Structures In Mixing Layer Research Articles

Effects of a Recess on Coaxial Cryogenic Injections at Supercritical Pressure

Effects of recess lengths on coaxial cryogenic jets under a supercritical pressure are numerically studied. A hybrid large-eddy simulation/Reynolds-averaged Navier–Stokes methodology is applied to capture unsteady jet behaviors of recessed injectors. For the injector with a moderate recess length, the dense-core length of the inner jet is shorter than that with a nonrecessed injector, demonstrating that recessing improves the mixing. The flowfields show that the vortex structures in an inner mixing layer are strongly developed within the recessed region, and the entrainment of the outer jet into the inner jet is strengthened. However, a further increase in the recess length turns to the increase of the dense-core length and deteriorates the inner jet decay along the centerline. This deterioration is caused by the separation of the outer jet flow on the wall in the recessed region. The turbulence generation in the outer mixing layer is suppressed because of the separation, resulting in less enhancement of the mixing in the downstream region. Therefore, the study demonstrates that there is an approximate recess length for optimum jet mixing. Finally, a correlation relationship is provided between the dense-core length and the momentum flux ratio from a comprehensive comparison of existing experimental and numerical data.

Impact of Uniaxial Mechanical Perturbation on Structural Properties and Smectite Porosity Features: Ion Exchanger Efficiency and Adsorption Performance Fate

The use of montmorillonite in the context of engineered barriers makes it possible to minimize the spread of heavy metals from industrial and even radioactive waste. An evaluation of the performance of the mechanisms controlling the clay‐environment interaction and predicting the dynamics/configuration of the interlayer space (IS) is required. This work focuses on a quantitative identification of the structural changes and porosity alteration in the case of heavy metal‐exchanged montmorillonite samples (Co2+ and Cd2+ cations) undergoing mechanical stresses (uniaxial oedometric test (loading/unloading)). Relationships between mechanical stress strength, intrinsic structural response, ion exchanger efficiency, and adsorption performance fate are investigated. This goal is achieved through the correlation of in situ quantitative X‐ray diffraction (XRD) analysis (under an extremely controlled atmosphere reached by varying relative humidity rate %rh) and porosity investigation (assured by combining outcomes from BET (Brunauer–Emmett–Teller) and BJH‐ (Barrett, Joyner, and Halenda‐) PSD (pore size distribution) analysis). Obtained results show an upsurge in the structural heterogeneities accompanying the theoretical increase in the mixed layer structure (MLS) number and developing an unconventional hydration behaviour after stress relaxation regardless of exchangeable cation nature. Experimental XRD patterns are reproduced using MLS, which suggests the coexistence of more than one “crystallite” specie and more than one exchangeable cation indicating a complex cation exchange capacity (CEC) saturation. For extremely low %rh value, a new homogeneous dehydrated state trend is observed in the case of the Co2+ cation. Porosity analysis shows mesopore volume growth for the stressed sample and confirms crystallite exfoliation layer trends, results of the layer cohesion damage, and subsequent constraint strength fluctuations.

The Mixed-Layer Structures of Ikunolite, Laitakarite, Joséite-B and Joséite-A

We used high-angle annular dark field scanning transmission electron microscopy (HAADF STEM) to image the crystal structures of four minerals in the Bi4X3 isoseries (X = Te, Se, S), a subgroup of the tetradymite homologous series: ikunolite (Bi4S3), laitakarite (Bi4Se2S), joséite-B (Bi4Te2S), and joséite-A (Bi4TeS2). The four minerals are isostructural and interpretable in terms of regular stacking of seven-atom packages: [Bi–S–Bi–S–Bi–S–Bi], [Bi–Se–Bi–S–Bi–Se–Bi], [Bi–Te–Bi–S–Bi–Te–Bi], and [Bi–S–Bi–Te–Bi–S–Bi], respectively. The four phases are mixed-layer structures representing the Bi2kTe3 (k = 2) module within the tetradymite series. Diffraction patterns confirm they are seven-fold superstructures of a rhombohedral subcell with c/3 = d~1.89–1.93 Å. Modulation along the d* interval matches calculations of reflection intensity using the fractional shift method for Bi4X3. Internal structures can be discerned by high-resolution HAADF STEM imaging and mapping. Paired bismuth atoms are positioned at the outside of each seven-atom layer, giving the minerals a modular structure that can also be considered as being composed of five-atom (X–Bi–X–Bi–X) and two-atom (Bi–Bi) sub-modules. The presence of mixed sites for substituting cations is shown, particularly for Pb. Moreover, Pb may be important in understanding the incorporation of Ag and Au in Bi–chalcogenides. Visualisation of crystal structures by HAADF STEM contributes to understanding relationships between phases in the tetradymite homologous series and will play an invaluable role in the characterization of potential additional members of the series.

