829 research outputs found
A one-dimensional Fermi accelerator model with moving wall described by a nonlinear van der Pol oscillator
A modification of the one-dimensional Fermi accelerator model is considered
in this work. The dynamics of a classical particle of mass , confined to
bounce elastically between two rigid walls where one is described by a
non-linear van der Pol type oscillator while the other one is fixed, working as
a re-injection mechanism of the particle for a next collision, is carefully
made by the use of a two-dimensional non-linear mapping. Two cases are
considered: (i) the situation where the particle has mass negligible as
compared to the mass of the moving wall and does not affect the motion of it;
(ii) the case where collisions of the particle does affect the movement of the
moving wall. For case (i) the phase space is of mixed type leading us to
observe a scaling of the average velocity as a function of the parameter
() controlling the non-linearity of the moving wall. For large
, a diffusion on the velocity is observed leading us to conclude that
Fermi acceleration is taking place. On the other hand for case (ii), the motion
of the moving wall is affected by collisions with the particle. However due to
the properties of the van der Pol oscillation, the moving wall relaxes again to
a limit cycle. Such kind of motion absorbs part of the energy of the particle
leading to a suppression of the unlimited energy gain as observed in case (i).
The phase space shows a set of attractors of different periods whose basin of
attraction has a complicate organization
Communion and Creation: Relational Theological Anthropology and the Flourishing of Creation
In this paper, I argue for an extension of relational accounts of the imago Dei which includes a kind of priestly relation to the created order. In this relation, humanity is intended to ensure the independent flourishing of creation in a way reflective of the kind of communion we ought to have with one another. Through an analysis of the brokenness of these relationships, I argue human oppression of other humans and ravaging of creation are born of the same brokenness in such a way that they contribute to one another as distortions of human teleological communion. By drawing on Social Identity Approach in group psychology, I can offer an account of shared human identity out of which humanity acts in distorted ways as a group. By describing oppression and ravaging in terms of broken communal telos and group action, I offer a way forward for relating humanity to the created order in a way that neither instrumentalizes creation, nor flattens the distinctiveness of human creation in the image of God
The dynamical evolution of massive black hole binaries - I. Hardening in a fixed stellar background
The stellar ejection rate and the rates of change of the binary semimajor
axis and eccentricity are derived from scattering experiments for the
restricted three-body problem. They are used to study the evolution of binaries
in simple models for galactic nuclei, starting soon after the black holes
become bound and continuing until the evolution is dominated by the emission of
gravitational radiation, or until the ejected mass is too large for the galaxy
to be considered fixed. The eccentricity growth is found to be unimportant
unless the binary forms with a large eccentricity. The scattering results are
compared with predictions from Chandrasekhar's dynamical-friction formula and
with previous work on the capture and scattering of comets by planetary
systems. They suggest that a binary with masses should not be
considered hard until its orbital velocity exceeds the background velocity
dispersion by a factor that scales as .Comment: includes 9 postscript figures, mn.sty, fixup.sty, psfig.st
Low alpha power (7.5–9.5 Hz) changes during positive and negative affective learning
There is evidence that the positive and the negative word lists of the Affective Auditory Verbal Learning Test (AAVL) are useful with regard to mood induction. To date, however, changes in brain activation, as indicated by quantitative electroencephalographic recording, have not been examined. Thus, changes in low alpha power (7.5–9.5 Hz) were examined during and after completion of the positive or the negative learning list of the AAVL among 37 undergraduate men and women. Three primary findings from the study include the following: (1) Previously reported recall patterns were replicated; (2) participants who completed the negative list reported a significant decline in mood state at the end of the session; and (3) participants who completed the negative word list evidenced a significant reduction in low alpha power (in comparison with baseline) within the parietal regions. The findings noted above are seemingly counter to contemporary theories of mood regulation (i.e., asymmetrical changes in anterior activity, rather than changes in parietal regions). Although the AAVL may have limited utility as a tool for mood induction, it may serve as a functional tool for examination of the cerebral processes associated with affective verbal memory
Constraining multiple systems with GAIA
GAIA will provide observations of some multiple asteroid and dwarf systems.
