Xesús Nogueira

Resolution of computational aeroacoustics problems on unstructured grids with a higher-order finite volume scheme

Computational fluid dynamics (CFD) has become increasingly used in the industry for the simulation of flows. Nevertheless, the complex configurations of real engineering problems make the application of very accurate methods that only work on structured grids difficult. From this point of view, the development of higher-order methods for unstructured grids is desirable. The finite volume method can be used with unstructured grids, but unfortunately it is difficult to achieve an order of accuracy higher than two, and the common approach is a simple extension of the one-dimensional case. The increase of the order of accuracy in finite volume methods on general unstructured grids has been limited due to the difficulty in the evaluation of field derivatives. This problem is overcome with the application of the Moving Least Squares (MLS) technique on a finite volume framework. In this work we present the application of this method (FV-MLS) to the solution of aeroacoustic problems.

Experimental and computational modeling of oscillatory flow within a baffled tube containing periodic-tri-orifice baffle geometries

Abstract This paper describes numerical simulation and matching experimental results for oscillatory flow within a baffled tube containing tri-orifice baffles. The numerical simulation implemented a non-standard approach based on Implicit Large Eddy Simulation (ILES) to predict the flow in a situation where complex eddy formation occurs due to periodic separation for each oscillation of the flow. The set of performed experiments showed that within the cavity between each baffle, the flow structure was complex with elements of periodicity and a wide range of length scales. Conventional Large Eddy Simulation and comparison with experiments were used to validate the ILES results. It was concluded that the gross flow structure was captured by ILES, indicating that the numerical scheme is suitable for computation in the complex flow situation. In addition, ILES methodology was used to study the effect of increasing the oscillation frequency on the radial and axial velocities.

Implicit large-Eddy simulation with a moving least squares-based finite volume method

In this contribution we present the application of a numerical method based on a mesh free interpolation technique (Moving Least-Squares (MLS)) in a finite volume framework to the computation of turbulent flows. Our approach is based on the monotonically implicit Large Eddy Simulation (MILES). The main idea of MILES methodology is the absence of any explicit subgrid scale (SGS) model in the numerical algorithm to solve turbulent flows. As MLS approximations are used to compute the gradients and higher-order derivatives, the parameters of the kernel function plays a crucial role on the properties of the numerical scheme, particularly on the dispersion and dissipation of the method. This allows us to define an implicit filter that acts as a SGS model in turbulence computations. We present the results for the decay of compressible isotropic turbulence, which are very promising, and the our first results computing turbulence flows in presence of walls.

Finite Volume Solvers and Moving Least Square Approximations for the Linearized Euler Equations on Unstructured Grids

This paper presents the application of a high‐order finite volume scheme based on the Moving Least Squares approximations (FVMLS) to solve Linearized Euler Equations (EEL) on unstructured grids. The proposed method allows the direct reconstruction of the (convective) fluxes using compact stencils, and without introducing new degrees of freedom, which is a real advantage over the most popular existing high‐order methods. This particularity confers to the proposed methodology a significant reduction in workload and memory storage capability. The proposed scheme is compared to a second‐order accurate finite volume scheme with centered fluxes around a uniform flow, with absorbing and/or reflecting boundary conditions. Some test cases are used to illustrate the potential of the FVMLS approximations on acoustic wave propagation. The ultimate objective of our research action is to simulate acoustic wave propagation into confined geometries and complex flows provided by subsonic turbomachines.

Smoothed Particle Hydrodynamics: A consistent model for interfacial multiphase fluid flow simulations

Abstract In this work, a consistent Smoothed Particle Hydrodynamics (SPH) model to deal with interfacial multiphase fluid flows simulation is proposed. A modification to the Continuum Stress Surface formulation (CSS) [1] to enhance the stability near the fluid interface is developed in the framework of the SPH method. A non-conservative first-order consistency operator is used to compute the divergence of stress surface tensor. This formulation benefits of all the advantages of the one proposed by Adami et al. [2] and, in addition, it can be applied to more than two phases fluid flow simulations. Moreover, the generalized wall boundary conditions [3] are modified in order to be well adapted to multiphase fluid flows with different density and viscosity. In order to allow the application of this technique to wall-bounded multiphase flows, a modification of generalized wall boundary conditions is presented here for using the SPH method. In this work we also present a particle redistribution strategy as an extension of the damping technique presented in [3] to smooth the initial transient phase of gravitational multiphase fluid flow simulations. Several computational tests are investigated to show the accuracy, convergence and applicability of the proposed SPH interfacial multiphase model.

