Uniform Gradient Research Articles

Enhancement of Mechanical Properties of NiCo Alloy Induced by Inhomogeneous Gradient of Solute Element Segregation

Inhomogeneous gradient nanocrystals have better mechanical properties than uniform gradient nanocrystals. The segregation of solute elements in inhomogeneous gradient nanocrystals is expected to induce the re‐enhancement of mechanical properties of alloys. Herein, solute element segregation structures of inhomogeneous gradient structure nanocrystalline NiCo alloy are established, simulating tensile deformation via molecular dynamics. The results show that the segregation of solute elements in the inhomogeneous gradient structure of nanocrystals can improve the mechanical properties of the alloy, especially the structure of the intragranular segregation. The intragranular segregation of solute elements induces the decrease of grain boundary energy and greatly enhances the stability of grain boundaries. In addition, the segregation of solute elements within grains can hinder the dislocation movement to a certain extent, and the hindering effect on dislocation movement of stable grain boundaries induced by intragranular segregation of solute elements further enhances the mechanical properties of nanocrystalline alloys. This strategy of combining heterogeneous gradient structure and solute element segregation structure provides a positive and interesting perspective for the design of advanced alloys with excellent properties.

Numerical simulation of the primary breakup of fuel jet with incoming positive velocity gradient

To study the breakup process of fuel jets in air crossflow with a positive velocity gradient, the Volume of Fluid (VOF) method and adaptive grid technology are combined to simulate the two-phase flow of gas and liquid. A comparative analysis is conducted on the breakup and corresponding flow characteristics of direct fuel jets under uniform and positive velocity gradient airflow. The simulation results demonstrate that the morphological changes of the fuel column are caused by factors such as gas-liquid shear and asymmetric airflow vortices. The fuel jet undergoes primary breakup, which mainly contains columnar and surface breakup. The columnar breakup is dominated by Rayleigh-Taylor (R-T) instability, while the surface breakup is dominated by Kelvin-Helmholtz (K-H) instability. Compared with uniform flow, the expansion angle in the positive velocity gradient incoming flow increases by an average of 9.2%, and the wavelength of the surface wave increases by an average of 34%.

Topology-optimized thermal metamaterials based on inhomogeneous and anisotropic unit array

In this paper, inhomogeneous and anisotropic thermal metamaterials are designed to achieve the manipulation of heat flow (MHF) at multiple points. The thermal metamaterial unit is constructed from mixtures of two solid-state materials with different thermal conductivities. A variable density topology optimization method is employed to design the filling fractions of the mixed materials within the unit. The thermal concentrator and cloak are designed through the optimization of the topological functional units (TFUs). The thermal gradients of 14 and 0.05 times the uniform thermal gradient are achieved by the concentrator and the cloak, respectively. The effectiveness of the proposed design method is validated through the excellent agreement between numerical simulations and experimental results.

Numerical investigation of unsteady MHD immiscible Casson micropolar and Jeffery fluid in a horizontal channel with heat transfer using MCB-DQM approach

The dynamics of immiscible Casson micropolar and Jeffery fluid flow offer new possibilities for enhancing momentum and heat transfer efficiencies. Motivated by numerous applications of immiscible fluid flows in natural and industrial systems, the current work presents a numerical investigation of unsteady, incompressible, magnetohydrodynamic (MHD) flow involving two immiscible fluids, namely, Casson micropolar and Jeffery fluid, in a horizontal channel. The flow, driven by a uniform pressure gradient, assumes no-slip conditions at the channel walls and a continuous, nondeformable interface between the immiscible fluids. The well-established rapid converging modified cubic B-spline differential quadrature method (MCB-DQM) is applied to solve the partial differential equations governing the flow. The impact of crucial fluid parameters comprising the Casson micropolar fluid parameter, Jeffery fluid parameter, thermal, magnetic, and various other fluid parameters on temperature, microrotation, and velocity profile is illustrated graphically. Notably, the parameters of each fluid influence the profiles in both fluid zones. The flow dynamics are significantly affected by viscous dissipation, Lorentz force, fluid–wall interactions, and interfacial conditions. Key parameters such as time, Jeffery fluid, Casson micropolar, ion slip and Hall parameters, Reynolds number, Eckert number, the ratio of viscosities, the ratio of densities, and pressure gradient show significant impacts on the velocity, microrotation, and temperature profiles. However, reverse or mixed behavior is observed for other parameters. The novelty of current work lies in modeling the unsteady, immiscible, MHD flow of Casson micropolar and Jeffery fluid and proposing a numerical solution by deploying the MCB-DQM method. The findings have potential applications in various industrial and engineering processes, such as the design of oil extraction, optimizing processes in the food processing and cosmetics industry, and developing effective strategies for environment conservation.

