Formation Of Rolls Research Articles

Convection Initiation Associated With Ambient Winds and Local Circulations Over a Tropical Island in South China

AbstractConvection is highly active on tropical islands. Based on a 3‐year radar climatology over Hainan Island, a mountainous tropical island in South China, we identified a distinct spatial dependence of convection initiation (CI) on the speed of the large‐scale low‐level flow. Convection initiates most prominently over the lee‐side plains in high‐wind conditions while over mountains in low‐wind conditions. Quasi‐idealized numerical simulations suggest that mountain ridges stimulate the formation and growth of horizontal convective rolls (HCRs) over the lee‐side plains in high‐wind conditions. These HCRs subsequently intersect with sea‐breeze fronts, which enhances the low‐level convergence favoring CI on the lee‐side plains of the island. During low‐wind conditions, thermally driven circulations control and favor CI over the mountains during the day. The findings highlight that the response of local circulations to the lower‐troposphere ambient flow and diurnal thermal heating is an important aspect of convection initiation on the island.

The Born–Oppenheimer approach to quantum cosmology

The scope of this paper is to compare two different approaches for solving the Wheeler–DeWitt (WDW) equation in the presence of homogeneous matter (inflaton) and perturbations around it. The standard Born–Oppenheimer (BO) decomposition, which consists of factorising out the gravitational wave function and then defining the flow of the time through it, and a more general BO decomposition where the whole minisuperspace wave function (i.e. scale factor and homogeneous scalar field) is factorized out. The two approaches are compared, for simplicity, in the case of a minimally coupled inflaton with a flat potential. Besides being exactly solvable, the model we study is also relevant as a toy model for ultra slow roll and primordial black holes formation. The consistency of the latter decomposition is checked against the former by comparing the resulting perturbation (Mukhanov–Sasaki) equations. Finally a few solutions to the homogeneous WDW not suitable for the traditional BO treatment are presented and the corresponding Mukhanov–Sasaki equations are evaluated.

Effective Cannula Vacuuming Maneuver to Unfold the Descemet Membrane Graft.

To present the cannula vacuuming maneuver (CVM) in the Descemet membrane endothelial keratoplasty (DMEK) and to investigate the efficacy of CVM in reducing the unfolding time (UT) of the graft in cases with an unstable anterior chamber (AC). Forty-four eyes of 44 patients, who underwent DMEK at the ophthalmology clinic of a training and research hospital, were included in this retrospective study. Surgical videos and patient charts were examined retrospectively. In 21 cases, only no-touch technique maneuvers were used (group 1), and in 23 cases, CVM was used in addition to the no-touch technique (group 2). Demographic data, the status of AC, graft diameter, width, and formation of graft rolls and UT were recorded and compared between the groups. The causes of increased UT and the reason for using CVM were investigated in group 2. Although an unstable AC was observed more frequently in group 2, the difference between the groups was not statistically significant (P=0.054). Unfolding time of rolls in group 2 was significantly shorter than group 1 (P=0.040). Cannula vacuuming maneuver shortens the UT of the graft roll in the AC and facilitates the graft unfolding maneuvers in complicated cases.

Characteristics of temperature fluctuation in two-dimensional turbulent Rayleigh-Bénard convection

We study the characteristics of temperature fluctuation in two-dimensional turbulent Rayleigh–Bénard convection in a square cavity by direct numerical simulations. The Rayleigh number range is 1 × 108 ≤ Ra ≤ 1 × 1013, and the Prandtl number is selected as Pr = 0.7 and Pr = 4.3. It is found that the temperature fluctuation profiles with respect to Ra exhibit two different distribution patterns. In the thermal boundary layer, the normalized fluctuation θ rms/θ rms,max is independent of Ra and a power law relation is identified, i.e., θ rms/θ rms,max∼ (z / δ)0.99 ± 0.01, where z / δ is a dimensionless distance to the boundary (δ is the thickness of thermal boundary layer). Out of the boundary layer, when Ra ≤ 5 × 109, the profiles of θ rms/θ rms,max descend, then ascend, and finally drop dramatically as z/δ increases. While for Ra ≥ 1 × 1010, the profiles continuously decrease and finally overlap with each other. The two different characteristics of temperature fluctuations are closely related to the formation of stable large-scale circulations and corner rolls. Besides, there is a critical value of Ra indicating the transition, beyond which the fluctuation 〈 θ rms〉 V has a power law dependence on Ra, given by 〈 θ rms〉 V ∼ Ra −0.14 ± 0.01.

