2,396 research outputs found
Random error propagation and uncertainty analysis in the dynamic characterization of Tilting Pad Journal Bearings
In this work a new statistical method for the determination of the dynamic coefficients of Tilting Pad Journal Bearings is described. The method is applied to the results obtained testing a 5 pads tilting pad journal bearing with 280 mm diameter. Tests were performed on an advanced experimental apparatus specifically realized for investigations on large size high performance bearings for turbomachinery. The linear coefficient identification procedure is based on the dynamic measurement of forces, accelerations and relative displacements of rotor and bearing, as function of excitation frequency for different operating conditions. The post-processing of the dynamic data is performed in the frequency domain using the Fast Fourier Transform. Along with a description of the experimental test and identification procedure, this paper presents a least-square minimization technique for determining the dynamic coefficients and a bootstrap statistical technique for estimating their confidence intervals
Phantom cosmologies and fermions
Form invariance transformations can be used for constructing phantom
cosmologies starting with conventional cosmological models. In this work we
reconsider the scalar field case and extend the discussion to fermionic fields,
where the "phantomization" process exhibits a new class of possible accelerated
regimes. As an application we analyze the cosmological constant group for a
fermionic seed fluid.Comment: 5 pages, version which was accepted for publication in CQ
Small Angle Polarization in High Energy P--P Scattering Through Nonperturbative Chiral Symmetry Breaking
We show that a large anomalous contribution due to nonperturbative
instanton-like gluonic field configurations to the axial charge of the proton
implies high-energy spin effects in elastic scattering. This is the same
mechanism which is responsible for anomalous baryon number violation at high
energy in the standard model. We compute the proton polarization due to these
effects and we show that it is proportional to the center-of-mass scattering
angle with a universal (energy-independent) slope of order unity.Comment: (13 pages, 2 figures
The Outskrits of Spiral Galaxies: Evidence for Multiple Stellar Populations
We present an analysis of the metallicity distribution functions of fields
projected along the minor axis for a sample of inclined spiral galaxies in
order to search for evidence of the presence of multiple stellar populations.
In all cases, the stellar populations appear to have asymmetric metallicity
distributions with very high confidence levels. The mean metallicities of both
stellar subpopulations, determined from mixture modelling of the metallicity
distribution functions, correlate with parent galaxy luminosity. This suggests
that the vast majority of field stars have probably formed in galactic
fragments that were already embedded in the dark matter halo of the final
galaxy. The steeper correlation between the mean stellar metallicity and parent
galaxy luminosity is driven by an increasing fraction of metal-rich stars with
increasing galaxy luminosity. Metal-poor components show larger dispersion in
metallicity than metal-rich components. These properties are in striking
similarity with those of globular cluster subpopulations around early-type
galaxies. The properties of field stars along the minor axis are consistent
with a formation scenario in which the metal-poor stars formed in all galaxies,
possibly as a result of tidal disruption of dwarf-like objects. An additional
metal-rich component might be related to the formation of the bulge and/or the
disk.Comment: 7 pages, 2 figures. ApJ, accepte
Magnetic Excitations in La2CuO4 probed by Indirect Resonant Inelastic X-ray Scattering
Recent experiments on LaCuO suggest that indirect resonant inelastic
X-ray scattering (RIXS) might provide a probe for transversal spin dynamics. We
present in detail a systematic expansion of the relevant magnetic RIXS cross
section by using the ultrashort core-hole lifetime (UCL) approximation. We
compute the scattering intensity and its momentum dependence in leading order
of the UCL expansion. The scattering is due to two-magnon processes and is
calculated within a linear spin-wave expansion of the Heisenberg spin model for
this compound, including longer range and cyclic spin interactions. We observe
that the latter terms in the Hamiltonian enhance the first moment of the
spectrum if they strengthen the antiferromagnetic ordering. The theoretical
spectra agree very well with experimental data, including the observation that
scattering intensity vanishes for the transferred momenta and
. We show that at finite temperature there is an
additional single-magnon contribution to the scattering with a spectral weight
proportional to . We also compute the leading corrections to the UCL
approximation and find them to be small, putting the UCL results on a solid
basis. All this univocally points to the conclusion that the observed low
temperature RIXS intensity in LaCuO is due to two-magnon scattering.Comment: 11 pages, 13 figures, Phys. Rev. B 77, 134428 (2008) (v4: corrected
figs 7
Commissioning of a Novel Test Apparatus for the Identification of the Dynamic Coefficients of Large Tilting Pad Journal Bearings
