Range Of Aspect Ratios Research Articles

Drag and heat transfer coefficients for axisymmetric nonspherical particles: A LBM study

In this work, the thermal lattice Boltzmann method with immersed moving boundary conditions has been employed to calculate the drag coefficients and Nusselt numbers for isolated axisymmetric nonspherical particles in uniform flow for a wide range of Reynolds numbers (Re) and aspect ratios (Ar). The simulation results demonstrate that the drag coefficient for ellipsoids and a spherocylinder evolves as sin2θ, viz. θ being the incident angle of the particle. However, the Nusselt number for prolate ellipsoids evolves as sinmθ, viz. m being a function of particle aspect ratio. A correlation for m has been developed for 1<Ar≤4 based on the simulation data. The Nusselt number for a spherocylinder of aspect ratio 4 evolves as sin1.278θ. Moreover, we observe that the drag coefficients and Nusselt numbers for a prolate ellipsoid of aspect ratio 4 and a spherocylinder of aspect ratio 4 are comparable with a maximal relative deviation 5%. Based on the simulation data and the data from literature, a drag correlation for oblate (0.2≤Ar<1 and 1≤Re≤100) and prolate (1≤Ar≤5 and 1≤Re≤2000) ellipsoids has been developed to broaden the range of applicability of existing drag correlations. The average relative deviation between the drag correlation and the fitting data is 9.4%. The Nusselt number correlation for prolate ellipsoids is valid for 1≤Ar≤4 and Re≤500. On the other hand, the Nusselt number correlation proposed for a spherocylinder is valid for Ar=4 and Re≤500. The new drag and Nusselt number correlations are expected to improve the accuracy of Euler-Lagrangian simulations of non-isothermal particulate flows comprising ellipsoids or spherocylinders.

Structure-controllable synthesis of ZnO nanowires using water vapor in an atmospheric-pressure microwave plasma system

Although the use of atmospheric-pressure microwave plasma to synthesize ZnO nanowires addresses various issues associated with conventional synthesis methods, it is difficult to obtain nanowires with controlled diameters and lengths. Herein, we investigated the effects of microwave input power and water vapor on the structure of ZnO nanowires grown using an atmospheric-pressure microwave plasma system. It was found that the aspect ratio of ZnO nanowires could be controlled by adjusting the plasma temperature and the amount of OH radicals. This method successfully produced ZnO nanowires in a structure-controllable manner with a wide range of aspect ratios from 10 to 340. Interestingly, we found that the sensitivity of the ZnO nanowires to UV irradiation was closely related to the morphology. In particular, the ZnO nanowires with high aspect ratios and densities showed excellent sensing characteristics.

Axis switching in low to moderate aspect ratio rectangular orifice synthetic jets

The behavior and performance of synthetic jets is closely tied to the dynamics of vortex rings, which are a dominant feature of the early jet. When a synthetic jet actuator employs a non-axisymmetric orifice, the self-induced deformations of the vortex rings are responsible for changes in the shape, momentum distribution, and entrainment rate of the jet. In this study, axis-switching of vortex rings in finite-span synthetic jets with a range of orifice aspect ratios was experimentally investigated using stereoscopic particle imaging velocimetry, and the results were compared to vortex dynamics simulations based on the Biot-Savart law to gain additional insight.

Spherical equivalence of cylindrical explosives: Effect of charge shape on deflection of blast-loaded plates

Quantification of near-field blast loading is a pressing issue for defence, transport security, and structural engineering. Realistic explosion scenarios often involve the detonation of non-spherical high explosive charges, rather than the idealised spherical/hemispherical explosives assumed in commonly employed semi-empirical approaches. Additionally, near-field effects are of great importance when assessing structural damage and injury risk from such an event. There is a need, therefore, to incorporate the effects of charge shape and the resulting loading distribution in simplified engineering-level tools using adjustments based on sound physical principles. This article details the development of an energy equivalent formulation to derive spherical equivalence factors, with the methodology illustrated for the scenario of a centrally detonated cylindrical explosive charge. A validated two-part numerical model is used to generate specific impulse distributions and quantify the resulting plate deformation, for a wide range of cylindrical aspect ratios (0.20≤L/D≤5), at a range of near-field scaled distances (0.108≤Z≤0.485 m/kg1/3) for different sized structural targets. A series of verification examples are used to demonstrate the accuracy of the method, with the peak deflection under the equivalent spherical charge matching peak deflection under the cylindrical charge to within ∼4%. The method developed in this article could be extended to find equivalence between any two systems with complex distributed loading, allowing for a fast running engineering approximation in cases where detailed modelling is inappropriate or infeasible.

