Pair Separation Distance Research Articles

Dynamic modeling of spur gear system considering the coupling effect between bearing deflection and gear mesh stiffness

Bearing deflection has a direct impact on the mesh condition of gear systems via the center distance of the gear pair and corner contact. These factors need to be considered to accurately model the time-varying mesh stiffness and dynamic responses of gears. To address this issue, a spatial coordinate conversion is used to obtain the instantaneous spatial position of the gear pair under the bearing deflection. The tooth pair separation distance is derived through the geometric relationship analysis to model the corner contact effect. A dynamic model is established which employs a step-to-step time integration algorithm that considers the coupling effect between bearing deflection and time-varying mesh stiffness to obtain the dynamic response of the spur gear system. The simulation results of the proposed model show good agreement with the experimental results which demonstrate that the proposed model enables a proper analysis of the influence of bearing deflection on gear mesh behavior. This study provides an original dynamic model for analyzing the dynamic performance of the gear system considering the influence of bearing deflection.

Photogeneration and excitons distribution function in organic bulk heterojunction solar cells

In this work, the excitons distribution function in organic bulk heterojunction solar cells, at a depth z has been determined from solving the charge continuity equation, by exploiting the Laplace transform with an appropriate boundary conditions. Next, the influence of the electron-hole pair separation distance on the excitons dissociation probability, the internal quantum efficiency and the binding energy, has been studied. The simulated results show that the probability density of the carriers photogenerated depend on the generation rate, excitons dissociation and the charge carriers in the cells. The potential improvement of the internal quantum efficiency of charge generation depends on electron-hole pair separation distance, the excitons dissociation probability into free charges and depends strongly on the optical absorption of the photons in the active layer.

X-ray radiography 4D particle tracking of heavy spheres suspended in a turbulent jet

The suspension of a heavy sphere by an upward jet is a classical fluid mechanics experiment to demonstrate the fluid forces acting on an object. In the range of the parameter space where the sphere can be suspended, the dynamics can either be regular, i.e., with oscillations around an equilibrium position, or chaotic, with extreme events leading to large deviations from that equilibrium region. The existence and characteristics of suspension regimes of several heavy spheres in such flow configurations remain open questions. Spheres compete for the equilibrium position and come very close to each other, resulting in large local particle concentrations that prevent direct imaging. Relatively high speed X-ray radiography along with the radioSphereanalysis technique is leveraged here to study the time-resolved 3D trajectory of each individual sphere in a vertical jet. radioSphereis an X-ray analysis method that retrieves the 3D information out of a single 2D radiography using a priori knowledge of the imaging geometry (E. Andò et al., 2021), which due to the imaging modality imposes no limitations on the optical properties of the water. The 3D ＋ time kinematics yield the evolution of the statistics of the position and velocity of the spheres as a function of the number of spheres and for two jet Reynolds numbers. Drastic changes in behavior occur when many spheres are present, leaving a clear signature on the temporal dynamics and on the exploration of the flow volume, where spheres can remain on the bottom of the vessel for long periods of time, resulting in only partial suspension. In addition to the suspension capacity, the interactions between spheres are explored with statistics of pair separation distances, which, together, allow for quantitative arguments to introduce suspension regimes of a collection of spheres in an upward vertical jet.

A Cohesive-Zone-Based Contact Mechanics Analysis of Delamination in Homogeneous and Layered Half-Spaces Subjected to Normal and Shear Surface Tractions

