accedaCRIShttps://accedacris.ulpgc.es/jspuiThe accedaCRIS digital repository system captures, stores, indexes, preserves, and distributes digital research material.Sat, 30 May 2020 12:18:40 GMT2020-05-30T12:18:40Z5091- New implementation of QMR-type algorithmshttp://hdl.handle.net/10553/47206Title: New implementation of QMR-type algorithms
Authors: García, M. D.; Florez, E.; Suárez Sarmiento, Antonio F.; González, L.; Montero, G.
Abstract: Quasi-minimal residual algorithms, these are QMR, TFQMR and QMRCGSTAB, are biorthogonalisation methods for solving nonsymmetric linear systems of equations which improve the irregular behaviour of BiCG, CGS and BiCGSTAB algorithms, respectively. They are based on the quasi-minimisation of the residual using the standard Givens rotations that lead to iterations with short term recurrences. In this paper, these quasi-minimisation problems are solved using a different direct solver which provides new versions of QMR-type methods, the modified QMR methods (MQMR). MQMR algorithms have different convergence behaviour in finite arithmetic although are equivalent to the standard ones in exact arithmetic. The new implementations may reduce the number of iterations in some cases. In addition, we study the effect of reordering and preconditioning with Jacobi, ILU, SSOR or sparse approximate inverse preconditioners on the performance of these algorithms. Some numerical experiments are solved in order to compare the results obtained by standard and modified algorithms.
Sat, 01 Jan 2005 00:00:00 GMThttp://hdl.handle.net/10553/472062005-01-01T00:00:00Z
- Algorithm comparing binary string probabilities in complex stochastic boolean systems using intrinsic order graphhttp://hdl.handle.net/10553/72585Title: Algorithm comparing binary string probabilities in complex stochastic boolean systems using intrinsic order graph
Authors: González, Luis
Abstract: This paper deals with a special kind of complex systems which depend on an arbitrary (and usually large) number n of random Boolean variables. The so-called complex stochastic Boolean systems often appear in many diff. erent scientifi. c, technical or social areas. Clearly, there are 2(n) binary states associated to such a complex system. Each one of them is given by a binary string u = (u1,..., u(n)) is an element of {0, 1}(n) of n bits, which has a certain occurrence probability Pr{u}. The behavior of a complex stochastic Boolean system is determined by the current values of its 2(n) binary n-tuple probabilities Pr{u} and by the ordering between pairs of them. Hence, the intrinsic order graph provides a useful representation of these systems by displaying (scaling) the 2(n) binary n-tuples which are ordered in decreasing probability of occurrence. The intrinsic order reduces the complexity of the problem from exponential (2(n) binary n-tuples) to linear (n Boolean variables). For any. xed binary n-tuple u, this paper presents a new, simple algorithm enabling rapid, elegant determination of all the binary n-tuples v with occurrence probabilities less than or equal to (greater than or equal to) Pr{u}. This algorithm is closely related to the lexicographic (truth- table) order in {0, 1} n, and this is illustrated through the connections (paths) in the intrinsic order graph.
Mon, 01 Jan 2007 00:00:00 GMThttp://hdl.handle.net/10553/725852007-01-01T00:00:00Z
- A finite difference model for air pollution simulationhttp://hdl.handle.net/10553/47203Title: A finite difference model for air pollution simulation
Authors: Sanín, N.; Montero, G.
Abstract: A 3-D model for atmospheric pollutant transport is proposed considering a set of coupled convection–diffusion–reaction equations. The convective phenomenon is mainly produced by a wind field obtained from a 3-D mass consistent model. In particular, the modelling of oxidation and hydrolysis of sulphur and nitrogen oxides released to the surface layer is carried out by using a linear module of chemical reactions. The dry deposition process, represented by the so-called deposition velocity, is introduced as a boundary condition. Moreover, the wet deposition is included in the source term of the governing equations using the washout coefficient. Before obtaining a numerical solution, the problem is transformed using a terrain conformal coordinate system. This allows to work with a simpler domain in order to build a mesh that provides finite difference schemes with high spatial accuracy. The convection–diffusion–reaction equations are solved with a high order accurate time-stepping discretization scheme which is constructed following the technique of Lax and Wendroff. Finally, the model is tested with a numerical experiment in La Palma Island (Canary Islands).
