452 research outputs found
Mesoscale model of dislocation motion and crystal plasticity
A consistent, small scale description of plastic motion in a crystalline
solid is presented based on a phase field description. By allowing for
independent mass motion given by the phase field, and lattice distortion, the
solid can remain in mechanical equilibrium on the timescale of plastic motion.
Singular (incompatible) strains are determined by the phase field, to which
smooth distortions are added to satisfy mechanical equilibrium. A numerical
implementation of the model is presented, and used to study a benchmark
problem: the motion of an edge dislocation dipole in a hexagonal lattice. The
time dependence of the dipole separation agrees with classical elasticity
without any adjustable parameters.Comment: 5 pages, 3 figure
Modelling the spatial variability of snow water equivalent at the catchment scale
International audienceThe spatial distribution of snow water equivalent (SWE) is modelled as a two parameter gamma distribution. The parameters of the distribution are dynamical in that they are functions of the number of accumulation and melting events and the temporal correlation of accumulation and melting events. The estimated spatial variability is compared to snow course observations from the alpine catchments Norefjell and Aursunden in Southern Norway. A fixed snow course at Norefjell was measured 26 times during the snow season and showed that the spatial coefficient of variation change during the snow season with a decreasing trend from the start of the accumulation period and a sharp increase in the melting period. The gamma distribution with dynamical parameters reproduced the observed spatial statistical features of SWE well both at Norefjell and Aursunden. Also the shape of simulated spatial distribution of SWE agreed well with the observed at Norefjell. The temporal correlation tends to be positive for both accumulation and melting events. However, at the start of melting, a better fit between modelled and observed spatial standard deviation of SWE is obtained by using negative correlation between SWE and melt
Forstå og håndtere utfordringer knyttet til sandkontroll for utvinning av olje
In the petroleum industry, sand production is a prominent technical and economic challenge that may lead to severe implications such as well abandonment and casing erosion. This challenge is particularly pronounced in unconsolidated reservoirs. The present study offers a thorough examination of sand production, detailing its causes, monitoring techniques, and detection methods. It further delves into the various measures implemented to mitigate the impact of sand production. A unique emphasis is laid on sand screens, recognized for their cost-effectiveness and efficiency in controlling sand production. This paper discusses the different categories of sand screens, highlighting their critical role in accomplishing successful gravel-packed completions. Moreover, it explores the real-world application of sand screen techniques in sand-producing reservoirs. Through these findings, the research provides invaluable insights into sand control methods, underlining the integral role of sand screens in optimizing operations within the petroleum industry
Classical analogies for the force acting on an impurity in a Bose-Einstein condensate
We study the hydrodynamic forces acting on a small impurity moving in a
two-dimensional Bose-Einstein condensate at non-zero temperature. The
condensate is modelled by the damped-Gross Pitaevskii (dGPE) equation and the
impurity by a Gaussian repulsive potential coupled to the condensate. For weak
coupling, we obtain analytical expressions for the forces acting on the
impurity, and compare them with those computed through direct numerical
simulations of the dGPE and with the corresponding expressions for classical
forces. For non-steady flows, there is a time-dependent force dominated by
inertial effects and which has a correspondence in the Maxey-Riley theory for
particles in classical fluids. In the steady-state regime, the force is
dominated by a self-induced drag. Unlike at zero temperature, where the drag
force vanishes below a critical velocity, at low temperatures the impurity
experiences a net drag even at small velocities, as a consequence of the energy
dissipation through interactions of the condensate with the thermal cloud. This
dissipative force due to thermal drag is similar to the classical Stokes' drag.
There is still a critical velocity above which steady-state drag is dominated
by acoustic excitations and behaves non-monotonically with impurity's speed.Comment: 21 pages, 4 figures. Supplementary movies available at:
https://cloud.ifisc.uib-csic.es/nextcloud/index.php/s/AFzw6JxNW77DT6
Machine learning depinning of dislocation pileups
We study a one-dimensional model of a dislocation pileup driven by an
external stress and interacting with random quenched disorder, focusing on
predictability of the plastic deformation process. Upon quasistatically ramping
up the externally applied stress from zero the system responds by exhibiting an
irregular stress--strain curve consisting of a sequence of strain bursts, i.e.,
critical-like dislocation avalanches. The strain bursts are power-law
distributed up to a cutoff scale which increases with the stress level up to a
critical flow stress value. There, the system undergoes a depinning phase
transition and the dislocations start moving indefinitely, i.e., the strain
burst size diverges. Using sample-specific information about the pinning
landscape as well as the initial dislocation configuration as input, we employ
predictive models such as linear regression, simple neural networks and
convolutional neural networks to study the predictability of the simulated
stress--strain curves of individual samples. Our results show that the response
of the system -- including the flow stress value -- can be predicted quite
well, with the correlation coefficient between predicted and actual stress
exhibiting a non-monotonic dependence on strain. We also discuss our attempts
to predict the individual strain bursts.Comment: 10 pages, 10 figure
Stress in ordered systems: Ginzburg-Landau type density field theory
We present a theoretical method for deriving the stress tensor and elastic
response of ordered systems within a Ginzburg-Landau type density field theory
in the linear regime. This is based on spatially coarse graining the
microscopic stress which is determined by the variation of a free energy with
respect to mass displacements. We find simple expressions for the stress tensor
for phase field crystal (PFC) models for different crystal symmetries in two
and three dimensions. Using tetradic product sums of reciprocal lattice
vectors, we calculate elastic constants and show that they are directly related
to the symmetries of the reciprocal lattices. We also show that except for bcc
lattices, there are regions of model parameters for which the elastic response
is isotropic. The predicted elastic stress-strain curves are verified by
numerical strain-controlled bulk and shear deformations. Since the method is
independent of a reference state, it extends also to defected crystals. We
exemplify this by considering an edge and screw dislocation in the simple cubic
lattice
Numerical modelling of fin side heat transfer and pressure loss for compact heat recovery steam generators
publishedVersio
Analytical computation of the demagnetizing energy of thin-film domain walls
Due to its nonlocal nature, calculating the demagnetizing field remains the biggest challenge in understanding domain structures in ferromagnetic materials. Analytical descriptions of demagnetizing effects typically approximate domain walls as uniformly magnetized ellipsoids, neglecting both the smooth rotation of magnetization from one domain to the other and the interaction between the two domains. Here, instead of the demagnetizing field, we compute analytically the demagnetizing energy of a straight domain wall described by the classical tanh magnetization profile in a thin film with perpendicular magnetic anisotropy. We then use our expression for the demagnetizing energy to derive an improved version of the 1D model of field-driven domain wall motion, resulting in accurate expressions for important properties of the domain wall such as the domain wall width and the Walker breakdown field. We verify the accuracy of our analytical results by micromagnetic simulations.Peer reviewe
- …