132 research outputs found
A summary of new predictive high frequency thermo-vibrational models in porous media
In this chapter, we consider the effect of mechanical vibration on the onset of convection in porous media. The porous media is saturated either by a pure fluid or by a binary mixture. The importance of transport model on stability diagrams are presented and discussed. The stability threshold for the Darcy-Brinkman case in the RaTc-R and kc-R diagrams are presented (where RaTc, kc and R are the critical Rayleigh number, the critical wave number and the vibration parameters respectively). It is shown that there is a significant deviation from the Darcy model. In the thermo-solutal case with the Soret effect, the influence of vibration on the reduction of multi-cellular convection is emphasized. A new analytical relation for obtaining the threshold of mono-cellular convection is derived. This relation shows how the separation factor Ψ is related to controlling parameters of the problem, Ψ = f (R, ε*, Le) when the wave number k -> 0. The importance of vibrational parameter definition is highlighted and it is shown how, by using a proper definition for vibrational parameter, we may obtain compact relationship. It is also shown how this result may be used to increase components separation
Search for eta-mesic 4He in the dd->3He n pi0 and dd->3He p pi- reactions with the WASA-at-COSY facility
The search for 4He-eta bound states was performed with the WASA-at-COSY
facility via the measurement of the excitation function for the dd->3He n pi0
and dd->3He p pi- processes. The beam momentum was varied continuously between
2.127 GeV/c and 2.422 GeV/c, corresponding to the excess energy for the dd->4He
eta reaction ranging from Q=-70 MeV to Q=30 MeV. The luminosity was determined
based on the dd->3He n reaction and quasi-free proton-proton scattering via
dd->pp n_spectator n_spectator reactions. The excitation functions determined
independently for the measured reactions do not reveal a structure which could
be interpreted as a narrow mesic nucleus. Therefore, the upper limits of the
total cross sections for the bound state production and decay in
dd->(4He-eta)_bound->3He n pi0 and dd->(4He-eta)_bound->3He p pi- processes
were determined taking into account the isospin relation between both the
channels considered. The results of the analysis depend on the assumptions of
the N* momentum distribution in the anticipated mesic-4He. Assuming as in the
previous works, that this is identical with the distribution of nucleons bound
with 20 MeV in 4He, we determined that (for the mesic bound state width in the
range from 5 MeV to 50 MeV) the upper limits at 90% confidence level are about
3 nb and about 6 nb for npi0 and ppi- channels, respectively. However, based on
the recent theoretical findings of the N*(1535) momentum distribution in the
N*-3He nucleus bound by 3.6 MeV, we find that the WASA-at-COSY detector
acceptance decreases and hence the corresponding upper limits are 5 nb and 10
nb for npi0 and ppi- channels respectively.Comment: This article will be submitted to JHE
Order-of-magnitude speedup for steady states and traveling waves via Stokes preconditioning in Channelflow and Openpipeflow
Steady states and traveling waves play a fundamental role in understanding
hydrodynamic problems. Even when unstable, these states provide the
bifurcation-theoretic explanation for the origin of the observed states. In
turbulent wall-bounded shear flows, these states have been hypothesized to be
saddle points organizing the trajectories within a chaotic attractor. These
states must be computed with Newton's method or one of its generalizations,
since time-integration cannot converge to unstable equilibria. The bottleneck
is the solution of linear systems involving the Jacobian of the Navier-Stokes
or Boussinesq equations. Originally such computations were carried out by
constructing and directly inverting the Jacobian, but this is unfeasible for
the matrices arising from three-dimensional hydrodynamic configurations in
large domains. A popular method is to seek states that are invariant under
numerical time integration. Surprisingly, equilibria may also be found by
seeking flows that are invariant under a single very large Backwards-Euler
Forwards-Euler timestep. We show that this method, called Stokes
preconditioning, is 10 to 50 times faster at computing steady states in plane
Couette flow and traveling waves in pipe flow. Moreover, it can be carried out
using Channelflow (by Gibson) and Openpipeflow (by Willis) without any changes
to these popular spectral codes. We explain the convergence rate as a function
of the integration period and Reynolds number by computing the full spectra of
the operators corresponding to the Jacobians of both methods.Comment: in Computational Modelling of Bifurcations and Instabilities in Fluid
Dynamics, ed. Alexander Gelfgat (Springer, 2018
ABC Effect and Resonance Structure in the Double-Pionic Fusion to He
Exclusive and kinematically complete measurements of the double pionic fusion
to He have been performed in the energy region of the so-called ABC effect,
which denotes a pronounced low-mass enhancement in the -invariant mass
spectrum. The experiments were carried out with the WASA detector setup at
COSY. Similar to the observations in the basic reaction
and in the He reaction, the data reveal a correlation
between the ABC effect and a resonance-like energy dependence in the total
cross section. Differential cross sections are well described by the hypothesis
of resonance formation during the reaction process in addition to the
conventional -channel mechanism. The deduced resonance
width can be understood from collision broadening due to Fermi motion of the
nucleons in initial and final nuclei
Cross section ratio and angular distributions of the reaction p + d -> 3He + eta at 48.8 MeV and 59.8 MeV excess energy
We present new data for angular distributions and on the cross section ratio
of the p + d -> 3He + eta reaction at excess energies of Q = 48.8 MeV and Q =
59.8 MeV. The data have been obtained at the WASA-at-COSY experiment
(Forschungszentrum J\"ulich) using a proton beam and a deuterium pellet target.
