall-bounded 8 × 8 cylindrical pins is studied. Large eddy simulations (LES) are performed for Reynolds numbers 3,000, 10,000 and 30,000. Three different subgrid-scale (SGS) models (WALE, QR, and VMS) are compared on a row-by-row study. Local values of velocity and pressure coefficient are depicted. Turbulence characteristics of the flow are well illustrated. The thermal field is analyzed and validated with the time-averaged Nusselt number at the end walls of the pin matrix. The capabilities of the methodology for predicting the turbulent flow features is also shown.

We investigate the electron spin relaxation between Zeeman sublevels of fully three-dimensional quantum dots. By going beyond the usual two-dimensional description of Rashba and Dresselhaus spin–orbit interactions (SOI), we provide a general overview of the effect of the quantum dot shape. It is shown that, in spherical quantum dots, the Dresselhaus SOI is severely suppressed, leading to slow relaxation rates and a strong (B9) dependence on the magnetic field.

The main objective of this paper is to study in detail the fluid flow and the heat transfer in plane impinging jets. Mean and fluctuating velocities and global parameters, i.e. the local Nusselt number, are analysed. The study is focused on a Reynolds number 20,000 (based on the bulk inlet velocity and the nozzle width, B) and dimensionless jet-to-surface spacing 4. As a first step, a reliable direct numerical simulation (DNS) has been performed. Then, the DNS results have been used as reference solution to assess the performance of several Reynolds-averaged Navier-Stokes (RANS) models. Namely, explicit algebraic Reynolds stress models and both non-linear and linear eddy viscosity models in conjunction with k- epsilon and k - omega platforms. Moreover, an overview of the numerical methods and the methodology used to verify the code and the simulations is also presented. Time-averaged DNS results have revealed that the main recirculating flow cannot be captured well unless the outflow is placed at least at 4013 from the jet centreline approximately. This suggests that previous experimental data may not be adequate to study the flow configuration far from the jet. Consequently, conclusions previously published by the authors on the performance of the tested RANS models have been necessarily revised. (C) 2011 Elsevier Ltd. All rights reserved.

This work is focussed on the development of a numerical simulation model that predicts the thermal and fluid-dynamic behaviour of the two-phase flow distribution in systems with multiple branching tubes like manifolds. The geometry of a simulated branching system is represented as a set of tubes connected together by means of junctions. On one side, the in-tube evaporation/condensation phenomena are simulated by means of a one-dimensional two-phase flow model, and on the other side, the splitting/converging flow phenomena occurring at junctions are predicted with appropriate junction models obtained from the technical literature. The global flow distribution is calculated using a semi-implicit pressure based method (SIMPLE-like algorithm) where the continuity and momentum equations of the whole domain are solved and linked with both the in-tube two-phase flow model and the junction models. In the present paper, the flow distribution model is described and its most significant aspects are detailed. Furthermore, the model is validated against experimental and numerical data found in the open literature. The numerical predictions are compared against an adiabatic single-phase flow manifold system working with water and also against a two-phase flow upwardly oriented manifold system working with carbon dioxide. In addition to this, a numerical comparison of a manifold system with two different orientations is carried out. Concluding remarks about the possibilities that this kind of model offers are presented in the last section.

This paper is devoted to the presentation of a new model for fin-and-tube evaporators, focusing on the solid core simulation and its integration with a quasi-homogeneous two-phase flow model for the in-tube refrigerant flow. Special attention is given to separated in-tube flow patterns (stratified, stratified-wavy), because of their importance in liquid overfeed and domestic refrigerator evaporators and the impact on the solid core temperature distribution. The paper presents the solid core formulation and numerical method, the in-tube two-phase flow model, and describes the proposed integration algorithm between them. A selected single-tube baseline case is analysed in full detail, showing the impact of stratified flow on the fin-and-tube temperature distributions. Additional studies are finally presented analysing different flow transitions (single phase to stratified flow, stratified-wavy flow to annular flow, annular flow to partial dry-out) and several operating parameters (flow regime, tube material, tube thickness). Special attention is given to the influence of the flow pattern on the fin-and-tube core temperature profiles.