Overexpression of VvPPR1, a DYW-type PPR protein in grape, affects the phenotype of Arabidopsis thaliana leaves

Pentatricopeptide repeat (PPR) proteins play important roles in plant growth and development. However, little is known about their functions in the leaf morphogenesis of Jingxiu grape (Vitis vinifera L.). Here, we explored the function of VvPPR1, which encodes a DYW-type PPR protein in grape. We showed that VvPPR1 is involved in the regulation of leaf rolling, anthocyanin accumulation, and trichome formation in Arabidopsis thaliana. Analysis of structural characteristics showed that VvPPR1 is a DYW-type PPR gene in the PLS subfamily consisting of 15 PPR motifs. The N-terminal had a targeted chloroplast site, and the C-terminal had a DYW domain. Quantitative PCR analysis revealed that the expression level of VvPPR1 was highest in grape leaves. Subcellular localization revealed that VvPPR1 is localized in the cytoplasm and chloroplast. VvPPR1-overexpressing plants had rolled leaves, high degrees of anthocyanin accumulation, and longer trichomes. The expression levels of genes related to these phenotypes were either significantly up-regulated or down-regulated. These results demonstrate that VvPPR1 is involved in leaf rolling, anthocyanin accumulation, and trichome formation in Arabidopsis; more generally, our findings indicate that VvPPR1 could be a target for improving the cultivation of horticultural crops.

A Novel Modified Shaving Die Design for Fabrication with Nearly Zero Die Roll Formations

In recent years, increasing levels of precision of the cut surface characteristics in the cutting process of a metal die have become required not only for crack prevention but also for die roll prevention. The conventional shaving die is unlikely to be applied to achieve these requirements. As an innovative design, a novel modified shaving die with two steps of tapering, press and cut tapers, was designed and proposed in this research. The finite element method (FEM) was used to clarify the cutting mechanism and identify the effects of the process parameters and geometry of the shaving die of interest on the cut surface characteristics based on material flow and stress distribution analyses. The press taper limits the die roll formation on the sheared edge during the first stage of the cutting phase, and the cut taper prevents a new die roll from forming during the second stage of the cutting phase. The results showed that by using the novel modified shaving die, die roll formation could be reduced. Furthermore, by suitably selecting the process parameters and geometry of the shaving die, nearly zero die roll (nZDR) could be achieved. Laboratory experiments were carried out to validate the FEM simulation results. The FEM simulation results showed good agreement with the experimental results in terms of the cut surface characteristics and the cutting forces.

Tribological behavior of polymeric 3D-printed surfaces with deterministic patterns inspired in snake skin morphology

Surface engineering can rely on biological systems to mimic characteristics of living specimens in order to develop and fabricate deterministic elements to modify friction. In this work, the tribological response of bio-inspired patterns obtained by 3D-printing for polymer-metal contact in dry conditions are discussed. The repetitive array of elements called ‘fibrils’ in the ventral scale of Phython regius snake was used as an inspiration to fabricate the deterministic surfaces. The texturized samples were 3D-printed with Nylon filament and the counterpart was AISI 304 stainless steel. The results showed that the friction coefficient was consistently lower in the tests carried out with the texturized samples in comparison to those performed with stochastic surfaces. Lower friction values were clearly correlated to geometrical descriptors of the texture patters including the Feature Slenderness Ratio (FSR) and the Feature Aspect Ratio (FAR). The analysis of polymer’s worn surfaces revealed adhesive damage, localized melting and roll formation as the main wear mechanisms.

Stepwise transitions in spin-up of rotating Rayleigh–Bénard convection

Abstract

Theoretical and Experimental Study of Thermal Convection in Rotating Horizontal Annulus

The paper presents the results of theoretical and experimental studies of thermal convection in a rotating horizontal cylindrical liquid layer. A layer with boundaries of different temperatures both when heating from inside and outside is considered. Steady thermal convection is determined by two mechanisms: thermovibrational and centrifugal ones. The first one appears due to oscillations of nonisothermal liquid in the cavity frame caused by gravity. Both mechanisms manifest themselves in the formation of two-dimensional convective rolls, extended along the axis of rotation. It is shown that the results of the experimental study of the layer quasiequilibrium stability are in a good agreement with the results obtained in the framework of numerical calculation and linear theory. The structure of thermal convection and heat transport in the supercritical area of parameters is studied numerically, taking into account the slow azimuthal drift of the entire liquid layer in the direction opposite to the rotation of the cavity. The results of numerical simulation and experimental studies are consistent in the area of moderate and large values of the dimensionless velocity of rotation.