AbstractA contact mechanics analysis of interfacial delamination in elastic and elastic-plastic homogeneous and layered half-spaces due to normal and shear surface tractions induced by indentation and sliding was performed using the finite element method. Surface separation at the delamination interface was controlled by a surface-based cohesive zone constitutive law. The instigation of interfacial delamination was determined by the critical separation distance of interface node pairs in mixed-mode loading based on a damage initiation criterion exemplified by a quadratic relation of the interfacial normal and shear tractions. Stiffness degradation was characterized by a linear relation of the interface cohesive strength and a scalar degradation parameter, which depended on the effective separation distances corresponding to the critical effective cohesive strength and the fully degraded stiffness, defined by a mixed-mode loading critical fracture energy criterion. Numerical solutions of the delamination profiles, the subsurface stress field, and the development of plasticity illuminated the effects of indentation depth and sliding distance on interfacial delamination in half-spaces with different elastic-plastic properties, interfacial cohesive strength, and layer thickness. Simulations yielded insight into the layer and substrate material property mismatch on interfacial delamination. A notable contribution of the present study is the establishment of a computational mechanics methodology for developing plasticity-induced cumulative damage models for multilayered structures.

Clustering and collision of Brownian particles in homogeneous and isotropic turbulence

The collision rate between primary nanoparticles in the turbulent flow is of significance for accurately predicting the growth rate of agglomerates during the flame synthesis process. In this work, the clustering and the collision of Brownian particles in the free-molecular regime in homogeneous isotropic turbulence are investigated using the direct numerical simulation and the Langevin dynamics. It is found that the Brownian motion can distribute the particles more uniformly, leading to the formation of a plateau on the curve of the radial distribution function (RDF). This anti-clustering effect is significant only at a small separation distance of particle pairs and cannot be observed when the separation is larger than a critical value. The radial relative velocity (RRV) is significantly enhanced, especially at the small separation distance, when the Brownian motion is taken into account. A velocity superposition analysis shows that the statistics of RRV of Brownian particles can be obtained by adding a random variable onto the RRV of non-Brownian particles with the same Stokes number. The increased radial relative velocity counteracts the anti-clustering effect of the Brownian motion and leads to the increase of the geometric collision kernel. Finally, the collision kernel of Brownian particles is formulated as a function of asymptotic values of RDF and RRV at a vanishing separation. The proposed collision kernel enables us to estimate the collision rate between nanoparticles that are much smaller than the Kolmogorov length scale.

You can go your own way: No evidence for social behavior based on kinship or familiarity in captive juvenile box turtles

Behavioral interactions between conspecific animals can be influenced by relatedness and familiarity. Compared to other vertebrate taxa, considering such aspects of social behavior when housing captive reptiles has received less attention, despite the implications this could have for informing husbandry practices, enhancing welfare, and influencing outcomes of conservation translocations. To test how kinship and familiarity influenced social behavior in juvenile Eastern Box Turtles (Terrapene carolina), we reared 16 captive-born individuals under semi-natural conditions in four equally sized groups, where each group comprised pairs of siblings and non-siblings. Using separation distance between pairs of turtles in rearing enclosures as a measure of gregariousness, we found no evidence suggesting siblings more frequently interacted with one another compared to non-relatives over the first five months of life (β = −0.016, 95% CI: −0.117 to 0.084). Average pair separation distance decreased during this time (β = −0.146, 95% CI: −0.228 to −0.063) but may have been due to turtles aggregating around concentrated resources like heat and moist retreat areas as cold winter temperatures approached. When subjects were eight months old, we measured repeated separation distances between unique pair combinations in an experimental environment and similarly found no support for gregariousness (associations) being influenced by kinship or familiarity (β = −1.554, 95% CI: −9.956 to 6.848). Additionally, neither differences in body size between pairs of turtles (β = −22.289, 95% CI: −68.448 to 23.870) nor the five-minute time interval during the 90-minute trial (P ≥ 0.18) had any apparent effect on associations. Agonistic interactions between individuals were never observed. Encouragingly, based on our results, group housing and rearing of juvenile box turtles did not appear to negatively impact their welfare. Unlike findings for other taxa, including some reptiles, our results suggest strategically housing groups of juvenile T. carolina to maintain social stability may not be an important husbandry consideration or necessary when planning releases of captive-reared individuals for conservation purposes.

