Scaling finite difference methods in large eddy simulation of jet engine noise to the petascale: numerical methods and their efficient and automated implementation

Reduction of jet engine noise has recently become a new arena of competition between aircraft manufacturers. As a relatively new field of research in computational fluid dynamics (CFD), computational aeroacoustics (CAA) prediction of jet engine noise based on large eddy simulation (LES) is a robust and accurate tool that complements the existing theoretical and experimental approaches. In order to satisfy the stringent requirements of CAA on numerical accuracy, finite difference methods in LES-based jet engine noise prediction rely on the implicitly formulated compact spatial partial differentiation and spatial filtering schemes, a crucial component of which is an embedded solver for tridiagonal linear systems spatially oriented along the three coordinate directions of the computational space. Traditionally, researchers and engineers in CAA have employed manually crafted implementations of solvers including the transposition method, the multiblock method and the Schur complement method. Algorithmically, these solvers force a trade-off between numerical accuracy and parallel scalability. Programmingwise, implementing them for each of the three coordinate directions is tediously repetitive and error-prone. ^ In this study, we attempt to tackle both of these two challenges faced by researchers and engineers. We first describe an accurate and scalable tridiagonal linear system solver as a specialization of the truncated SPIKE algorithm and strategies for efficient implementation of the compact spatial partial differentiation and spatial filtering schemes. We then elaborate on two programming models tailored for composing regular grid-based numerical applications including finite difference-based LES of jet engine noise, one based on generalized elemental subroutines and the other based on functional array programming, and the accompanying code optimization and generation methodologies. Through empirical experiments, we demonstrate that truncated SPIKE-based spatial partial differentiation and spatial filtering deliver the theoretically promised optimal scalability in weak scaling conditions and can be implemented using the two programming models with performance on par with handwritten code while significantly reducing the required programming effort

Parallel computation of aeroacoustics of industrially relevant complex-geometry aeroengine jets

Jet noise is still a distinct noise component when a commercial aircraft is taking off. A parallel high-fidelity simulation framework for industrial jet noise prediction is presented in this paper. This framework includes complex geometry meshing and Ffowcs Williams-Hawkings (FW-H) surface placement during preprocessing, a parallel hybrid RANS-LES flow solver coupled with an FW-H acoustic solver in the simulation and mean and unsteady data processing after the simulation. The use of this framework is demonstrated through two jet noise prediction cases: in-flight heated jets and installed ultra-high bypass-ratio (UHBPR) engines. These simulations can provide more insight than experimental tests into jet flow physics for engineering model improvement. Additional advantages are also shown in the cost and turn-around time. Thus there is great potential for high-fidelity jet noise simulations to partly replace rig tests for industrial use in the future

The prospect of using LES and DES in engineering design, and the research required to get there

In this paper we try to look into the future to divine how large eddy and detached eddy simulations (LES and DES, respectively) will be used in the engineering design process about 20-30 years from now. Some key challenges specific to the engineering design process are identified, and some of the critical outstanding problems and promising research directions are discussed.Comment: accepted for publication in the Royal Society Philosophical Transactions

Large Eddy Simulations of gaseous flames in gas turbine combustion chambers

Recent developments in numerical schemes, turbulent combustion models and the regular increase of computing power allow Large Eddy Simulation (LES) to be applied to real industrial burners. In this paper, two types of LES in complex geometry combustors and of specific interest for aeronautical gas turbine burners are reviewed: (1) laboratory-scale combustors, without compressor or turbine, in which advanced measurements are possible and (2) combustion chambers of existing engines operated in realistic operating conditions. Laboratory-scale burners are designed to assess modeling and funda- mental flow aspects in controlled configurations. They are necessary to gauge LES strategies and identify potential limitations. In specific circumstances, they even offer near model-free or DNS-like LES computations. LES in real engines illustrate the potential of the approach in the context of industrial burners but are more difficult to validate due to the limited set of available measurements. Usual approaches for turbulence and combustion sub-grid models including chemistry modeling are first recalled. Limiting cases and range of validity of the models are specifically recalled before a discussion on the numerical breakthrough which have allowed LES to be applied to these complex cases. Specific issues linked to real gas turbine chambers are discussed: multi-perforation, complex acoustic impedances at inlet and outlet, annular chambers.. Examples are provided for mean flow predictions (velocity, temperature and species) as well as unsteady mechanisms (quenching, ignition, combustion instabil- ities). Finally, potential perspectives are proposed to further improve the use of LES for real gas turbine combustor designs

Institute for Computational Mechanics in Propulsion (ICOMP)

The Institute for Computational Mechanics in Propulsion (ICOMP) was formed to develop techniques to improve problem-solving capabilities in all aspects of computational mechanics related to propulsion. ICOMP is operated by the Ohio Aerospace Institute (OAI) and funded via numerous cooperative agreements by the NASA Lewis Research Center in Cleveland, Ohio. This report describes the activities at ICOMP during 1997, the Institute's twelfth year of operation

A Computational Analysis of the Aerodynamics and Aeroacoustics of Jets with Fluid Injection

