Simulation of Cold Jet Installation Noise using a Stochastic Backscatter Model

This work presents results of Computational Aeroacoustics simulation for two different installation noise problems involving a cold jet interacting with a wing. Similar to very large-eddy simulation (VLES), the resolvable very large scales of turbulent fluctuations are directly calculated and the dissipation of the non-resolved scales is accounted for by a subfilter scale stress model. In addition, stochastic forcing in space and time is applied to model turbulent backscatter. The paper presents and discusses the rationale to explicitly realize turbulent backscatter along with details of the proposed stochastic backscatter model and its calibration. As a novel approach, the entire subfilter forcing function is modeled by means of an eddy-relaxation source term that provides forcing and dissipation as an entangled compound. The relaxation parameter defines the amount of correlation of the subfilter forcing with resolved quantities. Its proper calibration is achieved using decaying homogeneous isotropic turbulence. Further characteristics of the backscatter forcing are analyzed from synthetic turbulence data. The first jet-wing interaction problem studied is based on a generic static jet interacting with a non-inclined rectangular wing. The second problem deals with a dual-stream nozzle installed at a high-lift wing with deployed flap and slat in wind tunnel flow under approach conditions. For both problems installation noise from the airframe yields higher peak levels than the jet-noise contribution alone. For the first problem, relative to the corresponding jet spectrum a low-frequency narrow-band contribution is observed that can be attributed to coherent jet structures interacting with the airfoil trailing edge. Very good agreement with measured spectra is obtained. For the second problem a broadband airframe installation contribution to the overall spectrum is predicted with peak frequency above the jet contribution

„Sharp Immersed Boundary“ Implementierung zur Ermöglichung von Strömungsakustiksimulationen um komplexe Geometrien

Das Berechnen von Strömungsakustik um komplexe Geometrien scheitert oft an der Vernetzung. Blockstrukturierte Vernetzungen unter Berücksichtigung der Randbedingungen ist häufig schwierig und führt zu schlechter Gitterqualität. Unstrukturierte Verfahren benötigen teils aufwändige Aufbereitung der Geometrie um kleinste Elemente zu verhindern, welche den Zeitschritt limitieren würden. Die hier, in den strukturierten finiten Differenzen Computational AeroAcoustic (CAA) Code PIANO (basierend auf 4.Ordnung finite Dispersion Relation Preserving (DRP) Differenzen), implementierte Sharp Immersed Boundary (IBC) wird nur durch die Gitterfeinheit im Zeitschritt limitiert, nicht durch die Elementgrößen der Oberflächenbeschreibung. Das bedeutet, dass der Geometrieaufbereitungsaufwand sehr gering ist. In Kombination mit Hängenden Knoten ist die Gittergenerierung vollständig automatisierbar und dauert selbst für ganze Flugzeuge nur ein paar Minuten. Die Implementierung basiert auf Ghostpoints (GP), d.h. kartesische Netzpunkte, die unmittelbar an das Rechengebiet angrenzend, aber außerhalb verortet sind. Da mehr als ein GP an derselben Stelle existieren kann, oder auch im Fluid, sind auch sehr kantige oder dünne Geometrien handhabbar. Die Reduktion der Ordnung von 4.ter Ordnung DRP bis zu 2.Ordnung am Rand sorgt für hohe Stabilität. Eine Verfeinerung des Gitters um die Geometrie kann einen Genauigkeitsverlust verhindern. Die Implementierung wird vorgestellt und die Vorteile der Vernetzung an Hand einiger Beispiele dargestellt

3D Computation of Broadband Slat Noise from Swept and Unswept High-Lift Wing Sections

In previous work a RANS based simulation technique for the simulation of broadband slat noise was established. Good agreement was found between predicted and measured slat noise spectra. These predictions were based on 2D CAA computations and a connection to 3D measured data is only possible assuming a certain functional behavior of the spanwise coherence of the essential slat noise source. For this purpose, results from trailing edge noise measurements were used. In this work the simulation strategy is extended to 3D CAA computations, resolving the spanwise slat noise coherence as part of the CAA computations. The considered wing span is one main-chord, which is large enough to establish a realistic 3D problem for the turbulence as well as for the sound radiation. The Fast Random Particle Mesh (FRPM) method is applied for this study to generate fluctuating sound sources from steady RANS turbulence statistics. The study is conducted for the 30P30N airfoil with 0.457m main chord. The Mach number is 0.17 and the angle of attack is 4∘. Good agreement is found between the previous 2D and the 3D results as well as with unsteady simulations published in the literature. The influence of sweep on slat noise generation is studied

Stochastic approaches for airframe noise prediction

Stochastic approaches for airframe noise predictio

Modelling of combustion acoustics sources and their dynamics in the PRECCINSTA burner test case

A stochastic, hybrid computational fluid dynamics/computational combustion acoustics approach for combustion noise prediction is applied to the PRECCINSTA laboratory scale combustor (prediction and control of combustion instabilities in industrial gas turbines). The numerical method is validated for its ability to accurately reproduce broadband combustion noise levels from measurements. The approach is based on averaged flow field and turbulence statistics from computational fluid dynamics simulations. The three-dimensional fast random particle method for combustion noise prediction is employed for the modelling of time-resolved dynamics of sound sources and sound propagation via linearised Euler equations. A comprehensive analysis of simulated sound source dynamics is carried out in order to contribute to the understanding of combustion noise formation mechanisms. Therefrom gained knowledge can further on be incorporated for the investigation of onset of thermoacoustic phenomena. The method-inherent stochastic Langevin ansatz for the realisation of turbulence related source decay is analysed in terms of reproduction ability of local one- and two-point statistical input and therefore its applicability to complex test cases. Furthermore, input turbulence statistics are varied, in order to investigate the impact of turbulence on the resulting sound pressure spectra for a swirl stabilised, technically premixed combustor

Reformulated Flow-Acoustics Splitting for RANS/CAA based Acoustic Metrics: Tip Leakage Flow Problem

A novel acoustic metric is derived from reduced order modeling of turbulent near-field flow to characterize sources of sound caused by flow around complex geometry. The acoustic metric can serve as a physics based aeroacoustics cost function in 3-D optimization workflow. Quick turnaround times of less than 50-100 Core·h are deemed a crucial prerequisite for 3-D optimization in an industrial environment and can be accomplished with the new approach where an evaluation of the acoustic metric requires only the resolution of the source region of interest. The approach relies on an alternative flow/acoustics splitting of the Linearized Euler Equations (LEE) into Acoustic Perturbation Equations (APE) and linear equations for the split velocity fluctuations that govern vorticity transport. A specific dilatation based cross-coupling term can be identified that describes vortex sound generation. From this source term the acoustic metric is deduced. RANS informed stochastic forcing with the Fast Random Particle Mesh (FRPM) method is applied for obtaining inflow turbulence in the vortical velocity equations. In this paper, the acoustic metric is evaluated for a generic tip-gap flow problem typically encountered in industrial fan applications and for which noise reduction via geometrical optimization is of high interest. Besides the near-field simulations, additional acoustic simulations are carried out and resulting sound spectra are juxtaposed to near-field acoustic metric data by varying flow velocity and tip-gap size. Different discretization schemes for the split linear equations are applied, viz. a 4th order Discontinuous Galerkin based approach and a 4th order hierarchical-cartesian finite difference solver that combines the DRP scheme with an immersed/embedded boundary method. Proper trends are reproduced by the acoustic metric. The extension of the approach to a stochastically Forced Linear Advection Diffusion equation as the ultimate means to realize fully RANS-conformal synthetic turbulence on the foundations of well established numerics is discussed