Joan Baiges

The fixed-mesh ALE approach for the numerical approximation of flows in moving domains

In this paper we propose a method to approximate flow problems in moving domains using always a given grid for the spatial discretization, and therefore the formulation to be presented falls within the category of fixed-grid methods. Even though the imposition of boundary conditions is a key ingredient that is very often used to classify the fixed-grid method, our approach can be applied together with any technique to impose approximately boundary conditions, although we also describe the one we actually favor. Our main concern is to properly account for the advection of information as the domain boundary evolves. To achieve this, we use an arbitrary Lagrangian-Eulerian framework, the distinctive feature being that at each time step results are projected onto a fixed, background mesh, that is where the problem is actually solved.

Adaptive finite element simulation of incompressible flows by hybrid continuous-discontinuous Galerkin formulations

In this work we design hybrid continuous-discontinuous finite element spaces that permit discontinuities on nonmatching element interfaces of nonconforming meshes. Then we develop an equal-order stabilized finite element formulation for incompressible flows over these hybrid spaces, which combines the element interior stabilization of SUPG-type continuous Galerkin formulations and the jump stabilization of discontinuous Galerkin formulations. Optimal stability and convergence results are obtained. For the adaptive setting, we use a standard error estimator and marking strategy. Numerical experiments show the optimal accuracy of the hybrid algorithm for both uniformly and adaptively refined nonconforming meshes. The outcome of this work is a finite element formulation that can naturally be used on nonconforming meshes, as discontinuous Galerkin formulations, while keeping the much lower CPU cost of continuous Galerkin formulations.

Refficientlib: An Efficient Load-Rebalanced Adaptive Mesh Refinement Algorithm for High-Performance Computational Physics Meshes

In this paper we present a novel algorithm for adaptive mesh refinement in computational physics meshes in a distributed memory parallel setting. The proposed method is developed for nodally based parallel domain partitions where the nodes of the mesh belong to a single processor, whereas the elements can belong to multiple processors. Some of the main features of the algorithm presented in this paper are its capability of handling multiple types of elements in two and three dimensions (triangular, quadrilateral, tetrahedral, and hexahedral), the small amount of memory required per processor, and the parallel scalability up to thousands of processors. The presented algorithm is also capable of dealing with nonbalanced hierarchical refinement, where multirefinement level jumps are possible between neighbor elements. An algorithm for dealing with load rebalancing is also presented, which allows us to move the hierarchical data structure between processors so that load unbalancing is kept below an acceptable...

Variational multi-scale finite element approximation of the compressible Navier-Stokes equations

Purpose – The purpose of this paper is to apply the variational multi-scale framework to the finite element approximation of the compressible Navier-Stokes equations written in conservation form. Even though this formulation is relatively well known, some particular features that have been applied with great success in other flow problems are incorporated. Design/methodology/approach – The orthogonal subgrid scales, the non-linear tracking of these subscales, and their time evolution are applied. Moreover, a systematic way to design the matrix of algorithmic parameters from the perspective of a Fourier analysis is given, and the adjoint of the non-linear operator including the volumetric part of the convective term is defined. Because the subgrid stabilization method works in the streamline direction, an anisotropic shock capturing method that keeps the diffusion unaltered in the direction of the streamlines, but modifies the crosswind diffusion is implemented. The artificial shock capturing diffusivity i...

Reduced-Order Modelling Strategies for the Finite Element Approximation of the Incompressible Navier-Stokes Equations

In this chapter we present some Reduced-Order Modelling methods we have developed for the stabilized incompressible Navier-Stokes equations. In the first part of the chapter, we depart from the stabilized finite element approximation of incompressible flow equations and we build an explicit proper-orthogonal decomposition based reduced-order model. To do this, we treat the pressure and all the non-linear terms in an explicit way in the time integration scheme. This is possible due to the fact that the reduced model snapshots and basis functions do already fulfill an incompressibility constraint weakly. This allows a hyper-reduction approach in which only the right-hand-side vector needs to be reconstructed. In the second part of the chapter we present a domain decomposition approach for reduced-order models. The method consists in restricting the reduced-order basis functions to the nodes belonging to each of the subdomains. The method is extended to the particular case in which one of the subdomains is solved by using the high-fidelity, full-order model, while the other ones are solved by using the low-cost, reduced-order equations.

Unified solver for fluid dynamics and aeroacoustics in isentropic gas flows

The high computational cost of solving numerically the fully compressible Navier–Stokes equations, together with the poor performance of most numerical formulations for compressible flow in the low Mach number regime, has led to the necessity for more affordable numerical models for Computational Aeroacoustics. For low Mach number subsonic flows with neither shocks nor thermal coupling, both flow dynamics and wave propagation can be considered isentropic. Therefore, a joint isentropic formulation for flow and aeroacoustics can be devised which avoids the need for segregating flow and acoustic scales. Under these assumptions density and pressure fluctuations are directly proportional, and a two field velocity-pressure compressible formulation can be derived as an extension of an incompressible solver. Moreover, the linear system of equations which arises from the proposed isentropic formulation is better conditioned than the homologous incompressible one due to the presence of a pressure time derivative. Similarly to other compressible formulations the prescription of boundary conditions will have to deal with the backscattering of acoustic waves. In this sense, a separated imposition of boundary conditions for flow and acoustic scales which allows the evacuation of waves through Dirichlet boundaries without using any tailored damping model will be presented.

Simultaneous Finite Element Computation of Direct and Diffracted Flow Noise in Domains with Static and Moving Walls

Curle’s acoustic analogy allows one to compute aerodynamic noise due to flow motion in the presence of rigid bodies. However, the strength of the dipolar term in the analogy depends on the values of the total flow pressure on the body’s surface. At low Mach numbers, that pressure cannot be obtained from the computational fluid dynamics (CFD) simulation of an incompressible flow, because the acoustic component cannot be captured. To circumvent this problem, and still being able to separate the flow and body noise contributions at a far-field point, an alternative approach was recently proposed which does not rely on an integral formulation. Rather, the acoustic pressure is split into incident and diffracted components giving rise to two differential acoustic problems that are solved together with the flow dynamics, in a single finite element computational run. In this work, we will revisit the acoustics of that approach and show how it can be extended to predict the flow noise generated in domains with moving walls.

Local seeing determination by thermal-CFD analysis to optimize the European Solar Telescope image quality

The European Solar Telescope, EST, ([1], [2]) is a 4-meter solar telescope to be built in the Canary Islands in the near future. In order to select the best configuration for the EST telescope facilities, thermal and CFD analyses have been carried out to evaluate the seeing degradation produced by the telescope environment. The aim of this study is to calculate the values of optical parameters in different configurations and to find out which one causes the lowest image quality degradation. Starting from the determination of seeing degradation along the optical path by CFD techniques, several configurations have been compared making it possible to decide the future development line for the EST.