Structure and dielectric properties of solid solutions Bi7−2xNd2xTi4NbO21(x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0)

The electrophysical and structural characteristics of bismuth titanate oxides of a number of phases of solid solutions of the Aurivillius phases Bi[Formula: see text]Nd[Formula: see text]Ti4[Formula: see text] ([Formula: see text] = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) having a layered structure of the perovskite type have been investigated. According to the XRD data, all studied compounds are single-phase and have a mixed-layer structure of Aurivillius phases ([Formula: see text] = 2.5) with a rhombic crystal lattice (space group I2cm, [Formula: see text] = 2). A relationship has been established between changes in the chemical composition of solid solutions and orthorhombic and tetragonal distortions of perovskite-like layers. The temperature dependences of the relative permittivity [Formula: see text]/[Formula: see text](T) are measured. It was found that the change in the phase transition temperature — Curie temperature [Formula: see text] synthesized Aurivillius phases Bi[Formula: see text]Nd[Formula: see text]Ti4[Formula: see text] ([Formula: see text] = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) has a close to linear dependence on the change in the parameter [Formula: see text]. The activation energies of charge carriers in different temperature ranges were calculated. It was found that three clearly defined temperature ranges with different activation energies can be distinguished, which is associated with the different nature of charge carriers in the studied solid solutions of the perovskite type. The effect of substitution of Nd[Formula: see text] ions for Bi[Formula: see text] ions is investigated.

Synthesis of SnO2/CuO/SnO2 Multi-layered Structure for Photoabsorption: Compositional and Some Interfacial Structural Studies

Many metal oxide heterostructures have been synthesized as mixed oxides or layered structures for photocatalytic, photodegradation of pollutants and light-harvesting applications. However, in the layered structures the effects of interfacial properties and composition have largely not been explored. Hence, the effects of interfacial mixing and diffusion of sandwiched thin CuO layer on optical absorption of as-deposited and heat-treated multi-layered structured SnO2/CuO/SnO2 films were studied. The RBS analysis of the as-deposited films showed the presence of a minute amount of Cu in the surface and bottom SnO2 layers of the structure. We attributed this to inhomogeneous layer thickness evidenced by very low Sn/Cu atoms ratio of the CuO layer. However, the thermal treatment of the layered structure led to pronounced interlayer mixing and consequent formation of SnO2-CuO solid solutions throughout the layered structure. The layer integrity of the inserted CuO of the as-deposited films was very high and the as-deposited structure was far more optically absorbing. However, the annealed structure showed lesser optical absorption because of the onset of interfacial mixing and improved crystallization. This reflected in the optical bandgap variations of the as-deposited and annealed multilayered structures. The significance of this result is that the multi-layered films possess band narrowing – evidence of increased photon absorption - making it a better candidate than pure SnO2 oxide for photocatalysis, photodegradation and photodetection applications. It also pointed to the fact that attention must be paid to effects of heat treatments or annealing when inserting an absorbing layer into a photocatalyst or a material meant for photodegradation or any light-harvesting material.