Parametric study of natural convection heat transfer from an inclined rectangular cylinder embedded in a square enclosure

ABSTRACT This paper presents the effects of aspect ratio , Rayleigh number () and inclination angle of the heated rectangular cylinder on heat transfer and fluid flow characteristics due to natural convection in air around heated rectangular cylinders of different sizes inside a cold square enclosure. Galerkin finite element method was adopted for the solution of the developed model. The walls of the enclosure were maintained at an isothermal cold temperature while the boundaries of the rectangular cylinder were kept at a constant hot temperature. The results are presented in the form of isothermal contours, stream functions, local and average Nusselt numbers. It has been found that for the range of aspect ratio and Rayleigh number considered, the rate of heat transfer increases with increasing aspect ratio and Rayleigh number. Furthermore, the orientation angle of the rectangular cylinder was found to generally augment heat transfer rate except at value of , where the average Nusselt number declines beyond for all the range of Rayleigh numbers considered. Results from the present study could be applied to nuclear reactor technology and cooling of electronic chips system optimisation.

Analytical solution for residual stress and strain preserved in anisotropic inclusion entrapped in an isotropic host

Abstract. Raman elastic thermobarometry has recently been applied in many petrological studies to recover the pressure and temperature (P–T) conditions of mineral inclusion entrapment. Existing modelling methods in petrology either adopt an assumption of a spherical, isotropic inclusion embedded in an isotropic, infinite host or use numerical techniques such as the finite-element method to simulate the residual stress and strain state preserved in the non-spherical anisotropic inclusions. Here, we use the Eshelby solution to develop an analytical framework for calculating the residual stress and strain state of an elastically anisotropic, ellipsoidal inclusion in an infinite, isotropic host. The analytical solution is applicable to any class of inclusion symmetry and an arbitrary inclusion aspect ratio. Explicit expressions are derived for some symmetry classes, including tetragonal, hexagonal, and trigonal. The effect of changing the aspect ratio on residual stress is investigated, including quartz, zircon, rutile, apatite, and diamond inclusions in garnet host. Quartz is demonstrated to be the least affected, while rutile is the most affected. For prolate quartz inclusion (c axis longer than a axis), the effect of varying the aspect ratio on Raman shift is demonstrated to be insignificant. When c/a=5, only ca. 0.3 cm−1 wavenumber variation is induced as compared to the spherical inclusion shape. For oblate quartz inclusions, the effect is more significant, when c/a=0.5, ca. 0.8 cm−1 wavenumber variation for the 464 cm−1 band is induced compared to the reference spherical inclusion case. We also show that it is possible to fit an effective ellipsoid to obtain a proxy for the averaged residual stress or strain within a faceted inclusion. The difference between the volumetrically averaged stress of a faceted inclusion and the analytically calculated stress from the best-fitted effective ellipsoid is calculated to obtain the root-mean-square deviation (RMSD) for quartz, zircon, rutile, apatite, and diamond inclusions in garnet host. Based on the results of 500 randomly generated (a wide range of aspect ratio and random crystallographic orientation) faceted inclusions, we show that the volumetrically averaged stress serves as an excellent stress measure and the associated RMSD is less than 2 %, except for diamond, which has a systematically higher RMSD (ca. 8 %). This expands the applicability of the analytical solution for any arbitrary inclusion shape in practical Raman measurements.

Variations in the canopy shock structures of massive extraterrestrial plumes: Parametric DSMC simulation of 2007 Tvashtar observations

