4,547 research outputs found
Large-eddy simulations of high Reynolds number jets with a suitable subgrid-scale model for solver dependency study
Large-eddy simulations are performed of a turbulent
round jet at Ma = 0.5 and 0.9. The solver dependency
is explored on computationally affordable
grids of 5 and 20 million grid points, by taking advantage
of the consistency of the subgrid-scale sigma-model.
Three different solvers are tested. With all three, the
computed mean and second-order fluctuating quantities
of the turbulent near field compare favorably with
measurements, for both Mach numbers and both grids,
showing the strength of the sigma-model in adapting to different
flow conditions and grid refinements
Chaos and correlated avalanches in excitatory neural networks with synaptic plasticity
A collective chaotic phase with power law scaling of activity events is
observed in a disordered mean field network of purely excitatory leaky
integrate-and-fire neurons with short-term synaptic plasticity. The dynamical
phase diagram exhibits two transitions from quasi-synchronous and asynchronous
regimes to the nontrivial, collective, bursty regime with avalanches. In the
homogeneous case without disorder, the system synchronizes and the bursty
behavior is reflected into a doubling-period transition to chaos for a two
dimensional discrete map. Numerical simulations show that the bursty chaotic
phase with avalanches exhibits a spontaneous emergence of time correlations and
enhanced Kolmogorov complexity. Our analysis reveals a mechanism for the
generation of irregular avalanches that emerges from the combination of
disorder and deterministic underlying chaotic dynamics.Comment: 5 pages 5 figures; SI 26 pages 14 figures. Improved editing, 3
subsections added in S
Neutral theory and scale-free neural dynamics
Avalanches of electrochemical activity in brain networks have been
empirically reported to obey scale-invariant behavior --characterized by
power-law distributions up to some upper cut-off-- both in vitro and in vivo.
Elucidating whether such scaling laws stem from the underlying neural dynamics
operating at the edge of a phase transition is a fascinating possibility, as
systems poised at criticality have been argued to exhibit a number of important
functional advantages. Here we employ a well-known model for neural dynamics
with synaptic plasticity, to elucidate an alternative scenario in which
neuronal avalanches can coexist, overlapping in time, but still remaining
scale-free. Remarkably their scale-invariance does not stem from underlying
criticality nor self-organization at the edge of a continuous phase transition.
Instead, it emerges from the fact that perturbations to the system exhibit a
neutral drift --guided by demographic fluctuations-- with respect to endogenous
spontaneous activity. Such a neutral dynamics --similar to the one in neutral
theories of population genetics-- implies marginal propagation of activity,
characterized by power-law distributed causal avalanches. Importantly, our
results underline the importance of considering causal information --on which
neuron triggers the firing of which-- to properly estimate the statistics of
avalanches of neural activity. We discuss the implications of these findings
both in modeling and to elucidate experimental observations, as well as its
possible consequences for actual neural dynamics and information processing in
actual neural networks.Comment: Main text: 8 pages, 3 figures. Supplementary information: 5 pages, 4
figure
On the non-slip boundary condition enforcement in SPH methods.
