Divergent Flow Research Articles

A litho-tectonic event stratigraphy from dynamic Late Devensian ice flow of the North Sea Lobe, Tunstall, east Yorkshire, UK

The central sector of the British-Irish Ice Sheet during the last glaciation was characterised by complex ice-flow reflecting interacting ice streams and changing dominance of different ice dispersal centres. At Tunstall, east Yorkshire, two subglacial till units have been traditionally identified as the Late Devensian Skipsea and Withernsea tills, and thought to record two separate ice advances onto the Holderness coast, from divergent ice flow directions. Our study presents the first quantitative lithological, sedimentological and structural evaluation of glacial sediments at the site. The lithological composition of both till units suggests that ice extended southwards from southern Scotland, incorporating material from north-east England and the western margin of the North Sea Basin. Notably, the bulk lithological properties of both the Skipsea and Withernsea tills are very similar. Subtle variations in colour, texture and lithology that do occur simply appear to reflect spatial and temporal variability in subglacial entrainment along the flow path of the North Sea Lobe. The relative arrangements of the units plus the fracture sets also indicates phases of intra-till thrust-stacking and unloading (F2), consolidation and shrinkage (F1, F3) suggestive of cycles of ice re-advance (thrusting) and ice-marginal retreat (unloading and shrinkage) possibly relating to active recession. The findings from this study reveal a sedimentary and structural complexity that is not recognised by the current Late Devensian till stratigraphy of east Yorkshire.

Flexible filaments buckle into helicoidal shapes in strong compressional flows

The occurrence of coiled or helical morphologies is common in nature, from plant roots to DNA packaging into viral capsids, as well as in applications such as oil drilling processes. In many examples, chiral structures result from the buckling of a straight fiber either with intrinsic twist or to which end moments have been applied in addition to compression forces. Here, we elucidate a generic way to form regular helicoidal shapes from achiral straight filaments transported in viscous flows with free ends. Through a combination of experiments using fluorescently labeled actin filaments in microfluidic divergent flows and of two distinct sets of numerical simulations, we demonstrate the robustness of helix formation. A nonlinear stability analysis is performed and explains the emergence of such chiral structures from the nonlinear interaction of perpendicular planar buckling modes, an effect that solely requires a strong compressional flow, independent of the exact nature of the fiber or type of flow field. The fundamental mechanism for the uncovered morphological transition and characterization of the emerging conformations advance our understanding of several biological and industrial processes and can also be exploited for the controlled microfabrication of chiral objects.

Mixing and finger morphologies in miscible non-Newtonian solution displacement

Unexpected interface complexities were found resulting from the miscible displacement of saltwater by a miscible shear-thinning solution (xanthan gum) in a radial Hele-Shaw cell, for both convergent and divergent flow. Such complex patterns have not been described before for either Newtonian or non-Newtonian solutions. A more viscous solution was injected into the cell to displace a less viscous solution (divergent flow) and then withdrawn at the injection site (convergent flow). A variegated mixing fringe between the solutions developed during the injection phase, against the stabilizing viscous effects, which would tend to promote a stable piston-like displacement. This interface geometry is markedly different from what is seen in displacement experiments performed with glycerol under otherwise similar viscosity contrast and flow conditions. The concentration field heterogeneity resulting from the presence of the fringe, quantified using a spatial autocorrelation measure, is mostly controlled by the applied shear rate, or equivalently, by the ratio of the volumetric flow rate to the flow cell’s aperture. It is significantly correlated with the radial width of the mixing zone. In addition to producing a large volume of blended solution during the injection phase, the mixing fringe impacted the development of viscous fingers (VFs) during the withdrawal phase. Such VFs are expected due to the viscosity ratio, but the initial roughness of the interface from which they develop and, hence, their later dynamics are controlled by the geometry of the mixing fringe at the end of the injection. We characterize the dynamics as a function of the imposed flow rate and cell thickness. The observed complexities of miscible displacement involving shear-thinning solutions have implications for subsurface engineering applications such as oil recovery and groundwater remediation.