Heat transfer characteristics of topological latent heat storage systems based on optimization objectives

In order to improve the heat storage efficiency and shorten the heat storage/release time in latent heat storage (LHS) systems, topology optimization of heat transfer problems has been applied to LHS systems. The choice of the optimization objective determines the structure of the topology fins. In this paper, the topology generation process was analyzed using the mean temperature, the mean square deviation (MSD) of the mean temperature, and the multi-objective entropy depletion as the optimization objectives. In this way, a method for selecting the optimization objectives according to the design requirements was obtained. The multi-objective topological structure (Case 3) with uniform tree-shaped fins (Case 4) and gradient tree-shaped fins (Case 5) was applied to the LHS system, and the finite element analysis method was used to construct a mathematical model of the LHS system to comprehensively analyze the heat storage/release process. The results showed that the complete melting times for Case 3, Case 4, and Case 5 were 751 s, 1005 s, and 920 s, respectively. At 751 s, the average heat storage capacities for Case 3, Case 4, and Case 5 are 296.81 W, 216.94 W, and 238.92 W, respectively. Compared to Case 4, Case 3 decreased the complete melting time by 25.27 % and increased the average heat storage capacity by 36.82 %. At heat-releasing times of 500 s, 1000 s, 1500 s, and 2000 s, the maximum differences in liquid phase rates for the three models were 10.93 %, 11.97 %, 11.51 %, and 9.57 %, respectively. From the perspective of the field synergy, Case 3 showed a better heat transfer path and the optimal heat transfer effect considering the temperature, liquid phase rate, heat storage/release time, and heat storage/release capacity. In addition, the dimensionless criterion equation affecting the liquid phase rate was obtained by segmented fitting of the heat transfer process of the topology-optimized LHS system. The findings of the study can facilitate the promotion of topology-optimized structures for practical engineering applications in LHS systems.

Flat optics of nonuniform phase gradient metasurfaces

Flat optics of uniform phase gradient metasurfaces based on the generalized Snell’s law has been extensively studied. The optics of nonuniform phase gradient metasurfaces (NPGMs) is less clear. Here based on Huygens’ principle and finite-difference time-domain (FDTD) simulation, we explore the optical properties of NPGMs made of nanorods (meta-atoms), which can be tuned by modulation of propagation phase and geometric phase. It is found that the nonuniformity of phase gradient may lead to multichannel anomalous reflection/refraction. The multiple beam splitters with arbitrary intensity ratio can be achieved by using the amplitude/phase modulation. Based on our design principle, we can obtain different anomalous reflection patterns (with channels ±1, ±2, both ±1 and ±2, or no reflection at anomalous angle) by different arrangement of just two types of meta-atoms. In addition, we are able to achieve chiral anomalous reflections for designed NPGMs made of nanorods and L-shaped nanoparticles. Our formulism provides general design guidance for NPGMs and can be used to realize the beam splitting function with adjustable angle and intensity ratio.