The New Horizons flyby of the Jovian system yielded multiple images of the erupting Pele-class volcanic plume, Tvashtar. These images include a unique five-frame sequence or “movie” taken over 8 min that revealed short-term dynamical activity in the form of a traveling wave and evolving plume structure. The high-latitude Tvashtar plume was observed from various aspects during the flyby to exhibit a canopy shock and a depositional ring with thermal vent emission. The movie frames and other high-resolution images of Io taken during the New Horizons flyby by the Long Range Reconnaissance Imager (LORRI) were deconvolved to aid analysis of the plume dynamics.While our previous work (Hoey et al., 2016) modeled this plume's gas field in its steady state with rarefied gas dynamic methods, Tvashtar's virtual vent conditions were underconstrained by observation. Therefore, to constrain thermodynamic and geometric parameters at the virtual vent, we extend our models of the Tvashtar plume to perform a parametric series of 3-D simulations at much improved resolution. These simulations vary virtual vent aspect ratio and stagnation temperature, and implement new models for gas opacity in SO2 flows and for the condensation and sublimation of particles entrained in the plume.We arrive at a set of virtual vent conditions that incorporate or satisfy all available observational constraints and several novel ones, including the gas canopy height and aspect ratio and the depositional ring extent and orientation. The 2007 Tvashtar gas plume is shown to be consistent with a virtual vent of aspect ratio less than or equal to 3; stagnation temperature 1287 K; area 30 km2; and mass flow rate 2.0 × 105 kg/s SO2. Alternate canopy shock and depositional ring configurations are evaluated across a range of vent aspect ratios from 1 to 15 with results that will help to constrain the sources of other observed gas plumes.Finally, we develop an alternate hypothesis for the New Horizons observations of canopy unsteadiness in the Tvashtar plume; namely, that transient perturbations in the structure of the gas canopy shock itself produce a traveling notch-like structure that may even grow unstable, inducing temporary canopy collapse. This is as opposed to a high-amplitude density pulse at the source, or to source-entrained dust in a condensate model coupled to steady gas-field solutions.

Aspect ratio of nano/microstructures determines Staphylococcus aureus adhesion on PET and titanium surfaces.

Joint infections cause premature implant failure. The avoidance of bacterial colonization of implant materials by modification of the material surface is therefore the focus of current research. In this in vitro study the complex interaction of periodic structures on PET and titanium surfaces on the adhesion of Staphylococcus aureus is analysed. Using direct laser interference patterning as well as roll-to-roll hot embossing methods, structured periodic textures of different spatial distance were produced on surfaces and S. aureus were cultured for 24h on these. The amount of adhering bacteria was quantified using fluorescence microscopy and the local adhesion behaviour was investigated using scanning electron microscopy. For PET structures, minimal bacterial adhesion was identified for an aspect ratio of about 0·02. On titanium structures, S. aureus adhesion was significantly decreased for profile heights of < 200nm. Our results show a significantly decreased bacterial adhesion for structures with an aspect ratio range of 0·02 to 0·05. We show that structuring on surfaces can decrease the amount of S. aureus on titanium and PET as common implant materials. The study highlights the immense potential of applying specific structures to implant materials to prevent implant colonization with pathogen bacteria.

Thermal performance assessment of a thermal energy storage tank: effect of aspect ratio and tilted angle

Latent heat thermal energy storage is essential for a broad range of multidisciplinary thermal applications, due to its capability of keeping a relative constant temperature during thermal energy storage/release. The aspect ratio (AR) and installation angle (tilted angle) for a latent heat energy storage tank play important roles in addressing the issue of thermal energy storage/release efficiency. In this work, the coupled effects of tank aspect ratio and tilted angle upon melting phase change were both experimentally and numerically investigated. Wide range of tank aspect ratios from 0.1 to 8.0 was studied with respect to four representative tilted angles (0°, 30°, 60°, and 90°). The detailed features of melting front propagation, temperature distribution, and free convection development in the melt phase were quantified. Results indicated that melting heat transfer was markedly affected by the tilted angle, regardless of tank aspect ratio. However, the fastest melting occurred at different tilted angles, if different tank aspect ratios were considered. With AR < 1.0, the highest melting rate was found at 0° inclination. As for AR > 2.0, the fastest melting was achieved at an inclination of 60°.

Impinging planar jets: hysteretic behaviour and origin of the self-sustained oscillations

Abstract

CPP-GMR performance of electrodeposited metallic multilayered nanowires with a wide range of aspect ratios

Effect of the diameter on the CPP-GMR performance of electrodeposited Co/Cu multilayered nanowires was systematically investigated using our home-made anodized aluminum oxide (AAO) templates with the nanochannel diameter ranging from 35 to 130 nm. The nanochannel diameter was precisely controlled by tuning the anodization voltage ranging from 20 to 110 V. The Co/Cu multilayered nanowires with the aspect ratio of ~1000 were synthesized in the AAO nanochannels by applying a rectangular pulsed potential electrodeposition technique using an acidic aqueous solution containing Co2+ and Cu2+ ions. An ideal alternating Co/Cu multilayered structure with each layer thickness of several nanometers was clearly observed in the nanowires with the diameter ranging from 35 to 95 nm. The multilayered nanowires with the diameter ranging from 35 to 70 nm were spontaneously magnetized in the axial direction due to the preferential crystal orientation of hcp-Co (002) which was induced by the magnetic shape anisotropy. With decreasing the diameter down to 35 nm, the coercivity increased up to 1.37 kOe while the squareness reached 0.73 at the diameter of 50 nm. The CPP-GMR value reached 23.4% at room temperature in the Co/Cu multilayered nanowires with the diameter of 70 nm.