New scaling laws predicting turbulent particle pair diffusion, overcoming the limitations of the prevalent Richardson–Obukhov theory

Both the evolution of particle pair separation distance l in a turbulent flow and how different length scales affect l are major unresolved challenges. The reigning theory in this topic is that of Richardson and Obukhov (R-O theory). We propose a new theory of pair diffusion in homogeneous, isotropic turbulence hypothesizing that not only structures of size l, but much larger ones also induce significant pair separation—ignored in the R-O theory. We arrive at new scaling laws for the pair diffusivity K, leading to K∼lγ where γ depends on the size of the inertial subrange: for a short inertial subrange, we find from our simulations that K∼l1.44, and for an infinite inertial subrange, we find that K∼l1.556—these relations agree closely with data. We assert that the celebrated “R-O constant” gl is neither physically meaningful nor a constant as universally assumed; our theory leads to two new physically relevant constants: GK for pair diffusivity and Gl for pair separation—which asymptote to GK≈0.73 and Gl≈0.01 at high Reynolds numbers. We find that the particle dispersion is smaller by an order of magnitude compared to R-O prediction; this is significant in many applications such as sprays, and, in particular, the spread of biological contagions (e.g., COVID19) which persist longer and drift farther compared to R-O prediction. We find that the turbulent dispersion does not depend on the fine structure timescale—a striking result which would greatly facilitate turbulent diffusion modeling.

Molecular dynamics study on composition and temperature dependences of mechanical properties of CdTeSe nanowires under uniaxial stretching

In this study, a molecular dynamics (MD) study has been performed on composition and temperature dependences of mechanical properties of CdTe[Formula: see text]Se[Formula: see text] ([Formula: see text], 0.50 and 0.75) nanowires with a diameter of 6.93 nm. The simulation results show that CdTe[Formula: see text]Se[Formula: see text] nanowire seems to be more ductile, whereas CdTe[Formula: see text]Se[Formula: see text] nanowire seems to be more brittle at 1 K. Moreover, the temperature and composition exert significant effects on the mechanical properties of CdTeSe nanowires under stretching. We conclude that the dominancy of Se atoms yields a higher stability and strength at the lower temperature of 1 K, whilst it is the same for the nanowires with both higher Te and Se contents at the higher temperature of 300 K. The radial distribution functions (RDFs) have also been calculated for the CdTeSe nanowires based on the pair separation distance at 1 and 300 K.

Three dimensional numerical simulation of drops suspended in a channel under uniform electric field

Abstract In this paper, numerical study of emulsion of drops in a channel under uniform electric field is investigated. The electric field is created by imposing an electric potential difference across the channel walls. The leaky dielectric model developed by Taylor is used to compute the electric force. The electric conductivity ratio σ r = σ i σ o and electric permittivity ratio e r = e i e o are used to study oblate and prolate drops The interaction of drop-to-wall and drop-to-drop are investigated by changing the dimensionless numbers. Oblate drops and prolate drops with σ r e r , are always attracted to the walls, but prolate drops with σ r > e r , are attracted by or repelled from the walls depending on the initial distance and electric properties. Drop-to-drop interaction depends on the initial configuration of drops. Drop pairs that are placed at an angle relative to the direction of the electric field, are always attracted towards each other. If they have an initial separation distance in the direction of the electric field, they are attracted to the walls or towards each other depending on their separation distance. Prolate drops with σ r > e r , obtain a different configuration in their final state. They may stick together or have a small separation distance, depending on their electric properties. Increasing the electric capillary number, changes the vertical and horizontal separation distance of drop pairs at steady state configuration. It also decreases the time required to reach steady state configuration. The behavior of emulsion of drops is studied by considering oblate drops, prolate drops with σ r e r and prolate drops with σ r > e r . All drops form fiber chains in the direction of the electric field that will break in stronger Poiseuille flow. Oblate drops either settle on the walls or migrate to the channel center line, but prolate drops form chains of fibers that sometimes extend between the walls. Presence of drops will change the well-known parabolic velocity profile especially for prolate drops that are suspended in the channel. The channel flow rate is also studied for different emulsion of drops and electric field. At a constant electric field and pressure gradient, the channel flow rate for oblate emulsion is higher than prolate ones. As the electric field strength increases, the channel flow rate will also increase for oblate emulsions.