Mon, 01 Jan 2007 00:00:00 GMThttp://hdl.handle.net/10553/472032007-01-01T00:00:00Z
- Applications of genetic algorithms in realistic wind field simulationshttp://hdl.handle.net/10553/45239Title: Applications of genetic algorithms in realistic wind field simulations
Authors: Montenegro, Rafa; Montero, G.; Rodriguez, E.; Escobar, J. M.; González-Yuste, J. M.
Abstract: Mass consistent models have been widely use in 3-D wind modelling by finite element method. We have used a method for constructing tetrahedral meshes which are simultaneously adapted to the terrain orography and the roughness length by using a refinement/derefinement process in a 2-D mesh corresponding to the terrain surface, following the technique proposed in [14,15,18]. In this 2-D mesh we include a local refinement around several points which are previously defined by the user. Besides, we develop a technique for adapting the mesh to any contour that has an important role in the simulation, like shorelines or roughness length contours [3,4], and we refine the mesh locally for improving the numerical solution with the procedure proposed in [6]. This wind model introduces new aspects on that proposed in [16, 19, 20]. The characterization of the atmospheric stability is carried out by means of the experimental measures of the intensities of turbulence. On the other hand, since several measures are often available at a same vertical line, we have constructed a least square optimization of such measures for developing a vertical profile of wind velocities from an optimum friction velocity. Besides, the main parameters governing the model are estimated using genetic algorithms with a parallel implementation [12,20,26]. In order to test the model, some numerical experiments are presented, comparing the results with realistic measures.
Tue, 01 Jan 2008 00:00:00 GMThttp://hdl.handle.net/10553/452392008-01-01T00:00:00Z
- Quality improvement of surface triangulationshttp://hdl.handle.net/10553/45245Title: Quality improvement of surface triangulations
Authors: Montenegro, R.; Escobar, J. M.; Montero, G.; Rodriguez, E.
Abstract: This paper presents a new procedure to improve the quality of triangular meshes defined on surfaces. The improvement is obtained by an iterative process in which each node of the mesh is moved to a new position that minimizes certain objective function. This objective function is derived from an algebraic quality measures of the local mesh (the set of triangles connected to the adjustable or free node). The optimization is done in the parametric mesh, where the presence of barriers in the objective function maintains the free node inside the feasible region. In this way, the original problem on the surface is transformed into a two-dimensional one on the parametric space. In our case, the parametric space is a plane, chosen in terms of the local mesh, in such a way that this mesh can be optimally projected performing a valid mesh, that is, without inverted elements. In order to show the efficiency of this smoothing procedure, its application is presented.
Sat, 01 Jan 2005 00:00:00 GMThttp://hdl.handle.net/10553/452452005-01-01T00:00:00Z
- Implementation in ALBERTA of an automatic tetrahedral mesh generatorhttp://hdl.handle.net/10553/45242Title: Implementation in ALBERTA of an automatic tetrahedral mesh generator
Authors: Montenegro, R.; Cascón, J. M.; Escobar, J. M.; Rodriguez, E.; Montero, G.
Abstract: This paper introduces a new automatic tetrahedral mesh generator on the adaptive finite element ALBERTA code. The procedure can be applied to 3D domains with boundary surfaces which are projectable on faces of a cube. The generalization of the mesh generator for complex domains which can be split into cubes or hexahedra is straightforward. The domain surfaces must be given as analytical or discrete functions. Although we have worked with orthogonal and radial projections, any other one-to-one projection may be considered. The mesh generator starts from a coarse tetrahedral mesh which is automatically obtained by the subdivision of each cube into six tetrahedra. The main idea is to construct a sequence of nested meshes by refining only the tetrahedra which have a face on the cube projection faces. The virtual projection of external faces defines a triangulation on the domain boundary. The 3-D local refinement is carried out such that the approximation of domain boundary surfaces verifies a given precision. Once this objective is achieved reached, those nodes placed on the cube faces are projected on their corresponding true boundary surfaces, and inner nodes are relocated using a linear mapping. As the mesh topology is kept during node movement, poor quality or even inverted elements could appear in the resulting mesh. For this reason, a mesh optimization procedure must be applied. Finally, the efficiency of the proposed technique is shown with several applications.
Sun, 01 Jan 2006 00:00:00 GMThttp://hdl.handle.net/10553/452422006-01-01T00:00:00Z
- A new meccano technique for adaptive 3-D triangulationshttp://hdl.handle.net/10553/45238Title: A new meccano technique for adaptive 3-D triangulations
Authors: Cascón, J. M.; Montenegro, R.; Escobar, J. M.; Rodriguez, E.; Montero, G.