The implementation of boundary conditions is one of the points where the SPH methodology still has some work to do. The aim of the present work is to provide an in-depth analysis of the most representative mirroring techniques used in SPH to enforce boundary conditions (BC) along solid profiles. We specifically refer to dummy particles, ghost particles, and Takeda et al. [1] boundary integrals. A Pouseuille flow has been used as a example to gradually evaluate the accuracy of the different implementations. Our goal is to test the behavior of the second-order differential operator with the proposed boundary extensions when the smoothing length h and other dicretization parameters as dx/h tend simultaneously to zero. First, using a smoothed continuous approximation of the unidirectional Pouseuille problem, the evolution of the velocity profile has been studied focusing on the values of the velocity and the viscous shear at the boundaries, where the exact solution should be approximated as h decreases. Second, to evaluate the impact of the discretization of the problem, an Eulerian SPH discrete version of the former problem has been implemented and similar results have been monitored. Finally, for the sake of completeness, a 2D Lagrangian SPH implementation of the problem has been also studied to compare the consequences of the particle movemen
Seis2Rock: A Data-Driven Approach to Direct Petrophysical Inversion of Pre-Stack Seismic Data
The inversion of petrophysical parameters from seismic data represents a
fundamental step in the process of characterizing the subsurface. We propose a
novel, data-driven approach named Seis2Rock that utilizes optimal basis
functions learned from well log information to directly link band-limited
petrophysical reflectivities to pre-stack seismic data. Seis2Rock is composed
of two stages: training and inference. During training, a set of optimal basis
functions are identified by performing singular value decomposition on one or
more synthetic AVO gathers created from measured or rock-physics synthesized
elastic well-logs. In inference, seismic pre-stack data are first projected
into a set of band-limited petrophysical properties using the previously
computed basis functions; this is followed by regularized post-stack seismic
inversion of the individual properties. In this work, we apply the Seis2Rock
methodology to a synthetic dataset based on the Smeaheia reservoir model and
the open Volve field dataset. Numerical results reveal the ability of the
proposed method in recovering accurate porosity, shale content, and water
saturation models. Finally, the proposed methodology is applied in the context
of reservoir monitoring to invert time-lapse, pre-stack seismic data for water
saturation changes
Metabolic Complementation in Bacterial Communities: Necessary Conditions and Optimality
Bacterial communities may display metabolic complementation, in which different members of the association partially contribute to the same biosynthetic pathway. In this way, the end product of the pathway is synthesized by the community as a whole. However, the emergence and the benefits of such complementation are poorly understood. Herein, we present a simple model to analyze the metabolic interactions among bacteria, including the host in the case of endosymbiotic bacteria. The model considers two cell populations, with both cell types encoding for the same linear biosynthetic pathway. We have found that, for metabolic complementation to emerge as an optimal strategy, both product inhibition and large permeabilities are needed. In the light of these results, we then consider the patterns found in the case of tryptophan biosynthesis in the endosymbiont consortium hosted by the aphid Cinara cedri. Using in-silico computed physicochemical properties of metabolites of this and other biosynthetic pathways, we verified that the splitting point of the pathway corresponds to the most permeable intermediate.Financial support from Spanish Government (grant reference: BFU2012-39816-C02-01 co-financed by FEDER funds and Ministerio de Economía y Competitividad) and Generalitat Valenciana (grant reference: PROMETEOII/2014/065) is grateful acknowledged.Peer reviewe
Robin Based Semi-Implicit Coupling in Fluid-Structure Interaction: Stability Analysis and Numerics
International audienceIn this report, we propose a semi-implicit coupling scheme for the numerical simulation of fluid-structure interaction systems involving a viscous incompressible fluid. The scheme is stable irrespectively of the so-called added-mass effect and allows for conservative time-stepping within the structure. The efficiency of the scheme is based on the explicit splitting of the viscous effects and geometrical/convective non-linearities, through the use of the Chorin-Temam projection scheme within the fluid. Stability comes from the implicit pressure-solid coupling and a specific Robin treatment of the explicit viscous-solid coupling, derived from Nitsche's method
On the stability of underground caves in calcareous rocks due to long-term weathering
The final publication is available at Springer via http://dx.doi.org/10.1007/s00603-020-02142-yThis paper addresses the problem of the stability of structures on calcareous rocks due to long-term weathering processes. The case study consists of a building resting on a calcarenite rock formation where two abandoned man-made caves exist directly under the structure. The boundaries of the caves were exposed to a slightly acidic environment inducing time-dependent weathering. Analyses were performed following a semi-decoupled approach, where the weathering process, driven by a reactive transport mechanism, was first solved and its results were fed to the mechanical problem which hence accounted for the spatial and temporal evolution or rock damage. For the mechanical problem, a nonlocal constitutive model was employed for the objective simulation of localised deformations. Relevant outcomes are obtained regarding the evolution of the structure’s stability and about the importance of regularising the finite element solution in the presence of brittle materials.Peer ReviewedPostprint (author's final draft
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