Small Growth Rate Research Articles

Nonlinear Turbulent Dynamo during Gravitational Collapse

Abstract Via amplification by turbulent dynamo, magnetic fields can be potentially important for the formation of the first stars. To examine the dynamo behavior during the gravitational collapse of primordial gas, we extend the theory of the nonlinear turbulent dynamo to include the effect of gravitational compression. The relative importance between dynamo and compression varies during contraction, with the transition from dynamo- to compression-dominated amplification of magnetic fields with the increase of density. In the nonlinear stage of magnetic field amplification with the scale-by-scale energy equipartition between turbulence and magnetic fields, reconnection diffusion of magnetic fields in ideal magnetohydrodynamic turbulence becomes important. It causes the violation of the flux-freezing condition and accounts for (a) the small growth rate of the nonlinear dynamo, (b) the weak dependence of magnetic energy on density during contraction, (c) the saturated magnetic energy, and (d) the large correlation length of magnetic fields. The resulting magnetic field structure and the scaling of magnetic field strength with density are radically different from the expectations of flux freezing.

Optimization of the Epitaxial Growth of Undoped GaN Waveguides in GaN-Based Laser Diodes Evaluated by Photoluminescence

Non-intentionally doped c-plane GaN layers are generally employed as p-side waveguide layers in violet/blue-emitting laser diodes. The recombination and diffusion of charge carriers in the p-side GaN waveguide influence the injection efficiency of holes into the InGaN quantum wells of these devices. In this study, the non-radiative recombination and the diffusivity in the [000-1] direction for charge carriers in such GaN layers are investigated by the photoluminescence of buried InGaN quantum wells, in addition to the GaN photoluminescence. The vertical charge carrier diffusion length and the diffusion constant in GaN were determined by evaluating the intensity from InGaN quantum wells in different depths below a top GaN layer. Additionally, the intensity from the buried InGaN quantum wells was found to be more sensitive to variations in the non-radiative recombination rate in the GaN layer than the intensity from the GaN itself. The study enables conclusions to be drawn on how the growth of a p-side GaN waveguide layer has to be optimized: (1) The charge carrier diffusivity in the [000-1] direction at device operation temperature is limited by phonon scattering and can be only slightly improved by material quality. (2) The use of TMGa (trimethylgallium) instead of TEGa (triethylgallium) as a precursor for the growth of GaN lowers the background silicon doping level and is advantageous for a large hole diffusion length. (3) Small growth rates below 0.5 μm/h when using TMGa or below 0.12 μm/h when using TEGa enhance non-radiative recombination. (4) A V/III gas ratio of 2200 or more is needed for low non-radiative recombination rates in GaN.

Integral-equation approach to resonances in circular two-layer flows around an island with bottom topography

This paper presents an integral-equation approach to the linear instability problem of two-layer quasi-geostrophic flows around circular islands with radial offshore bottom slope. The flows are composed of concentric uniform potential-vorticity (PV) rings in each layer, with the PV of each ring being opposite in sign. The study extends an earlier similar barotropic model and focuses on the degree to which the topographic waves resonate with the deformation waves at the rings’ peripheries. The integral approach poses the instability problem in a physically elucidating way, whereby the resonating waves in the system are directly identified. Four types of instabilities are identified: instability caused by the resonance of waves at the liquid contours at the edge of each PV ring, instability caused by the resonance of the wave at the upper-layer contour and the topographic waves outside the lower-layer contour, a similar resonance of the lower-layer contour with the topographic waves, and a resonance between one of the eigenmodes of the contour subsystem with the topographic waves. The three latter resonances lead to critical layer instabilities and can be identified as resonances between the contour waves and a collection of singular topographic modes with a critical layer. The PV perturbations in the outer region can be represented asymptotically (far from the origin) as a combination of barotropic and baroclinic modes. Usually, the asymptotically barotropic mode is the mode in resonance with the contours, but, for small growth rates, the asymptotically baroclinic mode may be the dominant mode.

The Impact of Global Transcriptional Regulation on Bacterial Gene Order.

