530 research outputs found
Neutron scattering investigation of the low momentum collective ion dynamics in liquid potassium
No full textBrillouin neutron scattering in heavy water: Evidence for two-mode collective dynamics
No full textA high resolution (1.5 meV) inelastic neutron scattering experiment was carried out, aiming at an accurate
investigation of the high frequency and low momentum dynamic response in heavy water. The experimental
data confirm the existence of a dispersionless mode, besides the ordinary longitudinal collective dynamics. A
simplified model, based on the interaction of two vibrational branches, is proposed to interpret the observed
features of the dynamic spectra. The validity extent of this scheme is proved by applying it to room temperature
neutron and x-ray data, to temperature and pressure dependent x-ray data, and to room temperature
neutron data of vibrational density of states. The overall successful results provided by this model, in conjunction
with the combined analysis of the x-ray and neutron data on collective dynamics, enable a deeper
insight into the complex mechanisms of the water dynamics and provide a simple phenomenological explanation
for the transition from ordinary to fast sound
Neutron scattering investigation of low-momentum collective ion dynamics in liquid potassium
No full textThe low-momentum collective ion dynamics of molten potassium has been investigated by means of a high-resolution inelastic neutron-scattering experiment. Collective vibrational modes were found to contribute to the coherent response function over the wave-vector transfer region from 0.2 up to 1 Angstrom(-1). The analysis of the quasielastic features of the dynamic structure factor, due to mainly incoherent scattering, suggests that contributions other than the expected self-diffusion are present. An internally consistent description of the low momentum collective ion dynamics in molten alkali metals was obtained by applying the same data analysis scheme to the present data and those published on Li, Na, Rb, and Cs. The main conclusion is that the Bohm-Staver prescription for the collective-mode velocity does not describe the whole alkali metals series and approaches beyond the random-phase approximation are necessary to treat the electron gas screening of the ion interactions
Communication: Are metallic glasses different from other glasses? A closer look at their high frequency dynamics
No full textUsing high resolution inelastic x-ray scattering we studied the collective dynamics of the Pd77Si16.5Cu6.5 metallic glass, focusing on the energy-momentum region where the boson peak appears. The dispersion relation and the width of the acoustic excitations are determined showing how the longitudinal acoustic modes maintain their dispersive character for frequencies well above the boson peak frequencies. Moreover, we prove that close to these frequencies there is a softening of the apparent sound speed indicating a failure of the Debye continuum approximation at the boson peak frequencies and challenging previous results on other metallic glasses
Shape deformation of a piston ring groove by laser surface hardening
No full textWith laser beam heating it is possible to obtain a precisely controlled hardening area, in which the process parameters and material properties are important for the hardened area and deformed surface shape. In this study, the transient heat flow, thermal stress and deformed shape during laser surface hardening of the piston ring groove of a ship's engine were analysed by a finite element method. Using a modified two-dimensional finite element model, the heat-affected zone (HAZ) sizes and the deformed shapes in laser surface hardening were successively calculated. The effects of the process parameters and material properties on the deformed shape of the hardened part were investigated by using 2n − 1 ractional factorial design. The simulation results revealed that the process parameters such as laser power and traverse speed had a greater influence on the deformed shape of the surface-hardened part than the physical properties of the base metal. The displacement of the hardened surface was determined mainly by the bending and bulging phenomena which were due to the temperature gradient and martensitic phase transformation; the bending effect played a more important role than the bulging effect. Tensile residual stress occurred in the hardened area, while the maximum compressive residual stress occurred under the boundary of the hardened zone
Spectral features of non-nutritive suck dynamics in extremely preterm infants