A molecular dynamics study on the buckling behavior of x-graphyne based single- and multi-walled nanotubes

There are many two-dimensional (2D) structures of carbon allotropes which have been at the center of theoretical and experimental studies in recent years. α-, α2-, β- and γ-graphyne as the graphene allotropes with mixed sp1 and sp3 bonds are under study in the present work utilizing the molecular dynamics (MD) simulations. These thermodynamically stable forms of 2D carbon structure can also be rolled-up to generate novel nanotubes (NTs). Hence, the armchair (-A) and zigzag (-Z) NTs of the aforementioned one-atom-thick planar structures and their buckling behavior (critical force and strain) are considered. It is observed that γ-A- and γ-Z-graphyne NTs have the highest critical forces through the chosen range of aspect ratios (length/radius), whereas the α-graphyne NTs have the lowest critical forces. Also, α2- and β-graphyne NTs have the highest and lowest critical strains, respectively. Furthermore, the possible multi-walled (MW) NTs are simulated. In agreement with results of the single-walled (SW) NTs, the buckling analysis of MWNTs shows that γ-graphyne and α-graphyne MWNTs have the highest and the lowest mechanical stability under compression, correspondingly. In addition, the results of the critical strain for MWNTs are comparable with those of SWNTs.

Role of elastic strain energy in spheroidal precipitates revisited

Improving the treatment of thermodynamic modelling of precipitation kinetics makes it necessary to account for the stored elastic and interface energies of non-spherical strengthening particles. The existing kinetic concepts available for spherical precipitates can be directly adapted to spheroidal precipitates by introducing shape factors, thus approximating shapes differing from spheres. This paper provides shape factors for the stored elastic and interface energies over a wide range of aspect ratios as well as of elastic material properties.

Influence of size and shape on key performance metrics in spin-torque oscillators

Spin Torque Oscillators (STOs) are promising solutions in a wide variety of next generation technologies from read-head sensors in high-density magnetic recording technology to neural oscillator units for neuromorphic computing. There are several metrics that can be used to quantify the performance of an STO such as power, quality factor, frequency tunability, etc., most of which are dependent on the design of the STO device itself. Furthermore, determining the most important metric will be contingent on its desired application, meaning that it is crucial to understand how the STOs design parameters influence all aspects of its performance so that its design can be optimized to perform the desired function. In this work, we analyzed spin torque oscillations generated from 20 magnetic tunnel junctions with in-plane anisotropy and patterned into elliptical nano-pillars with a wide range of sizes and aspect ratios. For each device, we acquired 20 to 50 data sets at various bias fields and currents and used power spectral density plots to measure output power, frequency, linewidth, quality factor, and power-to-linewidth ratio for each set. We also analyzed each STOs performance in terms of the bias fields and bias currents required to maximize output power and signal quality as well as the frequency tunability with both field and current. By comparing all of these performance metrics between the 20 STOs tested, we studied the influence of device size and shape on all aspects of STO performance and used correlation coefficients to quantify relative magnitude of these effects.

Capillary-flow dynamics in open rectangular microchannels

Abstract

Thermal conductivity and elastic modulus of 3D porous/fractured media considering percolation

Pores, vugs, and cracks configurations affect physical and mechanical properties of porous and fractured (PF) media, specifically the percolations of pores/vugs/cracks trigger the dramatic change of conductive-like properties such as thermal conductivity, diffusivity, elastic modulus of PF media. It has been a key but unresolved issue how to accurately capture the percolation thresholds of these complex networks interacted by anisotropic-shaped pores and cracks, and their quantitative effects on the thermal conductivity and elastic modulus of PF media. This work proposes numerical and theoretical strategies for the accurate determination of percolation thresholds of penetrable spheroidal inclusions over a broad range of aspect ratios κ representing prolate pores (1 < κ ≤ 200), vugs (κ = 1) and oblate cracks (0.001 ≤ κ < 1), and the reliable predictions of effective thermal conductivity, diffusivity, and elastic modulus of PF media. The numerical and theoretical strategies include: (1) an extensive Monte Carlo finite-size scaling analysis (MCFSS) for obtaining the statistical values of percolation threshold of porous and fracture networks with κ from 200 to 0.001; (2) a Padé-type percolation model for predicting the percolation thresholds of penetrable spheroidal inclusions over a broad range of aspect ratios; (3) a continuum percolation-based generalized effective medium theory (CP-GEMT) for predicting effective thermal conductivity, diffusivity, and elastic modulus of PF media over the whole range of volume fractions of spheroidal inclusions, including near the percolation threshold. Comparison with extensive experimental, numerical and theoretical results confirms that the present models are capable of accurately determining the percolation thresholds of porous and fracture networks and the effective thermal conductivity, diffusivity, and elastic modulus of PF media as resemble to conductor-superconductor and insulator-conductor. The models can be regarded as a general theoretical framework to shed light on the intrinsic and complex interaction of components, structures and transport and elastic properties in porous and fracture materials, meanwhile the proposed models can be also well utilized to predict the percolation thresholds of spheroidal carbon nanotubes (CNTs) and graphenes, and the effective transport and elastic properties of CNT-/graphene-based composites.