Links Between Steepened Mach Waves and Coherent Structures for a Supersonic Jet

The links between coherent structures in the shear layer of an isothermal Mach 2 jet and steepened Mach waves just outside the jet are investigated using large-eddy simulation. Conditional averages triggered by the detection of high-intensity pressure peaks on a conical surface near the jet flow are computed. Averages obtained for peaks of different intensities are presented to identify the factors favoring the steepened aspect of the waves, which are not likely due to nonlinear propagation effects given the surface radial position. The Mach waves just outside the jet are shown to be correlated with coherent structures in the supersonic side of the mixing layers. Their steepened aspect is found to increase with their intensity. The variations of the wave properties with the convection speed, strength, and geometrical shape of the mixing layer structures are then discussed. These properties do not depend much on the convection speed but are closely related to the two other parameters. The wave intensity and steepened aspect increase because the shear-layer structures are stronger, but also because they are more inclined relative to the jet direction. In the latter case, the asymmetric shape of the structure appears to promote the formation of positively skewed Mach waves.

Seasonal variability of mixed layer depth from Argo floats in the central Red Sea

Argo floats are one of the eminent ocean observation systems which retrieve frequent measurements of temperature and salinity profiles in the remote marine environment and provide vital information about the world ocean with or without direct access to the human. The Argo floats available in the Red Sea are not well explored by the research community so far. In the present study, the temperature and salinity data from two Argo floats available in the central Red Sea for a period of 2 years are used to examine the mixed layer depth variability in the region. The mixed layer in the region is maximum during February associated with a decrease in static stability and shallowest during August due to an increase in static stability. A noticeable difference is observed in the existing mixed layer structure by the presence of eddies. The analysis of monthly mean thermal and haline structure showed that the warming of the surface layer is intense from March to August followed by cooling from September to February. The surface layer salinity increased from May to October and decreased in the following months. A noticeable east-west difference is observed in the thermal and haline structure, where the eastern side of the region is warmer by ~0.3 °C and less saline by ~0.1 PSU.

How the Solid/Liquid Ratio Affects the Cation Exchange Process and Porosity in the Case of Dioctahedral Smectite: Structural Analysis?

The performance of a clay mineral geomembrane used in the context of a geological barrier for industrial and radioactive waste confinement must pass through the understanding of its hydrous response as well as the limits of the cation exchange process which are closely related to the solid/liquid ratio constraint. The Na-rich montmorillonite is used, as starting material, to evaluate the link between the applied external constraint (variable solid/liquid ratio) and the structural response of the material. The geochemical constraint is realized at the laboratory scale, and the possible effects are investigated in the cases of Ba2+ and Ni2+ heavy metal cations. The structural analysis is achieved using the XRD profile modeling approach to quantify the interlayer space (IS) deformation. The quantitative XRD analysis, which consists of the comparison of experimental 001 reflections with the calculated ones deduced from structural models, allowed us to determine the optimal structural parameters describing IS configuration along the [Formula: see text] axis. The obtained result showed an interstratified hydration character, for both studied exchangeable cations, regardless of the solid/liquid ratio being described probably by a partial cation exchange process. The theoretical mixed layer structure (MLS) suggests the coexistence of more one cristallite species saturated by more than one exchangeable cations, indicating a partial saturation of all exchangeable sites. The optimum structural parameter values, from the theoretical model, allowed us to follow the evolution of several intrinsic properties versus the applied constraint strength. The variable solid/liquid ratio effect on the material porosity is examined by the BET-specific surface area and BJH pore size distribution (PSD) analyses. The adsorption measurement outcomes confirm XRD results concerning mainly the link between several intrinsic clay properties and the constraint strength.