Grounding Analysis in Heterogeneous Soil Models: Application to Underground Substations

Most of the research and development work done until now in earthing analysis is devoted to cases where the soil can be modelled in terms of an homogeneous and isotropic semi-infinite continuous medium, being the soil resistivity an order of magnitude higher than the resistivity of the electrode itself. Furthermore, all formulations of this classical (single-layer) type can be quite easily extended for solving more complex cases in which the soil is stratified in two or more layers of different resistivities. Assuming that the soil is regularized before installing the earthing grid and that the surface is leveled right after (prior to the equipment being assembled and brought into service) this approach is suitable for computing the equivalent resistance and the step and touch voltage in most large electrical over ground facilities. This important category includes all step up and step-down transmission substations, as well as a number of distribution substations indeed. Nevertheless, the current trend in electric power Engineering moves in another direction, due to the growing preference for smaller underground substations as a general rule. This pushes the need for developing new earthing analysis techniques that could be applied to facilities of this kind. In an attempt to do so, we present a mathematical model for earthing analysis in soils with enbedded finite regions of different resistivities. We also present the Boundary Element formulation that we propose for the numerical solution of the resulting equations, which implemention is being considered at present. Finally, we present some preliminary results of the earthing analysis of an underground substation. These results have been obtained by means of an approximated technique which details will be given in forthcoming publications.

Numerical Simulation of Multilayer Grounding Grids in a User-friendly Open-source CAD Interface

In this paper we present TOTBEM: a freeware application for the in-house computer aided design and analysis of grounding grids. The actual version of the software is available for testing purposes (and also use) at no cost and can be run on any basic personal computer (as of 2011) with no special requirements. The distribution kit consists in a single ISO bootable image file that can be freely downloaded from the Internet and copied into a DVD or a USB flash memory drive. The application runs on the Ubuntu 10.04 (Lucid Lynx) LTS release of Linux and can be easily started by just booting the system from the live DVD/USB that contains the downloaded file. This operation does not modify the native operative system nor installs any software in the computer, but the application is still fully operational while the live DVD/USB is taking control. The pre- and post-processing engines of the application have been built on top of the open source SALOME platform toolkit. On the other hand, the analysis part of the code is based on the Boundary Element Method (BEM). The implemented BEM formulation (that has been presented by the authors in previous publications) is suitable for computing the equivalent resistance and the step and touch voltage in most large electrical over ground substations with a grounding grid embedded in a single- or multi-layer stratified soil.

A Moving Least Squares-Based High-Order-Preserving Sliding Mesh Technique with No Intersections

The sliding mesh approach is widely used in numerical simulation of turbomachinery flows to take in to account the rotor/stator or rotor/rotor interaction. This technique allows relative sliding of one grid adjacent to another grid (static or in motion). However, when a high-order method is used, the interpolation used in the sliding mesh model needs to be of, at least, the same order than the numerical scheme, in order to prevent loss of accuracy. In this work we present a sliding mesh model based on the use of Moving Least Squares (MLS) approximations. It is used with a high-order ( > 2) finite volume method that computes the derivatives of the Taylor reconstruction inside each control volume using MLS approximants. Thus, this new sliding mesh model fits naturally in a high-order MLS-based finite volume framework (Cueto-Felgueroso et al., Comput Methods Appl Mech Eng 196:4712–4736, 2007; Khelladi et al., Comput Methods Appl Mech Eng 200:2348–2362, 2011) for the computation of acoustic wave propagation into turbomachinery.

A Higher-Order Finite Volume Method Using Multiresolution Reproducing Kernels

In this contribution we describe a numerical method based on the application of a mesh free interpolation technique (Moving Least Squares (MLS)) for the development of a higher-order finite volume discretization useful on structured and unstructured grids. With this procedure it is possible to build a higher-order scheme in which the computation of the derivatives is performed in a truly three-dimensional way. We use a MLS approach to compute the successive derivatives needed for the approximation of variables at element interfaces using Taylor series. Due to the use of cubic (or higher) reconstructions with the MLS technique, viscous fluxes are also approximated with higher-order accuracy and can be directly computed at edges.