Adaptive mesh refinement for computational aeroacoustics

UNIVERSITY OF SOUTHAMPTON ABSTRACT FACULTY OF ENGINEERING, SCIENCE & MATHEMATICS SCHOOL OF ENGINEERING SCIENCES Doctor of Philosophy ADAPTIVE MESH REFINEMENT FOR COMPUTATIONAL AEROACOUSTICS by Xun HuangThis thesis describes a parallel block-structured adaptive mesh refinement (AMR) method that is employed to solve some computational aeroacoustic problems with the aim of improving the computational efficiency. AMR adaptively refines and coarsens a computational mesh along with sound propagation to increase grid resolution only in the area of interest. While sharing many of the same features, there is a marked difference between the current and the established AMR approaches. Rather than low-order schemes generally used in the previous approaches, a high-order spatial difference scheme is employed to improve numerical dispersion and dissipation qualities. To use a high-order scheme with AMR, a number of numerical issues associated with fine-coarse block interfaces on an adaptively refined mesh, such as interpolations, filter and artificial selective damping techniques and accuracy are addressed. In addition, the asymptotic stability and the transient behaviour of a high-order spatial scheme on an adaptively refined mesh are also studied with eigenvalue analysis and pseudospectra analysis respectively. In addition, the fundamental AMR algorithm is simplified in order to make the work of implementation more manageable. Particular emphasis has been placed on solving sound radiation from generic aero-engine bypass geometry with mean flow. The approach of AMR is extended to support a body-fitted multi-block mesh. The radiation from an intake duct is modelled by the linearised Euler equations, while the radiation from an exhaust duct is modelled by the extended acoustic perturbation equations to suppress hydrodynamic instabilities generated in a sheared mean flow. After solving the near-field sound solution, the associated far-field sound directivity is estimated by solving the Ffowcs Williams-Hawkings equation. The overall results demonstrate the accuracy and the efficiency of the presented AMR method, but also reveal some limitations. The possible methods to avoid these limitations are given at the end of this thesis

Kompressible CFD-Simulationen von Aeroakustik für Automobilanwendungen

In this work, a direct noise computation method based on a low-Mach number flow solver is investigated. The new solver is implemented in the finite volume framework of the software OpenFOAM, accompanied with a new acoustic damping model for reducing spurious noise. The new solver is utilised to calculate noise generation and propagation for automotive applications. In order to validate the applicability of the low-Mach flow solver, a benchmark consisting of two-struts is calculated. The simulated aerodynamic near field as well as aeroacoustic far field are compared to wind tunnel measurements. The acoustic far field is computed using the direct method as well as a hybrid method. Both methods are evaluated based on comparing far field spectra and directivity patterns with experimental results. After validating the applicability of the low-Mach number solver, the topic of spurious noise generation in direct noise computation is addressed. Different spurious noise sources are presented and their generation mechanisms are investigated. Afterwards, two different strategies for spurious noise reduction, namely selective acoustic damping and numerical grid stretching, are discussed and validated. The acoustic damping model can substantially damp out spurious noise generated at grid interfaces without affecting the turbulence. It is also observed that the direction of grid refinement determines the direction of propagation of spurious noise. The strategies for spurious noise reduction are then applied on a side-mirror test case. For this, a new algorithm for automated and directional grid stretching is implemented. Spurious noise generation in the vicinity of the mirror’s surface as well as in the mirror’s wake could be substantially reduced and a quantitative analysis based on frequency-wavenumber spectra in the wake of the mirror is performed. Finally, the proposed flow solver, along with the strategies for spurious noise reduction, is used to directly compute noise generation on a generic vehicle model. Two different variants are calculated and the effect of the A-pillar and the side-mirror regarding their contribution to the acoustic waves on the side-window is investigated. Aerodynamic as well as aero- and vibroacoustic spectra on the side-window are calculated and compared to wind-tunnel measurements. For both variants, the results calculated using the direct method show good agreement with experimental data.In dieser Arbeit wird eine Methode zur direkten Berechnung von Aeroakustik basierend auf einen Strömungslöser für kleine Mach-Zahlen untersucht. Der Strömungslöser wird mit einem neuen Dämpfungsmodell für die Reduktion numerischer Schallwellen im finite Volumen Code OpenFOAM implementiert, und für die Berechnung der Entstehung und Ausbreitung von Schallwellen im Automobilbereich angewandt. Zur Validierung des neuen Strömungslösers, wird ein Benchmark, der aus zwei parallelen Streben besteht, berechnet. Das simulierte aerodynamische Nahfeld sowie das aeroakustische Fernfeld werden mit Windkanalmessungen verglichen. Das akustische Fernfeld wird mit der direkten sowie mit einer hybriden Methode berechnet. Beide Methoden werden anhand der Fernfeldspektren sowie der Richtcharakteristiken mit experimentellen Daten bewertet. Nach der Validierung des Strömungslösers wird die Entstehung von numerischen Störungen in der direkten Methode analysiert. Es werden verschiedene Quellen numerischer Störungen sowie deren Entstehungsmechanismen dargestellt. Anschließend werden zwei verschiedene Strategien zur Reduktion von Störungen diskutiert und validiert. Das Dämpfungsmodell zeigt sein Potenzial bei der Reduktion von numerischen Schallwellen ohne Beeinflussung der Turbulenz. Es wird außerdem gezeigt, dass die Richtung einer Verfeinerung des numerischen Gitters die Richtung der Ausbreitung numerischer Schallwellen bestimmt. Die Strategien zur Reduktion numerischer Störungen werden weiterhin an einem einzelnen Seitenspiegel angewandt. Dafür wird ein neuer Algorithmus für eine automatisierte und richtungsdefinierte Gitterexpansion implementiert. Die Amplitude numerischer Störungen, die im Spiegelnachlauf entstehen, werden mit Hilfe einer Frequenz-Wellenzahl Analyse quantitativ untersucht. Es zeigt sich, dass das Dämpfungsmodell diese Störungen deutlich reduziert. Abschließend wird der Strömungslöser zusammen mit den vorgeschlagenen Strategien in einer direkten Aeroakustikberechnung eines generischen Fahrzeugmodells angewandt. Es werden zwei unterschiedliche Varianten berechnet und der Einfluss der A-Säule und des Seitenspiegels bezüglich ihres akustischen Beitrags auf der Seitenscheibe untersucht. Sowohl aerodynamische als auch aero- und vibroakustische Spektren werden auf der Seitenscheibe berechnet und mit Windkanalmessungen verglichen. Für beide Varianten zeigen die Ergebnisse der direkten Methode gute Übereinstimmung mit den experimentellen Daten

Block-structured adaptive mesh refinement for computational aeroacoustics

High-order spatial discretization schemes based on block-structured adaptive mesh refinement (AMR) method are developed to solve computational aeroacoustics (CAA) problems, with the aim of improving efficiency and reducing costs. In this presentation we present several aspects of AMR application. Firstly, the AMR algorithm will be introduced. Basic work flow charts and pseudo codes are provided. The cost of parallel AMR operations is profiled to show that its burden is not significant. In the second part a number of numerical issues including implementation of high-order spatial schemes (dispersion-relation-preserving scheme and optimized prefactored compact scheme), fine-coarse interfaces under the AMR environment, stability analysis and filter/damping techniques are addressed. To demonstrate the feasibility and efficiency of the approach, the code is applied to some benchmark CAA problems. In particular the problem of spinning modal wave radiation from an unflanged duct is considered. The computed far-field sound directivity is found to agree well with analytical solutions, and the computing time is about one third of that on a uniform mesh. Finally a realistic engine intake problem is studied to show the working of parallel AMR code