Characterizing the spatial pattern of solar supergranulation using the bispectrum

Context. The spatial power spectrum of supergranulation does not fully characterize the underlying physics of turbulent convection. For example, it does not describe the non-Gaussianity in the horizontal flow divergence. Aims. Our aim is to statistically characterize the spatial pattern of solar supergranulation beyond the power spectrum. The next-order statistic is the bispectrum. It measures correlations of three Fourier components and is related to the nonlinearities in the underlying physics. It also characterizes how a skewness in the dataset is generated by the coupling of three Fourier components. Methods. We estimated the bispectrum of supergranular horizontal surface divergence maps that were obtained using local correlation tracking (LCT) and time-distance helioseismology (TD) from one year of data from the helioseismic and magnetic imager on-board the solar dynamics observatory starting in May 2010. Results. We find significantly nonzero and consistent estimates for the bispectrum using LCT and TD. The strongest nonlinearity is present when the three coupling wave vectors are at the supergranular scale. These are the same wave vectors that are present in regular hexagons, which have been used in analytical studies of solar convection. At these Fourier components, the bispectrum is positive, consistent with the positive skewness in the data and consistent with supergranules preferentially consisting of outflows surrounded by a network of inflows. We use the bispectral estimates to generate synthetic divergence maps that are very similar to the data. This is done by a model that consists of a Gaussian term and a weaker quadratic nonlinear component. Using this method, we estimate the fraction of the variance in the divergence maps from the nonlinear component to be of the order of 4–6%. Conclusions. We propose that bispectral analysis is useful for understanding the dynamics of solar turbulent convection, for example for comparing observations and numerical models of supergranular flows. This analysis may also be useful to generate synthetic flow fields.

Reduction of clean fracturing fluid filtration loss by viscosity enhancement using nanoparticles: Is it feasible?

Fracturing fluid filtration loss is a significant research topic related to fracture propagation, formation damage and the cost of hydraulic fracturing. To reduce the loss, nanoparticles have been used to enhance the fracturing fluid properties such as viscosity and well-building ability. In this work, silica nanoparticles (NPs) with different concentrations (0, 0.01, 0.03 wt%) were added into the viscoelastic surfactant (VES) fluid composed of cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal). A micro pore-throat structure was applied to investigate the flow characteristics of those three fracturing fluids with Deborah numbers (De) spanning from 1.52 to 73.2, and a Newtonian fluid was set as the control group. It was found that the flow patterns of VES solutions performed as the convergent flow similar to those of the Newtonian fluid at upstream, and turned into divergent flow with the gathering of streamlines at downstream. Surprisingly, the flow characteristics between VES solution with no nanoparticles and nanoparticle-enhanced the VES solution showed an insignificant difference with high flow rates (5 and 20 ml/h). The nanoparticle was unable to enhance or influence the properties of VES fluid when flowing through convergence–divergence and divergence–convergence structure. Neither the breakage nor the recombination of wormlike micellar were barely affected by the nanoparticle.

How do sovereign wealth funds pay their portfolio companies’ executives? Evidence from Kuwait

Sovereign wealth funds (SWFs) are major players in the global markets. We contribute to the corporate governance literature by examining the possible value SWFs bring to their domestic holdings. We specifically examine the impact of SWFs’ ownership on firm executive compensation. Using data on Kuwaiti SWFs, we find that the pay-performance relationship diminishes as the cash flow rights of the SWF are greater. Moreover, having the SWF as the ultimate owner in the ownership chain of the firm does not alleviate the adverse effects of the divergence in cash flow and control rights. This evidence supports the notion that SWFs impose agency costs on their targets.

Conserve the eco-evolutionary dynamic, not the subspecies: phenological divergence and gene flow between temporal cohorts of Euphilotes ancilla endemic to southern Nevada

Euphilotes ancilla purpura and cryptica (Lycaenidae), butterflies endemic to the Spring Mountains (Clark Co., Nevada), have been described as two univoltine, temporally isolated, sympatric taxa that utilize different early- and late-flowering larval host plant varieties (Eriogonum umbellatum). However, our results from field and laboratory indicate that this is not the case. The subspecies overlap in timing of adult reproductive flight (compilation of field records 1977 to 2018) and laboratory emergence of adults from early-season, non-diapause pupae indicate butterflies are not univoltine. Genetic samples collected from putative E. a. purpura (Early cohort) and cryptica (Late cohort) subpopulations show no evidence of genetic structure indicative of allochronic isolation in phylogenies of 26 mitochondrial DNA COI haplotypes and 18 nuclear ITS1 alleles. Analysis of molecular variance revealed 89% of mitochondrial DNA variation structured within and among subpopulations, with only 11% between the purportedly isolated subspecies. Analysis of isolation and migration indicated gene flow from the Early to Late cohort was 3 × greater than in the opposite direction. We conclude that, rather than two separate subspecies, Euphilotes ancilla exists in a network of partially interconnected subpopulations extending from 1750 to 3000 m across much of the Spring Mountains. Gene flow is related to the timing of adult flight and host plant flowering, contributing to the genetic variation in phenology necessary for evolutionary tracking of shifting flowering periods of larval host plants. Maintenance of connectivity and gene flow across the Spring Mountains is therefore essential for population persistence of both cohorts in the face of environmental change.

