A new construction of perfectly matched layers for the linearized Euler equations

Based on a PML for the advective wave equation, we propose two PML models for the linearized Euler equations. The derivation of the first model can be applied to other physical models. The second model was implemented. Numerical results are shown.Comment: submitted for publication on February 1st 2005 What's new: interface conditions for the first PML model, a 3D section, more numerical result

Remarks on the stability of Cartesian PMLs in corners

This work is a contribution to the understanding of the question of stability of Perfectly Matched Layers (PMLs) in corners, at continuous and discrete levels. First, stability results are presented for the Cartesian PMLs associated to a general first-order hyperbolic system. Then, in the context of the pressure-velocity formulation of the acoustic wave propagation, an unsplit PML formulation is discretized with spectral mixed finite elements in space and finite differences in time. It is shown, through the stability analysis of two different schemes, how a bad choice of the time discretization can deteriorate the CFL stability condition. Some numerical results are finally presented to illustrate these stability results

Analysis and Application of Perfectly Matched Layer Absorbing Boundary Conditions for Computational Aeroacoustics

The Perfectly Matched Layer (PML) was originally proposed by Berenger as an absorbing boundary condition for Maxwell\u27s equations in 1994 and is still used extensively in the field of electromagnetics. The idea was extended to Computational Aeroacoustics in 1996, when Hu applied the method to Euler\u27s equations. Since that time much of the work done on PML in the field of acoustics has been specific to the case where mean flow is perpendicular to a boundary, with an emphasis on Cartesian coordinates. The goal of this work is to further extend the PML methodology in a two-fold manner: First, to handle the more general case of an oblique mean flow, where mean velocities strike the boundary at an arbitrary angle, and second, to adapt the equations for use in a cylindrical coordinate system. These extensions to the PML methodology are effectively carried out in this dissertation. Perfectly Matched Layer absorbing boundary conditions are presented for the linearized and nonlinear Euler equations in two dimensions. Such boundary conditions are presented in both Cartesian and cylindrical coordinates for the case of an oblique mean flow. In Cartesian coordinates, the PML equations for the side layers and corner layers of a rectangular domain will be derived independently. The approach used in the formation of side layer equations guarantees that the side layers will be perfectly matched at the interface between the interior and PML regions. Because of the perfect matching of the side layers, the equations are guaranteed to be stable. However, a somewhat different approach is used in the formation of the corner layer equations. Therefore, the stability of linear waves in the corner layer is analyzed. The results of the analysis indicate that the proposed corner equations are indeed stable. For the PML equations in cylindrical coordinates, there is no need for separate derivations of side and corner layers, and in this case, the stability of the equations is achieved through an appropriate space-time transformation. As is shown, such a transformation is needed for correcting the inconsistencies in phase and group velocities which can negatively affect the stability of the equations. After this correction has been made, the cylindrical PML can be implemented without risk of instability. In both Cartesian and cylindrical coordinates, the PML for the linearized Euler equations are presented in primitive variables, while conservation form is used for the nonlinear Euler equations. Numerical examples are also included to support the validity of the proposed equations. Specifically, the equations are tested for a combination of acoustic, vorticity and entropy waves. In each example, high-accuracy solutions are obtained, indicating that the PML conditions are effective in minimizing boundary reflections

A perfectly matched layer approach to the linearized shallow water equations models

Monthly Weather Review, 132 No.6, (2004), 1369 – 1378.A limited-area model of linearized shallow water equations (SWE) on an f-plane for a rectangular domain is considered. The rectangular domain is extended to include the so-called perfectly matched layer (PML) as an absorbing boundary condition. Following the proponent of the original method, the equations are obtained in this layer by splitting the shallow water equations in the coordinate directions and introducing the absorption coefficients. The performance of the PML as an absorbing boundary treatment is demonstrated using a commonly employed bell-shaped Gaussian initially introduced at the center of the rectangular physical domain