Abstract: This paper introduces a new automatic strategy for adaptive tetrahedral mesh generation. A local refinement/derefinement algorithm for nested triangulations and a simultaneous untangling and smoothing procedure are the main involved techniques. The mesh generator is applied to 3-D complex domains whose boundaries are projectable on external faces of a coarse object meccano composed of cuboid pieces. The domain surfaces must be given by a mapping between meccano surfaces and object boundary. This mapping can be defined by analytical or discrete functions. At present we have fixed mappings with orthogonal, cylindrical and radial projections, but any other one-to-one projection may be considered. The mesh generator starts from a coarse tetrahedral mesh which is automatically obtained by the subdivision of each hexahedra, of a meccano hexahedral mesh, into six tetrahedra. The main idea is to construct a sequence of nested meshes by refining only those tetrahedra which have a face on the meccano boundary. The virtual projection of meccano external faces defines a valid triangulation on the domain boundary. Then a 3-D local refinement/derefinement is carried out such that the approximation of domain surfaces verifies a given precision. Once this objective is reached, those nodes placed on the meccano boundary are really projected on their corresponding true boundary, and inner nodes are relocated using a suitable mapping. As the mesh topology is kept during node movement, poor quality or even inverted elements could appear in the resulting mesh. For this reason, we finally apply a mesh optimization procedure. The efficiency of the proposed technique is shown with several applications to complex objects.
Tue, 01 Jan 2008 00:00:00 GMThttp://hdl.handle.net/10553/452382008-01-01T00:00:00Z
- Generalized top and bottom binary n-tupleshttp://hdl.handle.net/10553/44519Title: Generalized top and bottom binary n-tuples
Authors: González, L.
Abstract: A complex stochastic Boolean system (CSBS) depends on an arbitrary number n of random Boolean variables. The behavior of a CSBS is determined by the ordering between the occurrence probabilities Pr {u} of the 2 n associated binary strings u ∈ {0, 1} n. In this context, a binary n-tuple is called top (bottom, respectively) if its occurrence probability is always among the 2 n-1largest (smallest, respectively) ones. In this paper we generalize these n-tuples by defining and studying the k-top and k-bottom binary n-tuples, i.e., those whose occurrence probabilities are always among the k largest (smallest, respectively) ones (1 < k < 2 n). These results can be applied to the reliability analysis of many different technical systems, arising from diverse fields of Engineering. © 2008 Civil-Comp Press.
Tue, 01 Jan 2008 00:00:00 GMThttp://hdl.handle.net/10553/445192008-01-01T00:00:00Z
- An automatic strategy for adaptive tetrahedral mesh generationhttp://hdl.handle.net/10553/45234Title: An automatic strategy for adaptive tetrahedral mesh generation
Authors: Montenegro, R.; Cascón, J. M.; Escobar, J. M.; Rodriguez, E.; Montero, G.
Abstract: This paper introduces a new automatic strategy for adaptive tetrahedral mesh generation. A local refinement/derefinement algorithm for nested triangulations and a simultaneous untangling and smoothing procedure are the main involved techniques. The mesh generator is applied to 3-D complex domains whose boundaries are projectable on external faces of a meccano approximation composed of cuboids. The domain surfaces must be given by a mapping between meccano surfaces and object boundary. This mapping can be defined by analytical or discrete functions. At present, we have fixed mappings with orthogonal, cylindrical and radial projections, but any other one-to-one projection may be considered. The mesh generator starts from a coarse and valid hexahedral mesh that is obtained by an admissible subdivision of the meccano cuboids. The automatic subdivision of each hexahedron into six tetrahedra produces an initial tetrahedral mesh of the meccano approximation. The main idea is to construct a sequence of nested meshes by refining only those tetrahedra with a face on the meccano boundary. The virtual projection of meccano external faces defines a valid triangulation on the domain boundary. Then a 3-D local refinement/derefinement is carried out so that the approximation of domain surfaces verifies a given precision. Once this objective is reached, those nodes placed on the meccano boundary are really projected on their corresponding true boundary, and inner nodes are relocated using a suitable mapping. As the mesh topology is kept during node movement, poor quality or even inverted elements could appear in the resulting mesh; therefore, we finally apply a mesh optimization procedure. The efficiency of the proposed technique is shown with several applications to complex objects.
Thu, 01 Jan 2009 00:00:00 GMThttp://hdl.handle.net/10553/452342009-01-01T00:00:00Z