The Simulation Of Slat Noise Applying Stochastic Sound Sources Based On Solenoidal Digital Filters (SDF)

A new fast and cheap stochastic approach is introduced to model the unsteady turbulent sound sources in the slat-cove of a high-lift airfoil. It is based on the spatial filtering of a random white-noise field and incoporates information about the integral length scale and the turbulent kinetic energy from a steady RANS computation. The stochastic method yields a solenoidal velocity field that is capable to reproduce exactly the well known analytical solution of the second-order two-point correlation tensor in case of homogeneous isotropic turbulence. Results for the sound generation at the slat are given for the underlying RANS mean-flow field being based on a Menter SST turbulence model with Kato-Launder modification. The results for the modeled turbulent flow-field and the radiated acoustic field exhibit physical meaningful characteristics

Efficient prediction of broadband trailing edge noise and application to porous edge treatment

Trailing edge noise generated by turbulent flow traveling past an edge of an airfoil is one of the most essential aeroacoustic sound generation mechanisms. It is of great interest for noise problems in various areas of industrial application. First principle based CAA with short response time are needed in the industrial design process for reliable prediction of spectral differences in turbulent-boundary-layer trailing-edge noise due to design modifications. In this paper, an aeroacoustic method is studied, resting on a hybrid CFD/CAA procedure. In a first step RANS simulation provides a time-averaged solution, including the mean-flow and turbulence statistics such as length-scale, time-scale and turbulence kinetic energy. Based on these, fluctuating sound sources are then stochastically generated by the Fast Random Particle-Mesh Method to simulate in a second CAA step broadband aeroacoustic sound. From experimental findings it is well known that porous trailing edges significantly lower trailing edge noise level over a large range of frequencies reaching up to 8dB reduction. Furthermore, sound reduction depends on the porous material parameters, e.g. geometry, porosity, permeability and pore size. The paper presents first results for an extended hybrid CFD/CAA method including porous materials with prescribed parameters. To incorporate the effect of porosity, an extended formulation of the Acoustic Perturbation Equations with source terms is derived based on a reformulation of the volume averaged Navier-Stokes equations into perturbation form. Proper implementation of the Darcy and Forchheimer terms is verified for sound propagation in homogeneous and anisotropic porous medium. Sound generation is studied for a generic symmetric NACA0012 airfoil without lift to separate secondary effects of lift and camber on sound from those of the basic edge noise treatments.Comment: 37 page

Hydrodynamic/acoustic splitting approach with flow-acoustic feedback for universal subsonic noise computation

A generalized approach to decompose the compressible Navier-Stokes equations into an equivalent set of coupled equations for flow and acoustics is introduced. As a significant extension to standard hydrodynamic/acoustic splitting methods, the approach provides the essential coupling terms, which account for the feedback from the acoustics to the flow. A unique simplified version of the split equation system with feedback is derived that conforms to the compressible Navier-Stokes equations in the subsonic flow regime, where the feedback reduces to one additional term in the flow momentum equation. Subsonic simulations are conducted for flow-acoustic feedback cases using a scale-resolving run-time coupled hierarchical Cartesian mesh solver, which operates with different explicit time step sizes for incompressible flow and acoustics. The first simulation case focuses on the tone of a generic flute. With the major flow-acoustic feedback term included, the simulation yields the tone characteristics in agreement with reference results from K\"uhnelt based on Lattice-Boltzmann simulation. On the contrary, the standard hybrid hydrodynamic/acoustic method with the feedback-term switched off lacks the proper tone. As the second simulation case, a thick plate in a duct is studied at various low Mach numbers around the Parker-beta-mode resonance. The simulations reveal the flow-acoustic feedback mechanism in very good agreement with experimental data of Welsh et al. Simulations and theoretical considerations reveal that the feedback term does not reduce the stable convective flow based time step size of the flow equations.Comment: Submitted to Journal of Computational Physic

Akustische Black-Hole-Analogie mit exakter Raumzeitrepräsentation

Beginnend mit der Arbeit von W. Unruh 1981 [1] wird im Rahmen von Analogmodellen (Analogue Models) der intrinsische Zusammenhang zwischen Strömungsakustik und Wellenpropagation in gekrummter Raumzeit der Allgemeinen Relativitätstheorie (ART) studiert. Obwohl die Strömungsakustik auf klassisch nicht-relativistischer Newtonscher Physik in flacher Raumzeit basiert, kann die Ausbreitung von Schallwellen im strömenden Medium durch eine effektive akustische Metrik in einer 3+1-dimensionalen Lorentz-Raumzeit beschrieben werden, aus der ein Analogon zwischen Schwarzen Löchern (Black-Holes) mit Schwarzschild-Metrik und einer einfachen Uberschall-Senkenströmung (Sonic Black-Hole) hergeleitet werden kann. Die Erweiterung des akustischen Analogmodells mittels Acoustic Perturbation Equations (APE) zur exakten Repräsentation der SchwarzschildMetrik wird aufgezeigt. Simulationsergebnisse fur ein akustisches Black-Hole mittels Simulationsmethoden der Computational Aeroacoustics (CAA) werden vorgestellt

Exakte gekoppelte akustisch-inkompressible Lösung der kompressiblen Navier-Stokes Gl.

Hybride strömungsakustische Zweischritt-Verfahren basierend auf skalenauflösender inkompressibler Navier-Stokes Simulation (CFD) und akustischer Störungssimulation (CAA) stellen eine gut validierte Methodik für das breitbandige Umströmungsgeräuschproblem bei kleiner Machzahl dar. Hybride Verfahren sind jedoch nicht in der Lage, tonale Quellen aus strömungsakustischer Ruckkopplung zu beschreiben. Ausgehend von den kompressiblen Navier-Stokes Gleichungen wird in dieser Arbeit eine exakte Aufspaltung in zwei gekoppelte Gleichungssysteme für Strömung und Akustik aufgezeigt, welche die strömungsakustische Ruckkopplung berücksichtigt. Die akustische Propagation wird durch akustische Störungsgleichungen (APE) über der instationären Hintergrundströmung beschrieben, der strömungsmechanische Teil über eine den inkompressiblen Navier-Stokes Gleichungen äquivalente Form mit zusätzlichem CAA-nach-CFD Kopplungstermen. Das gekoppelte Gleichungssystem ist nicht in der Machzahl beschränkt. Die quasi-inkompressible Behandlung des Strömungsproblems bietet eine effiziente zeitgenaue Simulationsmethodik, die mit optimalen numerischen Verfahren fur jeden Gleichungstyp auf unterschiedlichen Netzen für Strömung und Akustik realisiert werden kann. Die neue Methode wurde zur Simulation von zwei Fällen mit strömungsakustischer Ruckkopplung eingesetzt (Parker-Mode im Kanal und Orgelpfeife). Die Ergebnisse werden mit Messungen bzw. der Lösung der kompressiblen Navier-Stokes Gleichungen verglichen

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