117 research outputs found
Paradox of inductionless magnetorotational instability in a Taylor-Couette flow with a helical magnetic field
We consider the magnetorotational instability (MRI) of a hydrodynamically
stable Taylor-Couette flow with a helical external magnetic field in the
inductionless approximation defined by a zero magnetic Prandtl number
(\Pm=0). This leads to a considerable simplification of the problem
eventually containing only hydrodynamic variables. First, we point out that the
energy of any perturbation growing in the presence of magnetic field has to
grow faster without the field. This is a paradox because the base flow is
stable without the magnetic while it is unstable in the presence of a helical
magnetic field without being modified by the latter as it has been found
recently by Hollerbach and Rudiger [Phys. Rev. Lett. 95, 124501 (2005)]. We
revisit this problem by using a Chebyshev collocation method to calculate the
eigenvalue spectrum of the linearized problem. In this way, we confirm that MRI
with helical magnetic field indeed works in the inductionless limit where the
destabilization effect appears as an effective shift of the Rayleigh line.
Second, we integrate the linearized equations in time to study the transient
behavior of small amplitude perturbations, thus showing that the energy
arguments are correct as well. However, there is no real contradiction between
both facts. The linear stability theory predicts the asymptotic development of
an arbitrary small-amplitude perturbation, while the energy stability theory
yields the instant growth rate of any particular perturbation, but it does not
account for the evolution of this perturbation.Comment: 4 pages, 3 figures, submitted to Phys. Rev.
Bonding performance of wood of fast-growing tree species eucalyptus (Eucalyptus grandis) and radiata pine (Pinus radiata D.Don) with polyvinyl acetate and emulsion polymer isocyanate adhesives
Received: July 11th, 2021 ; Accepted: December 5th, 2021 ; Published: January 25th, 2022 ; Correspondence: janis.iejavs@e–koks.lvFast-growing imported plantation tree species have become an available wood resource
for Europe’s wood industry in the last decades. This sustainable alternative may reduce the gap
between the increasing demand for and decreasing supply of the local tree species. The aim of
the study was to evaluate the performance of eucalyptus (Eucalyptus grandis) and radiata pine
(Pinus radiata D.Don) wood in face-bonding with polyvinyl acetate (PVAc) and emulsion
polymer isocyanate (EPI) adhesive for the production of non-structural semi-finished glued
laminated timber members for window manufacturing. Test specimen preparation and testing
were performed according to European standards. Tensile shear strength and wood failure
percentages were determined as bonding performance indicators for 3 adhesives and 3 selected
bonding parameters (pressure, pressing time and adhesive spread) in 27 variations after boiling
the specimens in water. According to the results, the bonding variables influence the glue-line
tensile shear strength and wood failure percentages. Bonding pressure and pressing time were
evaluated as the most significant factors influencing shear strength of bonded joints. For all
bonding variations the average level of shear strength from 3.45 to 5.23 MPa were reached for
PVAc adhesive and from 3.78 to 9.65 MPa for EPI adhesives. Both EPI adhesives provide higher
performance compared to PVAc adhesive. In the case of bonding fast-growing tree species, the
highest shear strength values were achieved using the lowest pressure of 0.8 MPa, adhesive
spread from 150 to 180 g m-2 and longest pressing time of 40 min. Based on the general
evaluation of the results, it can be stated that the wood of eucalyptus and radiata pine bonded with
both EPI adhesives presents great potential for non-structural semi-finished glued laminated
timber member production, especially for the use in humid conditions
Accumulation of radiation defects and modification of micromechanical properties under MgO crystal irradiation with swift 132Xe ions
This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 and 2019-2020 under grant agreement No. 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. A.A. also acknowledges support via the project GF AP05134257 of Ministry of Education and Science of the Republic of Kazakhstan .Accumulation of F-type defects under irradiation of MgO crystals by 0.23-GeV 132Xe ions with fluence varying by three orders of magnitude has been investigated via the spectra of optical absorption and low-temperature cathodoluminescence. The number of single centers continuously increases with fluence without any marks of saturation. At the highest fluence, a mean volume concentration of 3.1 × 1019 and 3.35 × 1019 cm−3 is reached for F and F+ centers, respectively. The F+ emission strongly dominates in the cathodoluminescence of irradiated MgO and its enhancement with fluence is detected. However, the creation efficiency of the F2 aggregate centers is very low and fluence dependence has a complicated shape. Radiation-induced changes of micro-mechanical properties of the same samples have been analysed; the depth profiles of hardening correlate with the ion energy loss. A joint contribution of ionization and impact mechanisms in the formation of structural defects under MgO irradiation with Xe ions is considered.H2020 Euratom 2019-2020,633053,2014-2018,GF AP05134257; Ministry of Education and Science of the Republic of Kazakhstan GF AP05134257; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART
Production properties of K*(892) vector mesons and their spin alignment as measured in the NOMAD experiment
First measurements of K*(892) mesons production properties and their spin
alignment in nu_mu charged current (CC) and neutral current (NC) interactions
are presented. The analysis of the full data sample of the NOMAD experiment is
performed in different kinematic regions. For K*+ and K*- mesons produced in
nu_mu CC interactions and decaying into K0 pi+/- we have found the following
yields per event: (2.6 +/- 0.2 (stat.) +/- 0.2 (syst.))% and (1.6 +/- 0.1
(stat.) +/- 0.1 (syst.))% respectively, while for the K*+ and K*- mesons
produced in nu NC interactions the corresponding yields per event are: (2.5 +/-
0.3 (stat.) +/- 0.3 (syst.))% and (1.0 +/- 0.3 (stat.) +/- 0.2 (syst.))%. The
results obtained for the rho00 parameter, 0.40 +/- 0.06 (stat) +/- 0.03 (syst)
and 0.28 +/- 0.07 (stat) +/- 0.03 (syst) for K*+ and K*- produced in nu_mu CC
interactions, are compared to theoretical predictions tuned on LEP measurements
in e+e- annihilation at the Z0 pole. For K*+ mesons produced in nu NC
interactions the measured rho00 parameter is 0.66 +/- 0.10 (stat) +/- 0.05
(syst).Comment: 20 p
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