A General Perfectly Matched Layer Model for Hyperbolic-Parabolic Systems

This paper describes a very general absorbing layer model for hyperbolic-parabolic systems of partial differential equations. For linear systems with constant coefficients it is shown that the model possesses the perfect matching property, i.e., it is a perfectly matched layer (PML). The model is applied to two linear systems: a linear wave equation with a viscous damping term and the linearized Navier–Stokes equations. The resulting perfectly matched layer for the viscous wave equation is proved to be stable. The paper also presents how the model can be used to construct an absorbing layer for the full compressible Navier–Stokes equations. For all three applications, numerical experiments are presented. Especially for the linear problems, the results are very promising. In one experiment, where the performance of a “hyperbolic PML” and the new hyperbolic-parabolic PML is compared for a hyperbolic-parabolic system, an improvement of six orders of magnitude is observed. For the compressible Navier–Stokes equations results obtained with the presented layer are competitive with existing methods

Finite element simulation of noise radiation through shear layers

Predicting sound propagation through the jet exhaust of an aero-engine presents the specific difficulty of representing the refraction effect of the mean flow shear. This is described in full in the linearised Euler equations but this model remains rather expensive to solve numerically. The other model commonly used in industry, the linearised potential theory, is faster to solve but needs to be modified to represent a shear layer. This paper presents a way to describe a vortex sheet in a finite element model based on the linearised potential theory. The key issues to address are the continuity of pressure and displacement that have to be enforced across the vortex sheet, as well as the implementation of the Kutta condition at the nozzle lip. Validation results are presented by comparison with analytical results. It is shown that the discretization of the continuity conditions is crucial to obtain a robust and accurate numerical model

Error Control with Perfectly Matched Layer or Damping Layer Treatments for Computational Aeroacoustics with Jet Flows

In this paper we show by means of numerical experiments that the error introduced in a numerical domain because of a Perfectly Matched Layer or Damping Layer boundary treatment can be controlled. These experimental demonstrations are for acoustic propagation with the Linearized Euler Equations with both uniform and steady jet flows. The propagating signal is driven by a time harmonic pressure source. Combinations of Perfectly Matched and Damping Layers are used with different damping profiles. These layer and profile combinations allow the relative error introduced by a layer to be kept as small as desired, in principle. Tradeoffs between error and cost are explored

Discontinuous Galerkin Methods for Solving Acoustic Problems

Parciální diferenciální rovnice hrají důležitou v inženýrských aplikacích. Často je možné tyto rovnice řešit pouze přibližně, tj. numericky. Z toho důvodu vzniklo množství diskretizačních metod pro řešení těchto rovnic. Uvedená nespojitá Galerkinova metoda se zdá jako velmi obecná metoda pro řešení těchto rovnic, především pak pro hyperbolické systémy. Naším cílem je řešit úlohy aeroakustiky, přičemž šíření akustických vln je popsáno pomocí linearizovaných Eulerových rovnic. A jelikož se jedná o hyperbolický systém, byla vybrána právě nespojitá Galerkinova metoda. Mezi nejdůležitější aspekty této metody patří schopnost pracovat s geometricky složitými oblastmi, možnost dosáhnout metody vysokého řádu a dále lokální charakter toho schématu umožnuje efektivní paralelizaci výpočtu. Nejprve uvedeme nespojitou Galerkinovu metodu v obecném pojetí pro jedno- a dvoudimenzionalní úlohy. Algoritmus následně otestujeme pro řešení rovnice advekce, která byla zvolena jako modelový případ hyperbolické rovnice. Metoda nakonec bude testována na řadě verifikačních úloh, které byly formulovány pro testování metod pro výpočetní aeroakustiku, včetně oveření okrajových podmínek, které, stejně jako v případě teorie proudění tekutin, jsou nedílnou součástí výpočetní aeroakustiky.Partial differential equations play an important role in engineering applications. It is often possible to solve these equations only approximately, i.e. numerically. Therefore number of successful discretization techniques has been developed to solve these equations. The presented discontinuous Galerkin method seems to be very general method to solve this type of equations, especially useful for hyperbolic systems. Our aim is to solve aeroacoustic problems, where propagation of acoustic waves is described using linearized Euler equations. This system of equations is indeed hyperbolic and therefore the discontinuous Galerkin method was chosen. The most important aspects of this method is ability to deal with complex geometries, possibility of high-order method and its local character enabling efficient computation parallelization. We first introduce the discontinuous Galerkin method in general for one- and two-dimensional problems. We then test the algorithm to solve advection equation, which was chosen as a model case of hyperbolic equation. The method will be finally tested using number of verification problems, which were formulated to test methods for computational equations, including verification of boundary conditions, which, similarly to computational fluid dynamics, are important part of computational aeroacoustics.