This work addresses the verification of multidimensional and transient numerical solutions based on Richardson extrapolation techniques. Alternative extrapolation strategies based on both simultaneous (SCR) and independent (ICR) space-and-time coordinates refinement studies are investigated. Extrapolation strategies studied encompass the generalized Richardson extrapolation (GRE) based on both the local and the global observed order of accuracy, and the mixed-order Richardson extrapolation (MORE). Performance of all verification methods investigated has been tested on a manufactured solution and on the flows in different piston–cylinder configurations. Results reveal that the global observed order of accuracy and the postprocessing estimators obtained from ICR studies can provide useful information on which the most suitable verification methods can be selected.

This article presents a model for the analysis of fin-and-tube heat exchangers, focusing on the heat conduction processes within the finned tube bundle. A cutting cell discretization has been proposed for the fins to adapt to the tubes shape, while the tubes have been discretized in axial and angular directions to consider complex heat transfer coefficient variations. A set of results is given on fin efficiency and transient response comparing with well-established methods. A full-scale condenser is also analyzed as an illustrative result, detecting important thermal bridges through the fins.

The transient cooling of a fluid initially at rest inside a vertical cylinder submitted to heat losses through the walls is studied. The study is restricted to laminar flow conditions. In order to identify the relevant non-dimensional groups that define the unsteady natural convection phenomenon that occurs, a non-dimensional analysis is carried out. The long-term behaviour of the fluid is modelled by formulating a prediction model based on global balances. A parametric study by means of several multidimensional numerical simulations led to correlate the Nusselt number and the transient mean fluid temperature, in order to feed the global model proposed. Special attention is given to the appropriateness of the spatial and time discretisation adopted, the verification of the numerical solutions and the post-processing tasks carried out in order to obtain the correlations. The most relevant particularities of the numerical model developed are also pointed out.

This paper focuses the attention on the drawbacks and abilities of wall-function techniques through an analysis of well-known wall-functions from literature. Besides this, some deeper analysis of these tools by means of physical and numerical considerations are carried out in order to improve their limitations when they are applied to predict heat transfer and fluid flow. Accuracy, grid-sensitivity, numerical behaviour and verification of numerical simulations are key aspects in this paper. The main purpose is to obtain tools which are able to predict both fluid flow and heat transfer with low CPU time consumption, reduced grid-sensitivity and a relatively good accuracy.

In this article, different impinging jet configurations are studied by means of time-averaged Navier-Stokes simulations. In this technique, explicit algebraic Reynolds stress models and both nonlinear and linear eddy–viscosity models are explored jointly with k − ∊ and k − ω platforms. The main object of this work is to study numerical performance and accuracy of models when they are used in the simulation of both plane and round impinging jets. With this purpose, results from numerical simulations, using different models, are compared among them and with experimental data available in the literature. Comparisons are performed in terms of mean and fluctuating velocities and global parameters, i.e., the local Nusselt number. A verification procedure is applied in order to ensure the credibility of the numerical solutions.

A detailed model for the simulation of automotive fin-and-tube heat exchangers previously developed by the authors (Part 1) has been experimentally validated for a wide range of geometries and boundary conditions. Some of this experimental information was collected in an academic testing unit, while a second group of experiments was conducted in the experimental set-up of an industrial partner. The data cover a wide range of operational conditions and analyze the effect of most influential geometric parameters. The paper presents the experimental units and shows the comparison between numerical predictions and experimental values. The numerical calculations have been carried out using the most suitable available air-side heat transfer and friction correlations. Although the comparison is reasonably accurate, the deviations appearing among the predictions using different correlations indicate that further research work is still necessary to describe in a more accurate way the fundamental behavior of the lo...

The aim of this paper is to present a developed detailed model for the simulation of automotive fin-and-tube heat exchangers (Compact Heat Exchanger Simulation Software, CHESS). The simulation strategy and the mathematical methodology are described in detail. The model is based on a 3D discretization around the tubes as small heat exchangers, where the appropriate governing equations (mass, momentum and energy) are applied for each control volume on the air-side, the solid elements, and the coolant side. Some verification and illustrative results are also provided to show the features of the model. A comparison between numerical simulation results and experimental data is presented in a companion paper (Part 2).