Genetic, morphological and ecological variation across a sharp hybrid zone between two alpine butterfly species.

Identifying the mechanisms involved in the formation and maintenance of species is a central question in evolutionary biology, and distinguishing the selective drivers of populations’ divergence from demographic processes is of particular interest to better understand the speciation process. Hybrid zones are recognized to provide ideal places to investigate the genetic architecture of speciation and to identify the mechanisms allowing diverging species to maintain their integrity in the face of gene flow. Here, we studied two alpine butterfly species, Coenonympha macromma and C. gardetta, which can be found flying together and hybridizing in narrow contact zones in the southern French Alps. We characterized the genomic composition of individuals, their morphology and their local habitat requirements, within and around a hybrid zone. Genetic diversity analysis at 794 SNPs revealed that all individuals within the hybrid zone were highly admixed, which was not the case outside the hybrid zone. Cline analysis showed that, despite ongoing hybridization, 56 out of 122 loci differentially fixed or nearly so between the two species were impermeable to introgression across the sharp hybrid zone (9 km wide). We also found concordance in cline position and width among genetic, morphological and environmental variation, suggesting a coupling of different reproductive barriers. Habitat characteristics such as the presence of trees and shrubs and the start of the growing season were strongly associated with the genetic variation, and we found evidence of divergence at genetic markers associated with morphology and physiology, putatively involved in visual or environmental reproductive isolation. We discuss the various behavioural and ecological factors that might interplay to maintain current levels of divergence and gene flow between this species pair.

Modeling Floodplain Inundation, Circulation and Residence Time Under Changing Tide and Sea-Levels.

Sea-level rise will have unknown effects on the structure and function of valuable tidal freshwater floodplains. One reason for this knowledge gap is our poor constraint on the physical controls on complex floodplain inundation and circulation processes. Here, a high-resolution light detection and ranging (lidar) digital elevation model (DEM) is applied to fine-scale numerical simulations of flow and tracer exchange in a 0.43 km2 river floodplain in Southeast Florida, USA. The sequence of inundation and associated circulation patterns is assessed at 1-hour intervals of the rising and falling tide in the context of floodplain geomorphic structure. The depth averaged velocity vectors show concomitant flow divergence and convergence over small spatial scales, and this complexity arises from the submergence and emergence of subtle floodplain topography over the tidal cycle. Tracer exchange and associated residence times highlight the controls of floodplain topography on water storage at the end of the ebb cycle, or during low river stages. The effects of a 0.2 m and 0.5 m increase in mean sea-level on inundation extent and water retention times were also assessed. Percent change in inundated area and associated e-folding times reveal greater lateral inundation extent and a 20% increase in water retention times with up to a 0.5 m increase in mean sea-level. This work reveals the topographic influence on how, when, and where sea-level rise will impact the freshwater floodplain through increased hydro period and salt-water intrusion, and the importance of evaluating floodplain restoration benefits in the context of fine-scale surface flow processes and sea-level rise.

Duan–Rach modified Adomian decomposition method (DRMA) for viscoelastic fluid flow between nonparallel plane walls

The main goal of this work is to examine the viscoelastic fluid flow velocity and skin friction coefficient through the Duan–Rach modified Adomian decomposition technique. The effects of various physical parameters of interest such as Reynolds number ($$ \text{Re} $$), channel half-angle ($$ \alpha $$) and Weissenberg number ($$ W_{i} $$) are investigated. Results revealed that the reversal flow is entirely excluded with the increase in Weissenberg number $$ W_{i} $$ for both convergent and divergent flows. Afterward, the earned analytical data are compared with numerical data and those available in the literature. From this comparison, results indicate a great agreement and therefore support the Duan–Rach approach’s applicability, validity and higher accuracy.

Mantle Convection Patterns Reveal the Mechanism of the Red Sea Rifting

AbstractWe present a new model of the stress state and present‐day tectonics of the Red Sea Rift (RSR) based on an instantaneous geodynamic mantle flow model. The initial density and viscosity variations in the mantle are derived from a joint inversion of gravity, residual topography, and tomography, which provides higher resolution than existing models. The calculated mantle flow shows clear distinctions along the rift axis. The tectonics of the southern part of the Red Sea is mainly controlled by the Afar plume and characterized by divergent mantle flow. The passive rifting along the central part of the RSR can be explained either by asthenospheric upwelling due to the Red Sea floor spreading or by the plume, rising from the transition zone and not directly related to the Afar plume. We also observed ridge‐axis‐aligned flow in the uppermost mantle in the northern part of the RSR.