A compact, low-pressure manifold with uniform flow at low Reynolds number

Manifolds that distribute fluid into or that collect fluid from a multiplicity of streams are ubiquitous. We introduce a new theory for manifold design to produce uniform flow among their streams. By constructing a tapered header region that feeds uniformly spaced diffuser channels (constraint A), flow uniformity can be achieved with less than a quarter of the footprint of bifurcating manifolds, provided that diffuser channels are arrayed in triangular form (constraint B) with a design-specific angle that satisfies a compatibility condition between its header and diffuser (constraint C). The associated theory harnesses creeping-flow hydraulics to induce a constant header pressure-gradient, in contrast with past theory that relied on the interplay between kinetic energy, pressure, and viscous losses to uniformize header pressure at finite Reynolds number. Experiments using dye-based flow visualization from manifolds incorporating these three design constraints are shown to produce uniform flows, while designs that violate any of the three constraints produce flow that is biased toward the manifold's ends or its center. Our experiments and three-dimensional simulations of such uniformizing manifolds show maximum deviations from uniformity of ∼10% for Reynolds number as high as ∼10. As expected from creeping-flow theory, simulations confirm that such flow uniformity is facilitated by a uniform header-pressure gradient. Finally, the associated uniformizing manifold is shown to produce lower hydraulic resistance than a rectangular manifold circumscribed around it. In addition to the theory's embodiment in the specific form tested here, it is readily applicable to a variety of header and diffuser-channel cross-sectional types.

Uniform gradient magnetic field and spatial localization method based on Maxwell coils for virtual surgery simulation

AbstractWith the development of virtual reality technology, simulation surgery has become a low‐risk surgical training method and high‐precision positioning of surgical instruments is required in virtual simulation surgery. In this paper we design and validate a novel electromagnetic positioning method based on a uniform gradient magnetic field. We employ Maxwell coils to generate the uniform gradient magnetic field and propose two positioning algorithms based on magnetic field, namely the linear equation positioning algorithm and the magnetic field fingerprint positioning algorithm. After validating the feasibility of proposed positioning system through simulation, we construct a prototype system and conduct practical experiments. The experimental results demonstrate that the positioning system exhibits excellent accuracy and speed in both simulation and real‐world applications. The positioning accuracy remains consistent and high, showing no significant variation with changes in the positions of surgical instruments.

Uniform boundary conditions on models of spherical particles through alpha shape surface tracking and Laguerre–Voronoi diagrams

This study describes the adaptation of the alpha-shape three-dimensional surface reconstruction technique to boundaries of spherical particle packings, such as those modeled using Discrete Elements. The alpha shape technique is employed to reconstruct a boundary surface for a packing as well as to assemble a Laguerre–Voronoi Diagram of the packing's boundary. The reconstructed surface can be used to track the evolution of the packing surface, while the surface diagram can be used to directly obtain the discrete force vectors that result from applying a uniform stress tensor (Static Uniform Boundary Condition), as well as the discrete velocity vectors that result from applying a uniform velocity gradient tensor (Kinematic Uniform Boundary Condition) to a model of spherical discrete particles. Doing so continuously throughout a simulation allows for the application of uniform boundary conditions on spherical discrete particles, whilst enabling precise tracking of stress and deformation tensors of the packing. An implementation of this novel approach in the Yade open-source DEM code is detailed here. The power and versatility of the real-time boundary tracking approach is demonstrated using some boundary value problem simulations on polydisperse spherical particle models.

Deep learning-assisted concentration gradient generation for the study of 3D cell cultures in hydrogel beads of varying stiffness.

The study of dose-response relationships underpins analytical biosciences. Droplet microfluidics platforms can automate the generation of microreactors encapsulating varying concentrations of an assay component, providing datasets across a large chemical space in a single experiment. A classical method consists in varying the flow rate of multiple solutions co-flowing into a single microchannel (producing different volume fractions) before encapsulating the contents into water-in-oil droplets. This process can be automated through controlling the pumping elements but lacks the ability to adapt to unpredictable experimental scenarios, often requiring constant human supervision. In this paper, we introduce an image-based, closed-loop control system for assessing and adjusting volume fractions, thereby generating unsupervised, uniform concentration gradients. We trained a shallow convolutional neural network to assess the position of the laminar flow interface between two co-flowing fluids and used this model to adjust flow rates in real-time. We apply the method to generate alginate microbeads in which HEK293FT cells could grow in three dimensions. The stiffnesses ranged from 50Pa to close to 1kPa in Young modulus and were encoded with a fluorescent marker. We trained deep learning models based on the YOLOv4 object detector to efficiently detect both microbeads and multicellular spheroids from high-content screening images. This allowed us to map relationships between hydrogel stiffness and multicellular spheroid growth.

