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Simulation and Modeling of a Helmholtz Resonator under Grazing Turbulent Flow

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Abstract

When gas flows along a surface containing a cavity or gap, this fluid-dynamical process is likely linked with acoustical effects. Primarily, in the transport and energy sector, the phenomenon occurs frequently: gas flows around land and air vehicles or streams inside duct systems and engines. A key challenge for these examples is either to prevent cavity noise before it arises or to reduce existing tonal noise by installing a cavity absorber. The present thesis deals with a Helmholtz resonator beneath a turbulent flat plate flow. This representative example includes all the mentioned phenomena of acoustic excitation or damping under realistic conditions. For the first time, a Direct Numerical Simulation of a three-dimensional Helmholtz resonator excited by a turbulent flow is conducted, and an unprecedented database is set up. To effectively simulate on a high performance computing center, a multi-block parallelization method is developed and implemented for complex geometries. A universal acoustic model of the Helmholtz resonator under grazing flow is derived, based on the new numerical database, previous theories by Howe, and experiments by Golliard. This acoustic model stands out through its uniquely defined and physically meaningful parameters, instead of fitted constants. Utilizing the lumped element method, the model consists of exchangeable impedance elements which guarantee a flexible use. The model enables the user to understand and to trace back how a modification of design parameters like the spatial form or the type of incoming flow affects the sound spectrum. The model is validated for low Mach number flows (M=0.01-0.14) and frequencies around the Helmholtz resonator base frequency. Hence, an industrial user is no longer dependent on expensive and time-consuming test series within this typical range of operation. A priori, rather than by trial-and-error approach, the sound absorption spectrum can be easily tuned for specific frequencies. Consequently, the developed model simplifies the design process of cavity absorbers. Furthermore, the model predicts fluid and acoustic resonance conditions and such allows the design engineer to avoid tonal cavity noise in advance. In doing so, the user of the model can circumvent noise pollution and material wear before it occurs.
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Chapter
The present project studies the sound pressure inside an acoustic resonant cavity driven by turbulent flow. With this numerical study as an interim result, the ultimate goal is to improve the industrial layout of resonant cavities, in general: For example to circumvent resonance conditions. The current rise of high-performance computing allows us to simulate the dynamics of the non-linear turbulence-acoustic interaction by the high-quality method of a Direct Numerical Simulation (DNS). For the first time, the three dimensional geometry investigated here can be studied numerically in full detail without simplification. So far numerical studies of Helmholtz resonators with resolved neck shape do not consider an inflowing turbulent boundary layer or do not resolve all system scales, but assume some form of turbulence model. To effectively run the DNS on a supercomputer, a multi-block parallelization method is newly implemented for complex geometries, which consist of multiple, different sized blocks. Both in strong and weak scaling test a previous single-block parallelization is outperformed. The optimal load of gridpoints per core is identified and a distinction between misleading and meaningful weak scaling tests is made.
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Thesis
Freistrahlen (engl. Jets) mit einer komplexen Stoßzellen-Struktur treten in vielen technischen Anwendung auf. Die meisten Überschallfreistrahlen in der Luftfahrt sind nicht perfekt angepasst, auch nicht solche aus sorgfältig gestalteten konvergent-divergenten Düsen. Die Anpassung an den Umgebungsdruck erfolgt in einer Abfolge von schiefen Verdichtungsstößen, die mit den freien Scherschichten interagieren und Lärm erzeugen. Dabei strahlt die Interaktion von Stoß und Scherschicht einen breitbandigen Lärm ab. Dies kann die dünne Scherschicht am Düsenaustritt anregen und eine Rückkopplungsschleife bilden, die einen diskreten Ton namens Screech (dt. Kreischen) hervorruft. Beide Komponenten sind aus strukturellen und umgebungsbedingten Gesichtspunkten unerwünscht (z. B. Kabinenlärm). Screech-Töne erzeugen Schalldruckpegel von 160 dB und darüber hinaus. Der Fokus der vorliegenden Arbeit liegt in der Minimierung von Überschall Jet-Lärm, insbesondere in der Minimierung von Jet-Screech. Da Screech – ein Phänomen, das noch nicht in allen Einzelheiten verstanden ist – durch die Geometrie der Jet-Düse beeinflusst wird, soll ein poröses Material an der Düse angebracht werden, um den Rückkopplungsmechanismus zu unterdrücken. Dadurch wird ebenfalls der Screech-Ton unterdrückt. Es ist keineswegs klar, wie die charakteristischen Eigenschaften des porösen Materials beschaffen sein sollten, um den Lärm zu minimieren. Zu diesem Zweck wird ein Optimierungsverfahren, basierend auf adjungierten Methoden, angewandt, um die Materialeigenschaften in Bezug auf den Lärm zu optimieren.
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