The electronic structure of monolayer MoS$_2$ nanoribbons and quantum dots have been investigated by means of an effective $\mathbf{k}\cdot \mathbf{p}$ two-band model. Both systems with borders exhibit states spatially localized on the edges and with energies lying in the band gap. We show that the conduction and valence band curvatures determine the presence/absence of these states whose origin has been related to the marginal topological properties of the MoS$_2$ single-valley Hamiltonian., Comment: submitted

Using multiband k·p calculations, we show that strain-engineered piezoelectricity is a powerful tool to modulate the electron−hole spatial separation in a wide class of wurtzite CdSe/CdS nanocrystals. The inherent anisotropy of the hexagonal crystal structure leads to anisotropic strain and, consequently, to a pronounced piezoelectric field along the c axis, which can be amplified or quenched through a proper design of the core−shell structure. The use of large cores and thick shells promotes a gradual departure from quantum confined nanocrystals to a regime dominated by piezoelectric confinement. This allows excitons to evolve from the usual type-I and quasi-type-II behavior to a type-II behavior in dot-in-dots, dot-in-rods, rod-in-rods, and dot-in-plates. Piezoelectric fields explain experimental observations for giant-shell nanocrystals, whose time-resolved photoluminescence reveals long exciton lifetimes for large cores, contrary to the expectations of standard quantum confinement models. They also explain the large differences in exciton lifetimes reported for different classes of CdSe/CdS nanocrystals. Support from MINECO project CTQ2014-60178-P, UJI project P1-1B2014-24 and a FPU grant (C.S.) is acknowledged. The present publication is further realized with the support of the Ministero degli Affari Esteri e della Cooperazione Internazionale (IONX-NC4SOL, I.M.).

Single and multi-band k⋅p Hamiltonians for GaAs crystal phase quantum dots are used to assess ongoing experimental activity on the role of such factors as quantum confinement, spontaneous polarization,valence band mixing, and exciton Coulomb interaction. Spontaneous polarization is found to be a dominating term. Together with the control of dot thickness [Vainorius et al., Nano Lett. 15, 2652 (2015)], it enables wide exciton wavelength and lifetime tunability. Several new phenomena are predicted for small diameter dots [Loitsch et al., Adv. Mater. 27, 2195 (2015)], including non-heavy hole ground state, strong hole spin admixture, and a type-II to type-I exciton transition, which can be used to improve the absorption strength and reduce the radiative lifetime of GaAs polytypes. We are grateful to P. Caroff, M. E. Pistol, and B. Loitsch for useful discussions. Support from UJI Project No. P1-1B2014-24, MINECO Project No. CTQ2014-60178-P, and a FPU grant (C.S.) is acknowledged.

The aim of this work is to study the adequacy of different RANS models in terms of accuracy and numerical performance in the description of turbulent internal forced convection flows. Within RANS modelizations, linear and non-linear eddy-viscosity models and explicit algebraic models are explored. A comparison of the suitability of different two-equation platforms such as k – ϵ and k – ω is also carried out. Three different internal forced convection flows are studied: turbulent plane channel, backward facing step, and confined impinging slot jet. The results are compared with DNS or experimental data available in the literature, reviewing mean and fluctuating velocities, turbulent stresses and global parameters like Nusselt number, skin friction coefficient or reattachment point. Governing partial differential equations are transformed to algebraic ones by a general fully implicit finite-volume method over structured and staggered grids. A segregated SIMPLE-like algorithm is used to solve pressure-velocity fields coupling. A verification procedure based on the generalised Richardson extrapolation is applied to ensure the credibility of the numerical solutions.

A set of complete two- and three-dimensional direct numerical simulations (DNS) in a differentially heated air-filled cavity of aspect ratio 4 with adiabatic horizontal walls is presented in this paper. Although the physical phenomenon is three-dimensional, owing to its prohibitive computational costs the majority of the previous DNS of turbulent and transition natural convection flows in enclosed cavities assumed a two-dimensional behaviour. The configurations selected here (Rayleigh number based on the cavity height 6.4 × 108, 2 × 109 and 1010, Pr = 0.71) are an extension to three dimensions of previous two-dimensional problems.An overview of the numerical algorithm and the methodology used to verify the code and the simulations is presented. The main features of the flow, including the time-averaged flow structure, the power spectra and probability density distributions of a set of selected monitoring points, the turbulent statistics, the global kinetic energy balances and the internal waves motion phenomenon are described and discussed.As expected, significant differences are observed between two- and three-dimensional results. For two-dimensional simulations the oscillations at the downstream part of the vertical boundary layer are clearly stronger, ejecting large eddies to the cavity core. In the three-dimensional simulations these large eddies do not persist and their energy is rapidly passed down to smaller scales of motion. It yields on a reduction of the large-scale mixing effect at the hot upper and cold lower regions and consequently the cavity core still remains almost motionless even for the highest Rayleigh number. The boundary layers remain laminar in their upstream parts up to the point where these eddies are ejected. The point where this phenomenon occurs clearly moves upstream for the three-dimensional simulations. It is also shown that, even for the three-dimensional simulations, these eddies are large enough to permanently excite an internal wave motion in the stratified core region. All these differences become more marked for the highest Rayleigh number.

