Numerical Simulation of Laminar and Turbulent Nozzle-Jet Flows and Their Sound

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1 Research Collection Doctoral Thesis Numerical Simulation of Laminar and Turbulent Nozzle-Jet Flows and Their Sound Author(s): Bühler, Stefan Publication Date: 2013 Permanent Link: Rights / License: In Copyright - Non-Commercial Use Permitted This page was generated automatically upon download from the ETH Zurich Research Collection. For more information please consult the Terms of use. ETH Library

2 Diss. ETH No NUMERICAL SIMULATION OF LAMINAR AND TURBULENT NOZZLE-JET FLOWS AND THEIR SOUND A dissertation submitted to ETH ZURICH for the degree of Doctor of Sciences presented by Stefan Bühler Dipl.-Ing., Universität Karlsruhe (TH) born on June 26, 1981 citizen of Germany accepted on the recommendation of Prof. Dr. L. Kleiser, examiner Prof. Dr. habil. (HDR) C. Bogey (EC Lyon), co-examiner 2013

3 Abstract Reduction of aerodynamically generated jet noise is a subject of great interest, as public concerns about noise emission have been rising steadily. With the tremendous growth of computing power in the past decades, numerical simulations of aeroacoustic flow problems have become feasible, which can help to give insight into basic noise generation mechanisms. To this end, a direct numerical simulation (DNS) framework is developed and validated for investigating a nozzle-jet flow configuration in which a cylindrical nozzle and the acoustic near-field are included in the simulation domain. The nozzle flow is modeled by a potential flow core and a developing turbulent wall boundary layer which is numerically resolved. The setup allows to create well-controlled physical nozzle-exit flow conditions and to examine their impact on near-nozzle flow dynamics, jet-flow development and the near-field sound. Turbulence at the nozzle inflow is generated by the Synthetic Eddy Method using flat-plate boundary-layer DNS data and imposed softly in a sponge layer. The jet Mach number in the present investigations is Ma = 0.9, the diameter-based jet Reynolds number is Re D = and the maximum axial RMS-fluctuations attain 13% at the nozzle exit. The accuracy of the numerical results is checked by varying grid resolution and computational domain size. The rapid flow development in the changeover region from wall turbulence to the turbulent free shear layer within about one nozzle diameter is documented in detail. Near-field sound pressure levels (SPLs) compare favorably with experimental reference data obtained at the much higher Reynolds number of This agreement is essentially attributed to a compensation of the effects of Reynolds number and turbulence level on the noise, for which an empirical scaling is derived from published data. Next, we investigate the effects of different jet inflows on the shearlayer and jet-flow development and the sound radiation, for the same Reynolds and Mach number. Within the nozzle, the flow is modeled by a potential flow core and a laminar, transitional or developing turbulent wall boundary layer. For laminar and transitional nozzle-exit conditions, transition to turbulence in the jet shear layer is governed by the development of Kelvin-Helmholtz instabilities. With the turbulent nozzle boundary layer, the jet flow development is characterized by a rapid changeover to a turbulent free shear layer. SPLs are strongly enhanced for laminar and transitional exit conditions compared to the turbulent case. However, a frequency and frequency-wavenumber analysis of the

4 near-field pressure indicates that the dominant sound radiation characteristics largely remain unaffected. By applying the developed scaling procedure we obtain a close match of the scaled near-field sound spectra for all nozzle-exit turbulence levels and also a reasonable agreement with experimental far-field data. Finally, we perform far-field computations based on a spectral formulation of Lighthill s acoustic analogy. Kurzfassung Die Aeroakustik von subsonischen Freistrahlen ist von besonderem Interesse im Hinblick auf die Reduzierung von Fluglärm. Grundlegende Fragen hinsichtlich Vorkommen und Form der Schallquellen und ihrer Relevanz bezüglich verschiedener Abstrahlrichtungen sind noch nicht geklärt. Durch den grossen Zuwachs an Rechenleistung in den letzten Jahren wurden numerische Untersuchungen rechenintensiver Akustik-Probleme möglich. Ziel der Arbeit ist die (parallele) numerische Simulation von Freistrahl-Strömungen mit Verfahren hoher Ordnung, die eine direkte Berechnung des akustischen Nahfeldes ermöglichen. Entscheidend für die Schallabstrahlung sind Fluktuationen im Strahl, wobei man zwischen turbulenten Fluktuationen und kohärenten Wirbelstrukturen unterscheidet, die gleichermassen relevant sein können. Um physikalisch realistische Fluktuationen in der Freistrahl-Entwicklung zu erzeugen und zu kontrollieren, wird die Düse in das Simulationsgebiet integriert. Von besonderem Interesse für die Schallentstehung sind turbulente Düsengrenzschichten, die zu einer turbulenten Entwicklung der freien Scherschicht direkt stromab der Düse führen. Durch Experimente motiviert wird die Strömung in der Düse zunächst als eine sich entwickelnde Rohrströmung mit turbulenter Wandgrenzschicht und Potentialkern modelliert und die Entwicklung des Strahls und des Schallfeldes für die Reynoldszahl Re D = und Machzahl Ma = 0.9 untersucht. Darauf aufbauend wird der Turbulenzgrad in der Grenzschicht sukzessiv reduziert, die daraus entstehende Strömungsund Schallfeldentwicklung untersucht und mit dem turbulenten Fall verglichen. Für laminare und transitionelle Düsengrenzschichten kommt es zur Ausbildung der bekannten Kelvin-Helmholtz-Instabiliät im Strahl, die zur laminar-turbulenten Transition in der Scherschicht und einer insgesamt stärkeren Schallabstrahlung führen. Zur Analyse

5 der aus den Simulationsdaten berechneten akustischen Quellterme wird eine spektrale Formulierung der Lighthill-Analogie genutzt und daraus die Schallabstrahlung ins Fernfeld berechnet und untersucht.