Binder-Free Electrode based on Electrospun-Fiber for Li Ion Batteries via a Simple Rolling Formation

With the demand for higher energy density and smaller size lithium-ion batteries (LIBs), the development of high specific capacity active materials and the reduction of the usage of inactive materials are the main directions. Herein, a universal method is developed for binder-free electrodes for excellent stable LIBs by rolling the electrospun membrane directly onto the commercial current collector. The rolling process only makes the fiber web denser without changing the fiber structure, and the fiber web still maintains a porous structure. This strategy significantly improves the structural stability of the membrane compared to the direct carbonized electrospun membrane. Moreover, this method is suitable for a variety of polymerizable adhesive polymers, and each polymer can be composited with different polymers, inorganic salts, etc. The electrode prepared by this method can be stably cycled for more than 2000 cycles at a current density of 2500 mA g−1. This study provides a cost-effective and versatile strategy to design the LIB electrode with high energy density and stability for experimental research and practical application.

The Formation of Hot Thermal Anomalies in Cold Subduction‐Influenced Regions of Earth's Lowermost Mantle

AbstractThe Earth's lowermost mantle is characterized by two large low shear velocity provinces (LLSVPs). The regions outside the LLSVPs have been suggested to be strongly influenced by subducted slabs and, therefore, much colder than the LLSVPs. However, localized low‐velocity seismic anomalies have been detected in the subduction‐influenced regions, whose origin remains unclear. Here, three‐dimensional geodynamic calculations are performed, and they show that linear, ridge‐like hot thermal anomalies, or thermal ridges, form in the relatively cold, downwelling regions of the lowermost mantle. Like the formation of Richter rolls due to sublithosphere small‐scale convection (SSC), the thermal ridges form as a result of SSC from the basal thermal boundary layer and they extend in directions parallel to the surrounding mantle flow. The formation of thermal ridges in subduction regions of the lowermost mantle is very sensitive to the thermal structures of the subducted materials, and thermal heterogeneities brought to the bottom of the mantle by subducting slabs greatly promote the formation of thermal ridges. The formation of thermal ridges is also facilitated by the increase of core‐mantle boundary heat flux and vigor of lowermost mantle convection. The thermal ridges may explain the low‐velocity seismic anomalies outside of the LLSVPs in the lowermost mantle. The results suggest that the relatively cold, subduction‐influenced regions of the Earth's lowermost mantle may contain localized hot anomalies.

Criteria for the onset of convection in the phase-change Rayleigh–Bénard system with moving melting-boundary

Here, for the first time, we report the criterion for the onset of convection in a low Prandtl number phase-change Rayleigh–Bénard (RB) system with an upward moving melt interface in a two-dimensional square box for a wide range of Rayleigh number Ra and Stefan number Ste (defined as the ratio between the sensible heat to the latent heat). High fidelity simulations were performed to study the phenomenon of the onset of convection. Unlike the classical RB system in the phase-change RB system, it was found that the onset of convection depended on Ste and Fourier number τ, in addition to Ra. The phase-change RB system with upward moving melt interface can be classified into two groups: slow expanding phase-change RB system (Ra ≤ 104) and moderate/fast melting phase-change RB system (Ra > 104). The slow melting phase-change RB system becomes unstable when the effective Rayleigh number based on the melt height is ≈1295.78, consistent with the finding by Vasil and Proctor [“Dynamic bifurcations and pattern formation in melting-boundary convection,” J. Fluid Mech. 686, 77 (2011)]; however, moderate and fast melting phase-change RB systems become unstable when the product of the local Rayleigh number Ra based on the melt-layer height hyt and the Fourier number based on the melt-layer height reaches a threshold value. Interestingly, it is seen that the criteria for the onset of convection for moderate and fast melting phase-change RB systems show a power law kind of form such that Racrτcr = aSteb + c. In addition, during the initial conduction regime before the onset of convection, it is seen that the Nusselt number at the hot wall is Nuh ∼ τ0.5, and during the onset of convection, i.e., during the formation of the initial convection rolls, the Nusselt number at the hot wall is Nuh ∼ τd, where the value of the exponent d is 2 for low Rayleigh numbers and 4 for higher Rayleigh numbers. This study reports some general characteristics of the onset of convection and some organized behavior in the transient melting phase-change RB system, which are not yet explored and reported in the open literature. This work may lead to significant understanding of different applications of fluid-dynamical melting phase-change RB systems in both natural and engineering systems.