Data-parallel query processing on non-uniform data

Graphics processing units (GPUs) promise spectacular performance advantages when used as database coprocessors. Their massive compute capacity, however, is often hampered by control flow divergence caused by non-uniform data distributions. When data-parallel work items demand for different amounts or types of processing, instructions execute with lowered efficiency. Query compilation techniques---a recent advance in GPU-accelerated database processing---suffer from the problem even more, because divergence effects are amplified during the execution of fused pipeline operators. In this work, we identify two types of control flow divergence--- filter divergence and expansion divergence ---that frequently occur in real world workloads. We quantify the problem for two poster cases and propose techniques to balance these divergence effects. By balancing divergence effects, our approach is able to restore processing efficiency even when pipelines contain heavily skewed operations. Our query compiler DogQC has a wider range of functionality than other query coprocessors and achieves performance improvements. We observe shorter execution times for TPC-H benchmark queries by factors up to 4.51x compared with existing GPU query compilers and by factors up to 4.54x compared with CPU-based systems.

PROGNOZOWANIE PROCESÓW FRONTOGENEZY I FRONTOLIZY METODĄ Q-WEKTORÓW Z ZASTOSOWANIEM PRZETWARZANIARÓWNOLEGŁEGO

Artykuł przedstawia autorską metodykę prognozowania procesów frontogenezy i frontolizy metodą Q-wektorów. Do jej realizacji wykorzystano zdjęcia satelitarne i dane numeryczne oraz zastosowano ideę przetwarzania równoległego. Metoda Q-wektorów służy do wyznaczania obszarów występowania prądów pionowych, określania ich kierunku i intensywności. Obszary Q-wektorów wskazują miejsca zbieżności i rozbieżności pola przepływu atmosfery, a tym samym wyznaczają rejony występowania prądów wstępujących i zstępujących. Obliczona na podstawie metody Q-wektorów funkcja frontogenetyczna wyznacza obszary frontogenezy i frontolizy. Przetwarzanie danych numerycznych wymaga zastosowania wydajnych systemów obliczeniowych. Zwiększanie wydajności współczesnych komputerów poprzez podwyższanie częstotliwości ich zegarów wewnętrznych w zasadzie dobiega końca. Właściwym kierunkiem rozwoju wydaje się być budowa komputerów o strukturach równoległych.

The Effect of a Conical Inner Electrode on the Mass-transfer Behavior in a Cylindrical Electrochemical Reactor under Single-Phase and Two-Phase (Gas-Liquid) Swirling Flow

The influence of a continuous change in the cross-section area on mass-transfer performance is analyzed in an electrochemical reactor under swirling flow. Thus, a conical frustum is used as an inner electrode in a cylindrical arrangement. A convergent flow is obtained by placing the conical frustum with its smaller base at the bottom of the reactor, or conversely, it produces a divergent flow. Experimental local mass-transfer coefficients along the electrode length are reported using the reduction of ferricyanide as a test reaction. The mass-transfer distribution as well as the global mass-transfer coefficient are both discussed in three geometric arrangements, i.e. constant interelectrode gap, convergent and divergent cases under single-phase and two-phase (gas-liquid) flow. The highest global mass-transfer coefficients are obtained for the convergent arrangement in the middle range of volumetric flow rates, giving out values 7.6% higher than those for the constant interelectrode gap case. Divergent geometry presents, as its main asset, the most uniform mass-transfer distribution in comparison with the other two configurations. The presence of the gas phase also has little influence on the mass-transfer performance of the reactor. Computational fluid dynamics calculations are in good agreement with experimental measurements.

Analysis of grinding process with the use of field theory

In this work were analyzed objective laws of material allowance removal that are common for different schemes of grinding with the use of field theory. Each level of the working surface of the circle corresponds to a certain scalar quantity, which characterizes the relative reference surface area of the diamond grains and the total grain width at this level. The total grains width determines the probability of material removal from the component surface in the process of grinding under the influence of the field of vectors of instrument’s grains. Changes which occur in the area of contact between material and grinding wheel lead to negative divergence of material flow and wheel’s grains. The change in the divergence of the fields is characterized respectively efficiency of machining and destruction of an instrument’s working area.