This article presents different interrelated numerical criteria for the analysis of both the diffusion and convection terms in convection-diffusion equations: gradient computation, interpolation procedures, diffusion schemes, convective schemes, and boundary conditions. Discretizations are performed on three-dimensional unstructured meshes composed of arbitrary convex polyhedra. A finite-volume cell-centered formulation is used. Most of the numerical criteria are well known in the technical literature, but a number of alternative approaches or variants are also presented. The objective of the article is to analyze these criteria in terms of accuracy, computational effort, and convergence characteristics. A comprehensive numerical study is performed in Part II of this article.

A model that considers the Navier-Stokes equations is used to study the influence of surfactants, substances that improve heat and mass transfer in absorption machines due to Marangoni effect. For solving the free surface location a surface tracking method is used, and for the calculation of surface tension forces, a continuum surface forces model is employed. The coupled equations are solved using the SIMPLEC procedure. Two common surfactants for LiBr aqueous solutions, 1-octanol and 2-ethyl-1-hexanol, are numerically tested and the absorption enhancement produced in a stagnant pool is compared with other researchers' results. For falling film absorption some other results are showed.

The majority of the direct numerical simulations of turbulent and transition natural- convection flows in cavities assume two-dimensional behavior. To investigate the effect of the three-dimensional fluctuations, a complete direct numerical simulation has been carried out, in a cavity with aspect ratio 4, Raz = 6.4 × 108 , and Pr = 0.71, using a low-cost PC cluster. A description of the parallel algorithm and the methodology used to verify the code and the accuracy of the statistics obtained is presented. The main features of the two- and three-dimensional flows are described and compared. Several first- and second-order statistic distributions have been evaluated, including the Reynolds stress tensor. Significant differences are observed between the second-order statistics of the two- and three- dimensional simulations.

Parallel computers based on PC-class hardware (Beowulf clusters) provide a matchless computing power per cost unit. However, their network performance tends to be too low for standard parallel computational fluid dynamics (CFD) algorithms. A relevant example is the solution of the Poisson equations. The subject of this article is a direct Schur-Fourier decomposition (DSFD) algorithm that, for certain three-dimensional flows, produces an accurate solution of each Poisson equation with just one message, providing speed-ups of at least 24 in a low-cost PC cluster with a conventional network and 36 processors. Direct Numerical Simulation (DNS) of turbulent natural convection flow is used as a benchmark problem.

We investigate the electron spin–orbit interaction anisotropy of pyramidal InAs quantum dots using a fully three-dimensional Hamiltonian. The dependence of the spin–orbit interaction strength on the orientation of externally applied in-plane magnetic fields is consistent with recent experiments, and it can be explained from the interplay between Rashba and Dresselhaus spin–orbit terms in dots with asymmetric confinement. Based on this, we propose manipulating the dot composition and height as efficient means for controlling the spin–orbit anisotropy. This work was supported by UJI-Bancaixa Project no. P1-1B2011-01, MINECO Project no. CTQ2011-27324, and FPU Grant (C. Segarra).

We report a systematic study of the spin relaxation anisotropy between single electron Zeeman sublevels in cuboidal GaAs quantum dots (QDs). The QDs are subject to an in-plane magnetic field. As the field orientation varies, the relaxation rate oscillates periodically, showing ``magic'' angles where the relaxation rate is suppressed by several orders of magnitude. This behavior is found in QDs with different shapes, heights, crystallographic orientations and external fields. The origin of these angles can be traced back to the symmetries of the spin admixing terms of the Hamiltonian. In [001] grown QDs, the suppression angles are different for Rashba and Dresselhaus spin-orbit terms. By contrast, in [111] grown QDs they are the same, which should facilitate a thorough suppression of spin-orbit induced relaxation. Our results evidence that cubic Dresselhaus terms play a critical role in determining the spin relaxation anisotropy even in quasi-2D QDs., Comment: 8 pages, 6 figs, submitted to PRB on 07/30/14