Rayleigh–Taylor instability with gravity reversal

We present results from Direct Numerical Simulations (DNS) of Rayleigh–Taylor instability at Atwood numbers up to 0.9. After the layer width had developed substantially, additional branched simulations have been run under reversed and zero gravity conditions. We focus on the modifications of the mixing layer structure and turbulence in response to the acceleration change. After the gravity reversal, the flow undergoes a complex transient process in which the vertical mass flux changes sign multiple times and, consequently, the buoyancy term in the turbulent kinetic energy transport equation changes its role back and forth from production to destruction. This behavior is examined in detail using the turbulent kinetic energy and mass flux transport equations and time instances when the vertical mass at the centerline crosses zero and reaches local minima and maxima. While the transient process significantly affects the flow anisotropy at all scales, other turbulence characteristics, like the alignment between the vorticity and eigenvectors of the strain rate tensor, retain their fully developed turbulence behavior in the interior of the layer. In addition, after the gravity reversal, the edges of the layer also exhibit characteristics closer to those of the turbulent interior, even as the fluids become more mixed. None of these changes affects the mean density profile, which still collapses among various cases. Such significant changes in some turbulence quantities and not others are difficult to capture with existing turbulence models.

Retraction: “Magnetic-field-induced ferroelectric domain dynamics and in-plane polarization in odd and mixed layered Aurivillius structures” [J. Appl. Phys. 126, 084104 (2019)

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Compressibility Effects on Large Structures and Entrainment Length Scales in Mixing Layers

In this work, three-component planar velocity measurements are analyzed to identify the effects of compressibility on mixing layer turbulence, with a focus on the evolution of large-scale structures, dominant spatial eigenmodes, and entrainment length scales. Velocity measurements obtained via stereoscopic particle image velocimetry for five different dual-stream air planar mixing layers with convective Mach numbers of , 0.38, 0.55, 0.69, and 0.88 are analyzed. Results of two-point correlations reveal that length scales of the streamwise velocity fluctuations increase in both the streamwise and transverse directions, whereas the length scales of the transverse velocity fluctuations decrease in the transverse direction. To further investigate these trends, the spatial organization, shape, and dynamics of large-scale turbulent structures are examined via two-dimensional spatial velocity correlations, proper orthogonal decomposition, and linear stochastic estimation. These quantitative techniques support qualitative findings in the literature that report coherent, round roller structures in incompressible mixing layers becoming flattened and elongated longitudinally with increased compressibility. This evolution is likely due to a dominant pulsing motion present for the higher cases and can be linked to compressibility effects on the entrainment mechanisms. Length scales of entrainment are determined using autocorrelations of normal flow velocity components along instantaneously identified turbulent/non-turbulent interfaces and are found to decrease with increasing . This result can heuristically be interpreted as small-scale nibbling being dominant for higher , whereas larger-scale engulfment contributes more in nearly incompressible mixing layers.

Near- and far-field structure of shallow mixing layers between parallel streams

Abstract

Unexpected deep mixing layer in the Sichuan Basin, China

In the Sichuan Basin (SCB), one of the four major basins in China, a one-year continuous observation study was performed using a ceilometer in Chengdu (2018.11–2019.12). The results show that the mixing layer height (MLH) in the SCB exhibits a bimodal seasonal variation pattern, with peaks in June (842 m) and October (704 m) and valleys in September (535 m) and January (607 m). Stable atmospheric conditions in September were the main reason for the decrease in MLH. Through comparison to other regions in China, it is found that the seasonal evolution of the MLH in the SCB is similar to that in the Yunnan-Guizhou Plateau region and more pronounced than that in the central and eastern plain regions of China. This indicates that the change in MLH is influenced by both the Western Pacific Subtropical High and South Asian High. Combined with an analysis of the fine-particulate matter concentration, it is found that the main influence factor of the air pollution in the SCB is not the atmospheric dilution capability, and the local contribution should be paid more attention to. The MLH as a meteorological index for air quality prediction varies from place to place. This study is of great importance to the understanding of the mixing layer structure in the basin and its influence on air pollution.