Near-Zone Mediation of RET by One and Two Proximal Particles.

The effect of two inert, electric dipole polarizable molecules in relaying electronic excitation energy resonantly between a donor-acceptor pair is studied within the framework of molecular QED theory. Since transfer is efficacious when the coupled particles are close to one another, the matrix element is calculated in the near-zone approximation by employing static dipolar interaction potentials and third-order diagrammatic perturbation theory. For isotropic species, the Fermi golden rule exchange rate exhibits a Förster-like inverse sixth power dependence on each pair separation distance, is proportional to the modulus squares of the transition electric dipole moments of donor and acceptor, and depends on the polarizabilities of the two mediators. Comparison is made with direct (2-body) and third-body mediated near-zone transfer. Matrix elements for these last two processes are used to evaluate contributions to the rate due to 2-body-3-body, 2-body-4-body, and 3-body-4-body interference terms. In each of these cases, an inverse cubic dependence on each of the relative particle displacements is found.

The Unified Theory of Resonance Energy Transfer According to Molecular Quantum Electrodynamics

An overview is given of the molecular quantum electrodynamical (QED) theory of resonance energy transfer (RET). In this quantized radiation field description, RET arises from the exchange of a single virtual photon between excited donor and unexcited acceptor species. Diagrammatic time-dependent perturbation theory is employed to calculate the transfer matrix element, from which the migration rate is obtained via the Fermi golden rule. Rate formulae for oriented and isotropic systems hold for all pair separation distances, R, beyond wave function overlap. The two well-known mechanisms associated with migration of energy, namely the R−6 radiationless transfer rate due to Förster and the R−2 radiative exchange, correspond to near- and far-zone asymptotes of the general result. Discriminatory pair transfer rates are also presented. The influence of an environment is accounted for by invoking the polariton, which mediates exchange and by introducing a complex refractive index to describe local field and screening effects. This macroscopic treatment is compared and contrasted with a microscopic analysis in which the role of a neutral, polarizable and passive third-particle in mediating transfer of energy is considered. Three possible coupling mechanisms arise, each requiring summation over 24 time-ordered diagrams at fourth-order of perturbation theory with the total rate being a sum of two- and various three-body terms.

Lagrangian Dispersion Characteristics in The Western Mediterranean

Dispersion characteristics in the Western Mediterranean are analyzed using data from Coastal Ocean Dynamics Experiment (CODE) and Surface Velocity Program (SVP) surface drifters deployed in the period 1986–2017. Results are presented in terms of absolute dispersion A2 (mean-squared displacement of drifter individuals) and of relative dispersion (D2; mean square separation distance of drifter pairs). Moreover, the dispersion characteristics are estimated for different initial separation distances (D0) between particles: smaller, larger, or comparable with the internal Rossby radius of deformation. Results show the presence of a quasiballistic regime for absolute dispersion at small time scales and the nonlocal relative dispersion regime related to the submesoscale activities for scales smaller than the internal Rossby radius. At intermediate times, two anomalous absolute dispersion regimes (elliptic and hyperbolic regimes) related with the flow topology are observed, although the relative dispersion involves the Richardson and shear/ballistic regimes only for D0 smaller than the Rossby radius. During the subsequent 20–30 days, absolute dispersion shows quasirandom walk regime and relative dispersion follows the diffusive regime for scales larger than 100 km for which pair velocities are uncorrelated.

Renal arteriovenous oxygen shunting.