Numerical study of the evaporative convection regimes in a three-dimensional channel for different types of liquid-phase coolant

Two-layer fluid flows with evaporation at the thermocapillary interface have been studied on the basis of the partially invariant solution of the convection equations. The modeling of the 3D fluid flows has been performed in an infinite channel of the rectangular cross section without assuming the axis-symmetrical character of the flows. The fluid flow patterns are determined by various thermal, mechanical and structural effects. Numerical investigations have been carried out for the liquid–gas system like ethanol–nitrogen and HFE-7100–nitrogen under terrestrial conditions. The solution takes into account the direct and inverse thermodiffusion effects in the gas–vapor phase. Influence of the applied thermal load, thickness of the liquid layer and thermophysical properties of the working media on topology and character of the flows is studied. It allows one to describe the formation of thermocapillary rolls and symmetrical multi-vortex structures in the systems with heat transfer liquids like the ethanol or HFE-7100. The appearance of the thermal and solutal shafts, and the near-wall condensation effects observed in real experiments are predicted by the solution.

Linear instability analysis of the onset of thermal convection in an Ekman-Couette-flow

The onset of thermal convection in a plane layer with rotation and shear is investigated. The boundaries of the layer are parallel to the x−y plane of the Cartesian coordinate system. The layer rotates at a constant angular velocity Ωz around the z axis, and is sheared by moving, along the x direction, the lower and upper boundaries parallel to themselves with constant velocity −Uw and Uw respectively. The temperature of the lower boundary is higher than the temperature of the upper boundary. The linear instability equations are solved by using the Tau-Chebyshev spectral numerical method. The critical parameters of the stationary, transverse and longitudinal convective rolls are presented. The relationship between the critical Rayleigh number and: (i) the Taylor number, (ii) the wavenumber vector, (iii) the Reynolds number, (iv) the kinetic energy norm and (v) the heat transfer at the sliding plates, is presented. We find the combination of the critical parameters that lead to the occurrence of two interesting situations, these are (a) a high value of the kinetic energy norm which promotes the formation of inertial longitudinal rolls, which are considered as hydrodynamic instabilities modified by the buoyancy force, and (b) a small value of the kinetic energy norm together with a high value of the heat transfer at the sliding plates. The latter situation, may be considered in the single-crystal growth from the melt processes.

Mechanical rolling formation of interpenetrated lithium metal/lithium tin alloy foil for ultrahigh-rate battery anode

To achieve good rate capability of lithium metal anodes for high-energy-density batteries, one fundamental challenge is the slow lithium diffusion at the interface. Here we report an interpenetrated, three-dimensional lithium metal/lithium tin alloy nanocomposite foil realized by a simple calendering and folding process of lithium and tin foils, and spontaneous alloying reactions. The strong affinity between the metallic lithium and lithium tin alloy as mixed electronic and ionic conducting networks, and their abundant interfaces enable ultrafast charger diffusion across the entire electrode. We demonstrate that a lithium/lithium tin alloy foil electrode sustains stable lithium stripping/plating under 30 mA cm−2 and 5 mAh cm−2 with a very low overpotential of 20 mV for 200 cycles in a commercial carbonate electrolyte. Cycled under 6 C (6.6 mA cm−2), a 1.0 mAh cm−2 LiNi0.6Co0.2Mn0.2O2 electrode maintains a substantial 74% of its capacity by pairing with such anode.

Formation of liquid marbles & aggregates: rolling and electrostatic formation using conductive hexagonal plates

A conductive coating is added to PET plates using a novel synthetic method and liquid marble manufactured using electrostatics forces.

The study of forming a roll of pressed hay and the influence of its density on quality parameters

Hay is one of the most valuable types of roughage in the diet of ruminants. During the winter period, animals get about half of the nutrients and digestible protein from hay. Leguminous-cereal hay is rich in protein, minerals, carbohydrates, vitamins. When harvesting hay in rolls, balers with a constant-volume pressure chamber are most often used. The roll forming process consists in continuously winding the flow of plant mass onto the core to a predetermined diameter. The process of roll formation occurs due to the gradual compaction of the roll from the outside, as a result of which rolls with a low density core are formed. Studies have shown that hay compaction from the outside leads to the fact that pressure transfer from the outer layers to the inner layers occurs with some delay (stress relaxation) and the location of stems in this case will have some significant effect on the density of hay inside the roll. Experimental studies have shown that the density of hay inside the roll also varies as for the width of the roll. In the middle part of the roll (in width), the density of hay is 2 to 3 times higher than along its edges, the zone of increased density is 0.7 m, which is 44 % of the width of the roll.