A detailed numerical analysis of fluidic injection as a tool to reduce noise emission is presented here. The noise reduction strategy, developed at the Pennsylvania State University, is based on injectors that blow air into the diverging section of the nozzle to emulate the effect of interior corrugation on the jet plume. The advantage is that the injection can be activated during takeoff and turned o_ during other phases of flight so that performance is not affected. Numerical simulations are performed on a military-style nozzle based on the GE F400-series engines, with a design Mach number of 1:65, for over-expanded jet conditions. The effectiveness of the fluidic injection as noise reduction technique is analyzed for heated and unheated jets. A high-order Large Eddy Simulation (LES) solver, developed originally at Purdue University, is used to analyze the flow-field and the acoustic field. New initial conditions and new boundary conditions are introduced. A set of Reynolds Averaged Navier-Stokes (RANS) simulations is used to set up the initial and boundary conditions for the LES runs. The numerical results are compared and validated with the outcome of experiments and RANS simulations performed at the Pennsylvania State University. The characteristics of unheated and heated jets are presented and compared. The higher temperatures do not modify the shock-cell structures, while they affect the jet development and the acoustic signature. The fluidic injection shows the potential of breaking down the shock-cells into smaller structures with lower strength, directly reducing the intensity of broadband shock associated noise. Moreover, the injectors are found to affect the development of the larger turbulent structures that generate the peak noise. For the cases tested the injectors reduce the peak noise by more than 1:5 dB for the unheated jet and by 3 dB for the heated jet, on the azimuthal plane in between two lines of injectors. The direction of maximum sound propagation moves from about 30_ to about 50_ as the jet gets heated. An analysis of the thrust changes due to activating the injectors is also presented for the heated and unheated jet conditions. The specific thrust is reduced by about 3% when the injectors are used

A coupled LES/high-order acoustic method for jet noise analysis

Aircraft noise is one of the main areas of active research for the aeronautical industry due to the increasingly stringent regulations on noise emission that aviation authorities are imposing. Among the different sources that contribute to the total emitted aircraft noise, jet noise is one of the most important during take-off. Furthermore, as the by-pass ratio of turbofan engines is increased, the interaction of the jet exhaust with the high-lift devices and the wing can potentially produce new mechanisms for noise generation. On the simulation front, the rapid increase of computing power over the last decades is enabling the use of high-fidelity simulations for the study of jet noise at both industrial and academic research levels. However, most of the numerical methods used by different research groups are either too dissipative for propagating the acoustic waves or are limited to the study of simple configurations. In many cases, surface integral methods have been the preferred choice with encouraging results for isolated jet configurations. Among these methods, the Ffowcs Williams-Hawkings (FWH) formulation has been commonly applied within research communities. However, applying them in complex configurations can be challenging, which may not provide sufficient information when it comes to studying noise generation mechanisms. The work reported in this thesis is devoted to the development of a coupling framework that is suitable for complex jet noise propagation cases. In this framework, the jet noise problem is divided into two different steps. First, the acoustic sources are computed using a robust compressible Large Eddy Simulation (LES) finite volume solver, which are then transferred to a spectral/hp high-order finite element Acoustic Perturbation Equations (APE) solver that propagates the sound waves to the far-field. Two different coupling strategies are investigated. Initially, a simple methodology based on the exchange of files between the solvers is implemented with only minor modifications made to the solvers’ source code. However, the poor efficiency of data transfer meant this method is applicable only to small problems. Thus, a more efficient parallel-interface coupling technique is developed to overcome this issue. With this technique all the required data is transferred via a parallel Message Passing Interface (MPI), avoiding the bottleneck of I/O and file systems. Both coupling techniques are validated with a 2-D cylinder case demonstrating the superiority of the parallel interface method. The parallel interface coupling framework is then tested on a low Reynolds number jet, being validated against experimental and numerical results in the literature, during which a well-established FWH method is used for references. More promising results are obtained using the LES/APE method than with the FWH method. The LES/APE method is then applied to the study of a more realistic isolated jet case and is compared to the experimental data obtained at NASA. A source analysis is further carried out, in this case, to reveal the distribution and convection of sources along the jet plume at different locations. The source distribution is in good agreement with the far-field noise results. Finally, the study of a jet-flat plate installed configuration is conducted. This simplified configuration is representative of a realistic installation scenario and is particularly useful to the understanding of the installation effects. The coupling framework captures these additional flow-acoustic effects demonstrating its potential to tackle complex configurations

Noise Characteristics of a Four-Jet Impingement Device Inside a Broadband Engine Noise Simulator

The noise generation mechanisms for four directly impinging supersonic jets are investigated employing implicit large eddy simulations with a higher-order accurate weighted essentially non-oscillatory shock-capturing scheme. Impinging jet devices are often used as an experimental apparatus to emulate a broadband noise source. Although such devices have been used in many experiments, a detailed investigation of the noise generation mechanisms has not been conducted before. Thus, the underlying physical mechanisms that are responsible for the generation of sound waves are not well understood. The flow field is highly complex and contains a wide range of temporal and spatial scales relevant for noise generation. Proper orthogonal decomposition of the flow field is utilized to characterize the unsteady nature of the flow field involving unsteady shock oscillations, large coherent turbulent flow structures, and the sporadic appearance of vortex tubes in the center of the impingement region. The causality method based on Lighthill's acoustic analogy is applied to link fluctuations of flow quantities inside the source region to the acoustic pressure in the far field. It will be demonstrated that the entropy fluctuation term in the Lighthill's stress tensor plays a vital role in the noise generation process. Consequently, the understanding of the noise generation mechanisms is employed to develop a reduced-order linear acoustic model of the four-jet impingement device. Finally, three linear acoustic FJID models are used as broadband noise sources inside an engine nacelle and the acoustic scattering results are validated against far-field acoustic experimental data

NAS Technical Summaries, March 1993 - February 1994

NASA created the Numerical Aerodynamic Simulation (NAS) Program in 1987 to focus resources on solving critical problems in aeroscience and related disciplines by utilizing the power of the most advanced supercomputers available. The NAS Program provides scientists with the necessary computing power to solve today's most demanding computational fluid dynamics problems and serves as a pathfinder in integrating leading-edge supercomputing technologies, thus benefitting other supercomputer centers in government and industry. The 1993-94 operational year concluded with 448 high-speed processor projects and 95 parallel projects representing NASA, the Department of Defense, other government agencies, private industry, and universities. This document provides a glimpse at some of the significant scientific results for the year