2,907 research outputs found
Wall effects on pressure fluctuations in turbulent channel flow
The purpose of the present paper is to study the influence of wall-echo on
pressure fluctuations , and on statistical correlations containing ,
{\em viz} redistribution , pressure diffusion , and
velocity/pressure-gradient . We extend the usual analysis of
turbulent correlations containing pressure fluctuations in wall-bounded
\tsc{dns} computations [Kim J.: {\em J. Fluid Mech.} {\bf 205} (1989)
421--451], separating not only into rapid and slow
parts [Chou P.Y.: {\em Quart. Appl. Math.} {\bf 3} (1945)
38--54], but further into volume ( and
) and surface (wall-echo;
and ) terms. An algorithm, based on a Green's function
approach, is developed to compute the above splittings for various correlations
containing pressure fluctuations (redistribution, pressure diffusion,
velocity/pressure-gradient), in fully developed turbulent plane channel flow.
This exact analysis confirms previous results based on a method-of-images
approximation [Manceau R., Wang M., Laurence D.: {\em J. Fluid Mech.} {\bf 438}
(2001) 307--338] showing that, at the wall, and
are usually of the same sign and approximately equal. The above
results are then used to study the contribution of each mechanism on the
pressure correlations in low Reynolds-number plane channel flow, and to discuss
standard second-moment-closure modelling practices
Measuring elemental abundance ratios in protoplanetary disks at millimeter wavelengths
During the million years of evolution, gas dust and ice in protoplanetary
disks can be chemically reprocessed. There are evidences that the gas-phase
carbon and oxygen abundances are sub-solar in disks belonging to nearby star
forming regions. These findings have a major impact on the composition of the
primary atmosphere of giant planets (but it may also be valid for super-Earths
and sub-Neptunes) as they accrete their gaseous envelopes from the surrounding
material in the disk. In this study, we performed a thermo-chemical modelling
analysis with the aim at testing how reliable and robust are the estimates of
elemental abundance ratios based on (sub-)millimeter observations of molecular
lines. We created a grid of disk models for the following different elemental
abundance ratios: C/O, N/O and S/O, and, we computed the line flux of a set of
carbon-, nitrogen and sulphur-bearing species, namely CN, HCN, NO, CH,
c--CH, HCO, HCN, CHCN, CS, SO, HS and
HCS, that have been detected with present (sub-)millimeter facilities
such as ALMA and NOEMA. We find that the line fluxes, once normalized to the
flux of the CO line, are sensitive to the elemental abundance
ratios. On the other hand, the stellar and disk physical parameters have only a
minor effect of the line flux ratios. Our results demonstrate that a
simultaneous analysis of multiple molecular transitions is a valid approach to
constrain the elemental abundance ratio in protoplanetary disks.Comment: Accepted for publication to A&
Four quadrant 120 A, 10 V power converters for LHC
The LHC (Large Hadron Collider) particle accelerator makes extensive use of true bipolar power converters, with a high precision regulated output current requirement. A special design and topology is required to allow high performance within the converter operating area, including quadrant transition. This paper presents the ±120A ±10V power converter, well represented in the LHC power converters (300 units). The design is adapted for a wide range of magnet loads [from 10mH to 4 Henry] (time constant load [0.1s..1050s]) with stringent EMC requirements. A quick-connect system was applied to the converter modules allowing easy installation and maintenance operations. Discussion of 4 quadrant control and practical results are presented
HCOOCH3 as a probe of temperature and structure of Orion-KL
We studied the O-bearing molecule HCOOCH3 to characterize the physical
conditions of the different molecular source components in Orion-KL. We
identify 28 methyl formate emission peaks throughout the 50" field of
observations. The two strongest peaks are in the Compact Ridge (MF1) and in the
SouthWest of the Hot Core (MF2). Spectral confusion is still prevailing as half
of the expected transitions are blended over the region. Assuming that the
transitions are thermalized, we derive the temperature at the five main
emission peaks. At the MF1 position we find a temperature of 80K in a 1.8"x0.8"
beam size and 120K on a larger scale (3.6" x2.2"), suggesting an external
source of heating, whereas the temperature is about 130K at the MF2 position on
both scales. Transitions of HCOOCH3 in vt=1 are detected as well and the good
agreement of the positions on the rotational diagrams between the vt=0 and the
vt=1 transitions suggests a similar temperature. The velocity of the gas is
between 7.5 and 8.0km/s depending on the positions and column density peaks
vary from 1.6x10^16 to 1.6x10^17cm^-2. A second velocity component is observed
around 9-10 km/s in a North-South structure stretching from the Compact Ridge
up to the BN object; this component is warmer at the MF1 peak. The two other
C2H4O2 isomers are not detected and the derived upper limit for the column
density is <3x10^14cm^-2 for glycolaldehyde and <2x10^15cm^-2 for acetic acid.