SummaryBacterial gene expression depends on the allocation of limited transcriptional resources provided a particular growth rate and growth condition. Early studies in a few genes suggested this global regulation to generate a unifying hyperbolic expression pattern. Here, we developed a large-scale method that generalizes these experiments to quantify the response to growth of over 700 genes that a priori do not exhibit any specific control. We distinguish a core subset following a promoter-specific hyperbolic response. Within this group, we sort genes with regard to their responsiveness to the global regulatory program to show that those with a particularly sensitive linear response are located near the origin of replication. We then find evidence that this genomic architecture is biologically significant by examining position conservation of E. coli genes in 100 bacteria. The response to the transcriptional resources of the cell results in an additional feature contributing to bacterial genome organization.

Cycle sequence sensitivity of near threshold fatigue under programmed loading – A fractographic study

Statistically representative programmed block renditions of a transport aircraft wing load spectrum were used to study cycle sequence effects on near-threshold fatigue crack growth rate. Less frequent higher load cycles served as markers that permitted unambiguous estimation of crack growth rate under individual load levels in the load spectrum. Small cycle crack growth rates down to threshold levels are extremely sensitive to the peak-valley sequence of preceding large load cycle, suggesting the need to model the effect of near-tip residual stress on threshold stress intensity in handling load spectra with a large statistical content of smaller load cycles.

A Novel Image Classification Approach via Dense-MobileNet Models

As a lightweight deep neural network, MobileNet has fewer parameters and higher classification accuracy. In order to further reduce the number of network parameters and improve the classification accuracy, dense blocks that are proposed in DenseNets are introduced into MobileNet. In Dense-MobileNet models, convolution layers with the same size of input feature maps in MobileNet models are taken as dense blocks, and dense connections are carried out within the dense blocks. The new network structure can make full use of the output feature maps generated by the previous convolution layers in dense blocks, so as to generate a large number of feature maps with fewer convolution cores and repeatedly use the features. By setting a small growth rate, the network further reduces the parameters and the computation cost. Two Dense-MobileNet models, Dense1-MobileNet and Dense2-MobileNet, are designed. Experiments show that Dense2-MobileNet can achieve higher recognition accuracy than MobileNet, while only with fewer parameters and computation cost.

Bifurcation analysis and dynamic behavior to a predator-prey model with Beddington-DeAngelis functional response and protection zone

<p style='text-indent:20px;'>In this paper we study the protection zone problem to a predator-prey model subject to Beddington-DeAngelis functional responses and small prey growth rate. This is a successive work to a previous paper of the authors [X. He, S. N. Zheng, Protection zone in a diffusive predator-prey model with Beddington-DeAngelis functional response, J. Math. Biol. 75 (2017) 239-257], where the model with large prey growth rate was considered. At first we establish the existence and multiplicity of positive steady state solutions, and then give the dynamic behavior of the evolution problem. It is proved that there may be no positive steady state, or may have at leat one, two, or even three positive steady states, depending on the parameters involved such as the growth rate, the predation rate, and the food handling time of the predators, the growth rate and the refuge ability of the preys, and the sizes of the habitat with protection zone. In addition, it is shown that the dynamics of the solutions rely on the initial state as well, e.g., though there could be multiple positive steady states, the prey will go to extinction as time tends to infinity if its initial value is small.

X-ray characterization of the micromechanical response ahead of a propagating small fatigue crack in a Ni-based superalloy

The small fatigue crack (SFC) growth regime in polycrystalline alloys is complex due to the heterogeneity in the local micromechanical fields, which result in high variability in crack propagation directions and growth rates. In this study, we employ a suite of techniques, based on high-energy synchrotron-based X-ray experiments that allow us to track a nucleated crack, propagating through the bulk of a Ni-based superalloy specimen during cyclic loading. Absorption contrast tomography is used to resolve the intricate 3D crack morphology and spatial position of the crack front. Initial near-field high-energy X-ray diffraction microscopy (HEDM) is used for high-resolution characterization of the grain structure, elucidating grain orientations, shapes, and boundaries. Cyclic loading is periodically interrupted to conduct far-field HEDM to determine the centroid position, average orientation, and average lattice strain tensor for each grain within the volume of interest. Reciprocal space analysis is used to further examine the deformation state of grains that plasticize in the vicinity of the crack. Analysis of the local micromechanical state in the grains ahead of the crack front is used to rationalize the advancing small crack path and growth rate. Specifically, the most active slip system in a grain, determined by the maximum resolved shear stress, aligns with the crack growth direction; and the degree of microplasticity ahead of the crack tip helps to identify directions for potential occurrences of crack arrest or propagation. The findings suggest that both the slip system level stresses and microplasticity events within grains are necessary to get a complete description of the SFC progression. Further, this detailed dataset, produced by a suite of X-ray characterization techniques, can provide the necessary validation, at the appropriate length-scale, for SFC models.