Towards an algebraic method of solar cycle prediction

We discuss the potential use of an algebraic method to compute the value of the solar axial dipole moment at solar minimum, widely considered to be the most reliable precursor of the activity level in the next solar cycle. The method consists of summing up the ultimate contributions of individual active regions to the solar axial dipole moment at the end of the cycle. A potential limitation of the approach is its dependence on the underlying surface flux transport (SFT) model details. We demonstrate by both analytical and numerical methods that the factor relating the initial and ultimate dipole moment contributions of an active region displays a Gaussian dependence on latitude with parameters that only depend on details of the SFT model through the parameter η/Δu where η is supergranular diffusivity and Δu is the divergence of the meridional flow on the equator. In a comparison with cycles simulated in the 2 × 2D dynamo model we further demonstrate that the inaccuracies associated with the algebraic method are minor and the method may be able to reproduce the dipole moment values in a large majority of cycles.

Identification of geodynamic movements based on the results of geodetic monitoring measurements

The methodology and results of an experimental study of modern geodynamic movements using the data from geodetic deformation monitoring are presented. The related issues of modeling natural, anthropogenic, and ecological systems are considered. A technique for visualizing geodynamic movements based on the results of cyclic geodetic measurements, which allows to reasonably identify active geological structures, blocks, tectonic faults, is presented. It is shown that as a key source of information on geodynamic movements over large spatio-temporal intervals, it is advisable to use not the absolute values of the vectors of displacement of observation points, but their velocities, normalized to the annual cycle. It is indicated that an important characteristic of the vector field of geodynamic movements is divergence, which characterizes the degree of convergence or divergence of the vector flow.

Aerodynamic Investigation of the Turbine Vane Row with a Flared Flow Path

The influence of a bilateral flare of the flow path of the turbine nozzle vane on the flow pattern and energy losses was investigated, a search was made for ways to improve the aerodynamic performance of a such kind of units. According to viscous flow calculations, the bilateral flared flow path affects the airfoil loading along the whole height of the vane, increasing velocities on both sides of the airfoil and the intensity of diffusion at divergent flow zones. The positive tip overlap considerably decreases energy losses in the vane.

Clustering of floating tracers in weakly divergent velocity fields.

This work deals with buoyant tracers floating at the ocean surface, where the geostrophic velocity component is two dimensional and rotational (nondivergent) and the ageostrophic component can contain rotational and potential (divergent) contributions that are comparable in size. We consider a random kinematic flow model and study the process of clustering, that is, aggregation of the floating tracer in localized spatial patches. In the long-time limit and in the cases of strongly and weakly divergent flows, the existing analytical theory predicts the process of exponential clustering, which is the emergence of spatial singularities containing all the available tracer. Here we confirm this analytical prediction, in numerical model solutions spanning different combinations of rotational and potential surface velocity components, and report that exponential clustering persists even in weakly divergent flows, however at significantly slower rates. For a wide range of parameters, we analyze not only the exponential clustering but also the other type of tracer aggregation, referred to as fragmentation clustering, as well as the coarse-graining effects on clustering. For the presented analysis we consider ensembles of Lagrangian particles and introduce and apply the statistical topography methodology.

Effects of thermodiffusive instability on the spherical premixed flames anchored to a porous-plug burner

Steady state burning and thermodiffusive instability of spherical premixed flames stabilized on a spherical porous-plug burner are analyzed using large-activation-energy asymptotic approach. The steady-state solution and linear stability solution are validated by comparing them to the published theoretical studies since there is limited experiment result. Within the frame of the flame-sheet model, the analytic solutions depend on the temperature and mass fraction within the flame front. The coupling between the temperature and the stand-off distance for the steady state are analyzed in detail. The spherical flames show two types of dual flame behavior and are stabilized by either heat losses to the burner or flow divergence. Subsequently, the growth rates of disturbances superimposed on the spherical burner-stabilized flames depend on the wavenumbers and are resolved. Results show that the flames are stabilized, lost stability and being re-stabilized with increasing flow velocity. It is further shown that, with increasing Lewis number (Le), the instability of the spherical flame is suppressed and only the pulsating instability appears once the Le exceeds unity. Increasing the heat release parameter and the radius of the porous-plug can enhance flame instability. The porosity μ and thickness h of the porous-plug burner have the contradictive effect on flame stability. Moreover, with increasing the ratio of μ/h, the flame becomes more stable.