We calculate the spin–orbit induced hole spin relaxation between Zeeman sublevels of vertically stacked InAs quantum dots. The widely used Luttinger–Kohn Hamiltonian, which considers coupling of heavy- and light-holes, reveals that hole spin lifetimes (T 1) of molecular states significantly exceed those of single quantum dot states. However, this effect can be overcome when cubic Dresselhaus spin–orbit interaction is strong. Misalignment of the dots along the stacking direction is also found to be an important source of spin relaxation.

We use spin-density-functional theory within an envelope function approach to calculate electronic states in a GaAs/InAs core-shell nanowire pierced by an axial magnetic field. Our fully three-dimensional quantum modeling includes explicitly a description of the realistic cross section and composition of the sample, and the electrostatic field induced by external gates in two different device geometries: gate-all-around and back-gate. At low magnetic fields, we investigate Aharonov-Bohm oscillations and signatures therein of the discrete symmetry of the electronic system, and we critically analyze recent magnetoconductance observations. At high magnetic fields, we find that several charge and spin transitions occur. We discuss the origin of these transitions in terms of different localization and Coulomb regimes, and we predict their signatures in magnetoconductance experiments. Universitat Jaume I P1-1B2011-01 P1.1B2014-24 MINECO CTQ2011-27324 Generalitat Valenciana Vali+d Grant APOSTD/2013/052 MINECO FPU Grant EU-FP7 Initial Training Network INDEX Universita di Modena e Reggio Emilia

A new parallel method for simulations with non-overlapping disconnected mesh domains but adjacent boundaries is presented and studied. This technique allows simulations using 3D unstructured meshes that are independent.

This paper reports the results of a numerical study on the transient removal of a contaminant from a two-dimensional enclosure with one inlet and one outlet. The influence of buoyancy forces due to thermal and concentration gradients, contaminant diffusivity, inlet velocity and outlet disposition over the cleaning-time are studied. The governing equations of the laminar flow (continuity, momentum, energy and contaminant concentration) are solved by means of the SIMPLEC algorithm. Several simulations of the same test case have been made, using two numerical schemes : PLDS and SMART, in order to determinate which one provides better performances. For isothermal situations, the time required to remove the contaminant is studied parametrically as a function of the outlet position and the governing dimensionless numbers (Reynolds number, Schmidt number and solutal Rayleigh number) , while the effect of horizontal temperature differences is studied in several situations in which it plays an important role. Buoyancy forces are found to have a strong influence over the flows and, in consequence, over the cleaning times.

Many applications of semiconductor quantum dots rely on the use of valence band holes. A prominent example is current endeavour to develop quantum information science using the spin of holes rather than that of electrons. Understanding the spin and orbital properties of holes is necessary for further progress. In self-assembled InAs/GaAs quantum dots, the hole ground state is mainly formed by the heavy hole subband. However, there is a finite mixing with the light-hole subband which has been shown to be critical in determining the hole spin properties. A large number of works have then investigated the influence of such coupling in dots. Based on k⋅p theory, in this chapter we study the influence of hole subband mixing in self-assembled quantum rings. It is shown that the inner cavity of the ring enhances the light hole component of the ground state. As the quasi-1D limit is approached, the light-hole character becomes comparable to that of the heavy hole. In InAs/GaAs quantum rings strain reduces the coupling, but the mixing is still larger than in quantum dots. Strain also gives rise to unusual phenomena, such as partial localization of the heavy holes inside the repulsive core region. Deviations of quantum rings from the perfect axial symmetry are shown to have a minor influence on the hole mixing.