This paper describes the commissioning of a novel test bench for the static and dynamic characterization of large tilting pad journal bearings, realized within a collaboration of the Department of Civil and Industrial Engineering of the University of Pisa, BHGE and AM Testing. The adopted test bench configuration has the test article (TA) floating at the mid-span of a rotor supported by two rolling bearings. The TA is statically loaded vertically upwards by a hydraulic actuator and excited dynamically by two orthogonal hydraulic actuators with multiple frequency sinusoidal forces. The test rig is capable of testing bearings with a diameter from 150 to 300 mm. It includes very complex mechanical, hydraulic, electrical and electronic components, and needs, for the whole plant, about 1 MW of electric power. The commissioning of the testing system involved several aspects and presented various issues. This work focuses on measuring systems and data acquisition of high-frequency data (forces, accelerations and relative displacements) and on data processing for the identification of the bearing dynamic coefficients. The identification procedure is based on the linearity assumption and the principle of superposition, operating in the frequency domain with the fast Fourier transforms of the applied forces and displacement signals. First results, referred to a 4-pad bearing, are in satisfactory agreement with theoretical ones
Conceptual design of the enhanced coolant purification systems for the European HCLL and HCPB test blanket modules
The Coolant Purification Systems (CPSs) is one of the most relevant ancillary systems of European Helium Cooled Lead Lithium (HCLL) and Helium Cooled Pebble Bed (HCPB) Test Blanket Modules (TBMs) which are currently in the preliminary design phase in view of their installation and operation in ITER. The CPS implements mainly two functions: the extraction and concentration of the tritium permeated from the TBM modules into the primary cooling circuit and the chemistry control of helium primary coolant. During the HCLL and HCPB-TBSs (Test Blanket Systems) Conceptual Design Review (CDR) in 2015 it was recognized the need of reducing the tritium permeation into the Port Cell #16 of ITER. To achieve this and, then, to lower the tritium partial pressure in the Helium Cooling Systems in normal operation, the helium flow-rate treated by each CPS has been increased of almost one order of magnitude. In 2017, to satisfy the CDR outcomes and the new design requirements requested by Fusion for Energy (F4E, the European Domestic Agency for ITER), ENEA performed a preliminary design of the “enhanced” CPSs. This paper presents the current design of the “enhanced” CPSs, focusing on design requirements, assumptions, selection of technologies and preliminary components sizing
The variations on the aerodynamics of a world-ranked wheelchair sprinter in the key-moments of the stroke cycle: a numerical simulation analysis
Biomechanics plays an important role helping Paralympic sprinters to excel, having the aerodynamic drag a significant impact on the athlete's performance. The aim of this study was to assess the aerodynamics in different key-moments of the stroke cycle by Computational Fluid Dynamics. A world-ranked wheelchair sprinter was scanned on the racing wheelchair wearing his competition gear and helmet. The sprinter was scanned in three different positions: (i) catch (hands in the 12h position on the hand-rim); (ii) the release (hands in the 18h position on the hand-rim) and; (iii) recovery phase (hands do not touch the hand-rim and are hyperextended backwards). The simulations were performed at 2.0, 3.5, 5.0 and 6.5 m/s. The mean viscous and pressure drag components, total drag force and effective area were retrieved after running the numerical simulations. The viscous drag ranged from 3.35 N to 2.94 N, pressure drag from 0.38 N to 5.51 N, total drag force from 0.72 N to 8.45 N and effective area from 0.24 to 0.41 m2. The results pointed out that the sprinter was submitted to less drag in the recovery phase, and higher drag in the catch. These findings suggest the importance of keeping an adequate body alignment to avoid an increase in the drag force.This project was supported by the
National Funds through FCT - Portuguese
Foundation for Science and Technology (UID/DTP/
04045/2013) - and the European Fund for regional
development (FEDER) allocated by European Union
through the COMPETE 2020 Programme (POCI-
01-0145-FEDER-006969). The funders had no role in study design, data collection and analysis,
decision to publish, or preparation of the
manuscript.info:eu-repo/semantics/publishedVersio
A primer to traction force microscopy
Traction force microscopy (TFM) has emerged as a versatile technique for the measurement of single-cell-generated forces. TFM has gained wide use among mechanobiology laboratories, and several variants of the original methodology have been proposed. However, issues related to the experimental setup and, most importantly, data analysis of cell traction datasets may restrain the adoption of TFM by a wider community. In this review, we summarize the state of the art in TFM-related research, with a focus on the analytical methods underlying data analysis. We aim to provide the reader with a friendly compendium underlying the potential of TFM and emphasizing the methodological framework required for a thorough understanding of experimental data. We also compile a list of data analytics tools freely available to the scientific community for the furtherance of knowledge on this powerful technique
- …