Gradual Distance Dispersal Shapes the Genetic Structure in an Alpine Grasshopper.

The location of the high mountains of southern Europe has been crucial in the phylogeography of most European species, but how extrinsic (topography of sky islands) and intrinsic features (dispersal dynamics) have interacted to shape the genetic structure in alpine restricted species is still poorly known. Here we investigated the mechanisms explaining the colonisation of Cantabrian sky islands in an endemic flightless grasshopper. We scrutinised the maternal genetic variability and haplotype structure, and we evaluated the fitting of two migration models to understand the extant genetic structure in these populations: Long-distance dispersal (LDD) and gradual distance dispersal (GDD). We found that GDD fits the real data better than the LDD model, with an onset of the expansion matching postglacial expansions after the retreat of the ice sheets. Our findings suggest a scenario with small carrying capacity, migration rates, and population growth rates, being compatible with a slow dispersal process. The gradual expansion process along the Cantabrian sky islands found here seems to be conditioned by the suitability of habitats and the presence of alpine corridors. Our findings shed light on our understanding about how organisms which have adapted to live in alpine habitats with limited dispersal abilities have faced new and suitable environmental conditions.

A Novel Physics-Based Predictive Model for Small Fatigue Crack Growth with Microstructural Sensitivity in Cast Aluminum Alloys

A novel predictive model for microstructurally small fatigue crack growth rates was developed using a three-part methodology. First, a deterministic model was created to predict microstructurally small fatigue crack growth behavior from long fatigue crack growth data using considerations of crack tip plasticity. Subsequently, microstructural barrier characteristic spacing/strength were modeled to introduce characteristic acceleration and deceleration mechanisms of grain- and secondary phase-controlled cracks. Finally, the deterministic model was coupled with a Monte Carlo technique, and used to make predictions of lifetime distributions and S–N curves with material and component specificity. Simple, metallographically measured parameters are used to make predictions, and the model provides insight into their respective roles in controlling fatigue crack growth lifetimes, and enables design practice optimization. The model predicts that, for cast Al-Si-Mg alloy A356, increasing matrix strength, grain size, and secondary dendrite arm spacing enhances overall fatigue crack growth resistance. Comparisons are made to experimental data, and show that successful predictions are made.

Pollution shapes the bacterial community of a river: a case study

Bacteria are involved in many vital biogeochemical functions in aquatic ecosystems. Owing to their small size and high growth rates, they tend to be highly responsive to any changes in their environment. This potential to reflect any kind of environmental change makes them sensitive indicators towards the effect of pollutants in aquatic systems. The present study focused on investigation of change in the bacterial community structure of Hindon River in Ghaziabad in North India, in relation to the river’s pollution levels. Both spatial and temporal variation in the resident as well as active bacterial community was determined using 16S rRNA gene and transcript as molecular markers through denaturing gradient gel electrophoresis. Sequence analysis of eluted bands showed that Bacteroidetes followed by γ-proteobacteria, Actinobacteria, β-proteobacteria, δ-proteobacteria, e-proteobacteria and Nitrospirae, represented the dominant phyla. Moreover, the effect of pollutants on the bacterial diversity was more conspicuous at RNA level. In addition, when assessing the diversity of culturables, 8 bacterial strains, having minimum inhibitory concentrations in the range of 200–250 mg l−1 for Ni, 75–200 mg l−1 for Co and 75–200 mg l−1 for Cd, were isolated and characterized. The identified bacteria have the potential to be used as sensitive biomarkers for the development of bioremediation strategies and their monitoring.

An investigation of small fatigue crack behavior in titanium alloy TC4 under different stress levels

In this paper, the small fatigue crack behavior in titanium alloy TC4 under different stress levels was investigated. Single-edge-notch tension specimens were axially fatigued with stress ratio of 0.1 at room temperature. Results show that the naturally initiated cracks nucleate from the α/β interfaces. When the crack is below a critical length of ∼200 µm, crack growth rates exhibit large fluctuations and temporary retardations due to microstructure effects. But once the crack length exceeds ∼200 µm, the fluctuations in crack growth rates die down and the crack grows more rapidly. There is no distinct effect of stress level on small crack growth rate. A linear relationship exists between the total fatigue life and crack initiation life.

