1,721,036 research outputs found
Uncertainty quantification analysis in discrete fracture network flow simulations
Full text linkIn the framework of underground flow simulations in fractured media, fractures may act as preferential paths, and may have a strong impact on the flow. The discrete fracture network (DFN) model allows for an explicit representation of the interconnected fractures. Flux on each fracture is assumed to be driven by Darcy Law, and suitable matching conditions are imposed along fracture intersections, ensuring flux balance and head continuity. The exact displacement of fractures in the network is usually not known, and networks are typically generated sampling geometrical and hydro-geological properties from probabilistic distributions; this stochastic generation is likely to generate geometrical configurations very challenging for the meshing process, which is a major issue in the framework of DFN simulations. Stochasticity in geometrical parameters may also result in a nonsmooth behavior of the quantity of interest with respect to the stochastic parameters.
In order to quantify the influence of these stochastic parameters on the output of DFN models, we propose an approach based on the geometric Multi Level Monte Carlo (MLMC) method, applied in conjunction with a well assessed underlying solver for performing DFN flow simulations. Key properties of the solver are its capability of circumventing the need of conforming meshes, and its consequent extreme robustness with respect to geometrical complexities in the network. These features make the solver quite suitable to be used in conjunction with MLMC for the effective application of uncertainty quantification strategies, as they allow to tackle complex geometrical configurations, also with very coarse meshes
A RANS approach to the meshless computation of pressure fields from Image Velocimetry
Full text linkWe propose a 3D meshless method to compute mean pressure fields in turbulent flows from image velocimetry. The method is an extension of the constrained Radial Basis Function (RBF) formulation by Sperotto et al. (2022) to a Reynolds Averaged Navier Stokes (RANS) framework. This is designed to handle both scattered data as in Particle
Tracking Velocimetry (PTV) and data in uniform grids as in correlation-based Particle Image Velocimetry (PIV). The RANS extension includes the Reynolds stresses into the constrained least square problem. We test the approach on a numerical database featuring a Backward Facing Step (BFS) with a Reynolds number of 6400 (defined with respect to the inlet velocity and step height), obtained via Direct Numerical Simulation (DNS)
DENPOL: A new program to determine electron densities of polypeptides using extremely localized molecular orbitals
No full textA new method to compute high-quality electron densities of polypeptides is proposed. The method is based on the transferability properties of extremely localized molecular orbitals, which can be used to describe with great accuracy the different functional groups of a molecule. It is therefore possible to generate a database of such orbitals, each of them associated with specific amino acids or with the peptide bond. A new program, DENPOL, has been written in order to build up the electron density of a generic polypeptide using this database. Due to both the large number of orbitals required to describe a polypeptide and the non-orthogonal nature of these orbitals, a Divide & Conquer strategy has been used to assemble the final electron density. The application of this approach is particularly efficient thanks to the extreme localization of the orbitals. The comparison with the corresponding electron densities generated by the Hartree-Fock method, shows the accuracy of the proposed approach and indicates that the electron densities generated by DENPOL are very close to those generated by an ab initio approach. © 2008 Elsevier B.V. All rights reserved
X-ray Constrained Spin-Coupled Wavefunction: a New Tool to Extract Chemical Information from X-ray Diffraction Data
No full textThe X-ray constrained wavefunction (XCW) approach is a reliable and widely used method of quantum crystallography that allows the determination of wavefunctions compatible with X-ray diffraction data. So far, all the existing XCW techniques have been developed in the framework of molecular orbital theory and, consequently, provide only pictures of the “experimental” electronic structures that are far from the traditional chemical perception. Here a new strategy is proposed that, by combining the XCW philosophy with the spin-coupled method of valence bond theory, enables direct extraction of traditional chemical information (e.g., weights of resonance structures) from X-ray diffraction measurements. Preliminary results have shown that the new technique is really able to efficiently capture the effects of the crystal environment on the electronic structure, and can be considered as a new useful tool to perform chemically sound analyses of the X-ray diffraction data
A new axially-chiral photochemical switch
No full textAxially chiral bis(azo) derivative 1 undergoes photochemical isomerisation, which can be seen with circular dichroism and pitch measurements of the induced cholesteric phases. © 2003 The Royal Society of Chemistry
Atomic level description of the protecting effect of osmolytes against thermal denaturation of proteins
No full textThe protecting effect of the osmolyte molecule taurine against thermal denaturation of the protein Chimotripsin Inhibitor 2 was modelled using Molecular Dynamics simulations. The protein was simulated in denaturing conditions at different taurine concentrations. Analysis of the molecular details of its behaviour shows that the protective effect of the osmolyte is concentration dependent. Moreover, the influence of taurine on the solvent structure was studied. A concentration dependent ordering effect of taurine on water molecules emerges from solvent structure analysis and is well correlated to the protecting effect observed. Based on these observations an interpretation of the osmoprotective effect is proposed. © 2007 Elsevier B.V. All rights reserved
Simulation of the steady-state flow in discrete fracture networks with non-conforming meshes and extended finite elements
No full textIn this paper a numerical method for the simulation of the steady-state fluid flow in discrete fracture networks is described. It is based on the use of non-conforming meshes, enrichment functions and an optimization procedure. The meshing process is performed on each fracture independently of the other fractures, i.e. without geometrical conformity at the intersections (traces). The slope discontinuities due to the flux exchange at the traces are then captured with the enrichment functions of the extended finite elements, and finally a functional is minimized by resorting to an optimization procedure. The method can be easily implemented for parallel computers being based on many small independent problems. In order to show the effectiveness of the method and the quality of the results, simulations of fluid flow in simple networks are illustrated
A three-field based optimization formulation for flow simulations in networks of fractures on non-conforming meshes
Full text linkA new numerical scheme is proposed for flow computation in complex discrete fracture networks. The method is based on a three-field domain decomposition framework, in which independent variables are introduced at the interfaces generated in the process of decoupling the original problem on the whole network into a set of fracture-local problems. A PDE-constrained formulation is then used to enforce compatibility conditions at the interfaces. The combination of the three-field domain decomposition and of the optimization based coupling strategy results in a novel method which can handle non-conforming meshes, independently built on each geometrical object of the computational domain, and ensures local mass conservation property at fracture intersections, which is of paramount importance for hydro-geological applications. An iterative solver is devised for the method, suitable for parallel implementation on parallel computing architectures
A novel extremely localized molecular orbitals based technique for the one-electron density matrix computation
No full textThe 'nearsightedness' of electronic structure is an underlying principle in many of the linear scaling methods recently developed to study large systems. Among them, there are strategies based on the transfer of orbitals strictly localized on molecular fragments, such as the extremely localized molecular orbitals (ELMOs). Unfortunately, due to the non-orthogonal nature of these orbitals, the density matrix calculation is computationally demanding, so preventing a straightforward application to very large molecules. In this Letter, we show how this problem can be overcome by a proper application of the 'Divide and Conquer' strategy to the ELMO approach. © 2005 Elsevier B.V. All rights reserved
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