28 research outputs found
On the numerical modelling of the Jet Erosion Test
International audienceEvaluating the erodibility of a soil, both in terms of erosion threshold (initiation) and erosion rate (progression), is critical for the evaluation of the safety of water retaining structures. Indeed different soils can erode at different rates. However, the relationship between the erosion parameters and the geotechnical and chemical properties of soils remains largely unknown. The jet erosion test appears to be an efficient and simple means for quantifying the two erosion parameters involved. The first parameter is the critical stress while the second parameter is the erosion coefficient. A simplified model of this test has been drawn up by G. Hanson et al. to interpret the experimental curves. Few attempts have been made so far to model the whole process, however. The aim of this study is to simulate the impinging jet and to take into account the erosion of the soil by means of computational fluid dynamics (CFD) numerical modelling. The key point was the time dependence of the problem, due to erosion processes, however the turbulent flow could be considered as steady because of the assumption of low kinetics erosion assumption. The results of the present modelling study are compared to the simplified model and to experimental data. This comparison is a first confirmation of the validity of the simplified model as a means of assessing the critical stress and the erosion coefficient with jet erosion tests
On the origin of heavy quasiparticles in LiV_2O_4
An explanation is provided for the heavy quasiparticle excitations in
LiV_2O_4. It differs considerably from that of other known heavy-fermion
systems. Main ingredients of our theory are the cubic spinel structure of the
material and strong short-range correlations of the d electrons. The large
gamma-coefficient is shown to result from excitations of Heisenberg spin 1/2
rings and chains. The required coupling constant is calculated from LDA+U
calculations and is found to be of the right size. Also the calculated
Sommerfeld-Wilson ratio is reasonably close to the observed one.Comment: REVTEX, 5 pages, 2 figure
Feed-back on the development of a small scale Contact Erosion Test in the laboratory (characteristic size ~ 30 cm)
To determine the hydraulic load requested to initiate contact erosion process, tests are performed with an apparatus called the “Contact Erosion Test”. This device originally results from research carried out by Grenoble University, Électricité de France and Compagnie Nationale du Rhône, at the scale of ~60 cm. It has been adapted to a smaller scale in geophyConsult laboratory to conduct tests on samples extracted from core drilling. The instrumentation was improved to enable a better control of the hydraulic loading and avoid biases. The test protocol was modified, especially to better constrain the soil density at the interface. From the first series of test, we drew conclusions on the test repeatability and on the influence of parameters of the soil state. Discrepancies with previous results obtained at the scale of ~60 cm were identified. Therefore, a new erosion test campaign was planned to confirm and determine the reasons for these differences
Corner transfer matrix renormalization group method for two-dimensional self-avoiding walks and other O(n) models
We present an extension of the corner transfer matrix renormalisation group
(CTMRG) method to O(n) invariant models, with particular interest in the
self-avoiding walk class of models (O(n=0)). The method is illustrated using an
interacting self-avoiding walk model. Based on the efficiency and versatility
when compared to other available numerical methods, we present CTMRG as the
method of choice for two-dimensional self-avoiding walk problems.Comment: 4 pages 7 figures Substantial rewrite of previous version to include
calculations of critical points and exponents. Final version accepted for
publication in PRE (Rapid Communications
Classical heisenberg antiferromagnet away from the pyrochlore lattice limit: entropic versus energetic selection
The stability of the disordered ground state of the classical Heisenberg
pyrochlore antiferromagnet is studied within extensive Monte Carlo simulations
by introducing an additional exchange interaction that interpolates
between the pyrochlore lattice () and the face-centered cubic lattice
(). It is found that for as low as , the system is
long range ordered : the disordered ground state of the pyrochlore
antiferromagnet is unstable when introducing very small deviations from the
pure limit. Furthermore, it is found that the selected phase is a
collinear state energetically greater than the incommensurate phase suggested
by a mean field analysis. To our knowledge this is the first example where
entropic selection prevails over the energetic one.Comment: 5 (two-column revtex4) pages, 1 table, 7 ps/eps figures. Submitted to
Phys. Rev.
Universality in Heavy Fermions Revisited
A previous scaling analysis of pressure experiments in heavy fermion is
reviewed and enlarged. We show that the critical exponents obtained from this
analysis indicate that a one-parameter scaling describes these experiments. We
obtain explicitly the enhancemente factors showing that these systems are
indeed near criticality and that the scaling approach is appropriate. The
physics responsible for the one-parameter scaling and breakdown of hyperscaling
is clarified. We discuss a microsocopic theory that is in agreement with the
experiments. The scaling theory is generalized for the case the shift and
crossover exponents are different. The exponents governing the physical
behavior along the non-Fermi liquid trajectory are obtained for this case.Comment: 7 pages, Latex, 3 Postscript figures, to be published in Physical
Review
Zero temperature phases of the frustrated J1-J2 antiferromagnetic spin-1/2 Heisenberg model on a simple cubic lattice
At zero temperature magnetic phases of the quantum spin-1/2 Heisenberg
antiferromagnet on a simple cubic lattice with competing first and second
neighbor exchanges (J1 and J2) is investigated using the non-linear spin wave
theory. We find existence of two phases: a two sublattice Neel phase for small
J2 (AF), and a collinear antiferromagnetic phase at large J2 (CAF). We obtain
the sublattice magnetizations and ground state energies for the two phases and
find that there exists a first order phase transition from the AF-phase to the
CAF-phase at the critical transition point, pc = 0.28. Our results for the
value of pc are in excellent agreement with results from Monte-Carlo
simulations and variational spin wave theory. We also show that the quartic 1/S
corrections due spin-wave interactions enhance the sublattice magnetization in
both the phases which causes the intermediate paramagnetic phase predicted from
linear spin wave theory to disappear.Comment: 19 pages, 4 figures, Fig. 1b modified, Appendix B text modifie