We study the effect of valence band spin-orbit interactions on the acoustic phonon assisted spin relaxation of holes confined in quantum dots. Heavy hole-light hole (hh-lh) mixing and all the spin-orbit terms arising from zinc-blende bulk inversion asymmetry (BIA) are considered on equal footing in a fully 3D Hamiltonian. We show that hh-lh mixing and BIA have comparable contributions to the hole spin relaxation in self-assembled QDs, but BIA becomes dominant in gated QDs. The dependence of the hole spin relaxation on the QD geometry and spin splitting energy is drastically different from that of electrons, with a non-monotonic behavior which results from the interplay between SOI terms. Our results reconcile contradictory predictions of previous theoretical works and are consistent with experiments., To be published in New Journal of Physics

This work is devoted to the study of turbulent natural convection flows in differentially heated cavities. The adopted configuration corresponds to an airfilled (Pr = 0.7) cavity of aspect ratio 5 and Rayleigh number Ra = 4.5 × 1010 (based on the cavity height). Firstly, a complete direct numerical simulation (DNS) has been performed. Then, the DNS results have been used as reference solution to assess the performance of symmetry-preserving regularization as a simulation shortcut: a novel class of regularization that restrain the convective production of small scales of motion in an unconditionally stable manner. In this way, the new set of equations is dynamically less complex than the original Navier-Stokes equations, and therefore more amenable to be numerically solved. Direct comparison with the DNS results shows fairly good agreement even for very coarse grids.

Few-electron states of AlAs-GaAs-AlAs hexagonal quantum rings pierced by an axial magnetic field are computed through full configuration interaction calculations. The quantum ring is in the low-density regime, populated with N = 1 up to N = 7 electrons. Similar to circular rings, the energy spectra of the hexagonal ones reflect an integer and fractional Aharonov-Bohm regular oscillation pattern for N = 1 and , respectively. Deviations from the regular fractional period with increasing electron density become apparent for larger N. Remarkably, for N = 6 the Aharonov-Bohm effect is completely suppressed. This is a unique symmetry-related feature of hexagonal rings that only can emerge in the low-density regime. © Copyright EPLA, 2013.

We study the spin purity of the hole ground state in nearly axially symmetric GaN/AlN quantum dots (QDs). To this end, we develop a six-band Burt-Foreman Hamiltonian describing the valence band structure of zinc-blende nanostructures with cylindrical symmetry, and calculate the effects of eccentricity variationally. We show that that the aspect ratio is a key factor for spin purity. In typical QDs with small aspect ratio the ground state is essentially a heavy hole (HH) whose spin purity is even higher than that of InGaAs QDs of similar size. When the aspect ratio increases, mixing with light-hole (LH) and split-off (SO) subbands becomes important and, additionally, the ground state becomes sensitive to QD anisotropy, which further enhances the mixing. We finally show that despite the large GaN hole effective mass, an efficient magnetic modulation is feasible in QDs with aspect ratio ∼ 1, which can be used to modify the ground state symmetry and hence the optical spectrum properties.

The direct numerical simulation of the flow over a sphere is performed. The computations are carried out in the sub-critical regime at Re = 3700 (based on the free-stream velocity and the sphere diameter). A parallel unstructured symmetry-preserving formulation is used for simulating the flow. At this Reynolds number, flow separates laminarly near the equator of the sphere and transition to turbulence occurs in the separated shear layer. The vortices formed are shed at a large-scale frequency, St = 0.215, and at random azimuthal locations in the shear layer, giving a helical-like appearance to the wake. The main features of the flow including the power spectra of a set of selected monitoring probes at different positions in the wake of the sphere are described and discussed in detail. In addition, a large number of turbulence statistics are computed and compared with previous experimental and numerical data at comparable Reynolds numbers. Particular attention is devoted to assessing the prediction of the mean flow parameters, such as wall-pressure distribution, skin friction, drag coefficient, among others, in order to provide reliable data for testing and developing statistical turbulence models. In addition to the presented results, the capability of the methodology used on unstructured grids for accurately solving flows in complex geometries is also pointed out.

This work is focused on the simulation and prediction of turbulent natural convection flows by means of two-equation eddy-viscosity models. In order to show the generality, precision, and numerical issues related to these models under natural convection, three different buoyancy-driven cavities have been simulated: a tall cavity with a 30:1 aspect ratio, a cavity with a 5:1 aspect ratio, and, finally, a 4:1 aspect ratio cavity. All cases are solved under moderate and∕or transitional Rayleigh numbers (2.43×1010, 5×1010, and 1×1010, respectively) and all simulations are compared to experimental and∕or direct numerical simulation data available in literature. These different situations allow to check the applicability of two-equation eddy-viscosity models in buoyancy-driven flows, giving criteria on computational effort∕precision and their physical behavior.

In the published article Rodriguez et al. (2011) one of the author’s names was misspelled and should have read Ricard Borrell.