Influence of the North Atlantic Oscillation on the Heat Budget of the Mixed Layer in the North Atlantic

The heat budget components that are involved in the formation of the tripole structure of the upper mixed layer (UML) temperature anomalies in the North Atlantic under the influence of the North Atlantic Oscillation (NAO) are analyzed using ORA-S3 oceanic reanalysis data for the winter period in 1959–2011. It is shown that the negative UML temperature anomaly in the western part of the tropical zone during the NAO intensification is mainly due to the anomalies of horizontal heat advection and eddy heat diffusion (i.e., nonlocal factors), as well as due to anomalous heat fluxes on the ocean surface (although with a lower significance level). Antiphase changes in the UML temperature in the eastern part of the tropical zone and in the western part of the subtropical zone are accompanied by the anomalies of the heat budget components of the opposite sign. In the subpolar zone, the anomalies of nonlocal components of heat budget also make significant contribution to the UML temperature variation which is in-phase with the one in the tropics. In the high-latitude areas of deep convection, the anomalous heat fluxes at the UML base are also important, and in the eastern part of the subpolar gyre, the anomalies of the net heat fluxes on the ocean surface are crucial too. The signs of the anomalies of the UML heat budget components forming the UML temperature fields in the North Atlantic during the periods with high and low values of the NAO index are opposite.

The role of hydrogen gas bubble in hydrophobic properties in mixed micro layer (Al2O3+Mg)

Purpose: To find out more about the role of hydrogen gas bubbles in improving the hydrophobic nature of a layer, especially in the layers of microparticles Alumina (Al2O3) with Magnesium (Mg). Design/methodology/approach: The method used is an experimental method by first conducting the SEM-Edx test, testing the content of the elements in the waxy layer and observing the topographic shape on the surface of the taro leaves. Then prepare a mixture of Alumina micro particles with Magnesium to investigate the hydrophobicity of the taro leaves. The mixed presentations between Alumina and Magnesium are: (0, 10, 20, 30, 40, 50, 60, 70, 80, 90 and 100%). Findings: The results of this study found three conditions of the Alumina and Magnesium mix layer when in contact with a droplet, namely: Hydrophobic conditions occur when the surface structure of the rough mixed micro layer forms micro crevices, then bubbles of hydrogen gas fill it to form trapped gases. When droplets come in contact with the surface of the mixed layer the effect of the gas being trapped is very effective at creating hydrophobic properties. While the transition conditions occur when more and more hydrogen gas bubbles along with the increasing percentage of Mg and the opposite occurs in micro particle fissures. Bubbles fill the micro-gap space fully so that the tops of the micro particles are covered by bubbles. This causes the droplet surface tension to weaken, so the droplet contact angle decreases. Furthermore, hydrophilic conditions occur when the micro gap is getting narrower as the percentage of Mg increases and the formation of hydrogen gas bubbles increases. The high level of bubble density in the micro gap closes the peaks of the micro particles, which results in the surface tension of the droplet getting weaker. In this weak surface tension condition, the hydrogen bubble can break through the droplet surface tension and change its hydrophobic nature to hydrophilic. Research limitations/implications: This research is limited to the hydrophobicity of Alumina and Magnesium materials, mainly to investigate the role of hydrogen gas in supporting the hydrophobic nature of taro leaves (Colocasia esculenta). Practical implications: The practical implication in this study is the use of hydrophobic membranes which are widely applied to filtration. Originality/value: Discovered the composition of a membrane mixture of Alumina (Al2O3) and Magnesium (Mg) to create hydrophilic and hydrophobic conditions.

Evolution characteristics of subsonic-supersonic mixing layer impinged by shock waves

The study of interaction between shock wave and mixing layer commonly existed in rocket based combined cycle engine is of profound significance for mixing enhancement in combustor. In this paper, the three-dimensional structures of subsonic-supersonic mixing layer impinged by shock waves induced by wedge were investigated experimentally employing the ice-cluster based planar laser scattering technique. The results showed that the mixing layer would partially deflect to the subsonic stream side and inhibit the development of K-H instability vortices under the interaction of induced shock wave. With the increasing of wedge angle (β), the interaction of shock waves could enlarge the global deflection angle of mixing layer and reduce the scale of large-scale vortices. Moreover, the T-shaped structures and horseshoe vortices were existed as usual topological structures in small-scale turbulent transport. Besides, the curved shock waves were generated under the influence of large-scale vortices. In addition, the comparison of mixing layer thickness indicated that the evolution of mixing layer could be promoted for β=5°, while it would be suppressed for β=15° in general.

