Predictive Large Eddy Simulation for Jet Aeroacoustics–Current Approach and Industrial Application (original) (raw)
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Predictive LES for Jet Aeroacoustics: Current Approach and Industrial Application
Volume 2A: Turbomachinery, 2016
The major techniques for measuring jet noise have significant drawbacks, especially when including engine installation effects such as jet-flap interaction noise. Numerical methods including low order correlations and Reynolds-Averaged Navier-Stokes (RANS) are known to be deficient for complex configurations and even simple jet flows. Using high fidelity numerical methods such as Large Eddy Simulation (LES) allow conditions to be carefully controlled and quantified. LES methods are more practical and affordable than experimental campaigns. The potential to use LES methods to predict noise, identify noise risks and thus modify designs before an engine or aircraft is built is a possibility in the near future. This is particularly true for applications at lower Reynolds numbers such as jet noise of business jets and jet-flap interaction noise for under-wing engine installations. Hence, we introduce our current approaches to predicting jet noise reliably and contrast the cost of RANS-Numerical-LES (RANS-NLES) with traditional methods. Our own predictions and existing literature are used to provide a current guide, encompassing numerical aspects, meshing and acoustics processing. Other approaches are also briefly considered. We also tackle * Address all correspondence to this author. the crucial issues of how codes can be validated and verified for acoustics and how LES based methods can be introduced into industry. We consider that hybrid RANS-(N)LES is now of use to industry and contrast costs, indicating the clear advantages of eddy resolving methods.
Towards the prediction of noise from jet engines
International Journal of Heat and Fluid Flow, 2003
We are in the initial stages of development of a ''non-empirical'' numerical tool for jet-noise prediction in the airline industry, ultimately to treat complex nacelles and nozzles. The non-empirical demand leads to compressible large-eddy simulations, followed by post-processing to produce the far-field sound. Here we treat a simple cold jet with an axisymmetric geometry. The simulations leave out the subgrid-scale model (which causes too much dissipation on the present grid as is often the case in transitional flows), use slightly upwind-biased high-order differencing, and are preliminary in that a grid-refinement has not yet been performed. We do not use any unsteady forcing. The initial instability remains grid-sensitive, but the region with developed turbulence gives accurate statistics. The sound seen in the simulations is also realistic. The far-field sound calculations use the Ffowcs Williams-Hawkings (FWH) equation with a control surface that encloses the turbulence as much as possible, and the outside quadrupoles omitted. We focus on the influence of the surface location and the problem of closing the FWH surface at the outflow of the simulation. Though many physical and numerical issues are only partly resolved, the agreement with experiment is quite good for the soundÕs level, directivity, and spectral content.
17th AIAA/CEAS Aeroacoustics Conference (32nd AIAA Aeroacoustics Conference), 2011
A novel numerical scheme for unstructured compressible large eddy simulation (LES) is developed. This method is low-dissipative and less sensitive to the quality of the computational grid and is targeted for performing large-scale, high-fidelity simulations of turbulent flows in complex configurations. The objective of this work is to introduce this method, present a rigorous validation study, and demonstrate the application to a variety of jet configurations. This technique is validated by predicting the flow and noise emitted from a single-stream pressure-matched hot supersonic jet. Nearfield flow as well as farfield noise computed using an acoustic projection method is studied and compared to experimental measurements obtained by Dr. James Bridges at NASA Glenn. Mesh refinement studies and sensitivity study on selecting the acoustic projection surface are provided. To test the method's performance in a variety of jet noise configurations, it is applied to a high bypass ratio dual-stream jet at sonic conditions, a vertical supersonic jet impinging on the ground, and a horizontal supersonic jet impinging on an angled jet blast deflector.