Does a Change of Government Influence Compliance with International Agreements? Empirical Evidence for the NATO Two Percent Target

ABSTRACT We examine whether changes of government influence compliance with international agreements. We investigate compliance with the NATO two percent target to which all NATO countries committed themselves during the NATO summit in Wales in 2014. The dataset includes the military expenditure by NATO countries over the period 2010–2018. The results suggest that countries that do not (yet) comply with the two percent target have smaller growth rates in military expenditure relative to GDP when they experienced a large change of government, e.g. a change from a rightwing to a leftwing government, than countries that did not experience such a large change of government since the NATO summit in 2014. Countries that experienced a large change of government are, thus, less likely to comply with the two percent target. Future research should examine the credibility problem of national governments in other international agreements too.

Bifurcation in the growth of continental crust

Is the present-day water-land ratio a necessary outcome of the evolution of plate tectonic planets with a similar age, volume, mass, and total water inventory as the Earth? This would be the case – largely independent of initial conditions – if Earth’s present-day continental volume were at a stable unique equilibrium with strong self-regulating mechanisms of continental growth steering the evolution to this state. In this paper, we question this conjecture. Instead we suggest that positive feedbacks in the plate tectonics model of continental production and erosion may dominate and show that such a model can explain the history of continental growth.We investigate the main mechanisms that contribute to the growth of the volume of the continental crust. In particular, we analyze the effect of the oceanic plate speed, depending on the area and thickness of thermally insulating continents, on production and erosion mechanisms. Effects that cause larger continental production rates for larger values of continental volume are positive feedbacks. In contrast, negative feedbacks act to stabilize the continental volume. They are provided by the increase of the rate of surface erosion, subduction erosion, and crustal delamination with the continental volume. We systematically analyze the strengths of positive and negative feedback contributions to the growth of the continental crust. Although the strengths of some feedbacks depend on poorly known parameters, we conclude that a net predominance of positive feedbacks is plausible. We explore the effect of the combined feedback strength on the feasibility of modeling the observed small positive net continental growth rate over the past 2–3 billion years. We show that a model with dominating positive feedbacks can readily explain this observation in spite of the cooling of the Earth’s mantle acting to reduce the continental production rate. In contrast, explaining this observation using a model with dominating negative feedbacks would require the continental erosion and production rates to both have the same or a sufficiently similar functional dependence on the thermal state of the mantle, which appears unreasonable considering erosion to be largely dominated by the surface relief and weathering.The suggested scenario of dominating positive feedbacks implies that the present volume of the continental crust and its evolution are strongly determined by initial conditions. Therefore, exoplanets with Earth-like masses and total water inventories may substantially differ from the Earth with respect to their relative land/surface ratios and their habitability.

In-Situ Stress Measurements during Cobalt Electrodeposition

In situ cantilever curvature is used to quantify the growth stress in Co thin films, electrodeposited from an electrolyte consisting of 0.5 mol/L Na2SO4, 0.5 mol/L H3BO3, and 0.1 mol/L CoSO4·7 H20. The average biaxial steady-state stress is measured as a function of the deposition potential and is examined as a function of growth rate. Stresses as low as +85 MPa (tensile) are obtained at small growth rate, increasing to a limiting value of 800 MPa as the growth rate is increased. The data is fit to a kinetic model that appears in the literature and treats the stress as a dynamic competition between coalescence-induced tensile stress and compressive stress due to insertion of atoms into the grain boundary. Kinetic parameters for Co indicate that stress development is dominated by nuclei coalescence and that ad-atom insertion into the grain boundary contributes to the overall stress only at very low growth rates.

Does a Change of Government Influence Compliance with International Agreements? Empirical Evidence for the NATO Two Percent Target

We examine whether changes of government influence compliance with international agreements. We investigate compliance with the NATO two percent target to which all NATO countries committed themselves during the NATO summit in Wales in 2014. The dataset includes the military expenditure by NATO countries over the period 2010–2018. The results suggest that countries that do not (yet) comply with the two percent target have smaller growth rates in military expenditure relative to GDP when they experienced a large change of government, e.g. a change from a rightwing to a leftwing government, than countries that did not experience such a large change of government since the NATO summit in 2014. Countries that experienced a large change of government are, thus, less likely to comply with the two percent target. Future research should examine the credibility problem of national governments in other international agreements too.