Structural Properties of Fe Doped TiO2 Nanorods Prepared by Low Cost Hydrothermal Method

In this work, titanium dioxide films were deposited on fluorine tin oxide (FTO)-glass substrates using Hydrothermal method. A low-cost homemade autoclave was used to fabricate pure TiO2 and Fe-doped (0.1%, 0.3%, 0.5%, 0.7% and 1.5%) films. X-ray diffraction patterns showed that the predominant phase is rutile (R-TiO2) with peaks at (101), (002) and (112). The Field Emission Scanning Electron Microscope (FESEM) top and cross-sectional images indicated that the films have vertically aligned nanorods structures with parallelogram cross-sectional areas and aspect ratio range (9.2-15.3).

Bed and wall shear stresses in smooth rectangular channels

Various empirical equations for mean bed and wall shear stresses have been proposed for smooth rectangular open channels. Considering that the distribution of bed shear and wall shear stresses is affected by the cross-sectional shape, the gravity force based on the flow depth used to non-dimensionalize shear stresses in previous studies is modified to a parameter that includes the width and wetted-perimeter. This yields a new set of dimensionless equations for mean bed and wall shear stresses, and the wall/bed shear force ratio. These equations compared well with all available experimental data over a wide range of aspect ratios from 0.31 to 50. These equations also yield new relationships for bed and wall friction factors.

Prediction of the settling velocity of the rod-shaped proppant in vertical fracture using artificial neural network

As a new technology proppant, rod-shaped proppant is receiving more and more attention because it can greatly improve the performance of hydraulic fracturing. With special shape, the rod-shaped proppant is subject to greater flow resistance, resulting in complex migration in fractures. Accurate prediction of the settling velocity of rod proppant in fracture is important for predicting the transport distance and bed height of rod proppant in hydraulic fractures to improve the fracture performance. In this study, high-speed photography and transparent parallel plates were used to simulate and record the settling process of proppant in vertical fractures. A total of 588 effective tests were conducted to reveal the influence of proppant shape, fracturing fluid properties, and the wall effect of fracture on the settling characteristics involving the range of proppant aspect ratio and Reynolds number are 1.5–5.0 and 0.001–172.54, respectively. It was found that the rod proppant settled vertically at the low Reynolds number and horizontally at the high Reynolds number. First, an artificial neural network model of settling orientation with the prediction accuracy of 100% is established as the basis for predicting settlement velocity. The topological structure of the neural network is further extended to establish the prediction model of settling velocity with mean relative error of 1.21%. The relative importance of each input factor to the settling velocity was analyzed by the connection weights algorithm, and the weight and bias of the neural network are provided for the application and generalization of the model. Finally, the neural network model is evaluated objectively by comparing with the mathematical model. This study is a successful exploration of artificial intelligence in particle migration and will provide guidance for improving the performance of rod proppant in hydraulic fracturing.

Preferential growth of specific crystal planes based on the dimension control of single crystal SnO2 nanobelts

In this paper, we have proposed a new crystal plane control technique of single crystal SnO2 nanobelts by thermal sublimation. The synthesis pressure was found to drastically influence the dimension of the cross-sectional aspect ratio (width–thickness) of the nanobelts. The pressure changed the surface free energy that contributes to the preferential growth of crystal planes of the nanobelt. The aspect ratio range of 1.2 to 7.39 for the nanobelts was achieved solely owing to the pressure variation. Also, as a catalyst style on the synthesis by thermal sublimation, the dispersed Au nanoparticles assisted the formation of uniform SnO2 nanobelts.