Theoretical and Computational Fluid Dynamics, 2003
This study investigates the noise radiated by a subsonic circular jet with a Mach number of 0.9 and a Reynolds number of 65 000 computed by a compressible Large Eddy Simulation (LES). First, it demonstrates the feasibility of using LES to predict accurately both the flow field and the sound radiation on a domain including the acoustic field. Mean flow parameters, turbulence intensities, velocity spectra and integral length scales are in very good agreement with experimental data. The noise generated by the jet, provided directly by the simulation, is also consistent with measurements in terms of sound pressure spectra, levels and directivity. The apparent location of the sound sources is at the end of the potential core in accordance with some experimental observations at similar Reynolds numbers and Mach numbers. Second, the noise generation mechanisms are discussed in an attempt to connect the flow field with the acoustic field. This study shows that for the simulated moderate Reynolds number jet, the predominant sound radiation in the downstream direction is associated with the breakdown of the shear layers in the central jet zone.
Effects of Inflow Forcing on Jet Noise Using Large Eddy Simulation
42nd AIAA Aerospace Sciences Meeting and Exhibit, 2004
Recent discoveries have shown that by adjusting selected inflow forcing parameters, properties such as turbulent flow development and most importantly jet noise are influenced to some extent. To implement fully a nozzle structure in a high-end simulation like Large Eddy Simulation (LES) would require a prohibitive number of grid points to resolve the boundary layer for realistic Reynolds numbers. Thus, inflow forcing currently seems to be a reasonable substitute for a nozzle geometry. However, the drawback of this approach is that the flow field results are sensitive to inflow forcing parameters used. With LES as an investigative tool, this paper studies the effects of inflow forcing with particular emphasis on the number of azimuthal modes. We find that by removing the first few modes results in the jet developing slower, i.e. longer potential core. Furthermore, the peak turbulence intensities increase when we remove the first 6 and 8 modes of forcing. Due to this high peak turbulence intensities we found that the overall sound pressure level (OASPL) also increases at all observation angles for a closed control surface using the Ffowcs Williams-Hawkings method.
Implicit Large-Eddy Simulation of Noise Radiated by a Subsonic Jet at High Reynolds Number
28th AIAA Applied Aerodynamics Conference, 2010
An implicit large-eddy simulation (LES) methodology was implemented to predict the noise radiated by a subsonic jet at high Reynolds number. Unlike subgrid eddy-viscosity type models, this approach assumes that the subgrid model may be determined by the structure of the resolved flow and, therefore, does not require any additional subgrid-scale stress or heat flux terms in the flow equations. The high-frequency content of the smallest unresolved subgrid scales was removed by the application of high-order filtering to the flow variables. The effect of these scales on the largest filtered scales was reconstructed by an approximate deconvolution model. The flow solution was computed in the physical domain by the non-conservative form of the fully compressible Navier-Stokes equations, without the need of Favre filtering average. High-order accurate compact schemes were used for spatial discretization and a fourth-order Runge-Kutta method was employed for time integration. Non-reflecting boundary conditions and buffer zone treatments were prescribed by a characteristic-based formulation and a conceptual model based on the characteristic analysis. By this modeling approach outgoing waves exit the domain without generating high-frequency spurious wave reflections, which can contaminate the acoustic field solution. Implicit LES were carried out in single-block sequential and multi-block MPI parallel solvers to predict the aerodynamic noise radiated by the well-known test case of a Mach 0.90 jet at Reynolds number 6.5 × 10 4. Effects of grid resolution on the jet shear-layer characteristics, such as the jet inlet shear-layer momentum thickness, were investigated.
Noise Prediction for Increasingly Complex Jets. Part I: Methods and Tests
International Journal of Aeroacoustics, 2005
This Part I presents a detailed description of a numerical system built and tested with the final goal of reaching an accuracy of 2–3 dB over a meaningful range of frequencies for airliner engine noise, while having low empiricism and a general-geometry capability. The turbulence is treated by Large-Eddy Simulation with grids of around 1 million points, slightly upwind-biased high-order differencing, and implicit time integration. The code can incorporate boundaries and multi-block grids (thus avoiding the centerline singularity), and capture shocks. The sub-grid scale model is de-activated, because on present grids it strongly interferes with transition in the mixing layer. Without unsteady inflow forcing, the shear-layer roll-up and three-dimensionalization are realistic and reasonably insensitive to the grid. The far-field noise is computed using the permeable Ffowcs-Williams/Hawkings (FWH) formulation without external quadrupoles. The treatment of the disk that closes the FWH su...