These observations are a way to determine and improve the quantification of
dynamical parameters, such as the masses and the gravity fields, in these
multiple systems. Here we investigate this problem in the cases of Pluto's and
Eugenia's system. We simulate observations reproducing an approximate planning
of the GAIA observations for both systems, as well as the New Horizons
observations of Pluto. We have developed a numerical model reproducing the
specific behavior of multiple asteroid system around the Sun and fit it to the
simulated observations using least-square method, giving the uncertainties on
the fitted parameters. We found that GAIA will improve significantly the
precision of Pluto's and Charon's mass, as well as Petit Prince's orbital
elements and Eugenia's polar oblateness.Comment: 5 pages, accepted by Planetary and Space Science, Gaia GREAT-SSO-Pis
Front-Line Employees’ Recognition And Empowerment Effect On Retail Bank Customers’ Perceived Value
Understanding how front-line bank employee recognition and empowerment impacts customer value is an important issue for the banking industry. Retail banking customers perceive little difference in services as competition has created more choices. When banks develop new products, they are easily duplicated by the competition. Banks can gain a competitive advantage by understanding the interrelationships that impact customers perceived value. The major providers of customer perceived value in retail banking are front-line employees. This research aims to better understand these interrelationships and examine how front-line retail bank employee recognition and empowerment relate to retail bank customer perceived value. Customers perceived value is defined in terms of relationship, social, emotional, service and empowerment. Survey findings illustrate that retail banks should concentrate more on employee recognition and empowerment to impact customers perceived value. According to the findings of the survey, front-line employee recognition and empowerment has a positive impact on customers perceived value
Comparative analysis of viruses in four bee species collected from agricultural, urban, and natural landscapes
Managed honey bees (Apis mellifera L.) and wild bees provide critical ecological services that shape and sustain natural, agricultural, and urban landscapes. In recent years, declines in bee populations have highlighted the importance of the pollination services they provide and the need for more research into the reasons for global bee losses. Several stressors cause declining populations of managed and wild bee species such as habitat degradation, pesticide exposure, and pathogens. Viruses, which have been implicated as a key stressor, are able to infect a wide range of species and can be transmitted both intra- and inter-specifically from infected bee species to uninfected bee species via vertical (from parent to offspring) and/or horizontal (between individuals via direct or indirect contact) transmission. To explore how viruses spread both intra- and inter-specifically within a community, we examined the impact of management, landscape type, and bee species on the transmission of four common viruses in Nebraska: Deformed wing virus (DWV), Israeli acute paralysis virus (IAPV), Black queen cell virus (BQCV), and Sacbrood virus (SBV). Results indicated the prevalence of viruses is significantly affected (P \u3c 0.005) by bee species, virus type, and season, but not by landscape or year (P = 0.290 and 0.065 respectively). The higher prevalence of DWV detected across bee species (10.4% on Apis mellifera, 5.3% on Bombus impatiens, 6.1% on Bombus griseocollis, and 22.44% on Halictus ligatus) and seasons (10.8% in earlymid summer and 11.4% in late summer) may indicate a higher risk of interspecific transmission of DWV. However, IAPV was predominately detected in Halictus ligatus (20.7%) and in late season collections (28.1%), which may suggest species-specific susceptibility and seasonal trends in infection rates associated with different virus types. However, there were limited detections of SBV and BQCV in bees collected during both sampling periods, indicating SBV and BQCV may be less prevalent among bee communities in this area
Comparative analysis of viruses in four bee species collected from agricultural, urban, and natural landscapes
Managed honey bees (Apis mellifera L.) and wild bees provide critical ecological services that shape and sustain natural, agricultural, and urban landscapes. In recent years, declines in bee populations have highlighted the importance of the pollination services they provide and the need for more research into the reasons for global bee losses. Several stressors cause declining populations of managed and wild bee species such as habitat degradation, pesticide exposure, and pathogens. Viruses, which have been implicated as a key stressor, are able to infect a wide range of species and can be transmitted both intra- and inter-specifically from infected bee species to uninfected bee species via vertical (from parent to offspring) and/or horizontal (between individuals via direct or indirect contact) transmission. To explore how viruses spread both intra- and inter-specifically within a community, we examined the impact of management, landscape type, and bee species on the transmission of four common viruses in Nebraska: Deformed wing virus (DWV), Israeli acute paralysis virus (IAPV), Black queen cell virus (BQCV), and Sacbrood virus (SBV). Results indicated the prevalence of viruses is significantly affected (P \u3c 0.005) by bee species, virus type, and season, but not by landscape or year (P = 0.290 and 0.065 respectively). The higher prevalence of DWV detected across bee species (10.4% on Apis mellifera, 5.3% on Bombus impatiens, 6.1% on Bombus griseocollis, and 22.44% on Halictus ligatus) and seasons (10.8% in earlymid summer and 11.4% in late summer) may indicate a higher risk of interspecific transmission of DWV. However, IAPV was predominately detected in Halictus ligatus (20.7%) and in late season collections (28.1%), which may suggest species-specific susceptibility and seasonal trends in infection rates associated with different virus types. However, there were limited detections of SBV and BQCV in bees collected during both sampling periods, indicating SBV and BQCV may be less prevalent among bee communities in this area
Ultrafast X-Ray Imaging of Laser-Metal Additive Manufacturing Processes
The high-speed synchrotron X-ray imaging technique was synchronized with a custom-built laser-melting setup to capture the dynamics of laser powder-bed fusion processes in situ. Various significant phenomena, including vapor-depression and melt-pool dynamics and powder-spatter ejection, were captured with high spatial and temporal resolution. Imaging frame rates of up to 10 MHz were used to capture the rapid changes in these highly dynamic phenomena. At the same time, relatively slow frame rates were employed to capture large-scale changes during the process. This experimental platform will be vital in the further understanding of laser additive manufacturing processes and will be particularly helpful in guiding efforts to reduce or eliminate microstructural defects in additively manufactured parts
Revealing Particle-Scale Powder Spreading Dynamics in Powder-Bed-Based Additive Manufacturing Process by High-Speed X-Ray Imaging
Powder spreading is a key step in the powder-bed-based additive manufacturing process, which determines the quality of the powder bed and, consequently, affects the quality of the manufactured part. However, powder spreading behavior under additive manufacturing condition is still not clear, largely because of the lack of particle-scale experimental study. Here, we studied particle-scale powder dynamics during the powder spreading process by using in-situ high-speed high-energy x-ray imaging. Evolution of the repose angle, slope surface speed, slope surface roughness, and the dynamics of powder clusters at the powder front were revealed and quantified. Interactions of the individual metal powders, with boundaries (substrate and container wall), were characterized, and coefficients of friction between the powders and boundaries were calculated. The effects of particle size on powder flow dynamics were revealed. The particle-scale powder spreading dynamics, reported here, are important for a thorough understanding of powder spreading behavior in the powder-bed-based additive manufacturing process, and are critical to the development and validation of models that can more accurately predict powder spreading behavior
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