While the shape of obtained angular distributions show only a slow variation
with the energy, the new results indicate a distinct and unexpected total cross
section fluctuation between Q = 20 MeV and Q = 60 MeV, which might indicate the
variation of the production mechanism within this energy interval.Comment: 9 pages, 9 figure
Measurement of the Reaction in Search for the Recently Observed Resonance Structure in and systems
Exclusive measurements of the quasi-free reaction have
been performed by means of collisions at = 1.2 GeV using the WASA
detector setup at COSY. Total and differential cross sections have been
obtained covering the energy region = (2.35 - 2.46) GeV, which
includes the region of the ABC effect and its associated resonance structure.
No ABC effect, {\it i.e.} low-mass enhancement is found in the
-invariant mass spectrum -- in agreement with the constraint from
Bose statistics that the isovector pion pair can not be in relative s-wave. At
the upper end of the covered energy region -channel processes for Roper,
and excitations provide a reasonable description
of the data, but at low energies the measured cross sections are much larger
than predicted by such processes. Adding a resonance amplitude for the
resonance at =~2.37 GeV with =~70 MeV and observed
recently in and reactions leads to an
agreement with the data also at low energies
Search for a dark photon in the decay
The presently world largest data sample of pi0 --> gamma e+e- decays
containing nearly 5E5 events was collected using the WASA detector at COSY. A
search for a dark photon U produced in the pi0 --> gamma U --> gamma e+e- decay
from the pp-->pp\pi^0 reaction was carried out. An upper limit on the square of
the U-gamma mixing strength parameter epsilon^2 of 5e-6 at 90% CL was obtained
for the mass range 20 MeV <M_U< 100 MeV. This result together with other recent
experimental limits significantly reduces the M_U vs. \epsilon^2 parameter
space preferred by the measured value of the muon anomalous magnetic moment.Comment: 16 pages, 8 figures; improved analysis extending the exclusion region
to 20 MeV<M_U< 100 MeV; implemented changes requested by referee
Neutron-Proton Scattering in the Context of the (2380) Resonance
New data on quasifree polarized neutron-proton scattering, in the region of
the recently observed resonance structure, have been obtained by
exclusive and kinematically complete high-statistics measurements with WASA at
COSY. This paper details the determination of the beam polarization, checks of
the quasifree character of the scattering process, on all obtained
angular distributions and on the new partial-wave analysis, which includes the
new data producing a resonance pole in the - coupled partial
waves at () MeV -- in accordance with the dibaryon
resonance hypothesis. The effect of the new partial-wave solution on the
description of total and differential cross section data as well as specific
combinations of spin-correlation and spin-transfer observables available from
COSY-ANKE measurements at = 2.27 GeV is discussed
Evidence for a New Resonance from Polarized Neutron-Proton Scattering
Exclusive and kinematically complete high-statistics measurements of
quasifree polarized scattering have been performed in the energy
region of the narrow resonance structure with , 2380 MeV/ and 70 MeV observed recently in the
double-pionic fusion channels and .
The experiment was carried out with the WASA detector setup at COSY having a
polarized deuteron beam impinged on the hydrogen pellet target and utilizing
the quasifree process . That way the
analyzing power was measured over a large angular range. The obtained
angular distributions deviate systematically from the current SAID SP07
NN partial-wave solution. Incorporating the new data into the SAID
analysis produces a pole in the waves as expected from the
resonance hypothesis
Search for the eta-mesic 4He with WASA-at-COSY detector
An exclusive measurement of the excitation function for the dd->3Heppi-
reaction was performed at the Cooler Synchrotron COSY-Juelich with the
WASA-at-COSY detection system. The data were taken during a slow acceleration
of the beam from 2.185 GeV/c to 2.400 GeV/c crossing the kinematic threshold
for the eta meson production in the dd->4He-eta reaction at 2.336 GeV/c. The
corresponding excess energy with respect to the 4He-eta system varied from
-51.4MeV to 22MeV. The integrated luminosity in the experiment was determined
using the dd->3Hen reaction. The shape of the excitation function for the
dd->3Heppi- was examined. No signal of the 4He-eta bound state was observed. An
upper limit for the cross-section for the bound state formation and decay in
the process dd->(4He-eta)bound->3Heppi- was determined on the 90% confidence
level and it varies from 20nb to 27nb for the bound state width ranging from
5MeV to 35MeV, respectively.Comment: 8 pages, 9 figure
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