Renal arteriovenous oxygen shunting has been proposed as a mechanism by which oxygen supplied to the kidney can bypass the renal parenchyma. Shunting could, therefore, play a crucial role in renal hypoxia and hyperoxia. In the absence of suitable quantitative experimental methods, computational modeling has been employed in recent years to estimate the extent and potential impact of oxygen shunting. Overestimation of the separation distance between arteries and veins was suggested to be responsible for previous findings that only negligible amounts of oxygen are shunted in the preglomerular vasculature. However, models considering the correct separation distance and wrapping of artery-vein pairs still showed shunting at negligible levels of less than 1% of total renal oxygen delivery. The effect of reverse CO2 shunting on the oxygen-hemoglobin dissociation curve was found to impair, rather than promote, preglomerular oxygen shunting. Oxygen is unlikely to be shunted along the preglomerular vasculature in sufficient quantities to affect renal oxygenation. There may be substantial shunting at the level of the postglomerular vasculature, but more extensive efforts in structural imaging and computational modeling are needed to quantify it reliably.

Half-Quantum Vortices in an Antiferromagnetic Spinor Bose-Einstein Condensate.

We report on the observation of half-quantum vortices (HQVs) in the easy-plane polar phase of an antiferromagnetic spinor Bose-Einstein condensate. Using insitu magnetization-sensitive imaging, we observe that pairs of HQVs with opposite core magnetization are generated when singly charged quantum vortices are injected into the condensate. The dynamics of HQV pair formation is characterized by measuring the temporal evolutions of the pair separation distance and the core magnetization, which reveals the short-range nature of the repulsive interactions between the HQVs. We find that spin fluctuations arising from thermal population of transverse magnon excitations do not significantly affect the HQV pair formation dynamics. Our results demonstrate the instability of a singly charged vortex in the antiferromagnetic spinor condensate.

Variable lag variography using k-means clustering

Experimental variography in three dimensions based on drillhole data and current modelling software requires the selection of particular directions (azimuth and plunge) and a basic lag distance. Variogram points are then calculated on distances which are multiples of that basic lag. As samples rarely follow a regular grid, directional and distance tolerances are applied in order to have sufficient pairs to calculate reliable variogram points. This process is adequate when drillholes follow a drilling pattern (even if not an exactly regular grid) but can be time consuming and hard when the drilling pattern is irregular or when drillhole orientations vary considerably. Having all variogram points being calculated on multiples of a fixed lag, and the same tolerance being applied throughout the range of distances used, can be very restrictive and a reason for considerable time wasting or even failure to calculate an interpretable experimental variogram. The method discussed in this paper is using k-means clustering of sample pairs based on pair separation distance leading to a number of clusters each representing a different variogram point. This way, lag parameters are adjusted automatically to match the spatial distribution of sample locations and the resulting variogram is improved. Case studies are provided showing the benefits of this method over current fixed-lag experimental variogram calculation techniques.

Dynamic, Charge Photogeneration and Excitons Distribution Function in Organic Bulk Heterojunction Solar Cells

In this work, the excitons distribution function in organic bulk hetero junction solar cells, at a depth z has been determined from solving the charge continuity equation, by exploiting the Laplace transform with appropriate boundary conditions. Next, the influence of the electron-hole pair separation distance on the excitons dissociation probability, the internal quantum efficiency and the binding energy, has been studied. The simulated results show that the probability density of the carriers photo generated depends on the generation rate, excitons dissociation and the charge carriers in the cells. The potential improvement of the internal quantum efficiency of charge generation depends on electron-hole pair separation distance, the excitons dissociation probability into free charges and depends strongly on the optical absorption of the photons in the active layer.

