1,721,050 research outputs found
Numerical models for the large-scale simulation of fault and fracture mechanics
Full text linkThe possible activation of pre-existing faults and the generation of new fractures in the subsurface may play a critical role in several fields of great social interest, such as the management and the exploitation of groundwater resources, especially in arid areas, the hydrocarbon recovery and storage, and the monitoring of the seismic activity in the Earth’s crust. The sliding and/or opening of a fault can create preferential leakage paths for the pore fluid escape, causing a matter of great concern in the process of storing fluids and hydrocarbons underground. The most challenging effect connected to a fault activation is the possible earthquake triggering. Many earthquakes associated with the production and injection of fluids have been recently reported. Similar issues arise also in the development of unconventional hydrocarbon reservoirs, that has recently experienced a dramatic increase thanks to the deployment of the “fracking” technology, which is based on the massive generation of fractures through the injection of fluids at high pressures. The use of this technique in densely populated areas has raised a large scientific debate on the possible connected environmental risks. The over-exploitation of fresh aquifers in arid regions has caused the generation of significant ground fissures. In this thesis, a novel formulation based on the use of Lagrange multipliers has been developed for the stable and robust numerical modeling of fault mechanics. A fault or fracture is simulated as a pair of inner surfaces included in a 3D geological formation where Lagrange multipliers are used to prescribe the contact constraints. The standard variational formulation of the contact problem with Lagrange multipliers is modified to take into account the energy dissipated by the frictional work along the activated fault portion. This term is computed by making use of the principle of maximum plastic dissipation, whose application defines the direction of the limiting shear stress vector. The novel approach has been verified against analytical solutions and applied in a number of real-world problems. In particular, we test the novel approach in four cases: (i) mechanics of two adjacent blocks, to investigate the numerical properties of the algorithm; (ii-iii) ground fractures due to groundwater withdrawal, with different geometries; (iv) fault reactivation in an underground reservoir subject to primary production and Underground Gas Storage cycles. The results are analyzed and commented. In the fourth case, the possible magnitude of the seismic events triggered by fault reactivation is computed, in order to evaluate whether underground human activities may generate seismicity.
The application of the fault model to large-scale problems gives rise to a set of sparse discrete systems of linearized equations with a generalized non-symmetric saddle point structure. The second part of this thesis is devoted to the development of efficient algorithms for the iterative solution of this kind of system. We focus on a preconditioning technique, denoted as “constraint preconditioning”, which exploits the native block structure of the Jacobian. The quality and performance of the preconditioner relies on two steps: (i) the preconditioning of the leading block and (ii) the Schur complement computation. In this work, novel preconditioning techniques for the leading block based on a multilevel framework are developed and tested. The main idea behind the multilevel preconditioner is to improve the quality of the factorized approximate inverses borrowing the scheme of incomplete factorizations, thus introducing some sequentially in perfectly parallelizable algorithms. The proposed approach is robust, from a theoretical point of view, and very efficient in parallel environment. As to the latter point, i.e. the Schur complement computation, it can be done with the aid of different approximations. The main difference is whether the Jacobian is symmetrized or not. The computation can be founded on the FSAI approximation of the leading block inverse or on a physically-based block diagonal block algorithm. The Schur complement must be inverted, thus other possibilities come in. The approximate Schur complement can be inverted through FSAI, if symmetric, or an incomplete factorization, if non-symmetric, but it can also be solved exactly, thanks to a direct solver.