In order to prevent entrainment, an air curtain should provide a jet with low turbulence level, and enough momentum to counteract pressure differences across the opening. Consequently, the analysis of the discharge plenum should be taken into consideration. Hence, the main object of this paper is to study the discharge chamber geometry and the presence of blades for flow orientation. This analysis is carried out in order to understand their influence on the jet produced. Studies presented are based on detailed numerical simulations and on experimental measurements. The influence of the turbulence model, boundary conditions and computational domain is investigated.

A fixed-grid-porous-media method capable of simulating the growth and densification of frost sheets is here presented. A velocity field is calculated across the entire domain, in which a porous media treatment is given to the ice-containing cells. The transported temperature and vapour density are used to define the thermophysical state of each cell, which might enable phase change. As an improvement to Bartrons et al., 2017, the method hereby presented accounts for solidification and sublimation phase transitions. The explicit time step has also been increased by using a semi-implicit treatment of the energy equation. Furthermore, a special boundary condition for cold surfaces has been developed in order to overcome the averaging effect that prevents ice formation in the cells adjacent to the wall. The method is then tested with a study case of a duct flow with a non-homogeneously cooled lower boundary. Several numerical tests are carried out in order to understand the capabilities of the model. The in¿uence of accounting for the convection, as well as the enhanced diffusion resistance factors within the frost layer, is studied by means of the calculated porosity and velocity fields throughout the domain.

The problem of melting inside an isothermal horizontal cylinder has been numerically studied, considering its three-dimensional behavior. The presented numerical results correspond to two different cases with Rayleigh numbers of 3.68 × 105 and 1.22 × 105, Stefan numbers of 0.04 and 0.133, and a common Prandtl number of 56.9. The flow pattern developed has been numerically visualized, and the melted mass fraction and the Nusselt number on the surface cylinder have been computed. The numerical results presented have been compared with a two-dimensional numerical prediction and with some experimental data.

The influence of the rotation ratio on the forces acting on a circular cylinder at Re = 5000 has been investigated by means of direct numerical simulations (DNS). Spin ratios in the range 0 ⩽ α = U T / U ref ⩽ 5 have been considered. The results showed that rotation causes vortex shedding to cease for spin ratios α ⩾ 2 , in very good agreement with previous numerical and experimental research. Furthermore, as a consequence of the increased angular velocity of the cylinder the onset of Taylor–Gortler structures is also observed. Symmetry in the flow is broken as rotation ratio increases. This is specially evident in the shear layers as they start to curve towards the side with the lower pressure gradient causing the shrinkage of the vortex formation region. As these changes occur, both the stagnation point and the saddle point, formed in the closure of the recirculation region, shift in location coming closer to each other as the rotation ratio increases. For larger rotational speeds, the recirculation area behind the cylinder disappears and shear layers roll over the cylinder creating a “circumvolving” layer that greatly changes the wake topology. For α ⩾ 4 , the circumvolving layer forces the stagnation point off the cylinder surface, whereas for α = 5 , the accumulation of vorticity close to the stagnation point makes vortices to be shed on one side of the cylinder. Moreover, the changes that rotation cause on the aerodynamic forces on the cylinder are analyzed and discussed in detail.

This study examines the resolution of the Navier-Stokes equations for laminar natural or forced incompressible flows by means of the domain decomposition method. Conservative and nonconservative interpolation schemes available in the literature are studied and compared to a formulation based completely on finite-volume techniques. The discretized governing equations are obtained in each subdomain using a finite-volume method on staggered grids and are solved adopting a pressure-based segregated algorithm. A mesh refinement study and the generalized Richardson extrapolation have been adopted to evaluate the errors and the order of accuracy of each interpolation scheme. Illustrative numerical results of a complex flow through a junction are shown.

This article is devoted to the study of air curtains applied to reduce the refrigerated chambers heat gains. The proposed strategy is based on the numerical simulation of air curtains by means of computational fluid dynamics (CFD) using RANS modeling and their corresponding experimental validation. Further work on the reduction of the detailed numerical results into overall energetic parameters is also presented. Unsteady three-dimensional numerical parametric studies are carried out, simulating the process of refrigerated chamber sudden door opening. The numerical solutions are verified and the influence of the turbulence model used is also investigated. The studies are centered on the influence of air curtain location, the air suction combination, and both the air discharge velocity and the discharge angle.