Enhanced mixing across the gyre boundary at the Gulf Stream front

The Gulf Stream front separates the North Atlantic subtropical and subpolar ocean gyres, water masses with distinct physical and biogeochemical properties. Exchange across the front is believed to be necessary to balance the freshwater budget of the subtropical gyre and to support the biological productivity of the region; however, the physical mechanisms responsible have been the subject of long-standing debate. Here, the evolution of a passive dye released within the north wall of the Gulf Stream provides direct observational evidence of enhanced mixing across the Gulf Stream front. Numerical simulations indicate that the observed rapid cross-frontal mixing occurs via shear dispersion, generated by frontal instabilities and episodic vertical mixing. This provides unique direct evidence for the role of submesoscale fronts in generating lateral mixing, a mechanism which has been hypothesized to be of general importance for setting the horizontal structure of the ocean mixed layer. Along the Gulf Stream front in the North Atlantic, these observations further suggest that shear dispersion at sharp fronts may provide a source of freshwater flux large enough to explain much of the freshwater deficit in the subtropical-mode water budget and a flux of nutrients comparable to other mechanisms believed to control primary productivity in the subtropical gyre.

Enhanced electrocatalytic activity on TEMPO mixed film grafted by diazonium reduction

Stable mixed organic films were formed on glassy carbon electrode surface via a two-step functionalization, using the electroreduction of a diazonium cation. First, the grafting of 4-azidobenzenediazonium was performed in the presence of a redox inhibitor, leading to the formation of an azido monolayer. Then, this monolayer was used as a platform able to react with a mixture of two acetylene molecules. TEMPO-based surfaces, with different controlled surface coverages, were obtained by the described method. TEMPO electrocatalytic properties were exploited and the interfacial reactivity of the functional groups was put into perspective with the mixed layer structure. In this context, the impact of the azido platform thickness was considered.

Effects of solid circular cylinders on the dynamics of the Richtmyer–Meshkov instability

We discuss the effect on the transient dynamics of the Richtmyer–Meshkov instability due to small-scale perturbations introduced by a set of solid circular cylinders located along the material interface. The arrangement of the cylinders mimics (in a two-dimensional domain) the presence of the solid supporting grid wires used in the formation of the material interface in an experimental setup. The numerical experiments are performed by utilizing an embedded boundary adaptive mesh refinement method. We quantify the effect of introducing the cylinders on the mixing layer growth and the mixedness level and qualitatively demonstrate the effect of the perturbation introduced by the cylinders on the mixing layer structure. We empirically model the effect of the solid cylinders based on their diameter and find that the effect of the cylinders on the mixing layer growth is a linear function of the cylinder diameter. The linear model allows us to predict the change in the mixing layer growth within a $$\pm 10\%$$ range, except for very small ratios of the initial perturbation amplitude of the density interface to the cylinder diameter.

Parameterization of Wave-Induced Mixing Using the Large Eddy Simulation (LES) (I)

Turbulent motions in the thin ocean surface boundary layer control exchanges of momentum, heat and trace gases between the atmosphere and ocean. However, present parametric equations of turbulent motions that are applied to global climate models result in systematic or substantial errors in the ocean surface boundary layer. Significant mixing caused by surface wave processes is missed in most parametric equations. A Large Eddy Simulation model is applied to investigate the wave-induced mixed layer structure. In the wave-averaged equations, wave effects are calculated as Stokes forces and breaking waves. To examine the effects of wave parameters on mixing, a series of wave conditions with varying wavelengths and heights are used to drive the model, resulting in a variety of Langmuir turbulence and wave breaking outcomes. These experiments suggest that wave-induced mixing is more sensitive to wave heights than to the wavelength. A series of numerical experiments with different wind intensities-induced Stokes drifts are also conducted to investigate wave-induced mixing. As the wind speed increases, the influence depth of Langmuir circulation deepens. Additionally, it is observed that breaking waves could destroy Langmuir cells mainly at the sea surface, rather than at deeper layers.