Magnetic quantum string waves in non-degenerate quantum plasma

Instabilities of transverse waves due to a constant magnetic field in a non-degenerate quantum electron-ion plasma are studied. A new type of cyclotron oscillation with a small growth rate is disclosed. Excitation of a new quantum mode is also revealed. Furthermore, the kinetic instabilities of Alfven waves are discussed and the growth rates are obtained. A novel branch of waves that we call magnetic quantum string waves is found.

Formation and growth of interfacial intermetallics in Sn-0.3Ag-0.7Cu-xCeO2/Cu solder joints during the reflow process

The effects of adding CeO2 nanoparticles on the formation and growth of interfacial intermetallic compounds (IMCs) between Sn-0.3Ag-0.7Cu-xCeO2 (x = 0, 0.05, 0.1, 0.15, 0.2, 0.3 wt%) (SAC0307-xCeO2) solders and Cu substrates during reflow soldering were investigated. Scanning electron microscopy (SEM) was used to observe the microstructural evolution of the solder joints and measure the thickness and grain size of the intermetallic layers. The results show that the thicknesses and grain sizes of the IMCs decrease with CeO2 nanoparticle addition to the SAC0307 solder and that these parameters decrease significantly at a particle concentration of approximately 0.15 wt%. At higher concentrations, the thicknesses and grain sizes of the IMCs increase slightly. The growth exponents for the IMC layers and grains are determined by curve fitting to study the growth kinetics of the IMCs during reflow soldering. The results reveal that the growth exponents of the IMC layers range from 0.335 to 0.374 and those of the IMC grains range from 0.302 to 0.318, thus suggesting that the growth of the IMCs is controlled by the combined kinetic processes of atomic interdiffusion, interfacial reactions, and grain ripening. These data also show that among the tested solders, the SAC0307–0.15CeO2 solder joint has the smallest growth rate and exhibits the most significant effects in IMC growth suppression and grain size refinement. Based on the theories of heterogeneous nucleation and adsorption, a mechanism for reducing the IMC growth rate by the addition of CeO2 nanoparticles to the SAC0307 solder is proposed.

Probabilistic prediction of minimum fatigue life behaviour in α + β titanium alloys

AbstractThe objective of this work was to develop and demonstrate a probabilistic life prediction method for the prediction of minimum fatigue lives that are typically used in the design of fracture critical rotating turbine engine components. A Monte Carlo analysis was used to predict the variability in fatigue lives based on the distribution of microstructural features that lead to early crack initiation as well as the variability in small fatigue crack growth rates. Two titanium alloys, both with bimodal microstructures, were tested and analysed in this study. The distribution of critical microstructural features was calibrated based on test results and understanding of microstructure neighbourhood effects. Testing was conducted on both alloys and included both smooth and notched specimens. The predictions are presented and compared with the data for smooth and notch geometries for the various loading conditions. A parametric study was performed to identify the importance of several model inputs and to identify areas for future improvement.

Retardation of Small Creep–Fatigue Crack in Gr. 91 Steel Through the Combined Effects of Stress Relaxation, Microstructural Evolution, and Oxidation

This investigation reports an unusual effect of hold time (up to 10 seconds) on retardation in the growth of creep–fatigue small cracks at 550 °C in Grade 91 steel. The observed phenomenon was interpreted by elucidating multiple processes that are active in the plastic zone at the crack tip. To this effect, microstructural and mechanical responses of the crack tip plastic zone were compared with the mechanical and microstructural responses during low cycle fatigue/creep–fatigue. It is proposed that the stress relaxation that occurs during the hold time can reduce the stress intensity in the plastic zone of crack tip thus, contributing to retardation in the small crack growth rate. Aside from stress relaxation, stress intensity in the crack tip can be further reduced by the phenomenon of cyclic softening that occurs because of plasticity-induced microstructural coarsening. Separately, the contribution of oxidation-induced crack tip shielding is also considered to explain the observed effects of hold time on crack growth rates. Taken together, a combination of stress relaxation, enhanced rate of cyclic softening, and higher degree of oxidation with the introduction of a hold time is demonstrated to be responsible for reduction in the crack growth under creep–fatigue conditions.