On the stages of vortex decay in an impulsively stopped, rotating cylinder

The flow within an infinitely long cylinder exhibiting solid-body rotation (SBR) is impulsively stopped. The complete decay of the initial SBR is captured by means of direct numerical simulations for a wide range of Reynolds numbers ( $Re$ ). Five distinct stages are identified during the decay process according to their flow structure and their underlying mechanisms of kinetic-energy dissipation. Initially, the laminar boundary layer undergoes a primary centrifugal instability, which causes the formation of coherent Taylor rolls. The flow then becomes turbulent, once the Taylor rolls are corrupted by secondary instabilities. Within the turbulent stage, two phases are distinguished. In the first turbulent phase, the SBR core is still intact and turbulence is sustained. The mean velocity profile is well described by the superposition of a near-wall region, a retracting SBR core and an intermediate region of constant angular momentum. In the latter region, the magnitude of angular momentum in viscous units $l^{+}(Re)$ is approximately constant in time. In the second turbulent phase, the SBR core breaks down, turbulence starts to decay exponentially and the kinetic energy of the mean flow decays logarithmically. Eventually, the flow relaminarises and the velocity profile of the analytical solution for purely laminar decay is recovered, albeit at an earlier temporal instant due to the net effect of transition and turbulence.

A review of liquid flow and heat transfer in microchannels with emphasis to electronic cooling

Since the realization of microchannel devices more than three and half decades ago with water as the cooling fluid providing heat transfer enhancement, significant progress has been made to improve the cooling performance. Thermal management for electronic devices with their ever-widening user profile remains the major driving force for performance improvement in terms of miniaturisation, long-term reliability, and ease of maintenance. The ever-increasing requirement of meeting higher heat flux density in more compact and powerful electronic systems calls for further innovative solutions. Some recent studies indicate the promise offered by processes with phase change and the use of active devices. But their adoption for electronic cooling still weighs unfavourably against long-term fluid stability and simplicity of device profile with moderate to high heat transfer capability. Applications and reviews of these promising research trends have been briefly visited in this work. The main focus of this review is the flow and heat transfer regime related to electronic cooling in evolving channel forms, whose fabrication are being enabled by the significant advancement in micro-technologies. Use of disruptive wall structures like ribs, cavities, dimples, protrusions, secondary channels and other interrupts along with smooth-walled channels with curved flow passages remain the two chief geometrical innovations envisaged for these applications. These innovations target higher thermal enhancement factor since this implies more heat transfer capability for the same pumping power in comparison with the corresponding straight-axis, smooth-wall channel configuration. The sophistication necessary to deal with the experimental uncertainties associated with the micron-level characteristic length scale of any microchannel device delayed the availability of results that exhibited acceptable matching with numerical investigations. It is indeed encouraging that the experimental results pertaining to simple smooth channels to grooved, ribbed and curved microchannels without unreasonable increase in pumping power have shown good agreement with conventional numerical analyses based on laminar-flow conjugate heat-transfer model with no-slip boundary condition. The flow mechanism with the different disruptive structures like dimple, cavity and rib, fin and interruption, vortex generator, converging-diverging side walls or curved axis are reviewed to augment the heat transfer. While the disruptions cause heat transfer enhancement by interrupting the boundary layer growth and promoting mixing by the shed vortices or secondary channel flow, the flow curvature brings in enhancement by the formation of secondary rolls culminating into chaotic advection at higher Reynolds number. Besides these revelations, the numerical studies helped in identifying the parameter ranges, promoting a particular enhancement mechanism. Also, the use of modern tools like Poincare section and the analysis of flow bifurcation leading to chaotic advection is discussed. Among the different disruptive structures, sidewall cavity with rib on the bottom wall within the cavity plays a significant role in augmenting the thermal performance. Among the different converging-diverging side walls or curved axis, the sinusoidal channel provides the highest mixing by the introduction of secondary vortices or dean vortices to augment the heat transfer with less pressure drop. The optimum geometry in terms of high heat transfer with low pressure plays a major role in the design of heat sink. Directions of some future research are provided at the end.

Airflow distribution in an apple storage room

In refrigerated stores of fruit and vegetables, cooling air is circulated by air coolers attached to the ceiling. Uniform airflow and even temperature distribution in the product stacks is important for quality maintenance. However, the air speed close to the produce inside the bins in industrial cold stores is unknown. Airflow distribution was measured with newly developed sensors at different ventilation levels inside bins and in vertical gaps between the bins in a common apple storage room. The air speed between the fruit was low (≤0.3 m/s) compared to the average air velocity in the neighboring gap (1.15 m/s) at 100% fan power. In the bins of the upper stack area, the air speed was about 7 times higher than that in the bins at the bottom. The airflow pattern in the vertical gaps showed the formation of an air roll with relatively uniform air velocity across the height of the bin stack. Reducing the fan power immediately lowered the airflow between the fruit and in the gaps. However, airflow was detected at all measuring positions, even when the fan power was reduced to 44%.