Starting from the 3D linearized Euler equations and decomposing the 3D perturbation quantities into a Fourier series in the azimuthal direction, a set of 2.5D linearized disturbance equations is derived which are valid for each azimuthal mode of fluctuation. The derivation is based on an axisymmetric mean flow and axisymmetric acoustic boundary conditions. A Computational Aeroacoustics (CAA) approach is applied to solve the 2.5D equation system. A fourth-order Dispersion-Relation-Preserving finite difference scheme is implemented for spatial discretization, whereas a 2N storage form Low Dissipation and Low Dispersion Runge-Kutta scheme is applied for time integration. Appropriate boundary conditions are prescribed at different boundary regions. The numerical procedure is firstly validated by a straight circular pipe and a straight annular duct subjected on subsonic uniform mean flows. Numerical results show very good agreement with the analytical solutions. Further numerical example is presented for an axisymmetric duct inlet with an aero-engine like geometry including a spinner inside. The aeroacoustic computation is based on an inviscid mean flow calculated by a 2nd order CFD solver. The CAA solutions agree rather well with the finite element results of Eversman as well as the semi-analytical multiple-scales solutions of Rienstra. In particular, a cut-on cut-off transition case has been simulated. This reveals the feasibility of the proposed theory and solution procedure.
Indirect combustion noise is investigated experimentally and numerically. This noise is generated in the outlet nozzle of combustion chambers if the entropy of the medium is nonuniform, which is the case in the exhaust of combustors. The contribution to the total noise emission of aeroengine combustors is not known. A test rig for the experimental investigation of this noise emission in the presence of swirl is first described. The indirect noise is generated in an exchangeable convergent-divergent nozzle at the exit of the combustor. The noise radiation is studied in a circular exhaust pipe with probe microphones using a radial mode analysis of the microphone signals. First results of the measured sound fields are reported. The experimental situation will be studied numerically with a 4 th order accurate CAA-method, which is first validated with theoretical results of the literature for the cases of a compact nozzle or diffuser and incoming entropy and sound waves in a one-dimensional mean flow. The agreement with the results of sound generation due to incoming entropy waves and the sound reflection and transmission for incoming sound waves is very good. The method is then applied to the more realistic cases of non-compact nozzles and it is found that the amplitudes of the generated waves are substantially smaller in comparison to the one-dimensional theory. The sound generation of real cases like a swirling hot-spot and entropy waves in a one-dimensional flow through a convergent-divergent nozzle as well as plain entropy waves in the swirl flow of the experimental setup are finally studied and the noise emission is computed.
Die Konstruktion leiser Flugzeuge gewinnt durch den ständig zunehmenden Flugverkehr und der damit einhergehenden Lärmbelastung immer mehr an Bedeutung. Eine wichtige Lärmquelle am Flugzeug, die besonders beim Start eine Rolle spielt, sind die Triebwerke. Durch die Einführung von Nebenstromtriebwerken konnte der Strahllärm der Triebwerke stark reduziert werden. Dies führte aber dazu, dass der durch den Fan verursachte tonale Lärm, der sich durch den Triebwerkseinlauf ausbreitet, zu einer wichtigen Komponente des Triebwerkslärms wurde. Für den Entwurfsprozeß bekommt die numerische Simulation der Schallausbreitung immer größere Bedeutung, da experimentelle Untersuchungen sehr aufwändig sind. Ziel ist die Simulation der Schallausbreitung von der Quelle bis ins Fernfeld mittels spezieller CAA‐Verfahren für axialsymmetrische Rohre.
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