Cosserat-phase-field modeling of grain nucleation in plastically deformed single crystals

Thermomechanical processing of crystalline materials induces microstructural evolution such as grain nucleation and growth. In the numerical simulation of these processes, grain nucleation is generally treated as an additional ad hoc step in which circular or spherical grains are added in regions where a critical dislocation density, stress or strain are reached. In this paper, systematic finite element simulations are performed showing that the Kobayashi–Warren–Carter (KWC) phase field model and its coupling with Cosserat crystal plasticity predict spontaneous nucleation of new grains in single crystals in the presence of lattice orientation/rotation gradients. The numerical analysis of the stability of gradients of lattice rotation and dislocation-based stored energy indicates that a gradient of stored energy alone is not sufficient to trigger grain formation. As an application, the KWC–Cosserat model is used to simulate the torsion and annealing of a copper single crystal bar with a circular cross section. This mechanical loading produces a large, fairly uniform axial rotation gradient which induces nucleation in the form of a stack of cylindrical grains. Plastic strain gradients in cross-sections predicted by the 3D finite element simulation, are not strong enough to compete with the longitudinal nucleation process, as confirmed by experimental observations from the literature.

Dynamic behaviors of sandwich panels with 3D-printed gradient auxetic cores subjected to blast load

This study experimentally and numerically investigated the blast resistance of sandwich panels composed of steel face sheets and 3D-printed gradient auxetic chiral core. Five gradient strategies were utilized in the design of auxetic core, including uniform gradient, positive gradient, negative gradient, center positive gradient and center negative gradient. Varying blast loadings were applied by adjusting the masses of explosive charges. The deformation/failure modes of sandwich panels were obtained in the experiments, in terms of the face sheets and gradient auxetic core. The effects of impulse and gradient type of auxetic core on dynamic responses of specimens were examined. In addition, the numerical simulation was employed to clarify the deformation process of sandwich panel, and to analyze the deformation mode of auxetic cores under sufficient compression. It was shown that higher impulse was associated with increased plastic deformation in the face sheet and more serious shear and tensile tearing within the auxetic core. The auxetic cores of varying gradient types displayed distinct compress deformation behaviors. Among them, the negative gradient variant exhibited the least permanent deflection, and the superior negative Poisson's ratio effect, indicating enhanced blast resistance. This work not only contributes to a strategy for designing gradient auxetic cores with enhanced blast resistance, but also provides valuable insights into experimentally understanding the response of complex auxetic configurations under blast loads.

Stratified substrates enhance water storage and distribution between irrigation events

AbstractThe specialty crop industry requires copious amounts of water to meet production needs; however, current substrates are highly porous to mitigate risks and are subsequently inefficient with regard to water use. Therefore, more sustainable soilless substrates are needed to ensure the future success/profitability of the horticultural industry, especially as finite fresh water sources become limiting. Substrate stratification (i.e., vertically layering unique substrates atop one another) provides opportunities to augment an existing system by maintaining substrate porosity, while strategically redistributing the air‐ and water‐filled pores, retaining more water when applied, and controlling vertical water distribution. The aim of this research was to further understand the complex stratified systems of hydraulics. Herein, the hydraulic properties of individual substrate components were measured and modeled using HYDRUS 1 d. Furthermore, matric tensions and volumetric water content (VWC) were measured continuously under two different irrigation schedules (single [1×] and cyclic applications [3×]) for 15 days with a fully established Dianthus barbatus ‘Amazon Neon Purple’ crop. The results showed that screened bark particles have more pore homogeneity assessed by steep declines in VWC with small changes in water potential. Moreover, HYDRUS 1 d modeling demonstrated that stratified substrates have more uniform hydraulic gradients (difference in moisture content from the top to the bottom of the substrate profile; 21%) present in the container profile when compared to non‐stratified systems (45%). Non‐stratified upper layers experienced greater diurnal tension fluctuations under single irrigation than in cyclic applications. Stratifying substrates resulted in a significant reduction in these fluctuations in the top layer when compared to non‐stratified systems. Additionally, when both substrate (stratifying) and irrigation (cyclic) management strategies are implemented, tension fluctuations can be even further reduced, and hydraulic gradients become more uniform due to improved moisture infiltration and distribution.