From the 223GHz continuum map, we identify several dust clumps with associated
gas masses in the range 0.8 to 5.8Msun. Assuming that the HCOOCH3 is spatially
distributed as the dust, we find relative abundances of HCOOCH3 in the range
<0.1x10^-8 to 5.2x10^-8. We suggest a relation between the methyl formate
distribution and shocks as traced by 2.12 mum H2 emission.Comment: Accepted for publication in A&
Vacuum energy in the presence of a magnetic string with delta function profile
We present a calculation of the ground state energy of massive spinor fields
and massive scalar fields in the background of an inhomogeneous magnetic string
with potential given by a delta function. The zeta functional regularization is
used and the lowest heat kernel coefficients are calculated. The rest of the
analytical calculation adopts the Jost function formalism. In the numerical
part of the work the renormalized vacuum energy as a function of the radius
of the string is calculated and plotted for various values of the strength of
the potential. The sign of the energy is found to change with the radius. For
both scalar and spinor fields the renormalized energy shows no logarithmic
behaviour in the limit , as was expected from the vanishing of the heat
kernel coefficient , which is not zero for other types of profiles.Comment: 30 pages, 10 figure
Dissipative Structures in Supersonic Turbulence
We show that density-weighted moments of the dissipation rate, ,
averaged over a scale , in supersonic turbulence can be successfully
explained by the She and L\'ev\^eque model [Phys. Rev. Lett. {\bf 72}, 336
(1994)]. A general method is developed to measure the two parameters of the
model, and , based directly on their physical interpretations as
the scaling exponent of the dissipation rate in the most intermittent
structures () and the dimension of the structures (). We find that
the best-fit parameters ( and ) derived from the
scalings in a simulation of supersonic turbulence at Mach 6 agree
with their direct measurements, confirming the validity of the model in
supersonic turbulence.Comment: 4 pages, 3 figures, accepted by Phys. Rev. Let
Application of large area SiPMs for the readout of a plastic scintillator based timing detector
In this study an array of eight 6 mm x 6 mm area SiPMs was coupled to the end
of a long plastic scintillator counter which was exposed to a 2.5 GeV/c muon
beam at the CERN PS. Timing characteristics of bars with dimensions 150 cm x 6
cm x 1 cm and 120 cm x 11 cm x 2.5 cm have been studied. An 8-channel SiPM
anode readout ASIC (MUSIC R1) based on a novel low input impedance current
conveyor has been used to read out and amplify SiPMs independently and sum the
signals at the end. Prospects for applications in large-scale particle physics
detectors with timing resolution below 100 ps are provided in light of the
results
Application of large area SiPMs for the readout of a plastic scintillator based timing detector
In this study an array of eight 6 mm x 6 mm area SiPMs was coupled to the end
of a long plastic scintillator counter which was exposed to a 2.5 GeV/c muon
beam at the CERN PS. Timing characteristics of bars with dimensions 150 cm x 6
cm x 1 cm and 120 cm x 11 cm x 2.5 cm have been studied. An 8-channel SiPM
anode readout ASIC (MUSIC R1) based on a novel low input impedance current
conveyor has been used to read out and amplify SiPMs independently and sum the
signals at the end. Prospects for applications in large-scale particle physics
detectors with timing resolution below 100 ps are provided in light of the
results
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