No full text© The Author(s) 2023. cc-by-nc-ndBackground: Non-nutritive suck (NNS) is used to promote ororhythmic patterning and assess oral feeding readiness in preterm infants in the neonatal intensive care unit (NICU). While time domain measures of NNS are available in real time at cribside, our understanding of suck pattern generation in the frequency domain is limited. The aim of this study is to model the development of NNS in the frequency domain using Fourier and machine learning (ML) techniques in extremely preterm infants (EPIs). Methods: A total of 117 EPIs were randomized to a pulsed or sham orocutaneous intervention during tube feedings 3 times/day for 4 weeks, beginning at 30 weeks post-menstrual age (PMA). Infants were assessed 3 times/week for NNS dynamics until they attained 100% oral feeding or NICU discharge. Digitized NNS signals were processed in the frequency domain using two transforms, including the Welch power spectral density (PSD) method, and the Yule-Walker PSD method. Data analysis proceeded in two stages. Stage 1: ML longitudinal cluster analysis was conducted to identify groups (classes) of infants, each showing a unique pattern of change in Welch and Yule-Walker calculations during the interventions. Stage 2: linear mixed modeling (LMM) was performed for the Welch and Yule-Walker dependent variables to examine the effects of gestationally-aged (GA), PMA, sex (male, female), patient type [respiratory distress syndrome (RDS), bronchopulmonary dysplasia (BPD)], treatment (NTrainer, Sham), intervention phase [1, 2, 3], cluster class, and phase-by-class interaction. Results: ML of Welch PSD method and Yule-Walker PSD method measures revealed three membership classes of NNS growth patterns. The dependent measures peak_Hz, PSD amplitude, and area under the curve (AUC) are highly dependent on PMA, but show little relation to respiratory status (RDS, BPD) or somatosensory intervention. Thus, neural regulation of NNS in the frequency domain is significantly different for each identified cluster (classes A, B, C) during this developmental period. Conclusions: Efforts to increase our knowledge of the evolution of the suck central pattern generator (sCPG) in preterm infants, including NNS rhythmogenesis will help us better understand the observed phenotypes of NNS production in both the frequency and time domains. Knowledge of those features of the NNS which are relatively invariant vs. other features which are modifiable by experience will likewise inform more effective treatment strategies in this fragile population
Identification of an ortholog of the eukaryotic RNA polymerase III subunit RPC34 in Crenarchaeota and Thaumarchaeota suggests specialization of RNA polymerases for coding and non-coding RNAs in Archaea.
No full textContains fulltext :
80405.pdf (Publisher’s version ) (Open Access)One of the hallmarks of eukaryotic information processing is the co-existence of 3 distinct, multi-subunit RNA polymerase complexes that are dedicated to the transcription of specific classes of coding or non-coding RNAs. Archaea encode only one RNA polymerase that resembles the eukaryotic RNA polymerase II with respect to the subunit composition. Here we identify archaeal orthologs of the eukaryotic RNA polymerase III subunit RPC34. Genome context analysis supports a function of this archaeal protein in the transcription of non-coding RNAs. These findings suggest that functional separation of RNA polymerases for protein-coding genes and non-coding RNAs might predate the origin of the Eukaryotes
Navigation Conditions, Suck Bend Reach, Chattahoochee River, Alabama and Georgia: Hydraulic Model Investigation
No full textSource: https://erdc-library.erdc.dren.mil/jspui/The Suck Bend Reach of the Chattahoochee River is in the upper end of Columbia Lock and Dam Pool about 1.5 miles downstream of the Walter F. George Lock and Dam. Suck Bend is essentially two adjacent and alternate bends in the form of a relatively flat "S" curve. Shoaling that occurs in the bend during flood flows and at times during powerhouse operation has been a problem for navigation when there is little or no flow from upstream. A movable-bed model reproducing about 1.5 miles of the Chattahoochee River to an undistorted scale of 1:72, model-to-prototype, was used to develop plans that would eliminate or reduce shoaling sufficiently to permit uninterrupted navigation and to improve navigation conditions for larger tows through the bend and in the approach to the proposed State Highway 10 bridge. The results of this investigation indicated the following: (A.) Navigation conditions in Suck Bend are affected by shoaling of the channel in the bend, elevation of bedrock, and adverse currents. (B.) Shoaling in the bend can be eliminated with training structures but the structures would not provide satisfactory navigation conditions through the bend and in the approach to the proposed State Highway 10 bridge for the larger tows because of the limited channel width and adverse currents. (C.) Satisfactory navigation conditions could be provided with training structures and with excavation and filling along the right bank in the bend to eliminate the irregularities in the bank line and increase the radius of curvature of the channel in the lower bend. (D.) Downbound tows of the larger size (435 ft by 78 ft) would have to flank to become properly aligned for passage through the bridge span even with the realigned right bank because of the short approach to the bridge after making the turn in the lower bend. (E.) The alignment of the currents through the proposed bridge could be improved with vane dikes along the left side of the channel upstream of the bridge but the dikes would reduce the maneuver area available and could be a navigation hazard
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