Sulfur Signature of a Mediterranean Wetland: The Case of Soils of the Inundated Floodplain Area at the Tablas de Daimiel National Park

In the Mediterranean region, numerous wetlands co-exist with competitive agricultural systems. Although wetlands’ hydrology (in water volume, source, and residence time terms) plays a major factor in their ecosystem structure, many of these wetlands have been subjected to different degradation processes. One such case is the Tablas de Daimiel National Park (TDNP), which witnesses one of its most environmentally sensitive times. In this context, scarce information is available on the sulfur (S) contents in this wetland. To diagnose this wetland’s degradation status, 43 soil samples were selected and analyzed to investigate S contents in the surface soils (0 to 30 cm) of the inundated floodplain area of the TDNP. They represent different degrees of exposure to pollutants and exposure to flooding. Sulfur determination was carried out by X-ray fluorescence. The S contents in the area range between 1.85 and 37.15 g kg−1, with a mean value of 13.71 g kg−1, which implies marked variability throughout this wetland. The highest values were observed near the wetland entrance via the Gigüela River, while the lowest ones appeared at its exit, with a relatively uniform gradient from the entrance to the exit. The relatively high S concentrations observed at sites other than the wetland entrance can be probably related to the continuous non-point source contamination with P, N, and S fertilizer applications, via runoff agricultural production systems. The results of this study are essential for understanding and predicting total S variability within the TDNP (Spain), which can interfere with soil management practices.

Overflow waters in the western Irminger Sea modify deep sound speed structure and convergence zone propagation

Deep sound speed structure in the western Irminger Sea is found to be highly dynamic in comparison to the adiabatic (uniform) sound speed gradient underpinning data assimilation and modeling efforts around the globe. A beamed source parabolic equation model is used to illustrate how the resulting non-uniform sound speed structure at 1 to 1.5 km in depth and sound speed inversion near the seafloor produce observable effects on acoustic signals between a shallow source and shallow vertical line array at convergence zone ranges. Beamforming analysis shows that a uniform sound speed gradient leads to “ideal” interference patterns that do not capture or represent modeled convergence zone properties, such as location, strength, and sharpness. Overall findings suggest that in situ information about sound speed below 1 km is necessary for low frequency, long-range propagation studies, particularly in areas of complex thermohaline circulation.

Directional guiding of mass and charge transfer via vertically aligned graphite nanosheets for high-rate and ultralong lifespan Dual-ion batteries

Cathode materials based on graphite for dual-ion batteries (DIBs) demonstrate multifarious merits, such as abundant resources and high anion interaction potential etc. However, the traditional irregular spherical graphite cathodes suffer from uneven anion migration through the highly tortuous pathways and the inductive anisotropic electric fields, leading to mass transfer non-uniformity and sluggish reaction kinetics. Simultaneously, large-sized anions and solvent co-intercalation restrict the anion intercalation behavior and cause severe intercalation volume expansion. To address these challenges, the vertical graphite nanosheets (VGNs) were designed to guide the anion transport directionally for the first time. The perpendicular open structures of VGNs regulate the uniform ion gradient and electric field for rapid anion diffusion by the directional and shortened ion channels with low electrode tortuosity. Meanwhile, the ordered tunnels buffer the intercalation volume expansion, enhancing the structural stability. Consequently, the DIBs with the VGNs as cathodes deliver an impressive discharge specific capacity of ∼ 220 mAh/g at 300 mA g−1, superior rate capability of up to 2000 mA g−1, and excellent cycling stability without obvious degradation after 2000 cycles, which surpasses those of reported DIBs based on anion intercalation cathodes significantly. This investigation on vertically aligned graphite nanosheets provides insights for the development of highly efficient nanomaterials for energy storage systems.