The performances of the proposed algorithms are finally investigated and discussed in a set of real-world numerical examples
Sketching music together: Mixed groups exploring melodic similarity and contrast using a digital tabletop
No full textIn this article, we investigate whether and how a purposely built digital tabletop musical instrument (DTMI) can help groups of novices and casual users to explore music composition. Working together in small groups around the DTMI, our participants explored how the musical concepts of melodic similarity and contrast can convey narrative through musical structure. We build on our previous work that investigated a one-to-one learner–tutor scenario and expanded it to groups of peers. Similarly to our previous study, we adopted an exploratory and primarily qualitative approach, involving 24 participants divided into eight groups of three each, sampled from the general population via flyers and word of mouth. We structured the sessions as a series of open-ended discussions of the notions of similarity and contrast, starting from a general point of view, leading up to the task of inventing a short story and composing a melody to describe it. Although the two studies may appear superficially similar, the group element represents a fundamental difference, as we found. The combination of technology and group setting was instrumental in helping less experienced participants discuss music with more experienced participants by using a simplified yet expressive representation of music that could be used to discuss complex aspects of melody and composition
Towards the re-Activation of La(m)Pelle di Ahmad (1979): from Digitization to Virtualization. An Interview with Roberto Taroni
No full textBlock preconditioning for fault/fracture mechanics saddle-point problems
Full text linkThe efficient simulation of fault and fracture mechanics is a key issue in several applications and is attracting a growing
interest by the scientific community. Using a formulation based on Lagrange multipliers, the Jacobian matrix
resulting from the Finite Element discretization of the governing equations has a non-symmetric generalized saddlepoint
structure. In this work, we propose a family of block preconditioners to accelerate the convergence of Krylov
methods for such problems. We critically review possible advantages and difficulties of using various Schur complement
approximations, based on both physical and algebraic considerations. The proposed approaches are tested
in a number of real-world applications, showing their robustness and efficiency also in large-size and ill-conditioned
problems
On the Development of Efficient Solvers for Real-World Coupled Hydromechanical Simulations
Full text linkLinear solvers usually are the most time- and memory-demanding part of a full coupled
hydromechanical simulation. The typical block structure of the linearized systems arising
from a fully-implicit solution approach requires the development of specialized algorithms,
ensuring both robustness and computational efficiency. In particular, the design of the
preconditioner to accelerate iterative methods based on Krylov subspaces is key for the
overall model effectiveness. This work introduces a unifying framework for the
development of preconditioning techniques in multi-physics problems, and specifically
in coupled poromechanics, with the aim to provide existing methods with a novel
interpretation. Three approaches, namely explicit, implicit and reverse, are considered
and compared in real-world challenging benchmarks, identifying merits and drawbacks of
each strategy. The proposed framework can open the way to a systematic comparison of
available preconditioning tools for coupled poromechanics and help generalize the existing
methods for the introduction of additional physical processes in the simulation
A scalable preconditioning framework for stabilized contact mechanics with hydraulically active fractures
Full text linkA preconditioning framework for the coupled problem of frictional contact
mechanics and fluid flow in the fracture network is presented. The porous
medium is discretized using low-order continuous finite elements, with
cell-centered Lagrange multipliers and pressure unknowns used to impose the
constraints and solve the fluid flow in the fractures, respectively. This
formulation does not require any interpolation between different fields, but is
not uniformly inf-sup stable and requires a stabilization. For the resulting 3
x 3 block Jacobian matrix, we design scalable preconditioning strategies, based
on the physically-informed block partitioning of the unknowns and
state-of-the-art multigrid preconditioners. The key idea is to restrict the
system to a single-physics problem, approximately solve it by an inner
algebraic multigrid approach, and finally prolong it back to the fully-coupled
problem. Two different techniques are presented, analyzed and compared by
changing the ordering of the restrictions. Numerical results illustrate the
algorithmic scalability, the impact of the relative number of fracture-based
unknowns, and the performance on a real-world problem.Comment: 25 pages, 11 figures, 7 table
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Recent advancements in preconditioning techniques for large size linear systems suited for high performance computing
Full text linkThe numerical simulations of real-world engineering problems create models with several millions or even billions of degrees of freedom. Most of these simulations are centered on the solution of systems of non-linear equations, that, once linearized, become a sequence of linear systems, whose solution is often the most time-demanding task. Thus, in order to increase the capability of modeling larger cases, it is of paramount importance to exploit the resources of High Performance Computing architectures. In this framework, the development of new algorithms to accelerate the solution of linear systems for many-core architectures is a really active research field. Our main focus is algebraic preconditioning and, among the various options, we elect to develop approximate inverses for symmetric and positive definite (SPD) linear systems, both as stand-alone preconditioner or smoother for AMG techniques. This choice is mainly supported by the almost perfect parallelism that intrinsically characterizes these algorithms. As basic kernel, the Factorized Sparse Approximate Inverse (FSAI) developed in its adaptive form by Janna and Ferronato is selected. Recent developments are i) a robust multilevel approach for SPD problems based on FSAI preconditioning, which eliminates the chance of algorithmic breakdowns independently of the preconditioner sparsity and ii) a novel AMG approach featuring the adaptive FSAI method as a flexible smoother as well as new approaches to adaptively compute the prolongation operator. In this latter work, a new technique to build the prolongation is also presented
- …