Simulations of Nearshore Particle-Pair Dispersion in Southern California

Abstract Knowledge of horizontal relative dispersion in nearshore oceans is important for many applications including the transport and fate of pollutants and the dynamics of nearshore ecosystems. Two-particle dispersion statistics are calculated from millions of synthetic particle trajectories from high-resolution numerical simulations of the Southern California Bight. The model horizontal resolution of 250 m allows the investigation of the two-particle dispersion, with an initial pair separation of 500 m. The relative dispersion is characterized with respect to the coastal geometry, bathymetry, eddy kinetic energy, and the relative magnitudes of strain and vorticity. Dispersion is dominated by the submesoscale, not by tides. In general, headlands are more energetic and dispersive than bays. Relative diffusivity estimates are smaller and more anisotropic close to shore. Farther from shore, the relative diffusivity increases and becomes less anisotropic, approaching isotropy ~10 km from the coast. The degree of anisotropy of the relative diffusivity is qualitatively consistent with that for eddy kinetic energy. The total relative diffusivity as a function of pair separation distance R is on average proportional to R5/4. Additional Lagrangian experiments at higher horizontal numerical resolution confirmed the robustness of these results. Structures of large vorticity are preferably elongated and aligned with the coastline nearshore, which may limit cross-shelf dispersion. The results provide useful information for the design of subgrid-scale mixing parameterizations as well as quantifying the transport and dispersal of dissolved pollutants and biological propagules.

Compositional patterning in immiscible alloys subjected to severe plastic deformation

Abstract

Anisotropy in pair dispersion of inertial particles in turbulent channel flow

The rate at which two particles separate in turbulent flows is of central importance to predict the inhomogeneities of particle spatial distribution and to characterize mixing. Pair separation is analyzed for the specific case of small, inertial particles in turbulent channel flow to examine the role of mean shear and small-scale turbulent velocity fluctuations. To this aim an Eulerian-Lagrangian approach based on pseudo-spectral direct numerical simulation (DNS) of fully developed gas-solid flow at shear Reynolds number Reτ = 150 is used. Pair separation statistics have been computed for particles with different inertia (and for inertialess tracers) released from different regions of the channel. Results confirm that shear-induced effects predominate when the pair separation distance becomes comparable to the largest scale of the flow. Results also reveal the fundamental role played by particles-turbulence interaction at the small scales in triggering separation during the initial stages of pair dispersion. These findings are discussed examining Lagrangian observables, including the mean square separation, which provide prima facie evidence that pair dispersion in non-homogeneous anisotropic turbulence has a superdiffusive nature and may generate non-Gaussian number density distributions of both particles and tracers. These features appear to persist even when the effects of shear dispersion are filtered out, and exhibit strong dependency on particle inertia. Application of present results is discussed in the context of modelling approaches for particle dispersion in wall-bounded turbulent flows.

Targeted Lagrangian sampling of submesoscale dispersion at a coastal frontal zone

The potential impact of rapidly‐evolving submesoscale motions on relative dispersion is at the forefront of physical oceanography, posing challenges for both observations and modeling. A persistent coastal front driven by river outflows in the North‐Western Mediterranean Sea is targeted by two observational cruises conducted in the summer of 2010. The frontal zone is sampled using drifters launched with a multi‐scale strategy consisting of modules of triplets, released on either side of the front by small boats. This experiment is original in that the submesoscale range of 100 m to 1000 m is directly targeted, and the results are expected to provide guidance for practical applications, such as prediction of the initial spreading of pollutants and biogeochemical tracers. The influence of submesoscale motions on relative dispersion is quantified using both particle mean square separation as a function of time, and scale‐dependent finite‐size Lyapunov exponents (FSLE,λ(δ)). Our main finding is the identification of a local dispersion regime with values reaching as high as λ ≈ 20 days−1 at drifter pair separation distances of δ < 100 m. This value is more than an order of magnitude greater than that obtained by drifters in the offshore Ligurian current. The Ligurian Sea circulation is modeled using a fully realistic Regional Ocean Modeling System (ROMS) with 1/60° horizontal resolution. It is found that the numerical model significantly underestimates the relative dispersion at submesoscales, indicating the need for particle dispersion parameterizations for unresolved processes.