Numerical analysis of demolition waste‐based thermal energy storage system for concentrated solar power plants

AbstractThermal energy storage (TES) is an enabling system that provides uninterrupted energy from concentrated solar power (CSP) plants. Packed‐bed TES systems have great opportunity to significantly enhance the cost‐effectiveness, efficiency, and sustainability of CSP plants by employing an affordable and sustainable packing material. The objective of this study is to design a demolition waste‐based packed bed TES system with a maximum storage capacity of 40 kWh, specifically tailored to store heat within the temperature range of 290°C to 565°C using solar salt as heat transfer fluid (HTF), thereby making it suitable for integration into CSP plants. Performance and thermal behavior of demolition waste‐based packed‐bed TES system was assessed through numerical analysis. The results demonstrate that a high discharging efficiency of 76.0% was achieved when the HTF flow rate was set at 100 kgh−1. However, it is important to note that at lower HTF flow rate, heat loss increases, leading to a decrease in discharging efficiency to 70%. The experiment also revealed a uniform thermal gradient within the packed‐bed TES system, up to a fluid flow rate of 300 kgh−1. It is worth mentioning that lower flow rates can further improve the stratification effect; however, they may also result in increased heat loss and reduced storage capacity. Based on these findings, an optimal flow rate range of 100 to 200 kgh−1 is recommended to achieve the best performance for the packed‐bed TES system.

Corrosion fatigue behavior of continuous gradient porous Ti‐15Mo alloy prepared by selective laser melting

AbstractIn this work, porous Ti‐15Mo scaffolds with different porosity gradient structures (uniform gradient structures [U‐BCC], level gradient structures [LG‐BCC], and vertical gradient structures [VG‐BCC]) were fabricated by selective laser melting, and the corrosion fatigue testing in Hank's solution was conducted for the first time. The fatigue strength order, when considering the same fatigue life, follows the sequence of LG‐BCC structure > U‐BCC structure > VG‐BCC structure. Cyclic ratcheting is a prominent factor leading to fatigue failure. The sequence of crack initiation and propagation in the latter stage of corrosion fatigue can be listed as follows: LG‐BCC structure < U‐BCC structure < VG‐BCC structure. During the corrosion fatigue process, the grain refinement occurs, and the deformation twinning is generated, leading to a dissipation of energy and an increase of the fatigue resistance. The corrosive effect of Hank's solution induces the surface roughening of the sample, accelerating crack initiation and propagation.

Computational Model of Effective Thermal Conductivity of Green Insulating Fibrous Media.

Modelling effective thermal properties is crucial for optimizing the thermal performance of materials such as new green insulating fibrous media. In this study, a numerical model is proposed to calculate the effective thermal conductivity of these materials. The fibers are considered to be non-overlapping and randomly oriented in space. The numerical model is based on the finite element method. Particular attention is paid to the accuracy of the results and the influence of the choice of the representative elementary volume (REV) for calculation (cubic or rectangular parallelepiped slice). The calculated effective thermal conductivity of fibrous media under different boundary conditions is also investigated. A set of usual mixed boundary conditions is considered, alongside the uniform temperature gradient conditions. The REV rectangular slice and uniform temperature gradient boundary conditions provide a more accurate estimate of the effective thermal conductivity and are therefore recommended for use in place of the usual cubic representative elementary volume and the usual mixed boundary conditions. This robust model represents a principal novelty of the work. The results are compared with experimental and analytical data previously obtained in the literature for juncus maritimus fibrous media, for different fiber volume fractions, with small relative deviations of 7%. Analytical laws are generally based on simplified assumptions such as infinitely long fibers, and may neglect heat transfer between different phases. Both short and long fiber cases are considered in numerical calculations.

Spontaneous oscillation of an active filament under viscosity gradients.

We investigate the effects of uniform viscosity gradients on the spontaneous oscillations of an elastic, active filament in viscous fluids. Combining numerical simulations and linear stability analysis, we demonstrate that a viscosity gradient increasing from the filament's base to tip destabilises the system, facilitating its self-oscillation. This effect is elucidated through a reduced-order model, highlighting the delicate balance between destabilising active forces and stabilising viscous forces. Additionally, we reveal that while a perpendicular viscosity gradient to the filament's orientation minimally affects instability, it induces asymmetric ciliary beating, thus generating a net flow along the gradient. Our findings offer new insights into the complex behaviours of biological and artificial filaments in complex fluid environments, contributing to the broader understanding of filament dynamics in heterogeneous viscous media.